Stavely Project – Regional 3D Geological Model

R.A. Cayley, M.A. McLean, P.B. Skladzien & C.P. Cairns

Stavely Project Report 3 Authorised by the Director, Geological Survey of Victoria Department of Economic Development, Jobs, Transport and Resources 1 Spring Street Melbourne Victoria 3000 Telephone (03) 9651 9999 © Copyright State of Victoria, 2018. Department of Economic Development, Jobs, Transport and Resources 2018 Except for any logos, emblems, trademarks, artwork and photography this this work is made available under the terms of the Creative Commons Attribution 4.0 Australia licence. To view a copy of this licence, visit creativecommons.org/licenses/by/4.0/. It is a condition of this Creative Commons Attribution 4.0 Licence that you must give credit to the original author who is the State of Victoria. Bibliographic reference: Cayley, R.A., McLean M.A., Skladzien P.B & Cairns C.P., 2018. Stavely Project – Regional 3D Geological Model. Stavely Project Report 3. Geological Survey of Victoria. Department of Economic Development, Jobs, Transport and Resources. ISSN 2207-8681 – Online (pdf / word) ISBN 978-1-925734-18-8 (pdf/online/MS word) GSV Catalogue Record #154990 Keywords: 3D geological modelling, potential field, gravity, magnetic, Stavely Project, Stavely Arc, Grampians Group, Cambrian, porphyry, volcanic massive hosted sulphide, mineral exploration, copper, Victoria. Acknowledgements: The development of the STAVELY 3D model has significantly benefited from the data and findings from the Southern Delamerian deep seismic reflection survey in 2009, a collaboration between the Geological Survey of Victoria, Geoscience Australia, AuScope Limited and the Department of Primary Industries and Regions (South Australia), and pre-competitive stratigraphic drilling undertaken as part of the Stavely Project, a collaboration between the Geological Survey of Victoria and Geoscience Australia. The skills, knowledge and expertise brought by individuals to these projects was fundamental in their successful delivery. Chris Osborne constructed the Rocklands Volcanic Group surface and refined the Newer Volcanic Group surface. James Goodwin (Geoscience Australia) completed early fault analysis and delineation and commenced modelling of the Grampians Group. Sarlae McAlpine (Geoscience Australia) identified, reviewed and compiled Newer Volcanic Group intersections from historical drill holes. David Taylor participated in technical discussions regarding the location and timing of faults and their influence on the regional geology. Lauren Wolfram, Ken Sherry and Taylor Ogden assisted in drafting figures. Stewart Govett assisted in preparation of the geological cross sections for publication. Navarre Minerals Ltd provided airborne magnetic data for modelling of the volcanic belts in the Black Range. Cover photo: Ross Cayley taking portable XRF measurement of botryoidal and vuggy silica alteration in Buckeran Diorite at the Lexington prospect, merged with image taken from the STAVELY 3D model (Phil Skladzien, Geological Survey of Victoria). Disclaimer: This publication may be of assistance to you but the State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication. The Victorian Government, authors and presenters do not accept any liability to any person for the information (or the use of the information) which is provided or referred to in the report. For more information about earth resources in Victoria visit the website at www.earthresources.vic.gov.au. Contents

Executive summary ...... xi Key findings for mineral exploration and targeting ...... xii 1. Introduction ...... 1 1.1 Background ...... 1 1.2 Geological context ...... 5 1.3 Aims ...... 6 1.4 Scientific and economic justification ...... 6 1.5 Philosophy ...... 9 2. Modelling methodology ...... 10 2.1 Geological rationale...... 10 2.2 Model extent ...... 10 2.3 Software ...... 11 2.4 Inputs and constraints ...... 11 2.4.1 Regional geological mapping and drilling ...... 13 2.4.2 Potential field data, processing and enhancement...... 13 2.4.2.1 Magnetic data ...... 13 2.4.2.2 Gravity data ...... 13 2.4.2.3 Data processing ...... 13 2.4.2.4 Image enhancement ...... 20 2.4.3 Key sites of constraint ...... 20 2.4.4 Rock properties ...... 31 2.4.5 Regional 2D seismic lines ...... 32 2.4.6 Existing 3D models ...... 35 2.5 Modelling workflow ...... 37 2.5.1 Modelling workflow summary...... 39 2.6 Interpretation methodology...... 39 2.6.1 Fault timing ...... 41 2.6.2 Fault shape and orientation ...... 41 2.6.3 Blind faults ...... 44 2.6.4 Interpretation of geophysical data ...... 44 2.6.4.1 Interpretation of fault network ...... 46 2.6.4.2 Interpretation of intrusive bodies ...... 48 2.6.5 Potential field data forward modelling and inversion ...... 48 2.6.5.1 Magnetic inversion modelling of the volcanic belts ...... 50 2.6.5.2 Gravity forward modelling – Serial cross sections ...... 53 3. Modelled surfaces and volumes ...... 61 3.1 Modelled geological units (surfaces) ...... 61 3.1.1 Newer Volcanic Group ...... 61 3.1.2 Murray Basin ...... 62 3.1.3 Otway Basin ...... 63 3.1.4 Rocklands Volcanic Group ...... 65 3.2 Modelled geological units (volumes) ...... 65 3.2.1 Intrusives – Devonian ...... 65 3.2.2 Grampians Group ...... 66 3.2.3 Intrusives – Cambrian (possible Ordovician) ...... 70 3.2.4 Nargoon Group ...... 72 3.2.5 Stavely Arc ...... 73 3.2.5.1 Black Range Belt ...... 80 3.5.2.2 Black Range West/Mitre Belt ...... 82 3.2.5.3 Boonawah Belt ...... 84 3.2.5.4 Brimpaen Belt ...... 84 3.2.5.5 Bunnugal Belt ...... 87 3.2.5.6 Caramut Belt ...... 93 3.2.5.7 Dimboola Belt ...... 94 3.2.5.8 Dryden Belt ...... 98 3.2.5.9 Dryden North Belt ...... 102 3.2.5.10 Elliot Belt ...... 104 3.2.5.11 Glenisla Belt ...... 105 3.2.5.12 Grampians ‘Deeps’ Belt ...... 107 3.2.5.13 Grampians ‘West’ Belt ...... 109 3.2.5.14 Narrapumelap Belt ...... 112 3.2.5.15 Pella Belt ...... 115 3.2.5.16 Stavely Belt ...... 115 3.2.5.17 Tyar Belt ...... 120 3.2.6 Kanmantoo Group ...... 121 3.2.7 Stawell Zone ...... 124 3.2.8 Glenelg Zone ...... 124 3.2.9 Unaffiliated highly reflective rocks ...... 124 3.2.10 Unaffiliated unreflective rocks ...... 124 3.2.11 Upper mantle ...... 125 3.3 Modelled faults ...... 125 3.3.1 Overview of faults ...... 126 3.3.1.1 Apsley Fault ...... 130 3.3.2 Regional-scale D1a faults ...... 130 3.3.2.1 Moyston Fault ...... 131 3.3.2.2 Yarrramyljup Fault ...... 133 3.3.3 District-scale D1a faults...... 135 3.3.3.1 Boonawah Fault East and West ...... 135 3.3.3.2 Bunnugal East Fault ...... 136 3.3.3.3 Bunnugal West Fault (new name) ...... 137 3.3.3.4 Dryden Fault ...... 138 3.3.3.5 Elliot North Fault (new name) ...... 138 3.3.3.6 Elliot South Fault ...... 139 3.3.3.7 D1a Escondida Fault ...... 139 3.3.3.8 Mouchong Fault System ...... 141 3.3.3.9 Muline Fault System ...... 142 3.3.3.10 Stavely Base Fault ...... 142 3.3.3.11 Narrapumelap North Fault (and Williamsons Fault correlates in the Narrapumelap Belt) ...... 143 3.3.3.12 Narrapumelap South Fault ...... 145 3.3.3.13 Stavely East Fault ...... 145 3.3.3.14 Stavely West Fault ...... 146 3.3.3.15 Tyar Fault System...... 146 3.3.3.16 Unnamed D1a faults beneath the Grampians Group – Grampians ‘Deeps’ and Brimpaen ...... 146 3.3.3.17 Williamsons Fault ...... 147 3.3.4 Regional-scale D3 and D4 faults ...... 148 3.3.4.1 Escondida Fault (reactivated) ...... 149 3.3.4.2 Golton Fault ...... 153 3.3.4.3 Henty Fault (new name) ...... 155 3.3.4.4 Henty West Fault (new name) ...... 155 3.3.4.5 Jalur Fault ...... 156 3.3.4.6 Marathon Fault ...... 156 3.3.4.7 Mehuse Fault ...... 158 3.3.4.8 Mosquito Creek Fault ...... 159 3.3.5 District-scale D3 and D4 faults ...... 159 3.3.5.1 Ashens Fault (new name) ...... 159 3.3.5.2 Angip Fault (new name) ...... 159 3.3.5.3 Babatchio Fault (new name) ...... 160 3.3.5.4 Barbican Fault and Cattle Camp Fault ...... 160 3.3.5.5 Cherrypool Fault ...... 160 3.3.5.6 Curtis Fault (new name) ...... 160 3.3.5.7 Dimboola Fault (new name) ...... 160 3.3.5.8 Dollin Fault (new name) ...... 161 3.3.5.9 Escondida East splay fault ...... 161 3.3.5.10 Glenlee Fault (new name) ...... 161 3.3.5.11 Grass Flat Fault (new name) ...... 161 3.3.5.12 Hopkins River Fault (new name) ...... 161 3.3.5.13 Latani Fault ...... 162 3.3.5.14 Log Hut Fault ...... 162 3.3.5.15 Lorquon Fault (new name) ...... 162 3.3.5.16 Lorquon West Fault (new name) ...... 162 3.3.5.17 Mackenzie River Fault ...... 162 3.3.5.18 Muirfoot Fault ...... 163 3.3.5.19 Nareeb Fault (new name) ...... 163 3.3.5.20 Olive Fault ...... 163 3.3.5.21 Tullyvea Fault (new name) ...... 164 3.3.5.22 Victoria Valley South Fault, Victoria Valley North Fault (new name) ...... 164 3.3.5.23 Wannon Fault (new name) ...... 166 3.3.5.24 West Wail Fault (new name) ...... 166 3.3.5.25 Winian East Fault (new name) ...... 166 3.3.5.26 Woorndoo Fault ...... 167 3.3.5.27 Yarrack Fault (new name) ...... 167 3.3.6 Post Devonian faults ...... 167 3.3.6.1 Glenthompson Fault ...... 168 3.3.6.2 Tarrington Fault ...... 168 3.4 Modelled folds ...... 168 3.4.1 Bepcha Fold ...... 174 3.4.2 Connangorach Fold ...... 174 3.4.3 Dollin Kink ...... 175 3.4.4 Mafeking Megakink ...... 175 3.4.5 Tyar Fold ...... 176 3.4.6 Wallup Fold ...... 176 3.4.7 Yarrack Orocline...... 177 4. Discussion ...... 178 4.1 D4 and D3 retrodeformation testing ...... 178 4.1.1 Mafeking Megakink retrodeformation ...... 181 4.1.2 Jalur Rift retrodeformation ...... 181 4.1.3 ‘Crab Nebula’ retrodeformation ...... 184 4.1.4 Dimboola Duplex retrodeformation ...... 184 4.1.5 Retrodeformation of locally overturned Cambrian strata in the Moyston Fault footwall ...... 185 4.1.6 Analysis of D4 and D3 retrodeformation results ...... 192 4.2 D2 retrodeformation testing ...... 196 4.3 D1 retrodeformation testing ...... 196 4.4 Implications for the architecture of the Stavely Arc through time ...... 196 4.4.1 Original configuration of the Stavely Arc ...... 197 4.4.2 Influence of deformation on Cambrian intrusives including porphyries ...... 197 4.4.3 Understanding the form and distribution of potential transfer structures ...... 198 4.4.4 Influence of deformation on Devonian intrusive rocks ...... 202 4.5 Strain (Stress) history mapping ...... 202 4.5.1 A scalable structural template for STAVELY through time ...... 209 4.5.2 Summary ...... 211 5. References ...... 212

Appendix 1 STAVELY 3D model export ...... 219

Appendix 2 Geological cross sections ...... 221

Appendix 3 Forward model sections ...... 228

Appendix 4 Geological units ...... 241

Appendix 5 Deformation history summary ...... 255

Appendix 6 Fault summary table ...... 259 Figures

Figure 1.1 Location of STAVELY underlain by 1:250 000 seamless geology of Victoria ...... 1 Figure 1.2 Location of STAVELY underlain by 1:50 000 seamless geology of Victoria ...... 2 Figure 1.3 Location of STAVELY showing distribution of Cambrian volcanic belts within the Grampians-Stavely Zone, with regional airborne magnetic data as background...... 3 Figure 1.4 Location of STAVELY showing distribution of Cambrian volcanic belts with gridded regional ground gravity data as background...... 4 Figure 1.5 The entire STAVELY 3D model, showing (A) all (pre)Cambrian – Silurian rock volumes to a depth of 40 km and (B) with Ordovician-Silurian strata omitted to show subsurface distribution of fault-belts of Cambrian Stavely Arc strata throughout STAVELY, infaulted with Glenthompson Sandstone and Nargoon Group...... 7 Figure 1.6 The STAVELY 3D model with most model volumes omitted to show the entire modelled fault network, and including magmatic rock volumes...... 8 Figure 2.1 Theoretical example showing the use of overprinting criteria to establish a geological history...... 10 Figure 2.2 Spatial extent of STAVELY underlain by 1:250 000 seamless surface geology, showing the Yarramyljup and Moyston faults and major towns...... 12 Figure 2.3 Key inputs and constraints used to develop the STAVELY 3D model ...... 14 Figure 2.4 Select images of the Southeast Australia shear wave velocity model of Young et al. (2013), showing A) oblique view of a depth-slice at 23 km within the model volume, and a vertical shear wave velocity profile through the same volume and B) plan view of the same 23 km depth-slice, depicting the interpreted positions, at that depth, of the Apsley Fault and the leading, eastern edge (the ‘Tasman Line’) of buried crystalline ‘continental’ Proterozoic crust that we interpret to form the foundation crust through which the Stavely Arc was erupted...... 15 Figure 2.5 Airborne magnetic data: A) Regional TMI data gridded at 50 m cells over STAVELY. Square shows location of detailed airborne magentic survey; B) Detailed (50 m line spacing) TMI data gridded at 12.5 m cells over the Black Range...... 17 Figure 2.6 Regional ground gravity: Complete Bouguer Anomaly data gridded at 300 m over STAVELY...... 18 Figure 2.7 Comparison of various filters applied to potential field data showing the Black Range region as an example...... 19 Figure 2.8 Wavelength filtering of magnetic data over the Cambrian-aged Boonawah and Bunnugal belts. Band pass filtering applied to TMI data (A) to remove the shorter wavelength, high frequency response of Newer Volcanic Group, and better delineate the longer wavelength, lower frequency response of the deeper volcanic belts (B and C)...... 21 Figure 2.9 Wavelength filtering of magnetic data west of the Grampians Ranges. Band pass filtering applied to TMI data (A) to remove the shorter wavelength, high frequency response of Rocklands Volcanic Group, accentuate the longer wavelength, lower frequency response of deeper sources, including potential Devonian intrusions (B and C)...... 21 Figure 2.10 Examples of remenant magnetisation within the Black Range Belt. Left hand panels show TMI; right hand panels show analytic signal. Arrows indicate location of probable reversely magnetised porphyries: A) TMI of northern Black Range Belt, B) Analytic signal of northern Black Range Belt, C) TMI of central Black Range Belt, D) Analytic signal of central Black Range Belt...... 22 Figure 2.11 Image enhancement – Magnetics: Image is a NE sun-shaded, partially-transparent histogram-equalised pseudocolour of tilt-angle filtered TMI (RTP), overlain on a 3 – 30 km band pass filtered, histogram-equalised pseudocolour image of TMI (RTP)...... 23 Figure 2.12 Image enhancement – Gravity: Image is a histogram-equalised pseudocolour layer of 30 km high pass filtered Bouguer gravity draped on NE sun-shaded TMI (RTP) intensity layer...... 24 Figure 2.13 A) Moyston Fault plane exposed in a creek near Lennox Springs Road, south of Moyston, B) Typical western Stawell Zone rocks: Carrolls Amphibolite typical of the Moyston Fault hangingwall, C) Thin section of Carrolls Amphibolite, showing layered, recrystallised hornblende-sphene-quartz-garnet metamorphic mineralogy, D) Typical western Stawell Zone rocks: Good Morning Bill Schist in the Moyston Fault hangingwall, showing boudinaged quartz vein fragments aligned in a mylonite foliation, E) Thin section of Good Morning Bill Schist showing S-C protomylonite fabrics...... 26 Figure 2.14 Escondida Fault exposed in a creek near Sheepwash Road, northwest of Wickliffe...... 27 Figure 2.15 Lineated fault plane of a Yarrack Fault splay within the Late Cambrian Buckeran Diorite ...... 27 Figure 2.16 Geology of the main Grampians Ranges depicting: the main stratigraphic subgroupings and structures within the ?Late Ordovician-Early Silurian Grampians Group; the distribution of Early Devonian igneous rocks; major structures and; schematic relationships with surrounding and underlying Cambrian geology ...... 28 Figure 2.17 Four-metre-thick fault breccia developed within a subhorizontal, possibly D3, fault zone in Grampians Group strata, at the ‘Chimney Pot’, Victoria Range, Grampians Ranges...... 29 Figure 2.18 Core straddling the Marathon Fault splay in drill hole STAVELY02 ...... 30 Figure 2.19 Dryden Belt facing directions determined from volcanic sediment A) Graded in west-facing subaqueous volcanolithic sandstone exposed on the eastern flank of McMurtrie Hill, B) Thin section of graded subaqueous volcanolithic sandstone...... 31 Figure 2.20 Uncleaved, low grade (sub-greenschist) but completely overturned Glenthompson Sandstone in the Moyston Fault footwall just west of Moyston on the Moyston-Willaura Road ...... 32 Figure 2.21 2-D deep seismic reflection line locations used in geological interpretation and development of the STAVELY 3D model...... 33 Figure 2.22 Deep seismic reflection transects 09GA-SD1 and 09GA-AR1, across the Grampians-Stavely and Glenelg zones, imaging the Stavely Arc edifice and related D1a fault slices...... 34 Figure 2.23 Seismic lines 97AGS-V1 and 97AGS-V2 re-processed and re-interpreted as part of the STAVELY 3D model ...... 35 Figure 2.24 Location (A), (B) and geological re-interpretation (C) of re-processed seismic reflection line MEMV96-09 that straddles part of the northern Grampians-Stavely Zone interior in northwest Victoria. Regional magnetic tilt and band pass filter pseudocolour location maps show relationship of seismic line to underlying magnetic rocks of the Dimboola Belt, part of the Stavely Arc...... 36 Figure 2.25 Perspective view of the 3D geological model of Victoria, sliced vertically east to west, with seamless geology shown at surface ...... 37 Figure 2.26 Serial full crustal-thickness cross section locations on A) TMI (RTP) and B) Complete Bouguer Anomaly images, including the location of volcanic belts of the Stavely Arc as unshaded regions...... 38 Figure 2.27 Map of the Grampians Ranges showing infill cross sections used to construct the STAVELY 3D model. A) sections shown on seamless surface geology, B) sections shown on 30 km high pass filtered gravity draped onto RTP magnetics layer revealing bedrock structures beneath cover units...... 38 Figure 2.28 Plan-view of STAVELY, showing the distribution of volcanic belts of the Stavely Arc and the interpreted network of Late-Cambrian (D1a) to Early Devonian (D4) faults included in the STAVELY 3D model. Cretaceous faults of more limited displacement related to Otway Basin formation in the south are also included. A) tilt and band pass filtered magnetics image and B) 30 km high pass filtered gravity image...... 40 Figure 2.29 RTP magnetic image of the Dryden and Dryden North belts (unshaded) in STAVELY, showing major towns, key locations where Dryden Belt strata crops out, and key stratigraphic drill holes into the Dryden North Belt...... 42 Figure 2.30 Example of the structural complexity evident within most of the volcanic belts of the Stavely Arc. The image shows the central Dimboola Belt, with major interpreted D4 faults that form part of the Dimboola Duplex, and other faults that appear to compartmentalise Dimboola Belt stratigraphy and may be of D1 age, on tilt and band pass filtered magnetic data ...... 43 Figure 2.31 Fault boundaries of the Grampians Deeps Belt buried beneath Grampians Group cover and the Victoria Valley Batholith, and of the Jalur Rift within which thick deformed Grampians Group strata is preserved. Background image is band pass filtered Complete Bouguer Anomaly...... 45 Figure 2.32 Oblique view of selected volumes and fault-surfaces from the STAVELY 3D model, illustrating the inferred regional-scale relationships between the east-dipping Dimboola Belt, the en-échelon steeply-dipping dextral strike-slip faults of the D4 Dimboola Duplex that segment it, and the interpreted south-dipping transtensional Jalur Fault ...... 46 Figure 2.33 Strain Ellipse theory ...... 47 Figure 2.34 TMI (RTP) image with interpreted faults and intrusions south of Glenthompson ...... 49 Figure 2.35 A) Locations of magnetic inversion modelling section lines. B) 3D perspective view showing tabular bodies derived from 2 ½-D inversions along profiles shown in (A)...... 51 Figure 2.36 Example of magnetic inversion section profiles across the Stavely Belt ...... 52 Figure 2.37 Serial cross section locations on gravity within STAVELY. A) location of serial cross sections and project area underlain by 30 km high pass filtered Complete Bouguer Anomaly map; B) serial cross sections in 3D space with a 30 km high pass filtered Complete Bouguer Anomaly surface, to show the true geographic relationships between sections...... 53 Figure 2.38 Example of forward gravity model section (serial cross section: Line 7) ...... 54 Figure 2.39 Locations of ground gravity stations used for gridding, interpretation and modelling of gravity data in STAVELY. Gravity forward modelled serial sections are also shown...... 57 Figure 2.40 Example of model mismatch associated with sparse ground gravity data distribution. A) serial section: Line 2, B) serial section: Line 3, profiles highlighting mis-match and C) gridded ground gravity with stations and serial cross section location...... 58 Figure 2.41 Example of gravity and magnetic forward modelling of Bushy Creek Granodiorite, indicating a potential later, inner intrusive phase ...... 60 Figure 3.1 Oblique view of geological cover units (basal surfaces) in the STAVELY 3D model. From oldest to youngest: Rocklands Volcanic Group, Otway Basin, Murray Basin and Newer Volcanic Group ...... 62 Figure 3.2 Otway Basin margin selected wells, bores and drill holes and modelled fault traces on a Gravity 30 km HP pseudocolour image...... 64 Figure 3.3 Oblique view of the STAVELY 3D model showing the modelled intrusive volumes ...... 66 Figure 3.4 Grampians Group A) View looking north from the summit of Mount Abrupt, near Dunkeld, B) Listric D3 thrust fault with minor displacement, Silverband Road, C) Grampians Group quartz arenite sandstone tilted to a sub-vertical attitude adjacent to the D3-D4 Log Hut Fault in Tulloh Creek, west of Mount Dundas ...... 67 Figure 3.5 Grampians Group cross sections drawn to approximate 8 km depth ...... 68 Figure 3.6 Perspective view of the STAVELY 3D model showing the Grampians Group basal contact surface (A) and model volume (B) for STAVELY ...... 69 Figure 3.7 Late Cambrian granites and relationships to other Cambrian rocks. A) Outcrop of fresh biotite granite (G393) in Chirrup Chirrup Creek, B) Pods of hydrothermally altered Lime Creek Granite aplite and feldspar veins intruding altered Glenthompson Sandstone, Lime Creek, C) Looking south at steeply east-dipping and west-facing Glenthompson Sandstone turbidites, altered to cordierite-spotted hornfels adjacent to a Late Cambrian granite sill, Chirrup Chirrup Creek ...... 71 Figure 3.8 Perspective view of the STAVELY 3D model showing the distribution of Nargoon Group volumes ...... 72 Figure 3.9 Volcanic belts of the Stavely Arc (A) shaded on a gravity pseudocolour map and (B) as a simplified cartoon map of the wider region with post-Middle-Cambrian intrusions and cover rocks omitted for clarity. The locations of major D4 structural basins, the Jalur, Lorquon, Cooac and Rocklands rifts are also shown...... 74 Figure 3.10 Oblique view of the STAVELY 3D model showing (A) all D1a volcanic belt-bounding faults and (B) all D1a volcanic belt volumes in STAVELY ...... 75 Figure 3.11 Oblique view of STAVELY 3D model volumes of the Boonawah, Bunnugal, Caramut and Stavely belts, showing the sub-parallel listric west-dipping imbricate character of the D1a fault-belts at crustal scale in the southern parts of STAVELY ...... 76 Figure 3.12 A) TMI (RTP) pseudocolour image of the Black Range showing the radial-arrangement of volcanic belts around a buried granitic body south of Glenisla – ‘Crab Nebula’. Inset (B) is an oblique view to show the geometry of model bodies extending to depth for the Tyar, Glenisla, Black Range and Black Range West/Mitre belts...... 77 Figure 3.13 Oblique parallel stereopair of the western part of the STAVELY 3D model, looking southeast at the ‘Crab Nebula’ of D1a volcanic belt volumes and bounding D1a fault surfaces (see Figure 3.12), and surface-traces of D3-D4 strike-slip faults. Note clockwise drag-folding of the western end of the Tyar Belt adjacent to the Henty Fault to form the steeply-plunging Tyar Fold ...... 78 Figure 3.14 Looking north at lower crustal volumes of the STAVELY 3D model that depict highly reflective rocks interpreted as autochthonous to para-autochthonous Stavely Arc deeply buried beneath overthrust material, including volcanic rocks in the Stavely, Narrapumelap, Dryden and Dimboola belts...... 78 Figure 3.15 Oblique view of the STAVELY 3D model showing the Dimboola Belt volume cut by the modelled subvertical D4 dextral-oblique transtensional fault surfaces of the Dimboola Duplex bound between the D4 Escondida Fault and related splays on the west and the D4 Golton Fault on the east ...... 79 Figure 3.16 Tilt and band pass filter of regional magnetic data in the Black Range, shaded to show the positions of the Black Range and Black Range West / Mitre belts, offset sinistrally by a minimum of 4 km across the subvertical ?D3/D4 Muirfoot Fault ...... 81 Figure 3.17 Example of inversion modelling of magnetic data over the Black Range and Black Range West/Mitre belts. A) Selected profiles over the Black Range Belt and section locations on detailed 1VD TMI data. Perspective view (B) shows all inverted bodies for the Black Range Belt – note the Bepcha Fold, C) Selected profiles over the Black Range West/Mitre Belt and section locations on regional 1VD TMI data, D) Location of all Stavely inversion sections on TMI (RTP), with detailed areas shown in (A) and (B) outlined...... 82 Figure 3.18 Black Range Belt inversion line 11 showing west dipping volcanic units within the northern Black Range Belt and the location of the sinistral Muirfoot Fault, which offsets the Black Range Belt from the Black Range West/Mitre Belt...... 83 Figure 3.19 Tilt and band pass filtered aeromagnetic data in the Black Range, shaded to show the position of the Boonawah Belt, its flanks bound by the west-dipping Boonawah West and East faults and key drill hole locations ...... 85 Figure 3.20 Brimpaen Belt highlighted by shading of a TMI (RTP) pseudocolour image, showing the positions of the Dollin Kink drag fold within the belt interior, and the interpreted position of the adjacent Dollin Fault ...... 86 Figure 3.21 Oblique view of STAVELY 3D model showing the east-dipping Brimpaen Belt volume bound on its west by the D3/D4/D5 Mosquito Creek Fault, and on its south by ?subvertical Fault 8 and the northwest-dipping Victoria Valley South Fault, both interpreted as D4 structures ...... 87 Figure 3.22 Oblique view parallel stereopair of the STAVELY 3D model, showing the east-dipping Brimpaen Belt volume with respect to the traced outlines of other volcanic belts of the Stavely Arc and major D4 faults that bound the belt...... 88 Figure 3.23 The Bunnugal, Elliot, Grampians ’Deeps’ and Brimpaen belts, superimposed on a regional magnetic (RTP) image...... 89 Figure 3.24 Oblique STAVELY 3D model view showing distribution of Bunnugal, Elliot, Grampians “Deeps’ and Brimpaen bounding D1a faults (A) and volumes (B) ...... 90 Figure 3.25 Oblique parallel stereopair of the STAVELY 3D model showing the Bunnugal and Stavely belt volumes to illustrate the point of convergence mapped between these belts north of Glenthompson ...... 91 Figure 3.26 A) Bunnugal Belt inversion line 2 showing west dipping magnetic units within the Bunnugal Belt, and anomalies derived from Newer Volcanic Group modelled by variably dipping bodies, B) Bunnugal Belt inversion line 7 showing predominantly west dipping magnetic units and the locations of Yarrack Fault system splays. The insert shows a plan view of the line location and Yarrack Fault on RTP magnetics, highlighting the Cambrian volcanic belts. C) Location of inversion sections on TMI (RTP)...... 92 Figure 3.27 Caramut Belt highlighted by shading of 30 km high pass filtered Bouguer gravity pseudocolour image ...... 93 Figure 3.28 Oblique view of the STAVELY 3D model showing the Bunnugal and Caramut belt volumes in southern STAVELY, illustrating the interpretation of these belts as parallel, but separate, D1a fault-slices ...... 94 Figure 3.29 Tilt and band pass filter image of TMI data, shaded to highlight the Dimboola Belt ...... 95 Figure 3.30 Oblique view of the STAVELY 3D model showing the extent of the upper part of the Dimboola Belt volume, confined to the Escondida Fault hangingwall ...... 96 Figure 3.31 A) Dimboola Belt inversion line 2, located in the northern section of the Dimboola Belt in STAVELY. B) Dimboola Belt inversion line 4 showing predominantly east dipping bodies within the central portion of the Dimboola Belt in STAVELY. C) Dimboola Belt inversion line 5 showing predominantly east dipping bodies within the southern portion of the Dimboola Belt in STAVELY. D) Location of inversion sections on TMI (RTP) ...... 97 Figure 3.32 The Stavely, Narrapumelap, Dryden and Dryden North belts, superimposed on a regional magnetic (RTP) image, including the locations of key outcrops and stratigraphy of the Dryden Belt and major towns ...... 99 Figure 3.33 A) Dryden Belt inversion line 3 showing west dipping volcanic units within the Dryden Belt; B) Dryden Belt inversion line 7 showing volcanic units of the Dryden Belt – western most unit dipping to the west while the eastern unit proximal to the Moyston Fault is overturned and dipping to the east; C) Dryden Belt inversion line 10 showing west dipping Dryden Belt volcanic units; D) Location of inversion sections on TMI (RTP). ....100 Figure 3.34 Mount Stavely Volcanic Complex – Dryden Belt A) Looking north from Mount Dryden summit, with tors of clinopyroxene and plagioclase-phyric andesite lava in the foreground; B) Andesite breccia exposed low on the northern flank of Mount Dryden; C) Tilted cooling columns in a sill-like body of low-Ti Andesite, high on the northern flank of Mount Dryden...... 101 Figure 3.35 Mount Stavely Volcanic Complex – Dryden Belt rhyolite A) Thin section of rhyolite from the Dryden Belt near Barton; B) Detail of bottom-right portion of (A)...... 102 Figure 3.36 Tilt and band pass filtered magnetics pseudocolour image shaded to depict the Jeparit Belt, a belt segment that is included in the Dryden North Belt in the STAVELY 3D model...... 103 Figure 3.37 Mount Stavely Volcanic Complex – Elliot Belt. A: Tors of andesite lava and breccia exposed on the summit of Mount Elliot; B: Thin section of totally undeformed, virtually unaltered albite and pyroxene-phyric andesite lava at Mount Elliot ...... 104 Figure 3.38 Example of inversion modelling of magnetic data over the Glenisla Belt. A) Selected profiles over the Glenisla Belt and section locations on detailed 1VD TMI data, highlighting the complex magnetic response associated with this belt. C) Location of all Stavely inversion sections on TMI (RTP)...... 105 Figure 3.39 Tilt filtered TMI pseudocolour image of 50m line-spacing aeromagnetic data over the Black Range and Glenisla belts, showing the internal structural complexity of the Glenisla Belt in particular ...... 106 Figure 3.40 Oblique parallel stereopairs of A) Glenisla Belt volume, showing relationships with various D1a and D3-D4 fault lines and bounding D1a and D3/D4 (pale blue) fault surfaces. B) Modelled Glenisla Belt fault surface, including a selection of anastomosing faults interpreted to disrupt the belt interior...... 107 Figure 3.41 The Grampians Deeps Belt buried beneath Grampians Group cover and the northern end of the Victoria Valley Batholith, in the interior of the Jalur Rift. Background image is 30 km high pass filtered Complete Bouguer Anomaly draped over shaded TMI (RTP) ...... 108 Figure 3.42 Grampians ‘West’ Belt highlighted on image of magnetics (RTP) showing an elevated magnetic response associated with buried Stavely Arc rocks, and drill hole Woohlpooer 6 ...... 110 Figure 3.43 Magnetics (RTP) pseudocolour image highlighting the different belt segments of Stavely Arc volcanics now thought to have been united within a single, simple D1a fault slice prior to the D3 and D4 deformations ...... 111 Figure 3.44 Tilt and band pass filtered regional magnetic image of the Mafeking Megakink, a large D4 structure hosted in Cambrian bedrock ...... 112 Figure 3.45 Oblique view of the STAVELY 3D model depicting the Narrapumelap Belt volume, with other near-surface belt boundaries shown for reference. The positions of Late Cambrian (D1b) intrusive rock volumes are also shown, including two that intrude the Narrapumelap Belt ...... 113 Figure 3.46 Oblique parallel stereopair of the STAVELY 3D model showing the volcanic belts that are deformed by the Mafeking Megakink ...... 114 Figure 3.47 Fault network on image of tilt and band pass filtered magnetics, with the Stavely Belt highlighted and showing a high degree of internal complexity and variable magnetic response of intercalated units. Thursday’s Gossan prospect is shown in the northern half of the belt...... 116 Figure 3.48 Stavely Belt geology map, showing internal stratigraphic units of the Mount Stavely Volcanic Complex, internal stratigraphic units of the Glenthompson Sandstone, Late Cambrian intrusive rocks, and the distribution of various cover-rock successions...... 118 Figure 3.49 Mount Stavely Volcanic Complex – Stavely Belt. A) typical outcrop of Fairview Andesite Breccia, Astons Road, Hopkins River; B) Well-bedded water-lain Towanway Tuff, Near Mount Stavely; C) Graded turbiditic (water-lain) Towanway Tuff exposed in Back Creek, adjacent to Drysdale Road ...... 119 Figure 3.50 A) Stavely Belt inversion profile 1 showing west dipping volcanic units within the Stavely Belt; B) Stavely Belt inversion profile 4 showing volcanic units of the Stavely Belt separated from the (north-)east dipping units of the central limb of the Mafeking Megakink – Narrapumelap Belt – by the regional dextral Escondida Fault; C) Location of inversion sections on TMI (RTP)...... 120 Figure 3.51 Example of inversion modelling of magnetic data over the Tyar Belt. A) Selected profiles over the Black Range West/Mitre Belt and section locations on regional 1VD TMI data; B) Location of all Stavely inversion sections on TMI (RTP)...... 121 Figure 3.52 Oblique STAVELY 3D model view showing the distribution of Kanmantoo Group (Glenthompson Sandstone) volumes, the single most extensive group of volumes in STAVELY ...... 122 Figure 3.53 Kanmantoo Group – Glenthompson Sandstone. A) Overturned well-bedded turbidites with typical thin-bedded A-C-E Bouma sequences, metamorphosed to hornfels by an underlying granite. Mount Drummond Quarry near Ledcourt; B) Thin section of the formation from the Lake Lonsdale spillway; C) Thicker-bedded facies exposed in a tributary of the Hopkins River near Nareeb; D) Coarse-grained, tabular-cross-bedded turbiditic volcanic-lithic gritstone, exposed in Back Creek just east of the Stavely Belt ...... 123 Figure 3.54 Oblique view of STAVELY 3D model looking north at extensive volumes interpreted as rocks of the Stavely Arc within the lower Dimboola Belt beneath the Escondida Fault footwall...... 127 Figure 3.55 Oblique model view showing distribution of Stavely (dark purple), Narrapumelap, Dryden and Dryden North bounding D1a faults (A) and volumes (B)...... 127 Figure 3.56 Oblique view of STAVELY 3D model, showing major (named) D3 and D4 fault surfaces, most of which dip eastwards and link at depth into the Moyston Fault footwall (blue translucent surface)...... 129 Figure 3.57 Oblique view of the southern STAVELY 3D model, showing predominantly south-dipping fault surfaces that combine to bound the northern margin of the Otway Basin...... 129 Figure 3.58 Moornambool Metamorphic Complex structures, typical of the Moyston Fault hangingwall high strain zone. A) Polydeformed psammitic Lexington Schist; B) Prominent steeply northwest-plunging stretching lineation in nearby mafic Carrolls Amphibolite on the crest of the Mount Ararat range; C) Tectonic mélange of Carrolls Amphibolite and Good Morning Bill Schist mylonite Good Morning Bill Creek; D) Polydeformed Carrols Amphibolite located just a few metres east of the Moyston Fault trace. Salt Creek ...... 132 Figure 3.59 Plan-view of a tilt and band pass filtered magnetics image including STAVELY shaded to highlight the volcanic belts of the Stavely Arc and the interpreted network of Late-Cambrian (D1a) to Early Devonian (D4) faults included in the STAVELY 3D model, and the location of the inset that includes the Wallup Fold, developed within the Stawell Zone...... 133 Figure 3.60 Tilt and band pass filtered magnetics pseudocolour image of central-west STAVELY, showing the interpreted position of the Yarramyljup Fault trace, and a selection of the key control points – outcrops in Yarramyljup Creek and the Glenelg River, and mineral exploration drill holes – that help constrain the fault position at surface and beneath Murray Basin cover...... 134 Figure 3.61 Glenelg River Metamorphic Complex A) Isoclinally-folded and fault-imbricated pegmatite veins in sillimanite schist in the Yarramyljup Fault hangingwall. Yarramyljup Creek. B) Core from diamond drill hole DD88BL215, northwest of Balmoral, showing layered folded migmatite and late pegmatite veins ...... 135 Figure 3.62 Oblique model view showing the distribution of the Boonawah (orange), Grampians ‘West’, Tyar, Cherrypool, Black Range and Black Range West / Mitre belt volumes...... 136 Figure 3.63 Buried magnetic intrusions, interpreted to be D1b porphyries, in the hangingwall of the Narrapumelap South Fault, within the Narrapumelap Belt...... 144 Figure 3.64 Tilt and band-pass filtered magnetics of the Brimpaen-Horsham region, with the Dimboola and Brimpaen belts highlighted, and illustrating complex overprinting criteria between the Escondida, Mosquito Creek and Mackenzie River faults...... 150 Figure 3.65 Structures in Grampians Group adjacent to the Golton Fault A) a north-verging thrust fault duplex that cuts subvertically-dipping Grampians Group strata in Golton Gorge; B) Panorama looking northwest towards the eastern flank of the Mount Difficult Range and Halls Gap, from Boronia Peak ...... 154 Figure 3.66 Mount Arapiles, Muirfoot Fault plane A) Looking southwest from the Natimuk-Frances Road at the northeast-facing >100 m high ‘Watchtower Face’; B) Oblique side-on view (location in A), looking upwards and southwards at ‘The Watchtower’ and at the large-amplitude fault-plane undulations that form the ‘North Watchtower Face’...... 164 Figure 3.67 Oblique view of the Bunnugal Belt volume, showing its northern termination across the north-dipping D4 Victoria Valley South Fault, and the conjugate relationship between the Victoria Valley South Fault and the subsidiary southeast-dipping Victoria Valley North Fault...... 165 Figure 3.68 Tilt and band pass filtered regional magnetic image of the D4 Yarrack Fault, with the Bunnugal Belt highlighted to show the nature of its disruption and dextral strike-slip offset across the Yarrack Fault. The stereonet depicts poles to Glenthompson Sandstone bedding measured in the Yarrack Orocline ...... 169 Figure 3.69 Yarrack fault splays. Tilt and band pass filtered magnetics image showing deformation by the Yarrack Fault and associated splay faults (thick lines)...... 170 Figure 3.70 Mesoscopic, upright D1a northeast-verging fold couple deforming Glenthompson Sandstone turbidites, exposed in a low cutting on Spittle Road ...... 171 Figure 3.71 Looking south at west-southwest dipping amalgamated sandstone beds in Glenthompson Sandstone, showing a pervasive spaced stylolitic S1a cleavage exposed on the bank of Bushy Creek ...... 171 Figure 3.72 Open folds in Mount Stavely Volcanic Complex tuff, intersected in mineral exploration drill hole SNDD3 (130.5m downhole)...... 171 Figure 3.73 Schematic block diagram that depicts the context of D4 oroclines developed within Grampians Group above subhorizontal D4 scissor faults that separate Grampians Group from coeval clockwise megakinks and dextral strike-slip faults developing in directly underlying Cambrian bedrock. Subvertical orocline axis plunges are indicated...... 173 Figure 3.74 Perspective 3D view looking north at Black Range region inversion models with TMI (RTP) magnetic image, showing strike-slip faults and associated drag folds. See Figure 2.35 for location...... 174 Figure 4.1 Regional magnetic first vertical derivative pseudocolour image of western Victoria, showing the location of the reference region that includes STAVELY, depicted in Figures 3.9, 4.2, 4.13, 4.15, 4.17, and used for the D3-D4 retrodeformation...... 179 Figure 4.2 Schematic present-day geology of the reference region that encompasses STAVELY and an additional northern part of the Grampians-Stavely Zone, depicting A) Regional tilt and band pass filtered magnetic data image and B) the distribution of all Stavely Arc D1a volcanic belt segments and major (named) faults that occur between the Moyston and Yarramyljup faults, with post-Cambrian cover and intrusive rocks omitted for clarity...... 180 Figure 4.3 The D4 Mafeking Megakink, showing restored and present-day tilt and band pass filtered regional magnetic data of the Stavely/ Bunnugal, Narrapumelap/Elliot and Dryden belts, and restored and present-day (right, but with strike-slip offsets of the megakink hinges omitted for clarity) schematic block-diagrams that depict how the once linear and combined D1a fault slices of the Stavely Arc came to be refolded and segmented into the Mafeking Megakink during D4, with Grampians Group strata partly detached along sub-horizontal scissor faults (in green) to form smoothly-curved oroclinal folds...... 182 Figure 4.4 Mafeking Megakink retrodeformation sequence...... 183 Figure 4.5 Oblique view of TMI pseudocolour image of the Black Range, showing the near surface traces of major Cambrian faults within the Cambrian volcanic belts, and the Siluro-Devonian faults that segment and offset them. The positions of major D3-D4 folds and the interpreted positions of Early Devonian granite intrusions are also shown. The tabular blocks depict the results of dip-modelling of multiple magnetic profiles constructed across each of the volcanic belts...... 185 Figure 4.6 A six-step plan-view retrodeformation sequence for D4 (and D3?) structures in the Black Range sub-region, applied to a tilt and band pass filtered magnetic image...... 186 Figure 4.7 Present day (left) and pre-D4-D3 plan views of geology in the Black Range, showing the positions of major D4 and D3 faults and folds...... 187 Figure 4.8 Locations of overturned Glenthompson Sandstone and Stavely Arc strata mapped in and east of the Dryden Belt along the Moyston Fault footwall, superimposed on tilt and band pass filtered regional magnetic data...... 188 Figure 4.9 Overturned moderately southeast-plunging synform in Glenthompson Sandstone, exposed adjacent to the Lake Lonsdale spillway. Overturning is attributed to D4 based on overprinting criteria...... 189 Figure 4.10 Oblique view of south-dipping D4 extensional faults modelled as an en-échelon array along the Moyston Fault footwall...... 190 Figure 4.11 Schematic block diagram illustrating the D4 dextral transtensional deformation sequence that segmented and overturned parts of the Glenthompson Sandstone and D1a structures including parts of the Dryden Belt...... 191 Figure 4.12 Schematic plan view of the D3-D4 retrodeformation applied to the same tilt and band pass filtered regional magnetic data image and D3-D4 fault network as Figure 4.2...... 193 Figure 4.13 Stepwise area-balanced D4- D3 plan-view retrodeformation of the reference region Cambrian geology from the present day to pre-Silurian configuration and showing a northeasterly-trending ‘releasing bend’ in the retrodeformed shape of the Moyston Fault trace...... 195 Figure 4.14 Retrodeformed regional magnetic tilt and band pass filtered pseudocolour image of STAVELY showing the pre-D4-D3 geometry of the northern Dimboola and Dryden North belts and spaced magmatic centres of the Stavely Arc ...... 198 Figure 4.15 D1a thrust belts in the Grampians Stavely Zone, including arc-oblique D1 transfer structures that explain Stavely Arc thrust-system vergence reversals. A) Proposed restored geometry of the Stavely Arc at D1a culmination at ~500Ma. The along-strike change thrust fault dip predominance implies an intervening oblique transfer structure active during D1a to facilitate opposing thrust-transport directions; B) Proposed geometry of the Stavely Arc at the end of D4 at ~400Ma ...... 200 Figure 4.16 Possible transfer fault segment within the Dryden Belt. Shaded tilt and band pass filtered regional magnetic image, including key Dryden Belt locations ...... 201 Figure 4.17 Stress history sequence for STAVELY (dashed borders depict present day area configuration for reference, based on the area of Figure 3.9 showing the interpreted plan-view structural evolution from ~505 – 500 Ma (D1a) to ~380 Ma (D5)...... 203–208 Figure 4.18 Geodynamic section series for STAVELY. Sections are schematic, depicting geology between Kangaroo Island (west) – Padthaway – Balmoral – Ararat – Castlemaine – Heathcote – Avanel (east) ...... 210 Tables

Table 2.1 Petrophysical properties and assumptions used for forward modelling of rock units in the development of the STAVELY 3D model. Also shown are the measured range and median values derived from scanning of core by AGOS’s Multi-Sensor Core Logger instrument...... 55 Table 3.1 Summary of deformation events within STAVELY for reference. Accompanying text is provided in Appendix 5 – Deformation history summary...... 125 Executive summary

The STAVELY 3D model is an explicit, full-crustal scale, geological unit and fault-framework model for the central Grampians-Stavely Zone of western Victoria that includes the Cambrian-aged Stavely Arc, prospective for arc-related mineral systems. Built using Paradigm’s GOCAD® software, the STAVELY 3D model honours surface geological constraints and integrates drill hole, geological interpretation of potential field, and deep seismic reflection data in and adjacent to the Grampians-Stavely Zone. It also estimates and illustrates how district to regional-scale faults and other structures and rock units mapped at surface, and modelled near-surface, might extend and interact at depth and influence the distribution of near surface geological units of the prospective Cambrian bedrock.

The STAVELY 3D model includes at least nineteen fault-bound volcanic belt segments of the Stavely Arc that contain various boninitic to calc-alkaline volcanic rocks and associated intrusions. The volcanic belts are largely concealed by younger Palaeozoic and Cenozoic cover, however where (poorly) exposed at surface they host numerous base and precious metal mineral occurrences. A primary aim of the STAVELY 3D model is to visualise, at crustal-scale, the surface and sub-surface arrangements and inter-relationships of the volcanic belts. The varied and complex distribution of the volcanic belts are the consequence of two main deformation events (D1a and D4), separated in time by 100 million years. Structural controls within Silurian Grampians Group cover constrains removal of D4 effects to reveal a simpler D1 imbricate thrust-geometry for the active Stavely Arc at ~505-500 Ma in just four or five long fault-slices. The D4 dextral transtensional event at 400 Ma segmented mineralised (D1a) fault slices of the Stavely Arc that had also been subject to a mineralising event during D1b.

D4 retrodeformation demonstrates how mineralised Cambrian calc-alkaline andesite and porphyry and related Stavely Arc associations in volcanic (fault) belts at Stavely and beneath the Black Range are direct correlates of at least seventeen other Cambrian volcanic belts dispersed under mostly shallow cover throughout the Grampians-Stavely Zone. This represents a nine-fold increase in the viable Cambrian arc-terrane search space for western Victoria. The STAVELY 3D model captures and communicates the interpreted geometries and potential stratigraphic facings of different volcanic (fault) belts of the Stavely Arc, the extent of these belts to depth and beneath younger cover, and the possible inter-relationships that existed between different belts of Stavely Arc at the time of mineralisation (in the Cambrian, up to D1b).

This is a new template for improved targeting and systematic mineral exploration of the Stavely Arc.

Due to the limited outcrop and poor exposure, and because few mid-or lower-crustal rocks are known within the Grampians-Stavely Zone, the level of uncertainty and ambiguity increases with depth within the model – a range of possible scenarios and inherent strengths and weaknesses are discussed, including a preferred scenario based on data available to the end of 2016, about which the STAVELY 3D model has been built. To limit the impact of this uncertainty on the STAVELY 3D model, wider geological understandings from regions adjacent to the Grampians-Stavely Zone are included; the back-arc and accretionary characteristics of the coeval Glenelg and Stawell zones to the west and east respectively; the wider geodynamic context of the eastern Gondwana margin during, and subsequent to, the Neoproterozoic breakup of the former supercontinent Rodinia; and the far-field geodynamics of the Lachlan Fold Belt farther east that superimposed post-Cambrian deformations onto the Grampians-Stavely Zone. These wider understandings have allowed a geological narrative to be developed that is constrained and consistent with all available data, but not necessarily a unique solution. This narrative was used to assign possible affiliations to buried geological elements that occur at deep levels in the crust, consistent with the range of petrophysical (rock) properties and geometries that are known.

Major implications for mineral prospectivity of the Stavely Arc have been realised from the development of the STAVELY 3D model – it is a predictive tool that allows for the direct correlation of Cambrian structures, intrusives, stratigraphy and related mineral systems between disrupted fault slices. Whilst the STAVELY 3D model is constructed at the regional scale to understand and communicate the greater geological architecture, the model has significant implications for area selection and mineral exploration targeting at the district (camp) to prospect/deposit (and even drill hole) scale – this is because of the scale-invariance that is inherent in geology.

Applications of the STAVELY 3D model includes, but is not limited to: construction of crustal-scale cross sections in any orientation or position; construction of depth-slices at any depth; identification of common plumbing systems for different or now-separated mineral systems; fluid flow modelling; examining regional structural controls on smaller-scale structural systems; providing a regional framework for detailed geophysical modelling, drill program orientation and optimising geophysical survey acquisition; depth to basement assessments. These applications are facilitated by flexible model portability. The STAVELY 3D model imports successfully into 3D applications including SurpacTM, Micromine, Vulcan and Leapfrog.

Key findings for mineral exploration and targeting

 The narrow surface width of the Grampians-Stavely Zone which contains the Stavely Arc is apparent. Overthrust margins conceal a much wider zone at depth that is directly comparable to modern continental margin magmatic arc system widths. Retrodeformation of post-Cambrian (post-D1) structures reveals a distinct, regular (~80 km) spacing to Stavely Arc volcanic centres, a spacing typical of modern magmatic arcs.

 At least one candidate for a large-scale arc-aged transfer structure is recognised in STAVELY, from reversals in Delamerian Orogeny (D1a) structural facing directions. Fragments of other possible arc-aged transfer structures occur within some volcanic belts of the Stavely Arc. The STAVELY 3D model captures and discusses their context, and how more may be recognised.

 The most prominent areas of oblique-trending Stavely Arc geology with the superficial appearance of ‘transfer structures’ coeval with arc activity are not D1 transfer structures. Instead they are caused by Siluro-Devonian (D4) kink-rotations that occurred nearly 100 million years after Stavely Arc extinction. This is a critical insight for mineral exploration, because arc ‘transfer structures’ are often targeted by mineral explorers and misattribution could seriously misdirect mineral exploration. The STAVELY 3D model clearly illustrates these effects and describes how to discriminate them.

 Interpretation of the regional structural architecture and history of the Grampians-Stavely Zone includes retrodeformation of major Siluro-Devonian deformation events (D3-D4). The STAVELY 3D model shows how Siluro-Devonian deformations changed four to five simple D1a Stavely Arc fault slices into (at least) nineteen dispersed post-D4 volcanic belt segments. The mineral prospectivity implications of this key realisation are profound - not only are all volcanic belts commonly derived from the Stavely Arc, Cambrian-aged structures, intrusives, stratigraphy and related mineral systems identified in one volcanic belt can now be correlated directly into adjacent but now disconnected belts using robust retrodeformation constraints.

 Structural systematics are typically scale-invariant. The timing and kinematic history of faults within the Grampians-Stavely Zone are scalable. Regional faults attributed in the STAVELY 3D model can be down-scaled to add context and insight to fault networks of comparable geometry and age encountered during mineral exploration (including drilling), improving prediction and targeting.

 Retrodeformation of post-Cambrian (post-D1) structures allows demonstrated mineral prospectivity of the Stavely Belt to be extended directly into the Narrapumelap, Dryden, Dryden North and Dimboola belts – the Dimboola Belt is a Dryden North Belt backthrust. Demonstrated Black Range Belt mineral prospectivity can be extended directly into the Black Range North / Mitre, Glenisla, Tyar, Grampians ‘West’ and Boonawah belts. Elliot Belt mineralisation can be extended directly into the Bunnugal and Brimpaen belts. The Caramut belt is untested. The collective strike length of restored Stavely Arc fault slices is approximately 1,160 km.

 Syn-volcanic mineralisation developed pre-Delamerian Orogeny is likely to now be steeply-dipping and faulted due to D1a effects. Later deformations (e.g. D4) caused further reorientations (e.g. Black Range Belt and equivalents), with implications for mineral exploration targeting (e.g. geophysical survey and drill hole design) of stratiform and stratabound systems.

 Mineralised porphyries (and related systems) emplaced late in D1, both within and outside of the volcanic belts, post-date the D1a Delamerian Orogeny phase and so are generally upright. Late Cambrian porphyries were subject to Early Devonian deformations and so can be locally rotated and/or segmented by subvertical strike-slip and low-angle dip-slip faults (Grampians Group deformation is a D3-D4 template).

 Post-Cambrian Palaeozoic magmatism in STAVELY is related to distal back-arc Macquarie Arc geodynamics.

 Understanding the tectonic context of the Cambrian versus Devonian intrusive phases allows the age of buried intrusions to be inferred from their shapes. Several potential Cambrian intrusions appear to remain untested under cover – these are described in the STAVELY 3D model.

 The STAVELY 3D model details the distribution and thickness of post-Cambrian cover rocks (Grampians Group, Rocklands Volcanic Group, Murray and Otway basins). Modelling shows cover successions are mostly relatively thin (<100 m - <500 m). Grampians Group cover is generally thin, but reaches several kilometres thick in distinct rifts (included in the STAVELY 3D model) which represent impenetrable barriers to exploration of underlying Cambrian bedrock. Some cover units are mineralised in their own right. The STAVELY 3D model is a foundation model into which more detailed camp-scale or prospect-scale 3D models can be embedded, so that faults and other geology encountered at local scale (including in drill hole intersections) can be placed into a wider context. 1. Introduction

This report outlines the philosophy, scope, methodology and key supporting geological and geophysical observations used to generate the regional-scale three dimensional (3D) geological model for the Stavely Project area (STAVELY), western Victoria. The implications of the modelling for our understanding of Victoria’s geology, including post mineral deformation of the Stavely Arc, is discussed.

1.1 Background

The Stavely Project is a collaborative geoscience research project between the Geological Survey of Victoria (GSV) and Geoscience Australia (GA) that was undertaken from December 2013 to December 2017 in western Victoria (Figure 1.1). The aim of the project was to investigate the geological architecture, geodynamic setting and mineral systems potential of the Cambrian-aged Stavely Arc. Findings from this program are summarised in Schofield et al., (2018). Figures 1.2, 1.3 and 1.4 show key geological units, the distribution of volcanic belts, and regional-scale geophysical features across STAVELY.

Less than 3% of STAVELY comprises (poorly) exposed Cambrian bedrock. Just 0.5% of STAVELY comprise rocks of the Stavely Arc, which is represented by the Mount Stavely Volcanic Complex (Figure 1.2). The 0.5% that is exposed has demonstrated prospectivity with mineralisation of porphyry copper(-gold-molybdenum), epithermal gold, and volcanic hosted massive sulphide affinity occurring with the small areas of Stavely Arc rocks (the Stavely and Black Range belts). The host rocks to these known occurrences of mineralisation extend under younger cover and remain under- or un-explored.

The Stavely Project seeks to help overcome the twin challenges of cover and geological complexity for mineral explorers operating in this Early Paleozoic arc terrane, by providing an improved regional-scale 3D and 4D (3D plus time) understanding of the distribution, context, and inter-relationships between different volcanic belts of the Stavely Arc and other strata, and an understanding of the controlling structural framework.

Figure 1.1 Location of STAVELY underlain by 1:250 000 seamless geology of Victoria (data from VandenBerg et al. 2000; Welch et al., 2011).

Figure 1.2 Location of STAVELY underlain by 1:50 000 seamless geology of Victoria (Higgins et al., 2014).

Figure 1.3 Location of STAVELY showing distribution of Cambrian volcanic belts within the Grampians-Stavely Zone, with regional airborne magnetic data as background. Volcanic belt colour groups (orange, green, purple, red) convey the interpreted inter-belt correlations at the time they formed as 4-5 large coherent, subparallel northerly-trending thrust slices during the Cambrian D1a deformation event. Subsequent deformations, most notably the Early Devonian D4 event, segmented and dismembered the D1a thrust slices resulting in the volcanic belts. Background is a histogram-equalised pseudocolour image of NE sun-shaded Total Magnetic Intensity (TMI), reduced to the pole (RTP). Blue colours indicate low magnetic response; red colours indicate high magnetic response.

Figure 1.4 Location of STAVELY showing distribution of Cambrian volcanic belts with gridded regional ground gravity data as background. Volcanic belt colour-groups (orange, green, purple, red) convey the interpreted inter-belt correlations at the time they formed as 4-5 large coherent, subparallel northerly-trending thrust slices during the Cambrian D1a deformation event. Subsequent deformations, most notably the Early Devonian D4 event, segmented and disrupted the D1a thrust slices resulting in the volcanic belts. Background is a histogram-equalised pseudocolour image of NE sun-shaded Complete Bouguer Anomaly. Blue colours indicate low gravity response; red colours indicate high gravity response.

1.2 Geological context

The regional geology, including the Grampians-Stavely Zone and surrounds, is summarised in Schofield et al. (2018). Stratigraphic units included in the STAVELY 3D model are summarised in Appendix 4 – Geological Units.

The Middle-Late Cambrian Stavely Arc is represented in STAVELY as nineteen discrete but discontinuous steeply-dipping volcanic belts that vary between 2 and 5 km in width, and are up to 150 km long (see Figure 1.3, Figure 1.4 and Figure 1.5). The volcanic belts have been delineated by mapping, drilling and geophysical data. Mapping shows the volcanic belts to be mostly fault-bounded and the volcanic rocks fault-intercalated with Cambrian marine metasedimentary rocks with markedly different geophysical characteristics. The age of this faulting is Late Cambrian as constrained by overprinting criteria. Geophysics and mapping define individual belt geometries.

Geochemistry, geochronology, and the lithological and structural characteristics of the Cambrian volcanic rocks show some commonalities across all the volcanic belts, suggesting that all nineteen belts contain portions of the Mount Stavely Volcanic Complex, together with portions of other rock types such as serpentinite and metasediment. The Mount Stavely Volcanic Complex is defined in just one volcanic belt, the Stavely Belt (Buckland, 1987; see Section 3.2.5.16 – Stavely Belt). Extensive tracts of volcanic rocks ranging in composition from mafic (tholeiite to boninite) to intermediate-felsic (calc-alkaline andesite to dacite) occur in all nineteen belts. Collectively, they define a broad, continuous geochemical spectrum that is consistent with a congruent evolving arc setting (see Schofield et al., 2018). The vesicularity and other textural complexities in even the more mafic volcanic components suggests low confining pressures, consistent with eruption relatively shallow water depths. This characteristic contrasts markedly with the generally low vesicularity of mafic rocks exposed in central Victoria that appear to have erupted in deep marine, possibly abyssal, settings (e.g. in the Heathcote Fault Zone). We therefore interpret all the Cambrian volcanic rocks in STAVELY as parts of a congruent shallow-marine volcanic-arc suite, discounting earlier interpretations that infer exotic (e.g. ophiolite) origins for large parts of the succession. Combined, the different datasets suggest that all the volcanic belts are derived from four to five strike persistent fault slices carved from the flanks of the Stavely Arc during a period of crustal shortening that accompanied the terminal stages of Cambrian arc development.

Scattered Middle-Late Cambrian intrusive rocks, including plutons and smaller porphyry stocks, some with Cu-Au-Mo mineralisation, intrude into the upturned volcanic belts, and into the adjacent Cambrian metasediments (Figure 1.6). The intrusive rocks are temporally and geochemically linked to the Mount Stavely Volcanic Complex, and are interpreted as a late phase of Stavely Arc activity. Subsequent pulses of Palaeozoic magmatism are related to back-arc transtension episodes in the distal back-arc of the Macquarie Arc.

The present-day distribution of the nineteen volcanic belts of the Stavely Arc is complex, apparently the product of multiple deformation events. Interpretation of the regional structural architecture and history of the Grampians-Stavely Zone has identified two major deformation episodes; D1a in the Middle-Late Cambrian when the active Stavely Arc was fault-imbricated and uplifted into a series of discrete fault slices; and D3-D4 in the Late Silurian-Early Devonian when the whole zone, including Grampians Group cover rocks, was radically reshaped by trans-current fault movements. Other deformation episodes – D1b, D2, D5 – are locally important and of economic significance, but did not significantly reshape the geology.

The Grampians Group cover rocks are a key constraint that allows for the effects of the major Siluro-Devonian D3-D4 deformation events, including that imparted on the underlying Cambrian bedrock, to be discriminated and retrodeformed. The trans-current nature of D3 and D4-aged faults means that retrodeformation of fault movements is confined to the sub-horizontal plane, so that potential field datasets (particularly regional magnetic data) can be used to extend the retrodeformation to include Cambrian bedrock that is entirely buried. The D3-D4 retrodeformation shows how Siluro-Devonian deformation events radically reshaped the plan-view of STAVELY, changing a few Cambrian-aged subparallel D1 fault slices of mineralised Stavely Arc into a plethora of widely dispersed and locally rotated D1 volcanic (fault) belts.

The implications of this key realisation for the mineral prospectivity of STAVELY are profound – not only are all the volcanic belts commonly derived from the Stavely Arc, they are all segments of just a few large, subparallel, Cambrian-aged D1 fault slices, which means that Cambrian structures, stratigraphy and related mineral systems identified in one volcanic belt can be correlated DIRECTLY into adjacent but now disconnected segments (volcanic belts) using the D3-D4 retrodeformation constraints. This has profound implications for prospectivity and mineral exploration targeting, particularly where the volcanic belt is partially or entirely buried.

The D3-D4 retrodeformation reveals the former D1 structural system that existed in the Late Cambrian, at the time of the last (D1b) phase of Stavely Arc magmatism, when metalliferous porphyries were emplaced. The Stavely Arc structural system in the Late Cambrian was simpler than today, comprising four – five strike-persistent, subparallel fault-bounded D1 fault slices with an estimated collective strike length of 1,160 km. Only two of these restored Cambrian fault slices are significantly exposed, albeit poorly, yet both contain multiple base and precious metal mineral occurrences – mineral prospectivity that can now be extended directly into buried portions (volcanic belts) of those Cambrian fault slices that were segmented by Siluro-Devonian deformation. Many of these buried portions of the Stavely Arc are not yet explored.

The geological data supports interpretation of the Stavely Arc as a continent-fringing west-Pacific-style suprasubduction zone system, aligned along the eastern flank of Gondwana above a west-dipping subduction zone. The subduction zone apparently established near the continent-ocean-boundary in the Early Cambrian, superimposing a magmatic arc onto the outer parts of the Neoproterozoic-Early Cambrian east-Gondwana passive margin succession. The geological history and geochronology supports interpretation of the arc as active over a time-span of up to 25 million years (e.g. Foden et al., 2006; Schofield et al., 2018). The Stavely Arc appears to have initiated as a mostly submarine Japan-style ‘island arc’ system in the Early-Middle Cambrian. It apparently evolved into an Andean-style system uplifted along the east Gondwana margin in the Middle-Late Cambrian. A terminal pulse of metalliferous porphyry magmas was introduced into the deformed arc succession in the Late Cambrian.

This history has strong temporal and lithological correlatives in the Mount Wright Arc of western NSW, in the richly-mineralised ‘Dundas Trough’ of western Tasmania (including extensions into the Selwyn Block of central Victoria), and in the Bowers Terrane of North Victoria Land in Antarctica, all of which appear to have lain directly along-strike of the Stavely Arc system in the Cambrian (Cayley, 2011). These correlations establish the continental scale and mineral prospectivity of the Stavely Arc system.

Detailed descriptions of major rock units (including volcanic belts of the Stavely Arc) and Late Cambrian through Middle Devonian deformation event characteristics – including elements included in, and omitted from, the STAVELY 3D model, and individual descriptions of faults are provided in Section 3 – Modelled surfaces and volumes. The age and kinematics of each fault modelled is provided in Appendix 6 – Fault summary table.

Pre-reading and reference to these documents and the appendices included in this report will provide the reader and users with the geological context required to understand the rationales used to develop the depicted interpretation, and to realise the full value of the knowledge contained within the STAVELY 3D model.

1.3 Aims

The STAVELY 3D model has been constructed at the regional scale to understand and communicate:

 the greater geological architecture of western Victoria, in particular the context and geodynamic setting for Proterozoic – Early Palaeozoic rocks within the Grampians-Stavely Zone,

 the surface, near-surface and sub-surface distribution of belts of mafic-intermediate igneous rocks interpreted to represent upturned fault-slices of a deformed continental margin magmatic arc – the Stavely Arc – that was active along the Victorian part of the eastern margin of Gondwana in the Middle Cambrian (Figure 1.5),

 the sub-surface extent of reflective crustal material imaged in various deep seismic reflection profiles across the region that may represent buried autochthonous to para-autochthonous remnants of an original Stavely Arc edifice that was erupted along the eastern margin of Gondwana in the Cambrian, and/or other igneous rock packages possibly related to previous and/or related episodes of rifting (e.g. Rodinia breakup, back-arc rifting) and accretion (e.g. emplacement of the Stawell Zone).

 the surface, near-surface and sub-surface distribution of Late Cambrian magmatic rocks, including mineralised and potentially mineralised porphyry stocks, that intrude the deformed arc succession, including any possible associations with individual fault belts and other structures (Figure 1.6),

 the distribution, extent and thickness of selected post-mineral rocks that either cover (e.g. Grampians Group, Rocklands Volcanic Group, Otway Basin, Newer Volcanic Group) or ‘burn out’ (e.g. Early Devonian intrusions) Cambrian arc stratigraphy,

 the influence and distribution of post-mineral deformation that has disrupted and reconfigured the prospective belts of Cambrian-aged Stavely Arc, a significant portion of which occurs within the Grampians-Stavely Zone (Figure 1.6), and

 the progressive evolution of structural systems within the Grampians-Stavely Zone through time from the Cambrian to the present day, developing a stress-history for the region that can help interpret displacement directions for structures that have controlled and disrupted mineralisation systems.

1.4 Scientific and economic justification

To successfully explore, discover and manage complex earth resources and better understand landscape evolution it is imperative that a regional geological context is established and well understood. This typically includes developing an understanding of the geodynamic setting – the physical processes that acted over millions of years to shape the geology into its present form. The first steps toward achieving this involves establishing the distribution and geometry of all the major stratigraphic and structural features identified within the area of interest.

Geological maps, geological cross sections, geophysical datasets and three dimensional geological models are powerful tools for geoscientists to investigate, analyse, visualise and communicate regional geological relationships, timing and constraints. These are all important considerations for an explorer with the significance and influence of regional controls often overlooked at the district (camp) and prospect (deposit) scale. Through improved understanding of the regional geology and the integration of maps and sections and other datasets into three dimensional models, reports and associated data comes an opportunity to improve exploration prediction and targeting and thereby reduce risk for industry and for land managers, and potential impacts on local communities (by improving exploration efficiency).

The model is designed to provide a regional-scale 3D framework into which more detailed district, camp and prospect-scale models can be embedded, so that the regional geological context of smaller models might be assessed. Assessments that can utilise this model include – but are not limited to: construction of crustal-scale cross sections in any orientation or position within the zone; construction of depth-slices at any position within the zone; exploring the possibility of correlation between different or widely separated mineral deposit or other resource locations; identification of common plumbing systems for different mineral systems (e.g. Williamsons Fault segments once united, now dispersed); examining the likely regional structural controls on smaller-scale structural systems (structural systems are typically scale-invariant); drilling program orientation; geophysical acquisition design; depth to basement assessments; link between bedrock and basin (e.g. Otway Basin) structure.

A robust 3D geological model forms the basic framework for fluid-flow modelling. It is a start-point for 4D reconstructions of the crust (progressive removal of the effects of structural systems from youngest to oldest, as already attempted in 3D (2D + time). Partial restoration of structures within the STAVELY 3D model (for example, the removal of D4 and D3 effects) reveals the 3D configuration of the Grampians-Stavely Zone in Late Cambrian times, a starting point for fluid-flow modelling of crustal configurations contemporaneous with porphyry mineralisation during deformation event D1b.

The value to mineral and energy explorers in understanding the geological context and geodynamic setting and, critically, the locations of target rock sequences, through the research and development of a crustal-scale 3D model has been demonstrated in Victoria with an improved understanding on the control and distribution of gold mineralisation, including implications for prediction and targeting by the GSV (see Willman et al., 2010; Cayley et al., 2011a; Rawling et al., 2011).

Figure 1.5 The entire STAVELY 3D model, showing (A) all (pre)Cambrian – Silurian rock volumes to a depth of 40 km and (B) with Ordovician-Silurian strata (Grampians Group in pink) omitted to show subsurface distribution of fault-belts of Cambrian Stavely Arc strata (purple, green, orange and red volumes) throughout STAVELY, infaulted with Glenthompson Sandstone (pale yellow volumes) and Nargoon Group (pale pink volumes). Portions of the overthrust Glenelg Zone (blue volumes) and Stawell Zone (bright yellow volumes) are included along the model flanks.

Figure 1.6 The STAVELY 3D model with most model volumes omitted to show the entire modelled fault network, and including magmatic rock volumes (red = Cambrian intrusions, pale pink = Early Devonian intrusions).

1.5 Philosophy

The modelling philosophy encompasses aspects such as the effective scale of the model, the depth of the model base, the workflow used to construct the model, the geological features captured, consolidation of stratigraphy and volumetric versus surface modelling.

The explicit modelling approach was favoured over implicit methods, because:

(1) the main datasets available for implicit modelling at crustal scale away from the few lines of seismic reflection data control are potential-field geophysics. Many potential field datasets (e.g. regional magnetic and gravity data) are inherently unable to resolve complex, moderate- to low-angle multi-layer geology at crustal scale – instead such potential field data tend to sum the petrophysical property response of multilayer elements. Other available potential-field datasets (e.g. magnetotellurics, EM) lack crustal-scale resolution. Shear wave velocity models derived from passive seismic data (e.g. Young et al., 2013) are also too coarse in resolution. Since we know for certain that the Cambrian crust of the Grampians-Stavely Zone comprises a crustal-scale, structurally-controlled multi-layer of linear, subparallel, steeply (at surface) to moderately- and shallowly (at depth)- dipping fault-belts of metaigneous rocks, regularly intercalated with metasedimentary rocks – as mapped in outcrop in the vicinity of the Grampians Ranges and imaged in deep seismic reflection data, modelling approaches (i.e. implicit modelling) that cannot fully capture or resolve these observations are limited in their application to regional 3D modelling, and:

(2) regional mapping in places of good exposure, such as within and south of the Grampians Ranges, demonstrates that many of the larger faults within the zone are very strike-persistent, yet these structures are only discriminated in geophysical datasets intermittently, at places where they separate rocks of contrasting geophysical character. Implicit modelling methods that are unable to connect fault segments in consistent, systematic, or meaningful ways fail to capture all the geological understandings gained from mapping into a coherent geological model, and:

(3) the preferred explicit approach allowed for: (i) careful step-by-step examination of interrelationships between all the different stratigraphic and structural packages, mapped in plan and in cross section, during model construction; (ii) reconciliation against field constraints, particularly against overprinting criteria established between different generations of rocks and structures; (iii) full categorisation of the range of theoretical assumptions (e.g. structural systematics) that were used to bridge the inevitable data-gaps that exist within the model volume and; (iv) a more specific ranking of confidence of the representation of different components contained within the model. 2. Modelling methodology

The STAVELY 3D model is designed to integrate and augment plan-view geology maps and interpretations (informed by field relationships and drilling), geophysical datasets and interpretations (including regional-scale deep seismic reflection and magnetotelluric transects) and crustal-scale cross sections. The STAVELY 3D model estimates and illustrates how district to regional-scale faults and other structures and rock units mapped at surface, and modelled near-surface (using geophysics), might extend and interact at depth, based on down-dip projections of near-surface constraints in conjunction with the appearance of geology imaged in deep seismic reflection data.

The 3D model was constructed by honouring the mapped surface distributions, drill intersections and interpretations (from geophysics) of various Cambrian – Devonian rock-types in STAVELY. Geometries imaged in regional deep seismic reflection profiles were delineated within the context of a structural history established for STAVELY using structural (surface) mapping, structural geophysics, including (observed) regional overprinting criteria, geochronology, geochemistry and paleoenvironment and paleogeography analysis.

2.1 Geological rationale

The Cambrian geology mapped at surface indicates that the continental crust within STAVELY comprises moderately- to steeply-dipping thrust sheets of Stavely Arc and metasedimentary rocks (see Appendix 4 – Geological Units), imbricated at crustal scale in the Late Cambrian, and modified during subsequent deformation events (see Appendix 5 – Deformation history summary). The thrust imbrication takes the form of a multi-layer. To capture this observation, an explicit modelling workflow extends the structural style and form mapped at surface and established from discrete regions of better control (e.g. deep seismic reflection lines) into the subsurface for the whole model volume.

Figure 2.1 Theoretical example showing the use of overprinting criteria to establish a geological history: Event 1: fault FA accompanied by early (pale) mineral growth. Event 2: episode of inclined (dark) mineral growth. Event 3: intrusion of granite body. Event 4: lateral offset across fault FB (coincidentally subparallel to Event 2). Event 5: lateral offset across fault FC. The workflow incorporates a set of structural overprinting criteria (Figure 2.1) and a stress (strain) history established from regional mapping (see Section 2.4.1 – Regional geological mapping and drilling), and the results of retrodeformations of some structural events (see Section 4 – Discussion) and uses these to infer and interpret the most likely fault inter-relationships and geometries at depth in the crust.

The workflow also incorporates the implications of wider geological understandings based on investigations of geology adjacent to the Grampians-Stavely Zone. These implications include the wider context of the eastern Gondwana margin during, and subsequent to, the breakup of the former supercontinent Rodinia in the Late Precambrian. The East Gondwana Neoproterozoic-Early Cambrian passive margin that resulted from Rodinia breakup apparently evolved into a convergent margin in the Early Cambrian. This convergent margin was apparently locally active until the end of the Cambrian, before convergence jumped farther east in the Ordovician. These wider geological understandings have allowed a geological narrative to be developed that is consistent with all available data, but not necessarily a unique solution. This narrative has been used to assign possible affiliations to buried geological elements that occur at deep levels in the crust, consistent with the range of petrophysical properties and shapes that are known.

The purpose of incorporating such elements in the model is to demonstrate the possibility that concepts, structures and relationships that are unable to be discriminated unambiguously on the basis of existing Grampians-Stavely Zone data may exist at depth, are a potential fit with the known geology, and may be important for understanding mineral potential. Where such assumptions impact on the 3D model they are identified and discussed in this report.

Regional geological relationships which underpin construction of the STAVELY 3D model are summarised in Schofield et al. (2018) and Duncan et al. (in prep).

2.2 Model extent

The STAVELY 3D model corresponds to STAVELY, which is bound on its western and eastern flanks by the Yarramyljup and Moyston faults respectively (Figure 2.2). The position, age and geometry of these bounding faults is constrained by field-mapping (Gibson & Nihill, 1992; Cayley & Taylor, 2000a, 2001; Morand et al., 2003), drilling, geochronology (Miller et al., 2005), potential field geophysics, and by several deep seismic reflection transects that directly image these faults and allow them to be traced into the lower crust (Korsch et al., 2002; Cayley et al., 2011a; Cayley et al., 2011b). The Moyston and Yarramyljup faults both strike northwest, and dip east and west respectively, away from the Grampians-Stavely Zone. This means that: (1) the Grampians-Stavely Zone represents a region of major structural vergence reversal in western Victoria, and (2) the width of the zone increases dramatically with depth.

Parts of the adjacent Glenelg and Stawell zones are included in the STAVELY 3D model to capture and communicate as much knowledge as possible of the Grampians-Stavely Zone, including those portions that occur in the overthrust zone-bounding fault footwalls.

The STAVELY 3D model is bound to the north and south by the extent of STAVELY (Figure 2.2), dictated in turn by the thickness of the post-Paleozoic cover-rock (~300 m cut-off) in the north and seismic lines BMR92-OT2 and BMR92-OT3 in the south. This cover comprises the rift-related Otway Basin in the south, and the sag-related Murray Basin in the north – the interpreted depth-to-base of these basins are included as surfaces in the model. Other cover units included in the STAVELY 3D model are the ?Late Ordovician-Silurian Grampians Group, and thin volcanic-dominated sequences – the Early Devonian Rocklands Volcanic Group that is apparently confined to a discrete region west of the main Grampians Ranges, and the Neogene-Quaternary Newer Volcanic Group generally confined to the south of the Grampians Ranges.

The STAVELY 3D model extends to full crustal depth (i.e. to the Moho) because: (i) deep seismic reflection data and shear wave velocity models derived from passive seismic data (Young et al., 2013) images to this depth and below in several locations, thus providing constraints on interpretations of the lower crust; (2) as also observed in adjacent zones (e.g. Cayley et al., 2011a,b), deep seismic reflection imagery of the Grampians-Stavely Zone shows that deformation of the Cambrian (and older) strata here is of thick-skinned character – key structures mapped at surface can be traced continuously into the lower crust and possibly below with little apparent change in character or form. For this reason, the structural history mapped at surface can be extrapolated into the mid- and lower crust to constrain the range of possible lower crustal geometry and geodynamic interpretations; (iii) balanced retrodeformations of thick-skinned terranes (including removal of the effects of D4) necessarily involve consideration of the whole crust; (iv) full crustal modelling was needed to better constrain the positions and interrelationships of faults that are known to be steep at the surface and are shown in deep seismic reflection data to decrease in dip magnitude to depth; (v) geodynamic interpretations involve the whole lithosphere; and; (vi) the Victorian 1:250 000 and 1: 1 000 000 3D geological models are full crustal thickness (Rawling et al., 2011), and the STAVELY 3D model supersedes the previous Grampians-Stavely Zone 3D model (which forms part of the GSV’s 3D Victoria model released in 2011 (see Rawling et al., 2011: 3D Geological model of Victoria 2011).

2.3 Software

The STAVELY 3D model was constructed using GOCAD (version 2009, patch 4). The GOCAD modelling system is a data-based, three-dimensional geometrical modelling scheme that integrates information through a discrete mathematical approach (Mallet, 1992). It allows interactive manipulation and visualisation of geological models comprising a variety of different model types. This software was used to import constraining data such as potential field data, structural mapping and interpreted cross sections, and build surfaces intended to characterise the geometry of faults and geological boundaries.

Gridded potential field data has been filtered and visually enhanced using the Discover PA (v2015.1) geophysical software package. Enhanced imagery produced in Discover PA was exported as georeferenced tiffs and imported into the GIS ESRI ArcMap software. Here multiple geophysical and geological datasets were interrogated to produce a final 2D geological interpretation. Potential field profile inversion and forward modelling has been carried out using ModelVision Pro (v15.00.13).

2.4 Inputs and constraints

The STAVELY 3D model is constrained by surface geology, surface structural measurements, drill hole data, interpretations of potential field geophysical data (including inversions, dip modelling and forward modelling), the geometries imaged in seismic reflection data in and adjacent to the Grampians-Stavely Zone, and by overprinting criteria which allow for discrimination between successive depositional, deformational and intrusive events in the region (Figure 2.3, also see Appendix 5 – Deformation history summary). Regional scale shear wave velocity volumes derived from passive seismic data (Young et al., 2013; Figure 2.4) are coarse in resolution, but do image changes coincident with major structures (e.g. Apsley Fault) and so provide additional constraints on orientation and dip magnitudes of some structures confined to, or extending into, the mid and lower crust.

Because few rocks that can be proven to have originated from mid-or lower-crustal levels are known within this zone, the level of uncertainty and ambiguity rises with increasing depth within the model – a range of possible scenarios and their inherent strengths and weaknesses are discussed, including a preferred scenario based on data available to 2018, about which the 3D model has been built. The levels of uncertainty and ambiguity are lowest where surface geological control can be traced directly to depth, for example in regions of outcrop along and adjacent to deep seismic reflection transects where faults and stratigraphy mapped at surface can be traced without break to mid-crustal levels or deeper. Even in regions imaged by seismic reflector data, some significant reflector packages are confined to the lower and mid-crust, blind to the surface. Therefore, their precise nature and origin remains ambiguous. Because of their depth, they cannot be discriminated sufficiently in existing geophysics data to be modelled implicitly.

Figure 2.2 Spatial extent of STAVELY underlain by 1:250 000 seamless surface geology (see Welch et al., 2011), showing the Yarramyljup and Moyston faults and major towns.

2.4.1 Regional geological mapping and drilling

The starting framework for the STAVELY 3D model was development of a new plan-view line interpretation of Palaeozoic geology assembled at approximately 1:50 000 – 1:100 000 scale, depicting the surface positions of pre-Carboniferous rock-units and the faults that affect these rocks, and the near-surface positions where these rocks lie beneath younger cover rocks. This new interpretation is constrained by previous surface geological mapping where a range of Early Palaeozoic rocks are exposed for direct study (e.g. in the Willaura region; Buckland, 1987; Stuart-Smith & Black, 1999; Cayley et al., in prep.: in the Black Range region; McArthur, 1990; Cayley & Taylor, 1997a, 1997c; Morand et al., 2003: in the Grampians Ranges; Spencer-Jones, 1965; Cayley & Taylor, 1997a: and east of the Grampians; Buckland 1987; Wilson et al., 1992; Cayley & Taylor, 2001 – and numerous additional references cited therein). The new plan-view line interpretation is additionally constrained by regional compilation maps that rationalise previous geological interpretations, including of geophysical data (e.g. Simons & Moore, 1999; Welch et al., 2011), and by stratigraphic, water bore and mineral exploration drill hole data where the Early Palaeozoic rocks are concealed by younger units such as the Murray and Otway Basins and SRTM data.

2.4.2 Potential field data, processing and enhancement

The structural interpretation in STAVELY was extended beneath areas of cover utilising geophysical data constrained in places by drill hole data. Geophysical data sets available for this study include regional government airborne magnetic, radiometric (100-400 m line spacing) and ground gravity data (nominal 1.5 km station spacing), as well as open-file company data (detailed ground and airborne magnetics, detailed ground gravity) and deep seismic reflection transects (Cairns et al., 2018). Regional-scale magnetotelluric data spans much of the zone, acquired along the deep seismic-reflection transect locations and as long-period data collected as part of the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) survey.

2.4.2.1 Magnetic data

Magnetic data was sourced from VicMag (2005), Victoria’s state-wide open file database of airborne magnetic data managed by the GSV, merged and gridded at a cell size of 50 m (Figure 2.5A). The merged database consists of data acquired by government, researchers and industry (see Cairns et al., 2018 for details of individual surveys within and immediately adjacent STAVELY). Over STAVELY the flight line spacing is generally 200 m, with sparser 400 m line spacing in the north. The nominal flight height for both line spacings is 80 m. A limited number of local surveys have been undertaken by industry and range in specification, the smallest of which is 40 m line spacing and 40 m flight height. Where located line data was used, this was sourced directly from the relevant individual survey. In the Black Range region recent detailed airborne data (50 m line spacing and 50 m flight height) was supplied to the GSV by Navarre Minerals Ltd to aid the detailed interpretation and modelling of the Black Range and Glenisla belts (Figure 2.5B). This enabled the identification of faults otherwise not evident in the regional dataset. After standard corrections the final dataset forming the basis for further filtering, interpretation and modelling was Total Magnetic Intensity (TMI) (nT).

2.4.2.2 Gravity data

Gravity data was sourced from VicGrav (2008), Victoria’s state-wide open file database of ground gravity data managed by the GSV, merged and gridded at a cell size of 300 m (Figure 2.6). The merged database consists of data acquired by government, researchers and industry (see Cairns et al., 2018 for details of individual survey details within and immediately adjacent STAVELY). Individual surveys have been tied to the Australian Fundamental Gravity Network (AFGN). Within STAVELY the located data has a nominal station spacing of 1.5 km, but can be up to 5 – 8 km in some areas of poor coverage such as national parks. After standard reductions and corrections (including terrain correction) the final dataset forming the basis for further filtering, interpretation and modelling was the Complete Bouguer Anomaly (m/s2).

2.4.2.3 Data processing Processing of available potential field datasets (and/or seismic or electrical) can provide additional information for interpretation, which can be used to constrain aspects of 3D model development. Mathematical filtering and visual enhancement of potential field (ground gravity and airborne magnetic) data enables the accentuation of information not easily discerned in the raw data, therefore allowing a more robust interpretation of the underlying geology. Utilising these techniques a more rigorous analysis of the three-dimensional physical property distribution within the crust can be undertaken, more accurately delineating and constraining the geometries, spatial distribution and boundaries of the interpreted source bodies (different rock types) producing the observed geophysical anomalies. The result is a more informed understanding of the whole crust.

The extensive and varied nature of cover sequences that overlie the prospective Cambrian bedrock in STAVELY mean that filtering and enhancement of potential field data has played an indispensable role in facilitating the structural interpretation of Cambrian rocks and associated D1a and D1b structures, as well as younger units and D2-D5 structures which have subsequently deformed the basement and some overlying cover units (e.g. Grampians Group). These datasets have enabled a palinspastic retrodeformation of D4 and D3 structures to be undertaken, which may otherwise have been impossible (see Section 4.1 – D4 and D3 retrodeformation testing).

Figure 2.3 Key inputs and constraints used to develop the STAVELY 3D model. In addition to the selected mineral exploration drill holes depicted in this figure are thousands of water bores and other drill holes with lithological information used to further constrain the STAVELY 3D model.

Figure 2.4 Select images of the Southeast Australia shear wave velocity model of Young et al. (2013), showing A) oblique view of a depth-slice at 23 km within the model volume, and a vertical shear wave velocity profile through the same volume that illustrates the overall upward-decrease in shear wave velocity, and overall easterly-dip of velocity features of Precambrian-Cambrian rocks related to the east-Gondwana margin beneath the position of the Stavely Arc system. Modelled surfaces are the ‘Torrens Hinge Zone’ in the west, and the (Silurian) Bootheragandra Fault in the east. B) plan view of the same 23 km depth-slice, depicting the interpreted positions, at that depth, of the Apsley Fault and the leading, eastern edge (the ‘Tasman Line’) of buried crystalline ‘continental’ Proterozoic crust that we interpret to form the foundation crust through which the Stavely Arc was erupted. Both structures are coincident with marked changes in shear wave velocity – the higher velocities farther east coincide with imbricated Cambrian mafic igneous material in the lower crust of the Stawell and Bendigo zones (e.g. Cayley et al., 2011a) and with Selwyn Block crust.

2.4.2.3.1 Filtering

Several geophysical filters were applied to potential field data in STAVELY (Figure 2.7), combining to form an integral part of the interpretation process. Mathematical filtering of gridded potential field data was undertaken using algorithms within the Discover PA Pro geophysical software package, while filtering of profile data (applied during forward modelling and inversion operations) was undertaken using the ModelVision Pro software package. In many places the resultant datasets have been able to resolve the distribution of contrasting sequences of Cambrian geology within and adjacent areas of (poor) exposure and/or drill hole control. This has resulted in an extension of the Cambrian geology under younger cover and an increased confidence in the interpretation. The following section outlines the geophysical filters which proved most valuable in the geological interpretation and resulting development of the STAVELY 3D model.

2.4.2.3.1.1 Reduction to pole filter

The reduction to pole (RTP) filter removes the asymmetry of a TMI anomaly associated with an inclined inducing field (i.e. the Earth’ s magnetic field at latitudes away from the poles), and represents what the anomaly would look like at the magnetic poles (Figure 2.7). The filter theoretically positions the peak of the anomaly directly over the centre of the magnetic source body. Resolution of closely spaced anomalies may also be improved (Dentith & Mudge, 2014). The RTP transformation assumes the body’s magnetisation is the result of induction alone, and therefore parallel to the inducing geomagnetic field. Where remanence is present anomalies will not be correctly transformed, potentially resulting in offsets between the centre of the anomaly and the centre of the source body. The RTP transform is a standard filter applied to magnetic data when undertaking interpretation, and was applied to magnetic datasets in STAVELY. Subsequent filtering operations applied to magnetic data were typically applied to RTP data.

2.4.2.3.1.2 Vertical derivative filters

In STAVELY the first vertical derivative (1VD) filter has been applied to gridded magnetic data to aid in the interpretation of relatively shallow structures, and mapping the edges of magnetic units with a high frequency response (e.g. Rocklands Volcanic Group, Newer Volcanic Group, and some intrusive rocks) (Figure 2.7). The 1VD filter has also been applied to magnetic profile data and inverted upon during inversion modelling of volcanic belts, concurrently with TMI inversions (see Section 2.6.5.1 – Magnetic inversion modelling of the volcanic belts). This formed additional constraints in the inversion process, and enabled the separation of anomalies not resolved in the TMI data. Due to the density of ground gravity data in STAVELY the 1VD filter was generally not useful in resolving finer detail in the data and was seldom applied.

2.4.2.3.1.3 Wavelength filters

A separation of deeper versus shallower magnetic sources has proven useful in highlighting the spatial distribution of the Cambrian volcanic belts, particularly beneath magnetic cover in the south of STAVELY. Band pass filtering enabled the delineation of the Boonawah and Bunnugal volcanic belts (and coincident intrusions) beneath the high frequency magnetic response of the Newer Volcanic Group (Figure 2.8). A qualitative interpretation of the distribution of (magnetic) Cambrian volcanic belts at depth was guided by the application of band-pass and upward continuation filters (Figure 2.7). The utilisation of wavelength separation filters on magnetic data has also enabled the identification of intrusive rocks, and/or potential volcanics beneath highly magnetic Rocklands Volcanic Group west of the Grampians Ranges (Figure 2.9). These features are inferred to be Devonian in age due to their orientation and general north-east alignment, and may be the source of the eruptive materials that formed the overlying Rocklands Volcanic Group. The most useful wavelength filters applied to magnetic data were 1 – 10 km; 3 – 30 km and 10 – 50 km band pass filters, and 500 m and 1000 m upward continuation filters.

Wavelength filtering of gravity data in STAVELY was applied extensively (Figure 2.7). Of particular use were the high pass filters – 15 km High Pass; 30 km High Pass and 50 km High Pass. Application of these filters to gridded ground gravity data helped in mapping structures controlling the Otway Basin (including basin bounding faults), intrusive rocks (including a qualitative interpretation of thicknesses and geometries), density contrasts associated with regional faults, and the distribution of the Stavely Arc volcanic belts – particularly the Grampians Deeps Belt buried beneath the Grampians Ranges (see Figure 1.3), which shows little in the way of magnetic anomalism. Filtered ground gravity data also provided additional control in determining the distribution of Grampians Group rocks beneath younger cover – for example in the Lorquon Rift. Since caution needs to be applied in regions of sparse data density, where gridding artefacts may be accentuated by filtering, ground gravity station locations were regularly interrogated to confirm the validity of anomalies being interpreted (see Section 2.6.5.2 – Gravity forward modelling – Serial cross sections).

In-line wavelength filtering was applied during forward modelling of the serial cross sections developed to constrain and provide a framework for the development of the STAVELY 3D model (see Section 2.6.5.2– Gravity forward modelling – Serial cross sections). In addition to modelling the terrain corrected Bouguer anomaly, 20 km low pass filtered and 15 km high pass filtered gravity data were also simultaneously forward modelled along serial section profiles, providing additional constraints to the modelled section for both the deep, regional bodies, and the shallower localised bodies.

Figure 2.5 Airborne magnetic data: A) Regional TMI data gridded at 50 m cells over STAVELY. Square shows location of detailed airborne magentic survey; B) Detailed (50 m line spacing) TMI data gridded at 12.5 m cells over the Black Range (data courtesy of Navarre Minerals Ltd).

Figure 2.6 Regional ground gravity: Complete Bouguer Anomaly data gridded at 300 m over STAVELY.

Figure 2.7 Comparison of various filters applied to potential field data. The square on the location image shows the region over the Black Range for which filtered grids are shown. The top seven panels show TMI and filtered magnetic data; bottom three panels show Complete Bouguer Anomaly and filtered gravity data. From left to right: Row 1 – TMI; TMI Reduced to Pole (RTP); 1VD of TMI (RTP); Row 2 – Analytic Signal; Tilt-angle filter; Row 3 – 3-30 km Band Pass filtered TMI (RTP); 1 km Upward Continued TMI (RTP); Row 4 – Complete Bouguer Anomaly; 15 km High Pass filtered Complete Bouguer Anomaly; 30 km High Pass filtered Complete Bouguer Anomaly.

2.4.2.3.1.4 Analytic signal filter

In STAVELY the analytic signal filter was applied to gridded airborne magnetic data (Figure 2.7) and was an important tool utilised in the interpretation of both the faults bounding the volcanic belts of the Stavely Arc, and fault networks within the belts. In places, discontinuity or the apparent termination of units in the filtered data, which otherwise appeared continuous in unfiltered data, provided additional constraints for the structural interpretation. The distribution of possible localised remanent magnetisation was also identified by application of the analytic signal filter, expressing as spatial distortions of the magnetic response of bodies and, in extreme cases, a complete reversal of the resultant magnetisation vector when compared to unfiltered TMI data (Figure 2.10). The filter was also useful in defining the edges of intrusions, particularly when applied in combination with other enhancement techniques such as the 1VD and tilt-angle filters. 2.4.2.3.1.5 Tilt-angle filter

In STAVELY the tilt-angle filter was applied to gridded magnetic data and used in the interpretation of linear magnetic units within the volcanic belts, highlighting faulted offsets in these units (Figure 2.7). The filter also proved useful in the delineation of intrusions, including different intrusive phases. The combination of the tilt-angle filter with band pass filtered magnetic data was a particularly powerful interpretation tool (see Section 2.4.2.4 – Image enhancement).

2.4.2.4 Image enhancement

Further to mathematical filtering, visual enhancement can be a powerful technique in aiding the qualitative interpretation of gridded geophysical imagery. These enhancements can include applying various colour stretches (e.g. histogram; linear) and look up tables, sun-shade illumination, transparency overlays and drapes of various combinations of filtered and non-filtered dataset images, and transparency overlays of different potential field dataset images.

The combination of tilt-angle filtered and band-pass (3 – 30 km) filtered magnetic data was one of the main convergent data displays used for qualitative interpretation and palinspastic retrodeformation in STAVELY (Figure 2.11). The combination of these two representations of the magnetic field enable a simultaneous visualisation of both deeper and shallower magnetic features, reinforcing where the two responses coincide and superimpose. This technique proved particularly useful in highlighting the Cambrian volcanic belts. This enhanced image is effective in delineating the Bunnugal and Boonawah volcanic belts beneath Newer Volcanic Group, as well as outlining intrusions, and/or volcanic centres, beneath the Rocklands Volcanic Group. The image was produced by applying transparency to a pseudocolour tilt-angle filtered image and overlaying it over a pseudocolour band-pass filtered image.

Another enhancement that was frequently utilised was a combination of high pass (30 km) filtered gravity data displayed as pseudocolour draped over a sun-shaded reduced to pole TMI surface (Figure 2.12). This allowed for a visual correlation of the magnetic and gravity responses, which aided the interpretation of geological units and faults under cover.

2.4.3 Key sites of constraint

In several localities, the positions of major fault-lines interpreted from potential field geophysics – for example where the trend of a magnetic unit terminates abruptly or where sharp boundaries exist between domains of contrasting geophysical character or where units of different orientation are juxtaposed – coincide with the mapped positions of exposed faults and with breaks in reflectivity imaged in deep seismic reflection data. These, and locations of geochronological, palaeontological and/or geochemical control, are key locations of confidence that help constrain the geometry, timing and movement history of faults, the interpretation of the structural/tectono-stratigraphic history, and hence the modelling of subsurface fault geometries.

Figure 2.8 Wavelength filtering of magnetic data over the Cambrian-aged Boonawah and Bunnugal belts. Band pass filtering applied to TMI data (A) to remove the shorter wavelength, high frequency response of Newer Volcanic Group, and better delineate the longer wavelength, lower frequency response of the deeper volcanic belts (B and C).

Figure 2.9 Wavelength filtering of magnetic data west of the Grampians Ranges. Band pass filtering applied to TMI data (A) to remove the shorter wavelength, high frequency response of Rocklands Volcanic Group, accentuate the longer wavelength, lower frequency response of deeper sources, including potential Devonian intrusions (B and C).

Figure 2.10 Examples of remenant magnetisation within the Black Range Belt. Left hand panels show TMI; right hand panels show analytic signal. Arrows indicate location of probable reversely magnetised porphyries: A) TMI of northern Black Range Belt, B) Analytic signal of northern Black Range Belt (see Figure 3.18 for inversion modelling); C) TMI of central Black Range Belt, D) Analytic signal of central Black Range Belt.

Figure 2.11 Image enhancement – Magnetics: Image is a NE sun-shaded, partially-transparent histogram-equalised pseudocolour of tilt-angle filtered TMI (RTP), overlain on a 3 – 30 km band pass filtered, histogram-equalised pseudocolour image of TMI (RTP). The tilt-angle filtered data enhance shallow magnetic bodies and their edges, while the band pass filtered data enhance deeper magnetic bodies and highlight their distribution in relation to the response of shallower units. Blue colours indicate lower magnetic intensity and phase angle values; red colours indicate higher magnetic intensity and phase angle values. Figure 2.12 Image enhancement – Gravity: Image is a histogram-equalised pseudocolour layer of 30 km high pass filtered Bouguer gravity draped on NE sun-shaded TMI (RTP) intensity layer. Blue colours indicate low gravity response; red colours indicate high gravity response.

Structural examples include, but are not limited to: Early Cambrian (D1a) thrust imbrication

 the Moyston Fault, an east-dipping thrust fault that juxtaposes Cambrian high-grade metamorphic rocks in its hangingwall over Cambrian low-grade Glenthompson Sandstone and Stavely Arc succession in its footwall, exposed near Moyston (Figure 2.13) and at Mount Drummond (Cayley & Taylor, 2000c, 2001);

 The Yarramyljup Fault, a west-dipping thrust fault that juxtaposes Cambrian high-grade metamorphic rocks in its hangingwall against Cambrian low-grade Stavely Arc succession in its footwall, unconformably overlain by Grampians Group south of Balmoral (Gibson & Nihill, 1992; Morand et al., 2003);

 the Williamsons Road Fault, a west-dipping thrust fault that incorporates a fault-slice of exotic serpentinite within the interior of the Stavely Belt, age constrained to D1a by the Late Cambrian Lalkaldarno Porphyry that intrudes across the fault plane, west of Wickliffe (Buckland, 1985, 1987; Stuart-Smith & Black, 1999).

 The Narrapumelap South Fault, a splay of a northeast-dipping thrust fault with down-dip fault striations, part of the fault system that separates volcanics of the Stavely Arc in the Narrapumelap Belt to the north from low-grade Glenthompson Sandstone to the south, exposed east of Gayton Road, near Wickliffe.

 Stratigraphy disrupted across unnamed faults within belts of steeply-dipping Mount Stavely Volcanic Complex rocks, including a high proportion of low-grade Glenthompson Sandstone and other sediments incorporated as fault-slices within many of the belts. This is demonstrated by drill hole intersections in the Stavely, Narrapumelap, Elliot, Dryden, Brimpaen and Glenisla belts. Shale units – including packages of pyritic black shale – are mapped within the Stavely Belt (e.g. Glenronald Shale Member), and more widely within the Black Range Belt, appear exotic and are localised along steeply-dipping faults.

 Regions between volcanic belts are occupied by belts of low-grade Glenthompson Sandstone (Kanmantoo Group) and Nargoon Group metasediments that are up-turned to steep dips closely comparable to the adjacent volcanic belts. The metasediments are generally homoclinally dipping and facing and generally upright, as expected in an imbricated thrust system. Facing reversals in the sediments, and occasional fold closures show upright chevron-style folds to occur locally (see Figure 3.70).

 Original stratigraphic relationships between different Cambrian strata inferred from facing-directions preserved within fault-belts and from studies of rock provenance. These studies suggest that parts of the low-grade Glenthompson Sandstone are older than the Mount Stavely Volcanic Complex (and so lack any detrital input from the volcanics, as seen in detrital zircon geochronology; Lewis et al., 2016), so that where Glenthompson Sandstone overlies units of the Mount Stavely Volcanic Complex, this is likely to be a consequence of overthrusting.

Early Devonian (D3, D4) transtensional deformation

 the Escondida Fault, a sub-vertical dextral strike-slip fault that cuts and offsets Cambrian Stavely Arc strata, exposed near Wickliffe (Figure 2.14), and is also developed in the Grampians Group north of Yarram Park, where it truncates the western end of the Mafeking Orocline;

 the Yarrack Fault, a sub-vertical dextral strike-slip fault that cuts and offsets Cambrian Stavely Arc rocks south of Glenthompson (see Section 3.3.5.27 – Yarrack Fault), cuts and offsets Cambrian metasediments and Late Cambrian (D1b) intrusives south of Glenthompson (Figure 2.15) and is also expressed as a dextral kink of lesser magnitude in overlying Grampians Group (see Figure 2.16);

 the Mosquito Creek Fault, a subvertical strike-slip fault that bounds the western flank of the Brimpaen Belt and is also exposed in the Victoria Range in the western Grampians Ranges;

 the Muirfoot Fault, a subvertical sinistral strike-slip fault developed in Grampians Group in the western Black Range (Cayley & Taylor, 1997a, 1997c) and cuts and offsets the underlying Black Range Belt (see Section 3.3.5.18 – Muirfoot Fault);

 the Latani Fault, a subvertical sinistral strike-slip fault developed in Grampians Group and in Cambrian metamorphic rocks, exposed south of Balmoral (Morand et al., 2003) (see Section 3.3.5.13 Latani Fault);

 the Log Hut Fault, a dip-slip fault that juxtaposes Grampians Group against Cambrian metamorphic and granitic rocks northwest of Hamilton (Simpson & Woodfull, 1994) (see Section 3.3.5.14 Log Hut Fault),

 the Golton Fault, an east-dipping thrust developed along the eastern flank of the Grampians Ranges (Spencer-Jones, 1965; Wilson, 1988; Wilson et al., 1992; Cayley et al., 2011b) that juxtaposes subvertical Grampians Group against Cambrian strata southwest of Mt Dryden (see Section 3.3.4.2 – Golton Fault);

 numerous faults that deform Grampians Group strata, exposed in three-dimensions in many places within the interior of the Grampians Ranges (Figure 2.17). These inform the movement histories of buried portions of the faults that are imaged in potential field data and in deep seismic reflection data, including in the underlying Cambrian bedrock. It is now recognised that high-angle and moderate-angle faults developed within the Grampians Group have propagated directly up from the underlying Cambrian bedrock. This additional control has enabled the construction of eighteen additional cross sections within the Grampians Ranges to mid-crustal depth to inform the STAVELY 3D model.

Figure 2.13 A) Moyston Fault plane exposed in a creek near Lennox Springs Road, south of Moyston. MGA 54 656321 5867678. The fault dips moderately southeast at this locality, and is the sharp, left-dipping surface at the hammer base that thrusts greenish polydeformed, chloritised, Carrolls Amphibolite schist, carbonate cataclasite and deformed quartz veins in the hangingwall (Moornambool Metamorphic Complex of the Stawell Zone, above) over brown, low-grade, uniform uncleaved Glenthompson Sandstone in the footwall (Grampians-Stavely Zone, below; see also Figure 2.20). B) Typical western Stawell Zone rocks: Carrolls Amphibolite typical of the Moyston Fault hangingwall. Excluding introduced quartz veining, this high-grade, polydeformed hornblende-garnet-albite -calc-silicate (± epidote) rock is geochemically similar to the MORB-like Early Cambrian Magdala Basalt, and is interpreted to be derived from it. Strong fabrics and relict fold hinges are attributed to polydeformation in the hangingwall of the Moyston Fault during D1a. Carrolls Cutting, Ararat-Moyston Road, MGA 54 664622 6870828. C) Thin section of Carrolls Amphibolite, showing layered, recrystallised hornblende-sphene-quartz-garnet metamorphic mineralogy with relict twinned igneous plagioclase and prominent euhedral garnet porphyroblasts. Near Mount Moornambool, MGA 54 660863 5855190. Crossed polarised light, field of view 7 x 4.2mm. D) Typical western Stawell Zone rocks: Good Morning Bill Schist in the Moyston Fault hangingwall, showing boudinaged quartz vein fragments aligned in a mylonite foliation. This Cambrian high-grade polydeformed rock is geochemically similar to pelitic Cambrian Saint Arnaud Group and is interpreted to be derived from it. It is structurally intercalated with Carrolls Amphibolite and shares a common structural and metamorphic history. Andrews Lane, MGA 54 660471 5856928. E) Thin section of Good Morning Bill Schist showing S-C protomylonite fabrics. Schistosity planes are defined by aligned coarse muscovite fish (brightly coloured birefringence), cut by oblique shear planes. Grey groundmass is predominantly recrystallised quartz. Minor mm-scale garnets are black dots (garnet is isotropic). Cross polarised light. Field of view 16 x 11mm.

Figure 2.14 Escondida Fault exposed in a creek near Sheepwash Road, northwest of Wickliffe. MGA 54 647510 5830763. Photo and sketch looking down (and towards the southeast) at sheared and foliated Glenthompson Sandstone cataclasite within a 100m+ wide subvertical D4 dextral shear zone developed along the trace of the northerly-trending Escondida Fault. Stratigraphic facing is both northeast (younging in thin, graded sandstone beds indicated at bottom right) and southwest at different places across the width of the exposure, indicating that the shear zone was superimposed over a succession already deformed by upright, probable F1a, folds. Competent sandstone bed fragments in the cataclastic mélange contain stylolytic to slaty cleavage relicts typical of S1a. The cleaved sandstone bed fragments are elongate, aligned parallel to the D4 shear zone boundaries. They display abrupt, fracture-bound terminations such as the bottom of the thick bed at right. Enclosing the sandstone bed fragments, an incompetent siltstone with relict bedding layers displays a prominent low-temperature S4 stylolytic/cataclastic shear-fabric that trends northwest and cuts obliquely across the shear zone and across the sandstone breccia fragments aligned within it (from top-left to bottom-right in this image), thus defining a dextral shear-sense. The axial planes of subvertical F4 kink-folds lie within the shear-fabric, as at bottom right, and also have dextral asymmetry.

Figure 2.15 Lineated fault plane of a Yarrack Fault splay within the Late Cambrian Buckeran Diorite. The lineations (parallel to pen) are interpreted as slickenside striations, and plunge northwest on this moderately west-dipping D4 fault plane. They constrain a dextral-transtensional displacement on the fault plane. This minor fault is interpreted as a splay of the Yarrack Fault, the main trace of which cuts and completely offsets the diorite nearby (MGA 54 645240 5824230).

Figure 2.16 Geology of the main Grampians Ranges depicting: the main stratigraphic subgroupings and structures within the ?Late Ordovician-Early Silurian Grampians Group; the distribution of Early Devonian igneous rocks; major structures and; schematic relationships with surrounding and underlying Cambrian geology. Indicated cross-section locations are depicted in Figure 3.5. Of particular note are: major strike-slip structures such as the Escondida, Golton, Mosquito Creek, Yarrack, Henty and Muirfoot faults and the Mafeking Megakink / Orocline that deform both Grampians Group and the underlying Cambrian bedrock and predate Early Devonian magmatism and so have a tightly constrained Late Silurian – Early Devonian (D3, D4) age. Also of note: the distribution of the subhorizontal D4 Marathon Fault, a transtensional detachment that separates Grampians Group from underlying Cambrian bedrock, exposed around the margins of the main Grampians Ranges and intersected in drill hole STAVELY02 (see Figure 2.18) and; splays up into the Grampians Group from the Marathon Fault detachment – the Cranage and Big Cord splays. The interpreted lateral extents of footwall Marathon Fault planes are depicted with green shading and arrows that illustrate interpreted fault footwall slip vectors during D4. Q symbols and open arrows relate this mapped geology to a schematic block-diagram of the interpreted structural evolution, illustrated in Figure 3.73.

Figure 2.17 Four-metre-thick fault breccia developed within a subhorizontal, possibly D3, fault zone in Grampians Group strata, exposed at base of massive, coherent quartz arenite sandstone beds that form the ‘Chimney Pot’, Victoria Range, Grampians Ranges. MGA 54 609600 5862250. Numerous other fault planes have been intersected by drilling, examples include:

 the D4 Marathon Fault that separates Grampians Group from underlying Cambrian bedrock intersected in drill hole STAVELY02 (Figure 2.18) and;  D1a faults that juxtapose fault slices of different Cambrian stratigraphy intersected in mineral exploration drill holes in the Stavely Belt interior and;

 post-D1 structures that cut and displace hydrothermal alteration assemblages related to D1b porphyries.

In all these instances, direct field-measurements of fault size, age, geometry, overprinting criteria and (dip-slip and often strike-slip) movement history can be extrapolated with confidence from the point of ground-control into areas of younger cover using geophysics.

In addition to fault plane exposures, regions of unusually oriented or disordered stratigraphy give insight into larger scale processes, and can be used to constrain fault geometries included in the STAVELY 3D model. Examples are:

 a thick, strike-persistent panel of west-facing and dipping Glenthompson Sandstone exposed east of, and stratigraphically below, the Stavely (and Dryden) Belt of Stavely Arc volcanics (between the Stavely East and Escondida faults). The realisation that Glenthompson Sandstone might be older than the Stavely Arc volcanics (see Appendix 4 – Geological Units, Glenthompson Sandstone (Kanmantoo Group)) requires inference of a large west-dipping D1a thrust fault, the Stavely Base Fault, beneath both the Stavely Belt and the adjacent Glenthompson Sandstone to allow interpretation of the large underlying buried reflective rock-package as a younger, autochthonous Stavely Arc edifice;

 a thick panel of consistently west-facing and dipping Glenthompson Sandstone to the west of the Stavely Belt of Stavely Arc volcanics, exposed near Chatsworth and intersected in drill hole STAVELY17 (see Section 3.4.4 – Mafeking Megakink). This panel structurally overlies the Stavely Belt, yet is most likely stratigraphically older than the Stavely Arc succession. This relationship implies crustal-scale D1a overthrusting of Glenthompson Sandstone across the Stavely West Fault (included in the STAVELY 3D model), together with D1a internal thrust imbrication of Glenthompson Sandstone stratigraphy (not included in the STAVELY 3D model) to explain the large (~10 km) apparent thickness of steeply dipping Glenthompson Sandstone strata that is intruded by Late Cambrian D1b granites.

 consistent west-facing of Mount Stavely Volcanic Complex strata exposed in different parts of the Dryden Belt, one of the least internally deformed of the exposed belts of the Stavely Arc (Figure 2.19; see also Figure 3.33). The consistent west facing of Dryden Belt strata along many kilometres of strike (Buckland, 1987; Cayley & Taylor, 2001) gives confidence to our interpretation of an overall westerly dip and facing of the stratigraphy within the Dryden Belt. This control can be extended into other volcanic belts that appear to have been part of the Dryden Belt when it was forming as a fault-slice in the Cambrian – e.g. the Dryden North Belt (under cover), the Narrapumelap Belt, and the Stavely Belt. The Stavely Belt is poorly exposed and internally faulted and folded, giving rise to an ambiguous overall facing direction. Inference of an overall westerly facing for the Stavely Belt is based on direct comparison with the simpler Dryden Belt and is consistent with the overall westerly facing observed in the panels of Glenthompson Sandstone that enclose the Stavely Belt.

 thick panels of generally northeasterly-facing and dipping Glenthompson Sandstone that occur south and north of the Narrapumelap Belt and as fault slices within the Narrapumelap Belt interior, discontinuously exposed across a region that spans between the Escondida and Golton faults southeast of the Grampians Ranges (see Section 3.4.4 – Mafeking Megakink). This geometry contrasts strongly with the overall westerly dips and westerly stratigraphic facings of the Dryden Belt and related sediments just to the east and the sediments adjacent to the Stavely Belt just to the west, and was a key dataset that constrained the new interpretation of the Narrapumelap and Elliot belts as lying within the central limb of the large, steeply plunging, fault-disrupted Mafeking Megakink. The megakink interpretation allowed, in turn, for direct correlation between the Stavely, Narrapumelap and Dryden belts, and between the Bunnugal and Elliot belts, and has allowed a linkage to be established between late deformations seen in the Cambrian bedrock and in Grampians Group rocks that directly overlie it.

Figure 2.18 Core straddling the Marathon Fault splay in drill hole STAVELY02. A few metres true thickness of both Grampians Group conglomerate (hangingwall) and Mount Stavely Volcanic Complex (footwall) are sheared and altered adjacent to the fault plane – the Cambrian volcanics in particular are strongly oxidised and brecciated adjacent to the fault. The drill core is oriented; Grampians Group bedding is upright and moderately east-dipping. Slickenside lineations and steps on a range of fault surfaces that cut both Grampians Group and Mount Stavely Volcanic Complex and are subsidiary to the main Marathon Fault plane show dip-slip extensional displacements.

 a progressive decrease in dip-magnitude with depth is established for Narrapumelap Belt orientation by direct measurements of strata at surface, compared to dip-modelling of magnetic data (1-2 km depth) and triangulation from surface to the offset position of the Narrapumelap Belt as imaged in deep seismic reflection line 09GA-AR1 (~5 km depth). The systematic dip-magnitude decrease with depth defines a distinctly listric profile to the Narrapumelap Belt to at least approximately 5 km depth. Conservation of volume arguments require this listric profile to be extended into adjacent belts of Cambrian strata. Using this methodology, most of the west-dipping Cambrian structures mapped within the Cambrian bedrock across the width of the Grampians-Stavely Zone can be demonstrated to have pronounced listric profiles at crustal scale, all shallowing in dip magnitude to depth. This is consistent with the constraints provided by available deep seismic reflection data, and formed a key premise that controls the construction of the serial full crustal cross sections used to construct the STAVELY 3D model volumes.

 abrupt, drag-folded terminations of D1a fault-belts of Stavely Arc stratigraphy both mapped and visible in potential field data. Examples include the radial array of fault-belt segments in the Black Range region (the ‘Crab Nebula’; see Section 3.4 – Modelled Folds and Section 4.1.3 – ‘Crab Nebula’ retrodeformation), the warped northern termination of the Stavely Belt buried beneath Grampians Group west of Mafeking (see Section 3.4.4 – Mafeking Megakink) and the low-dipping belt segments that span obliquely between the Stavely and Dryden belts (the Elliot and Narrapumelap belts now interpreted as the middle limb of the dextral Mafeking Megakink – see Section 3.4.4; Mafeking Megakink), all attributable to D4 faulting and drag-folding and;

 south- and east-convex oroclinally folded warps of Grampians Group stratigraphy (Figure 2.16). These are congruent with late structures preserved in the Cambrian bedrock, which directly underlie Grampians Group and imply late (D4) large-scale clockwise rotations about subvertical axes (Cayley & Taylor, 1997a). Thus, timing is constrained to D4 with the formation of subvertically-plunging dextral megakinks (e.g. Mafeking Megakink) and large structural basins (e.g. Jalur Rift) developed in the Cambrian bedrock, and;

 panels of low-dipping but downward facing Glenthompson Sandstone (and volcanics of the Stavely Arc). These are mapped in the footwall of the Moyston Fault, arranged in en-échelon fashion from Mount Drummond to Moyston (Figure 2.20). These panels are bounded by northeast-trending faults that cut and offset the Dryden Belt at high strike-angle (see Section 3.2.5.8 – Dryden Belt) and thus have post-D1 timing and appear to be a consequence of block-overturning at kilometre-scales.

These examples and others are discussed in detail in this report. The structural scenarios developed to explain them form a key part of the rationale used to construct the STAVELY 3D model.

Dip-modelling of belts of magnetic strata provides upper-crustal constraints on the dip direction and dip magnitude of the faults that bound the belts (see Section 2.6.4 – Interpretation of geophysical data and Section 3.2.5 Stavely Arc). Additionally, the dip direction and magnitude of some faults is directly imaged in several deep seismic reflection profiles that transect the whole region. The cross-strike offset of some faults from their positions imaged in deep seismic reflection data, to their positions mapped at surface (e.g. the Narrapumelap North and Narrapumelap South faults) provides the opportunity to triangulate averaged dips for these structures at crustal scale. These dips help constrain the geometry of adjacent geology. In such ways, deep seismic data and velocity changes apparent in shear wave velocity volumes calculated over the region (e.g. Young et al., 2013), provide information on how some fault surfaces change in dip magnitude with depth, and how some faults interact at depth.

Figure 2.19 Dryden Belt facing directions determined from volcanic sediment A) Graded subaqueous volcanolithic sandstone exposed on the eastern flank of McMurtrie Hill. The grading shows the unit is west-facing, so that the steeply east-dipping stratigraphy here is overturned. B) Thin section (cross polarised light, field of view 25 x 17 mm) of graded subaqueous volcanolithic sandstone from MGA 54 644571 5891258, showing upward-fining beds comprising juvenile, angular volcanic plagioclase, augite and subordinate quartz. The top bed is quartz dominated, in contrast to the feldspar-dominated lower beds.

2.4.4 Rock properties

A key input in the development of 3D models is knowledge of the petrophysical attributes (rock property values) of the subsurface geology. Accurate density measurements (g/cm3) and magnetic susceptibility (SI) data are essential to inform and constrain the interpretation and modelling of gravity and magnetic potential field data, crucial to the development of a robust framework from which a 3D geological model can be constructed.

Rock property values in STAVELY were obtained from GSV’s existing central database including in-situ and laboratory measurements (Skladzien, 2007; Grant, 2002), and by direct measurements of drill core using the Australian Geophysical Observing System (AGOS) GeoTEK Multi-Sensor Core Logger (MSCL) at The University of Melbourne (Skladzien, et al. 2016a; Skladzien, in prep).

Detailed petrophysical data was obtained for Early Palaeozoic rocks in STAVELY. Limited petrophysical data is available for Murray Basin sediments due to the unconsolidated nature of the lithology, and sonic drill core being too large for to scan with the MSCL equipment.

Statistical analysis of rock property values constrained geophysical modelling (Appendix 3 – Forward model sections), including estimates of depth to basement (i.e. thickness of cover) in areas lacking drill hole control.

2.4.5 Regional 2D seismic lines The STAVELY 3D model is constrained in cross section by a single deep seismic reflection transect southeast of the Grampians Ranges (09GA-AR1), and by combined seismic transects that span the full width of the Grampians-Stavely Zone just to the north of the Grampians Ranges (97AGS-V1, 97AGS-V2; 09GA-SD1). These seismic transects represent key tie-points between the horizontal and vertical interpretations (Figure 2.21, Figure 2.22 and Figure 2.23).

Complimentary datasets that aided the seismic and geological interpretation are:

 Reprocessed Murray seismic – MEMV96-09 (Figure 2.24)

 Otway margin transect – BMR92-OT2 and OT3

 Regional 3D shear wave velocity models (Young et al., 2013) (Figure 2.4)

Figure 2.20 Uncleaved, low grade (sub-greenschist) but completely overturned Glenthompson Sandstone in the Moyston Fault footwall just west of Moyston on the Moyston-Willaura Road. MGA 54 655893 5870248. The downward facing is clearly shown by prominent grading and cross-lamination truncations in the turbiditic sandstone beds, for example in the bed near the hammer handle which is coarsest at the top. Similarly, overturned Glenthompson Sandstone beds occur elsewhere along the Moyston Fault footwall, such as at Mount Drummond (see Figure 3.53A)

Figure 2.21 2D deep seismic reflection line locations used in geological interpretation and development of the STAVELY 3D model.

Figure 2.22 Deep seismic reflection transects 09GA-SD1 and 09GA-AR1, across the Grampians-Stavely and Glenelg zones, imaging the Stavely Arc edifice and related D1a fault slices (in pale green and pale blue). Major structures are labelled. Interpretation adapted from Cayley et al. (2011b). AF = Apsley Fault. Refer to source reference for other fault abbreviations.

Figure 2.23 Seismic lines 97AGS-V1 and 97AGS-V2 (Korsch et al., 2002) were re-processed and re-interpreted as part of the STAVELY 3D model (see McLean et al., in prep). Particular changes from the reinterpretation relate to repositioning the Escondida Fault to the base of highly reflective crust, which is included as part of the Dimboola Belt, and recognising that numerous subvertical strike-slip faults (part of the D4 Dimboola Duplex) that cannot be imaged by the seismic data (because of their steep dip) have nevertheless resulted in large apparent depth-offsets of other structures, including the Escondida Fault, changing the positions of depth-projections. KG = Kanmantoo Group. HFS = Henty Fault System.

2.4.6 Existing 3D models

The 3D Geological Modelling Project, undertaken as part the Rediscover Victoria Initiative, resulted in the development of a 3D geological model for Victoria at 1:250 000 that integrated modern geological surface mapping and geophysical (magnetic, radiometric, gravity and seismic) datasets (see Rawling et al. 2011) (Figure 2.25).

The STAVELY 3D model has been constructed in much greater detail than the 3D Victoria model. While consideration has been given to the relationships of rock units and faults constructed with adjacent structural zones, the STAVELY 3D model is yet to be integrated into the current 3D Victoria model. The intention is to complete this in the future to capture and present updated geological knowledge and interpretation(s).

Other available 3D resources that informed the interpretation and construction of parts of the STAVELY 3D model include:

 Three-dimensional perspective block-diagrams conceptualising the internal geology of parts of the Grampians Ranges, and of the adjacent Moornambool Metamorphic Complex, in Cayley & Taylor (1997a), Cayley & Taylor (2001) and Morand et al. (2003).

 The Murray Basin 3D model, constrained by sparse 2D seismic, abundant drilling through to basement, surface mapping, and a depth-to-magnetic-basement analysis (McLean, 2010).

 The detailed Otway Basin 3D model, constrained by a detailed 2D and 3D seismic reflection transect network and by abundant drilling (GSV, unpublished).

Relevant parts of existing basin models have been adapted and incorporated into the STAVELY 3D model as surfaces (not as volumes, as the basins themselves are generally too thin to be meaningfully represented at the 500 m3 cell-size used to generate the STAVELY model voxet).

Figure 2.24 Location (A), (B) and geological re-interpretation (as part of this project) (C) of re-processed seismic reflection line MEMV96-09 that straddles part of the northern Grampians-Stavely Zone interior in northwest Victoria. Regional magnetic tilt and band pass filter pseudocolour location maps show relationship of seismic line to underlying magnetic rocks of the Dimboola Belt, part of the Stavely Arc. The deep seismic reflection line images well-bedded flat-lying Paleogene-Neogene Murray Basin sediments and Permian Urana Formation fluvioglacial sediments, and weakly-reflective flat-lying Silurian Grampians Group siliciclastics. These cover rocks unconformably overlie highly and relatively uniformly-reflective Cambrian bedrock (Stavely Arc) containing subtle east- and west-dipping fault-breaks that cut low-angle reflectors within the bedrock, and variously cut, or are overlain by, Grampians Group and Urana Formation strata. Interpretation control is provided by stratigraphy encountered in nearby petroleum exploration well Gunamalary 2, whose location and projected location are depicted on (B) and (C) respectively. Data from the Geological Survey of Victoria and Geoscience Australia. All grid coordinates MGA Zone 54.

Figure 2.25 Perspective view of the 3D geological model of Victoria, sliced vertically east to west, with seamless geology shown at surface. (Rawling et al., 2011).

2.5 Modelling workflow

Eleven serial cross sections at full crustal thickness (~40 km) were constructed covering the width of STAVELY, with some sections extending slightly beyond the project area (Figure 2.26).

Two of the serial cross sections were designed to coincide with the 1997 and 2009 deep seismic (+/- magnetotellurics) transect lines V1, V2, SD1 and AR1. Crustal-scale cross sections drawn along the deep seismic reflection profile locations were reconciled against the geology that has been mapped in outcrop along and adjacent to the transects. The two detailed sections were further constrained by forward-modelling and dip-analysis, using rock properties (petrophysical measurements) and structural measurements projected to these locations. These two tightly-constrained cross sections formed the reference sections for nine additional serial cross sections, the volume-balanced geometrical constraints projected progressively north and south along-strike away from the locations of greatest control. Crustal-scale cross sections were spaced between 20 km to 35 km apart.

The additional nine crustal-scale cross sections were orientated east-west to northeast-southwest, orthogonal to the major structural trends of the Cambrian volcanic belts, and were located in areas where the geophysical anomalies appeared relatively “clean” (i.e. minimal influence from magnetic cover rocks and post-Cambrian magmatic features). This ensured that the additional cross sections could also be constrained by forward-modelling and dip analysis. Areas of geological interest and complexity apparent in potential field datasets were also a factor in determining the positions of the serial cross sections, so that the geological understanding could be further tested in the third dimension.

Interpretation templates were developed and printed at 1:250 000 scale for each serial cross section. The templates included strip maps of surface geology, filtered magnetics and gravity and profiles of TMI (RTP). Initial cross section interpretations were developed manually, with element-orientations informed by directly-measured structural data were available (e.g. dip and strike data measured in outcrop and/or drill core), nearby structural data projected along-strike (generally using magnetic data to constrain projection-directions) and/or from dip-modelling of potential field data and/or projections to surface from seismic reflection data. The manual hand-drawn cross sections were scanned, georeferenced and digitised to provide the initial interpretation that was refined by forward modelling of profile gravity data along the sections.

An additional seventeen shorter cross sections constructed to mid-crustal depths (~8-10 km) were adapted by reinterpreting and extending previously-published or draft cross sections produced as part of the Grampians, Ararat, Glenelg and Willaura mapping projects (Cayley & Taylor, 1997a,b,c, Taylor & Cayley, 1997; Cayley & Taylor, 2001, Morand et al., 2003 and Cayley et al. in prep). These sections infill regions between the larger serial cross sections in areas of good geological outcrop between Chatsworth and Stavely, along the eastern margin of the arc-terrane, and in and beneath the Grampians Ranges (Figure 2.27).

Figure 2.26 Serial full crustal-thickness cross section locations on A) TMI (RTP) and B) Complete Bouguer Anomaly images, including the location of volcanic belts of the Stavely Arc as unshaded regions.

Figure 2.27 Map of the Grampians Ranges showing infill cross sections used to construct the STAVELY 3D model. A) sections shown on seamless surface geology, B) sections shown on 30 km high pass filtered gravity draped onto RTP magnetics layer revealing bedrock structures beneath cover units. A selection of the infill cross sections are included in Figure 3.5

2.5.1 Modelling workflow summary

The workflow adopted for the STAVELY 3D model can be summarised as follows:

1. Compile previous geological mapping, geophysical (magnetic, gravity and seismic) data and interpretation

2. Field mapping including field checking, infill and detailed analysis in key areas of known and potential constraint 3. Linework interpretation of gridded potential field data including update of previous interpretation where appropriate

4. Correlate, check and update interpretation against available drill hole logs and data

5. Targeted modelling / inversion and local filtering/enhancement of potential field data to resolve localised interpretation issues in areas of geological complexity

6. Magnetic data inversion (of volcanic belts) throughout project area – dip / geometry modelling

7. Design serial sections

8. Produce preliminary geological serial section interpretation

9. Gravity forward modelling of serial sections based on preliminary geological interpretation and median density values for units derived from rock property (petrophysical measurement) data

10. Refine geological serial section interpretation based on forward modelling results (as required)

11. Refine structural linework interpretation based on final serial sections (as required)

12. Import surface linework and hang sections in 3D modelling software as constraints for 3D model development

13. Refine linework/forward models in conjunction with 3D model development as required, while adhering to “hard” constraints (e.g. mapping, drilling)

2.6 Interpretation methodology

The new near-surface plan-view fault network mapped and interpreted for STAVELY is constrained by regional magnetic and gravity data , drilling (mineral, water and petroleum) data, and by modern geological mapping in the southeast and central portions (Figure 2.28).

The fault network is constrained in cross section by a single deep seismic reflection transect southeast of the Grampians Ranges, and by combined deep seismic transects that span the full width of the Grampians-Stavely Zone just to the north of the Grampians Ranges (Figure 2.22 and Figure 2.23). These deep seismic reflection transects represent key tie-points between the horizontal and vertical interpretations.

The plan-view pattern of geology revealed by regional magnetic and gravity data and lithology that is intermittently exposed and now further constrained by STAVELY stratigraphic drilling indicates that, local areas of complexity notwithstanding, the overall geological and structural system does not change much in character from south to north. For example, the Dryden Belt can be traced in regional magnetic data north from its outcropping position east of the Grampians Ranges as a single coherent entity into the far-north of the zone, indicating that the structural system that controlled its formation is also strike-persistent (Figure 2.29). Igneous lithologies intersected under cover in drill hole STAVELY16, within the Hindmarsh Belt in the far-north of STAVELY, are indistinguishable from volcanic rocks of the Cambrian bedrock exposed in the Dryden Belt east of the Grampians Ranges (e.g. at Jallukar; Buckland & Ramsay, 1982): and in the Stavely Belt south of the Grampians Ranges (e.g. at Mount Stavely and further south under cover in drill hole STAVELY02).

A key implication of a relatively strike-persistent geological and structural style in plan-view is that the geological and structural style is also highly likely to be strike-persistent in cross section. Thus, rocks of the Dryden Belt and their controlling structural system are likely to be similar, and have similar origin, in all successive serial cross sections that intersect it. Only where the Dryden Belt appears truncated or absent (e.g. in the region south of the Mafeking Megakink), or duplicated (e.g. north of the Tullyvea Fault; see Figure 2.29), or reoriented (e.g. overturned adjacent to the Moyston Fault at Bellellen) is the interpretation of additional structural complexity warranted.

In many cases the nature of the additional complexity (e.g. offset across a subsequent fault) can be determined with some confidence because overprinting relationships with other units, including buried Grampians Group, are apparent in outcrop and/or geophysical data.

The methodology and rationale presented here allows the geological units and geometries mapped in areas of outcrop, and imaged in cross section at deep seismic reflection transect locations, to be extended beyond the areas of greatest control using other geophysical data and drill hole control to inform construction of additional serial cross sections. This is particularly important in the north of STAVELY where the Cambrian bedrock is buried beneath the Murray Basin.

The starting principle for the construction of each cross section is that the geology of the previous cross section must carry across, modified only to remain consistent with progressive changes of geology that are apparent in plan view, as constrained by potential field data, mapping and drill holes (when present), and modified only in a way that maintains a volume-balance.

Figure 2.28 Plan-view of STAVELY, showing the distribution of volcanic belts of the Stavely Arc and the interpreted network of Late-Cambrian (D1a) to Early Devonian (D4) faults that are included in the STAVELY 3D model. Cretaceous faults of more limited displacement that are related to Otway Basin formation in the south are also included. A) tilt and band pass filtered magnetics image and B) 30 km high pass filtered gravity image.

2.6.1 Fault timing

Relative fault timings are constrained by overprinting criteria directly observed in areas of outcrop, and inferred from juxtapositions of rock-packages of contrasting geophysical character. The various overprinting criteria used include:

 where one fault or fold is observed to cut, truncate, or otherwise deform another to reveal a relative age;

 where a younger body of undeformed rock rests unconformably on an older body of deformed rock, thereby placing an upper limit on structure age;

 where alteration (e.g. contact metamorphism or other intrusion related hydrothermal alteration) is superimposed onto older structures in a rock, thereby placing an upper limit on structure age.

 Fossil control and/or geochronology that locally provides defined age constraints on other overprinting criteria.

A key stratigraphic package that clearly discriminates relative fault timings is the ?Late Ordovician- Silurian-aged Grampians Group, which variously unconformably overlays early (mostly Cambrian) faults and folds, and is affected by later (mostly Early Devonian) faults and folds (Figure 2.16). These various-aged structures can be interpreted in gravity, magnetic and seismic data away from areas of outcrop because the Grampians Group is characterised by relatively lower overall density and a low overall magnetic susceptibility and distinctive reflectivity compared to the Cambrian bedrock. The positions of some underlying Cambrian structures, stratigraphic units and sequences can be determined from attenuated features visible in regional magnetic and gravity data even in areas of very thick Grampians Group cover.

2.6.2 Fault shape and orientation

The shapes and orientations of fault surfaces (fault strike, fault dip direction and magnitude, fault profile shapes, etc) within the STAVELY 3D model – are constrained by:

14. surface geological mapping, where the geometry (dip and strike) of some fault surfaces have been directly observed and measured;

15. dip-direction and dip-magnitude modelling of the (faulted) boundaries between rock packages of contrasting geophysical (often magnetic) character;

16. by direct observation of fault planes where they are imaged by deep seismic reflection transects or implied by velocity differences within shear velocity volume models, including in the lower crust;

17. by inference, from determinations of the orientations of structural enveloping surfaces within fault-bounded rock packages, usually by direct geological outcrop mapping, and finally

18. by volume-balancing requirements – the assumption that the model volume cannot contain voids or overlaps so that the geometry determined (observed – e.g. in deep seismic reflection data) for one fault or tectonostratigraphic element might serve to constrain the range of possibilities for modelling of an adjacent fault or element.

Another key rationale for 3D model construction is adherence to the fundamental principle that – with few exceptions – faults propagate upwards from depth as they form, whether in response to crustal extension (e.g. as ‘normal’ faults), shortening (as ‘thrust’ faults), strike-slip, or any combination. This rationale is underpinned by the notion that crustal deformation is driven by plate tectonic processes that originate in the mantle. This rationale means that faults mapped at the surface are assumed to either continue – and increase in displacement – to depth and to beyond the limits of the model, to merge at some depth within the model volume with related faults, or to be truncated and offset at depth by unrelated structures formed during subsequent episodes of deformation. In the latter case, blind fault continuations of similar character are anticipated to continue beneath the unrelated younger structures in offset positions, and are modelled as such wherever they are considered to exist within the model volume (see Section 2.6.3 – Blind faults). General observations:

19. most faults determined to have moderate to steep dips at surface tend to reduce in dip-magnitude with depth. In other words, faults are generally listric in shape, as observed in deep seismic reflection data across the Grampians-Stavely Zone, and measured for structures such as the Narrapumelap North and Narrapumelap South faults, and;

20. families of similar age and similar-style faults observed at surface and in the upper-crust tend to converge and merge into fewer, lower-angle master faults at depth. It is these master faults that can be discriminated in deep seismic reflection data and regional scale velocity volumes. The master faults appear to project out laterally towards the limits of the STAVELY 3D model, typically linking into the bounding Moyston and Yarramyljup fault surfaces in the mid- and lower crust. As well as matching direct observations of fault movement histories, geometries and dips established from field-mapping, drilling, potential field and deep seismic reflection data, the modelled fault geometries are consistent with modern theories of fault formation and propagation, and the idea that the Grampians-Stavely Zone is just one component of larger geological system.

21. The STAVELY 3D model is too coarse and regional in scale to represent all faults (and stratigraphic units) mapped or interpreted within STAVELY. For this reason, faults confined within the many different stratigraphic units have been omitted from the model (e.g. faults internal to the volcanic belts of the Stavely Arc; faults within the Glenthompson Sandstone; faults confined to the Grampians Group; Figure 2.30), in order to better represent the larger faults responsible for imbrication, juxtaposition and/or segmentation of the main contrasting packages of Cambrian strata. A part of the Glenisla Belt has been modelled in more detail to include some internal faults evident in high resolution magnetic data (Figure 3.39 and Figure 3.40), as an example of the structural complexity that is found within most of the volcanic belts of the Stavely Arc (e.g. Figure 2.30). A higher resolution model to represent faults and stratigraphy in the upper crust of WILLAURA in the southeast of STAVELY is in preparation (Cayley et al. in prep.).

Figure 2.29 RTP magnetic image of the Dryden and Dryden North belts (unshaded) in STAVELY, showing major towns, key locations where Dryden Belt strata crops out, and key stratigraphic drill holes into the Dryden North Belt. The distinction between the Dryden and Dryden North belts is taken to be the fault-disruption of the belt just south of drill hole VIMP9.

Figure 2.30 Example of the structural complexity evident within most of the volcanic belts of the Stavely Arc. The image shows the central Dimboola Belt, with major interpreted D4 faults that form part of the Dimboola Duplex, and other faults that appear to compartmentalise Dimboola Belt stratigraphy and may be of D1 age, on tilt and band pass filtered magnetic data. While the major Dimboola Duplex faults were modelled, it was beyond the scope of the regional STAVELY 3D model to capture the the detailed D1a fault complexity within most of the volcanic belts; typically only belt bounding D1a faults have been modelled.

2.6.3 Blind faults

Where deep seismic reflection data in the Grampians-Stavely Zone has imaged large blind faults confined to the mid-lower crust, these faults have been included in the STAVELY 3D model volume. In most instances, such faults are only modelled in close proximity to the areas of control along deep seismic reflection line locations. The deep seismic reflection data is 2D, lacking control on the strike-direction of imaged faults. In most instances little other data exists to constrain the strike or lateral extent of these blind faults.

An exception is the Apsley Fault, imaged in 09GA seismic reflection data as being confined to the mid-lower crust of the Glenelg and Grampians-Stavely zones (see Figure 2.22). The Apsley Fault is a large and obvious structure that separates sequences of strikingly contrasting seismic character. The Apsley Fault is therefore very likely to have a large and persistent strike-extent. The position of the Apsley Fault imaged in orthogonal seismic reflection lines 09GA-SD1 and 09GA-SD2 coincides with a dramatic change in shear wave velocity (Young et al., 2013; see Figure 2.4). The shear wave velocity volume affords a degree of local 3D control on the fault-plane strike direction. An attenuated regional change in gravity data also coincides with the fault-plane strike established from seismic reflection and shear wave velocity data. This combination has allowed the Apsley Fault position to be extrapolated away from the deep seismic reflection line locations, despite the fault not being expressed at surface. The accuracy of interpreted position and geometry of the Apsley Fault decreases with increasing distance from the areas of greatest control.

Other major blind faults interpreted within the STAVELY 3D model include:

22. D1a fault segments buried beneath Grampians Group cover in the interior of the Jalur Rift (see Figure 3.9). These faults bound the flanks of the Grampians Deeps Belt (Figure 2.31). The flank locations and dip-orientations of the Grampians Deeps Belt were modelled from gravity and magnetic data. Their interpretation as faults is based on direct comparison (and interpreted equivalence) with the D1a faulted flanks of other volcanic belts of the Stavely Arc mapped nearby (e.g. in the Bunnugal Belt, in the Stavely Belt and in the Narrapumelap Belt):

23. D1a transform-type faults required to separate regions of opposing but coeval dip and tectonic transport direction, for example the blind Stavely Transform interpreted to separate west-dipping (e.g. Stavely, Bunnugal, Boonawah) and east-dipping (e.g. Escondida) thrust systems that apparently developed in the south and north of STAVELY concurrently during D1a.

24. D4 faults – most notably the south-dipping Jalur Fault (Figure 2.32) – required beneath thick Grampians Group in the core of the Jalur Rift to explain the juxtaposition of clockwise-rotated crustal elements within the Mafeking Megakink and Jalur Rift core against translated but not rotated crustal elements farther north (the Dimboola Duplex).

Errors in interpretation of fault dip-direction or incorrect assumptions of overprinting relationships between faults at depth were revealed during the model-build process as impossible (overlapping) model volumes at depth. In most instances where this occurred only a limited range of alternate geometrical solutions could adequately resolve the issue, boosting confidence in the validity of the final model result. In most cases, errors could be solved by changing fault hierarchies. In a few places, misfits within the model could only be resolved by inferring a structure completely blind-to-surface – the Jalur Fault is one such example. The rational and details of these instances are discussed case-by-case (see Section 3.3 – Modelled Faults).

The progressive and iterative problem-solving component to model-build represents, in its own right, an important validation step in the explicit modelling process, and in the robustness of the final STAVELY 3D model volume at the scale of modelling.

2.6.4 Interpretation of geophysical data

An interpretation of the sub-surface geology can be derived, in part, by the enhancement and analysis of potential field data via the application of both qualitative and quantitative methods.

In STAVELY the regional-scale gridded and enhanced magnetic and gravity datasets have been used to derive a qualitative interpretation involving spatial pattern recognition. Identification and interpretation of various structural features including faults, folds, lineaments (e.g. strand lines) has been undertaken. Characteristic magnetic and gravity responses, which in places can be correlated with direct observations (e.g. outcrop, drilling), have enabled the mapping of regional stratigraphic units such as Cambrian-aged Glenthompson Sandstone (and Nargoon Group), the Cambrian-aged Stavely Arc volcanics (Mount Stavely Volcanic Complex), Cambrian and Devonian intrusions, younger sedimentary cover (e.g. ?Late Ordovician- to Silurian-aged Grampians Group) and both young and old volcanic flows (e.g. Newer Volcanic Group and Rocklands Volcanic Group). The correlation of known surface exposure with geophysical responses provides a reference and constraint for extrapolation of these rocks under cover with a high degree of confidence.

Potential field data also has the capacity to image through some of the Early Palaeozoic cover successions. The distribution of some Cambrian bedrock has been mapped beneath Grampians Group and Rocklands Volcanic Group cover, particularly in areas where these cover units appear to be relatively thin. Examples of this style of mapping include delineating the subsurface extent of the northern extensions and terminations of the Stavely and Bunnugal belts beneath the Serra and Mt William ranges, and identification of the ‘Grampians Deeps Belt’ which is entirely concealed beneath the Grampians Group rocks of the Victoria Range and the Victoria Valley.

A qualitative interpretation of potential field data can provide an indication of relative ages (overprinting criteria), relative depths to sources (wavelengths and frequency information) and geometries (e.g. symmetry vs asymmetry of anomalies).

This qualitative regional interpretation has been complimented and in places refined by quantitative interpretations of the potential field data utilising forward modelling and inversion methods targeting specific geophysical anomalies or localised areas. The interpretation has been constrained where possible by surface mapping, drill holes and rock property (petrophysical measurement) data. This type of analysis can provide important depth and geometry information – particularly in the upper few kilometres of the crust.

Regional gravity and magnetic data both inform the thickness of some cover rocks (e.g. Grampians Group, Murray Basin – see McLean (2010), and on the geometry of other rock units (e.g. the overall dip of denser and more-magnetic Cambrian igneous rocks contained within the volcanic belts, versus adjacent less-dense and less magnetic Cambrian sedimentary rocks).

Figure 2.31 Fault boundaries of the Grampians Deeps Belt (unshaded) buried beneath Grampians Group cover and the Victoria Valley Batholith, and of the Jalur Rift (heavy fault lines; see also Figure 3.9), within which thick deformed Grampians Group strata is preserved. Background image is band pass filtered Complete Bouguer Anomaly.

Figure 2.32 Oblique view of selected volumes and fault-surfaces from the STAVELY 3D model, illustrating the inferred regional-scale relationships between the east-dipping Dimboola Belt (red volume) and the en-échelon steeply-dipping dextral strike-slip faults of the D4 Dimboola Duplex that segment it (yellow, blue, green faults; see also Figure 3.15). The interpreted south-dipping transtensional Jalur Fault truncates the southern margin of the Dimboola Belt and juxtaposes it against the northeast-dipping Elliot (green) and Narrapumelap (purple) belts that occupy the clockwise-rotated middle limb of the D4 Mafeking Megakink to south. The surface outlines of the Stavely, Narrapumelap and Dryden/Dryden North belts are shown for reference.

2.6.4.1 Interpretation of fault network

A re-interpretation of the existing GSV pre-Permian structural and stratigraphic interpretations (including updated interpretations derived for the 3D Victoria project; Rawling et al., 2011) have been undertaken in light of new knowledge gained as part of field work, data collection and development of the STAVELY 3D model (see Section 2.4 – Inputs and constraints).

The identification of regional, late strike-slip structures in outcrop and in geophysical data has allowed for the discrimination of Cambrian D1a & D1b structures from later D2 – D5 structures, guided by structural principles derived from the use of overprinting criteria (see Figure 2.1) and the systematic application of finite strain ellipse theory (Figure 2.33). Fault traces have been interpreted away from regions of greatest control (field mapping, drilling and deep seismic reflection transects) by utilising gridded potential field data. The nature of typical interactions of sets of contractional, extensional and transtensional fault networks established experimentally and documented in other regions worldwide (e.g. Boyer & Elliott, 1982; Sylvestor, 1988) has been used to inform the interpretation of possible fault-interactions at depth within the STAVELY 3D model.

Figure 2.33 Strain Ellipse theory. A) Strain ellipse theory (and the stress field required to induce it) is typically represented schematically by illustrating the magnitude of material shape change from a spherical (x=y=z) start-point to an ellipsoidal (x>y>z) end-point in three-dimensions, or a circle (e.g. x=z) start-point to an elliptical (e.g. x>z) end-point in any two of three dimensions. An end-point of identical shape can be induced in response to either ‘pure’ shear with no bulk rotation or ‘simple’ shear with bulk rotation. In ‘simple’ shear, the directions of maximum flattening (z) and maximum stretching (x) are inclined at an angle to the plane of shearing – in this example ‘dextral’ strike-slip shear. The symmetry of the strain ellipse is mirror-imaged for sinistral shear. Faults (B) develop as a brittle response to stress. Normal (extensional) faults develop orthogonal to the long-axis of a plan-view strain ellipse (e.g. ‘x’ in A). Thrust (contractional) faults develop orthogonal to the short-axis of a plan-view strain ellipse (e.g. ‘z’ in A). Strike-slip faults develop at oblique angles to these axes. All can form simultaneously within a single strain ellipse (C), as can related ductile structures (e.g. folds). The instantaneous stress and strain conditions dictate which types of structure are predominant at any given time. Wrench-dominated ‘simple shear’ stress systems are associated with a rotational component to strain (CA-B-C). Rotational strain is typically domanial – i.e. confined within or adjacent to specific structures such as fold and kink limbs or rift interiors. Ongoing rotation associated with ongoing shear-induced flattening or extension can serve to progressively rotate early-formed structures out of favourable orientations, so that early dip-slip structures evolve into oblique-slip and eventually strike-slip structures and vice-versa (CA-B-C) as strain accumulates. This leads to locally complex strain histories accumulated in structures mapped in otherwise simple strike-slip dominated systems – this is what we interpret in STAVELY, for D4 in particular. Strain ellipse theory helps place different types of coeval faults into context with one another, and provides a logic flow to constrain retrodeformation (see Section 4.1 – D4 and D3 retrodeformation testing).

2.6.4.2 Interpretation of intrusive bodies

The areal extent of intrusive bodies has been interpreted in plan using gridded potential field data, and verified by field mapping and drilling where available. Intrusive bodies have been predominantly interpreted from regional magnetic data. The higher spatial density of airborne magnetic data compared to ground gravity measurements enables a greater resolution in defining a contact between the intrusive body and host rocks, providing there is a discernible contrast in magnetic properties of the two rock types.

The majority of intrusive bodies in STAVELY display a strong magnetic character, in contrast to the non- to weakly-magnetic sedimentary and metasedimentary host rocks of the Grampians, Kanmantoo and Nargoon groups which they commonly intrude (Figure 2.34). This becomes more problematic where the host and intrusive rocks display a similar magnetic response, for example magnetic intrusions intruding the volcanic belts, e.g. Dryden, Dimboola, Tyar, Bunnugal, Black Range West/Mitre. Differences in the general character and shapes of anomalies, and overprinting criteria (the majority of intrusives are of post-D1a age and so cut across D1a structures) can be useful in distinguishing between the different units in these cases. Where magnetic intrusive bodies have intruded the (magnetic) volcanic belts, contrasts in magnetic character, together with a gravity response if discernible, have been used to map the intrusive.

Numerous intrusive bodies in STAVELY display internally varying magnetic intensity and/or character, signifying various intrusive phases – these have not been discriminated in the STAVELY 3D model. Wavelength (band pass and low pass) and continuation filtering of magnetic data has proven useful in identifying intrusive bodies beneath the high frequency magnetic response of the Newer and Rocklands Volcanic Group (e.g. Figure 2.8 and Figure 2.9).

Less common non-magnetic intrusives (including some porphyries within the volcanic belts – e.g. Lalkaldarno and ‘Victor’) can be inferred from magnetic data by overprinting criteria where magnetic units have been “burnt out” by the intrusive body (Figure 2.34), or by the presence of a magnetic aureole around the periphery of the intrusive body (e.g. Stawell Granite). If the intrusive body is of sufficient size, there is a density contrast with the host rock and gravity data is detailed enough, the individual intrusive body (or intrusive complex) is generally expressed as a region of low relative response in gridded gravity data – for example in the Black Range West/Mitre Belt. High pass filtered gravity data has proven a particularly useful in the interpretation and delineation of intrusive bodies with these characteristics in STAVELY.

Qualitative analysis of gravity (and to a lesser degree magnetic) data has been undertaken in STAVELY to estimate relative intrusive thicknesses and subsurface shapes (e.g. outward/inward dipping boundaries), which has informed the modelling of intrusive bodies in section and in 3D. The complex and variable nature of intrusive bodies – host rock spatial positioning/interaction, large variability in geophysical response between different intrusions, and different phases of individual intrusions, has proven problematic in the modelling and inversion of intrusive bodies in STAVELY.

2.6.5 Potential field data forward modelling and inversion

Forward modelling and inversion of magnetic and gravity anomalies was carried out along numerous profiles within STAVELY to indirectly derive information about the distribution, geometries and depths of geological units producing the response observed in these potential field data. Typically, the various deformed parts of the Stavely Arc exhibit a marked increase in both gravity and magnetic response, relative to the surrounding sedimentary and meta-sedimentary rocks. The difference in responses reflects the contrasting petrophysical properties of these rock types, with the volcanics usually comprising of rocks with relatively higher density and magnetic susceptibility. Intrusive rocks exhibit variable petrophysical properties with respect to the surrounding host rocks, but commonly display a medium to high magnetic response, and low gravity response.

The most useful potential field dataset for deriving maximum geological information is dependent on the local geology (e.g. gravity may be more useful in areas where Newer Volcanic Group cover masks the magnetic response of deeper units). The spatial distribution of the data is also an important factor in determining the resolution in a particular dataset. The spatial distribution of regional airborne magnetic data is tighter than that of the regional ground gravity measurements, thus often enabling a more detailed interpretation of the upper crust.

Localised targeted magnetic data inversions were undertaken to determine the geometries (dips) and provide indicative relative depths and thicknesses of segmented volcanic belts along lengths of strike where magnetic data was considered “clean” enough to give reliable results (i.e. the magnetic contribution from neighbouring anomalies was negligible). Near surface (< 6 km) magnetic sources were prioritised for this approach. All bodies derived from the modelling have been included as objects within the 3D model (see Appendix 1 – STAVELY 3D model export). These results provided a framework for informing the regional geological interpretation of the volcanic belts, their geometries and how they may relate to each other (see Section 2.6.5.1 – Magnetic inversion modelling of the volcanic belts).

Forward modelling of gravity has proven useful in testing, constraining and refining the development of regional, full crustal geological cross sections and deep crustal reflection seismic interpretations (Rawling et al., 2011; Cayley et al., 2011a; Cayley et al., 2011b). A similar methodology has been applied to the development of STAVELY serial cross sections. Forward modelling of gravity data along the eleven serial geological cross sections was undertaken. Initial geological cross section development incorporated the results of the magnetic inversion modelling and, where available, seismic transect data interpretations. The sections were then refined based on the results of gravity forward modelling. Petrophysical properties for rock types and major stratigraphic units were compiled for STAVELY to inform and constrain forward modelling and inversion (see Section 2.4.4 – Rock properties).

Potential field data is inherently ambiguous, meaning that an infinite number of models can produce a calculated response that fits the observed data. As such, independent constraints such as field mapping and seismic data increases our understanding of the geology and can provide a greater level of confidence that an a priori model is a close representation of the actual geology.

Figure 2.34 TMI (RTP) image with interpreted faults (black) and intrusions (red) south of Glenthompson. Over-printing criteria indicate the Cambrian-aged granites, including the multi-phase Bushy Creek Granodiorite and the Buckeran Diorite in the north of the image, have intruded deformed and upturned metasediment and metavolcanic units. The Cambrian intrusions have “burnt out” the magnetic response of the host lithology, particularly evident in the centre and south of the image where both magnetic and non-magnetic intrusions have intruded the linear, sub-parallel Chatsworth Basalt (thin N-S trending anomalies within unmagnetic Glenthompson Sandstone; see Appendix 4 – Geological Units).

2.6.5.1 Magnetic inversion modelling of the volcanic belts

Numerous profiles have been extracted from the VicMag (2005) state-wide regional (airborne) magnetic database and modelled. The primary purpose of inverting the magnetic data was to determine the angle (dip magnitude) and dip directions of the volcanic belts within STAVELY. The orientation of the volcanic belts and/or segments therein has been used as a proxy for the orientation of faults that bound the belts, except where field data indicate otherwise (e.g. the east-dipping Mehuse Fault bounding the upper Dryden Belt that contains west-dipping and facing strata). This information has been used in constraining and testing the validity of the geological interpretation and in the construction of cross-sections providing the framework for construction and development of the STAVELY 3D model.

Cover sequences within STAVELY are generally non-magnetic, except for the Rocklands Volcanic Group in the south west, and the Newer Volcanic Group in the south (Figure 2.8 and Figure 2.9). This has allowed for the modelling of the Cambrian magnetic units under cover to reveal the geometries of the volcanic belts across the region. The Newer Volcanic Group completely cover the Boonawah Belt south of the Grampians Ranges and therefore magnetic data inversion was not carried out over this belt.

Profile locations (Figure 2.35) were selected based on a qualitative assessment of the gridded magnetic data. Anomalies deemed to be “clean” were prioritised for inversion modelling – i.e. magnetic mafic to ultramafic volcanic units, juxtaposed to non-magnetic felsic volcanic units or Cambrian metasediments, producing linear discrete anomalies or multiple non-overlapping anomalies. Magnetic profiles were extracted from gridded data to ensure modelled sections were orthogonal to the local strike of magnetic units to minimise off-section effects, therefore providing the best opportunity to determine reliable dip geometries.

The Quick Invert tool within the ModelVision software package proved to be a very useful way of quickly interpreting magnetic profile data, particularly where the geology is associated with steeply dipping magnetic rock units. The tool is optimised for inversion of magnetic anomalies using a tabular source body. The volcanic belts are generally linear, steeply dipping, planar bodies with limited width and extensive depth and strike length. The tabular source body types used in inversions were found to effectively represent these volcanic units, producing a calculated response closely matching the observed data.

Modelling of the 1VD of TMI was carried out concurrently with modelling of TMI to optimise the inversion (Figure 2.36). This provided additional constraints on the possible configuration of the modelled bodies, particularly their geometries near surface. Bodies for which a good fit between the observed and calculated response was achieved for both TMI and 1VD are considered more reliable than those for which a fit of the calculated to observed TMI response was achieved, but for which the 1VD response did not fit closely.

Where inverting only on the TMI response produced ambiguous results, simultaneously inverting the 1VD was valuable in separating overlapping anomalies and constraining the dip of bodies. The 1VD in-line filter is one-dimensional and is computed on the assumption that there is no variation perpendicular to the profile. Therefore, it was important to ensure modelled sections were orthogonal to the local strike of magnetic units to minimise off-section effects. This was achieved by using the gridded data which enabled profile extraction at any arbitrary direction.

Inversion modelling was focussed on the shallow parts of the magnetic units responsible for producing the high frequency signal contained in the magnetic data. Modelled depth extents were generally between 500 m and 6 km. As such, regional surfaces were calculated using a second order polynomial to approximate the regional component within the observed data (Figure 2.36). This allowed for modelling of only the shorter wavelength anomalies derived from the relatively shallow units of the volcanic belts. Geometries derived for shallow units have been extrapolated to depth using qualitative interpretation of filtered magnetic/gravity data, and gravity forward modelling along the serial cross sections and deep seismic reflection and shear wave velocity data where applicable.

Inversions were completed for each anomaly on any given profile, enabling a gradual build-up of bodies that could be used to constrain cross section construction. After calculating a regional surface for the profile being modelled, a tabular body was generated for the anomaly of interest, and an iterative process of inverting on TMI and 1VD for magnetic susceptibility, position along the profile, depth (to top of body), depth extent and width of the body was performed. Strike length and azimuth were fixed and the dip was initially set to 90° (vertical).

Once an approximate fit was achieved the dip of the body was allowed to vary while again inverting iteratively on TMI and 1VD. If required, the regional level was also freed at this point to refine the TMI fit while leaving the 1VD fit unaltered.

Once a good fit for an anomaly was achieved for both the TMI and 1VD responses, and further iterations resulted in only minimal improvements in the mismatch (Root Mean Square) between the calculated and observed responses, the process was repeated for the next anomaly on the cross section.

An iterative process of re-visiting and re-inverting all the bodies on the serial cross sections was undertaken as a final step once all other bodies had been created and individually inverted to refine any minor mismatches resulting from the influences of additional neighbouring bodies.

Figure 2.35 A) Locations of magnetic inversion modelling section lines. B) 3D perspective view showing tabular bodies derived from 2 ½-D inversions along profiles shown in (A). Background images are TMI (RTP) with un-shaded areas in (A) highlighting the Stavely Arc volcanic belts. The brown rectangular outline represents the field of view (towards the northwest) seen in the 3D perspective image of dip-modelled anomalies in the Black Range region in Figure 3.12 and Figure 4.5. Results from selected inversion sections are discussed in Section 3.2.5 – Stavely Arc.

Figure 2.36 Example of magnetic inversion section profiles across the Stavely Belt. Bottom panel shows inverted magnetic bodies within the Stavely Belt and associated magnetic susceptibility values (SI). The insert shows the location of the profile (red line) and interpreted faults overlain on an image of TMI. The middle panel shows observed and caluculated TMI profiles and the regional profile (pink). The top panel shows observed and caluculated 1VD of TMI profiles. Inversion results indicate a west dipping geometry for the fault bound Stavely Belt.

Magnetic susceptibility values assigned to the tabular bodies representing the volcanic belts (from inversion on this parameter) were analysed during the inversion process and restricted to ensure values were consistent with geologically realistic values derived from drill core measurements and surface exposure (see Section 2.4.4 – Rock properties).

The Brimpaen, Grampians Deeps, Caramut, Elliot and Boonawah belts have not been inverted due to younger magnetic cover masking and/or interfering with any significant suitable magnetic anomalies from deeper sources of interest. Examples of selected inversion profile results are described and shown below in Section 3.2.5 – Stavely Arc.

2.6.5.2 Gravity forward modelling – Serial cross sections

Forward modelling of the eleven serial cross sections was carried out on profiles extracted from the VicGrav (2011) statewide database (Figure 2.37). Forward modelled serial sections profiles, locations and density values used are also presented in Appendix 3 – Forward model sections. Terrain corrected Complete Bouguer Anomaly data was modelled1. The aim of forward gravity modelling was to test, constrain and refine a priori crustal-scale geological cross sections developed with inputs from field observations, drilling, seismic, qualitative potential field interpretations and magnetic inversion modelling. Alterations to Stavely Arc volcanic belt geometries, thicknesses, depths and degrees of offset along faults were incorporated into the final geological serial sections (see Appendix 2 – Geological cross sections), providing the framework from which a full crustal 3D model of STAVELY was developed. Gravity forward modelling has also indirectly informed assumed lithological characteristics of various arc belt segments (i.e. relatively more or less mafic; relative volumes of intercalated metasediment), based on the relative density values that were required to be assigned to bodies to produce a satisfactory fit to the observed data. The regional scale of the serial sections and STAVELY 3D model has meant that the relatively thin cover units were not forward modelled.

The top of the forward modelled sections was set to 0 m AHD, and topography was ignored since terrain corrected data was being utilised. This was not ideal, particularly for serial sections transecting areas of elevated topographic relief such as the Grampians Ranges and Black Range. However, given the relatively flat topography throughout STAVELY, and the crustal-scale geological model being developed, it was deemed to be an acceptable compromise. A regional removal was undertaken by calculating first or second order polynomial surfaces for each section line and manually adjusting if required. After assigning measured median density values to the a priori model a regional level inversion was run (datum shift) to allow the average calculated response level to more closely match the average observed response level.

Modelling of Complete Bouguer Anomaly data was refined by simultaneously modelling 15 km high pass and 20 km low pass filtered profiles (Figure 2.38). This allowed for additional constraints when modelling the shallower, shorter

1 Data acquired as part of the Stavely Project ground gravity traverses (Haydon, et al., 2016) was not available at the time of modelling.

wavelength and deeper, broader wavelength anomalies, respectively, compared to modelling the Complete Bouguer Anomaly alone. Given that the high pass filtered profile modelling is little affected by compensation for the regional field (i.e. regional removal process), it provides an independent constraint on the validity of near-surface bodies. The additional resolution and anomaly separation evident in the high pass filtered data, which needs to be adequately addressed by the modelled section, was not always evident in the unfiltered data. The low pass filtered profile provided confidence in the modelled section in relation to the broader scale density distribution, particularly in the mid to lower crustal levels.

Figure 2.37 Serial cross section locations on gravity within STAVELY. A) location of serial cross sections and project area underlain by 30 km high pass filtered Complete Bouguer Anomaly map; B) serial cross sections in 3D space with a 30 km high pass filtered Complete Bouguer Anomaly surface, to show the true geographic relationships between sections. Forward modelled gravity profiles are presented for each section. See Appendix 3 detailed forward modelled profiles and petrophysical measurements used to inform the modelling.

Figure 2.38 Example of forward gravity model section (serial cross section: Line 7). See Appendix 3 for all forward modelled sections. The range of (measured) density values used to forward model the gravity of all the different rock units in the cross section are listed in Table 2.1. Note that the Stavely Arc (Mount Stavely Volcanic Complex) was subdivided into two main units for the purposes of forward modelling; a ultramafic-mafic dominated sequence (in blue), and an intermediate-felsic dominated sequence (in green). These subdivisions were assigned different density ranges, based on the differences in density measured directly in rocks of the Stavely Arc (Table 2.1). The near-surface distribution of these subdivisions was constrained by mapping and drilling. The subsurface distribution was constrained by geophysics and by inferences from mapping (e.g. older (= lower) arc stratigraphy is more mafic – boninitic), but was allowed to move as part of the modelling process. The Stavely Arc subdivisions were not captured as part of the STAVELY 3D model.

The forward modelling workflow for obtaining an acceptable fit between the observed and calculated gravity response involved iteratively moving from a simple to more complex representation of the interpreted geology for each section. The RMS error of the fit between modelled and observed profiles was also calculated and used to guide the modelling process. Final RMS errors are given for the Bouguer and 15km high pass modelled data in Table 2.1 and Appendix 3 – Forward model sections. To streamline modelling and interpretation similar rock types separated by faults in the original hand drawn sections were combined, where practical, into single bodies and assigned a common density. If necessary these bodies were later split along the interpreted faults and assigned different densities to produce a better fit to observed data. An attempt was made to keep the differences in density for similar geological units to a minimum (Table 2.1). In general, when forward modelling representative bodies of tectonostratigraphy in the serial cross sections, median values initially assigned were those obtained from direct measurements of density for the corresponding representative stratigraphy. The median density values were then varied within the 25th to 75th percentile of the sampled data as necessary to achieve the best possible fit without altering body geometries. However, on occasion density values were assigned outside of this range – but only within a range consistent with published data for the corresponding rock type (Table 2.1). Typically, the assigned density of bodies representing similar lithologies were increased marginally with increasing in depth. The final forward modelled sections generally provide a good fit between observed and modelled gravity. Most of the geometric variations from the original hand drawn serial cross sections predominantly consisted of alteration to the depth and distribution of Grampians Group rocks and intrusive bodies. ptions used for forward modelling of rock units in the development of the STAVELY 3D model. Also shown are the measured ng of core by AGOS’s Multi-Sensor Core Logger instrument.

Section 2 Section 3 Section 4 Section 5 Section 6 Section 7 Section 8 Section 9 Section 10 Section 11 AGOS median (n) AGOS range (AR1) (SD1) (25-75%) 4.35 1.91 0.81 2.13 1.25 1.16 2.34 2.33 1.29 0.94 8.69 4.12 2.05 9.11 4.07 4.45 2.31 3.31 2.95 2.66

2.40 2.50 1.81 (366) 1.76 2.50 1.86 2.60 2.75 2.50 2.65 2.45 2.65 2.60 2.61 2.59 (19) 2.29 2.67 2.76 2.66 2.75 2.64 2.71

2.64 2.60 2.62 2.67 2.63 2.65 2.65 2.65 2.60 2.60 2.78 (783) 2.77 2.76 2.65 2.75 2.68 2.68 2.68 2.65 2.70 2.80

2.76 2.79 2.82 2.79 2.73 2.79 2.78 2.69 2.83 2.88 3.03 (5452) 2.90 2.78 2.86 2.91 2.88 2.80 2.85 2.80 2.77 3.11 2.79 2.68 2.69 2.70 2.84 2.78 2.77 2.81 2.76 2.81 2.80 2.72 2.80 2.80 2.81 2.87 2.85 2.70 2.70 2.75 2.75 2.73 2.72 2.73 2.70 2.77 2.72 (2210) 2.40 2.82 2.80 2.78 2.77 2.77 2.80 2.76 2.79 2.81

2.76 2.75 2.62 2.71 2.72 2.70 2.75 2.75 2.74 2.76 2.80 (35773) 2.63 Mafic Pre-Cambrian 2.90 2.95 2.90 2.90 3.10 2.90 3.00 3.00 2.90 2.90 2.85 Undifferentiated Pre-Cambrian 3.10 3.10 3.10 3.10 3.10 3.10 3.10 3.10 3.10 3.10 3.10 Hummocks Serpentinite Min 2.80 2.83 2.91 Max 2.84 3.00 Mantle 3.25 3.25 3.25 3.25 3.25 3.25 3.25 3.25 3.25 3.25 3.25

Background density: 2.67 g/cm3

Modelled density lower than 25th percentile AGOS measurements

Modelled density higher than 75th percentile AGOS measurements

Forward models along the serial cross sections typically consisted of the following eleven generalised stratigraphic units (detailed geological descriptions of these units are provided in Appendix 4 – Geological units.) :

1. Otway Basin sediments

2. Grampians Group

3. Granite intrusions (Cambrian and Devonian)

4. Moornambool Metamorphic Complex (Stawell Zone)

5. Glenelg River Metamorphic Complex (Glenelg Zone)

6. Nargoon Group

7. Mount Stavely Volcanic Complex (felsic dominant)

8. Mount Stavely Volcanic Complex (mafic dominant)

9. Kanmantoo Group (Glenthompson Sandstone)

10. Pre-Cambrian (pre-Delamerian)

11. Mantle

In summary the generalised gravity forward modelling workflow consisted of the following steps:

12. digitise initial body polygons from hand drawn serial cross sections (informed by magnetic inversions) displayed as background BMP files in profiles within ModelVision,

13. assign median density values derived from pre-competitive stratigraphic drilling,

14. regional removal / datum shift,

15. vary density values as necessary within the 25th – 75th percentile ranges of measured densities,

16. divide bodies as necessary along faults and alter assigned densities,

17. alter body geometries, thicknesses, depths as necessary,

18. alter assigned densities outside of measured 25th – 75th percentile range as necessary – iterative process with geometry changes,

19. introduce new bodies if required while adhering to existing constraints (e.g. intrusives; additional fault slices of the Stavely Arc ; Grampians Group),

20. confirm consistency of model with interpretation line-work; confirm validity in 3D model (i.e. consistency between cross sections), and

21. update serial cross sections – if necessary alter model geometries for geological consistency and re-test.

Forward modelling considerations

Potential field forward modelling contains an inherent problem of ambiguity. To minimise this, complimentary datasets were interrogated to inform the forward modelling of gravity. Of particular importance was the magnetic data, though it was not forward modelled directly along the serial sections due to the model bodies representing a simplified regional scale crustal architecture, while the magnetic response comprises a substantial contribution from smaller scale, shallower units, often producing a complex high frequency response which is problematic to model with regional bodies. The magnetic inversion modelling of these various units (see Section 3.2.5 – Stavely Arc) has informed the geological sections and forward gravity models. Broader magnetic anomalies helped guide and constrain regional gravity modelling.

Close attention was paid to the spatial distribution of gravity data in relation to the serial cross section lines (Figure 2.39), taking note of gridding artefacts and interpolations resulting from sparse data distribution. Where such areas were identified, precedence was given to the geological interpretation informed by magnetic data or mapping as opposed to attempting to fit the spurious gravity profile (Figure 2.40).

To remove edge effects resulting from the density contrast associated with the end of the modelled line, bodies were extended by approximately 20 km beyond the ends of each serial cross section. The model geometries in the off-section segments were informed by gridded gravity and magnetic data. Where a significant influence from off-line source bodies (intrusives) was deemed to affect the observed section profile (e.g. serial sections 4, 5 and 7), off-section bodies were modelled based on a qualitative interpretation of gridded gravity (and magnetic) data. These bodies do not appear in the depth sections.

Assumptions made during the forward modelling process:

 Forward modelling of gravity was used to determine large scale structures and geometries, incorporating results derived from magnetic inversion modelling.

 Major geometries and structures interpolated north and south from seismic lines have been constrained by surface mapping and geophysical trends.

 A background density of 2.67 T/m3 was used for forward modelling of gravity data.

 Terrain corrected Bouguer anomaly (Complete Bouguer Anomaly) data were used for forward gravity modelling. Theoretically this dataset has had topographic effects removed and thus the top of each modelled section is 0 m AHD. Topographic effects are unlikely to have been totally compensated for over the Grampians Ranges and/or the Black Range.

 Cover units including the Murray Basin, Newer Volcanic Group, Rocklands Volcanic Group and Quaternary deposits were not included in forward gravity modelling. The thickness of these units was negligible compared to the full crustal (40 km) serial cross sections. The typically sparse spatial density of regional gravity data does not enable high frequency variations in the gravity signature derived from these units to be sampled. Instead modelling was focussed on larger, more regional anomalies.

 Densities of major units are assumed to remain relatively constant along strike throughout STAVELY.

 Densities of major units are assumed to typically increase with depth (decreased porosity, increased metamorphic grade).

 Small scale structures were not considered to significantly influence the modelled gravity response.

 Lithological boundaries were generalised for the modelling process.

 Where large off–section intrusions are assumed to influence the gravity response along the section, bodies approximating the intrusions have been included in the modelling (but are not displayed in modelled section images). Off-line bodies were modelled on serial cross sections 4 and 7.

 Bodies were extended beyond end of sections by 10 to 20 km to avoid “edge effects” at section ends. Geometries and depths of extended units were informed by gridded gravity and magnetic data.

Notable forward modelling issues:

 Sparse gravity data distribution causing potential gridding artefacts which translated onto the profile section. Where gridded interpolation appeared conformable with surrounding data points and assumed geology an attempt was made to fit the extracted observed profile. Where no evidence for an interpolated gravity anomaly was evident, no attempt was made to model the anomaly – most notably the eastern extent of the large anomaly at 60000 m in serial cross section 2 (Figure 2.40).

 An attempt was made to distinguish between more mafic (blue – higher density) and more felsic (green – lower density) units within the volcanic belts during modelling (Appendix 3 – Forward model sections) and to keep these units consistent between serial cross sections. Internal intercalation of these units within the volcanic belts resulted in some overlap in the range of densities assigned (Table 2.1). Thus, in places more mafic blue units were assigned a lower density than proximal more felsic green units, and vice versa. Lithology assigned to measured density data from drill core for the MSVC did not distinguish between felsic – mafic units.

 There is limited density data for units of the Glenelg River Metamorphic Complex (e.g. pelitic schist, gneiss and granite) of the Glenelg Zone (Skladzien, 2007). Modelled densities range between 2.69 – 2.87 g/cm3 (Table 2.1). In serial cross sections 6, 7 and 8 the Hummocks Serpentinite was modelled as an individual unit, with modelled densities of 2.79 – 3.0 g/cm3. Lower importance was placed on modelling the Glenelg River Metamorphic Complex given that it was not the focus of the STAVELY 3D model. Modelling of the Glenelg River Metamorphic Complex was primarily focussed on matching regional anomalies and reducing edge effects (as was the case with the Stawell Zone rocks to the east).

 Saint Arnaud Group and Moornambool Metamorphic Complex rocks were modelled as separate units within the Stawell Zone in serial cross sections 5 and 6. For remaining serial cross sections the modelled Stawell Zone consisted entirely of Moornambool Metamorphic Complex bodies (see Appendix 3 – Forward model sections).

 An enigmatic low gravity response, both in profile and in gridded data, directly to the west of the Boonawah Belt on serial cross section 3 is attributed to low density rocks of the Nargoon Group. No Grampians Group or Otway Basin sediments have been intersected above the Nargoon Group in proximal drilling west of the Mosquito Creek Fault. Measured density data from drill core for rocks of the Nargoon Group indicates a bi-modal distribution (Skladzien, in prep) with peaks centred around 2.0 and 2.82 g/cm3. A relatively low density of 2.70 g/cm3 was assigned to a body representing Nargoon Group to better match the observed data.

 A low density (2.62 g/cm3) body of felsic Mount Stavely Volcanic Complex (Stavely Arc), and a low density (2.65 g/cm3) body of mafic Mount Stavely Volcanic Complex (Stavely Arc) were modelled in serial cross sections 4 and 5, respectively. These are interpreted as probable intercalated units of Williamsons Road Serpentinite within the Stavely Belt. Measured density data from drill core for rocks of the Mount Stavely Volcanic Complex indicates a bi-modal distribution (Skladzien, in prep.) with peaks centred around 2.62 and 3.10 g/cm3, consistent with the low densities modelled.

Figure 2.39 Locations of ground gravity stations used for gridding, interpretation and modelling of gravity data in STAVELY. Gravity forward modelled serial sections are also shown. Background image is gridded Complete Bouguer Anomaly.

Figure 2.40 Example of model mismatch associated with sparse ground gravity data distribution. A) serial section: Line 2, B) serial section: Line 3, profiles highlighting mis-match and C) gridded ground gravity with stations and serial cross section location.

Variations in assigned model densities

Assigned density values were kept within the 25th to 75th percentile range of measured density values from drill core densities for each unit modelled where possible (Table 2.1; Appendix 3 – Forward model sections). Table 2.1 highlights assigned minimum and maximum density values for each unit which fall outside of this range, for each serial cross section. Stratigraphic units for which the modelled density range consistently extended beyond the measured percentile range include:

 Otway Basin (sediments) – modelled with higher density values than the measured 75th percentile in all serial cross sections. Sample bias (lithological) from limited density measurements may be a factor. The shallow depth of burial of measured drill core samples may also be an influence – AGOS measured values give a median of 1.81 g/cm3, with a minor population at 2.65 g/cm3 – deeper samples from drill holes PHE1 and PHE2 (Skladzien, in prep.) returned density values closer to those used for modelling (2.40 – 2.50 g/cm3).

 Grampians Group – consistently modelled using density values lower than measured. Sample bias (lithological) from limited measurements (sandstone and minor conglomerate) may be a factor – however would expect values lower than the 2.78 g/cm3 measured median for these quartz arenite-dominated lithologies.

 Kanmantoo Group (Glenthompson Sandstone) – maximum of modelled range densities higher than 75th percentile in all sections except for serial cross section 2. Probable density increase with burial depth.

 Moornambool Metamorphic Complex – range lower than measured – sample bias (lithological) from limited available drill core; intrusions within the Moornambool Metamorphic Complex were not included in the measured sample population.

 Intrusives – Density values of some intrusions modelled are higher than the measured 75th percentile – this may be due to sample bias as measured data is limited to Bushy Creek Granodiorite (19 measurements only).

Limitations

The following are some key limitations associated with the interpretation:

 Due to time constraints, reconciliation of serial cross section rock-types and geometries against potential field geophysics (gravity and magnetics) overlapped in the project timeline with construction of the STAVELY 3D model surfaces. However, most geometry changes that resulted from forward-modelling along the serial cross sections have been incorporated into the STAVELY 3D model. Time limitations meant that some small geometrical differences remain in localised areas. For example; the thickness of the western part of the Narrapumelap Belt on serial cross section 3. Other small differences can be identified by comparing forward-modelled cross sections with surfaces in the STAVELY 3D model.

 Time did not permit inversions to be undertaken for each of the individual intrusive bodies identified within STAVELY. An effort was made to forward model discrete intrusives where the potential field response suggested a marked property contrast between the host and intrusive body (e.g. Figure 2.41). Magnetic profile inversion modelling incorporated intrusive bodies if encountered within the profile, however the tabular body type used in the inversions is not considered an ideal representation. Nevertheless, some general information could be derived from these results, e.g. depth and depth extent estimates and broad geometries.

 Three dimensional geophysical inversions were not deemed useful because the complex nature of regional deformation (and correspondingly complex geophysical response) in STAVELY has resulted in a large spatial distribution of similar rock types that in places display internally heterogeneous rock properties.

Figure 2.41 Example of gravity and magnetic forward modelling of Bushy Creek Granodiorite, indicating a potential later, inner intrusive phase (red). Magnetic susceptibility (S – in SI units) and density (D – in g/cm3) values are shown for reference.

3. Modelled surfaces and volumes

The STAVELY 3D model comprises stratigraphic volumes, stratigraphic surfaces and fault surfaces at the crustal scale. This section introduces the contents of the STAVELY 3D model, with individual descriptions of the volumes (comprised of groupings of voxels) and surfaces. Descriptions of stratigraphic volumes and surfaces include a summary of their geology, the nature of the generalisations used in model building, a description of the constraining geological and geophysical data within each volume or associated with each stratigraphic surface, and descriptions of relationships with adjacent model volumes and surfaces. Additional details for each of the geological units modelled, and those not modelled but considered regionally significant, are provided in Appendix 4 – Geological units.

Model volumes are apparently separated from one another by faults in most instances, although stratigraphic contacts are demonstrated in some places (e.g. Grampians Group unconformably overlying Cambrian bedrock near Willaura; intrusive bodies intruding host strata) and/or inferred in others (e.g. volcanic rocks of the Stavely Arc conformably overlying Glenthompson Sandstone at depth).

Detailed individual descriptions of faults within the STAVELY 3D model are restricted to named faults that are interpreted to have large displacements and/or are understood in detail from outcrop mapping, geophysics or other means. The named fault descriptions include a summary of their distribution, key outcrops, interpreted age (overprinting criteria), stratigraphic units affected, constraints on interpretation of geometry, interpreted movement history and inter-relationships with other structures. All faults, both named and unnamed, are summarised in Appendix 6 – Fault summary table.

The stratigraphic surfaces and volumes are described in order of stratigraphic age (youngest to oldest, and in alphabetical order within a stratigraphic unit). The named fault surfaces are described individually, grouped by age (oldest to youngest) and by size (significance), arranged alphabetically within subgroupings.

3.1 Modelled geological units (surfaces)

Certain geological cover units have not been modelled as volumes and are represented in the STAVELY 3D model as surfaces only (Figure 3.1). The main reasoning behind this approach is the limited depth extent of these units, when compared to the total depth extent of the crustal-scale model (~40 km). Due to the large area and depth extent of the STAVELY 3D model it was impractical to discretise the model volume into voxels smaller than 500 m, a value larger than the thickness of several cover units. The lateral spatial extent of these units has been interpreted from surface exposure and field observations, drilling and geophysical data. Depth extent is controlled by drilling, seismic and geophysical modelling and inversion.

3.1.1 Newer Volcanic Group

The Miocene to Holocene Newer Volcanic Province is a thin, laterally extensive intraplate alkaline to tholeiitic basalt province (Price et al., 2003; Gray & McDougall, 2009; Boyce et al., 2014) formed from the amalgamation of hundreds of small point-source and fissure eruptions and related lava flows of the Newer Volcanic Group. The province covers 15 000 km2 in western Victoria, including a substantial area in the south of STAVELY.

The Newer Volcanic Group is the youngest rock type included in the STAVELY 3D model. Most units of the Newer Volcanic Group in STAVELY are dated between 2-4 Ma (Gray & McDougall, 2009), but the youngest volcanic activity in STAVELY is dated at just tens of thousands of years old, for example the upstanding volcanic edifices of Mount Napier and Mount Rouse. Because of this, the upper surface of the Newer Volcanic Group is the present-day land surface. The margins of the basalt plains are locally covered by thin alluvium, a consequence of the disruption and damming of pre-existing drainage systems by the lava flows themselves (e.g. the southern Victoria Valley). In such instances, the extent of the buried lava flows used to construct the STAVELY 3D model surface were interpreted from drilling and from regional magnetic data, where the flows typically produce a high amplitude, high frequency magnetic response (see Figure 2.8).

Drilling confirms that much of the Newer Volcanic Group cover in STAVELY is generally less than 20 m thick, increasing to a maximum of 145 m in places where pre-existing valleys were filled and overtopped, such as in the ancestral Salt Creek near Lake Bolac (Raiber & Webb, 2008). All these thicknesses are far too thin to be represented by volumes in the STAVELY 3D model, consequently the extent of the Newer Volcanic Group is only represented by a basal surface (Figure 3.1).

Drilling and field relationships both confirm that the Newer Volcanic Group directly overlies Otway Basin rocks in many places, but rests directly and unconformably on Paleozoic bedrock in other places (e.g. adjacent to the Hopkins River near Chatsworth and south of Dunkeld).

Where lava units of the Newer Volcanic Group have drowned Miocene-Pliocene strandline topography of the last Otway Basin marine incursion (i.e. the upper surface of the Brighton Group), the basal surface of the Newer Volcanic Group has a regularly undulating shape, dictated by the pre-existing topography of the underlying aligned Brighton Group that reflects former sand dunes. The amplitude of these undulations is generally less than 20 m, which means that they are far too small to be resolved by the basal surface included in the STAVELY 3D model. The undulations are a significant consideration for mineral exploration and groundwater drilling, as basalt flows covering dune crests are up to 20 m thinner than flows in adjacent interdune swales. Fortunately, the position of the dune crests and swales is easy to interpret from regional magnetic data, as the thicker interdune basalt corresponds to a locally increased magnetic intensity. From this data it is possible to interpret buried strandlines of the Otway Basin, which extend north from the southern edge of STAVELY to beneath Hamilton and Cavendish, and to the north of Lake Bolac, and beyond to the vicinity of Skipton. This feature and observation has helped define, in turn, the lateral extent of the Otway Basin basal surface (see Section 3.1.3 – Otway Basin).

The regional-scale base of the Newer Volcanic Group surface built by McAlpine and Goodwin (2015) was included in the STAVELY 3D model. The surface was constrained by surface geology, geophysics, a solid geology interpretation and >600 bores and drill holes that intersect the Newer Volcanic Group (McAlpine & Goodwin, 2015).

Figure 3.1 Oblique view of geological cover units (basal surfaces) in the STAVELY 3D model. From oldest to youngest: Rocklands Volcanic Group (green), Otway Basin (purple), Murray Basin (red) and Newer Volcanic Group (blue). Vacant regions represent the places where Cambrian bedrock and/or Grampians Group is exposed at surface, with one clarification: thin (<20 m) and young alluvial and colluvial deposits, including a veneer of ‘Brighton Group’ sand extends near-continuously from the Otway Basin to the Murray Basin west of the Grampians Ranges, and this veneer is not included in the STAVELY 3D model.

3.1.2 Murray Basin

The Murray Basin is a thin Cretaceous to Recent intracratonic sag basin in the north of the STAVELY. The entire stratigraphic sequence represents an initial fluvial system (Renmark Group) overrun by a shallow marine carbonate system (Murray Group) that was in turn overlain by beach to fluvial systems (Wunghu Group) as the sea retreated. The Victorian portion of the Murray Basin is detailed in Lawrence (1975) with a brief overview in Holdgate & Gallagher (2003). The entire regional framework including good unit descriptions is contained in the synthesis volume of Brown and Stephenson (1991).

The top surface of the Murray Basin is the current flat landscape, mostly comprising a strand-line landform formed on Loxton Sand. The Murray Basin onlaps against the Paleozoic bedrock and conceals a significant portion of the Stavely Arc in the northwest of STAVELY. The Murray Basin onlaps against the Cambrian bedrock exposure along the northern end of the Grampians Ranges near Horsham and gently thickens northwards to attain a thickness of about 300 m at the north end of STAVELY.

At a maximum of 200-300m thick in STAVELY, the Murray Basin remains too thin to be represented by a volume in the STAVELY 3D model – only a basal surface is included. The surface is intended to be a regional-scale top of Palaeozoic bedrock surface and was developed by integrating existing water bore, drill hole and petroleum well data with depth to magnetic source solutions derived using the AutoMagTM automatic analysis program (Shi & Boyd, 1993). The Murray Basin stratigraphy is typically non-magnetic and of relatively uniform density, so that underlying bedrock geology and drill targets can be interpreted from geophysics with confidence. The surface is identical to that generated by McLean (2010), however it has been cut using the Stavely project area. Across the entire Victorian part of the Murray Basin, which includes STAVELY, a total of 993 boreholes and more than 650 AutoMagTM depth to magnetic source estimates were used to constrain the surface. This surface has additional attributes: “depth below ground”, “DTM”, and “true depth” which enable it to be converted into a depth to basement map.

A 3D model of the Murray Basin that includes the main stratigraphic groups is available. This model is well constrained by numerous water bores and petroleum wells (McLean, 2011).

The basal surface of the Murray Basin is undulating, reflecting topography of the underlying Palaeozoic bedrock, including valleys eroded by the initial fluvial systems that deposited the Renmark Group. The undulating basal surface slopes gently to the northwest at a dip of about three degrees (McLean, 2010). Bedrock ‘highs’ poke up right through the Murray Basin strata, particularly around the basin margins; Mount Arapiles, located in northwest STAVELY, is one example. These bedrock highs are represented as ‘holes’ in the Murray Basin basal surface. 3.1.3 Otway Basin

The Otway Basin fringes the southern edge of STAVELY and is thicker and contains more diverse and complex geology than the Murray Basin to the north. The Otway Basin contains a Cretaceous rift phase (Otway and Sherbrook groups) associated with the continental rifting between Australia and Antarctica and a Cenozoic thermal sag phase following continental breakup (Wangerrip, Nirranda, Heytesbury and Brighton groups). Good descriptions of the basin development and complete stratigraphy are available in Woollands & Wong (2001), Duddy (2003) and Holdgate & Gallagher (2003), with recent updating of stratigraphic nomenclature in VandenBerg (2009).

Only small portions of the northern margin of the Otway Basin extend north into the southern parts of STAVELY. Here, Otway Basin strata is almost completely covered by the Newer Volcanic Group. The only Otway Basin strata exposed in STAVELY is Brighton Group, which extends as a sand-sheet with a strandline morphology beyond the limits of the Newer Volcanic Group, for example near Woorndoo. Brighton Group is time equivalent to, and continuous with, the Loxton Sand in the Murray Basin (Morand et al., 2003).

Other Otway Basin stratigraphy in STAVELY is only known from drilling. Drill hole STAVELY01 (Schofield et al., 2015b) and mineral exploration drill holes south and east of Dunkeld show that Otway Basin strata in southern STAVELY is generally less than 200 m thick. Some deeper drill holes here pass through the Brighton Group to intersect siliciclastic or marly carbonate sediments of older Otway Basin stratigraphy (e.g. PRC-05, PRC-06, PRC-07) (Figure 3.2). West of Dunkeld and south of Hamilton, Otway Basin strata is locally thicker, with deep petroleum wells (Moyne Falls 1, Hawkesdale 1, PHE 1, PHE 2 and Garvoc 1) intersecting at least 1000 m of typical Otway Group before intersecting underlying Cambrian bedrock. Some drill holes south of Hamilton (e.g. Hamilton South 3) intersect Older Volcanic Group basalts amongst the sedimentary sequences – these volcanics are responsible for the patchy magnetic character west of the Boonawah Belt.

Although locally thicker than 500 m in STAVELY, Otway Basin strata is generally too thin for the basin to be represented as a volume. A regional scale top of Palaeozoic bedrock surface defines the base of the Otway Basin in the Stavely 3D model. This surface was generated by Frogtech Pty Ltd using their “SEEBASETM” methodology (Structurally Enhanced view of Economic Basement) (see Jorand et al., 2010).

Figure 3.2 Otway Basin margin with selected wells, bores and drill holes and modelled fault traces on a Gravity 30 km HP pseudocolour image. Drill holes such as Hamilton South 3 and PRC5 do not intersect significant Otway Basin stratigraphy, but instead pass downwards into Palaeozoic rocks. These drill holes lie just north of the east-west – trending Tarrington Fault, which appears to define the northern Otway Basin margin. The Tarrington Fault coincides with an abrupt change in gravity. South of the Tarrington Fault, drill holes Moyne Falls and Hawkesdale and others pass through more than one thousand metres of Otway Basin stratigraphy, before intersecting Cambrian bedrock. High-grade Cambrian migmatite bedrock in Moyne Falls contrasts with low-grade Cambrian metasediment (sandstone) bedrock in Hawkesdale, constraining the position of the Cambrian Yarramyljup Fault – the western boundary of STAVELY – beneath thick Otway Basin strata.

3.1.4 Rocklands Volcanic Group

Felsic volcanic rocks of the Early Devonian Rocklands Volcanic Group occur in the southwest of STAVELY where they unconformably overlie the Grampians Group and Cambrian bedrock. The Rocklands Volcanic Group occupies an area of 1500 km2, and is confined to a region approximately bounded by Hamilton in the south, the Yarramyljup Fault in the west, the Black Range in the north and the Victoria Range in the east. The northern and southern margins of the Rocklands Volcanic Group are unconformably overlain by Murray Basin sediments, and Otway Basin and Newer Volcanic Group strata, respectively.

The sequence is sub-horizontal with a maximum exposed thickness of 250 m, comprised mostly of thick, densely welded ignimbrites (Simpson, 1997). The Rocklands Volcanic Group has been intersected by several drill holes, some of which penetrate through to underlying Cambrian bedrock and/or Grampians Group (e.g. Woohlpooer 6). Drill hole intersections demonstrate that units of the Rocklands Volcanic Group form flat-lying sheets generally less than 100 m thick and more typically only a few tens of metres thick. The Rocklands Volcanic Group is therefore too thin to be represented as a volume in the STAVELY 3D model. The lateral extent of the Rocklands Volcanic Group basal surface was interpreted from mapping, drill hole intersections and from regional magnetic data.

Gravity and magnetic data image circular-shaped bodies beneath the Rocklands Volcanic Group, which may represent related (possibly parental) intrusions (Figure 2.9). These have been modelled as separate volumes in the STAVELY 3D model (see Section 3.2.1 – Intrusives – Devonian).

3.2 Modelled geological units (volumes)

Visualising the STAVELY 3D model as a complete set of surfaces is particularly complex. It is difficult to appreciate the geometry of individual geological units from visualisation of the surfaces alone. One way of simplifying this is to only visualise the Cambrian faults (for example) since these were responsible for emplacing the fault-slices of Cambrian Stavely Arc volcanics, which are now recognised as the volcanic belts. This approach, however, makes understanding how the various volcanic belt relate to each other in their present configuration difficult. Therefore it is valuable to be able to visualise the geological units (volumes) which are bound by these faults. This was done using fault surfaces as boundaries to divide up the voxet (or block model) into a number of regions (groups of cells, or voxels). Each region represents a different geological unit (volume), and each was assigned a different colour (Figure 1.5).

A three-dimensional volumetric voxet model enables the attribution of the various regions (i.e. geological units) with physical rock properties, allowing a range of quantitative modelling procedures including (but not limited to), heat flow modelling, numerical simulation, fluid flow modelling, volume estimation and further potential field modelling involving regional geophysical forward and/or inversion modelling. Three-dimensional inversion modelling could be used to test the validity of cross sections of the current model, or as a regional basis from which more detailed refinements of particular areas of interest could be undertaken.

The following sections describe the significance, structural context, geometry, and distribution of each of the regions modelled.

3.2.1 Intrusives – Devonian

Granite bodies constrained by geochronology to be Early Devonian in age in STAVELY are centred around the Jalur Rift (intruding Grampians Group in the Grampians Ranges (see Figures 2.16 and 3.9) and Cooac Rift (beneath and north of Mount Arapiles, where Grampians Group is contact metamorphosed; see Figure 3.9 for location), and the Rocklands Rift beneath the co-magmatic Rocklands Volcanic Group in the southwest of STAVELY (Figures 2.9, 3.9). Overprinting relationships suggest that additional stocks of Early Devonian age intrude parts of the Dimboola Belt near Woorak and north of Netherby. Several base metal and precious metal prospects are associated with exposed Early Devonian intrusions (e.g. Mafeking, Mount Mackersay, see Schofield et al., 2018 Section 3.1.3.1 – Intrusion-related mineral prospects).

Although geochemically similar to Late Cambrian intrusives (see Section 3.2.3 – Intrusives – Cambrian (possible Ordovician)), the Early Devonian intrusions show different shape and orientation characteristics, related to intrusion within a different (D4) stress field. The Early Devonian intrusions tend to be elongated northeast-southwest, indicating that the geometry of D4 rifts played an important role in localising these intrusions. Some Early Devonian intrusions are more circular in shape, for example at Mafeking and McKenzie River. These intrude the cores of D4 oroclines, and are associated with comagmatic sills which are also oroclinally curved so that oroclinal rotation accompanying intrusion may have resulted in ‘averaged’ overall shapes for these examples. The distinctive northeast-southwest elongation of most Early Devonian intrusions serves to discriminate them from Late Cambrian intrusives, and has been used as a method to interpret the former beneath cover units of the Otway and Murray basins in the south and north of STAVELY. These are included as volumes in the STAVELY 3D model (Figure 3.3).

Figure 3.3 Oblique view of the STAVELY 3D model showing the modelled intrusive volumes. Late Cambrian intrusives (dark red) are generally elongated north-south or occur in north-south oriented clusters, whereas Early Devonian intrusives (pink) are elongated northeast-southwest, and are clustered within and adjacent to D4 rift-centres.

3.2.2 Grampians Group

The ?Late Ordovician- to Silurian-aged Grampians Group is the oldest cover unit overlying Cambrian bedrock in STAVELY (Figure 1.2, Figure 3.4 and Figure 3.5; Cayley & Taylor, 1997a; Miller et al., 2001). The purpose of the Grampians Group volume included in the STAVELY 3D model is to depict the lateral extent of the unit, including beneath the Murray and Otway basins, and to highlight areas where the unit is expected to be too thick to be explored through (Figure 3.6).

The Grampians Group is a fluvial to shallow marine, red bed sandstone and mudstone. Many of the sandstones are quartz-rich and are very resistant to erosion, forming the Grampians Ranges and related hills (Figure 3.4A). Other Grampians Group has been eroded and downthrown, so that it underlies parts of the Murray and Otway basins (Figure 3.4C) and presents an additional impediment to mineral exploration of the underlying Cambrian bedrock.

Thickness of Grampians Group is highly variable, and dependent on two factors: 1 the local stratigraphic thickness, which remains relevant in places where a basal unconformity with Cambrian bedrock appears to be preserved more-or-less intact – for example beneath the Willaura Syncline southeast of Mount Elliot (see Figure 2.16), and 2: the local geometry of extensional faults related to the Marathon Fault, relevant in places where the contact between the Grampians Group and underlying Cambrian bedrock is faulted. Because of this complexity, Grampians Group thickness and distribution is impossible to estimate by down-dip projections of mapped stratigraphic relationships alone. In some places where Grampians Group stratigraphy appears to be potentially thick, it is faulted-out at depth and relatively thin (<100 m). The maximum preserved demonstrated stratigraphic thickness is about 3700 m in the heart of the Grampians Ranges, although the actual true thickness is highly variable. Gravity, passive seismic, electrical methods and drilling may be some of the techniques that could be potentially used to establish the local thickness of Grampians Group.

Figure 3.4 Grampians Group A) View looking north from the summit of Mount Abrupt, near Dunkeld, showing the classic cuesta-form of peaks in the Serra Range, with western dip-slopes developed on resistant, tilted Serra Sandstone quartz arenite. B) Listric D3 thrust fault with minor displacement, climbing from a bedding-parallel detachment located in thinly-bedded Silverband Formation (dark) into overlying thick-bedded quartz-arenite of the Glen Hills Sandstone Member (pale – a sandy lens within the Silverband Formation). This fault is one of several splay faults related to the nearby Leuctra Fault. Silverband Road, MGA 54 633982 5883408. C) Grampians Group quartz arenite sandstone tilted to a sub-vertical attitude adjacent to the D3-D4 Log Hut Fault in Tulloh Creek, west of Mount Dundas. MGA 54 575820 5841178. Flat-lying Loxton Sand unconformably overlies the Grampians Group at this locality (pasture and brown rubbly scours above outcrops).

Figure 3.5 Cross sections adapted from Cayley & Taylor (1997b, 1997c) and Taylor & Cayley (1997), drawn to approximate 8 km depth to illustrate: the remarkable layer-cake succession that is the internal Grampians Group stratigraphy; the locally revised thicknesses of the succession; the main structures that deform it; relationships with underlying Cambrian strata, including with underlying D1a volcanic (fault) belts of Stavely Arc (depicted in orange, green, purple as for Figure 3.9). Stratigraphic codes correspond to Welch et al. (2011). The deformed Grampians Group succession is reinterpreted to be kilometres-thick in the Jalur Rift interior (sections A, B, D; from modelling of gravity data; see also Figure 3.9), and to be thin to absent outside the rift margins (e.g. section C; interpretation controlled by mapping and drilling data). Out-of-section rotational fault movements related to D4 growth of the Mafeking (section A), Big Cord (section D) and Cranage (section B) oroclines are invoked to explain unusual internal Grampians Group geometries depicted on these section lines – particularly the lateral termination of the D4 Cranage Splay scissor-fault against the dextral D4 Escondida Fault beneath the Asses Ears Range illustrated in section B. Similar angular relationships occur across the Thermopylae Fault in the Mafeking Orocline (section A). Relationships at depth in these sections are constrained by direct lateral projection from adjacent areas of excellent outcrop. See Figure 3.73 and Figure 4.3 for an explanation of the context of these oroclines and scissor faults.

Figure 3.6 Perspective view of the STAVELY 3D model showing the Grampians Group basal contact surface (A) and model volume (B) for STAVELY. Most of the basal contact surfaces are fault splays of the D4 Marathon Fault. A minority of the basal contact surfaces preserve an intact unconformity, as exposed near Willaura (Stuart-Smith & Black, 1999) and south of Balmoral (Morand et al., 2003). Minor differences between the lateral-extents of the surfaces and volumes are due to the model volume voxel-size – 500 m3. Grampians Group thicknesses and widths of less than 500 m cannot be represented in STAVELY model volumes but are implied from the wider distribution of the surfaces. The Grampians Group model volume was constructed by combining field map constraints (Cayley & Taylor, 1997a; Buckland & Stuart-Smith, 2000; Stuart-Smith & Black, 1999; Morand et al., 2003), drilling that intersects Grampians Group strata, and interpretation of Grampians Group strata from geophysics to form an extent surface, with Grampians Group thickness profiles within that calculated from mapping and from forward modelling of gravity and magnetic data.

Reinterpretation of the Grampians Group distribution using improved geophysical coverage acquired since detailed geological mapping has refined the estimation of Grampians Group thickness. The Grampians Group is thicker than previously estimated in the main ranges, but thin in the north and west of STAVELY where high frequency magnetic features of the underlying Cambrian bedrock are visible. The Grampians Group volume has been modelled using density values that range from 2.6 to 2.76 g/cm3, with a median density of approximately 2.65 g/cm3, reflecting the dominant quartz-arenite sandstone constituent rocks, with subordinate conglomerate and mudstone of higher density distributed throughout the multilayer sequence.

The coarse voxel size and regional scale of the STAVELY 3D model limits the ability of the model volume to capture and communicate the finer details of Grampians Group true thickness where it is thin (< 500 m). However, these details can be extrapolated from the more detailed geological cross sections that are included in the model, and from the lateral extent of the Marathon Fault surfaces, which in most cases are proxy for a basal Grampians Group surface. Where Marathon Fault surfaces locally extend beyond the limits of the Grampians Group volume they depict Grampians Group cover interpreted to be less than 500m thick.

Grampians Group internal stratigraphy is not captured in the generalised model volume, but is depicted in Figures 2.16 and Figure 3.5. Such detail cannot be meaningfully discriminated at the scale of the STAVELY 3D model, and are not considered important for mineral exploration of the Cambrian bedrock.

While an unconformable relationship between the Grampians Group and the Cambrian bedrock is exposed near Willaura and south of Balmoral, most other contacts between Grampians Group and underlying rocks appear to be fault-controlled, mainly along Marathon Fault splays. Drill hole STAVELY02 (south of Mount Stavely) intersected less than 100 m thickness of Grampians Group above a splay of the Marathon Fault (Figure 2.18). Similar thicknesses of Grampians Group have been penetrated by mineral exploration drilling in the Black Range region. Although the nature of the contact between the Cambrian bedrock and overlying Grampians Group is variable (i.e. fault-related versus unconformable), these results show that prospective Cambrian bedrock occurs at explorable depths beneath at least some parts of the Grampians Group.

About 70 km north of STAVELY, seismic reflection line MEMV96-09 images Grampians Group beneath the Murray Basin. The presence of Grampians Group here is constrained by drill hole Gunamalary 2, located nine kilometres to the northwest of the westerly end of the seismic reflection line. Gunamalary 2 intersected Murray Basin stratigraphy from surface to a depth of 479 m, Permian stratigraphy (Urana Formation) from 479 m to 576 m, and terminated in Grampians Group stratigraphy at 717.6 m. The seismic interpretation indicates a total thickness of Grampians Group here of about 250 m, and a total depth of cover of approximately 825 m (Figure 2.24).

Attenuated high-frequency signals in magnetic data in the northwest of STAVELY are comparable with the magnetic signature constrained for seismic reflection line MEMV96-09 and indicate that a similar thickness of cover (approximately 825 m), probably including Permian rocks and Grampians Group, extends to near Netherby. Grampians Group appears to be absent nearly everywhere east of the Golton Fault in STAVELY, with drill hole STAVELY16 and mineral exploration drill hole WI-1-6 both passing directly from Murray Basin stratigraphy into underlying Cambrian volcanic rocks at depths of 300 m and 359 m respectively.

Most refinements to the original serial cross sections are based on gravity forward modelling. Previously published Grampians Group geological cross sections (see Cayley & Taylor, 1997a) have also been refined following the interpretation of more recent and enhanced ground gravity data (Figure 2.27), and were used to inform development of the serial cross sections and the STAVELY 3D model. Most refinements involve alterations to the bodies that represent the Grampians Group and intrusives. These included changes to the thickness and geometry of Grampians Group (e.g. serial cross sections 8 and 9), as well as its spatial distribution (e.g. serial cross sections 4 and 5). In the north of STAVELY new extents of Grampians Group were introduced to account for anomalies observed in the gravity data (e.g. serial cross sections 10 and 11). Sporadic and probably localised Permian deposits up to hundreds of meters thickness have been intersected in drill holes in this area (e.g. Figure 2.24; drill hole Warraquil 3) and may locally contribute to the gravity lows observed in the data and interpreted here as Grampians Group, but generally the regional extent of these anomalies implies a substantial volume of spatially continuous lower density material as the source of these gravity lows, and the Grampians Group is a likely candidate.

3.2.3 Intrusives – Cambrian (possible Ordovician)

Distinguishing Cambrian from younger intrusive igneous rocks without geochronological data is difficult, owing to similar geophysical responses (see Section 2.6.4.2 – Interpretation of intrusive bodies), although there is potential to discriminate between Cambrian and Early Devonian intrusive phases using overprinting criteria and stress history mapping (see Section 4.5 – Strain (Stress) history mapping and Schofield et al., 2018). Cambrian and Devonian intrusions have been discriminated in the STAVELY 3D model (Figure 3.3).

Most of the Late Cambrian intrusive rocks are buried beneath younger cover, however some examples are known from outcrop (Figure 3.7) and intersections in mineral exploration drilling. Just west of the Stavely Belt are the Bushy Creek Granodiorite and Buckeran Diorite, which have been grouped together and referred to as the Bushy Creek Igneous Complex by Whelan et al. (2007). Intrusions of the Bushy Creek Igneous Complex are sub-circular and intrude upturned, thrusted Glenthompson Sandstone (Figure 3.7C), and are interpreted to be post-tectonic (e.g. VandenBerg et al., 2000). The cluster of Late Cambrian intrusives here is aligned and elongated north-south (Figure 2.34), and may represent a D1b rift-controlled magmatic grouping, with their geometry controlled by the transtensional stress conditions interpreted to have accompanied D1b. Similarly, north-south aligned magmatic groupings occur elsewhere in STAVELY, such as within and adjacent to the Black Range West / Mitre Belt and southeast of Kiata, and we interpret these as likely to be Late Cambrian in age on this basis. No D1b rift-bounding structures have been included in the STAVELY 3D model, since defined D1b rift margins have not yet been identified.

Figure 3.7 Late Cambrian granites and relationships to other Cambrian rocks. A) Outcrop of fresh biotite granite (G393) in Chirrup Chirrup Creek (MGA 54 641055 5812183) B) Pods of hydrothermally altered Lime Creek Granite aplite and feldspar veins intruding altered Glenthompson Sandstone, Lime Creek (MGA 54 638920 5805292). C) Looking south at well-bedded, steeply east-dipping and west-facing Glenthompson Sandstone turbidites, altered to cordierite-spotted hornfels adjacent to a Late Cambrian granite sill, which cuts across bedding and overprints a spaced S1a cleavage developed in the sandstone, Chirrup Chirrup Creek (MGA 54 641396 5811969).

Geochemical features of the Bushy Creek Igneous Complex suggest similarities with subduction-related magmatism, and may share a genetic link with the Mount Stavely Volcanic Complex, although petrogenesis of the Bushy Creek Igneous Complex is complicated by mixing with a more evolved, crustal component (Whelan et al., 2007). The Bushy Creek Igneous Complex also shares remarkably similar geochemical and isotopic characteristics with the nearby Early Devonian Victoria Valley Complex (Hergt et al., 2007; Whelan et al., 2007).

Porphyries of dominantly tonalitic or dacitic composition are known in the south of STAVELY. The porphyries intrude deformed rocks of the Mount Stavely Volcanic Complex within the Stavely Belt (e.g. ‘Victor Porphyry’, Thursday’s Gossan Prospect), and have also been noted outside of the Stavely Belt, where they intrude the Buckeran Diorite (Lexington Prospect) and Glenthompson Sandstone (Junction Prospect; drill hole STAVELY17) (Costelloe, 1994; Taylor et al., 2014; Schofield et al., 2015a; Taylor et al., 2015; Skladzien et al., 2016). These porphyries are considered prospective with several known to host porphyry-style mineralisation (see Schofield et al., 2018 Section 3.1.1.1 – Porphyry-related occurences). Like the larger Late Cambrian granite bodies, the porphyries show a distinct north-south elongation and alignment, including the Lalkaldarno Porphyry stocks mapped in the Stavely Belt (Buckland, 1985, Buckland 1987), intruding but apparently aligned along the Williamsons Fault. Similar porphyry stocks of Late Cambrian age are interpreted to be aligned along a segment of the Williamsons Fault in the interior of the Narrapumelap Belt, subsequently rotated into a northwest orientation within the core of the Mafeking Megakink (Figure 3.44).

While no Ordovician-age intrusives are confirmed in STAVELY, a voluminous tranche of Ordovician-age intrusives extends from the Glenelg Zone west into South Australia (Padthaway Ridge, including, in Victoria, the Dergholm Granite dated at between 459 ±2 Ma Ar/Ar and 488 ± 5 Ma K/Ar; see Morand et al., 2003). This means that intrusives of similar age are possible within STAVELY. A prolonged deformation and uplift/subsidence hiatus indicated in the Stawell Zone (the Stavely Arc accretionary wedge), in the Grampians-Stavely Zone, and in the Glenelg Zone (back arc basin of the Stavely Arc) between ~495 and ~450 Ma demonstrates that local subduction beneath the Stavely Arc ceased around ~490 Ma and did not recommence. For this reason, localised (in time and space) pulses of Ordovician magmatism in western Victoria most likely represent rift-localised second-stage decompression melts related to transient phases of crustal extensional associated with Macquarie Arc subduction system geodynamics - the extinct Stavely Arc system lay in the distal back-arc of the Macquarie Arc which was active throughout the Ordovician.

A distal back-arc, rift-related setting for Ordovician magmatism is consistent with overprinting criteria, the elongated shapes, and the ‘A’-type geochemistry of ‘Padthaway Ridge’ intrusives (Foden et al., 1990). As for subsequent Early Devonian magmatism, any Ordovician-age intrusives in STAVELY are likely to show arc-related geochemistry inherited from the reworking of Stavely Arc material at depth.

3.2.4 Nargoon Group

The Nargoon Group comprises Middle Cambrian marine sediments interpreted to overlie the Mount Stavely Volcanic Complex. The Glenthompson Sandstone is now correlated with the Kanmantoo Group and considered older than the Mount Stavely Volcanic Complex and so is described separately (see Section 3.2.6 – Kanmantoo Group).

The recently redefined Nargoon Group is interpreted to represent a separate phase of post-arc Late Cambrian synorogenic sedimentation (Taylor et al., 2015). This reinterpretation explains the slightly different composition of the Nargoon Group, which contains abundant detrital albite and chlorite together with detrital amphibole and garnet, a composition that is notably different to the micaceous Glenthompson Sandstone. The geochemistry of the Nargoon Group is also different being higher in Cr than the Glenthompson Sandstone. Nargoon Group has therefore been modelled as a discrete volume in the STAVELY 3D model. Although younger and slightly different compositionally compared to the Glenthompson Sandstone, the overall range of lithologies included in the Nargoon Group are similar, as are the range of modelled densities of 2.70-2.82 g/cm3.

The extent of the Nargoon Group appears confined to the uppermost tectonostratigraphic position preserved within Cambrian rocks in the Grampians-Stavely Zone, generally the region in the western Grampians-Stavely Zone that lies between the Boonawah-Grampians West-Tyar-Glenisla-Black Range-Black Range West/Mitre belts and the Yarramyljup Fault (Figure 3.8). This region includes the Nargoon Group type locality south of Balmoral. Nargoon Group at the type locality consists of clastic sediments and displays sedimentary features such as low angle cross-bedding, suggestive of shallow water deposition (Morand et al., 2003). Near Balmoral, Nargoon Group sediments are subvertically-dipping, folded and weakly cleaved, and unconformably underlie flat-lying Grampians Group stratigraphy. This indicates that the Nargoon Group was deformed prior to deposition of the Grampians Group, most likely during the Cambrian (Morand et al., 2003). Nargoon Group has been intersected in Stavely Project drill holes STAVELY05 and STAVELY14.

North of Tooan, the Black Range West/Mitre Belt appears to pinch out within the Mouchong fault system. North of this region, Nargoon Group appears to be fault-juxtaposed against Glenthompson Sandstone with the intervening Mount Stavely Volcanic Complex locally absent at the current level of exposure. This relationship is captured and modelled in serial cross section 10. In serial cross section 11 this relationship is overthrust by the Yarramyljup Fault, and lies outside the limits of the STAVELY 3D model.

Subvertical D3-D4 strike-slip faults that segment volcanic belts comprising upturned rocks of the Stavely Arc in the Black Range also segment Nargoon Group lying stratigraphically above the Mount Stavely Volcanic Complex, locally juxtaposing Nargoon Group against Glenthompson Sandstone, such as across the sinistral Cherrypool Fault (see Section 3.3.5.5 – Cherrypool Fault). Other juxtapositions of Nargoon Group and Glenthompson Sandstone volumes occur across the Muirfoot and Henty faults. These juxtapositions are captured in serial cross sections 6-8 (Appendix 2 – Geological cross sections).

Additional volumes of Nargoon Group are predicted at depth in the southeast of STAVELY, where the unit is inferred to overlie the crest of the in-situ Stavely Arc edifice, in the footwall of the Stavely Base Fault. The volumes of Nargoon Group here are inferred based on stratigraphic thickness arguments – basically the sediment pile here appears too thick to be able to be explained entirely by Glenthompson Sandstone alone. There are no other constraints on these volumes – they are entirely conceptual.

Figure 3.8 Perspective view of the STAVELY 3D model showing the distribution of Nargoon Group volumes. The large western volumes (encompassing the type locality south of Balmoral) are confined between the Boonawah – Grampians ‘West’ – Tyar – Glenisla – Black Range and Black Range West / Mitre belts to the east, which they may conformably overlie, and the Yarramyljup Fault to the west (not shown). Complexities in these volumes occur where the Cambrian sequence is laterally offset across D3 and D4 faults in the vicinity of the Black Range. The smaller Nargoon Group volume in the southeast is conceptual, interpreted to conformably overlie autochthonous Stavely Arc rocks in the mid-crust to help account for the large apparent thickness of Cambrian metasediments in this part of the model.

3.2.5 Stavely Arc

The Cambrian-aged Stavely Arc is represented in STAVELY as nineteen discrete but discontinuous steeply-dipping volcanic (fault) belts (see Section 1.2 – Geological Context) (Figure 1.3 and Figure 1.4). Individual volcanic belts are tens of kilometres long (20-150 km), are of similar width (generally ~2.5 to 5 km, up to 10 km in the Dimboola Belt), and have highly varying strikes and dips (Figure 3.9 and Figure 3.10). Although steeply dipping, stratigraphy within the volcanic belts appears to be generally upright. Local exceptions occur where later deformation appears to have caused overturning and disordering of stratigraphy (e.g. at Bellellen, in the Dryden Belt, in the Moyston Fault footwall; Cayley & Taylor, 2000c, 2001; and along parts of the Tyar and Black Range belts).

The margins of the volcanic belts are poorly exposed, but occasional outcrops of fault-zones, drilling intersections, and the disordered nature of Cambrian stratigraphy within the volcanic belts themselves all support the interpretation of most external and internal belt boundaries as faults. These structures are locally cross-cut by Late Cambrian (D1b) intrusions, and are unconformably overlain by Grampians Group sediments, constraining their ages to D1a. D1a fault development was accompanied by local chevron-style folding and slaty cleavage development. Dip-slip dominant displacement is indicated by the generally sub-horizontal plunge of F1a fold axes developed in Glenthompson Sandstone, by the strike-parallelism of S1a cleavage with belts of Cambrian stratigraphy contained within D1a faults, and by inference from adjacent zones, where sinistral-oblique thrust-displacements of faults of proven Cambrian age has been mapped.

The mix of igneous stratigraphy and the structural style observed within each of the volcanic belts is similar overall, including for many of the buried belts that were investigated by pre-competitive stratigraphic drilling undertaken as part of the Stavely Project. Therefore, all volcanic belts are best interpreted as fault-repeats, uplifted parts of a wider Stavely Arc stratigraphy, most of which probably lies at depth.

The volcanic belts south of the Grampians Ranges (Stavely, (Caramut), Bunnugal and Boonawah belts) are west-dipping and have features characteristic of high-angle thrust systems, for example common westward-facing, coupled with alternations of stratigraphy that imply fault-repetition (Figure 3.11). The regular, subparallel strike spacing is consistent with simple imbrication of Stavely Arc stratigraphy within relatively narrow, steeply west-dipping fault-slices, thrust to surface over the western flank and crest of a large arc-edifice that is deeply buried, imaged in the mid-to lower crust by deep seismic reflection data (Figure 2.22). A steep-dip to the enveloping surfaces of volcanic units within the belts, and of the metasediments that separate them is mapped in outcrop south of the Grampians Ranges and modelled in the Black Range region. It is consistent with the steepening of strata seen in strongly emergent imbricate thrust systems. The D1a faults were formed in response to an episode of crustal shortening and thickening.

This simple pattern appears to change east of Stavely, where the Elliot and Narrapumelap belts dip northeast, and beneath the Grampians Ranges, where the buried Grampians ‘Deeps’ Belt has a similar orientation. West of the Grampians Ranges, the fault belts appear as shorter segments. In the Black Range region, four volcanic belts are arranged in plan in a radial-configuration like spokes in a wheel – a feature referred to as the ‘Crab Nebula’ by CRAE in the 1980’s.

The fault belts segments in the ‘Crab Nebula’ (Grampians West, Tyar, Black Range and Glenisla belts) show a variety of dips (Figure 3.12 and 3.13), but predominantly dip to the west, with predominant westerly facing (Skladzien et al., 2016). Drill hole intersections into a poorly-exposed package of interbedded volcanics and sediments along the south-east flank of the Black Range Belt show that parts of the succession dip steeply west and face west (e.g. CRAE drill hole DD96GM69; from flow top – flow base transitions described in Navarre Minerals drill hole10BR001), while other parts appear to face east (e.g. from flow tops in Navarre Minerals drill hole10BR002). The local facing reversals here are due either to folding, faulting or both.

In the north of STAVELY some volcanic belts appear to die out (e.g. Black Range West/Mitre) while another wide, strike-persistent belt (the Dimboola Belt) appears with an overall easterly dip that contrasts with most of the other belts. Despite these changes, other belts appear to persist from the south largely unchanged into the north, with westerly dip and facing (e.g. the Dryden and Dryden North belts).

Internal stratigraphy and structure of each of the different volcanic belts are not represented in the STAVELY 3D model. Only generalised volcanic belt volumes could be captured at the scale of the model. However, a summary of known internal stratigraphy and structure within each of the volcanic belts is provided below.

Figure 3.9 Volcanic belts of the Stavely Arc (A) shaded on a gravity pseudocolour map and (B) as a simplified cartoon map of the wider region (see Figure 4.1 for location) with all post-Middle-Cambrian (post- D1a) intrusions and cover rocks omitted for clarity, with all the volcanic belts named, major faults named, and including the bounding Moyston and Yarramyljup faults. The volcanic belts are correlated into four main groupings (orange, green, red, purple), based on their interpreted pre-D4 inter-relationships (see Figures 4.12 and 4.13). The locations of major D4 structural basins, the Jalur, Lorquon (bounded by the Lorquon and Glenlee faults), Cooac (bounded by the Winian East and Grass Flat faults) and Rocklands (adjacent to the Wannon Fault) rifts are also shown.

Figure 3.10 Oblique view of the STAVELY 3D model showing (A) all D1a volcanic belt-bounding faults and (B) all D1a volcanic belt volumes in STAVELY. Colour codes correspond to those used in Figure 3.9.

The simplest initial working assumption for the STAVELY 3D model – that the buried arc-foundation edifice imaged in deep seismic reflection data in the south of STAVELY, beneath the imbricated volcanic belts (see Figure 2.22), might pass north along-strike to directly correlate with the broad igneous body that forms the northern end of the Dimboola Belt in the far north of STAVELY and visible in regional magnetic data beneath the Murray Basin was impossible to sustain geometrically during model construction.

This is because it is clear from mapping, deep seismic reflection and potential field data that the buried edifice imaged in the south of STAVELY lies in a footwall position east of the west-dipping Stavely and Dryden belts, whereas the Dimboola Belt and associated rocks imaged in the north of STAVELY lies in a hangingwall position west of the Dryden and Dryden North belts.

Taking the effects of D4 deformation into account, the Stavely, Narrapumelap, Dryden and Dryden North belts are interpreted to represent segments of a single fault slice that was likely to have been continuous and west-dipping along the full length of STAVELY when the fault systems within which it is contained was active during D1a.

Because of this key constraint, any northern continuation of the buried arc edifice imaged in the south of STAVELY must continue to lie in a footwall position to the east of the Dryden North Belt. This places it beneath the strike-persistent package of Glenthompson Sandstone that lies to the east of the Dryden North, Hindmarsh and Jeparit belts, in the Moyston Fault footwall (Figure 3.14).

The Dimboola Belt is east dipping. It sits in a hangingwall position relative to the west-dipping Stavely – Narrapumelap – Dryden – Dryden North – Hindmarsh fault belts. The Dimboola Belt is large, but does not have the strike-persistence of the Stavely – Narrapumelap – Dryden – Dryden North – Hindmarsh fault belts, instead terminating beneath the Grampians Ranges.

Because of its large strike-persistence and unchanging geophysical character from the regions of outcrop constraint in the south, the Stavely – Narrapumelap – Dryden – Dryden North – Hindmarsh fault belt is interpreted to retain a westerly dip and facing through the centre of the STAVELY 3D model volume. This geometry suggests down-dip continuity with the Dimboola Belt, so that direct correlation with the large igneous bodies exposed to the base of the Murray Basin at the northern end of the Dimboola Belt is possible. Comparable stratigraphy is proven by drilling.

In this style of interpretation, the Dimboola Belt is considered as a back-thrust succession that is equivalent to, but structurally subsidiary to, the Stavely – Narrapumelap – Dryden – Dryden North – Hindmarsh fault belts, all formed in the Late Cambrian, during the Delamerian Orogeny (D1a). The Dimboola Belt is regarded as a back-thrust succession because of its opposing geometry, despite its large width and prominence in potential field data. This key set of constraints forms the fundamental basis for construction of the northern half of the STAVELY 3D model (Figure 3.15).

The high degree of additional structural complexity in the middle of STAVELY is a consequence of subsequent orogenic events, particularly the Siluro-Devonian Bindian Orogeny, which is expressed within the Grampians-Stavely Zone as faults and folds of D3 and D4 age (Cayley & Taylor, 1997a; Miller et al., 2001, 2006), and, in the underlying Cambrian bedrock as complex transtensional structures (D4) that have dismembered, offset, rotated and tilted Cambrian structures to varying degrees.

Detailed descriptions of the individual volcanic belts are described below in alphabetical order with further background provided in Appendix 4 – Geological Units.

Figure 3.11 Oblique view of STAVELY 3D model volumes of the Boonawah (yellow), Bunnugal (lighter green), Caramut (dark green) and Stavely (purple) belts, showing the sub-parallel listric west-dipping imbricate character of the D1a fault-belts at crustal scale in the southern parts of STAVELY, in places where they appear not to have been significantly modified by subsequent deformations. Near-surface outlines of other volcanic belts are depicted for reference.

Figure 3.12 A) TMI (RTP) pseudocolour image of the Black Range showing the radial-arrangement of volcanic belts around a buried granitic body south of Glenisla (circled in red), a pattern colloquially known as the ‘Crab Nebula’. This pattern is interpreted to be a consequence of conjugate strike-slip (D3, D4) fault segmentations of a single, simple parent D1a fault slice (see Figure 4.6 and Figure 4.7). Inset (B) is an oblique view (from the south) of the central part of the main image, utilising a transparent magnetic (RTP) image to show the geometry of model bodies extending to depth for the Tyar, Glenisla, Black Range and Black Range West/Mitre belts. The bodies illustrate the results of dip-modelling inversions of multiple magnetic profiles constructed at right-angles to local belt strike (see Figure 3.17, Figure 3.38 and Figure 3.51). The variations in dip between the different bodies highlight an overall westerly-dip to the volcanic belts, interpreted as relict from the original single west-dipping D1a fault slice. Portions are locally overturned to dip steeply east, particularly adjacent to the fault-truncated and drag-folded ends of the volcanic belt segment.

Figure 3.13 Oblique parallel stereopair of the western part of the STAVELY 3D model, looking southeast at the ‘Crab Nebula’ of D1a volcanic belt volumes and bounding D1a fault surfaces (see Figure 3.12), and surface-traces of D3-D4 strike-slip faults (in red). The Grampians ‘West’ Belt (dark orange) passes north along-strike into the Tyar Belt (pale orange). The western end of the Tyar Belt is offset dextrally from the southern end of the Glenisla Belt (dark yellow) across the D4 Henty Fault. The northern end of the Glenisla Belt is offset sinistrally from the southern end of the Black Range Belt (yellow) across the D3-D4 Cherrypool Fault. The northern end of the Black Range Belt is offset sinistrally from the southern end of the Black Range West / Mitre Belt (palest yellow) across the D3-D4 Muirfoot Fault (see Figure 3.16 and Figure 3.39). Note clockwise drag-folding of the western end of the Tyar Belt adjacent to the Henty Fault to form the steeply-plunging Tyar Fold (see Figure 4.5). The dragged-portion of the Tyar Belt is modelled to extend deep beneath the Henty Fault footwall, balancing the Glenisla Belt volume in the Henty Fault hangingwall.

Figure 3.14 Looking north at lower crustal volumes of the STAVELY 3D model that depict highly reflective rocks (in deep seismic reflection data) interpreted as autochthonous to para-autochthonous Stavely Arc deeply buried beneath overthrust material, including volcanic rocks in the Stavely, Narrapumelap, Dryden (pink outlines at surface) and Dimboola (red outlines near-surface) belts. The eastern flank of the main volume supports the Moyston Fault footwall (in dark blue). The western flank of the main volume supports the footwalls of imbricated west-dipping D1a thrust systems. The Stavely East Fault and Dryden Fault plane positions (translucent pink surfaces) are illustrated as one example.

Figure 3.15 Oblique view of the STAVELY 3D model showing the Dimboola Belt volume (red) cut by the modelled subvertical D4 dextral-oblique transtensional fault surfaces of the Dimboola Duplex. The Dimboola Duplex appears bound between the D4 Escondida Fault and related splays on the west (orange, translucent white) and the D4 Golton Fault on the east (pink translucent mesh). The en-échelon geometry of the fault duplex is consistent with formation in response to overall dextral transtensional shear. The abrupt southern end of the Dimboola Belt volume is interpreted as truncated by the Jalur Fault (not shown; see Figure 2.32). South of the Jalur Fault, the Escondida and Golton faults converge to enclose the core of the Mafeking Megakink (not shown; see Figure 3.44, Figure 3.46 and Figure 4.3).

3.2.5.1 Black Range Belt

The Black Range Belt (belt 17; Figure 3.9) extends from beneath Mount Bepcha and the Rocklands Reservoir in the south towards Noradjuha 11 km south of Natimuk in the north. Magnetic data suggests that the southern end of belt is abruptly truncated against Cherrypool Fault just east of Mount Bepcha (Figure 3.16). The northern end of belt is truncated obliquely against the Muirfoot Fault on its western flank, where it is apparently offset sinistrally from the Black Range West/Mitre Belt (see below). East of the Muirfoot Fault, part of the Black Range Belt persists to eventually plunge north beneath Nargoon Group metasediments near Noradjuha. In the Black Range, west of the Grampians Ranges, the Black Range Belt underlies Grampians Group, against which it appears to be faulted along a splay of the Marathon Fault (Cayley & Taylor, 1997c). The Black Range Belt is structurally complex, but appears to be bounded on its flanks by west-dipping D1a faults of the Mouchong Fault System. The total strike length of the Black Range Belt is approximately 45 km.

Cambrian igneous rocks within the Black Range Belt are discontinuously exposed in the valleys of the Mouchong and Mount Talbot creeks within the Black Range, and in tributaries to the Glenelg River along the southeastern flank of the range. These creeks and their tributaries have eroded through the Grampians Group and down into the underlying bedrock (Spencer-Jones, 1965). Mafic Cambrian volcanics are exposed in dam excavations and in shallow pits dug by nineteenth century gold prospectors (Cayley & Taylor, 1997a).

Although outcrop is poor, the moderate to high magnetic susceptibilities of intermediate to mafic volcanics within the Black Range Belt make its position and geometry easy to interpret in regional magnetic data, even where buried beneath Grampians Group (Figure 3.16). The Black Range Belt is approximately 3 km wide where it underlies the Black Range, and this part of the belt appears remarkably similar to the adjacent Tyar and Glenisla belts. Further north, the Black Range Belt broadens to over 6 km wide near Connangorach. North of Connangorach, the belt apparently breaks into a discontinuous series of sub-belts up to 2.5 km wide that step north in a right-stepping en-échelon pattern to near Noradjuha.

The southern half of the Black Range Belt has been extensively drilled owing to the discovery of several VHMS and/or porphyry-related prospects. Rocks encountered in drilling vary from gabbro to basalt to trachyandesite along the western side of the belt, and andesite, dacite and rhyolite along the eastern side. Basaltic breccia also occurs, and many of the volcanic rocks are richly pyritic (Cowan, 1988). Subordinate rocks along the western side of the belt include serpentinite and minor mafic intrusives. The intermediate succession along the eastern side of the belt includes a suite of greenish grey, altered, glassy dacite to rhyolite lavas, which have been brecciated or autobrecciated in places. The generally glassy groundmass in these rocks is characteristic of lava flows. The glass is now devitrified to fine grained aggregates of quartz, albite, sericite, minor chlorite and leucoxene. Phyllic and propylitic hydrothermal alteration assemblages in this succession is common (Crawford, 1994). Minor rocks in the eastern part of the belt include andesite breccia and black shale. A summary of existing petrological work is given in Cayley & Taylor (1997a), and Crawford et al., (2003).

Further north, drilling has intersected andesite within the Black Range Belt at Nurrabiel. Mineral exploration drilling (e.g. Thompson, 2014) and gravity data show the Black Range Belt to be dominated by Cambrian mafic to intermediate altered volcanics, with andesite breccia reminiscent of the Fairview Andesite Breccia of the Stavely Belt (drill holes SVO8009, SVO8025, SVO8027 and SVO8029), interspersed with subordinate andesite lava flows (e.g. drill hole SVO8028). Structural complexity reminiscent of that seen in the Stavely Belt is implied by slices of Glenthompson Sandstone metasediments incorporated into the interior of the volcanic succession (e.g. drill holes SVO8013, SVO8016). Relatively low gravity response along the western side of the belt implies the presence of metasediments and possibly felsic igneous successions of lower density (Figure 2.6, Figure 2.12 and Figure 2.28B).

Further north at Noradjuha, drill hole VIMP3 intersected hydrothermally altered, vesicular plagioclase-augite-phyric andesite breccia and lava that are very similar to the Mount Stavely Volcanic Complex (Maher et al., 1997). The high vesicularity of this lava suggests shallow water eruption. The whole rock geochemistry of this rock is typical of a low-Ti andesite, and is similar to the low-Ti andesites at Mount Dryden (Maher et al., 1997).

Modelling of detailed airborne magnetic data suggests that the Black Range Belt dips generally west, apart from near its southern end where it appears to be locally steepened and in places overturned to dip east in the Bepcha Fold adjacent to the Cherrypool Fault (Figure 3.17A and Figure 3.17B). Because of the overall westerly dip, the Black Range Belt succession is assumed to face west, although this has not been confirmed from available drill hole data. The west dip and assumed west facing of the succession here appears at odds with geochemistry that is generally expected to evolve from mafic towards intermediate-felsic in the proposed arc-setting. This disparity suggests the possibility of disordered stratigraphy within the Black Range Belt, or that the succession is east-facing, but is now overturned. Localised remanent magnetisation has been identified within STAVELY. Black Range Belt inversion line 11 intersects one such unit – potentially a Cambrian porphyritic intrusion, or perhaps a younger basaltic plug unrelated to the Black Range Belt – characterised as a magnetic low in TMI data, and a high in analytical signal filtered data (Figure 3.18). In magnetic profile data this feature produces a discrete low in both TMI and 1VD data and has been assigned a negative magnetic susceptibility (by the inversion algorithm) to enable a fit of observed to calculated data (Figure 3.18).

Figure 3.16 Tilt and band pass filter of regional magnetic data in the Black Range, shaded to show the positions of the Black Range and Black Range West / Mitre belts, offset sinistrally from each other by a minimum of 4 km across the subvertical ?D3/D4 Muirfoot Fault. The Muirfoot Fault is exposed in the Black Range where it sinistrally offsets Grampians Group (Cayley & Taylor 1997a), and at Mount Arapiles (see Figure 3.66). The southern end of the Muirfoot Fault abuts the D4 Henty Fault. Regional magnetic data makes it clear that there is no offset within the Tyar Belt directly south along-strike of the Muirfoot Fault, so that any southern extension to the Muirfoot Fault must be in an offset position. Dextral displacement across the Henty Fault during D4 implies that the southerly continuation to the Muirfoot Fault must lie west of the Tyar Belt, a position occupied by the Latani Fault. The Cherrypool Fault, which truncates the southern end of the Black Range Belt, has similar relationships to the Muirfoot Fault.

Figure 3.17 Example of inversion modelling of magnetic data over the Black Range and Black Range West/Mitre belts. A) Selected profiles over the Black Range Belt and section locations on detailed 1VD TMI data. Perspective view (B) shows all inverted bodies for the Black Range Belt – note the change in dip of magnetic bodies at the southern end of the belt associated with the Bepcha Fold (see Section 3.4.1). C) Selected profiles over the Black Range West/Mitre Belt and section locations on regional 1VD TMI data. Top profile panels show first vertical derivative of TMI; middle panels show TMI; bottom panels show inversion bodies. D) Location of all Stavely inversion sections on TMI (RTP), with detailed areas shown in (A) and (B) outlined.

Figure 3.18 Black Range Belt inversion line 11 showing west dipping volcanic units within the northern Black Range Belt and the location of the sinistral Muirfoot Fault, which offsets the Black Range Belt from the Black Range West/Mitre Belt. Line 11 intersects the location of a reversely magnetised body as shown by the white arrows in the inserts of TMI and analytic signal (AS) gridded data, and indicated in the profile. Top profile panels show first vertical derivative of TMI; middle panels show TMI; bottom panels show inversion bodies with assigned magnetic susceptibility values (SI) – note negative susceptibility value assigned to reversely magnetised body. Pink line in TMI panel shows the regional.

3.5.2.2 Black Range West/Mitre Belt

The Black Range West/Mitre Belt (belt 18; Figure 3.9) is completely buried beneath Murray Basin and/or Grampians Group cover, but the high magnetic susceptibility of the igneous rocks within it make it easy to interpret from regional magnetic data (Figure 3.16). The Black Range West Belt extends from approximately 12 km southeast of Tolondoo, where magnetic data show it to be offset from the Black Range Belt across the Muirfoot Fault (Cayley & Taylor, 1997a; Figure 3.16), north towards Clear Lake. The Black Range West Belt appears to die out approximately 9 km north of Clear Lake. The total strike length of the Black Range West/Mitre Belt is approximately 28 km.

The abrupt western margin of the Black Range West/Mitre Belt is characterised by a near-continuous cuspate magnetic high approximately 1 km wide that runs northwest through Toonlondo to Clear Lake. Drill hole RC83GM014 at the southern end of this high penetrated 31 m of Murray Basin sediments before intersecting magnetic basalt and basaltic breccia on the magnetic high along the western margin of the belt (Harvey, 1984). This lithology compares well to metabasalt and trachybasalt intersected along the western flank of the Black Range Belt, south along-strike. Drill hole SVO8019 at the northern end of this magnetic high intersected andesite breccia (Thompson, 2014) similar to that seen in the Black Range Belt. Overall this part of the Black Range West/Mitre Belt is reminiscent of the steeply-dipping, fault-bounded mafic to intermediate metavolcanics in the Black Range Belt.

Further north the Black Range West/Mitre Belt is characterised by low gravity and a blobby, dispersed magnetic character more consistent with felsic intrusives or other igneous rocks that have not been subjected to faulting or other significant internal deformation. This part of the Black Range West/Mitre Belt is wider than most of the Black Range Belt, reaching nearly 7 km in places. In this part of the Black Range West/Mitre Belt, drill hole RC84GM007 encountered non-magnetic ‘acid volcanic’ rock beneath 38 m of Murray Basin cover (Williams, 1985) in a region of low magnetic response. Groundwater bore TOOLONDO 1 intersected weathered and fresh ‘greenstone’, presumably metabasalt or metaandesite, beneath approximately 60 m of Murray Basin cover in the middle of the Black Range West/Mitre Belt. Drill hole SVO8023 encountered andesite lava (Thompson, 2014), indicating that volcanic rocks of the Stavely Arc span the full width of the Black Range West/Mitre Belt, including near its northernmost end. The circular magnetic character of these basalt intercepts suggests that these features could be younger plugs unrelated to the Black Range West/Mitre Belt.

Physical connectivity of the Black Range West/Mitre and Black Range belts across the Early Devonian Muirfoot Fault near Connangorach mean that it is highly likely that these volcanic belts were continuous pre-D4. Therefore, the stratigraphy described within the Black Range Belt is highly likely to continue into the Black Range West/Mitre Belt. Magnetic inversions again indicate an overall west dip for the Black Range West/Mitre Belt, and steepened to overturned units adjacent to the Muirfoot Fault at the southern end of the belt (Figure 3.17C). Differences between the Black Range West/Mitre and Black Range belts seem to be in the percentage of the belts that now comprises felsic intrusions or other igneous rocks, the lower overall gravity of the Black Range West/Mitre Belt suggests the possibility of a greater proportion of felsic igneous rocks (Figure 2.28B). The general northern elongation of intrusive bodies within the Black Range West/Mitre Belt compares favourably to northerly-elongated intrusives of known Cambrian age within and adjacent the Stavely Belt. We relate these geometries to the stress-field that accompanied intrusion during D1b. The similar elongation for intrusives in the Black Range West/Mitre Belt interior implies that they may be of D1b age, and therefore likely to be prospective for mineralisation related to evolution of the Stavely Arc. The only other voluminous intrusives in STAVELY are Early Devonian in age, and these granites tend to be elongated in a southwest-northeast direction, which we relate to the completely different stress-field that accompanied D4.

3.2.5.3 Boonawah Belt

The Boonawah Belt (belt 13; Figure 3.9) is an approximately 5 km wide, north-northwest-striking belt of magnetic rocks that is entirely buried beneath basalt flows of the Newer Volcanic Group. The Boonawah Belt is obvious in regional magnetic data because of the relatively high magnetic susceptibility of the igneous stratigraphy within the Boonawah Belt (Figure 2.8 and Figure 3.19). Many of the rocks within the Boonawah Belt are also relatively dense, and the belt is coincident with a prominent gravity high (Figure 2.28B). The Boonawah Belt lies 2.3 km west of Dunkeld, 5 km east of Penshurst, near Minhamite, and 5.5 km east of Hawkesdale where it is deeply buried beneath the Otway Basin. While the Boonawah Belt becomes untraceable in magnetic data south of Hawkesdale, the gravity high associated with the belt can be traced south beneath the Otway Basin all the way to the coast just west of Warrnambool. The total strike length is approximately 84 km.

The Boonawah Belt is interpreted to lie within the western part of the Grampians-Stavely Zone, since it lies east of low-grade ?Nargoon Group metasediments intersected beneath the Otway Basin in petroleum well Hawkesdale 1 in the footwall of the Yarramyljup Fault. The Boonawah Belt is apparently cut and slightly offset by fault splays related to the Mosquito Creek Fault in the vicinity of Dunkeld (Figure 3.19). Further north it is separated from the southern end of the Grampians ’West’ Belt, with which it is interpreted to have been continuous pre-D4.

The Boonawah Belt was first investigated by Penshurst Resources Limited, who drilled a series of mineral exploration holes targeting magnetic stratigraphy and a prominent gravity high within the belt approximately 10 km north of Penshurst (see Evans & Cuffley, 2008; Figure 3.19). Drilling intersected a suite of ultramafic serpentinite rocks with geochemistry comparable to peridotite and the Hummocks Serpentinite (drill hole PRC-04; Bailey et al., 2016; see also Schofield et al., 2018 Section 2.6 – Geochemistry of the Stavely Arc), tholeiitic basalt and mafic intrusions (drill holes PRC-02 and PRC- 03; see Schofield et al., 2018 Section 2.6 – Geochemistry of the Stavely Arc) and a suite of quartz dolerite intrusions (drill hole PRC-01). A felsic phase of one of the quartz dolerite intrusions in drill hole PRC-01 has been dated at approximately 510 Ma (Lewis et al., 2016), an age equivalent to an unnamed felsic intrusive incorporated as a fault slice in the Stavely Belt (Lewis et al., 2015), and to the peak of metamorphism in the adjacent Glenelg River Metamorphic Complex (Morand et al., 2003). Basalt lavas in the Boonawah Belt are pillowed in places, suggesting eruption into a subaqueous environment. Intermediate to felsic rocks with a clear subduction signature have yet to be identified from available drill hole samples, although are present in the Grampians ‘West’ Belt with which it is interpreted to have been contiguous along strike pre-D4 (see Schofield et al., 2018 Section 2.6 – Geochemistry of the Stavely Arc)

The magnetic high associated with the Boonawah Belt is asymmetric, with highest values along the eastern side of the belt and lower, but still elevated, values along the western side of the belt (Figure 2.8). Evans and Cuffley (2008) interpreted the belt to dip west. The region of higher magnetic intensity was interpreted by Evans and Cuffley (2008) to reflect magnetic mafic or ultramafic igneous stratigraphy as seen in drill holes PRC-02 to PRC-04. Separate elongated magnetic bodies located just to the east of the main Boonawah Belt, for example the 7 km long, 300 m wide belt about 4 km south of Dunkeld, may represent slivers of the same magnetic igneous stratigraphy hosted by fault-splays of the Boonawah East Fault. Due to the Boonawah Belt being completely covered by basalt flows of the Newer Volcanic Group over practically its entire length, detailed magnetic inversion modelling was not carried out.

An approximately 2 km wide region of lower magnetic intensity along the western side of the belt was interpreted to potentially reflect intermediate-felsic igneous rocks comparable to the upper parts of the Mount Stavely Volcanic Complex stratigraphy, although no geochemical correlatives have yet been identified (see Schofield et al., 2018 Section 2.6 – Geochemistry of the Stavely Arc). Patchy zones of low magnetic intensity in the western part of the Boonawah Belt are coincident with small gravity lows and may reflect demagnetisation caused by magnetite-destructive hydrothermal alteration, and/or the influence of felsic intrusions in the succession.

3.2.5.4 Brimpaen Belt

The Brimpaen Belt (belt 11; Figure 3.9) comprises a complex series of magnetic rocks that lie buried beneath Grampians Group and the Murray Basin in the vicinity of Brimpaen, Zumsteins and Laharum just northwest of the Grampians Ranges. Although rocks in the Brimpaen Belt are yet to be dated radiometrically, they unconformably and/or disconformably underlie the Ordovician-Silurian Grampians Group, and so cannot be younger than Ordovician (Cayley & Taylor, 2001). The range of lithologies encountered in the Brimpaen Belt, including in pre-competitive stratigraphic drill holes in STAVELY, invites direct correlation with stratigraphy of the Stavely Arc.

The Brimpaen Belt extends from near Glenisla Crossing in the south (intersected in drill hole STAVELY04) to near Wonwondah North in the north, a distance of 32 km. Unlike most of the other volcanic belts, the Brimpaen Belt is highly internally convoluted and disrupted, with a combination of northerly and east-west strikes and folds (Figure 3.20). The thickness of the Brimpaen Belt varies from less than 1.5 km thick where it is northerly-trending near Glenisla Crossing and Wonwondah, to more than 9 km near Brimpaen, where the belt appears to have been widened by internal faults and folds. Large parts of the belt appear to strike nearly east-west, for example within the 3.5 km long southern limb of the Dollin Kink.

The northern and eastern edges of the Brimpaen Belt are apparently truncated against the Escondida Fault (Figure 3.20), east of which lies the western flank more coherent magnetic stratigraphy of the Dimboola Belt. The southern and western edges of the Brimpaen Belt are apparently overlain and truncated by the Marathon and Mosquito Creek faults respectively. The Brimpaen Belt appears to be separated from the Grampians ‘Deeps’ Belt by the Fault 08, and other unnamed structures beneath the Grampians Group (Figure 3.21). All these structures were active during D4, implying that the internal complexity of the Brimpaen Belt may be partly a consequence of D4 deformation. The Brimpaen Belt plunges south beneath a northeast-trending splay of the Marathon Fault, and possibly underlies the Asses Ears Range – modelling of gravity data suggests a considerable thickness of Grampians Group strata overlies the Brimpaen Belt at this location.

The Cambrian igneous stratigraphy of the Brimpaen Belt was first investigated by North Exploration Limited as part of their Wartook mineral exploration campaign (O’Neill, 1994; Cayley & Taylor, 1997a). Drilling encountered small intrusives of diorite to monzogranite and associated volcanic breccias of andesite to dacite which are geochemically and lithologically similar to rocks described within the Mount Stavely Volcanic Complex in the Stavely Belt. Drilling also encountered highly magnetic ultramafic rocks (serpentinite), and mafic high-Mg tholeiitic rocks similar to those used to originally define the Dimboola Igneous Complex (see Section 3.2.5.7 -Dimboola Belt) interspersed with the intermediate volcanic and plutonic rocks.

The Brimpaen Belt was most recently investigated by two pre-competitive stratigraphic drill holes (STAVELY04 and STAVELY06), with the geochemical results (e.g. STAVELY06; Schofield et al., 2018 Section 2.6 – Geochemistry of the Stavely Arc) confirming a clear arc affiliation for intermediate igneous rocks within the succession. Drill hole STAVELY04 intersected an isotopically juvenile, high-Al basalt with a weakly-developed subduction signature (see Schofield et al., 2018 Section 2.6 – Geochemistry of the Stavely Arc). These results are consistent with the volcanic and intrusive rocks within the Brimpaen Belt belonging to the Mount Stavely Volcanic Complex.

The Brimpaen Belt appears to lie partly within the interior of the Jalur Rift, as it is bounded by the D4 Mosquito Creek and Escondida faults (Figure 3.22, see also Figure 3.9). Retrodeformation of D4 structures indicates that the Brimpaen Belt likely represents the northernmost segment of a D1a fault-belt that included the Grampians ‘Deeps’, Elliot and Bunnugal belts (see Section 4.1 – D4 and D3 retrodeformation testing). The Brimpaen Belt appears to be the northernmost of the volcanic belt segments that experienced clockwise rotation comparable to the Mafeking Megakink during D4. As for the Elliot Belt (overlain by the Mafeking Orocline) and the Grampians ‘Deeps’ Belt (overlain by the Big Cord Orocline), the Brimpaen Belt is overlain by the western flank of the Cranage Orocline (see Figure 3.73).

Retrodeformation of the clockwise rotations attributed to D4 show that the Brimpaen, Grampians ‘Deeps’ and Elliot belt segments may all have originated as a west-dipping D1a fault system. For this reason, the Brimpaen Belt is not directly correlated with the adjacent Dimboola Belt which, despite containing similar arc stratigraphy, appears to have had a primary east-dip during D1a and so most likely developed as a separate structure.

Figure 3.19 Tilt and band pass filtered aeromagnetic data in the Black Range, shaded to show the position of the Boonawah Belt, its flanks bound by the west-dipping Boonawah West and East faults and key drill hole locations. All other faults included in the STAVELY 3D model are also depicted. The Boonawah Belt apparently continues north beneath Grampians Group cover as the Grampians ‘West’ Belt – the boundary between the two belts is taken as the Cattle Camp Fault.

Figure 3.20 Brimpaen Belt highlighted by shading of a TMI (RTP) pseudocolour image, showing the positions of the Dollin Kink drag fold within the belt interior, and the interpreted position of the adjacent Dollin Fault. Sinistral displacement on the Dollin Fault may have induced the drag-folding, but this cannot be confirmed, because both ends of the Dollin Fault and of the Dollin Kink are now truncated at high angles by the Mosquito Creek Fault to the west and the Escondida Fault to the east. These relationships implying a D3 age for the Dollin Kink and Dollin Fault. The Brimpaen Belt extends south adjacent to and beneath the Marathon Fault footwall to underlie the Asses Ears Range and to be intersected in drill hole STAVELY04 (see Figure 3.21).

Figure 3.21 Oblique view of STAVELY 3D model showing the east-dipping Brimpaen Belt volume (green) bound on its west by the D3/D4/D5 Mosquito Creek Fault (colour-coded for depth), and on its south by ?subvertical Fault 8 and the northwest-dipping Victoria Valley South Fault, both interpreted as D4 structures. Surface trace positions of other volcanic belts are shown for reference. South of Brimpaen, and north of Fault 8, the southern part of the Brimpaen Belt lies at depth in the southeast-dipping Marathon Fault footwall (not depicted; see Figure 3.22), beneath thick Grampians Group cover in the Asses Ears Range – the thickness of Grampians Group here is constrained by gravity data.

3.2.5.5 Bunnugal Belt Magnetic data shows that the northern end of the 4 – 4.5 km wide Bunnugal Belt (belt 8; Figure 3.9) emerges from beneath the Grampians Ranges, where it has apparently been truncated at depth by the Victoria Valley South Fault, and trends south, passing between Strathmore, Glenthompson and Dunkeld and persisting south to near Caramut (Figure 3.23 and Figure 3.24). South of Caramut, the Bunnugal Belt cannot be clearly traced beneath thick cover comprising Newer Volcanic Group and Otway Basin, and may die out.

The Bunnugal Belt is flanked along its eastern and western sides by panels of Glenthompson Sandstone, except for a region between Laidlaw Road (approximately 10 km north of Glenthompson) and Yarram Park, where the eastern flank of the Bunnugal Belt appears to be fault-juxtaposed directly against the Stavely Belt, with the intervening panel of Glenthompson Sandstone locally omitted (Figure 3.25).

Figure 3.22 Oblique view parallel stereopair of the STAVELY 3D model, showing the east-dipping Brimpaen Belt volume (in green) with respect to the traced outlines of other volcanic belts of the Stavely Arc (in yellow, green, pink and red) and major D4 faults that bound the belt. The Brimpaen Belt occupies the northern part of the D4 Jalur Rift, bound on its west by the Mosquito Creek Fault (brown), on its east by the Escondida Fault (orange), and on its south by Fault 8 (pale green) and the Victoria Valley South Fault at depth beneath the Escondida Fault footwall (darker green; see also Figure 3.21). The distribution of the base of the overlying Grampians Group is shown by the pink surfaces – most of these surfaces are splays of the D4 Marathon Fault, confined to the uppermost crust. The Brimpaen Belt is faulted directly against Grampians Group across a Marathon Fault splay beneath the Asses Ears Range.

The Bunnugal Belt is virtually entirely buried beneath younger, highly magnetic basalt flows of the Newer Volcanic Group. This has meant modelling of the Cambrian volcanic belts beneath the Newer Volcanic Group has been problematic using magnetic data. Where the Bunnugal Belt is exposed west and north-west of Glenthompson, inversion modelling was carried out along several section lines. Modelling of magnetic stratigraphy within the Bunnugal Belt suggests it has a moderate to steep westerly dip overall. Figure 3.26 shows two example inversion profiles. Inversion line 2 (A) shows west dipping volcanic units within the eastern part of the belt, while further west higher amplitude and frequency anomalies are interpreted to be the product of Newer Volcanic Group, or more likely a combined response from the Newer Volcanic Group and the underlying Cambrian volcanic belt. Further north, inversion line 7 (B) traverse’s cleaner anomalies resulting from magnetic units within the volcanic belts that have been emplaced in the weakly magnetic Glenthompson Sandstone. Along this profile the Bunnugal Belt is offset by the dextral Yarrack Fault and associated splay faults, which strike obliquely to the belt (see Section 3.3.5.27 – Yarrack Fault). Inversion line 7 shows Bunnugal Belt units directly to the west of the fault appear to have undergone fault drag causing the units to become steepened and overturned in the vicinity of the fault. In plan view (see Figure 3.26B) deformation resulting from movement along the Yarrack Fault system is also evident as arcuate anomalies to the north of the fault (corresponding to shallower dipping units in profile).

Rubbly subcrop of basaltic rocks and mafic lapilli tuffs southwest of Glenthompson (Stuart-Smith & Black, 1999) are the only known exposures of igneous rocks in the Bunnugal Belt. A thickly-bedded metabasaltic lapilli tuff, with low-K tholeiitic metabasalt clasts up to 20 cm in a foliated matrix of biotite and actinolite crops out along Bushy Creek, about 1.4 km northeast of Scrubby Hill. This outcrop lies on the eastern periphery of the Bunnugal Belt and may be unrelated to it since additional sampling of these most easterly occurrences show they have the same geochemistry as the nearby (Chatsworth Basalt; Taylor et al., 2015). The Chatsworth Basalt is likely to predate eruption of the Mount Stavely Volcanic Complex (see Appendix 4 – Geological Units).

The main body of the Bunnugal Belt to the west does contain rocks of the Mount Stavely Volcanic Complex, intersected in a series of mineral exploration drill holes in the vicinity of Strathmore. Drill holes STAVRA 257-259 and STAVRA 589 intersect a succession of intermediate tuffs and brecciated porphyritic dacitic lavas (Horvath, 1994) that are comparable to the Towanway Tuff succession in the Stavely Belt. Other drill holes intersected intercalated fault slices of Glenthompson Sandstone incorporated within the belt interior (STAVRA 287, 604, 587-592) or failed to reach basement (STAVRA 646-652). Magnetic and gravity data suggests that considerable stratigraphic and structural complexity exists within the Bunnugal Belt, comparable to the adjacent Stavely Belt and the Dimboola Belt. Semi-circular regions of low gravity coincide with broad magnetic highs and suggest the possibility that late felsic intrusions occur here. Non-magnetic portions of the Bunnugal Belt are likely to be in-faulted metasediments as drilled and seen in adjacent volcanic belts, and/or are related to magnetite-destructive hydrothermal alteration.

Figure 3.23 The Bunnugal, Elliot, Grampians ’Deeps’ and Brimpaen belts, superimposed on a regional magnetic (RTP) image. These volcanic belts are interpreted to be D4 fault segments of a single, continuous, northerly-trending and west-dipping parent D1a fault belt (see Figure 4.13). The Elliot, Grampians ‘Deeps’ and Brimpaen belts experienced faulting, clockwise rotations and significant southerly translations during D4 (to form the Jalur Rift and Mafeking Megakink). The Bunnugal Belt apparently lay mostly outside the influence of D4, except where cut and offset across the Yarrack Fault (see Figure 3.68).

Figure 3.24 Oblique STAVELY 3D model view showing distribution of Bunnugal (dark green), Elliot, Grampians “Deeps’ and Brimpaen (pale green) bounding D1a faults (A) and volumes (B). These volcanic belts were likely all continuous within a single west-dipping D1a fault slice pre-D4, but are now locally reoriented to northeast dips as the Elliot Belt in the middle limb of the Mafeking Megakink (see also Figure 3.44 and Figure 3.46), and as the clockwise-rotated Grampians ‘Deeps’ and Brimpaen belts in the core of the Jalur Rift.

Figure 3.25 Oblique parallel stereopair of the STAVELY 3D model showing the Bunnugal and Stavely belt volumes to illustrate the point of convergence mapped between these belts north of Glenthompson. Other volcanic belt surface outlines shown for reference. At the point of convergence, the Bunnugal and Stavely belts are in direct fault-contact with intervening Glenthompson Sandstone strata absent. The footwall-propagation model for imbricate thrust systems predicts the Stavely West Fault to have formed later in D1a than the Bunnugal East Fault, so that local omission of Glenthompson Sandstone is most likely related to a localised lateral ramp-up of the Stavely West Fault, although a previous local down-cut of the Bunnugua East Fault cannot be discounted – note that regional potential field data show the Bunnugal and Stavely belts to diverge northwards beneath Grampians Group cover and towards the truncations of the Bunnugal and Stavely belts by the Victoria Valley South and Escondida faults respectively, so that the convergence is only a localised feature.

Although poorly exposed and sparsely drilled, the likely diversity of Bunnugal Belt stratigraphy can be inferred from the volcanic belts with which the Bunnugal Belt appears to have been continuous along-strike prior to D4 deformation; namely the Elliot and Brimpaen belts. The Grampians ‘Deeps’ Belt is also a potential segment correlate with the Bunnugal Belt, but is not exposed and lacks drilling (see Figure 3.23). The Elliot Belt is well exposed on Mount Elliot, with andesitic volcanic breccia similar to Fairview Andesite Breccia, including a massive mottled green feldspar- and hornblende-phyric andesite lava, and occasional feldspar porphyry dykes (Buckland, 1987). The Brimpaen Belt contains similar rocks, together with a diversity of other mafic and ultramafic stratigraphy and late porphyry stocks (e.g. STAVELY06). All these lithologies are likely to occur within the Bunnugal Belt.

The northern end of the Bunnugal Belt is truncated by the D4 Victoria Valley South Fault. The nature of the pre-D4 continuation of the Bunnugal Belt north of this fault as other volcanic belt segment(s) is dependent on developing an understanding of the D4 event.

The Grampians ’Deeps’ Belt segment occurs just to the north of the Victoria Valley South Fault, but this segment has a different dip-direction to the Bunnugal Belt precluding direct correlation. The Grampians ‘Deeps’ Belt segment is overlain by the Big Cord Orocline in Grampians Group, a D4 structure that testifies to an approximate 180° clockwise rotation of the underlying crust north of the Victoria Valley South Fault during D4 (see Figure 3.73). This rotation is not recorded in the Bunnugal Belt, or in directly overlying Grampians Group exposed in the Serra Range, further precluding direct correlation.

A better rationale for volcanic belt-segment correlation pre-D4 is to consider the west-dipping Bunnugal Belt segment in concert with the adjacent and structurally underlying west-dipping Stavely Belt segment, since both were formed during D1a and retain D1a relationships. As for the Bunnugal Belt, the northern end of the Stavely Belt is truncated beneath Grampians Group, but against the D4 Escondida Fault, a dextral strike-slip structure which occupies the western axis of the Mafeking Megakink/Orocline (see Section 4.1.1 – Mafeking Megakink retrodeformation). The position of the Escondida Fault within the megakink axis means that the pre-D4 northern continuation to the Stavely Belt now lies within the clockwise-rotated central limb of the Mafeking Megakink. Two volcanic belt segments occur in the central limb of the Mafeking Megakink – the Elliot and Narrapumelap belts.

The Elliot and Narrapumelap belts, together with intervening Glenthompson Sandstone strata, dip and face northeast overall within the central megakink limb. This means that the Narrapumelap Belt structurally underlies the Elliot Belt. Overprinting criteria indicates a D1 age for this relationship. It is this relationship that is the key for direct pre-D4 correlations with the Stavely and Bunnugal belts.

Because the Narrapumelap Belt occupies the lowest tectonostratigraphic position within the Mafeking Megakink central limb, it compares directly in position with the adjacent west-dipping Stavely Belt which occupies the lowest (= easternmost) tectonostratigraphic position in the D1a west-dipping imbricate thrust-stack remnant to the west of the megakink. The Bunnugal Belt segment occupies the next-highest tectonostratigraphic position to the Stavely Belt here, and so correlates with the Elliot Belt segment which occupies the next-highest tectonostratigraphic position to the Narrapumelap Belt. This is the rationale behind the correlation of the northern end of the Bunnugal Belt with the western end of the Elliot Belt across the position of the Victoria Valley South Fault pre-D4, and correlation of the northern end of the Stavely Belt segment with the western end of the Narrapumelap Belt across the position of the Escondida Fault pre-D4 (and correlation of the southern end of the Dryden Belt segment with the eastern end of the Narrapumelap Belt segment pre-D4).

Mafeking Megakink initiation early in D4 appears to have involved rift-rupture of the Bunnugal Belt from the Elliot Belt across the position of the Victoria Valley South Fault (see Figure 4.3 and Figure 4.4). The eastern end of the Elliot Belt appears to have been additionally internally ruptured into at least two additional segments in D4; the main Elliot Belt segment appears to have rotated together with the Narrapumelap Belt to form the middle limb of the Mafeking Megakink, while the additional segments appear to have rotated and translated separately within the core of the Jalur Rift to become the east-dipping ‘Grampians Deeps’ and Brimpaen belts. Subsequent dextral strike-slip offsets across the dextral Escondida and Golton faults further ruptured the kink axes, causing offset of the western end of the Narrapumelap Belt from the northern end of the Stavely Belt, and pulling the Elliot Belt well south of the northern end of the Stavely Belt, obscuring any simple correlation with the Bunnugal Belt. A regional-scale palinspastic restoration of the whole terrain reveals the likely correlation of the Bunnugal Belt with the Elliot Belt (see Section 4.1.1 – Mafeking Megakink retrodeformation).

Figure 3.26 A) Bunnugal Belt inversion line 2 showing west dipping magnetic units within the Bunnugal Belt (blue), and anomalies derived from Newer Volcanic Group (and potentially deeper sources) modelled by variably dipping bodies (green); B) Bunnugal Belt inversion line 7 showing predominantly west dipping magnetic units and the locations of Yarrack Fault system splays. The steep easterly dipping unit in the centre of the profile has been overturned by fault drag associated with dextral movement along the Yarrack Fault system. The insert shows a plan view of the line location and Yarrack Fault on RTP magnetics, highlighting the Cambrian volcanic belts. Top profile panels show first vertical derivative of TMI; middle panels show TMI; bottom panels show inversion bodies with assigned magnetic susceptibility values (SI). Pink line in TMI panel shows the regional. Note the difference in vertical exaggeration between profiles A and B. C) Location of inversion sections on TMI (RTP) – sections presented here are shown in white.

3.2.5.6 Caramut Belt

The Caramut Belt is buried entirely beneath Newer Volcanic Group and/or Otway Basin sediments, and only poorly imaged by geophysics as a localised northerly-trending magnetic and gravity high approximately 4.5 km east of Caramut, close to the southern margin of STAVELY. The Caramut Belt has not been drilled, but is assumed to comprise dense, magnetic igneous rocks in a D1a fault belt that we correlate with rocks of the Stavely Arc.

The Caramut Belt lies almost south along-strike, slightly offset to the west from the westernmost main sill of Chatsworth Basalt rocks that intrude Glenthompson Sandstone, between the Stavely and Bunnugal belts (Figure 3.27). The Chatsworth Basalt does not share any geochemical fingerprints of subduction-related melts, instead comprising MORB-related rocks interpreted to have a rift origin and is possibly unrelated to (older than) the Stavely Arc. However, sills of the Chatsworth Basalt (hosted by Glenthompson Sandstone) are narrow, quite unlike the broader volcanic belts of the Stavely Arc such as those in the adjacent Stavely and Bunnugal belts. Geophysics suggests that the Caramut Belt is approximately 2.5-4 km wide south of Caramut.

Figure 3.27 Caramut Belt highlighted by shading of 30 km high pass filtered Bouguer gravity pseudocolour image. The Caramut Belt is the most speculative of all the Stavely Arc volcanic belts, as it is buried beneath progressively thicker sequences of Otway Basin sediments towards the south. Consequently, the Caramut Belt displays a subdued magnetic response, except for its northern extent where an intrusive body is interpreted to have intruded the belt.

Figure 3.28 Oblique view of the STAVELY 3D model showing the Bunnugal and Caramut belt volumes in southern STAVELY, illustrating the interpretation of these belts as parallel, but separate, D1a fault-slices. The top of the Caramut Belt is modelled to plunge north beneath deformed Glenthompson Sandstone, while the base of the Bunnugal Belt also likely plunges north.

The northern end of the Caramut Belt peters out just north of Caramut. North of this position the bedrock lithology is dominated by faulted Glenthompson Sandstone. Similarly, the adjacent Bunnugal Belt cannot be traced in geophysics far south of the Minhamite-Caramut Road. Although the coincidence in position of the southern termination of the Bunnugal Belt with the northern termination of the Caramut Belt invites the interpretation of the Caramut Belt as a lateral offset of the southern Bunnugal Belt across a subsequent structure this seems unlikely because geophysics shows that the enclosing Boonawah and Stavely belts persist southwards beyond the southern boundary of STAVELY with no such evidence of significant lateral offset.

Instead, our preliminary interpretation of igneous rocks within the Caramut Belt is as a separate fault-slice of Stavely Arc hosted by a separate D1a fault system that loses relative displacement northwards, so that the volcanic belt within it plunges to depth with enclosing, possibly older, Glenthompson Sandstone (Figure 3.28). North of the point of plunge, Glenthompson Sandstone becomes the only lithology exposed north of Caramut – this area contains significant thrust faults of D1a age with potential to host rocks of the Stavely Arc (Cayley et al., in prep). The same (but opposite) scenario is invoked to explain the apparent southern termination of the Bunnugal Belt – continuation of the fault system that hosts the belt, but downward plunge of the igneous arc-strata within it. These are the geometries modelled in the STAVELY 3D model.

The Caramut Belt had not been interpreted when Schofield et al. (2018) was written, which is why it is not included in that report.

3.2.5.7 Dimboola Belt

The large Dimboola Belt (belt 12; Figure 3.9, Figure 3.29) was originally described as the Dimboola Igneous Complex, conceived to describe mafic, tholeiitic and boninitic rocks intersected in several mineral exploration drill holes beneath the southern margin of the Murray Basin (VandenBerg et al., 2000). In the STAVELY 3D model the Dimboola Belt (including rocks formerly included in the Dimboola Igneous Complex) modelled as a single distinct volume in the Escondida Fault hanging wall (Figure 3.30). Geochemically, these rocks share an affinity with Cambrian mafic volcanics in central Victoria and forearc ophiolites in western Tasmania, and were regarded as different to the calc-alkaline rocks within the Mount Stavely Volcanic Complex (Crawford et al., 1996; Cayley & Taylor, 2001). Subsequently, the Dimboola Belt magnetic body was attributed to the presence of a thick ‘seaward-dipping reflector package’ related to the Neoproterozoic rifted margin of southeastern Australia, based mostly on modelling of geophysical data (Direen & Crawford, 2003). The stratigraphy encountered in drill holes STAVELY09, STAVELY10 and STAVELY12, coupled with the higher vesicularity of most of the mafic volcanics in the belt, unlike central Victoria or western Tasmania, indicate that the Dimboola Belt includes rocks that correlate closely with the Mount Stavely Volcanic Complex and shares a shallower-water eruptive setting, and as such the Dimboola Belt is considered another volcanic belt (fault slice) of the Stavely Arc.

The Dimboola Belt varies from around 5.5 km wide in the vicinity of Laharum to approximately 10 km wide near Pimpinio, to over 20 km wide across-strike near Netherby, and over 30 km wide near Murrayville north of STAVELY. The Dimboola Belt has a strike length of approximately 250 km between Zumsteins and Murrayville, with a further 50 km of estimated strike length of shallowly buried, and possibly related, igneous bodies farther north again. The general non-magnetic nature of Murray Basin and Grampians Group cover rocks has allowed for magnetic inversion modelling of the Dimboola Belt to be undertaken. Modelling was focussed on the northern section of the Dimboola Belt within STAVELY (Figure 2.35) where arcuate anomalies form large semi-circular magnetic features, modelled as radially outward-dipping bodies (e.g. Figure 3.31A). Intrusive bodies were also interpreted and modelled in this section of the Dimboola Belt. North of Netherby, a more symmetrical, upright Dimboola Belt appears to be buried intermittently beneath a sub-horizontal veneer of Grampians Group (see Figure 2.24).

In the south the Dimboola Belt appears more coherent in magnetic data, with relatively consistent east dipping magnetic stratigraphy east of the Escondida Fault, which marks the western margin of the belt (Figure 3.31B and Figure 3.31C). Here, between the Grampians Ranges and Netherby, the eastern flank of the Dimboola Belt, east dipping in the hangingwall of the Escondida Fault, appears to be buried beneath a wedge of Grampians Group. Increasing depths to modelled magnetic bodies towards the eastern margin of the belt, together with an increasingly subdued, lower frequency magnetic response in gridded data suggests a thickening of Grampians Group toward the eastern boundary of the Dimboola Belt, and further east to the Golton Fault forming the western margin of the west dipping Dryden Belt (Figure 3.31B and Figure 3.31C). The overall east-dip of the Dimboola Belt here is also imaged in deep seismic reflection line 97GA-V1.

Magnetic patterns for the Dimboola Belt show a thick northwest-striking sequence of banded igneous tectonostratigraphy partially disrupted by a series of northerly-trending en-échelon faults (Figure 3.29). Despite these later faults, magnetic patterns are very coherent and strike-persistent, allowing tectonostratigraphy to be extrapolated for a considerable distance from the limited drill hole constraints. Magnetic stratigraphy in the southern Dimboola Belt and the Escondida Fault hangingwall has a steep but consistent east-dipping geometry. At a broad scale, the tectonostratigraphy consists of basal ultramafic rocks (intersected in drill hole STAVELY10) passing upwards (eastwards) into geochemically depleted volcanic and intrusive, including tholeiitic rocks (drill holes STAVELY09; SV10011; Thompson, 2014), and eventually into andesite and dacite (drill holes VIMP8; STAVELY07, STAVELY12; DD89HS1234-1237, Allnut & Weber, 1989). Although the transition is complex and fault-disrupted, with andesitic volcanics reported from drill holes west and east of drill hole STAVELY10 (e.g. mineral exploration drill holes SV10009 and SV10010; Thompson, 2014), the overall trend is consistent with the idea of an intermediate, calc-alkaline, subduction-related succession (drill holes STAVELY12, DD89HS1234-1237) superimposed over a pre-existing mafic rift or incipient (juvenile) arc margin succession. The geochemical evolution of the Dimboola Belt is discussed further in Schofield et al. (2018) (Section 2.6 – Geochemistry of the Stavely Arc).

The rocks intersected in drill hole VIMP8 mark the transition, as they lie towards the base of the (magnetic) Stavely Arc stratigraphy. Rocks intersected in VIMP8 comprise low-Ti andesite and dacite geochemically similar to the basal rocks at Mount Dryden (Maher et al., 1997; see also Schofield et al., 2018 Section 2.6 – Geochemistry of the Stavely Arc). Arc-related intermediate volcanic rocks intersected in drill hole STAVELY12 are directly comparable to rocks of the Mount Stavely Volcanic Complex described in the other volcanic belts. Thus, there are key similarities between the overall stratigraphic succession and geochemical evolution seen in the Dimboola, Dryden, Stavely, Black Range and Brimpaen belts (STAVELY04 and STAVELY06; see O’Neill, 1994; Cayley & Taylor, 1997a). These results show that the Dimboola Belt shares a common diversity of rocks with the Mount Stavely Volcanic Complex. Given the lithological and geochemical and textural similarities with other volcanic belts, the Dimboola Igneous Complex has been subsumed as the junior synonym. The region coincident with the large magnetic high beneath the Murray Basin is referred to as the Dimboola Belt.

The Dimboola Belt contains the most internal structural complexity of the volcanic belts. The Dimboola Belt apparently transitions from an apparent east-dipping fault belt in the south (bound along its western flank by the Escondida Fault; Cayley & Taylor, 1996, 1997a; Moore, 1996a, 1996b; Korsch et al., 2002), to a broad, more symmetrical body farther north. The large, semi-circular magnetic features north of Netherby may represent para-autochthonous igneous rocks of the Stavely Arc edifice below the Grampians Group and/or Murray Basin (see Section 4.4.1 – Original configuration of the Stavely Arc).

Although the northern end of the Dimboola Belt is large and wide, the simplest initial working assumption for the STAVELY 3D model – that these rocks might represent a near-surface continuation to the deeply buried arc edifice imaged in deep seismic reflection data in the south of STAVELY– was impossible to sustain geometrically during model construction. This is because it is clear from mapping and from deep seismic reflection and potential field data that the buried arc edifice imaged in the south of STAVELY lies in a footwall position east of the west-dipping Stavely and Dryden belts, whereas the Dimboola Belt and associated rocks imaged in the north of STAVELY are located in a hangingwall position west of the Dryden and Dryden North belts (see Appendix 2 – Geological cross sections).

Because of this key constraint, any northern continuation of the buried edifice imaged in the south of STAVELY must continue to lie in a footwall position to the east of the Dryden North Belt. The Dimboola Belt, including its wide, divergently dipping northern end, occupies a hangingwall position to the west of the Dryden and Dryden North belts, and may simply represent a backthrust to that strike-persistent structure.

Retrodeformation of the D4 fault network places the Dimboola Belt nearly directly north along-strike from the Brimpaen, Grampians ‘Deeps’, Elliot and Bunnugal belts farther south, and this is interpreted to be the position of the Dimboola Belt when it first formed during D1a. However, the primary (D1a) east-dip apparent the Dimboola Belt contrasts with the primary (D1a) west-dip apparent for the other volcanic belts, and this strongly implies that they are not direct correlates within a single structural system. Instead, we interpret the Dimboola Belt as a backthrust-slice of Stavely Arc stratigraphy that is associated with the Dryden and Dryden North belts, whereas the Brimpaen-Grampians ’Deeps’-Elliot-Bunnugal belts are interpreted as a separate west-dipping fault slice of material that probably developed slightly earlier during D1a thrust fault imbrication. Due to their opposite structural vergence and similar age, the changes from predominantly easterly dips in the Dimboola Belt to predominantly westerly dips in the (restored) Elliot and Bunnugal belts suggest a complex transition across a transform or some other accommodating structure, now fully concealed beneath the Grampians Ranges. This is discussed in Section 4.4.3 – Understanding the form and distribution of potential transfer structures.

Figure 3.29 Tilt and band pass filter image of TMI data, shaded to highlight the Dimboola Belt. Note the apparently east-dipping Escondida Fault forms the sharp western boundary to the southern Dimboola Belt, but appears to pass into the interior of the belt near Nhill, north of which the Dimboola Belt has a more symmetrical appearance, dipping west along its western flank. Key town locations are shown for reference, as are all faults included in the STAVELY 3D model. North- and northeast-trending en-échelon faults within the interior of the Dimboola Belt are part of the Dimboola Duplex, a D4-fault system that also deforms Grampians Group cover. Key stratigraphic and mineral exploration drill holes discussed in the text are also included.

Figure 3.30 Oblique view of the STAVELY 3D model showing the extent of the upper part of the Dimboola Belt volume, confined to the Escondida Fault hangingwall. The Dimboola Belt was likely east-dipping and east-facing pre-D4, a geometry that sets it distinctly apart from other volcanic belts of the Stavely Arc. Other parts of the Dimboola Belt lie offset at depth in the Escondida Fault footwall (see Figure 3.10). The Dimboola Belt continues north beyond the limit of STAVELY (see Figure 2.24). The southern end of the belt is abruptly truncated against the Jalur Fault (see Figure 2.32)

Figure 3.31 A) Dimboola Belt inversion line 2, located in the northern section of the Dimboola Belt in STAVELY. Modelling shows west dipping bodies in the western half of the belt, while east dipping bodies are modelled in the eastern parts of the belt. The Escondida Fault dextrally off-sets the western extremity of the belt into the plane of the section; B) Dimboola Belt inversion line 4 showing predominantly east dipping bodies within the central portion of the Dimboola Belt in STAVELY. The position of the Escondida and Golton faults are also shown, with the west dipping Dryden Belt modelled east of the latter. The olive-green body at the western end of the section is interpreted as an intrusive body intruding the Kanmantoo Group; C) Dimboola Belt inversion line 5 showing predominantly east dipping bodies within the southern portion of the Dimboola Belt in STAVELY. The position of the Escondida and Golton faults are also shown, with the west dipping Dryden Belt modelled east of the latter. Note the bodies intersecting the Escondida Fault in Dimboola Belt inversion lines 2 (A) and 4 (B) are not interpreted as post-dating movement on the fault but are assumed to be truncated at the fault surface. Top profile panels show first vertical derivative of TMI; middle panels show TMI; bottom panels show inversion bodies with assigned magnetic susceptibility values (SI). Pink line in TMI panel shows the regional. D) Location of inversion sections on TMI (RTP) – sections presented here are shown in white.

3.2.5.8 Dryden Belt

The Dryden Belt (belt 3; Figure 3.9 and Figure 2.32) is a northerly-trending belt of Mount Stavely Volcanic Complex rocks that crop out as a series of separate low hills that expose different stratigraphic levels of a low metamorphic grade Cambrian-aged andesite-dacite-rhyolite igneous succession. The Dryden Belt contains a thick, coherent succession that comprises a variety of interlayered magnetic (locally pillowed) basalt and andesite, and less magnetic dacite and rhyolite lava flows, subaqueous volcanic breccia, subaqueous volcanic sandstone, thick sills and dykes of andesitic composition with prominent cooling-columns and diorite/gabbro. The main outcrops are, from north to south Lake Lonsdale/Mount Asler (exposed as a separate fault block – see Section 4.1.5 – Retrodeformation of locally overturned Cambrian strata in the Moyston Fault footwall), Mount Dryden, Fyans, McMurtrie Hill, Bellellen, Jallukar, Barton and, much farther south, the isolated exposure of Lake Bolac (Ramsay, 1981; Buckland, 1981; Buckland, 1987; Cayley & Taylor, 2001).

Continuity of volcanic stratigraphy between these outcrops is demonstrated by magnetic and gravity data (Figure 2.28) and by mapping, where distinct intermediate volcanic and intrusive packages can be defined and traced (Buckland, 1987; McDonald & Whitehead, 1994; Cayley & Taylor, 2000a, 2001). Gravity, magnetic, deep seismic reflection data and mapping constraints show the Dryden Belt to be fault-bounded on both flanks near-surface (Cayley & Taylor, 2000a, 2001; Korsch et al., 2002). Magnetic data shows the width of the Dryden Belt to vary between 4 km (e.g. at Mount Dryden and Barton) and 1.5-1.8 km (e.g. Bellellen and Jallukar). The Dryden Belt has a strike length of approximately 115 km.

Although quite narrow, the Dryden Belt is remarkably strike-persistent. The Dryden Belt, and others directly related to it (Dryden North Belt, Hindmarsh Belt, Jeparit Belt), collectively extend from near the southern edge to well beyond the northern edge of STAVELY. Geophysical data and mineral exploration drilling constrain the buried continuation of the Dryden Belt to the north of Mount Dryden – the isolated location of the distinctive magnetic and gravity high associated with the Dryden Belt allows for a confident interpretation into the Dryden North Belt beneath the Murray Basin, as far north as Antwerp and beyond. Geophysics and mineral exploration drilling show that parts of the Dryden Belt may have been imbricated across a series of strike-slip faults in the vicinity of Antwerp, Glenlee and Hindmarsh. Although combined into a single entity in the STAVELY 3D model, these volcanic belt segments are described separately as the Hindmarsh and Jeparit belts in this section.

Geophysical data do not constrain any buried continuation of the Dryden Belt south of Woorndoo, the interpreted position of its intersection with the Escondida Fault, either because the south end of the belt is truncated by the Escondida and/or Golton faults, with its continuation now represented by the Narrapumelap (and Stavely) belts – our preferred interpretation; or because any southern extension of the Dryden Belt is just difficult to discriminate beneath magnetic Newer Volcanic Group cover. Further south drill hole STAVELY01 intersected metamorphic rocks related to the Moornambool Metamorphic Complex.

The Dryden Belt is a very important volcanic belt for understanding the Mount Stavely Volcanic Complex because it contains the best and most coherent exposures of the Cambrian igneous stratigraphy. These exposures show that stratigraphy in the Dryden Belt faces consistently westward. Facing is indicated by grading in marine volcanic sandstones interbedded with volcanic lava and breccia (Figure 2.19). Direct measurements of outcrop, and inversion modelling of magnetic stratigraphy completed approximately every 20 to 30 km along the length of the Dryden Belt (Figure 3.33) show that westerly dips are also predominant, except in close proximity to the Moyston Fault where the volcanic stratigraphy is locally overturned to dip east (Cayley & Taylor, 2001; Figure 3.33B).

The best outcrops of the Dryden Belt at Mount Dryden, McMurtrie Hill and Jallukar show a change from low-Ti boninite-like mafic-intermediate basaltic andesite at Mount Dryden (Tolliday, 1978; Crawford, 1982) to calc-alkaline intermediate-felsic dacite and rhyolite with strong continental affinity at Jallukar (Ramsay 1986). This range of rock types encompasses the full compositional variation seen in the Mount Stavely Volcanic Complex.

The lowest stratigraphy in the Dryden Belt comprises ultramafic rocks at the Fryingpan Prospect east of Bellellen, adjacent to the Moyston Fault footwall (Cayley & Taylor, 2001). These rocks do not crop out, but are intersected in mineral exploration drill holes (e.g. CRAE drill hole DD94 AA150). These rocks consist of olivine and chromite-bearing ultramafic to mafic glassy lavas with a low-Ti, high-Mg boninitic composition (Crawford, 1994) and are comparable to mafic, boninite-like low-Ti andesites and dacites of the Mount Dryden succession, and may underlie them stratigraphically (Cayley & Taylor, 2001). Sediments associated with the igneous rocks are fossiliferous (acritarch’s, Radiolaria), and are characteristic of pelagic sediments accumulating in deep marine, sediment-starved environments. These sediments therefore probably pre-date the onset of local magmatism. Mafic (basaltic) rocks at Mount Asler, northeast of Mount Dryden, are also likely to represent a lower part of the Dryden Belt stratigraphy (Cayley & Taylor, 2001; see Section 4.1.5 – Retrodeformation of locally overturned Cambrian strata in the Moyston Fault footwall).

Figure 3.32 The Stavely, Narrapumelap, Dryden and Dryden North belts, superimposed on a regional magnetic (RTP) image, including the locations of key outcrops and stratigraphy of the Dryden Belt and major towns. All these volcanic belts are interpreted to be D4 fault segments of an originally continuous, single, northerly-trending and west-dipping D1a fault slice (see Figure 4.3 and Figure 4.4). The Stavely Belt apparently lay mostly outside the influence of D4. The Narrapumelap Belt experienced clockwise kink-rotation and fault-offset in the Mafeking Megakink interior. The Dryden and Dryden North belts experienced significant southerly D4 translations relative to the Stavely Belt.

Figure 3.33 A) Dryden Belt inversion line 3 showing west dipping volcanic units within the Dryden Belt; B) Dryden Belt inversion line 7 showing volcanic units of the Dryden Belt – western most unit dipping to the west (purple colour) while the eastern unit proximal to the Moyston Fault is overturned and dipping to the east (green colour). The Moyston Fault position is shown with magnetic bodies in the hanging wall (blue colour); C) Dryden Belt inversion line 10 showing west dipping Dryden Belt volcanic units; Top profile panels show first vertical derivative of TMI; middle panels show TMI; bottom panels show inversion bodies with assigned magnetic susceptibility values (SI). Pink line in TMI panel shows the regional. D) Location of inversion sections on TMI (RTP) – sections presented here are shown in white.

Andesite and dacite breccia is common in the stratigraphic succession exposed at Mount Dryden (Tolliday, 1978; Crawford, 1982; Buckland, 1987; Figure 3.34A). Low-Ti andesitic lava occurs in flow and/or sill packages up to 200 m thick, locally columnar jointed (Figure 3.34C). These flows are interspersed with volcaniclastic breccia horizons up to 100 m thick. The breccia comprises angular clasts of lava up to 50 cm in size set in a poorly-sorted crystal-rich sandy groundmass (Figure 3.34B). Crawford (1988) reported a conformable ‘transition’ up into turbiditic metasediments (of the Glenthompson Sandstone), however Cayley & Taylor (2001) regarded the sediments found in the west of the Dryden Belt as volcanogenic and an integral part of the igneous succession. A change westward to Glenthompson Sandstone does occur but is well west of Mount Dryden, is not exposed, and appears faulted in magnetic and deep seismic reflection data (Korsch et al., 2002), with an east-dipping orientation that is in marked contrast to bedding – this is the interpreted position of the Mehuse Fault.

Rhyodacite and dacite are the predominant lithologies exposed further south at McMurtrie Hill and Jallukar, and rhyolite also occurs locally (Figure 3.35). These show similar geochemical compositions and identical rare earth element patterns to rocks of the Mount Stavely Volcanic Complex in the Stavely Belt (Schofield et al., 2018). These rocks are also described in Buckland (1987) and Cayley & Taylor (2001).

The cross-sectional shape of the Dryden Belt is complex. Although strata exposed within the Dryden Belt is consistently west-facing and generally west-dipping, the upper few kilometres of the belt apparently has a keel-like configuration, caused by the down-dip convergence of the bounding Dryden and Mehuse (and Golton) faults. The east-dip of the Mehuse Fault, and the keel-like geometry of the portion of the Dryden Belt above it, is constrained by regional magnetic and gravity data (Cayley & Taylor, 2001), and by deep seismic reflection data (Korsch et al., 2002). This relationship appears to be strike-persistent. Overprinting criteria indicate that the east-dipping Mehuse and Golton faults are substantially younger than the D1a deformation that was responsible for the initial emplacement of the volcanic belts, including the Dryden Belt. This implies that the current east-dipping configuration of the western flank of the Dryden Belt is a consequence of subsequent deformations, and is not a primary feature. This explains the contradiction of the present-day east-dip of the western bounding fault compared to the west-facing and predominant west-dip of the Mount Stavely Volcanic Complex stratigraphy within the Dryden Belt.

Figure 3.34 Mount Stavely Volcanic Complex – Dryden Belt. A) Looking north from Mount Dryden summit, with tors of clinopyroxene and plagioclase-phyric andesite lava in the foreground. The Mount Difficult Range dominates the background. B) Andesite breccia exposed low on the northern flank of Mount Dryden. This rock is morphologically similar to Fairview Andesite Breccia, but comprises low-Ti andesite. Note darker (glassy) margins on some clasts, consistent with the chilled margins of pillow-basalts, and consistent with the subaqueous origin also indicated by interbedded volcanic sediments. C) Tilted cooling columns in a sill-like body of low-Ti Andesite, high on the northern flank of Mount Dryden. The columns plunge gently northeast, consistent with a moderate to steep southwest dip to the sill, and to the enclosing volcanic stratigraphy. David Taylor for scale.

Regional geometrical constraints show that the Golton Fault is a D4 structure that experienced dextral transtension, whereas the Mehuse Fault was a D3 fault that experienced sinistral transpression. Both apparently link east into the Moyston Fault footwall. The most likely geometrical solution of this combination of displacements is that early overthrusting of the upper part of the Dryden Belt along the Mehuse Fault is responsible for the current observed configuration. In such a scenario, a west-dipping down-dip continuation to the Dryden Belt, complete with west-dipping bounding D1a thrust faults, is expected to continue to depth below the Mehuse and Golton faults. This is precisely analogous to the geometry proven by mineral exploration drilling and underground mining at Stawell for the Cambrian ‘Magdala Antiform’ displaced across the younger, east-dipping South Fault (Miller & Wilson, 2004), but at a much larger scale.

With displacements of the Dryden Belt across the Mehuse and Golton faults taken into account, the overall geometry of the Dryden Belt appears very similar to the Stavely Belt, with which the Dryden Belt is interpreted to have been continuous, pre-D4 deformation. This is the overall geometry captured in the STAVELY 3D model.

The demonstration of an along-strike physical continuity between the Dryden Belt and the Stavely Belt via the Narrapumelap Belt prior to the D4 deformation and formation of the Mafeking Megakink, has helped resolve the relationships between these different volcanic belts, and between the volcanic belts and rocks of the Dimboola Belt (formerly included into the separate Dimboola Igneous Complex). New geochemistry and stratigraphic drilling data strengthens the interpretation that all the volcanic belts exposed within the Grampians-Stavely Zone are parts of a single magmatic complex, the Mount Stavely Volcanic Complex (Schofield et al., 2018).

The realisation that the Narrapumelap Belt represents a D4 refolded segment of the D1a Stavely and Dryden belts means that the listric dip-profiles constrained for the Narrapumelap Belt and intervening Cambrian sequences within the central limb of the Mafeking Megakink (see Section 3.2.5.14 – Narrapumelap Belt) can be extrapolated around the position of the eastern Mafeking Megakink hinge (the position of the Golton Fault) to constrain a similarly listric profile for the Dryden Belt and the D1a thrust faults that bound it, prior to internal deformation of this belt across subsequent structures such as the Mehuse and Golton faults.

Projection of these constraints means that most of the west-dipping D1a structures mapped within the Cambrian bedrock across the width of the Grampians-Stavely Zone can be demonstrated to have listric profiles at the crustal scale, all shallowing in dip magnitude to depth. This is consistent with the constraints provided by available deep seismic reflection data across the width of the zone, and forms a key premise that controls the construction of the serial full crustal-scale cross sections used to construct the STAVELY 3D model, including the volumes.

Figure 3.35 Mount Stavely Volcanic Complex – Dryden Belt rhyolite A) Thin section (cross-polarised light, field of view 18 x 12 mm) of rhyolite from the Dryden Belt near Barton. Note rounded quartz phenocrysts and some feldspar, set in a fine-grained chlorite-albite-quartz groundmass. Much of the quartz in the groundmass may be secondary. B) Detail of bottom-right portion of (A), (cross polarised light, field of view 7 x 4.2 mm) showing rounded quartz phenocrysts, euhedral hornblende and plagioclase (albite) phenocrysts, and including an angular feldspathic granitic fragment incorporated into the lava (at top-right).

3.2.5.9 Dryden North Belt

The Dryden North Belt (belts 4, 5, 6; Figure 3.9 and Figure 2.29) is defined as the semi-continuous strike-extension to the Dryden Belt that extends from a prominent late fault-disruption across the belt approximately 9 km northwest of Dadswells Bridge north to Antwerp. The series of subparallel volcanic belts buried beneath the Murray Basin that extend from Antwerp, Glenlee, Hindmarsh to Kumbrunin and beneath Big Desert are all interpreted as fault-repeats of the Dryden North Belt, strike-slip imbricated across a number of D4 faults. These buried volcanic belts, the Hindmarsh and Jeparit belts (Figure 3.36), are depicted and named separately on plan maps, but are all incorporated into a single Dryden North Belt volume in the STAVELY 3D model.

Figure 3.36 Tilt and band pass filtered magnetics pseudocolour image shaded to depict the Jeparit Belt (see Figure 3.9), a belt segment that is included in the Dryden North Belt in the STAVELY 3D model. Apart from D4 complexity at its northernmost end, the Dryden North Belt has a reasonably consistent width of approximately 2.5-3 km between Dadswells Bridge and Antwerp and a strike-length of approximately 85 km. The distinct character and isolation of the magnetic high associated with these rocks allows for its confident interpretation, even though the northern extension of this belt is entirely concealed beneath the Murray Basin.

As for the Dryden Belt, regional magnetic and gravity data over the Dryden North Belt indicate a predominant steep west-dip for internal strata along the belt, but a keel-shape in cross section for its upper few kilometres. As for the Dryden Belt, we interpret this keel-shape to be caused by down-dip convergence of the bounding west-dipping Dryden and east-dipping Mehuse (and Golton) faults, with the upper part of the Dryden North Belt offset across the younger east-dipping Mehuse and Golton faults. A thicker west-dipping D1a Dryden North Belt is interpreted to continue to depth in the footwall of these faults.

Seven kilometres north of the southern end of the Dryden North Belt, drill hole VIMP9 intersected a succession of red and grey shale, siltstone and tholeiitic basalt within the belt (Maher et al., 1997). Possible hyaloclastite textures and peperitic contacts between the basalt and the sediments suggest the eruption of the volcanics into a juvenile seafloor sediment pile. These rocks have direct along-strike correlates in the basaltic rocks exposed as the lowest levels of the stratigraphy along the eastern side of the Dryden Belt and at Mount Asler.

Five kilometres southeast of Antwerp, drill hole VIMP6 intersected serpentinised harzbergite (Maher et al., 1997) within the interior of the Dryden North Belt. The abundant magnetite within this rock gives the rock a high magnetic susceptibility, the cause of the prominent magnetic high within the belt interior here. The abundant high-Cr chrome spinel within this serpentinite are distinctive, and support direct correlation with Williamsons Road Serpentinite correlates (as fault slices) in the Dryden Belt (e.g. Fryingpan Prospect), within the interior of the Stavely Belt (see Bailey et al., 2016), that within the intervening Narrapumelap Belt.

Approximately 10 km west northwest from Antwerp, drill hole STAVELY16 intersected a calc-alkaline andesitic to dacitic volcanic breccia in the southern end of the 2.1 km wide Hindmarsh Belt, interpreted to be a D4 strike-slip imbrication of the Dryden North Belt (belt 5; Figure 3.9). This segment of the Dryden North Belt is separated from the main Dryden North Belt by the Babatchio and Tullyvea faults, interpreted to have been active in D4. The Hindmarsh Belt flanks the Dimboola Belt, separated by a fault that may correlate with the Golton Fault.

The rocks intersected in drill hole STAVELY16 are strongly reminiscent of Mount Stavely Volcanic Complex stratigraphy in the Stavely and Dryden belts, supporting the idea that the Dryden North Belt and the Hindmarsh and Jeparit belts all represent fault slices of a common Stavely Arc stratigraphy that persists largely unchanged along almost the entire length of the Grampians-Stavely Zone. The rocks intersected in STAVELY16 can be traced a further 44 km north in regional magnetic and gravity data, to a point where the belt appears to pinch out beneath the Big Desert.

3.2.5.10 Elliot Belt

Magnetic data shows the Elliot Belt (belt 9; Figure 3.9, Figure 3.23) to strike northwest-southeast. The Elliot Belt is truncated by the Escondida Fault at an oblique angle against the eastern flank of the Stavely Belt near Yarram Park, and truncated by the Golton Fault at an oblique angle against the western flank of the Dryden Belt southeast of Willaura (Figure 3.9). Between these bounding structures the Elliot Belt has a strike length of about 26 km, and is approximately 5.5 km wide. The Elliot Belt dips moderately northeast, a geometry constrained by qualitative interpretations of magnetic data, and by the position of the belt where it is imaged as prominent reflectors in deep seismic reflection data a short distance north of its position at surface (Cayley et al., 2011b). This dip magnitude is considered a proxy for the D1a faults that bound the Elliot Belt.

The listric profile calculated for the Narrapumelap Belt where imaged in deep seismic reflection line 09GA-AR1 (Figure 2.22) is conferred, via the intervening panel of unreflective Glenthompson Sandstone, into the Elliot Belt. These constraints indicate a similarly listric profile for the D1a Elliot South and Elliot North faults, constraints that helped constrain the 3D modelling of all the D1a elements that together comprise the central limb of the Mafeking Megakink.

Lithologies in the Elliot Belt are known from a single outcrop on Mount Elliot (Figure 3.37A), and from a limited series of mineral exploration drill holes that span the width and length of the belt. Exposed on Mount Elliot is an andesitic volcanic breccia very similar to the Fairview Andesite Breccia in the Stavely Belt (Buckland, 1987). Other rocks exposed include massive andesite lava flows comparable to those interbedded within the Fairview Andesite Breccia (Figure 3.37B) and thin feldspar-quartz porphyry dykes of unknown age or affiliation that intrude the succession (Buckland, 1987).

Mineral exploration drill holes in the south side of the Elliot Belt (e.g. STAVRA 220-230) intersected a suite of porphyritic trachyte and dacite lavas and intercalated tuffs and other igneous rocks that we correlate with the Narrapumelap Road Dacite Member and Towanway Tuff (Buckland, 1987), and with the Fairview Andesite Breccia (e.g. STAVRA 582-583) and Lalkaldarno Porphyry (STAVRA 581). Drill holes along the north side of the Elliot Belt intersect tuffs, rhyolite and intrusive igneous rocks reminiscent of the Narrapumelap Road, Nanapundah Tuff, and Lalkaldarno Porphyry, including occasional black carbonaceous shale (e.g. STAVRA 230-232) that may be equivalent to the Glenronald Shale Member of the Stavely Belt (Buckland, 1987). Correlatives of nearly all the stratigraphy formalised within the Stavely Belt have thus been encountered within the Elliot Belt.

The Elliot Belt is regarded as a D4 fault-offset from the northern end of the Bunnugal Belt and of the buried Grampians ‘Deeps’ Belt, as described in Section 3.2.5.5 – Bunnugal Belt. Therefore, the lithologies within the Elliot Belt can be used to constrain the range of lithologies expected within these other belts. The listric geometry established for the Elliot Belt at depth can also be projected around the position of the western Mafeking Megakink hinge (the Escondida Fault) to constrain the subsurface geometry of the Bunnugal Belt.

Figure 3.37 Mount Stavely Volcanic Complex – Elliot Belt. A: Tors of andesite lava and breccia exposed on the summit of Mount Elliot (MGA 54 647121 5849678). The Grampians Ranges form the skyline, dominated by Mount William and the Major Mitchell Plateau. B: Thin section of totally undeformed, virtually unaltered albite and pyroxene-phyric andesite lava (original glassy groundmass now devitrified to dark spherulitic micro-crystalline material) at Mount Elliot (cross-polarised light, field of view 18 x 12 mm).

3.2.5.11 Glenisla Belt

The Glenisla Belt (belt 16; Figure 3.9) is a northerly-trending belt of Cambrian igneous rocks that extends from Glenisla in the south to the vicinity of Brimpaen-Laharum Road in the north, a strike length of approximately 20 km. The belt passes just east of Cherrypool. Most of the belt is buried beneath the Murray Basin, but the medium- to high-magnetic susceptibility of igneous rocks within it make its position beneath this cover easy to interpret in regional magnetic data (Figure 3.12). A portion of the Glenisla Belt between the Henty Highway and the northern reaches of the Rocklands Reservoir subcrops discontinuously as weathered scattered rubble. Although very poorly exposed, the range of rock types within this part of the Glenisla Belt are reasonably well known due to a reconnaissance mineral exploration drilling programme undertaken by CRAE in the 1980s to 1990s (e.g. Weber, 1989; Radojkovic, 1996). In addition to a series of RAB and aircore transects across the southern part of the Glenisla Belt, several diamond and percussion drill holes penetrate the succession. The parts of the Glenisla Belt east and north of Cherrypool remain poorly known.

Drilling across the width and much of the length of the Glenisla Belt shows it to comprise fine-grained basaltic to (altered) andesitic lava, volcanic sandstone and volcanic breccia, thick sequences of tuffaceous sandstone, pyroclastic sediments with shard and lithic fragments, and in-faulted slices of psammitic and pelitic metasedimentary rocks. The largest known sliver of exotic metasedimentary rocks is intersected in RAB traverse lines MB4 to MB8. Minor constituent rock types include porphyritic quartz microgabbro and quartz dolerite, and thin fault-slices of exotic ultramafic rocks (serpentinite and hornblende pyroxenite). The most mafic rocks appear to lie mostly along the eastern side of the Glenisla Belt. Inversion modelling of magnetic anomalies associated with the Glenisla Belt suggest that it dips steeply west (Figure 3.38), a geometry corroborated by west-dipping cleavage in diamond drill holes into the belt (Cayley & Taylor, 1997a). The range of lithologies encountered by mineral exploration drilling to date are a close match for the succession seen in the adjacent Black Range and Tyar belts, and for the stratigraphy formalised as the Mount Stavely Volcanic Complex. Complex magnetic highs and lows within parts of this part of the Glenisla Belt are interpreted to reflect demagnetisation caused by magnetite-destructive hydrothermal alteration, and/or the influence of small felsic intrusions into the succession. Few of these magnetic highs and lows have been drill tested.

The Glenisla Belt is structurally complex, but appears to be bounded on its flanks by west-dipping D1a faults of the Muline Fault System. The northern end of the Glenisla Belt (and of the Muline Fault System) is obliquely truncated by a younger fault approximately 2.5 km southwest of Brimpaen (Figure 3.39 and Figure 3.43). Rocks north and west of this fault are metasediments, probably Glenthompson Sandstone correlatives. Cayley & Taylor (1997a) attributed this truncation to a splay of the Mosquito Creek Fault. Recent high resolution (50 m line spacing) regional magnetic data in the region shows that this truncation is caused by the north-northeasterly-trending Cherrypool Fault, the location of which was slightly adjusted using the new data (Figure 3.39). The southern end of the Glenisla Belt (and Muline Fault System) is intruded by a granite buried beneath the Rocklands Volcanic Group, 1 km south of Glenisla. The Henty Fault also strikes into this vicinity. Drilling and magnetic data show the Glenisla Belt to vary in width from approximately 1 km to 2.5 km, probably due to second-order faulting within it, some of which is depicted in the STAVELY 3D model as an example of the structural complexity seen in many of the volcanic belts (Figure 3.40).

Figure 3.38 Example of inversion modelling of magnetic data over the Glenisla Belt. A) Selected profiles over the Glenisla Belt and section locations on detailed 1VD TMI data, highlighting the complex magnetic response associated with this belt. Top profile panels show first vertical derivative of TMI; middle panels show TMI; bottom panels show inversion bodies. C) Location of all Stavely inversion sections on TMI (RTP), with detailed area (A) outlined.

Figure 3.39 Tilt filtered TMI pseudocolour image of 50m line-spacing aeromagnetic data over the Black Range and Glenisla belts (data courtesy of Navarre Minerals Ltd), showing the internal structural complexity of the Glenisla Belt in particular (see also Figure 3.40). The high-quality data discriminates shallow paleochannels with concentrations of magnetic mineral grains, and the Cherrypool Fault position cutting through Glenthompson Group and Nargoon Group metasediments. The reinterpretation of the Cherrypool Fault position allowed correlation of the truncated southern end of the Black Range Belt with the truncated northern end of the Glenisla Belt, offset sinistrally across the Cherrypool Fault during D3-D4. Splays from the Cherrypool Fault link towards the Eclipse (McRaes) Prospect, with a geometry consistent with termination in a small D3-D4 extensional basin developed along the western flank of the fault. This geometry explains a local deepening of Grampians Group cover in this vicinity (Cayley & Taylor, 1997a).

Figure 3.40 Oblique parallel stereopairs of A) Glenisla Belt volume, showing relationships with various D1a (blue) and D3-D4 fault (red) lines and bounding D1a (yellow) and D3/D4 (pale blue) fault surfaces. B) Modelled Glenisla Belt fault surfaces (D1a – yellow; D3-D4 – pale blue), including a selection of anastomosing faults interpreted to disrupt the belt interior (see Figure 3.39). The Glenisla Belt is the only volcanic belt where internal D1a fault surfaces have been included in the STAVELY 3D model, as an example of the style of deformation found in all the volcanic belts (the near-surface traces of significant internal faults are included in other belts, in blue). Restoration of the D4 fault network (see Section 4.1 – D4 and D3 retrodeformation testing) suggests that, prior to the Early Devonian, the Glenisla Belt was contiguous along-strike with the northwestern end of the Tyar Belt to the south and with the southern end of Black Range Belt to the north as a single D1a fault slice of the Stavely Arc (see Figure 4.7). This interpretation accords with the common spectrum of rocks within all three belts, and suggests that the Glenisla, (mineralised) Black Range and Tyar belts share similar mineral potential.

3.2.5.12 Grampians ‘Deeps’ Belt

The Grampians ‘Deeps’ Belt (belt 10; Figure 3.9) is not exposed, but its presence and steep north-easterly dip is revealed by coincident elongate magnetic and gravity highs that extend beneath the northern Victoria Range and beneath the eastern end of the Victoria Valley Batholith (Figure 3.41). The Grampians ’Deeps’ Belt appears to comprise two sub-belts, each approximately 5km in width, that are partially offset from one another across the northern splay of the northeast-trending Victoria Valley Fault system (Figure 3.41).

The southern end of the Grampians ‘Deeps’ Belt appears to terminate abruptly against nonmagnetic rocks along the southern splay of the Victoria Valley Fault system approximately 4 km northwest of Jimmy Creek (Figure 3.41). The belt trends northwest for approximately 11 km, apparently underlying the eastern end of the (Early Devonian) Victoria Valley Batholith and adjacent Grampians Group at apparently shallow depth. The Grampians ‘Deeps’ Belt has a small left-lateral offset beneath Big Cord, and continues a further 15 km northwest beneath the northern Victoria Range. The northern end of the Grampians ‘Deeps’ Belt is poorly defined, but is apparently truncated obliquely against the Mosquito Creek Fault and against Fault 08 (see Figure 3.21), all buried beneath Grampians Group cover. The faults truncating the Grampians ‘Deeps’ Belt ends are all constrained to D4, which indicates that the Grampians ‘Deeps’ Belt was mobile during the D4 deformation.

Figure 3.41 The Grampians Deeps Belt (unshaded) buried beneath Grampians Group cover and the northern end of the Victoria Valley Batholith (red outline), in the interior of the Jalur Rift (heavy fault lines; see also Figure 3.9), within which thick deformed Grampians Group strata is preserved. Background image is 30 km high pass filtered Complete Bouguer Anomaly draped over shaded TMI (RTP) – the gravity high is attributed to mafic-intermediate volcanics of the Stavely Arc in the belt.

The Grampians ‘Deeps’ Belt has not been intersected by drilling and so the rock types contained within are unknown. The highly magnetic character and higher density of the Grampians ‘Deeps’ Belt suggest that mafic rocks may be predominant. The Grampians ‘Deeps’ belt is interpreted to have been continuous, pre-D4, with the Brimpaen Belt to the north, and the Elliot Belt to the south, so that the range of Stavely Arc stratigraphy known in these adjacent belts is considered as likely to exist within the Grampians ‘Deeps’ Belt. The large width of the Grampians ‘Deeps’ Belt compares closely with that of Stavely Arc stratigraphy contained within the Elliot, Brimpaen and Bunnugal belts.

The northeast dip of the Grampians ‘Deeps’ Belt is different to the westerly dips of the adjacent Bunnugal and Grampians ‘West’ belts, but is similar to the Elliot Belt that was apparently rotated clockwise within the core of the Mafeking Megakink during D4 (see Section 4.1.1 – Mafeking Megakink retrodeformation). Given its location within the heart of the Jalur Rift, and the presence of the vertically-plunging, clockwise-rotated Big Cord Orocline developed in directly overlying Grampians Group in the northern Victoria Range (Figure 2.16), we envisage a similarly partitioned strain history of clockwise rotation for the crustal block that contains the Grampians ‘Deeps’ Belt within the heart of the Jalur Rift during D4. Realisation that the Grampians ‘Deeps’ and Elliot belts are equivalent means that the listric D1a dip-profile established for the Elliot Belt and intervening Cambrian sequences can be extrapolated to constrain a similarly listric dip-profile for the Grampians ’Deeps’ Belt and its bounding D1a fault segments.

3.2.5.13 Grampians ‘West’ Belt

The Grampians ‘West’ Belt (belt 14; Figure 3.9) is entirely concealed beneath the Newer Volcanic Group, Rocklands Volcanic Group and Grampians Group, but contains highly magnetic rocks that are clearly visible in regional magnetic data (Figure 3.42). The Grampians ‘West’ Belt trends north-northeast, extending from near Victoria Point in the south to near Glenisla in the north, approximately parallel to the adjacent Victoria Range. The Grampians ‘West’ Belt lies approximately 2 km east of Moralla and Woohlpooer, where magnetic data shows it to be approximately 4.5-5 km wide, and has a strike length of approximately 33 km.

The Grampians ‘West’ Belt appears truncated at its southern end along the Cattle Camp Fault (Figure 3.42 and Figure 3.9) and, farther south, by the Mosquito Creek Fault which is buried beneath younger rocks in the Victoria Valley but marked by a zone of demagnetisation approximately 4 km wide near Karabeal. These faults appear to separate the Grampians ‘West’ Belt from the Boonawah Belt directly along-strike to the south. The northern end of the Grampians ‘West’ Belt appears to have been intruded by a buried, circular intrusion south of Glenisla (Figure 3.12 and Figure 3.43). The Wannon Fault also trends into this region and may cut the Grampians ‘West’ Belt. East of Mona Park, the Grampians ‘West’ Belt appears to have been affected by the Cattle Camp Fault mapped in the overlying Grampians Group (Taylor & Cayley, 1997; Cayley & Taylor, 1997a). This fault appears to have imparted a slight dog-leg to the Grampians ‘West’ and Boonawah belts (Figure 3.42 and Figure 3.43).

The eastern (approximate) 2.5 km of the Grampians ‘West’ Belt is highly magnetic (Figure 3.42). These magnetic rocks are buried beneath outwash colluvium flanking the Victoria Range, Rocklands Volcanic Group and Grampians Group and have yet to be tested. Drill hole VIMP18, designed to test the southern end of the Grampians ‘West’ Belt, failed to penetrate through the Grampians Group (Maher et al., 1997). The western (approximate) 2.5 km of the Grampians ‘West’ Belt has a complex, variably magnetic character, interpreted to reflect the presence of a variably-magnetic igneous stratigraphy (Figure 3.42). Complex magnetic highs and lows within this part of the Grampians ‘West’ Belt may reflect demagnetisation caused by magnetite-destructive hydrothermal alteration, and/or the influence of felsic intrusions into the succession. None have been tested. A small magnetic high intersected by groundwater bore Woohlpooer 6 in the western part of the Grampians ‘West’ Belt intersected intermediate volcanics which show geochemical features typical of subduction zone magmas (e.g. negative Nb and Ti anomalies, high LILE/HFSE). However, the volcanic rocks in Woohlpooer 6 differ from known volcanic rocks of the Stavely Arc (see Schofield et al., 2018, Section 2.6 – Geochemistry of the Stavely Arc for an overview). In comparison, volcanic rocks intersected in Woohlpooer 6, have higher TiO2 (1.15 wt %), Zr (232 ppm), flatter MREE-HREE (Gd/YbN = 1.7), and lack the characteristic MREE-HREE-Y depletion relative to N-MORB present in all other known Stavely Arc rocks, with concentrations of those elements close to N-MORB. Therefore, although a Stavely Arc-related origin is plausible, it is likely that the intermediate volcanics in Woohlpooer 6 have a different petrogenesis to known volcanic rocks of the Stavely Arc.

Figure 3.42 Grampians ‘West’ Belt highlighted on image of magnetics (RTP) showing an elevated magnetic response associated with buried Stavely Arc rocks, which appears relatively subdued beneath Grampians Group cover. Drill hole Woohlpooer 6 passed through a thin veneer of welded Rocklands Volcanics ignimbrite to intersect intermediate volcanics with boninitic geochemical features typical of juvenile subduction zone magmas (e.g. negative Nb and Ti anomalies, high LILE/HFSE). Drillhole VIMP 18 failed to penetrate Grampians Group cover.

Figure 3.43 Magnetics (RTP) pseudocolour image highlighting the different belt segments of Stavely Arc volcanics now thought to have been united within a single, simple D1a fault slice in the west of STAVELY prior to the D3 and D4 deformations (see Figure 3.62 and 4.6).

3.2.5.14 Narrapumelap Belt

The Narrapumelap Belt (belt 2; Figure 3.9) is poorly exposed. Highly weathered outcrops of intermediate igneous rocks in paddocks between Stavely and Narrapumelap are the main exposures of Mount Stavely Volcanic Complex rocks within the Narrapumelap Belt. Most other outcrops within the Narrapumelap Belt are of faulted Glenthompson Sandstone and rhyolitic dykes of unknown age. Magnetic data, sparse mineral exploration drilling, deep seismic reflection data (Line 09GA-AR1), and outcrop show that the Narrapumelap Belt is approximately 25 km long, and up to 3.5 km wide (Figure 3.44). It strikes northwest-southeast from Stavely to near Lake Bolac, 5.5 km south of, and subparallel to, the Elliot Belt. Like the Elliot Belt, the Narrapumelap Belt appears to be bound at its western and eastern ends by the D4 Escondida and Golton faults respectively.

Figure 3.44 Tilt and band pass filtered regional magnetic image of the Mafeking Megakink, a large D4 structure hosted in Cambrian bedrock. Shading depicts the Stavely, Narrapumelap and Dryden belts (interpreted to have been united pre-D4; see Figure 4.3 and Figure 4.4) in the western, central and eastern megakink limbs respectively. Reversals of overall stratigraphic facing directions (Y), determined from mapping of outcrops and from drilling, help define a megakink superimposed on a Cambrian succession that was, prior to D4, predominantly steeply west-facing (as seen in and adjacent to the Stavely Belt including drill hole STAVELY17, and in the Dryden Belt, e.g. Figure 2.19). Predominantly northeast-facings only occur in and adjacent to the Narrapumelap Belt in the interior limb of the Mafeking Megakink. The averaged orientations and dip magnitudes obtained for the upper Stavely Belt (dark purple = measured in outcrop; pale purple = dip determined from modelling of magnetic data) and upper Narrapumelap Belt (dark green = measured in outcrop; pale green = dip determined from modelling of magnetic data) show that D4 megakinking was associated with a dramatic change in belt strike, but virtually no change in overall pre-existing belt dip-magnitude. This indicates that D4 megakinking occurred about a subvertical axis of rotation. The lower-dipping portion of the Narrapumelap Belt (aqua plane in top stereonet) represents the average dip of the Narrapumelap Belt measured between its surface position and its offset subsurface position as imaged in deep seismic reflection line 09GA-AR1 (see Figure 2.22) – this lower dip magnitude reflects a listric profile to the Narrapumelap Belt (see Figure 3.45) and has no bearing on the orientation of the Mafeking Megakink axes. The megakink axes are now fault-offset, with large (> 10 km) dextral strike-slip fault-displacements mapped across the D4 Escondida (green) and Golton (orange) faults. Magnetic and gravity data show the northern end of the Stavely Belt to be deflected 6-7 km eastward (clockwise) towards the megakink. Overprinting criteria suggest this warping closely followed D4 strike-slip offset of the western megakink axis across the Escondida Fault with a geometry consistent with overall Mafeking Megakink geometry, suggesting all occurred successively during D4. The fault-displaced Elliot Belt also occupies the Mafeking Megakink middle limb – prior to D4, the Elliot Belt is interpreted to have been continuous along-strike with the Bunnugal Belt across the present-day position of the Victoria Valley South Fault. Potential field data indicates an overall moderate north-northeast dip to the Narrapumelap Belt. This orientation is corroborated by occasional outcrops of Glenthompson Sandstone within the belt with bedding dip and facing predominantly towards the north-northeast. The Narrapumelap Belt projects down-dip to a position coincident with a reflective tabular-shaped package imaged in deep seismic reflection line GA09-AR1 that underlies the Elliot Belt (that locally crops out at Mount Elliot) but is grossly similar to it in size, character and geometry, and this gives a tight constraint on the dip magnitude of the Narrapumelap Belt in the mid- to upper crust (Cayley et al., 2011b). This dip magnitude is considered a proxy for the D1a faults that bound the belt.

The orientation of the Narrapumelap Belt has been measured by several different means, and at different depths. The different results give insight into the subsurface shape of the Narrapumelap Belt and, by inference, adjacent D1a volcanic belts of the Stavely Arc. The geometry of (upright) bedding of Glenthompson Sandstone and volcanic rocks incorporated into the Narrapumelap Belt interior has been directly measured at surface in several places in the centre-west of the Narrapumelap Belt. An averaged directly-measured dip/dip-direction for bedding of approximately 65-70° towards 035° (grid north) is taken as a proxy for the overall orientation of this portion of the Narrapumelap Belt at surface (Figure 3.45).

The geometry of the upper 1-2 km of the Narrapumelap Belt has been modelled from regional magnetic data, giving an averaged modelled dip/dip-direction of approximately 55° to 62° towards 052° (grid north) for magnetic igneous units within the interior of the Narrapumelap Belt (e.g. Figure 3.50B), in the same portion of the Narrapumelap Belt where the orientation of stratigraphy incorporated within the belt has been measured directly. The averaged modelled dip-magnitude is lower than the averaged dip-magnitude measured directly at surface, implying that the overall dip magnitude of this part of the Narrapumelap Belt decreases with depth (Figure 3.44).

An average geometry of the upper 5 km of the Narrapumelap Belt can be accurately determined by across-strike triangulation from the surface positions of the Narrapumelap Belt flanks (occupied by the Narrapumelap North and South faults) to the buried positions of the same Narrapumelap Belt flanks as imaged in deep seismic reflection line 09GA-AR1. The upper boundary of the Narrapumelap Belt reflector in 09GA-AR1 (which correlates with the Narrapumelap North Fault) at its centre-point is located at a depth of approximately 4.5 km. The across-strike distance from directly above this point on 09GA-AR1, to the surface position of the northeastern belt flank of the Narrapumelap Belt where it is exposed near Stavely, is 12.5 km.

Figure 3.45 Oblique view of the STAVELY 3D model depicting the Narrapumelap Belt volume, with other near-surface belt boundaries shown for reference. The positions of Late Cambrian (D1b) intrusive rock volumes are also shown, including two that intrude the Narrapumelap Belt (see Figure 3.63). The stereoplot (note different orientation) depicts three averaged geometries determined for the Narrapumelap Belt, illustrating its listric profile with a steep northeast dip-magnitude at surface, and a much lower northeast dip-magnitude at depth.

Figure 3.46 Oblique parallel stereopair of the STAVELY 3D model showing the volcanic belts that are deformed by the Mafeking Megakink. The modelled west-dipping and west-facing Stavely (dark purple) and Bunnugal (dark green) belt volumes in the west limb, the northeast-dipping and facing Narrapumelap (medium purple) and Elliot (medium green) belt volumes in the fault-offset middle limb, and the west-dipping and west-facing Dryden Belt volume (pale purple) in the east limb. The east-limb equivalents for the Bunnugal and Elliot belts are the Grampians ’Deeps’ and Brimpaen belts which are separate rotated fault blocks contained within the Jalur Rift interior (not shown). Stereoplot illustrates the measured orientations of the uppermost Stavely (purple) and Narrapumelap (green) belts in positions close to the middle megakink limb. All belts have a listric form, decreasing in dip-magnitude to depth. This is illustrated in Figure 3.11 and Figure 3.45.

Triangulation from these constraints gives an averaged orientation of the Narrapumelap North Fault between surface (at its western end) and 4.5 km depth of approximately 20° towards 030° (grid north). This dip is much lower than measured at surface, or modelled from regional magnetic data, and indicates that the overall dip magnitude of the Narrapumelap Belt, and of the faults that bound it, decreases dramatically below approximately 2 km depth (Figure 3.45). The overall decrease in dip-magnitude from surface to depth for the northern flank of the Narrapumelap Belt indicates conclusively that the Narrapumelap Belt and its flank-bounding D1a faults have a distinctly listric profile to at least a depth of approximately 5 km.

With the overall geometry of the Narrapumelap Belt taken into account, the true width measured at surface is a close match for the calculated true width of the belt imaged in 09GA-AR1. This indicates that the D1a Narrapumelap South Fault retains the same separation from the D1a Narrapumelap North Fault to depth and therefore must share a similarly listric profile.

The (reflective) Narrapumelap Belt imaged in GA09-AR1 is underlain and overlain by less-reflective sequences. These are interpreted to represent uniform, homoclinally-dipping and facing panels of Glenthompson Sandstone given that these sequences project laterally to surface north and south of the Narrapumelap Belt, where northeast dipping and facing Glenthompson Sandstone has been mapped at surface and intersected in near-surface drilling. The true width of the panel of northeast dipping and facing Glenthompson Sandstone that separates the Narrapumelap and Elliot belts measured at surface, is a close match for the calculated true width of the unreflective region that separates the Narrapumelap and Elliot belt reflectors imaged in 09GA-AR1. This indicates that this panel of Glenthompson Sandstone retains its thickness to depth and must therefore share the same listric profile as established for the Narrapumelap Belt.

Rock types intersected in drill holes into the Narrapumelap Belt include non-magnetic Glenthompson Sandstone, magnetic intermediate volcanics and felsic tuff that correlate with stratigraphy of the Mount Stavely Volcanic Complex (e.g. correlatives of the Towanway Tuff intersected in drill holes TG-750E to TG-950E), and discontinuous fault-bounded lenses of serpentinite that correlate with the Williamsons Road Serpentinite in the interior of the belt (e.g. drill holes TG-1000E to TG-1500E). Several highly remanently-magnetised intrusions occur within the Narrapumelap Belt, for example north of Wickliffe and northwest of Lake Bolac. These are undated, but are likely to be of Late Cambrian (D1b) age.

Occurrences of serpentinite within the Narrapumelap Belt serve as a useful marker horizon that supports interpretation of the Narrapumelap Belt as representing, pre-D4, a direct along-strike continuation of the northerly-trending and west-dipping (mineralised) Stavely Belt. The overall northeast dip of Narrapumelap Belt and northeast facing of adjacent Glenthompson Sandstone contrasts with the steep west dip and facing of the adjacent Stavely and Dryden belts, and is consistent with the interpretation of clockwise kink-rotation of the Narrapumelap Belt within the central limb of the Mafeking Megakink about a sub-vertical axis during D4 (see Section 4.1.1 – Mafeking Megakink retrodeformation; Figure 3.46 and Figure 3.44). Subsequent dextral strike-slip rupture of the kink axes across the dextral Escondida and Golton faults (late D4) explains the offsets of the western end of the Narrapumelap Belt from the northern end of the Stavely Belt, and eastern end from the southern end of the Dryden Belt (see Section 4.1.1 – Mafeking Megakink retrodeformation).

The dextral geometry, D4 timing and magnitude of the Mafeking Megakink is constrained by the coincident Mafeking Orocline that is developed in Grampians Group that overlies the northern end of the Mafeking Megakink central limb, and is also confined between the Escondida and Golton faults (Figure 2.16). This convex-south oroclinal structure has warped moderately-dipping Grampians Group stratigraphy through more than 100° around a sub-vertical axis of rotation, which constrains a minimum magnitude of rotation for the central limb of the underlying megakink (see Figure 3.73 and Figure 4.3).

The northeast-dipping Narrapumelap Belt occupies the lowest tectonostratigraphic position within the Mafeking Megakink central limb, sitting below the Elliot Belt. Thus, the Narrapumelap Belt compares directly in position with the adjacent west-dipping Stavely Belt which occupies the lowest (= easternmost) tectonostratigraphic position of a D1a imbricate fault stack that includes the west-dipping Bunnugal and Boonawah belts. The Narrapumelap Belt also compares directly with the west-dipping Dryden Belt for the same reason. This is the reason we correlate both the Stavely and Dryden belts with the Narrapumelap Belt, rather than with the Elliot Belt. The Elliot Belt is instead correlated with the Bunnugal Belt (see Section 3.2.5.5 – Bunnugal Belt).

3.2.5.15 Pella Belt

The Pella Belt (belt 7; Figure 3.9) lies just outside the northern edge of STAVELY and is not included in the STAVELY 3D model volume. However, the Pella Belt is significant as a near-continuous belt of buried coherent magnetic igneous rocks amidst non-magnetic, presumed metasedimentary rocks that extends north from Pella, approximately 7 km northwest of Rainbow, towards Tutye and Boinka on the Mallee Highway. It has a strike length of nearly 90 km. The belt is approximately 1-1.4 km wide, with a symmetrical magnetic character that suggests a steep, near subvertical attitude for the magnetic stratigraphy within it.

The nature of the rocks within the Pella Belt are unknown, but the coherence of the magnetic stratigraphy contrasts with that of the adjacent Moornambool Metamorphic Complex of the western Stawell Zone where metamorphic stratigraphy is fault-disrupted. Therefore, the Moyston Fault is interpreted to lie just to the east of the Pella Belt, and the Pella Belt is inferred to contain upturned magnetic rocks of the Stavely Arc.

The overall geophysical character of the Pella Belt is similar to the adjacent Dryden North Belt, including the rocks intersected in drill hole STAVELY16. While it is possible that the Pella Belt may represent an additional D4 dextral strike-slip imbricate of the Dryden North Belt, such an interpretation has no additional constraints. We instead interpret the Pella Belt as an additional D1a fault belt of Stavely Arc volcanics thrust to near-surface within the eastern Grampians-Stavely Zone.

3.2.5.16 Stavely Belt

Along with outcrops at Mount Dryden and Jallukar, Cambrian igneous rocks in the Stavely Belt (belt 1; Figure 3.9) were recognised during early geological mapping of Victoria (e.g. Krause, 1873). These rocks include coherent volcanics, volcanic breccias and volcaniclastic sediments of the Mount Stavely Volcanic Complex, and porphyry intrusives, which crop out on Mount Stavely and on hills and in creeks along-strike to the south. Other rocks defined within the Mount Stavely Volcanic Complex are the Williamsons Road Serpentinite and Lalkaldarno Porphyry. These rocks are described in Schofield et al. (2018), and the porphyry intrusions are captured as separate volumes as they are slightly younger (see Section 3.2.3 – Intrusions – Cambrian (possible Ordovician)). The igneous rocks within the Stavely Belt are variably magnetic. Moderately to highly magnetic basalt, andesite and serpentinite (e.g. see Stuart-Smith & Black, 1999; Skladzien et al., 2016a) within the Stavely Belt explain the obvious magnetic high associated with it along its length (Figure 3.47). This means that, even where outcrop is poor or the belt is buried, it can be traced easily using regional magnetic data.

The Stavely Belt trends approximately north-south from the Grampians Ranges to beyond the southern limit of STAVELY. Regional magnetic data shows that the northern end of the Stavely Belt is truncated beneath the Grampians Ranges northwest of Mafeking (Figure 3.47). This termination occurs abruptly across the D4 Escondida Fault, which, further south, curves to bound the northeastern flank of the Stavely Belt for a significant portion of its length.

The Stavely Belt emerges from beneath the Grampians Group in the vicinity of Yarram Park, and trends southeast through Mount Stavely towards Hexam in the south, passing beneath the Hopkins River, and 5 km east of Chatsworth en-route. The Stavely Belt is intermittently exposed from Yarram Park to the Hopkins River. South of here, the Stavely Belt is concealed beneath increasing thicknesses of the Newer Volcanic Group, Otway Basin sediments (Brighton Group) and, south of Chatsworth, Grampians Group (intersected in drill hole STAVELY02). A gravity high associated with the Stavely Belt can be traced south through Hexam and beneath the Otway Basin to the vicinity of Ellerslie and Purnim, west of Terang (Figure 3.9 and Figure 2.28B). The strike length of the Stavely Belt is over 110 km.

The width of the Stavely Belt varies from a little over 1 km at its northern extent where the eastern margin is bound by the D4 Escondida Fault, to approximately 2 km where both flanking D1a bounding faults appear intact and unmodified, to over 5 km wide near the Hopkins River where it has been widened by fault complexity related to splays of the D4 Yarrack Fault. South of Chatsworth, the Stavely Belt appears to narrow back to an average width of around 2 km.

The lithologies in the Stavely Belt are well known from outcrops between Stavely and the Hopkins River, and this is the location of all type localities within the Mount Stavely Volcanic Complex (Figure 3.48). Although outcrop north of Stavely is poor, the Stavely Belt has been intersected by many mineral exploration drill holes owing to the presence of known porphyry, epithermal and VHMS prospects (see Cairns et al., 2018, Schofield et al., 2018 Section 3.1 – Known mineral occurrences in STAVELY). These drill holes demonstrate that all the main stratigraphic units defined in the area of outcrop persist into northernmost parts of the belt. A series of reconnaissance mineral exploration drill transects at the southern end of the belt show similar stratigraphic persistence.

Within the Mount Stavely Volcanic Complex, the Fairview Andesite Breccia (Figure 3.49A) and related andesite lava flows is the most voluminous unit, comprising most of the outcrop in the Stavely Belt. It forms a large volcanic pile of approximately 2500 m apparent thickness. This unit tends to occur in the western side of the volcanic sequence along much of the Stavely Belt between the Hopkins River and Mount Stavely, faulted against Glenthompson Sandstone along the Stavely West Fault and related splays. North of Stavely, the Fairview Andesite Breccia appears to be increasingly fault-intercalated with the Glenthompson Sandstone and Towanway Tuff within the belt.

Figure 3.47 Fault network on image of tilt and band pass filtered magnetics, with the Stavely Belt highlighted and showing a high degree of internal complexity and variable magnetic response of intercalated units. Thursday’s Gossan prospect is shown in the northern half of the belt.

A thin fossiliferous shale package, the Glenronald Shale Member, occurs within the Fairview Andesite Breccia. Bedding within the Glenronald Shale Member dips west and east in different places, but tends to dip predominantly steeply east in the southern part of the Stavely Belt. Buckland (1987) interpreted the Glenronald Shale to form a remarkably strike-persistent marker-horizon that was conformable within the Fairview Andesite Breccia, and noted that the shale appeared to be ‘in contact’ with Nanapundah Tuff in places. Subsequent mapping, geophysics and mineral exploration drilling indicate that the shale is not continuous along strike. Its position within the stratigraphy varies along strike and is structurally controlled. The Glenronald Shale Member lies along the Stavely West Fault in the south and is hosted by several different unnamed faults within the belt at different places along its length farther north. Some of these faults are intruded by Late Cambrian porphyries (intruded during D1b) which constrains the age of shale faulting to D1a. The Glenronald Shale Member is weak and therefore likely to have hosted faults during D1a deformation. This implies that the Glenronald Shale Member is allochthonous with respect to the enclosing breccia, and is rather possibly part of an underlying pre-volcanic marine stratigraphy. The possibility of faulting within the Fairview Andesite Breccia succession suggests that its large thickness in parts of the Stavely Belt may be apparent, a consequence of imbrication.

The Fairview Andesite Breccia passes east into the Nanapundah Tuff south and north of Mount Stavely. Originally described as overlying the Fairview Andesite Breccia (Buckland, 1987; Stuart-Smith & Black, 1999), the change to dark grey-green massive, poorly sorted, medium to coarse-grained andesitic crystal lithic volcanic sandstone of the Nanapundah Tuff appears to be fault-controlled in most places, and bedded tuff occurs to the east and west of outcrops of Fairview Andesite Breccia. The change from the Fairview Andesite Breccia to the Nanapundah Tuff occurs across fault slices of exotic Williamsons Road Serpentinite in several other places.

The Towanway Tuff is the second most extensive unit in the Stavely Belt, occupying much of the eastern flank of the belt south of Mount Stavely, and large tracts of the belt north of Stavely. The type locality of the Towanway Tuff is separated from the rest of the volcanic stratigraphy in the Stavely Belt by the Williamsons Fault and Williamsons Road Serpentinite. Elsewhere, the Towanway Tuff appears to overlie the Nanapundah Tuff and Fairview Andesite Breccia to the west (Buckland, 1987; Stuart-Smith & Black, 1999), although these other contacts within the belt appear also to be faulted. Mineral exploration drilling in the vicinity of the Thursday’s Gossan Prospect, and to the north of Mount Stavely has encountered tracts of felsic tuffs that correlate best with the Towanway Tuff.

On Drysdale Road, thinly bedded Towanway Tuff adjacent to the Escondida/Stavely East faults (see Figure 3.49C) face west, suggesting that the tuff may overlie Glenthompson Sandstone stratigraphy that lies farther east. These rocks are overprinted by open, steeply plunging crenulation folds and dextral shear zones that probably relate to strike-slip movements along the adjacent Escondida Fault during D4. Stuart-Smith & Black (1999) noted that cherty tuff clasts incorporated in Glenthompson Sandstone that crops out on Drysdale Road adjacent to the tuff were identical to the Towanway Tuff. Elsewhere the Towanway Tuff is typically folded by upright folds which makes interpretation of overall facing directions for the formation ambiguous. A tuff intersected in drill hole SNDD3 at 128 m appears to face down-hole, raising the possibility that parts of this, and other, formations within the Mount Stavely Volcanic Complex have been overturned.

South of the Glenelg Highway and east of its type locality, the Towanway Tuff is faulted against the Glenthompson Sandstone along the Stavely East Fault. An original gradational sedimentary contact here can be inferred by the presence of interbedded rhyolitic volcaniclastic sandstone beds within Glenthompson Sandstone east of the fault, exposed in a tributary of Back Creek (Figure 3.53D). The Glenthompson Sandstone here is overturned to face east closest to the fault footwall, but is upright and west-facing just to the east, suggesting that the overturning is a local phenomenon related to reverse movements along the Stavely East Fault and that, prior to faulting, Towanway Tuff may once have overlain the Glenthompson Sandstone and so be the younger unit.

Figure 3.48 Stavely Belt geology map, showing internal stratigraphic units of the Mount Stavely Volcanic Complex, internal stratigraphic units of the Glenthompson Sandstone, Late Cambrian intrusive rocks, and the distribution of various cover-rock successions. Based on Buckland (1985) and Buckland & Stuart-Smith (2000), incorporating additional information from subsequent GSV mapping (Cayley et al., in prep), mineral exploration drilling and geophysics data.

Figure 3.49 Mount Stavely Volcanic Complex – Stavely Belt. A) typical outcrop of Fairview Andesite Breccia, showing large clasts of andesite, dacite and minor basalt lava set in a volcanolithic matrix. Astons Road, Hopkins River (MGA 54 648219 5821656); B) Well-bedded water-lain Towanway Tuff, locally east-facing bedding (from grading and cross-lamination truncations) overturned to dip steeply west. Near Mount Stavely (MGA 54 644686 5831012); C) Graded turbiditic (water-lain) Towanway Tuff exposed in Back Creek, adjacent to Drysdale Road, close to the faulted eastern edge of the Stavely Belt. Grading shows west-facing (younging to top of picture) for the beds here (MGA 54 644664 5835036).

Bedding in the Towanway Tuff dips moderately west with unclear facing at the type locality. The interpretation of an overall westward facing for the Towanway Tuff is opposite to the interpretation of Stuart-Smith & Black (1999), but helps explain the interlayered, folded contact between the Towanway Tuff and Glenthompson Sandstone that is shown by mineral exploration drilling along the eastern flank of the Stavely Belt. For example, south of the Glenelg Highway a suite of mineral exploration drill holes pass from Towanway Tuff east into Glenthompson Sandstone. Occasional intersections of igneous rocks (e.g. tuff in VICT2RA21) within the Glenthompson Sandstone east of the main belt suggest that a fault-modified and folded transition from Glenthompson Sandstone up into the Mount Stavely Volcanic Complex is retained partially intact here.

The overall west-dip of magnetic units (Figure 3.50) and stratigraphy within the Stavely Belt (Figure 3.11), coupled with the overall west dip and west facing of Glenthompson Sandstone ‘panels’ that lie west and east of the Stavely Belt suggest that stratigraphy within the belt is probably also west-facing overall. This interpretation is consistent with the proven west dip and facing of comparable igneous stratigraphy in the Dryden Belt, which lies north along strike from the Stavely Belt in pre-D4 structural interpretations (see Section 4.1.6 – Analysis of D4 and D3 retrodeformation results).

The realisation that the Narrapumelap Belt represents a D4 refolded segment of the D1a Stavely and Dryden belts means that the listric dip-profiles constrained for the Narrapumelap Belt and intervening Cambrian packages within the central limb of the Mafeking Megakink (see Section 3.2.5.14 – Narrapumelap Belt) can be extrapolated around the position of the western Mafeking Megakink hinge to constrain a similarly listric profile for the Stavely Belt and the D1a thrust faults that bound it.

Projection of these constraints means that most of the west-dipping D1a structures mapped within the Cambrian bedrock across the width of the Grampians-Stavely Zone can be demonstrated to have listric profiles at the crustal-scale, all shallowing in dip magnitude to depth (see Figure 3.10 and Figure 3.11). This is consistent with the constraints provided by available deep seismic reflection data across the width of the Grampians-Stavely Zone, and forms a key premise that controls the construction of the serial full crustal cross sections used to inform development of the STAVELY 3D model volumes (see Appendix 2 – Geological cross sections). The Stavely Belt differs from the Dryden Belt in that the most primitive rocks (e.g. Fairview Andesite Breccia and basaltic lavas within) tend to lie along the west of the belt, while the most evolved (calc-alkaline) rocks (e.g. Towanway Tuff and dacitic lavas within) occur in the east. This configuration is counter-intuitive with the overall west facing and dip indicated for the Stavely Belt, since a progression from mafic to more intermediate and felsic compositions may be expected as the arc matures. The appearance of felsic Towanway Tuff as perhaps the first formation of the series in the Stavely Belt suggests that local magmatism here may have commenced with more evolved calc-alkaline geochemistry in a mature arc setting, and that local magmatism in this part of the Stavely Arc was somewhat younger than that recorded by the mafic, low-Ti rocks preserved in the Dryden, Black Range and Dimboola belts farther north.

Exotic fault-slices of ultramafic rock (Williamson Road Serpentinite), pelagic sediments (Glenronald Shale Member), and other sedimentary rocks (Glenthompson Sandstone), incorporated into the interior of the calc-alkaline igneous succession of the Stavely Belt suggest that the arc stratigraphy may be disordered within the Stavely Belt, across structures such as the Williamsons Fault. Fault slices of such exotic materials tend to separate the major bodies of mafic and intermediate-felsic Cambrian stratigraphy. The age of structural complexity and stratigraphic disordering within the Stavely Belt is constrained to the culmination of the Delamerian Orogeny (D1a) by the late Cambrian (D1b) Lalkaldarno Porphyry, lobes of which intrude across faults and exotic fault-slices within the belt (Figure 3.47 and Figure 3.48).

Figure 3.50 A) Stavely Belt inversion profile 1 showing west dipping volcanic units within the Stavely Belt; B) Stavely Belt inversion profile 4 showing volcanic units (blue colours) of the Stavely Belt separated from the (north-)east dipping units of the central limb of the Mafeking Megakink – Narrapumelap Belt (purple and brown colours) – by the regional dextral Escondida Fault. Top profile panels show first vertical derivative of TMI; middle panels show TMI; bottom panels show inversion bodies with assigned magnetic susceptibility values (SI). Pink line in TMI panel shows the regional. C) Location of inversion sections on TMI (RTP) – sections presented here are shown in white.

3.2.5.17 Tyar Belt

The Tyar Belt (belt 15; Figure 3.9) is entirely concealed beneath younger cover rocks. The Tyar Belt contains a coherent, steeply southwest-dipping magnetic sequence that comprises several highly magnetic units interlayered with less magnetic units, a characteristic that allows the belt to be readily interpreted in magnetic data (Figure 3.12, Figure 3.16 and Figure 3.43). The Tyar Belt is approximately 3.5 km wide, and approximately 22 km long. The overall magnetic character, width and steep dip of the Tyar Belt is very similar to the adjacent Black Range and Glenisla belts.

The Tyar Belt trends northwest-southeast along the southwest flank of the Black Range, from which it is separated across the Henty Fault (Figure 3.12, Figure 3.16). At its northern end, the Tyar Belt appears to bend abruptly towards the Henty Fault around the Tyar Fold (Figure 3.74), and is truncated abruptly by the Henty Fault near Roy Blakes Road. At its southeastern end, the Tyar Belt has been intruded by a buried intrusion south of Glenisla, and possibly also truncated and offset by the Wannon Fault (see Figure 3.42). Magnetic inversion modelling of the Tyar Belt (Figure 3.12 and Figure 3.51) shows an overall southwesterly dip, although the northwestern end of the belt warps into local east dips in the vicinity of the Tyar Fold and its point of truncation by the Henty Fault, which suggests that drag-folding and fault-disruption of the Tyar Belt was accompanied by local warping and overturning of the entire belt (Figure 3.13).

The only constraints on the composition of rocks within the Tyar Belt are a series of RAB traverses undertaken by CRAE across its southeastern end, and from a small number of mineral exploration drill holes that intersect the northern end. RAB traverse AC96GM408-458 intersected an interlayered succession of metasediments, intermediate tuffaceous sandstone, volcaniclastics including volcanic sandstone and conglomerate, and andesite lava beneath a thin veneer of Murray Basin cover. RAB traverse AC96GM398-407 intersected andesite lava (Radojkovic, 1996; Cayley & Taylor, 1997a, 1997c). Mineral exploration drilling in the northern end of the Tyar Belt intersected interlayered dolerite, shale and porphyritic mafic intrusives beneath the Murray Basin and Rocklands Volcanic Group cover. Overall, this succession is closely comparable to similar Cambrian volcanics in the adjacent Black Range and Glenisla belts, and to the Mount Stavely Volcanic Complex in the Stavely Belt. These rocks are undated, but are assumed to be Cambrian by virtue of their context with other Cambrian volcanic rocks in the region (Cayley & Taylor, 1997a).

Complex magnetic highs and lows within the Tyar Belt may reflect demagnetisation caused by magnetite-destructive hydrothermal alteration, and/or the influence of felsic intrusions into the succession. None have been tested.

Figure 3.51 Example of inversion modelling of magnetic data over the Tyar Belt. A) Selected profiles over the Black Range West/Mitre Belt and section locations on regional 1VD TMI data. Top profile panels show first vertical derivative of TMI; middle panels show TMI; bottom panels show inversion bodies. B) Location of all Stavely inversion sections on TMI (RTP), with detailed area (A) outlined.

3.2.6 Kanmantoo Group

Cambrian-aged Glenthompson Sandstone occupies a number of discrete fault-bounded ‘panels’ that separate the different volcanic belts of the Stavely Arc. These panels are all steeply, but generally homoclinally dipping, comprised of tilted, folded and faulted clastic stratigraphy, and are described collectively. The panels are represented in the STAVELY 3D model as separate volumes that occupy the voids between the (D1a) volcanic belts (Figure 3.52).

The Glenthompson Sandstone comprises a turbiditic, deep marine metasedimentary package comprising interlayered, terrigenous thinly- to thickly bedded micaceous quartz arenite and quartz lithic sandstone and siltstone (Figure 3.53), with minor interbeds of shale and pelitic mudstone and calcareous siltstone. The sequence is intruded in places by basaltic sills – the Chatsworth Basalt, related basaltic lavas may have erupted onto the depositional surface during sedimentation (see Appendix 4 – Geological Units).

Away from the disruptive influence of D4 faults, Glenthompson Sandstone stratigraphy is coherent and layer-cake and consistent with deposition in a stable basin or sea-floor environment with a minimum of sediment reworking. The strata exposed at surface is generally of low regional metamorphic grade. The homoclinal dip and facing of Glenthompson Sandstone exposed in several different panels of the stratigraphy is consistent with a fault-dominated D1a deformation style, and gives insight into regional-scale geometries for structures in STAVELY, including the likely facing directions of adjacent volcanic belts. Large-scale reversals in facing within Glenthompson Sandstone are a key constraint on the D4 Mafeking Megakink (see Section 4.1.1 – Mafeking Megakink retrodeformation). Glenthompson Sandstone is generally upright. Local overturning occurs adjacent to faults.

The overall stratigraphic thickness of Glenthompson Sandstone within the panels is unknown. An apparent stratigraphic thickness of over 10 km exposed in the large, consistently west-dipping and west-facing panel of Glenthompson Sandstone that separates the Stavely and Bunnugal belts west of Chatsworth is most likely to be a consequence of subsequent deformation, since the constant low regional metamorphic grade of this succession across its width is inconsistent with a 10 km variation in burial depth. In addition, there is evidence of several D1a faults within the succession that are likely to have caused internal thrust-repetition of parts of the sequence, including the Chatsworth Basalt marker horizons. D1a thrust faults confined within Glenthompson Sandstone have not been included in the STAVELY 3D model.

Glenthompson Sandstone is an extensive unit in STAVELY. In addition to outcrop, Glenthompson Sandstone is intersected in drill holes STAVELY11 and STAVELY17 (see Schofield et al., 2015b) and in numerous mineral exploration drill holes and groundwater bores. Glenthompson Sandstone is mostly non-magnetic (see Skladzien et al., 2016a), and so can be clearly distinguished from fault belts of more magnetic Stavely Arc volcanic rocks in magnetic data, even where the volcanic belts are deeply buried beneath younger rocks. Glenthompson Sandstone is much less dense than Stavely Arc rocks, and so can be distinguished using gravity data. Glenthompson Sandstone is unreflective in deep seismic reflection data, so can be distinguished from more reflective Stavely Arc stratigraphy.

Figure 3.52 Oblique STAVELY 3D model view showing the distribution of Kanmantoo Group (Glenthompson Sandstone) volumes, the single most extensive group of volumes in STAVELY. The volumes span the footwall of the west-dipping Boonawah Belt in the west to the footwall of the Moyston Fault in the east, punctuated only by the intervening volcanic belts of the Stavely Arc, including the Bunnugal, Stavely, Elliot, Narrapumelap, Dryden, Grampians ‘Deeps’, Brimpaen, Dryden North and Dimboola belts. These are the north-trending voids in the Kanmantoo Group volumes. As for the volcanic belts of the Stavely Arc, deep seismic reflection data indicate that many of the volumes of Glenthompson Sandstone mapped at surface extend down-dip into the lower crust. As for the volcanic belts of the Stavely Arc, complexities to Kanmantoo Group volumes occur where the Cambrian sequence is segmented, laterally offset and rotated across D3 and D4 faults. The smaller, separate Kanmantoo Group volume confined to the lower crust of the model is conceptual, interpreted to conformally underlie the buried autochthonous Stavely Arc volume and overlie Proterozoic material, based on extrapolation from tectonostratigraphic relationships mapped along the eastern flank of the Stavely Belt, and from wider tectonostratigraphic relationships for the Kanmantoo Group established in South Australia.

In addition to the large coherent panels of Glenthompson Sandstone between the volcanic belts that are represented as separate model volumes, numerous smaller (sub-kilometre to kilometre-scale) lenticular bodies of Glenthompson Sandstone are incorporated into the interior of the volcanic belts themselves, most notably within the Stavely and Narrapumelap belts (Figure 3.48). These juxtapositions appear to be mostly as fault slices formed during the D1a regional deformation, since Glenthompson Sandstone within the volcanic belts typically shows no evidence of any stratigraphic mixing with the enclosing volcanic rocks. Such fault slices cannot be resolved at the regional scale of the STAVELY 3D model, and so fault slices of Glenthompson Sandstone within the volcanic belts are just subsumed within the Stavely Arc belt volumes.

The absolute age of the Glenthompson Sandstone is unknown. Glenthompson Sandstone was previously assumed to have been rapidly deposited following eruption of the Mount Stavely Volcanic Complex but prior to Late Cambrian deformation and subsequent intrusion of the Bushy Creek Granodiorite and Buckeran Diorite, both of which are Late Cambrian (~502-498 Ma; Lewis et al., 2016; VandenBerg et al., 2000; Crawford et al., 2003).

However, the overall character of the Glenthompson Sandstone is inconsistent with synorogenic deposition, and new field constraints and geochronology and geochemistry indicate that the Glenthompson Sandstone is much more likely to be older than the Mount Stavely Volcanic Complex, a direct correlate of the Kanmantoo Group (Lewis et al., 2015; Lewis et al., 2016; Maas & Taylor, in prep.). Thin Chatsworth Basalt sills and flows that intrude and intercalate with the Glenthompson Sandstone have geochemical similarities to Kanmantoo Group dykes dated at approximately 510 Ma in South Australia (Taylor et al., 2015; Schofield et al., 2018), strengthening this correlation and indicating a likely depositional age for the Glenthompson Sandstone of approximately 520-505 Ma. These relationships suggest that the Glenthompson Sandstone was at least coeval with Stavely Arc volcanism, and most of the formation may in fact be older, and marks a departure from the previously accepted post-volcanic timing for deposition.

Figure 3.53 Kanmantoo Group – Glenthompson Sandstone. A) Well-bedded turbidites with typical thin-bedded A-C-E Bouma sequences, metamorphosed to hornfels by an underlying granite. Mount Drummond Quarry near Ledcourt. Although weakly internally deformed and uncleaved, grading shows the Mount Drummond Quarry strata to be completely overturned. These rocks lie in the Moyston Fault footwall, as for similarly overturned Glenthompson Sandstone in the Moyston Fault footwall farther south (see Figure 2.20 and Figure 4.9). B) Thin section (cross-polarized light, field of view 7 x 4.2 mm) of the formation from the Lake Lonsdale spillway (MGA 54 641231 5901198) showing a typical detrital mineralogy in sandstones – angular quartz grains and polycrystalline aggregates and albite (twinned) and K-feldspar grains, set in a dirty biotite/chlorite (altered clay minerals) groundmass. C) Thicker-bedded facies exposed in a tributary of the Hopkins River near Nareeb, showing A-E Bouma sequences typical of much of the formation. Late joints that cut bedding at a high angle lie parallel to the hammer handle (MGA 54 639106 5809371). D) Coarse-grained, tabular-cross-bedded turbiditic gritstone, the basal part of thick-bedded B-C-E Bouma sequences exposed in Back Creek just east of the Stavely Belt (MGA 54 646258 5832700). The gritstone is filled with centimetre-sized, angular quartz, feldspar and volcanic-lithic detritus, including fragments of dark green andesite lava. The diversity and angularity of the clasts indicates a juvenile sediment source, with the adjacent Mount Stavely Volcanic Complex implicated. These beds are locally overturned to dip steeply west adjacent to the Stavely Belt. Farther from the Stavely Belt to the east, Glenthompson Sandstone beds dip and face west, and grade to more typical lithologies.

This reinterpretation fundamentally changes the construction of the serial cross sections used to build the STAVELY 3D model, opening the possibility that Glenthompson Sandstone may form part of the foundation of the Stavely Arc. Unreflective lower crust imaged beneath the Stavely Arc edifice in deep seismic reflection data across STAVELY may include Glenthompson Sandstone. This possibility is captured in the serial cross sections used to construct the STAVELY 3D model, and has resulted in the modelling of several laterally extensive Glenthompson Sandstone volumes that are confined entirely to the mid and lower crust of the STAVELY 3D model. These are conceptual, impossible to distinguish geophysically or conceptually from Precambrian continental crustal blocks also interpreted to be present beneath this region and which, by comparison with geology exposed in South Australia, the Glenthompson Sandstone may be deposited directly and unconformably upon.

This reinterpretation had significant consequences for the construction of the STAVELY 3D model. It shifts the large D1a thrust fault displacement magnitudes away from faulted contacts where volcanic belts of the Stavely Arc are seen to overlie Glenthompson Sandstone panels (e.g. Stavely East Fault, Narrapumelap South Fault, Dryden Fault) to faulted contacts where volcanic belts of the Stavely Arc are seen to underlie Glenthompson Sandstone panels, a geometry that is now recognised as a disordered stratigraphic succession (e.g. Stavely West Fault, Narrapumelap North Fault, Bunnugal West Fault).

3.2.7 Stawell Zone

The Stawell Zone bounds the eastern flank of the Grampians-Stavely Zone along the Moyston Fault. The Stawell Zone has been modelled as a single volume in the STAVELY 3D model. Rocks of the Moornambool Metamorphic Complex dominate this volume and are briefly described below with further details provided in Appendix 4 – Geological Units.

The Moornambool Metamorphic Complex (Cayley & Taylor, 2001) is a wedge of fault-intermixed metamorphosed Cambrian turbidites (Saint Arnaud Group, metamorphosed to biotite, muscovite-sillimanite/muscovite-garnet-staurolite schists: Lexington Schist; Good Morning Bill Schist) and mafic volcanic rocks (Magdala Basalt, metamorphosed to hornblende-garnet-diopside bearing Carrolls Amphibolite) that comprises the western margin of the Stawell Zone, forming a region that is approximately 15 km wide at surface, but likely narrows in width with depth (Cayley & Taylor, 2000a). The Stawell Zone, and the Moornambool Metamorphic Complex within it, bounds the eastern flank of the Grampians-Stavely Zone along the Moyston Fault. Thus, the Moornambool Metamorphic Complex defines the eastern flank of the STAVELY 3D model.

The Moyston Fault dips moderately to steeply east, so that Grampians-Stavely Zone rocks extend east in the fault footwall, beneath overthrust Moornambool Metamorphic Complex rocks. The STAVELY 3D model overlaps the surface position of the Moyston Fault to illustrate this relationship.

The Moyston Fault is regarded as the effective eastern edge of the exploration search space for Cambrian arc-related mineral systems – the rocks further east have allochthonous accretionary wedge characteristics. Thus, the Moornambool Metamorphic Complex has not been internally differentiated within the STAVELY 3D model. The Stawell Zone was modelled using averaged density (2.68 – 2.91 g/cm3) and magnetic susceptibility values for the rock successions exposed within it (summarised individually in Appendix 4). Further east, the Moornambool Metamorphic Complex passes into low-grade metasediments (turbidites) of the Saint Arnaud Group across the Coongee Fault (Cayley & Taylor, 2001). Saint Arnaud Group turbidites of lower density than their metamorphosed equivalents are modelled in serial cross sections 5 and 6 (Appendix 3).

3.2.8 Glenelg Zone

The Glenelg Zone, and the Glenelg River Metamorphic Complex within it, bounds the western flank of the Grampians-Stavely Zone along the Yarramyljup Fault. Thus, the Glenelg Zone defines the western flank of the STAVELY 3D model. The Glenelg Zone has been modelled as a single volume in the STAVELY 3D model. Rocks of the Glenelg River Metamorphic Complex dominate this volume and are summarised in Appendix 4 – Geological Units.

The Yarramyljup Fault is regarded as the effective western edge of the Cambrian arc-hosted mineral systems search space. Thus, the Glenelg Zone geology has not been internally differentiated within the STAVELY 3D model, other than fault-slices of ultramafic rocks (correlates of the Hummocks Serpentinite – see Appendix 4 Geological Units), which have been incorporated into some of the serial cross sections. The Glenelg Zone was modelled using a range of averaged density values (2.69 – 2.87 g/cm3) for the rock successions.

The Yarramyljup Fault dips west, so that Grampians-Stavely Zone rocks extend west in the fault footwall, beneath overthrust Glenelg Zone rocks. The STAVELY 3D overlaps the surface position of the eastern Glenelg Zone boundary to illustrate this relationship.

3.2.9 Unaffiliated highly reflective rocks

The rocks that form the footwall to the Apsley Fault lie deeply buried beneath the Glenelg Zone and beneath the western Grampians-Stavely Zone and, given the Neoproterozoic ages determined for parts of the overlying Glenelg Zone stratigraphy (Morand & Fanning, 2006, 2009) are interpreted as Paleoproterozoic – Paleoproterozoic continental crust (see 3.2.10 Unafilliated unreflective rock packages). The basal few kilometres thickness of this package is highly reflective, and can be traced in the footwall of the Apsley Fault beneath STAVELY. The higher reflectivity reflects a significantly higher velocity (i.e. density) for these rocks compared to exposed sialic Gawler Craton rocks. Increased density indicates a more mafic chemistry, likely comparable to the known mafic-ultramafic igneous successions of similar reflectivity that occur within fault-slices in the overlying Glenelg Zone. A range of density values (2.85 – 3.1 g/cm3) have been used to model the highly reflective rock packages that occupy the footwall of the Apsley Fault in STAVELY.

3.2.10 Unaffiliated unreflective rocks

The Proterozoic age and rifted continental character assigned to an elongated, upward-convex ribbon of unreflective crust buried deeply beneath the Stavely Arc in STAVELY is based on its uniform unreflective appearance in deep seismic reflection data (a characteristic of sialic crystalline rocks), its density (Murphy et al. 2006), its geometry, its tectonostratigraphic position, and the continental geochemistry of the overlying Stavely Arc which demands a continental margin setting for the arc in the Cambrian.

The western side of this buried continental ribbon is directly underlain by the Apsley Fault, the footwall of which comprises highly reflective rocks that thicken westwards and upwards, passing into unreflective crust beneath the greater Glenelg Zone farther west that is comparable in appearance and context to the crust beneath the Stavely Arc (Cayley et al., 2011b). The rocks that form the footwall to the Apsley Fault lie deeply buried beneath the Glenelg Zone, but are visible in deep seismic reflection data and can be traced near-continuously in legacy shallow marine seismic reflection data adjacent to The Coorong towards a surface position the Fleurieu Peninsula in South Australia, where inliers of Paleoproterozoic crust are exposed in the interior of the Adelaide fold and thrust belt. The Gawler Craton is directly observed to unconformably underlie the Adelaidean succession in South Australia, and the upper parts of the Adelaidean succession have now been traced into the Glenelg Zone of western Victoria (Morand & Fanning, 2006, 2009; Schofield et al., 2018). For these reasons, we interpret the crust in the Apsley Fault footwall, and beneath the Stavely Arc as Paleoproterozoic – Mesoproterozoic ‘Gawler Craton’ continental crust.

Continental breakups often result in microcontinental ribbons, and this is the interpretation we favour for the unreflective rock packages beneath the Stavely Arc edifice. The lower crust of the Grampians-Stavely Zone is likely to share the same rock properties as the uppermost crust lying in the footwall to the Apsley Fault further west in such a scenario. Petrophysical properties can thus be estimated from direct examination of exposed Paleoproterozoic-Mesoproterozoic Gawler Craton rocks in South Australia. An assumed average density of 3.1 g/cm3 has been used for unaffiliated unreflective rock packages in the STAVELY 3D model.

3.2.11 Upper mantle

Upper mantle rocks (below the Moho) in western Victoria are modelled as Proterozoic peridotite. These rocks are likely direct equivalents of the olivine-orthopyroxene-chrome spinel mantle rocks that have been faulted up into the directly overlying crust as the Hummocks Serpentinite (see Appendix 4 – Geological Units) and as related unnamed pyroxenites and metamorphic derivatives mapped in the Glenelg Zone.

Peridotites in western Victoria apparently range in composition from pyroxenite (typical density 3.1 – 3.6 g/cm3) as exposed in some Glenelg Zone fault zones (Morand et al., 2003), to dunite (typical density 2.84 – 2.85 g/cm3) as seen in olivine-dominated peridotite xenoliths carried to surface from the upper mantle by nearby Newer Volcanic Group eruptions (e.g. Mt Shadwell, near Mortlake in southern STAVELY).

The average density used for modelling the upper mantle rocks in STAVELY is 3.25 g/cm3. This reflects the likely composition that is equivalent to the Hummocks Serpentinite, in combination with the other end-member peridotite compositions known to occur beneath the region.

3.3 Modelled faults

The Grampians-Stavely Zone has been subjected to multiple deformation events (see Table 3.1), the most significant constrained by overprinting criteria and geochronology to the Mid-Late Cambrian (D1a transpression – late Delamerian Orogeny) and Siluro-Devonian (D3 transpression and D4 transtension – Bindian Orogeny-equivalent). The deformation style within STAVELY appears to be fault-dominated. The largest faults were formed during the most significant deformation events, and thus faults formed during D1a, D3 and D4 are the predominant structures included in the STAVELY 3D model.

Table 3.1 Summary of deformation events within STAVELY for reference. Accompanying text is provided in Appendix 5 – Deformation history summary.

Lesser deformation events in the Late Cambrian (D1b transtension – latest Delamerian Orogeny), Late Ordovician (D2 transpression; Benambran Orogeny equivalent) and Middle Devonian (D5 transpression; Tabberabberan Orogeny equivalent) appear to have mainly involved reactivations of pre-existing structures, and these are discussed on a case-by-case basis. Selected faults associated with formation of the Otway Basin are also included in the STAVELY 3D model.

Faulting is observed at all scales, but only the largest faults – typically those large enough to be resolved in regional-scale geophysical data – are captured in the STAVELY 3D model. The largest are named and described individually below, grouped according to size/importance and relative age. Smaller faults, including splay faults to named faults, are not named, but are numbered, and their interpreted characteristics (age, geometry, movement history, constraining data etc) are listed in Appendix 6 – Fault summary table.

All fault surfaces included in the STAVELY 3D model are depicted in Figure 1.6. The practical aspects of recognising structures that have deformed the Grampians-Stavely Zone, including STAVELY, are provided in Duncan et al. (in prep).

3.3.1 Overview of faults

Although internally structurally complex, the overthrust boundaries of the Grampians-Stavely Zone – the Yarramyljup and Moyston faults – are relatively simple and subparallel in strike, and serve to define limits to STAVELY and the area considered most prospective for rocks of the Stavely Arc and related minerals systems (Figure 2.2). The Yarramyljup and Moyston faults are both of D1a age, but dip away from the Grampians-Stavely Zone, meaning that the Grampians-Stavely Zone represents a region of major structural vergence change in western Victoria, and that the zone widens dramatically with increasing depth.

The D1a vergence change across the Grampians-Stavely Zone may be controlled in part by the large magmatic arc edifices within the zone that are imaged in deep seismic reflection data in the south, and in regional magnetic data in the north. These edifices are depicted in the STAVELY 3D model as autochthonous Stavely Arc and as Stavely Belt equivalent quasi-allochthonous volumes (Figure 3.54). They appear to have acted as structural buttresses that controlled the geometry of overlying and adjacent fault-systems. The arc edifices and the underlying, possibly Proterozoic, crystalline crust foundation they appear to overlie, may combine to form a rigid buttress that acted as a backstop to the more intense Cambrian deformation and related metamorphism of the adjacent structural zones.

In broader terms, convergent plate boundaries are typically associated with structural vergence changes related to subduction-related processes – and the data available for the Grampians-Stavely Zone suggests a continent-dipping suprasubduction zone setting for the Cambrian (see Schofield et al., 2018).

In general terms, most Cambrian-age faults (D1a thrusts) within the STAVELY interior dip west and appear to merge and link west into lower-crustal levels of the west-dipping Yarramyljup Fault footwall as a single imbricated network (Figure 3.10A). Although the east-dipping Moyston Fault was also formed at this time, few D1a faults within STAVELY can be traced east into this structure. For example, the east-dipping Escondida Fault segment north of the Grampians Ranges appears to have been active during D1, but is separated from the Moyston Fault by the west-dipping Dryden Belt and related D1a bounding faults. The persistence of D1a faults bounding the Dryden Belt along the length of STAVELY implies that the D1a Escondida Fault may be a backthrust to these faults, rather than a structure that linked farther east into the Moyston Fault footwall during D1a. Other easterly D1a fault dips are local, and generally confined to a region beneath and southeast of the Grampians Ranges. These can be explained as a consequence of subsequent deformations (see below).

The realisation that the northeast-dipping Narrapumelap and Elliot belts and bounding D1a faults are segments within a D4 megakink that were once continuous with the northerly, west-dipping D1a structures, means that the listric dip-profiles constrained by geophysical data for the Narrapumelap and Elliot belts and D1a bounding faults within the central limb of the Mafeking Megakink can be extrapolated around the positions of the western and eastern Mafeking Megakink hinges to constrain similarly listric profiles for the equivalent west-dipping D1a thrust fault segments (Figure 3.55). In this way, most of the west-dipping D1a structures mapped within the Cambrian bedrock across the width of STAVELY can be demonstrated to have listric profiles at crustal scale, all shallowing in dip magnitude to depth. This is consistent with the constraints provided by available deep seismic reflection data across the width of the Grampians-Stavely Zone, and forms a key premise that controls the construction of the serial crustal-scale cross sections used to construct the STAVELY 3D model surfaces and volumes.

No distinct D1b (rift) structures could be discriminated with sufficient confidence to be modelled. While the overall northerly alignment of Late Cambrian magmatic complexes may be related to underlying D1b rift structures, compartmentalised reactivation of D1a structures appears to be the predominant expression of D1b – this is shown by the close association of clusters of D1b intrusions to certain D1a faults within D1a volcanic (fault) belts.

In contrast to the predominant west-dip of D1a structures, almost all Early Devonian-age faults (D3 sinistral transpressional and D4 dextral transtensional faults) appear to dip east and merge and link east into the lower-crustal levels of the Moyston Fault footwall (Figure 3.56). D3 structures confined within the Grampians Group form a discrete network but have not been modelled. Large bedrock-hosted sinistral strike-slip faults such as the Latani, Cherrypool and Muirfoot faults are modelled and may be of D3 age. D4 structures are widely distributed. They may comprise one single fault network related to dextral transtension.

Figure 3.54 Oblique view of STAVELY 3D model looking north at extensive volumes interpreted as rocks of the Stavely Arc within the lower Dimboola Belt (purple) beneath the Escondida Fault footwall. This volume rises to the base of Murray Basin cover close to the northern edge of STAVELY. These volumes appear faulted over autochthonous Stavely Arc (in blue see Figure 3.14). All volumes are bound between the Yarramyljup (pale surface mesh to west) and Moyston (blue surface mesh to east) faults.

Figure 3.55 Oblique model view showing distribution of Stavely (dark purple), Narrapumelap, Dryden and Dryden North (pale purple) bounding D1a faults (A) and volumes (B). These volcanic belts were likely all continuous within a single west-dipping D1a fault slice pre-D4, but are now locally reoriented to northeast dips as for the Narrapumelap Belt in the middle limb of the Mafeking Megakink (see also Figure 3.45). The Moyston Fault appears to have been significantly reactivated during D2, D3 and D4. The Escondida Fault in particular preserves clear evidence of large scale reactivation and strike growth during D3-D4. The Escondida Fault appears to have established linkage east into the Moyston Fault footwall during D3-D4, as imaged in deep seismic reflection data in lines 97GA-V1, 97GA-V2 and 09GA-AR1 (Figure 2.22 and Figure 2.23). The Henty, Mosquito Creek, and Latani Faults and related structures were also linked to this large structure at this time, and are interpreted as splays.

Dextral transtensional displacement across the Escondida, Golton and Henty faults during D4 is spatially and temporally linked to formation of the Mafeking Megakink. The Mafeking Megakink has disrupted a single, linear D1a fault belt into the Stavely, Narrapumelap and Dryden belts. The middle limb of the megakink contains the Narrapumelap and Elliot belt segments and their respective bounding D1a faults, all locally rotated clockwise into northeast-dipping orientations (Figure 3.46). Megakink growth appears to have accompanied the formation of the adjacent Jalur Rift structural basin beneath the Grampians Ranges, within which clockwise rotations of other segments of D1a faults also occurred in the ‘Grampians Deeps’ and Brimpaen belts (see Section 4.1.1 – Mafeking Megakink retrodeformation). The structural style changes north along-strike to be dominated by strike-slip fault translation, as seen in the D4 fault network of the Dimboola Duplex which segments the Dimboola Belt (Figure 2.32).

Sinistral D3 offsets across the Latani, Cherrypool and Muirfoot faults, followed by dextral offsets across the Henty Fault during D4 appear to have disrupted a single linear D1a fault belt in the western Grampians-Stavely Zone into a fanning-array of fault-segments comprising the Grampians West, Tyar, Glenisla, Black Range and Black Range West/Mitre belts (Figure 3.13). The tectonics and constraints of this interpretation are discussed in more detail in Section 4.1 – D4 and D3 retrodeformation testing. The possible geodynamics are explained in more detail in Section 4.4 – Implications for the architecture of the Stavely Arc through time (see also Schofield et al., 2018 and Duncan et al., in prep).

Few D5 structures (sinistral-transpressional faults and related joint networks, including in granite intrusions) are large enough to be included in the STAVELY 3D model. One exception is the Mosquito Creek Fault. Although much of its displacement in the Cambrian bedrock is probably older (D3, D4), the truncation of D3 and D4 structures in the Grampians Group (e.g. western flank of D4 Big Cord Orocline; see Figure 2.16) shows that the last sinistral strike-slip movements on the Mosquito Creek Fault are of D5 age.

Late east-west trending faults with small displacements are related to the rift and sag-related formation of the Murray (and underlying ‘Netherby’) and Otway basins. Most Otway Basin faults appear to dip south towards the present-day Australian continental margin (Figure 3.57). Steps in sub-basin margins within the Otway Basin are locally coincident with D1a faults.

Some of the key structural characteristics include:

1. The predominant west dip of the D1a thrust fault network that developed within the Grampians-Stavely Zone when the Stavely Arc was active indicates that the fault network propagated from the west; i.e. from beneath the back-arc region of the Stavely Arc and from the Yarramyljup Fault footwall during the last time it was active. Although the Moyston Fault apparently also formed as a thrust during D1a, few faults of D1a age within STAVELY can be traced into this structure.

2. The D3-D4 (+D5) fault networks (and possibly also D1b and D2 faults – none of which have been modelled in STAVELY) that developed within the Grampians-Stavely Zone during and after the Stavely Arc had become extinct appear to have propagated from the east; i.e. from the Moyston Fault footwall and from beneath the region most affected by D2-D5 age (Lachlan) orogenesis. The Moyston Fault appears to have undergone significant reactivation during all subsequent deformation episodes (particularly D2-D4). There is no evidence of significant reactivation of the Yarramyljup Fault post-D1.

3. Late, small-displacement extensional faults related to the Otway Basin rifting appear to have propagated into STAVELY from the south. The uneven distribution of Otway Basin stratigraphy shows that rifting was not evenly distributed within STAVELY. The margin of the Otway Basin has stepped offsets that locally coincide in position with D1a – D4 fault segments that lie at a high strike-angle to the overall axis of basin rifting. This indicates that parts of these older structures underwent compartmentalised reactivations as ‘transfer’-type structures during the Otway Basin rift event.

4. Fault network interactions within the STAVELY 3D model volume are very complex, particularly because there are multiple different fault generations of varying and contrasting dip and strike. Faults with northeast-southwest strikes at surface (e.g. the Victoria Valley South, Ashens, Winian East, Lorquon, Hopkins River, Marathon faults) and east-west strikes (e.g. Otway Basin faults) strike sub-parallel to the serial cross sections that underpin model construction. The positions of such faults within the lower crust were estimated by downward lateral projection though multiple serial cross sections, and then refined iteratively from preliminary fault-surfaces built within the model volume. Establishing the correct dip-direction of fault planes at surface was critical, since this is generally – but not always – assumed to be maintained to depth.

5. It is likely that many faults present within the lower to mid-crust of the Grampians-Stavely Zone do not extend to the current levels of exposure, and so have not been recognised. Such ‘blind’ faults generally cannot be mapped from surface and are only visible where datasets exist that can directly image the subsurface – typically (deep) seismic reflection data. This means that the STAVELY 3D model is likely to significantly underestimate the true number of faults and related structures within STAVELY.

Figure 3.56 Oblique view of STAVELY 3D model, showing major (named) D3 and D4 fault surfaces, most of which dip eastwards and link at depth into the Moyston Fault footwall (blue translucent surface).

Figure 3.57 Oblique view of the southern STAVELY 3D model, showing predominantly south-dipping fault surfaces that combine to bound the northern margin of the Otway Basin. Otway Basin stratigraphy that is thicker on the hangingwall sides of these faults show them to be Cretaceous normal (extensional) faults. Some (e.g. C2, C3) are experiencing minor inversion in the in-situ compressive stress-field. Many of the Cretaceous faults terminate against the Bunnugal East Fault (mesh surface, colour coded for depth), implicating transtensional reactivation of portions of this D1a structure during Cretaceous opening of the Otway Basin. This shows that D1a structures in the Cambrian bedrock exerted a direct control on the locations and shapes of Otway sub-basins. These relationships show why developing an understanding of Cambrian-Devonian basement structural architecture is critical for developing a full understanding of the overlying Otway Basin.

3.3.1.1 Apsley Fault

The Apsley Fault (Cayley et al., 2011b) is nowhere exposed, but is interpreted to extend into the lower crust of STAVELY from the west, and is included in the STAVELY 3D model as a discrete surface. The Apsley Fault has been directly imaged in the lower crust here in deep seismic reflection data, and coincides with an abrupt change in shear velocity (Figure 2.4).

The Apsley Fault position is revealed in deep seismic reflection line 09GA-SD1 where the west-dipping Yarramyljup Fault and associated reflective mafic-igneous rocks in the Glenelg River Metamorphic Complex in its hangingwall, abruptly terminate down-dip against a moderately east-dipping boundary that is underlain by less-reflective rocks west of STAVELY (Figure 2.22). The termination of the Yarramyljup Fault occurs at ~ 7 seconds TWT (or ~21 km) deep, and is interpreted to occur along the Apsley Fault. The fundamental change in seismic character and geometry across the Apsley Fault position is a very prominent feature, and indicates a major, crustal-scale, tectonic boundary.

The Apsley Fault appears to represent the uppermost fault of a subparallel network of predominantly east-dipping faults confined entirely to the middle and lower crust below the Glenelg Zone. The lower crust here is interpreted as Mesoproterozoic cratonic rocks, since the deep seismic reflection data shows it to continue west into South Australia, rising towards surface to a position located directly southeast along-strike from Paleoproterozoic – Mesoproterozoic cratonic blocks with Gawler Craton affinities exposed as structural inliers within the Cambrian-aged Adelaide Fold-Thrust Belt. A key inference from the Apsley Fault now bounding the upper surface of these Mesoproterozoic cratonic rocks is that, prior to formation of the fault, thick Mesoproterozoic cratonic rocks must have extended further east. This is one key line of argument for interpreting a Mesoproterozoic continental crust foundation buried deep beneath the Stavely Arc.

The Apsley Fault projects eastwards below the Stavely Arc at 8-9 seconds TWT (equating to a depth of 26-28 km), and continues to dip east to reach the Moho at around 11 seconds TWT (~33 km). In the lowest parts of the crust the Apsley Fault position coincides with a region of uniformly reflective rocks which makes discriminating the fault difficult. Nevertheless, pronounced changes in dip and reflective character lie along the projected trace of the fault to the base of the crust, and so the Apsley Fault is interpreted to continue east beyond and below the eastern edge of deep seismic reflection line 09GA-SD1. Faults generally propagate upwards as they form, implying that the Apsley Fault trace continues downwards and eastwards into the lithospheric mantle and below the base of the STAVELY 3D model.

The point that the Apsley Fault trace reaches the Moho lies beneath the centre of STAVELY. The directly overlying lower crustal material, in the Aspley Fault hangingwall, shows similar reflectivity (and modelled density) characteristics as seen for the interpreted Mesoproterozoic rocks in the Apsley Fault footwall farther west. This sequence is included in the STAVELY 3D model as a Mesoproterozoic continental foundation of Gawler Craton affinity, upon which the autochthonous Stavely Arc edifice was deposited. The northwesterly trend of the Apsley Fault lies subparallel to the Grampians-Stavely Zone, implying that the cratonic foundation in its hangingwall is a ‘ribbon’ that underlies the Stavely Arc along the whole length of the Grampians-Stavely Zone.

The magnitude of apparent lateral offset between interpreted Mesoproterozoic cratonic ‘ribbon’ beneath the Stavely Arc in STAVELY (in the Apsley Fault hangingwall), and footwall rocks of similar character and likely Gawler Craton provenance beneath the Glenelg Zone further west is approximately 100 km. The lateral offset has an extensional geometry that is at odds with the predominant thrust-geometry seen for D1a faults in the Grampians-Stavely Zone and in the Glenelg Zone to the west, including the Yarramyljup Fault. The implication is that, prior to significant inversion during D1a shortening, lateral extensional offset across the Apsley Fault was even larger. The apparent lateral offset across the Apsley Fault position prior to D1a inversion, combined with the depth-extent of faulting is reminiscent of lithospheric-scale extensional rifting. The Glenelg and Grampians-Stavely zones are both confined to the Aspley Fault hangingwall, and contain ultramafic rocks (e.g. Hummocks Serpentinite; Williamsons Road Serpentinite) of inferred Neoproterozoic age and with characteristics of mantle rocks exhumed by hyperextension (GSV unpublished data; Gibson et al., 2015). The overall geometry, rock-associations and age are consistent with crustal-scale hyperextensional rifting across the Apsley Fault position in the Neoproterozoic.

The east-dipping Apsley Fault, and subparallel structures in its footwall, are probably buried remnants of a seaward-dipping continental margin extensional system that formed in the Gawler Craton during the rift breakup of Rodinia (see Section 4.5.1 – A scalable structural template for STAVELY through time, Figure 4.18 – 1). Such structures have been speculated to lie at depth here, based on the passive margin character of the Neoproterozoic-Cambrian Glenelg Zone stratigraphy (e.g. VandenBerg et al., 2000, Figure 5.7), but the deep seismic reflection data acquired in 2009 provided the first evidence that such structures in ancient crust exist beneath western Victoria.

The Neoproterozoic age and passive-margin origin indicated for the Apsley Fault sets it apart from all other faults modelled included in the STAVELY 3D model. This fault apparently preceded initiation of convergence along the margin, and eruption of the Stavely Arc.

3.3.2 Regional-scale D1a faults

Regional-scale D1a (Grampians-Stavely Zone-bounding) structures are described in the following section and presented with district-scale faults and volcanic belts of the Stavely Arc in Figure 1.6 and Figure 2.28. The age of the D1a faults is constrained by overprinting criteria and by context to a period between approximately 505 Ma and approximately 500 Ma. The constraints are the age of folding and faulting of ≥500 Ma metamorphic rocks deformed and truncated by the faults, namely rocks of the Glenelg River Metamorphic Complex in the hangingwall of the Yarramyljup Fault in the west of STAVELY (Gibson & Nihill, 1992; Morand et al., 2003), and rocks of the Moornambool Metamorphic Complex in the hangingwall of the Moyston Fault in the east of STAVELY (Miller et al., 2005; Cayley et al., 2011a).

Most D1a structures within STAVELY are unconformably overlain by Grampians Group, which provides an additional age constraint that is widespread geographically, but too broad in time to discriminate from D1b.

With this age, the D1a faults appear to have developed late in the Delamerian Orogeny. The Delamerian Orogeny is a protracted and complex event that began around 514 Ma or earlier (e.g. see Foden et al., 1999; Morand et al., 2003; Schofield et al., 2018), and includes a discrete phase of crustal shortening and thickening that occurred towards the end of the Delamerian Orogeny (D1a in STAVELY). The Delamerian Orogeny appears to have terminated at approximately 495 Ma (e.g. Foden et al., 2006) with a pulse of apparently transtensional deformation temporally and geographically associated with magmatism (D1b in STAVELY).

3.3.2.1 Moyston Fault

The Moyston Fault is considered a very significant structure in western Victoria because it has the largest vertical throw apparent in any fault within the Tasman Fold Belt system. East-side up vertical displacement of more than 20 km in the Late Cambrian (i.e. D1a) is indicated by the juxtaposition of upper amphibolite facies rocks of the Moornambool Metamorphic Complex of the Stawell Zone in the hangingwall to the east, against sub-greenschist facies rocks of the Glenthompson Sandstone and volcanics of the Stavely Arc in the Dryden Belt in the footwall to the west (Radojkovic, 1989; Cayley, 1995; McKnight et al., 2000; Cayley & Taylor, 2001; Phillips et al., 2002; Miller et al., 2005).

The position, dip-direction and overall geometry of the Moyston Fault is constrained by fault plane outcrops near Moyston (Figure 2.13) and near Mount Drummond (Cayley & Taylor, 2001), and by mineral exploration drill holes that straddle and/or penetrate the fault plane to the north and south. Deep seismic reflection lines also cross the fault, and provide additional regional constraints of fault geometry. South of Moyston, the fault is buried beneath Newer Volcanic Group, however in addition to mineral exploration drilling its position can be traced in regional magnetic and gravity data. The map-trace of the Moyston Fault is relatively linear, with occasional offsets and dog-legs where cut and modified by younger structures.

An east-dipping fault geometry, and a cross-cutting relationship at depth with contemporaneous west-dipping faults within its hangingwall, are demonstrated for the Moyston Fault from geological mapping (Cayley & Taylor, 2001) and interpretation of regional potential field datasets (e.g. Section A-B, Figure 97 of Cayley & Taylor, 2001). In deep seismic reflection data, the Moyston Fault occurs as a major set of east-dipping reflectors that continue east and down beneath the Stawell and Bendigo zones to intersect the Moho at a depth of approximately 42 km depth (Cayley et al., 2011a).

The Moyston Fault has a complex movement history. Cooling of mylonitic metamorphic rocks in its hangingwall (Figure 2.13) is dated at approximately 500 Ma from metamorphic hornblende and muscovite (Miller et al., 2003), indicating that the major uplift and unroofing of the Moornambool Metamorphic Complex relative to the Grampians-Stavely Zone footwall occurred up until this time. This suggests a Cambrian age for the majority of movement across the Moyston Fault, consistent with the idea that the Moyston Fault developed outboard of the Stavely Arc, synchronous with its formation and subsequent D1a accretion along the east Gondwanaland margin (Cayley et al., 2011a, 2011b). Stretching lineations developed in the mylonitic rocks in the fault hangingwall (Figure 3.58 B) are consistent with uplift of the Moornambool Metamorphic Complex in response to sinistral transpression (Cayley & Taylor, 2001).

The Moyston Fault has experienced multiple episodes of reactivation that appear to have had significant influence on both the deposition (D2) and deformation (D3-D4) of the ?Late Ordovician-Late Silurian Grampians Group. The strong deformation imparted on the Grampians Group during D4 resulted from the reactivation and/or development of large east dipping bedrock structures, such as the Escondida and Golton faults (Section 3.3.4.1 – Escondida Fault (reactivated) and Section 3.3.4.2 – Golton Fault), that reside in the footwall of the Moyston Fault and appear to link into the Moyston Fault plane at depth as indicated in deep seismic reflection imagery.

The trace of the Moyston Fault itself is disrupted and offset by D4 faults in several places, for example where splays of the Mehuse or Golton faults cut across and offset the Moyston Fault trace north of Dadswells Bridge, and near Moyston where the main fault outcrop (see Figure 2.13 A) has a southeasterly dip that is anomalous to the overall fault trend and dip-direction, and is suggestive of subsequent clockwise block-rotation of a monolithic segment of the fault interface, possibly in response to dextral shearing. This notion is consistent with en-échelon block-overturning of fault-bounded Grampians-Stavely Zone bedrock in the fault footwall (Cayley & Taylor, 2001), which we now also attribute to D4 (see Section 4.1.5 – Retrodeformation of locally overturned Cambrian strata in the Moyston Fault footwall).

A few km south of Wallup, magnetic data reveals a near-isoclinal J-shaped fold of over 8 km apparent amplitude developed within magnetic Moornambool Metamorphic Complex stratigraphy adjacent to the Moyston Fault position buried beneath the Murray Basin (Figure 3.59). The asymmetry of the Wallup Fold (see Section 3.4.6 – Wallup Fold) suggests a drag-induced formation in response to dextral shear. This, the block-reorientations of parts of the Moyston Fault trace, en-échelon block-overturning of segments of the eastern Grampians-Stavely Zone Cambrian strata, and an even larger J-shaped drag fold that overprints the Coongee Fault extension northeast of Warracknabeal, are evidence of a significant late episode of dextral shear along the Moyston Fault, which we assign to reactivation during D4. The modifying influence of D4 on the Moyston Fault is further described in Section 4.1.6 – Analysis of D4 and D3 retrodeformation results.

Figure 3.58 Moornambool Metamorphic Complex structures, typical of the Moyston Fault hangingwall high strain zone. A) Polydeformed psammitic Lexington Schist adjacent to the Mount Ararat Fault, a major west-dipping back-thrust to the Moyston Fault located in the centre of the metamorphic complex, about 8 km east of the Moyston Fault trace. Note sub-vertically plunging refolded crenulation folds and multiple generations of boudinaged quartz veins and spaced alternating quartz-biotite metamorphic segregations. Mount Ararat, MGA 54 664121 5869028. B) Prominent steeply northwest-plunging stretching lineation in nearby mafic Carrolls Amphibolite on the crest of the Mount Ararat range (MGA 54 664270 5869028). This lineation is defined, across the range, by an alignment of metamorphic minerals, quartz veins and the rotated axes of crenulation folds, including the folds in intercalated Lexington Schist seen in A. The geometry is consistent, and interpreted as recording sinistral transpression on west-dipping backthrusts in the Moyston Fault hangingwall during D1a – a proxy for interpreting sinistral transpression along the Moyston Fault plane during D1a. C) Tectonic mélange of Carrolls Amphibolite (dark green – a metamorphosed and polydeformed MORB) and Good Morning Bill Schist mylonite (pale – a metamorphosed and polydeformed siliciclastic rock). These contrasting rock-types are fault-intermixed at all scales throughout the western Moornambool Metamorphic Complex, typical of the mid-crustal levels of an accretionary wedge. Good Morning Bill Creek, MGA 54 660721 5864378. D) Polydeformed Carrols Amphibolite located just a few metres east of the Moyston Fault trace. Note the strong shear foliation defined by hornblende and thin parallel quartz veins, overprinted by upright crenulation folds. Salt Creek, MGA 54 655821 5871678.

Figure 3.59 Plan-view of a tilt and band pass filtered magnetics image including STAVELY shaded to highlight the volcanic belts of the Stavely Arc and the interpreted network of Late-Cambrian (D1a) to Early Devonian (D4) faults included in the STAVELY 3D model, and the location of the inset that includes the Wallup Fold, developed within the Stawell Zone. Inset showing the tight Wallup Fold closure in the Moyston Fault hangingwall. The ‘J’-shape of the Wallup Fold closure is consistent with formation in response to dextral displacement along the Moyston Fault. The geometry, large size and coherence of the Wallup Fold is inconsistent with the typical style of Cambrian (D1a) structures in the Moyston Fault hangingwall (see Figure 3.58), and so is attributed instead to late deformation associated with D4. A larger amplitude faulted fold of similar geometry is associated with dextral offset and refolding of the D1a Coongee Fault across the Angip Fault, also attributed to D4.

3.3.2.2 Yarrramyljup Fault

The Yarramyljup Fault (Gibson & Nihill, 1992) is a buried north-trending fault that separates sillimanite-bearing schist of the Glenelg River Metamorphic Complex (Glenelg Zone) to the west from low-grade metamorphosed rocks of the Nargoon and Kanmantoo groups and Mount Stavely Volcanic Complex of the Grampians-Stavely Zone to the east. The position of the Yarramyljup Fault, as far north as Clear Lake, is constrained by regional magnetic data, deep seismic reflection data, outcrop in Yarramyljup Creek (south of Balmoral) and several mineral exploration drill holes (Morand et al., 2003; Cayley et al., 2011b; Figure 3.60). The Yarramyljup Fault must pass west of the Black Range (drill hole STAVELY05 intersects low grade metasediments of the Grampians-Stavely Zone), east of the Dundas Ranges (schist of Glenelg River Metamorphic Complex crops out in dam excavations east of Mount Dundas; Morand et al., 2003), and west of the southern tip of the Serra Range (where low grade Glenthompson Sandstone is faulted against Grampians Group; Spencer-Jones, 1965; Taylor & Cayley, 1997; and Nargoon Group metasediments under younger cover rocks are intersected in a number of drill holes).

Figure 3.60 Tilt and band pass filtered magnetics pseudocolour image of central-west STAVELY, showing the interpreted position of the Yarramyljup Fault trace, and a selection of the key control points – outcrops in Yarramyljup Creek and the Glenelg River, and mineral exploration drill holes – that help constrain the fault position at surface and beneath Murray Basin cover. See Figure 3.2 for drill hole constraints on the Yarramyljup Fault position beneath Otway Basin cover. The geometry of the fault at-depth is constrained by a single deep seismic reflection line (see Figure 2.22) Gibson & Nihill (1992) interpreted a steep westerly dip for the Yarramyljup Fault, and considered the much higher grade of Glenelg River Metamorphic Complex rocks exposed to the west to indicate a substantial west-side up displacement. East-verging crenulation folds in the high-grade hangingwall (e.g. Figure 3.61) are compatible with the fault being an east-vergent thrust fault, although these high-grade hangingwall structures are clearly cut by the Yarramyljup Fault and therefore predate final movements on the fault. Deep seismic reflection line 09GA-SD1 images the Yarramyljup Fault as moderately west-dipping into mid-crust (Cayley et al., 2011b).

Major movement on the Yarramyljup Fault appears to be D1a. Structures related to its formation clearly overprint peak metamorphism and associated deformation in the Glenelg River Metamorphic Complex (Morand et al., 2003). The fault plane, and the rocks adjacent to them, appear quite planar in deep seismic reflection data, and so the fault appears not to have undergone significant refolding or reactivation, although the fault trace is cut by the West Henty Fault, and appears to steepen across the position of this younger, D4 strike-slip structure. The Yarramyljup Fault trace is overlain non-conformably by the Grampians Group in several places, which constrains the final fault movements to pre-Silurian, and possibly pre-Late Ordovician.

While the Yarramyljup Fault trace is relatively linear, it is disrupted and offset by younger, D4, faults in several places, for example by the Latani Fault near Cadden Flat, and by a buried, northerly-trending fault north of Cavendish. Between Clear Lake and Yanac the trace of the D4 Henty Fault lies subparallel to the Yarramyljup Fault but, unlike the related West Henty Fault, does not appear to have interacted with the Yarramyljup Fault.

3.3.3 District-scale D1a faults

The main D1a district-scale faults included in the STAVELY 3D model have deformed the Stavely Arc and are described below from east to west across the width of the Grampians-Stavely Zone. District-scale D1a faults are presented with regional-scale D1a faults and volcanic belts of the Stavely Arc in Figure 1.6 and Figure 2.28. Several new district-scale D1a faults identified during the new interpretation and construction of the STAVELY 3D model are named and described here for the first time. The district-scale D1a faults are constrained by overprinting criteria and geochronology and are equivalent in age to regional-scale D1a faults (see Section 3.3.2 – Regional-scale D1a faults).

The Late Cambrian Lalkaldarno Porphyry and Bushy Creek Granodiorite (~502-498 Ma; Lewis et al., 2016) intrude D1a district-scale faults that cut and bound upturned volcanic belts of the Stavely Arc and intervening Cambrian metasedimentary strata and thereby provide a local upper age constraint. The Middle Cambrian age (≥511 Ma – ~502 Ma; Lewis et al., 2016) of rocks deformed by D1a, including the greater Mount Stavely Volcanic Complex, provides a local lower age constraint. With these age constraints, the timing of district-scale D1a faults in STAVELY is constrained to the interval 505-500 Ma, overlapping in time with Stavely Arc magmatic activity.

Figure 3.61 Glenelg River Metamorphic Complex A) Isoclinally-folded and fault-imbricated pegmatite veins in sillimanite schist in the Yarramyljup Fault hangingwall. The folds verge east towards the fault and are likely to be of D1a age. Yarramyljup Creek, MGA 54 576871 5871478 B) Core from diamond drill hole DD88BL215, northwest of Balmoral, showing layered folded migmatite and late pegmatite veins that are typical of the eastern parts of the metamorphic complex (MGA 54 555121 5902178).

3.3.3.1 Boonawah Fault East and West

The Boonawah East and Boonawah West faults are concealed beneath Rocklands Volcanic Group and/or Newer Volcanic Group. The existence of these faults is inferred from the presence of igneous rocks – including serpentinised peridotite, Cambrian diorite with Stavely Arc affinities, and other mafic igneous rocks interpreted to be part of the same Cambrian volcanic sequence – within the north-trending Boonawah Belt. The Boonawah Belt is the most westerly volcanic belt rocks currently known in STAVELY. These rocks have been intersected in several mineral exploration drill holes beneath Newer Volcanics Group and thin Otway Basin sediments south of Dunkeld.

The Boonawah East Fault is interpreted to bound the eastern flank of the Boonawah Belt, and to separate the Boonawah Belt to the west from metasediments (of either the Kanmantoo or Nargoon group affinity) to the east. The Boonawah West Fault is interpreted to bound the western flank of the Boonawah Belt, separating the belt from Nargoon Group metasediments farther west (intersected in several drill holes). Little is known about the dip of the Boonawah East and West faults, but the buried fault-traces appear to exhibit a northerly trend, parallel to the strike of the Boonawah Belt.

The Boonawah Belt and its bounding faults appear to have been cut and offset across the buried trace of the Mosquito Creek Fault near Karabeal. The Boonawah Belt and its bounding faults persist farther north, visible as a magnetic body partly buried beneath the southern Victoria Range, with rocks of boninitic affinity intersected in groundwater bore 104667 near Mooralla (see Crawford, 2016) – this part of the belt is described separately as the Grampians ‘West’ Belt. Overprinting relationships suggest that the Boonawah faults are Cambrian structures – there is no expression of them in the overlying Grampians Group and D3-D4 structures that deform the Grampians Group here also deform and offset the underlying Boonawah and Grampians ‘West’ belts and, by inference, the faults that bound their flanks. Near Woohlpooer farther north, the Boonawah East and West faults are apparently cut by the Wannon Fault which now separates these faults from the eastern end of the Tyar Belt and the Tyar Fault System, with which they may have been continuous pre-Early Devonian D4 (Figure 3.62).

The Boonawah Fault system is interpreted to have developed as a west-dipping and east-vergent thrust system during D1a, responsible for emplacing a fault-slice of the Stavely Arc and associated rocks into a metasediment-dominated succession. The Boonawah Fault system may represent a segment of the first of a sequence of imbricate thrust-splays that emanated successively from the footwall of the large Yarramyljup Fault during D1a.

Figure 3.62 Oblique model view showing the distribution of the Boonawah (orange), Grampians ‘West’, Tyar, Cherrypool, Black Range and Black Range West / Mitre (pale yellow) belt volumes (see Figure 3.43). These volcanic belts were likely all continuous within a single east-dipping listric D1a fault system pre-D4, but have been segmented, reoriented and offset by D3 and D4 strike-slip faults in the vicinity of the Black Range (see Figures 3.12 and 3.13).

3.3.3.2 Bunnugal East Fault

The Bunnugal East Fault is interpreted to bound the eastern edge of the poorly-exposed Bunnugal Belt. The fault is interpreted to dip steeply west based on the modelled dip of magnetic Mount Stavely Volcanic Complex rocks within the Bunnugal Belt (e.g. Figure 3.26), and the predominant west-dip of Glenthompson Sandstone stratigraphy to the east of the fault. The sense and magnitude of displacement on the Bunnugal East Fault is difficult to determine. Since the Glenthompson Sandstone to its east is now regarded as Kanmantoo Group and possibly older than the Stavely Arc, it is possible that the westwards transition from west-facing Glenthompson Sandstone to Mount Stavely Volcanic Complex in the Bunnugal Belt is virtually conformable. All the rocks were strongly tilted and folded during D1a however, and the different rheological behaviour of metasediments versus metavolcanics suggests that some differential displacement is very likely to have occurred across the Bunnugal East Fault position during D1a.

North of Glenthompson the Bunnugal East Fault position converges towards the Stavely West Fault, with the intervening panel of Glenthompson Sandstone thinning progressively northwards until it is absent 12 km north of Glenthompson. North of this position of confluence, the Bunnugal and Stavely Belts of volcanics lie in direct fault-contact with one another. Since the reflective character of Cambrian igneous rocks in the Bunnugal and Stavely belts is similar, the Bunnugal East Fault is poorly imaged in deep seismic reflection line 09GA-AR1 where the two volcanic belts lie in direct contact. The along-strike convergence of the Bunnugal and Stavely belts demonstrates that substantial displacement must exist on one or both of the belt-bounding faults, however the actual point of belt-confluence in plan-view appears arbitrary, a function of the present-day level of erosion. For this reason, the Stavely West and Bunnugal East faults are interpreted to diverge at depth in the STAVELY 3D model, similar to the relationship seen in plan (Figure 3.25 and Figure 3.47).

The northern end of the Bunnugal East Fault position is buried beneath the Grampians Ranges and not exposed. Magnetic data indicate that the northern end of the Bunnugal Belt terminates abruptly and at a high-angle against a north-east-trending structure – the D4 Victoria Valley South Fault – close to the margin of the Victoria Valley Batholith, buried beneath Grampians Group (see Figure 3.68). The Bunnugal East Fault is therefore likely also terminated here. The Bunnugal East Fault has also been cut and offset dextrally by several kilometres across the D4 Yarrack Fault. These cross-cutting relationships clearly constrain a pre-D4 and pre-Grampians Group age for the Bunnugal East Fault.

Retrodeformation of D4 displacements across the Victoria Valley Fault, the Early Devonian Mafeking Megakink and the Jalur Rift suggest that, prior to the Devonian, the Bunnugal East Fault continued north as the Elliot South Fault (Section 3.3.3.6 – Elliot South Fault). In its present position the Elliot South Fault is separated from the Narrapumelap North Fault (the pre-D4 equivalent of the Stavely West Fault) by a panel of northeast dipping and facing Glenthompson Sandstone >3.5 km wide. This relationship is a clockwise-rotated equivalent to that seen between the Bunnugal East and Stavely West faults south of Glenthompson, further evidence that the confluence of the Bunnugal and Stavely Belts seen north of Glenthompson is a localised phenomenon rather than a consequence of persistent thrust-ramping or any other type of major structural transition between different volcanic belts.

Correlates of the Bunnugal East Fault in the Grampians ‘Deeps’ and Brimpaen belts lie deeply buried beneath Grampians Group sediments. Evidence of clockwise rotation of both the Grampians ‘Deeps’ and Brimpaen belts around sub-vertical axes during D4 (e.g. the Big Cord Orocline; see Figure 3.73) rifting means that any equivalent of the Bunnugal East Fault in these volcanic belts is likely to occur on the western side of the rotated, northeast-dipping belt segments that are included in the Grampians ‘Deeps’ Belt in the STAVELY 3D model. Near surface, the position of any correlate of the Bunnugal West Fault beneath the Grampians Ranges and in the Brimpaen Belt is now occupied by younger faults such as the Mosquito Creek Fault (at Brimpaen). These younger (D4) faults are subvertical near-surface, apparently shallowing to east-dipping at depth.

3.3.3.3 Bunnugal West Fault (new name)

The Bunnugal West Fault is interpreted from regional magnetic, seismic reflection and gravity data to bound the western flank of the Bunnugal Belt. The Bunnugal West Fault is the westernmost of a series of steeply-dipping faults imaged by deep seismic line GA09-AR1 above 3 s TWT (ca. 9 km; Cayley et al., 2011b). These faults all splay upwards from the western flank of the underlying reflective Stavely Arc edifice, with the Bunnugal West Fault rising to surface in the vicinity of CDP 2220 (Figure 2.22). The Bunnugal West Fault separates less-reflective metasedimentary rocks to the west from seismically reflective Cambrian igneous rocks close to the surface to the east.

The precise affiliations of non-magnetic Cambrian metasediments west of the Bunnugal West Fault, such as those exposed adjacent to the Bencke Quarry (Spencer-Jones, 1965; Cayley & Taylor, 1997a; Taylor & Cayley, 1997), and intersected in several mineral exploration drill holes, are unknown. If these Cambrian metasediments are Nargoon Group and younger than the Mount Stavely Volcanic Complex (e.g. Morand et al., 2003), the Bunnugal West Fault may have a limited displacement. If the metasediments are Glenthompson Sandstone and Kanmantoo Group-equivalent they may be older than the Mount Stavely Volcanic Complex, requiring a considerable overthrust displacement across a west-dipping fault in this position.

The Bunnugal West Fault is concealed beneath Grampians Group in the Serra Range in the north. The Bunnugal West Fault is probably Late Cambrian (D1a) in age given that it is not expressed in the overlying Grampians Group. In the vicinity of Mirranatwa, regional magnetic data shows the Bunnugal West Fault position to be truncated at a high angle by the north-east trending D4 Victoria Valley South Fault. Retrodeformation of the effects of D4, including the Mafeking Megakink and the Victoria Valley Fault suggests that, prior to formation of the Jalur Rift, the Bunnugal West Fault was continuous with the Elliot North Fault.

South of Strathmore, the Bunnugal West Fault is entirely concealed beneath Newer Volcanic Group. Its position is interpreted from regional magnetic and gravity data to pass west of Woodhouse and towards Minjah, where it apparently converges towards the interpreted position of the Boonawah East Fault. South of Minjah the volcanic rocks of the Bunnugal Belt appear to thin and disappear, implying that the Bunnugal Fault system has lost displacement so that Stavely Arc rocks are no longer reaching towards surface. This interpretation is reflected in the STAVELY 3D model. Gravity data shows an additional belt of Cambrian(?) igneous rocks occurs beneath Newer Volcanic Group just east of Caramut. This volcanic belt is unlikely to represent a late lateral fault-offset of the Bunnugal Belt, since no similar lateral offset occurs to the adjacent Boonawah Belt. Instead, the volcanic belt east of Caramut is attributed to displacement on faults within the panel of Glenthompson Sandstone that separates the Bunnugal and Stavely belts – an interpretation of how we think this may occur is also represented in the STAVELY 3D model (Figure 3.28).

Correlates of the Bunnugal West Fault in the north-east and east-dipping Grampians ‘Deeps’ and Brimpaen belts lie deeply buried beneath Grampians Group sediments. Evidence of clockwise rotation of both the Grampians ‘Deeps’ and Brimpaen belt segments around sub-vertical axes within the core of the Jalur Rift during D4 and Mafeking Megakink formation, means that any correlation of the Bunnugal West Fault in these volcanic belts is likely to lie buried beneath the northern Victoria Valley, bounding the eastern side of the rotated belt segments and dipping east, as depicted in the STAVELY 3D model and described in more detail below.

3.3.3.4 Dryden Fault

The Dryden Fault bounds the eastern side of the Dryden Belt, a west-facing Cambrian metavolcanic sequence exposed at Mount Dryden and in other hills south along-strike. The Dryden Fault is interpreted as a steep, west-side-up fault, probably originally a thrust (Cayley & Taylor, 2001). It is imaged as west-dipping in the vicinity of Mount Dryden in deep seismic reflection line 97AGS-V1 (Korsch et al., 2002). Farther south, proximal to the Moyston Fault, the Dryden Belt, and therefore likely the fault itself, become subvertical and locally overturned to steep east-dips. Farther south again, the Dryden Fault is modelled to dip west. The Dryden Fault can be interpreted in potential field data as a simple west-dipping structure that bounds the eastern edge of the Dryden Belt, and the Dryden North Belt, far north beneath the Murray Basin (Simons & Moore, 1999). In the vicinity of Hindmarsh, the Dryden Fault appears to be segmented and repeated across a series of younger strike-slip faults such as the Babatchio Fault. A segment of the Dryden Fault likely bounds the western edge of the belt of Cambrian igneous rocks intersected by drill hole STAVELY16.

The magnitude of displacement across the Dryden Fault is dependent on the age-relationships that exist between the Mount Stavely Volcanic Complex and the Glenthompson Sandstone. Previous interpretation-styles that supposed the Glenthompson Sandstone to the east was younger than the volcanics in the Dryden Belt (Buckland, 1987; Stuart-Smith & Black, 1999; Cayley & Taylor, 2001) imply the possibility of substantial west-side-up displacement across the Dryden Fault position. Reinterpretation of the Glenthompson Sandstone as belonging to the older Kanmantoo Group, based on new geochronology, geochemistry and stratigraphic relationships east of Stavely, means that there is no requirement for major stratigraphic displacement across the Dryden Fault splay that bounds the eastern margin of the Dryden and Dryden North belts – the fault here may just represent shearing along a previous conformable transition upwards from Glenthompson Sandstone into igneous rocks of the Stavely Arc.

Most of the D1a thrust displacements associated with emplacement of the Dryden Belt are now ascribed to a west-dipping fault equivalent to the Stavely West Fault that is assumed to exist along the western side of the Dryden and Dryden North belts (but now truncated at surface by the Mehuse and Golton faults), and to an underlying thrust fault, the Stavely Base Fault.

In the vicinity of deep seismic reflection line 09GA-AR1, the Dryden Belt becomes internally structurally complex with several subparallel faults that may represent hangingwall or footwall splays of the main Dryden Fault (Figure 4.16). The eastern edge of the Dryden Belt can be traced to a depth of approximately 1.3 s TWT (ca. 4 km) in 09GA-AR1 data (see Figure 2.22), before it is apparently truncated and presumably offset at a moderate angle by the east-dipping Golton and Mehuse faults. The Mehuse and Golton faults are likewise interpreted to offset the internal D1a faults, and the D1a fault inferred to bound the western flank of the Dryden Belt. This is a similar relationship to that described for the Dryden Fault in Cayley & Taylor (2001) and Korsch et al. (2002).

The lack of exposure, or evidence in potential field data, of any continuation of the Dryden Fault or of the Dryden Belt west of the Mehuse Fault along its considerable length implies that the Dryden Fault and the Dryden Belt has been overthrust across the Mehuse Fault, so that the continuation of the Dryden Fault and Dryden Belt in the footwall of the Mehuse Fault is now deeply buried, directly below or even to the east of the surface fault position. The precise geometry of the Dryden Fault in the Mehuse Fault footwall is not known, but is inferred to continue to depth in a reverse-offset position. A hint of the presence of an offset of the Dryden Fault and the Dryden Belt in the Mehuse Fault footwall may be the broad, low-amplitude magnetic highs that lie east of the near-surface Dryden Belt and Dryden Fault between Dimboola and Jeparit and east of Wail and Pimpinio.

3.3.3.5 Elliot North Fault (new name)

The Elliot North Fault is buried beneath Newer Volcanic Group and not exposed. It is interpreted as the northernmost flank of a small number of poorly-exposed anomalously southeast-striking belts of Mount Stavely Volcanic Complex rocks located southeast of the Grampians Ranges. North of the Elliot North Fault, scattered outcrop and mineral exploration drill hole intersections show the Cambrian bedrock to be non-magnetic Glenthompson Sandstone. The Elliot North Fault is therefore interpreted to separate Glenthompson Sandstone from rocks of the Mount Stavely Volcanic Complex to the south. The Elliot North Fault is interpreted as a Cambrian reverse fault, responsible for thrusting a panel of older Glenthompson Sandstone stratigraphy over a fault slice of Stavely Arc volcanics.

Although the sense and magnitude of displacement is poorly constrained, regional potential field and deep seismic reflection data show the Elliot North Fault to have a west-northwest strike and a low to moderate north-northeast dip. Potential field and deep seismic reflection data image the fault as bounding the northeast (upper) flank of a buried west-northwest-striking magnetic and acoustically-reflective rock package (near CDP 3150 in deep seismic reflection line 09GA-AR1; see Figure 2.22) and suggest a low to moderate northnortheast dip to the structure. Several shallow mineral exploration drill holes south of the fault trace intersect magnetic mafic-intermediate igneous rocks that are comparable to Cambrian low-Ti andesite that crops out farther south on Mount Elliot. The Elliot North Fault trace projects northwest towards and beneath the Grampians Ranges in the vicinity of Mafeking, at a strike-angle highly oblique to the Escondida Fault against which magnetic data suggest the Elliot North Fault abuts beneath Grampians Group.

The Elliot North Fault shares its orientation with overlying, unusually oriented Grampians Group stratigraphy in the southern limb of the Mafeking Orocline adjacent to Mafeking, and this coincidence suggests a linked structural history. Unfolding of locally curved Grampians Group bedding trends deformed during D4 involves regional scale anticlockwise rotation about a subvertical axis (Figure 3.73). Applying the same style and magnitude of retrodeformation to the underlying Elliot North Fault and Elliot Belt suggests the likely pre-Silurian orientation and position of the Elliot Belt was as a steep westerly-dipping and northerly striking volcanic belt positioned directly north along-strike from the Bunnugal Belt. The retrodeformed Elliot North Fault position suggests continuity with the west-dipping Bunnugal West Fault during D1a (Section 3.3.3.3 – Bunnugal West Fault). The retrodeformation suggests that during the Early Devonian D4 event the Elliot North segment of this D1a fault and volcanic belt was ruptured, translated and reoriented to its present configuration within the middle limb of the large Mafeking Megakink. Megakink formation was accompanied by formation of the smoothly-curved Mafeking Orocline in the overlying Grampians Group, the differences in fold-style accommodated across a splay of the Marathon Fault (see Section 3.3.4.6 – Marathon Fault).

3.3.3.6 Elliot South Fault

The Elliot South Fault does not crop out but is interpreted from regional magnetic, gravity and deep seismic reflection data to lie along the southern margin of the west-northwest-striking Elliot Belt, separating highly magnetic Cambrian mafic-intermediate (basalt-andesite-dacite) metavolcanic rocks within the belt (Buckland, 1987; Cayley & Taylor, 2001) to the north from Glenthompson Sandstone farther south.

The age of the Elliot South Fault is poorly constrained, but since it juxtaposes a belt of Mount Stavely Volcanic Complex rocks against Glenthompson Sandstone, and the Cambrian volcanics uplifted in its hangingwall are unconformably overlain by discontinuous outcrops of Grampians Group, which also straddle the fault trace, the Elliot South Fault is interpreted to have formed during D1a in the Late Cambrian. South of Willaura, a re-joining splay from the fault footwall apparently juxtaposes Grampians Group against Nargoon Group and Mount Stavely Volcanic Complex stratigraphy, indicating that some post-Silurian (D3-D4) movement on the Elliot South Fault may also have occurred.

The Elliot South Fault is interpreted to underlie the uppermost reflective layer visible in deep seismic reflection line GA09-AR1 (see Figure 2.22). This reflective package projects to surface in the vicinity of Mount Elliot, where Cambrian calc-alkaline andesite and dacite crop out (Buckland, 1987). The unreflective sequence below the Elliot South Fault imaged in deep seismic reflection line 09GA-AR1 can be correlated confidently with Glenthompson Sandstone, since this region projects south to surface in an area south of Mount Elliot where generally northeast-dipping and facing Glenthompson Sandstone crops out and is intersected in several historical mineral exploration drill holes.

Drill hole, magnetic and deep seismic reflection (line 09GA-AR1) data suggest that the western end of this fault terminates against the Escondida Fault (a position formerly interpreted as part of the Mount Stavely East Fault; Cayley & Taylor, 2001) in the vicinity of Yarram Park. The eastern end of the Elliot South Fault – like the eastern end of the Elliot North Fault – converges at an oblique strike-angle towards the interpreted position of the Golton Fault and the Dryden Belt near Willaura. The Golton Fault is interpreted to cut and offset the eastern end of the Elliot South Fault.

Like the Elliot North Fault, the Elliot South Fault is interpreted to dip moderately north-northeast, based on geometrical arguments, deep seismic reflection data, and the asymmetry of potential field data over the Mount Elliot Belt. The apparent very low dip of the Elliot South Fault as imaged in deep seismic reflection line 09GA-AR1 (see Figure 2.22) is an artefact of the highly oblique strike of the reflective Elliot Belt relative to the seismic line that imaged it. Nevertheless, based on comparison with the adjacent Narrapumelap North Fault, the Elliot South Fault is likely to have a listric profile, with a dip-magnitude that decreases markedly with depth. The Elliot South Fault is interpreted as a Cambrian reverse fault, separating a fault slice of Stavely Arc from underlying Glenthompson Sandstone stratigraphy.

Along with the Elliot North Fault, the Elliot South Fault sits within a block of Cambrian rocks that exhibit an unusual strike oblique to the northwestern structural trends typical of the bedrock within the Grampians-Stavely Zone. The block is bound on the western and eastern sides by the Escondida and Golton faults respectively, faults with an Early Devonian dextral transtensional movement history related to formation of the large scale, Z-shaped Mafeking Megakink. Retrodeformation of megakink rotation and of dextral strike-slip rupture of the kink hinges restores the Elliot South Fault northwest to a pre-Early Devonian configuration along-strike from the Bunnugal East Fault. The Bunnugal East Fault currently terminates against the Victoria Valley South Fault (Section 3.3.3.2 – Bunnugal East Fault; see Figure 3.67 and Figure 3.67), a structure that controls the southern margin of the Victoria Valley Batholith and is therefore interpreted to be a rift-structure of D4 age.

3.3.3.7 D1a Escondida Fault

Although entirely buried north of the Grampians Ranges, the Escondida Fault is the most obvious structure in the regional potential field data covering STAVELY. The Escondida Fault bounds much of the western flank of the huge Dimboola Belt with the fault trace and hangingwall occupied by highly magnetic mafic to ultramafic rocks (Moore, 1996; Cayley & Taylor, 1996, 1997a; Figure 3.29). Regional magnetics, gravity (Cayley & Taylor, 1996), deep seismic reflection data (Korsch et al., 2002) and inversion modelling of individual magnetic units within the Dimboola Belt interior (e.g. Figure 3.31) all show that the Escondida Fault near Horsham dips moderately northeast, with rocks of the Dimboola Belt in its hangingwall unconformably overlain by Grampians Group sandstone.

Although significant displacements and fault extensions occurred on the Escondida Fault when it was reactivated and extended in strike-length towards the south during D4 (described separately in Section 3.3.4.1 – Escondida Fault (reactivated)), the geometry and movement history of the D1a portion of the Escondida Fault north of the Grampians Ranges can be deduced from the geometry of Cambrian stratigraphy within the Dimboola Belt in its hangingwall, and its relationship to overlying Grampians Group.

Modelling of regional gravity and magnetic data, and deep seismic reflection data (97GA-V1; see Figure 2.23) show that the contact between non-magnetic Grampians Group and underlying magnetic, deformed Cambrian volcanics in the Dimboola Belt dips moderately to the east. An unconformable nature for this contact is supported by dip modelling of magnetic rocks of the Dimboola Belt which shows that the Cambrian stratigraphy dips much more steeply east than either the contact or the dip of strata in the overlying Grampians Group. Regional scale unconformities such as this are likely to have been low-dipping at the time of deposition upon them. The present-day regional-scale moderate east-dip of the basal Grampians Group contact above the Dimboola Belt is therefore likely to be subsequent, due to eastward tilting of the entire Escondida Fault hangingwall succession post-Grampians Group deposition.

Restoration of the Grampians Group basal contact towards a more sub-horizontal attitude pre-D3-D4 involves proportionate restoration of the steeply-dipping magnetic stratigraphy in the underlying Dimboola Belt. Restored, the magnetic stratigraphy dip moderately to steeply but universally eastward in northern STAVELY, everywhere south of Netherby. This geometry suggests that the Cambrian igneous stratigraphy within southern Dimboola Belt is likely to occupy the hangingwall of an east-dipping D1a fault – we interpret this to be the D1a Escondida Fault.

The abrupt truncation of Dimboola Belt volcanic stratigraphy and associated potential field characteristics across the Escondida Fault trace between Zumsteins and Nhill implies that the net displacement across this part of the Escondida Fault during D1a must have been reverse. This is because the alternative – net extensional displacement – should surely have exhumed a fault footwall to the west that comprised even more of the magnetic Stavely Arc bedrock than is seen in the hangingwall. Confidence that more volcanic rocks of the Stavely Arc lie to the west of the Dimboola Belt at depth stems from the presence of the additional fault slices of Stavely Arc in the Black Range, and from deep seismic reflection data that images reflective (igneous) material at depth beneath the entire region. At surface, the footwall region of the Escondida Fault is dominated near-surface by non-magnetic, low grade metasediments, encountered in several mineral exploration drill holes and groundwater bores. These sediments are correlated with Glenthompson Sandstone, which we now interpret to be older than the Stavely Arc. Such relationships imply that the D1a Escondida Fault offsets an already disordered stratigraphy. This is consistent with interpretation of the D1a Escondida Fault as a backthrust to an adjacent west-dipping D1a thrust fault, such as the Dryden fault system, across which regional mapping demonstrates stratigraphic disordering does occur.

Overall, the relationships established near-surface and from deep seismic data across the Escondida Fault near Horsham suggest that any footwall extension to the Dimboola Belt must have been overthrust during D1a, so that buried Dimboola Belt correlates reside beneath the Escondida Fault footwall, but are deeply buried. This is also the position occupied by depth-projections of the Dryden and Dryden North belts. The down-dip convergent geometry and comparable movement timing suggests that D1a Dimboola Belt faults linked east into the hangingwall of the Dryden Fault, possibly formed as a major back-thrust to this structure developed during D1a. The strike-persistence of the west-dipping and facing Dryden and Dryden North belts precludes depth projections of the Escondida Fault further east into the footwall of the Moyston Fault at D1a time. This geometry matches patterns seen in the 97GA-V1 deep seismic reflection line (Korsch et al., 2002), once the out-of-plane movement effects of subsequent D4 structures – such as the dextral Mckenzie Creek Fault – are taken into account.

The D1a Escondida Fault appears to lose displacement beyond the northern limit of STAVELY, where the Dimboola Belt broadens and appears to dip both east and west. The fault apparently cuts into the interior of the Dimboola Belt near Netherby. Here, a strip of Dimboola Belt volcanic rocks emerges in the Escondida Fault footwall, widening progressively along-strike towards the northwest. Regional potential field data shows that these footwall volcanics dip west, quite different to the east dip of the Escondida Fault and the southern Dimboola Belt, but comparable to the dip of the Dryden and Dryden North belts.

The east-dip interpreted for D1a faults in the southern Dimboola Belt is in stark contrast to the westerly dip of D1a faults and stratigraphy interpreted in the Bunnugal Belt south of the Grampians Ranges, which we consider to have lain almost directly south along-strike prior to D4. There is no evidence of any large east-dipping faults of D1a age south of the Grampians Ranges – overprinting criteria show that the parts of the Escondida Fault within the Grampians Group and farther south developed during D4. Major reversals in regional-scale structural vergence and dip-direction, such as implied between the Escondida Fault beneath the Dimboola Belt and the Bunnugal Belt faults (and faults related to the Grampians ‘Deeps’/Brimpaen belts) during D1a are typically accommodated across regional-scale transform structures or equivalent transitional structural systems. Although any evidence is now concealed beneath the Grampians Ranges, we interpret relicts of D1a transform structures at depth in the vicinity of the Brimpaen Belt to explain the observed D1a structural vergence reversals. The transform structures required to explain these localised vergence and dip differences need not be depth-persistent, since the Stavely-Dryden fault belt and related fault systems appears to persist with unchanged dip and facing for the full strike-length of STAVELY.

The along-strike transition from east-dipping Dimboola Belt D1a faults to west-dipping Elliot-Bunnugal belt D1a faults appears to coincide with the position of the Jalur Rift (see Figure 3.9), which developed over the intervening Brimpaen and Grampians ‘Deeps’ belts during D4. Reactivation of crustal systems that were inherently weak because of pre-existing transform systems may explain why this region was predisposed to become the locus of subsequent transtensional deformations that formed the Jalur Rift and related rocks during D4 (see Section 4.4.3 – Understanding the form and distribution of potential transfer structures). The structural history of the Escondida Fault during D4 is discussed in Section 4.1.1 – Mafeking Megakink retrodeformation, Section 4.1.4 – Dimboola Duplex retrodeformation).

3.3.3.8 Mouchong Fault System

The main Mouchong Fault System faults are interpreted to bound the western and eastern flanks of basaltic to dacitic metavolcanics of the Black Range Belt. Other related fault splays deform the belt interior, but these have not been modelled. As for the related Tyar, Muline and Boonawah fault systems, rocks of the Mount Stavely Volcanic Complex appear to have been thrust to surface within the Mouchong Fault System. The Black Range Belt and the Mouchong Fault System are straddled, with little relief, by Grampians Group. Therefore, any vertical displacements along the Mouchong Fault System must predate the Silurian.

The faults bounding the flanks of the Black Range Belt were originally interpreted to dip west by Cayley & Taylor (1997a, 1997c) based on several mineral exploration diamond drill hole intersections and sparse outcrop, all of which show west-dipping structural fabrics. Murphy et al. (2006) modelled the fault system as east-dipping. Modelling of TMI profiles across the Black Range Belt as part of the STAVELY 3D model reaffirms an overall steep westerly dip for the metavolcanics within at least the upper 3 km of the fault system, but with the southern end of the belt locally overturned towards the east within a sinistral drag-fold – the D4 Bepcha Hinge – adjacent to where the belt is truncated by the sinistral Cherrypool Fault, also interpreted as a D4 structure (Figure 3.12 and Figure 3.17A; Skladzien, et al. 2016b).

Where the position of the shallowly buried Mouchong Fault System is crossed by deep seismic reflection line 09GA-SD1 near CDP 8600, a series of moderately west-dipping breaks in reflectivity rise to near surface, interpreted to represent the igneous rocks and the Mouchong Fault System that controls their distribution. Here, the Mouchong Fault System is overlain by a thin (0.3 s TWT, ca. 1 km) veneer of non-reflective material interpreted as Grampians Group. The breaks along the Mouchong Fault System can be traced downwards until ca. 2.8 s TWT (ca. 7.5 km). Below 3 s TWT (ca. 9 km), the Mouchong Fault System appears to merge into a region of relatively uniform high reflectivity, interpreted to include parts of the Stavely Arc continuous with the Stavely-Narrapumelap-Dryden-Dryden North fault slice. Overall, the Mouchong Fault System can be described as a west-dipping imbricate thrust system which cuts the western flank of the Stavely Arc, and appears to have brought a portion of the Stavely Arc volcanics and ultramafic rocks to the surface as the Black Range Belt. In this way, the origin, age and position of the Mouchong Fault System appears identical to that interpreted for the Muline, Tyar, and Boonawah fault systems.

The Mouchong Fault System is partially truncated and sinistrally offset by approximately 8.5 km across the Muirfoot Fault (Figure 3.16). The Muirfoot Fault also cuts the Grampians Group (Cayley & Taylor, 1997a; 1997c) and is therefore a D3 or D4 structure. Mesoscopic structures developed in Grampians Group strata along and adjacent to the Muirfoot Fault trace position show a sinistral shear-sense. An offset segment of the Mouchong Fault System is interpreted to continue west of the Muirfoot Fault to bound a complex, highly-magnetic belt of Cambrian igneous rocks trending northwest through Toolondo and Clear Lake. This is the Black Range West/Mitre Belt. Mineral exploration drilling into Black Range West/Mitre Belt intersects a mix of Stavely Arc rocks including andesite and intermediate-felsic plutonic rocks (e.g. Crystal Mining mineral exploration drill holes 8019 and 8023). The northern continuation of this part of the belt appears to be dominated by Cambrian granitic rocks, with the belt coincident with a conspicuous gravity ‘low’ (Figure 2.28B and Figure 3.9A). The Mouchong Fault System trends towards the Henty Fault and appears truncated and offset across it. The Mouchong Fault System also trends towards the buried trace of the Yarramyljup Fault which possibly overthrusts and thus conceals the northern extents of the Mouchong Fault System.

East of the Muirfoot Fault, the Mouchong Fault System appears to broaden and possibly lose displacement northwards. Volcanic rocks are intersected by Crystal Mining mineral exploration drill holes 8027 and 8025 in the vicinity of Noradjuha in a broad gravity high beneath Grampians Group. North of this region, the Black Range Belt appears to plunge beneath Nargoon Group metasediments, as intersected near Natimuk in drill hole STAVELY11. The Mouchong Fault System may continue north of this point, but the uniform nature of the metasedimentary stratigraphy across the northern parts of the fault system makes it difficult to distinguish in geophysical data.

At its southern end the Mouchong Fault System appears to be overprinted by the sinistral Beptcha Fold, and is truncated by the Cherrypool Fault, now interpreted as a D3 and/or D4 sinistral strike-slip structure (Section 3.3.5.5 – Cherrypool Fault). The Cherrypool Fault trends north-northeast, and offsets the southern end of the Mouchong Fault System from the northern end of the Muline Fault System (Figure 3.13). The southern end of the Cherrypool Fault is truncated by the D4 Henty Fault. The Mouchong and Muline fault systems are offset sinistrally by a little over 18 km across the Cherrypool Fault. Prior to D4 and D3, these fault systems were likely continuous (see Figure 4.7).

3.3.3.9 Muline Fault System

The Muline Fault System encloses and deforms the Glenisla Belt, a northerly trending segment of Cambrian volcanics that is dominated by altered andesitic lava and volcanic breccia. The Muline Fault System was interpreted as west-dipping by Cayley & Taylor (1997a; 1997c), based on historical mineral exploration drill hole intersections (e.g. CRAE, EL 3009), which encountered steeply west-dipping cleavage. The Glenisla Belt is internally structurally complex (Figure 3.39). Reconnaissance mineral exploration drilling shows infaulted slivers of metasediment and serpentinite, and the belt has a complex appearance in regional magnetic data. We attribute much of this complexity to rejoining splay faults within the Muline Fault System. Some of this complexity is included in the STAVELY 3D model as an example of the structural complexity developed in many of the D1a volcanic belts.

The northern end of the Muline Fault System appears to be truncated obliquely by the northerly-trending D3 – D4 Cherrypool Fault. The southern end of the Muline Fault System is intruded by a granite buried beneath Early Devonian Rocklands Volcanic Group, in the vicinity of the D4 Henty Fault (see Figure 3.12). This overprinting relationship shows that the Muline Fault System and Glenisla Belt pre-date the Early Devonian, and is most likely a D1a structure. Restoration of the D3 and D4 fault networks show that the Muline Fault System was continuous with the Tyar Fault System to the south and the Mouchong Fault System to the north when these fault segments were active during D1a (see Figure 4.7). Like the Tyar and Mouchong fault systems, the Muline Fault System is interpreted as a segment of a larger thrust system that emplaced a fault slice of Stavely Arc rocks up into the metasedimentary package during D1a.

3.3.3.10 Stavely Base Fault

A prominent basement high of reflective rocks imaged by deep seismic reflection line 09GA-AR1 (Figure 2.22) occupies the middle crust beneath the central and eastern Grampians-Stavely Zone, forming the footwall to the Escondida Fault that extends east towards the Moyston Fault. This basement high is locally overlain by variably-reflective Stavely Arc rocks in the Stavely Belt and other volcanic belts, and by unreflective metasedimentary rocks that crop out as Glenthompson Sandstone, for example to the east of the Stavely Belt, and to the east of the Dryden Belt.

The exposed Stavely Arc rocks have a similar density and seismic reflectivity character to that imaged and modelled for the basement high, which is why we interpret the reflective upper part of this basement high to represent an autochthonous Stavely Arc edifice, captured as a separate volume in the STAVELY 3D model (Figure 3.14). We interpret the volcanic belts of Stavely Arc rocks to represent fault-slices of this edifice, progressively gouged from its western flank during D1a and thrust towards surface. This interpretation is consistent with the imbricate thrust-geometries mapped for the D1a faults that contain the volcanic belts. The reflective rocks within the buried basement high that are interpreted to be Stavely Arc rocks overlie less-reflective rocks in the lower crust.

New data, including the zircon provenance of the Glenthompson Sandstone, a mapped upwards transition from Glenthompson Sandstone into units dominated by volcanic detritus that locally tectono-stratigraphically underlie the Mount Stavely Volcanic Complex in the Stavely Belt, and the occurrence of MORB-like basalts within the Glenthompson Sandstone, has necessitated reinterpretation of the Glenthompson Sandstone as part of the Kanmantoo Group which is older than, or equal in age to, the Stavely Arc (Taylor et al., 2015; see Section 3.2.6 – Kanmantoo Group). This means that if the reflective parts of the basement high imaged by deep seismic reflection line 09GA-AR1 represent buried parts of the Stavely Arc edifice, then some of the unreflective rocks beneath and adjacent to the basement high may represent a substrate of less-reflective Kanmantoo Group, including Glenthompson Sandstone.

One consequence of the reinterpretation of relative age is that the Glenthompson Sandstone sequences that are exposed at surface must have been structurally emplaced to their current position high in the crust during D1a. This means that the largest stratigraphic displacements occur on faults that lie stratigraphically above the D1a fault slices of Stavely Arc rocks, for example the Stavely West and Bunnugal West faults – both these faults are west-dipping and appear to have thrust packages of west-facing Glenthompson Sandstone up and over the Stavely and Bunnugal belts in the footwalls to their east respectively.

The occurrence of Glenthompson Sandstone stratigraphy on top of in-situ Stavely Arc rocks, in outcrop, and possibly also imaged above the basement high in deep seismic reflection line 09GA-AR1, necessitates the interpretation of an additional major D1a thrust fault that is blind to surface, but that separates older overlying Glenthompson Sandstone from younger underlying autochthonous rocks of the Stavely Arc. This inferred fault is the Stavely Base Fault. It is the deepest of the thrust structures interpreted to facilitate the emplacement of older sedimentary rocks over younger Stavely Arc rocks. The Stavely Base Fault has not been observed in outcrop, is unable to be discriminated in any geophysical data, but is required geometrically to explain the observed geological relationships at surface. This concept is built into all the geological serial cross sections prepared for construction of the STAVELY 3D model (see Appendix 2 – Geological cross sections). The persistence of Glenthompson Sandstone on both flanks of the Dryden and Dryden North belts into the far north of STAVELY indicates a similar lateral persistence for the Stavely Base Fault. Therefore, the Stavely Base Fault is modelled as an extensive structure. In places where the thickness of unreflective crust imaged in deep seismic reflection lines 09GA-AR1 and 97GA-V1 above the buried reflective basement high exceeds the mapped thickness of imbricated Glenthompson Sandstone, we infer that the Glenthompson Sandstone has overthrust buried Nargoon Group sediments of similar seismic reflectivity across the Stavely Base Fault. Nargoon Group appears to stratigraphically overlie the Stavely Arc in the Boonawah, Grampians ‘West’ and Tyar belts, and a similar relationship is implied at depth beneath the rest of the Grampians-Stavely Zone.

3.3.3.11 Narrapumelap North Fault (and Williamsons Fault correlates in the Narrapumelap Belt)

South of Mount Elliot, and south of a poorly-exposed panel of Glenthompson Sandstone, a sub-parallel westnorthwest-trending belt of magnetic Cambrian meta-igneous rocks and intercalated Cambrian metasediments, the Narrapumelap Belt, crops out poorly. The meta-igneous rocks are variably magnetic, are visible in potential field data, and have been intersected in several mineral exploration drill holes. Rock-types intersected in drill holes into the Narrapumelap Belt include non-magnetic Glenthompson Sandstone, magnetic intermediate metavolcanics and felsic tuff that correlate with Mount Stavely Volcanic Complex stratigraphy, and serpentinite that correlates with the Williamsons Road Serpentinite (for example, intersected in Pennzoil mineral exploration drill holes TG-1000E – TG-1500E). The Narrapumelap North Fault is interpreted to occupy the interface between non-magnetic metasedimentary rocks to the north of the Narrapumelap Belt, and magnetic rocks within the belt.

Surface measurements of exposed strata (occasional outcrops of Glenthompson Sandstone within the belt which show predominantly north-northeast bedding dip and facing), potential field, and deep seismic reflection data combine to indicate an overall listric profile for the Narrapumelap Belt, characterised by steeper north-northeast dips near surface (65° – 70°) which transition to moderate to low north-northeast dips at depth (Figure 3.44). This geometry is a proxy for the geometry of the Narrapumelap North Fault that bounds the upper flank of the Narrapumelap Belt. The Narrapumelap Belt outcrops project down-dip to a position coincident with a reflective package visible in deep seismic reflection line GA09-AR1 that underlies the Elliot Belt but is grossly similar to it in size, character and geometry. The intervening, less reflective, rock package imaged in deep seismic reflection line 09GA-AR1 is interpreted to represent the intervening Glenthompson Sandstone metasediments, since this package projects to surface south of Mount Elliot and north of the Narrapumelap North Fault in a region where monotonous Glenthompson Sandstone is mapped and drilled.

The upper boundary of the Narrapumelap Belt reflector imaged in deep seismic reflection line 09GA-AR1 (which correlates with the Narrapumelap North Fault) lies at approximately 4.5 km depth at its centre-point. The across-strike distance, from directly above this point on deep seismic reflection line 09GA-AR1 to the surface position of the Narrapumelap North Fault near Stavely, is 12.5 km. Triangulation from these constraints gives an averaged orientation of the Narrapumelap North Fault between surface (at its western end) and 4.5 km depth of approximately 20° towards 030° (grid north). This dip is much lower than measured at surface, or modelled from magnetics, and indicates that the overall dip magnitude of the Narrapumelap North Fault decreases dramatically below approximately 2 km depth (Figure 3.45). The overall decrease in dip-magnitude from surface to depth for the Narrapumelap North Fault indicates conclusively that it has a distinctly listric profile to at least approximately 5 km depth.

The Narrapumelap North Fault is interpreted as a Cambrian reverse fault, responsible for thrusting the panel of Glenthompson Sandstone stratigraphy to the north of the belt over the Narrapumelap Belt. The age of this displacement is constrained by Grampians Group which unconformably straddles the fault at its southeastern end, thereby placing an upper limit on displacement at Early Silurian or possibly Ordovician. This relationship, and a wider context with adjacent faults in the Cambrian bedrock, suggests that the Narrapumelap North Fault developed during D1a.

Retrodeformation of the Mafeking Megakink moves and rotates the Narrapumelap Belt northwest to a pre-Early Devonian configuration aligned along-strike from the truncated northern end of the Stavely Belt, now buried beneath the Grampians Ranges but clearly visible in regional magnetic data (Figure 3.47). In its retrodeformed position, the Narrapumelap North Fault is northerly-trending and steeply west-dipping (at surface), similar to the autochthonous portion of the Mount Stavely West Fault with which it is directly correlated.

Because of their exotic nature, fault-slices of Williamsons Road Serpentinite within the volcanic belts (see Schofield et al., 2018 for further discussion) serve as distinct marker horizons that allow correlation between the fault slices of the Stavely Arc rocks segmented post-D1a.

A thin fault slice of serpentinite occurs in the interior of the Narrapumelap Belt, apparently faulted against dacite and tuff just to the north (e.g. Pennzoil mineral exploration drill holes TG-750E – TG-950E). Although this fault slice is too thin to be captured in the STAVELY 3D model, the position of the fault that hosts it within the interior of the Narrapumelap Belt is distinctive and a close match for the Stavely Belt, where Williamsons Road Serpentinite is hosted by the D1a-aged Williamsons Fault which deforms Towanway Tuff in the Stavely Belt. The similarity suggests that the Williamsons Fault and its associated serpentinite fault slices within the Stavely Belt were continuous with the Narrapumelap Belt during D1a, but were subsequently segmented, rotated and offset during D4 Mafeking Megakink folding and faulting. As for the Stavely Belt, small intrusions (that show magnetic remanence and so are interpreted to be Late Cambrian) intrude the interior of the Narrapumelap Belt and are aligned along the trace of the Williamsons Fault extension (Figure 3.63). The coincidence of small intrusions and the serpentinite suggest that, as noted for the Stavely Belt, weak serpentinites contained within faults in the Narrapumelap Belt interior facilitated reactivation during D1b, with local dilations along the fault plane acting as potential magmatic fluid pathways (see Section 3.3.3.17 – Williamsons Fault).

Figure 3.63 Buried magnetic intrusions, interpreted to be D1b porphyries, in the hangingwall of the Narrapumelap South Fault, within the Narrapumelap Belt. The western intrusion was not intersected by mineral exploration drill hole WD001 – the drill hole position appears to miss the associated magnetic anomaly position in RTP data, and so the anomaly remains untested. The interpreted intrusions are aligned along the position of a fault (WFE) within the Narrapumelap Belt interior. Mineral exploration drilling shows this fault to contain slices of serpentinite (equivalent to Williamsons Road Serpentinite). The context of serpentinite fault slices within the interior of the Narrapumelap Belt is reminiscent of the Stavely Belt, which contains Williamsons Road Serpentinite within the Williamsons Fault. The Williamsons Fault segment that lies within the Stavely Belt was apparently reactivated during D1b to facilitate intrusion of stocks of Lalkaldarno Porphyry, including the Thursday’s Gossan ‘Victor’ porphyry stock, along the fault trace. The coincidence of magnetic intrusions with serpentinite in fault-slices in the interior of the Narrapumelap Belt suggests that the Narrapumelap Belt porphyries are a direct along-strike continuation of the Stavely Belt porphyries, the different overall orientation of the Narrapumelap Belt segment attributable to subsequent (D4) rotation and strike-slip translation within the Mafeking Megakink.

3.3.3.12 Narrapumelap South Fault

The Narrapumelap South Fault bounds the southern flank of the Narrapumelap Belt, separating it from a region of monotonous Glenthompson Sandstone just to the south. The position of the fault is interpreted from regional magnetic data as the boundary between magnetic rocks of the Narrapumelap Belt to the north and non-magnetic metasediments mapped in outcrop just to the south. The main fault plane is buried, but a secondary, east-dipping fault developed in weathered rocks of the Mount Stavely Volcanic Complex within the Narrapumelap Belt adjacent to the main fault plane (MGA 54 646609 5834417) has north-east-plunging oblique slickenside lineations that are consistent with dip-slip movements along the main fault plane. Other secondary faults along the trace of the main fault are developed in Glenthompson Sandstone. These display a variety of dip-directions but share common easterly to northeasterly slickenside lineation azimuths, suggesting a common origin.

The Narrapumelap South Fault is truncated at its western end by the Escondida Fault. The eastern end of the Narrapumelap South Fault is concealed beneath Newer Volcanic Group, but appears to be offset across the Hopkins River Fault and likely terminates, further east, against the Golton Fault. The Narrapumelap South Fault is interpreted as a Cambrian fault that separates Mount Stavely Volcanic Complex from Glenthompson Sandstone stratigraphy in its footwall. At its eastern end, the fault trace is poorly exposed. A splay from the fault coincides with a poorly exposed faulted interface between Glenthompson Sandstone and Grampians Group and felsic igneous rocks along the Hopkins River, and this suggests that parts of the Narrapumelap South fault system have undergone limited post-Silurian (D3-D4) reactivation.

The true width of Narrapumelap Belt measured at surface is a close match for the calculated true width of the Narrapumelap Belt where it is imaged in deep seismic reflection line 09GA-AR1. This indicates that the Narrapumelap Belt retains its thickness to depth, so that the Narrapumelap South Fault must share the same listric-shaped dip-profile calculated for the Narrapumelap North Fault.

Retrodeformation of the Mafeking Megakink moves and rotates and steepens the western end of the Narrapumelap South Fault northwest together with the Narrapumelap Belt to a pre-D4 configuration north along-strike from the truncated northern end of the Stavely East Fault, now buried beneath the Grampians Ranges west of Mafeking. The eastern end of the Narrapumelap South Fault restores to the truncated southern end of the Dryden Fault, now buried beneath Newer Volcanic Group south of Lake Bolac. The restored pre-D4 configuration suggests that the Narrapumelap South Fault is a segment of a continuous fault that bounded the eastern flank of the Stavely, Narrapumelap and Dryden belts in D1a-D1b. At this time, the Narrapumelap South Fault occupied the region between the Stavely East Fault and Dryden Fault. The listric profile established for the Narrapumelap South Fault may therefore inform the expected profiles of the Stavely East and Dryden fault segments. As for the Stavely East and Dryden faults, the Narrapumelap South Fault may not be large in displacement. The Glenthompson Sandstone is now regarded as stratigraphically older than the rocks of the Stavely Arc volcanics that occupy the volcanic belts, so that little or no stratigraphic disruption is required across this fault position.

3.3.3.13 Stavely East Fault

The Stavely East Fault occurs along the eastern boundary of the Mount Stavely Volcanic Complex (Buckland, 1987; Stuart-Smith & Black, 1999). Although it does not crop out, it’s orientation can be estimated because it bounds much of the eastern flank of the magnetic Stavely Belt. The Stavely Belt orientation is moderately to steeply west-dipping to several kilometres depth, modelled from regional magnetic and gravity data (Figure 3.50 and Appendix 3 – Forward model sections) and consistent with the moderate to steep westerly dips of bedding in enclosing belts of Glenthompson Sandstone. Deep seismic reflection data indicate that the Stavely Belt (and fault) dip magnitude possibly lessens with depth, to link west into other listric thrusts that, collectively, appear to have been responsible for thrusting multiple sub-parallel fault slices of the Stavely Arc against Glenthompson Sandstone.

North of Stavely, the surface trace of the Stavely East Fault appears to have been truncated by a younger fault, the steeply east-dipping D4 Escondida Fault. The Escondida Fault lies close and subparallel to the projected position of the Stavely East Fault between Stavely and Yarram Park, but is seen in magnetic data to cut right across the projected trace of the Stavely East Fault where the Stavely Belt warps eastward and terminates beneath Grampians Group cover northwest of Mafeking (Figure 3.44).

South of Stavely, Glenthompson Sandstone just east of the Stavely East Fault variably dips and faces both west and east at different places, although westerly dips and facing are predominant. At MGA 54 646258 5832700, just south of the position where the Stavely East Fault is truncated by the Escondida Fault, the west limb of a north-trending syncline is developed in Glenthompson Sandstone with a distinctive coarse angular volcanic component. This limb is overturned and dips moderately west in the footwall of the Stavely East Fault. The east-facing of turbidites closest to the Stavely East Fault at this locality is consistent with either the turbidites being younger than the adjacent Cambrian volcanics or, more likely, being older but with local overturning of folded turbidites in the footwall of the fault in response to reverse, west-over-east movements along it.

Chert, tuff, and other volcanics of the Mount Stavely Volcanic Complex west of the Stavely East Fault trace dip and face predominantly west, consistent with the modelled overall westerly dip of magnetic rocks within the Stavely Belt. This orientation is also consistent with interpretation of the Stavely East Fault as west-dipping and reverse, with the hangingwall occupied by the Stavely Belt and the footwall occupied by Glenthompson Sandstone.

The magnitude of displacement across the Stavely East Fault is dependent on the age-relationships that exist between the Mount Stavely Volcanic Complex and the Glenthompson Sandstone. Previous interpretation-styles that supposed the Glenthompson Sandstone was younger than the Mount Stavely Volcanic Complex (Buckland, 1987; Stuart-Smith & Black, 1999) implied the possibility of substantial west-side-up displacement across the Stavely East Fault position. Reinterpretation of the Glenthompson Sandstone as belonging to the older Kanmantoo Group, based on new geochronology, geochemistry and stratigraphic relationships east of Stavely, means that there is no requirement for major stratigraphic displacement across the position of the Stavely East Fault. There has, however, been sufficient reverse displacement across this fault position to generate the reclined to overturned folds that are observed in Glenthompson Sandstone in the fault footwall.

The Stavely East Fault is almost certainly Late Cambrian (D1a) in age since splay faults within the hangingwall succession – for example those that host the Williamsons Road Serpentinite – are intruded and ‘stitched’ by Late Cambrian granite and porphyry stocks intruded during D1b.

Prior to truncation by the Escondida Fault and formation of the Mafeking Megakink during D4, the Stavely East Fault was likely continuous with the Narrapumelap South Fault and, further north, with the Dryden Fault.

3.3.3.14 Stavely West Fault

The Stavely West Fault (Buckland, 1987) is interpreted to bound the western flank of the Stavely Belt, separating Mount Stavely Volcanic Complex within it from Glenthompson Sandstone farther west (see Figure 3.47, Figure 3.48 and Figure 3.9). Drill hole STAVELY17 was designed to test this fault but was collared too far west and so failed to intersect it, so the precise nature of the fault remains speculative. The Stavely West Fault is interpreted to dip west overall, since modelling of magnetic data suggests a west-dip for Cambrian strata within the Stavely Belt, and Glenthompson Sandstone intersected in STAVELY17 just to the west of the Stavely West Fault also dips and faces west. The Stavely West Fault is interpreted as a Cambrian reverse fault, responsible for thrusting a panel of Glenthompson Sandstone stratigraphy over the Stavely Belt. Although apparently west-dipping along much of its length, portions of the fault and/or rejoining backthrusts in the fault hangingwall, appear to locally dip east. For example, just south of Astons Road the fault trace position coincides with a steeply east-dipping fault slice of the Glenronald Shale Member. Drill intersections of diorite (e.g. mineral exploration drill hole STAVRA276) and other rocks of the Stavely Arc intercalated with Glenthompson Sandstone metasediments approximately 5.5 km east of Glenthompson occur on a series of narrow magnetic highs that are also modelled as east dipping. These rocks are interpreted to lie in subsidiary east-dipping, rejoining back-thrusts developed in the Stavely West Fault hangingwall.

The magnitude of displacement across the Stavely West Fault is dependent on the age-relationships that exist between the Mount Stavely Volcanic Complex and the Glenthompson Sandstone. In previous interpretation-styles that supposed the Glenthompson Sandstone to be younger than the Mount Stavely Volcanic Complex (Buckland, 1987; Stuart-Smith & Black, 1999) the fault position could have been occupied by a conformable transition from volcanics up into sediment, given a westerly overall facing for both stratigraphic packages. Reinterpretation of the Glenthompson Sandstone as belonging to the older Kanmantoo Group, based on new geochronology, geochemistry and stratigraphic relationships east of Stavely, now implies the possibility of disordered stratigraphy, necessitating substantial west-side-up displacement across the Stavely West Fault position, with older Glenthompson Sandstone and fragments of Glenronald Shale Member fault-uplifted against and over the younger Mount Stavely Volcanic Complex.

3.3.3.15 Tyar Fault System

The Tyar Fault System (Cayley & Taylor, 1997a, 1997c) bounds the southwestern and northeastern flanks of a narrow northwest-striking belt of Cambrian intermediate metavolcanic rocks (the Tyar Belt) which lies buried at shallow depth beneath Rocklands Volcanic Group and/or Pliocene sand and laterite, along the western side of the Black Range, west of the Grampians Ranges (see Section 3.2.3.17 – Tyar Belt). The Tyar Fault System is assumed to be a Cambrian southwest-dipping thrust system (Cayley & Taylor,1997a), based on comparison with the adjacent Muline and Mouchong faults which both contain west-dipping structural fabrics and locally underlie Grampians Group. A southwest dip for the Tyar Fault System along parts of its length is indicated by modelling of magnetic data (Figure 3.12 and Figure 3.51). The northwestern end of this fault system progressively changes to a local easterly dip where it has been refolded about the D4 Tyar Hinge and truncated by the D4 Henty Fault that also cuts into and bounds the Grampians Group.

The southeastern end of the Tyar Fault System is intruded by an interpreted Early Devonian pluton buried beneath Rocklands Volcanic Group (Figure 3.16 and Figure 3.43). The D4 Wannon Fault also trends into this region. Taking offset and kinking across the Wannon Fault into account, the Tyar Fault System may correlate with the Boonawah Fault system, and have the same Cambrian origin. The northwestern end of the Tyar Fault System is truncated by the Henty Fault, and offset laterally from its interpreted continuation in the southern end of the Muline Fault System (Glenisla Belt) by approximately 21 km. We interpret this offset to reflect dextral displacement across the Henty Fault during D4, a displacement sense that is consistent with formation of the Tyar Hinge as a drag-fold accompanying fault offset.

3.3.3.16 Unnamed D1a faults beneath the Grampians Group – Grampians ‘Deeps’ and Brimpaen

Potential field data clearly show the presence of buried linear belts of magnetic and dense rocks in the Cambrian bedrock underlying the Serra, Victoria and Asses Ears ranges, one belt apparently intruded in part by the Victoria Valley Batholith (Figure 3.23 and Figure 3.41). This region is a localised, kilometres-deep structural low – the Jalur Rift – which apparently developed during D4, subsequent to Grampians Group deposition (D2) and initial shortening deformation (D3). Parts of the already-deformed Grampians Group allochthon appear to have subsided into the Jalur Rift during D4 (see Section 4.1.2 – Jalur Rift retrodeformation), simultaneous with formation of the Victoria Valley South, Victoria Valley North, Mosquito Creek, Escondida, Golton and Marathon faults, and intrusion of the large Victoria Valley, McKenzie River, Stony Creek and Mafeking batholiths into the rift interior.

The width, length and overall geophysical character of these buried belts of dense, magnetic material are closely comparable to other volcanic belts of Stavely Arc rocks mapped and delineated within STAVELY. Geophysics allows the belts to be interpreted to the northwest flank of the Grampians Ranges, where they abut Cambrian mafic boninitic and calc-alkaline volcanics intersected in drill holes STAVELY04 and STAVELY06 within the Brimpaen Belt, of similar geophysical character. For these reasons, the linear belts beneath the Grampians Ranges are defined as volcanic belts of Stavely Arc rocks – the Grampians ‘Deeps’ Belt (see Section 3.2.5.12 – Grampians ‘Deeps’ Belt).

Although there are no definite constraints for a structural nature to Grampians ‘Deeps’ Belt margins, retrodeformation of D4 structures suggests that the Grampians ‘Deeps’ Belt was originally continuous along-strike with the Elliot and Narrapumelap belts and, to its north, the Brimpaen Belt. All appear to be segments of elongate D1a, west-dipping fault-slices of Stavely Arc volcanics that included, from south-to-north, the Bunnugal, Elliot, Grampians ‘Deeps’, and Brimpaen belts and, possibly at greater depth, slices of the Stavely-Narrrapumelap-Dryden belt (see Figure 3.23 and Figure 3.24, Figure 4.13; Appendix 2 – Geological cross sections). Thus, the Grampians ‘Deeps’ Belt likely had similarly faulted margins to those seen for the Elliot, Brimpaen and Bunnugal belts. Although in its retrodeformed pre-D4 position the D1a volcanic belt lies virtually along-strike from the Dimboola Belt, the Dimboola Belt dips east, and seems to have been emplaced towards surface along a separate thrust system of opposite vergence, possibly a back-thrust to the Dryden and Dryden North belts.

Potential field data suggest that the Grampians ‘Deeps’ Belt dips steeply north-east. Therefore, a north-east dip is also likely for the D1a faults interpreted to bound the Grampians ‘Deeps’ Belt. This geometry is similar to the adjacent Elliot North and Elliot South faults, and may have originated in a similar way – by clockwise block-rotation, about a subvertical axis during D4, of originally west-dipping D1a thrust faults. The Grampians ‘Deeps’ Belt appears to be truncated at its western end by the Mosquito Creek Fault, cut and internally offset by the Victoria Valley North Fault system, and truncated at its southeastern end by the Escondida Fault and by the Victoria Valley South Fault, all D4 structures that also deform the overlying Grampians Group.

The Brimpaen Belt has been so strongly deformed and disrupted by D4 structures (e.g. Dollin Fault, Dollin Kink, Mosquito Creek Fault, Escondida Fault; see Section 3.2.5.4 – Brimpaen Belt) that no original D1a faults in this belt can be interpreted. The local complexity of D4 here may reflect the complexity of pre-existing D1a structures. The Brimpaen and Grampians ‘Deeps’ belt appear to have occupied an along-strike transition from D1a fault systems that dipped west in the south (e.g. in the Bunnugal and Elliot belts) and dipped east in the north (e.g. in the Dimboola Belt). The region may thus have developed adjacent to a transform fault or equivalent transitional structural system during D1a, a complexity compounded during subsequent D4 deformations (see Section 4.4.3 – Understanding the form and distribution of potential transfer structures).

3.3.3.17 Williamsons Fault

The interior of the Stavely Belt contains an anastomosing array of northwest-trending faults that splay from, and rejoin, the main belt-bounding structures. They have not been included in the STAVELY 3D model, but are described here because they provide key timing constraints for deformation, and also form part of a series of distinctive structures that has allowed for correlation between the different volcanic belts. These faults are virtually unexposed, visible only in magnetic data where different stratigraphic sequences are juxtaposed, and intersected in drill core. The largest and most prominent of these ‘interior’ faults is the Williamsons Fault, an anastomosing series of faults easily identified because of slices of highly-distinctive serpentinised ultramafic rocks contained along much of its length – the Williamsons Road Serpentinite (see Schofield et al. 2018 for description). The serpentinite is highly magnetic, and modelling of the associated anomaly (Figure 3.50) derives a steep westerly dip for the serpentinite unit, which is also taken to represent the dip of the Williamsons Fault series.

The Williamsons Fault and related splays has been intruded, ‘stitched’, in a few locations by porphyry stocks of the Lalkaldarno Porphyry, including the non-mineralised type locality (Buckland, 1987) and mineralised stocks such as the informally named ‘Victor’ porphyry at the Thursday’s Gossan Prospect. These Late Cambrian intrusive rocks were emplaced during D1b, and so constrain the preceding major contractional movements along the Williamsons Fault group to D1a.

The close association of the Williamsons Fault series with multiple spaced, northerly-elongated stocks of Late Cambrian porphyry implicate this fault group as one that experienced transtensional reactivation during D1b, with associated dilations across the fault plane facilitating the porphyry intrusions. Sinistral transtension is indicated for D1b, consistent with the idea that local dilation zones may have developed at left-stepping jogs in the fault plane during D1b. Strike-slip dominated reactivation of steeply-dipping faults can form pull-aparts that are steeply-plunging, with potential to act as deeply-tapping conduits for rising magmas associated with porphyry intrusion.

Serpentinite is a particularly weak rock-type, making faults within it especially prone to reactivation, as described further below. At the time of D1a and D1b porphyry intrusion the D4 Mafeking Megakink was yet to form, so that strike-extensions to the Williamsons Fault system within the Narrapumelap and Dryden belts lay directly north along-strike from the Stavely Belt, presumably experiencing the same D1 structural history. The interior of the Narrapumelap Belt contains fault-slices of serpentinite, and localised ?porphyry intrusions, so that the porphyry prospectivity already demonstrated for the Stavely Belt is likely to be able to be extended directly into the Narrapumelap Belt with the additional complexity of subsequent rotation and tilting during D4.

The Williamsons Road Serpentinite stratigraphy contained within the Williamsons Fault is exotic to the surrounding Stavely Arc stratigraphy in the Stavely and Narrapumelap belts. This suggests that vertical movements along the Williamsons Fault during D1a were large enough to have introduced parts of an underlying or adjacent stratigraphy into the interior of both these belt segments. In the Stavely Belt, mafic arc stratigraphy – for example the Fairview Andesite Breccia – predominates to the west of the Williamsons Fault, while intermediate to felsic arc stratigraphy – for example the Towanway Tuff – predominates to the east of the Williamsons Fault. This configuration is counter-intuitive with the overall west facing and dip interpreted for the Stavely Belt, given that arc volcanic successions typically begin as mafic and evolve towards intermediate and felsic compositions as they mature. The implication is that the arc stratigraphic order may be disrupted within the Stavely Belt, across structures such as the Williamsons Fault.

Other, subparallel unnamed D1a faults within the Stavely Belt similarly enclose fault slices of exotic material – mostly metasedimentary rocks such as Glenthompson Sandstone and the Glenronald Shale Member (see Schofield et al., 2018 for descriptions) – within the interior of the Mount Stavely Volcanic Complex at several different structural levels.

North of the Maroona-Glenthompson Road, the eastern flank of the Stavely Belt has been faulted out by the Escondida Fault. Between here and Yarram Park the Escondida Fault bounds the Stavely Belt, and appears coincident in position with the eastern flank of the Williamsons Fault system. Deep seismic reflection data indicates an overall easterly dip for the Escondida Fault, so that these faults likely diverge at depth.

The Williamsons Road Serpentinite within the Stavely Belt contains prominent S-C fabrics which define a dextral shear-sense (Stuart-Smith & Black, 1999). As mentioned above, serpentinite is a very weak rock so it is likely that these fabrics developed during the last significant deformation to affect the area. This appears to be D4, and dextral fabrics are consistent with large dextral translations indicated on adjacent D4 faults such as the Escondida and Yarrack faults (Sections 3.3.4.1 – Escondida Fault (reactivated) and Section 3.3.5.27 – Yarrack Fault). Evidence of late dextral reactivation of parts of the Williamsons Fault during D4 may help explain the context of late brittle faults seen in drill core to segment and offset stocks of the Lalkaldarno porphyry at places like Thursday’s Gossan. These present a significant challenge to targeting during mineral exploration.

3.3.4 Regional-scale D3 and D4 faults

D3 (sinistral transpressional) and D4 (dextral transtensional) structures are described together in this section, and the following district-scale D3 and D4 structures section, because of the uncertainty that exists as to the precise affiliations of some of the D3 structures – they may be of either D3 or D4 age, or both.

Structures within the Grampians-Stavely Zone that appear to be related to crustal shortening and can be unambiguously assigned to D3 include early bedding-parallel faults with thrust-characteristics within the Grampians Group and the large Wartook and Geerak synclines that overprint those folds (Cayley & Taylor, 1997a; see Figure 2.16 and Figure 3.5), but these structures are not included in the STAVELY 3D model. Equivalent D3 structures are seldom preserved unmodified within the underlying Cambrian bedrock. Thrust-displacements interpreted for the Escondida and Mehuse faults are an example of effects attributed to D3, but can only be discriminated once the effects of subsequent D4 and D5 deformations have been taken into account.

Most other faults and structures assigned to D3 are related to sinistral strike-slip, but are ambiguous – the predominantly-north trending geometries and strike-slip movement histories of some D3 faults is consistent with either a synthetic response to a prevailing sinistral-tranpressional (D3) stress field, or an antithetic response to a prevailing dextral-transtensional (D4) stress-field. It is therefore probable that synthetic D3 faults underwent significant reactivation(s) of similar sense during D4, but as antithetic structures.

Although most structures interpreted to belong to D3 appear to be truncated by D4 faults to varying degrees (e.g. the D3 sinistral Latani, Cherrypool and Muirfoot faults cut and displaced across the D4 dextral Henty Fault – see Figure 3.13 and Figure 4.5), it has proven difficult to determine if such overprinting was the result of successive separate deformation events, or because displacements across synthetic D4 master faults outlived displacements across subsidiary antithetic D4 faults, or a combination of the two (see Figure 4.7). Wholesale rotations of crustal blocks during D4 are a further complication, since they serve to disguise the original orientations of many contained structures, D3 structures included. Some D3 and/or D4 structures appear to have undergone additional reactivation during D5 – (e.g. the Golton and Mosquito Creek faults). Retrodeformation of the effects of D4 and/or D3 structures suggests that a combination of displacements on most structures during D3 and D4 (and locally, D5) is most likely (see Section 4.1.3 – ‘Crab Nebula’ retrodeformation). Because of this, the relative ages and timing constraints of regional-scale D3 and D4 faults are discussed on a case-by-case basis. Individual D3 and D4 faults are described below in alphabetical order and presented in Figure 3.9.

3.3.4.1 Escondida Fault (reactivated)

The reactivated Escondida Fault is the most significant D3-D4 structure within STAVELY with a strike length of more than 260 km (Figure 3.9, Figure 3.15 and Figure 3.22). It preserves evidence of a complex, extended movement history, and different parts of the Escondida Fault appear to have formed and reactivated at different times.

The segment of the Escondida Fault that extends north of the Grampians Ranges and the Jalur Rift and bounds the western flank of the Dimboola Belt appears to have formed as an east-dipping thrust fault during D1a (see Figure 3.29). The D1a part of the Escondida Fault movement history is discussed separately in Section 3.3.3.7 – D1a Escondida Fault). The northern part of the Escondida Fault shows evidence of multiple subsequent reactivations during D3 and then D4.

The segment of the Escondida Fault that extends from within the Jalur Rift south to bound part of the eastern flank of part of the Stavely Belt, truncate the western ends of the Elliot and Narrapumelap belts, and link southeast into the Moyston Fault footwall (see Figure 3.9) appears to have first developed during D3 and D4. This segment of the Escondida Fault is spatially and temporally associated with the Mafeking Megakink (in Cambrian bedrock) and with the overlying Mafeking and Cranage oroclines (in Grampians Group cover). The southern segment of the Escondida Fault occupies the western axis of the Mafeking Megakink, and dextral strike-slip displacement across the fault here appears to disrupt and offset this megakink axis.

The D3 and D4 fault movement history proposed here for the Escondida Fault is younger and considerably more complex than previous interpretations which implied that all movement along the Escondida Fault was Cambrian (e.g. Cayley & Taylor, 1997a; Korsch et al., 2002). The complex D1a, D3 and D4 movement history is based on field control gained from additional mapping in the Cambrian bedrock in STAVELY. New mapping has found exposures of the Escondida Fault trace within the Cambrian bedrock to the east of the Stavely Belt (see Figure 2.14), not to the west as previously inferred. The new location coincides with the position of subvertical faults not previously recognised in the overlying Grampians Group (see Figure 2.16). The coincidence of subvertical faults in the Cambrian bedrock and in overlying Grampians Group, and additional control provided by deep seismic reflection line 09GA-AR1 means that the D3-D4 age and movement history of the Escondida Fault are well constrained. The crustal-scale fault geometry is based on deep seismic reflection data and dip-modelling and inversions of regional potential field data.

Formation of the southern D4 Escondida Fault extension is linked to transtensional reactivation of the Moyston Fault in D4, and to initiation of the Mafeking Megakink as a rotational rift-structure that developed in its footwall. Northwards propagation of the southern Escondida Fault segment through the Jalur Rift and towards the pre-existing D1a Escondida Fault segment position north of the Grampians Ranges, as the Mafeking Megakink and Jalur Rift grew in size, is implicated as the facilitator of strike-slip reactivation of the northern Escondida Fault, and for the development of the D4 Dimboola Duplex that segmented the Dimboola Belt. This is discussed in more detail in Section 4.4.3 – Understanding the form and distribution of potential transfer structures. Due to this compound movement history, the Escondida Fault shows considerable variation in style, effect, and apparent movement timing along its length. Descriptions of the Escondida Fault in this section are split into D3-D4 reactivation of the northern segment, and D4 growth of the southern segment.

3.3.4.1.1 Escondida Fault reactivation north of Jalur Rift

North of the Grampians Ranges at Laharum, Horsham and Dimboola regional magnetic, gravity (Cayley & Taylor, 1996) and deep seismic reflection data (Korsch et al., 2002) all show that the Escondida Fault dips moderately northeast, and defines the faulted (western) boundary between the northwest striking highly magnetic Dimboola Belt to the east, and the less magnetic, more northerly-striking Glenthompson Sandstone and/or Nargoon Group rocks to the west. The Dimboola Belt here is overlain, apparently unconformably, by Grampians Group. Grampians Group, its basal unconformity and the Dimboola Belt all apparently dip moderately east, subparallel to the dip of the underlying Escondida Fault. The whole succession of Cambrian and Silurian rocks here therefore appears to have been block-rotated to an east-dipping orientation in the Escondida Fault hangingwall post-Grampians Group deposition. The overall east-dip of the hangingwall succession is consistent with tilting early in D3, a geometry consistent with reverse movement during reactivation of the Escondida Fault.

Regional magnetic data (Figure 3.64) clearly shows that the southern end of the Mackenzie River Fault, and any connectivity with fault extensions farther south along-strike, is abruptly truncated and cross-cut by a narrow, northwest-trending fault slice of foliated and fractured sequence of ultramafic-mafic rocks that occupy the hangingwall of the Escondida Fault (intersected in drill hole STAVELY10; see Figure 3.29). The regional magnetic data also show that the Escondida Fault cuts across the trace of the Mosquito Creek Fault. The Mosquito Creek and Mackenzie River faults both deform Grampians Group, and so are themselves structures that formed during D3 and D4 respectively, relatively late within the Grampians structural system (Cayley & Taylor, 1997a). This combination of overprinting criteria requires that, in addition thrusting early in D3, the segment of the Escondida Fault north of the Grampians Ranges experienced trans-tensional reactivation in D4 that accompanied and then outlived movements along the D3 Mosquito Creek and D4 Mackenzie River faults. Figure 3.64 Tilt and band-pass filtered magnetics of the Brimpaen-Horsham region, with the Dimboola and Brimpaen belts highlighted, and illustrating complex overprinting criteria between the Escondida, Mosquito Creek and Mackenzie River faults. The Mosquito Creek and Mackenzie River faults both cut and offset Grampians Group and so are of D3-D4 age. The alignment of these faults is coincidental, with both dextral and sinistral strike-slip displacement indicated for the Mosquito Creek Fault, only dextral strike-slip displacement indicated for the Mackenzie Creek Fault, and strike-slip offset indicated for the intervening Escondida Fault. The Mosquito Creek and Mackenzie River faults are both cut by magnetic ultramafic rocks aligned along the Escondida Fault trace, indicating that Escondida Fault displacement outlasted movements on both these faults. Overprinting criteria with Grampians Group indicates the Escondida Fault has an additional early (D3 (D2), D1) movement history. Late (D4) reactivation of the Escondida Fault appears to have been dextral transtensional, so that any northern extension to an early Mosquito Creek Fault is expected to have been displaced south as, for example, the Olive Fault.

North of Dimboola, the Escondida Fault appears to lose displacement, and the fault apparently cuts into the interior of the Dimboola Belt near Netherby (see Figure 3.29). Here, a strip of magnetic volcanics of the Dimboola Belt emerges in the fault footwall, widening progressively to the northwest. North of Netherby where the apparent vertical offset across the Escondida Fault appears small, the Dimboola Belt interior appears to have remained largely unmodified by the effects of either D1 or D3-D4.

Where vertical offset associated with the Escondida Fault diminishes north of Netherby the overall dip of the Grampians Group and its basal unconformity appears to flatten to subhorizontal. Simple, flat-lying Grampians Group and a subhorizontal basal unconformity are imaged in seismic reflection line MEMV96-09 (Figure 2.24). The coincidence of degree of tilting and structural complexity within the Grampians Group to the size and proximity of D3 and D4 movements along the Escondida Fault implies linkage – structural complexity within the Grampians Group is closely related to the magnitude of reactivation of the underlying Escondida Fault.

3.3.4.1.2 D4 Escondida Fault segments associated with the Jalur Rift and Mafeking Megakink

The segment of the Escondida Fault that extends south from within the Jalur Rift beneath the Grampians Ranges to bound part of the eastern flank of the Stavely Belt, and link southeast into the Moyston Fault footwall appears to have first developed during D3 and D4. The age of initiation of this structure is tightly constrained to D4 by overprinting criteria – this fault segment deforms both Cambrian bedrock and overlying ?Late Ordovician-Early Silurian Grampians Group in complementary ways. The last fault movements on this segment either immediately preceded or accompanied intrusion of the McKenzie Creek and Mafeking granites. These granites stitch the Escondida Fault in several places, thus providing an upper age constraint on D4 fault movement timing of approximately 403–400 Ma (Lewis et al., 2016). This appears to be the last phase of movement that occurred along the Escondida Fault.

Retrodeformation of the effects of the Mafeking Megakink shows that the Stavely, Narrapumelap and Dryden Belts were united in one west-dipping fault-belt pre-D4 (see Figure 4.4). There is no evidence or need for any significant pre-D4 east-dipping structure in the position currently occupied by the southern segment of the Escondida Fault. This segment cuts obliquely across the position of pre-existing west-dipping D1a structures in Cambrian bedrock in plan, most notably, the truncated northern end of the Stavely Belt beneath Grampians Group cover near Mafeking (Figure 3.44), and the truncated northwestern ends of the Elliot and Narrapumelap belts. Therefore, it likely also cuts across D1a structures in dip – and this relationship is imaged in deep seismic reflection data (Figure 2.22). The D4 Escondida Fault splays can be traced further north into and beneath the Grampians Ranges to link along-strike into the D1a Escondida Fault segment near Zumsteins, possibly explaining the context of D4 reactivation of the older northern segment.

The coincidence of the Jalur Rift and Early Devonian granites with the D4 Escondida Fault extensions implicates this fault activity directly to formation of the Jalur Rift. The Escondida Fault is expressed within the Grampians Group as an anastomosing array of subvertical faults, typically intruded by porphyry dykes. These are superimposed over much of the rest of the complex D3 structure developed within the Grampians Group. The D4 Escondida Fault splays trend southeast through Flat Rock Crossing, past Mount Wrrinaburb (formerly Mount Lubra), to link into the Marathon/Thermopylae Fault west of Mafeking, a few kilometres northwest along-strike of exposures of D4 Escondida Fault structures in the Cambrian bedrock. Deep seismic reflection line 09GA-AR1 crosses the Escondida Fault near Yarram Park, and images the Escondida Fault as a large, listric, east-dipping structure, linking east into the Moyston Fault footwall in the lower crust (see Figure 2.22).

Monotonous Grampians Group stratigraphy, bedding-dips that lie sub-parallel to the direction of apparent offset across the Escondida Fault splays, and the presence of a series of coeval dip-slip splays of the Marathon Fault series make the effects of Escondida Fault segment displacements hard to quantify in the Grampians Group, other than where the fault cuts across the western end of locally east-west striking Grampians Group stratigraphy at Mafeking. Here, the Thermopylae Fault (Taylor & Cayley, 1997; Cayley & Taylor, 1997a) appears to overthrust the western end of the Mafeking Orocline, suggesting localised late dextral transpression along this part of the Escondida Fault. The Thermopylae Fault position lies exactly above the trace of the Escondida Fault in the underlying Cambrian bedrock, and coincides with the western axis of the Mafeking Megakink.

Beneath the Grampians Ranges the position of the Escondida Fault in Cambrian bedrock is constrained by gravity data. Grampians Group thickness is seen to change across the trace positions of the largest underlying Escondida Fault splays (see Figure 2.27). This data implies that changes in the depth of the basal unconformity and/or basal Marathon Fault topography occur across the Escondida Fault position. Gravity data shows the greatest thicknesses of Grampians Group occurs to the east of the Escondida Fault trace, in the Mount Difficult and Mount William ranges, and at Mackenzie Creek. The eastern side of the Escondida Fault is the hangingwall side, so a greater thickness of cover here is consistent with a downthrown hangingwall side, and with an extensional movement history for the Escondida Fault in D4.

South of the Grampians Ranges, the position and character of the Escondida Fault has been reinterpreted. This reinterpretation is based on mapping, drilling, deep seismic reflection data, and dip-modelling of potential field data all of which now show that no large east-dipping fault that can correlate with the Escondida Fault occurs to the west of the Stavely Belt, the position originally interpreted for the Escondida Fault (Cayley & Taylor, 1996, 1997a; Stuart-Smith & Black, 1999). Instead, deep seismic reflection data and field mapping of a wide dextral strike-slip fault-network exposed in a tributary of Back Creek east of Mount Stavely show that a large, steeply east-dipping fault – which we correlate with the Escondida Fault – lies just to the east of the Stavely Belt and in places bounds the eastern margin of the Stavely Belt. Several outcrops of the Escondida Fault trace here constrain a steep near-surface dip, and a complex and substantial D4 dextral strike-slip movement history.

The best exposure of D4 Escondida Fault deformation is in Glenthompson Sandstone to the east of the Stavely Belt. The fault zone is well exposed in a creek near Sheepwash Road. Here the Escondida Fault comprises a large, steeply-dipping strike-slip fault-mélange with a strong, oblique, scaly to cataclastic to stylolitic cleavage developed in the low-grade rocks with a geometry consistent with dextral shear (see Figure 2.14). The axial planes of small, sub-vertically plunging dextral kink folds within the mélange lie within the plane of the cleavage. The cataclastic and stylolitic deformation style of the fault mélange contrasts strongly with the more ductile structural style and pervasive metamorphic textures (including slaty chlorite cleavage growth and quartz-mobility) typically associated with D1a in the Glenthompson Sandstone. This indicates that the fault mélange formed under cooler and lower confining pressure conditions, consistent with the fault zone being considerably younger.

Multiple facing reversals in Glenthompson Sandstone bedding fragments incorporated within the Escondida Fault zone suggest that the D4 fault mélange and scaly fabrics are superimposed over a succession that had already been folded by pre-existing upright D1a folds. The D4 Escondida Fault zone here is over 150 m wide, is subvertical and strikes northwest towards the eastern flank of the Stavely Belt, which it apparently truncates near Spittle Road. North of this point, the Escondida Fault bounds the eastern flank of the belt to Yarram Park and beneath the Grampians Ranges. The Escondida Fault here also truncates the western ends of the Narrapumelap and Elliot belts at a high strike-angle. South of the exposures near Sheepwash Road, the Escondida Fault is enclosed within Glenthompson Sandstone and can be traced in potential field data southeast to Wickliffe and towards the buried traces of the Golton, Mehuse and Moyston faults farther east, with which it may merge under cover.

Outcrops of Williamsons Road Serpentinite in the Williamsons Fault of the Stavely Belt adjacent to the Escondida Fault preserve well-developed dextral S-C fabrics (Stuart-Smith & Black, 1999). Serpentinite is a weak rock and tends to preserve evidence of only the last fault movements. The dextral shear components preserved within the Williamsons Road Serpentinite are consistent with the D4 dextral displacements preserved along the Escondida Fault, which truncates the Williamsons Fault along much of its length north of Stavely. Stuart-Smith & Black (1999) associated these displacements with wrench movements during deformation of the Grampians Group, and we concur with this interpretation.

3.3.4.1.3 Regional context for interpretation of the D3-D4 Escondida Fault

We interpret the development of the Escondida Fault during D3 and D4 to involve two distinct movement episodes, both associated with deformation of the Grampians Group, and developed sequentially. The first movement episode (D3) appears to have only involved reactivation of pre-existing D1a northern Escondida Fault segments. It apparently occurred in response to a regional stress regime of sinistral transpression in the Late Silurian, during which early thrust-imbrication along bedding-parallel faults occurred in the overlying Grampians Ranges (Cayley & Taylor, 1997a), followed by thick-skinned folding of the Grampians Group thrust stack to form large features such as the Wartook Syncline (Spencer-Jones, 1965; Cayley & Taylor, 1997b).

During D3, the northern Escondida Fault segment appears to have been reactivated as an east-dipping sinistral-oblique thrust, possibly in response to thrust-reactivation of the Moyston Fault (Cayley et al., 2011b). The Mehuse Fault appears to have formed at this time (Cayley & Taylor, 2001). The Escondida and Mehuse faults both cut east through steeply dipping volcanic belts of the Stavely Arc – the Dimboola and Dryden belts respectively. The hangingwalls of both volcanic belts have apparently been thrust towards the northwest, a scenario that explains the truncated western margins of Cambrian stratigraphy in the Mehuse and Escondida fault hangingwalls (Cayley & Taylor 2001, Korsch et al., 2002). We interpret these thrusts to have locally propagated up into the overlying Grampians Group, providing context for the origin of early low-angle faults which appear to have thickened and shortened this succession in D3 (Cayley & Taylor, 1997a).

Evidence of westward thrusting on east-dipping structures within STAVELY during D3 diminishes to the south. In the Cambrian bedrock west-dipping D1a (thrust) belts survive intact as the Boonawah, Bunnugal and Stavely belts, and there is no evidence of any post-D1 – pre-D4 deformation. There is no evidence of D3 thrusts or disordered stratigraphy in the outcrops of Grampians Group that extend from near Wickliffe south towards Woorndoo and Hexam (Stuart-Smith & Black, 1997).

D4 movement along the Escondida Fault appears to have occurred in response to a regional stress regime of dextral transtension in the Early Devonian. This timing is constrained by the development of splays of the Escondida Fault within the Grampians Group, where they clearly overprint earlier structures like bedding parallel faults and the Wartook Syncline. D4 structures in the Grampians Group appear to have been associated with transtensional and strike-slip segmentation of the Grampians Group (Cayley & Taylor, 1997a), and including: the formation of a structural basin, the Jalur Rift (see Figure 3.9) ; refolding of earlier structures about large oroclinal warps with subvertical axes (the Mafeking, Cranage and Big Cord oroclines; see Figure 3.73) and; structural dislocation between the Grampians Group and underlying Cambrian bedrock along splays of the Marathon Fault. The D4 movement history of the Escondida Fault appears associated with formation of the subparallel Golton Fault which also cuts and bounds the Grampians Group (Spencer-Jones, 1965; Wilson, 1988; Wilson et al., 1992; Cayley & Taylor, 2001) and lies close to the position of the Mehuse Fault.

During D4 the Escondida Fault appears to have been active as an east-dipping dextral-transtensional fault. D4 appears to have involved the formation of a new fault extension that linked from the Moyston Fault footwall northwest into the Jalur Rift beneath the Grampians Ranges. Movements on these new structures may have facilitated further reactivation of the existing northern Escondida Fault segments.

D4 movements along the southern parts of the Escondida and Golton faults at this time appear to be associated with the development of the dextral Z-shaped Mafeking Megakink in the Cambrian bedrock (see Section 3.4.4 -Mafeking Megakink). The Escondida Fault appears to occupy and rupture the western axis of the Mafeking Megakink. The Golton Fault ruptures the eastern megakink axis (see Section 3.3.4.2 – Golton Fault). The central limb of the Mafeking Megakink is occupied by a rotated block of Cambrian bedrock that includes the Elliot and Narrapumelap belts.

Megakinks with central limbs rotated beyond 90° (as seen in the Mafeking Megakink; see Figure 4.4, timeslices 402 – 399 Ma) are typically characterised by late local transpression due to convergence that becomes progressively confined within the kink band. Both the Escondida and Golton faults display local characteristics consistent with late transpression, with scaly cleavage formation and overthrusting (Thermopylae Fault) on the Escondida Fault, and late up-turning and thrusting within the Grampians Group (Golton Fault, Fyans Fault) adjacent to the Golton Fault. Whether-or-not this local transpression occurred late in D4, or during a subsequent separate compressional deformation event (e.g. D5) is poorly constrained. Evidence of sinistral transpression along the Fyans and Golton faults that is preserved in the Northern Grampians Ranges (e.g. Wilson et al., 1992; Cayley & Taylor, 1997a) suggests that this late transpression may have occurred subsequent to D4, during the D5 Tabberabberan Orogeny. The Tabberabberan Orogeny is characterised more regionally by sinistral transpression. Major late sinistral fault movements, such as seen in the Mosquito Creek Fault, may also have occurred during D5.

North of the Grampians Ranges, the style of D4 deformation associated with movements along the Escondida Fault appears to transition to translation. D4 reactivation of the Escondida Fault appears to be associated with development of the Dimboola Duplex, a dextral strike-slip duplex of subparallel, en-échelon strike-slip faults that includes the McKenzie River, Dimboola and West Wail faults. The Dimboola Duplex cuts Grampians Group, is of D4 relative age, and is bound by, and links into, the northern Escondida Fault segment. Splays of the Dimboola Duplex bound local D4 structural sub-basin deeps filled with thick Grampians Group successions, such as to the east of the Mackenzie River Fault and, further north, to the southeast of the Lorquon Fault (the Lorquon Rift).

Segmentation and transtensional lateral collapse of Dimboola Belt crust southwards into the northern flank of the Jalur Rift as it opened behind the Mafeking Megakink during D4 may explain the origin and geodynamic context of the Dimboola Duplex. In this style of interpretation, a structural separation is required to accommodate the different local tectonics between rotated (Mafeking Megakink, Jalur Rift) and translated (Dimboola Duplex) elements of the D4 structural system. This separation is the inferred Jalur Fault, hosted by Cambrian bedrock beneath the Grampians Ranges, described separately.

3.3.4.2 Golton Fault The Golton Fault (Spencer-Jones, 1965; Wilson, 1988) bounds the eastern side of the Grampians Ranges and juxtaposes Grampians Group against rocks of the Mount Stavely Volcanic Complex farther east. This relationship suggests an overall east-side-up movement history, possibly with a significant strike-slip movement component. An estimate of the magnitude of sub-horizontal (strike-slip) displacement on the Golton Fault is given by the amplitude of the Mafeking Megakink and by offset of its eastern hinge.

Grampians Group adjacent to the Golton Fault has a subvertical orientation, in places overturned to dip steeply east (Spencer-Jones, 1965; Cayley & Taylor, 1997a, 1997b; Figure 3.65B). The Golton Fault is associated with deformation of Grampians Group sediments and so must have a substantial Late Silurian-Early Devonian D3-D4 (possibly also D5) movement history. The strain history in the Grampians Ranges and relationships to the Mafeking Megakink imply significant dextral transtensional offset across the Golton Fault early in D4, followed, within the megakink, by subordinate local dextral transpression late in D4. Tilting of the Grampians Group adjacent to the Golton Fault is a late (D4-D5) phenomenon, since the tilting affects bedding-parallel faults and other structures within the Grampians Group that are attributed to D3. Structures developed in subvertical sedimentary rocks of the Grampians Group adjacent to the Golton Fault reflect south-over-north and east-over-west thrust movements (Figure 3.65A). This is interpreted to reflect a sub-vertical to steep easterly dip to the Golton Fault, and suggests a local, late-D4 to D5 reverse/oblique displacement movement history. A sinistral component of deformation preserved in late structures deforming Grampians Group at Golton Gorge in the north of the Grampians Ranges (e.g. Wilson & Watchorn, 1988; Wilson et al., 1992; Cayley & Taylor, 1997) suggests that this late transpression may have occurred subsequent to D4, during the D5 Tabberabberan Orogeny. The Tabberabberan Orogeny is characterised more regionally by sinistral transpression.

The Golton Fault is imaged as east-dipping in deep seismic reflection line 97AGS-V1 just north of the Grampians Ranges – deep seismic reflection data shows the fault to converge down-dip towards the Mehuse Fault at approximately 2.8 s TWT (ca. 7.5 km) depth in the crust (Korsch et al., 2002). An along-strike extension of the Golton Fault is imaged as east-dipping in deep seismic reflection data (line 09GA-AR1; Cayley et al., 2011b). Linkage between the different east-dipping faults indicates that structures like the Golton and Mehuse faults are all footwall splays emanating from mid-crustal levels of the Moyston Fault footwall, presumably formed during D3 – D4 reactivation of that structure. North of Kalkee the Mehuse and Golton faults cannot be distinguished and may be coincident – low gravity response adjacent to the Dryden Belt here suggests that Grampians Group is juxtaposed hard against the Mount Stavely Volcanic Complex.

North of Dimboola, the position of the Golton Fault appears to merge with, or be offset and juxtaposed across several complex, poorly-understood structures that are likely of D4 age, such as the Babatchio, Tullyvea and Glenlee Faults. The deformation pattern here has the characteristic shape of a strike-slip duplex. Apparent offsets of the Golton Fault position within this part of the duplex include a mix of large-scale dextral (Babatchio Fault) and sinistral (Tullyvea) offsets, suggesting that they may be directly related to the larger-scale dextral transtension mapped in other parts of the Dimboola Duplex.

Southeast of the Grampians Ranges the Golton Fault is imaged as east-dipping in deep seismic reflection line 09GA-AR1 (Cayley et al., 2011b). Here, and east of Mount Stavely, the interpreted position of the Golton Fault bounds the eastern edge of the block of poorly exposed Cambrian bedrock that contains the obliquely-striking Elliot and Narrapumelap belts, interpreted as the central limb of the Z-shaped Mafeking Megakink. The abrupt change in strike and dip of D1a structures across the trace of the Golton Fault here suggests that the fault may occupy the faulted eastern hinge of the D4 megakink (Section 3.4.4 – Mafeking Megakink). The amplitude of the Mafeking Megakink, and right-lateral offset of the Dryden Belt from the Narrapumelap Belt both suggests a substantial component of dextral strike-slip translation occurred along the Golton Fault during D4. Farther south, the Golton Fault trace strikes towards the tract-positions of the Escondida Fault and the Moyston Fault, with which it may merge beneath Newer Volcanic Group cover.

Figure 3.65 Structures in Grampians Group adjacent to the Golton Fault A) looking west at David Taylor admiring a north-verging thrust fault duplex comprising stacked lens-shaped fault slices, that cuts subvertically-dipping Grampians Group strata in Golton Gorge (MGA 54 626900 5912700). This brittle fault system has down-dip slickenside striations on fault surfaces, constraining dip-slip reverse displacement. The late timing and transpressional structural geometry of this fault duplex is at odds with the overall dextral transtensional stress-field indicated for D4, but is consistent with the sinistral transpressional stress field indicated for D5 (Tabberabberan Orogeny). B) Panorama looking northwest towards the eastern flank of the Mount Difficult Range and Halls Gap, from Boronia Peak (MGA 54 656500 5887500). Boronia Peak is a high point on ‘The Terraces’, formed of deformed, northwest-trending Grampians Group strata upended to vertical and slightly-overturned dips (visible in foreground). The subvertical beds structurally underlie gently-dipping Grampians Group strata in the Mount Difficult Range (background), faulted across the Fyans Fault. The subvertical beds likely persist north at depth since they re-emerge from beneath the Mt Difficult range with the same geometry in the vicinity of Golton Gorge (Cayley & Taylor, 1997b). Strata upending is attributed to late (D5) overthrusting of the adjacent east-dipping Golton Fault, the trace of which lies just a few kilometres to the east of The Terraces.

3.3.4.3 Henty Fault (new name) The Henty Fault does not crop out, but its position is interpreted from the abrupt truncation of Grampians Group along the southwestern margin of the Black Range, where Grampians Group is juxtaposed against Cambrian rocks in the Tyar Belt across the Henty Fault position, and from regional magnetic data which shows the northwestern end of the Tyar Belt to truncate abruptly and at high strike angle against a steeply-dipping northwest-trending structure (see Figure 3.16 and Figure 4.5). The asymmetry of the Tyar Hinge (see Section 3.4.5 – Tyar Fold) suggests that the Henty Fault is a dextral strike-slip fault. Because the Henty Fault truncates Grampians Group, and cuts and displaces sinistral D3 strike-slip faults also developed in Grampians Group (e.g. the southern end of the Muirfoot Fault) it is interpreted as an Early Devonian D4 structure.

The western end of the Henty Fault can be traced on regional magnetic data towards the position of the Latani Fault, which it apparently truncates west of Toolondo. North of this point, the Henty Fault persists beneath the Murray Basin in the footwall of the Yarramyljup Fault, towards which it converges to likely truncate and merge with the Henty West Fault north of Boyeo.

East of the Black Range, the Henty Fault appears to truncate the southern end of the Muirfoot Fault and the southern end of the Cherrypool Fault, before passing beneath Rocklands Volcanic Group, where it appears to have been intruded by a buried ?Early Devonian intrusion just south of Glenisla. Any farther eastwards extension of the Henty Fault appears to be truncated by the Mosquito Creek Fault. The southern end of the Glenisla Belt trends towards the Henty Fault position – the Henty Fault is interpreted to have truncated the southern end of this belt prior to the emplacement of the intrusive.

Restoration of dextral displacement across the Henty Fault, constrained by in-line restoration of the Tyar and Glenisla Belts, in combination with in-line restoration of the sinistral Cherrypool and Latani faults, suggests that a little over 22 km of dextral displacement may have occurred across this portion of the Henty Fault during D4. The amplitude of the Tyar Hinge adjacent to the Henty Fault (see Section 3.4.5 – Tyar Fold), accounts for a couple of kilometres of this displacement.

Drill hole STAVELY05 was designed to test for the possibility of offset of the Yarramyljup Fault across the dextral Henty Fault. Low-grade rocks encountered in drill hole STAVELY05 confirm that no significant lateral offset of the Yarramyljup Fault occurs across the Henty Fault – west of the point where the Henty Fault cuts the Tyar Belt the fault strike must swing north to subparallel with the Yarramyljup Fault. West-dipping reflectors beneath the position of the Muirfoot Fault imaged on deep seismic reflection line SD1 instead relate to Cambrian volcanic stratigraphy in the Black Range and Black Range West/Mitre belts, and do not represent a footwall splay from the Cambrian Yarramyljup Fault (as proposed by Cayley et al., 2011b).

3.3.4.4 Henty West Fault (new name)

The Henty West Fault does not crop out, but is interpreted from deep seismic reflection data, to explain the abrupt change in dip in reflective rocks in the hangingwall of the Yarramyljup Fault and the Yarramyljup Fault plane beneath CDP 7000 in line 09GA-SD1 (see Figure 2.22). This point projects down-dip to the east to link into one of several late faults that offset the deeply buried crest of reflective rocks interpreted to represent buried Stavely Arc volcanics beneath CDP 7750. This point projects up-dip to the west to surface into the vicinity of a subtle change in the character of regional magnetic data within the interior of the eastern Glenelg River Metamorphic Complex, across which previous magnetic units within the metamorphic complex appear to be truncated (Figure 2.28A). This truncation is interpreted to represent the Henty West Fault trace buried beneath Murray Basin cover rocks. Thus the Henty West Fault is modelled as an east-dipping listric fault that cuts deep into the interior of STAVELY, linking into other D4-age faults in the mid-crust. The locally steeper dip of the Yarramyljup Fault segment in the Henty West Fault hangingwall is consistent with roll-over, and indicates transtensional displacement across the listric Henty West Fault.

The Henty West Fault can be traced north in regional magnetic data, from deep seismic reflection line 09GA-SD1, beneath Murray Basin cover. The Henty West Fault trace lies subparallel to and slightly west of the Yarramyljup Fault position. The Henty West Fault apparently cuts across the Yarramyljup Fault trace to merge with the Henty Fault approximately 18 km northwest of Nhill.

The Henty West Fault can be traced in south in regional magnetic data beneath Balmoral, where it trends subparallel to the Yarramyljup and Latani faults to its east. The Henty West Fault has not been recognised in outcrop here. It apparently links into the Latani Fault near Gatum, south of which no distinction between the Latani and Henty West faults can be recognised.

The point of convergence between the Henty West and Latani faults coincides with a major disruption in the Yarramyljup Fault trace, with part of the Yarramyljup Fault reoriented and locally offset into a local northwest-southeast trend adjacent to (beneath) Mount Dundas. The Latani and/or Henty West faults are implicated in this reorientation, but insufficient data currently exists to allow a precise interpretation. The overall geometry appears similar to the reorientation seen for the Tyar Belt adjacent to the Henty Fault.

The Henty West Fault is interpreted to share a common (D3-D4) age with the Henty and/or Latani Faults. The movement history of the Henty West Fault is not well constrained, because its surface position is confined entirely to the Glenelg River Metamorphic Complex and there are few sufficiently distinctive marker horizons within the metamorphic complex that can be used to match offsets across the trace of the Henty West Fault. However, the local geometrical similarities with the Henty West Fault at different places along its length imply a similar D4 dextral strike-slip movement history.

3.3.4.5 Jalur Fault

The Jalur Fault is a steep (at surface) south-dipping, transtensional, sinistral oblique-slip fault of large displacement that is interpreted to span east-west between the Golton and Escondida faults in a position entirely concealed beneath Grampians Group strata in the eastern Grampians Ranges (Figure 2.32).

Although not imaged, a fault with large, sinistral trans-tensional scissor-style D4 displacement is required to account for the huge difference in D4 movement history that exists in Cambrian bedrock south and north of this position: the difference is between the tens-of-kilometre-scale clockwise-rotated middle limb of the Mafeking Megakink to the south, and the south-translated but unrotated Dimboola Duplex fault system confined within the Dimboola Belt to the north.

The Jalur Fault is entirely buried and so is not observed, but its position is constrained by regional potential field data which allows, even beneath considerable thickness of Grampians Group cover, for the lateral projection of Cambrian bedrock with differing structural characteristics from north and from south. An overall south-dipping geometry and D4 sinistral transtensional displacement history for the Jalur Fault has been deduced by a process of elimination.

The key constraint on Jalur Fault geometry is the large scale rotational character of the large block of Cambrian bedrock that forms the Mafeking Megakink middle limb. This is confined to the south side of the Jalur Fault position. Interpreting a northerly dip for the Jalur Fault would place the rotated Cambrian bedrock into the fault footwall, necessitating, in turn, the projection of rotated Cambrian bedrock north to depth, where it would underlie the southern Dimboola Belt. Because the Southern Dimboola Belt shows no evidence of rotation during, or prior, or subsequent, to D4, it unlikely that any Cambrian bedrock beneath the southern Dimboola Belt has rotated. Thus, the Jalur Fault is unlikely to dip north.

Instead, a southerly dip and D4 sinistral transtensional displacement history for the Jalur Fault (as modelled) is indicated. A southerly dip is consistent with the idea that the clockwise-rotated block of Cambrian bedrock in its southern flank is confined to the Jalur Fault hangingwall, and with the idea of antithetic sinistral displacement across the fault position during D4.

A south-dipping Jalur Fault detachment forms a logical lower boundary surface for the central limb of the Mafeking Megakink, a key rotational component within the D4 Jalur Rift (see Figure 3.9). A south-dipping Jalur Fault can merge congruently at depth into other D4 fault planes of similar history that also control and define the wider Jalur Rift margins, specifically the Mosquito Creek, Escondida, and Golton faults, and Fault 8 (see Appendix 6 – Fault summary table). Deep seismic reflection line 09GA-AR1 images the Escondida Fault as the base of the central Mafeking Megakink limb, which indicates that the Jalur Fault likely merges with the Escondida Fault north of 09GA-AR1.

All the D4 structures modelled within the Jalur Rift share an overall displacement sense that involves clockwise rotational translation of hangingwall materials towards the southeast and towards the Moyston Fault position.

Grampians Group within and adjacent to the Jalur Rift appears to be largely allochthonous with respect to the locally underlying Cambrian bedrock, separated across various splays of the D4 Marathon Fault system which show complex scissor-displacement histories. It is therefore expected that the Jalur Fault also likely links into select splays of the Marathon Fault system, particularly in those splays that are interpreted to straddle across the Jalur Fault position (see Figures 3.22 and Figure 3.73). The Jalur Fault is not expressed in the overlying Grampians Group as steep structures. This precludes interpretation of simple post-D4 displacements across the Jalur Fault position.

Fault 08 (see Figure 3.21 and Appendix 6 – Fault summary table) is considered to be a subsidiary splay of the Jalur Fault. Fault 08 separates the Grampians ‘Deeps’ and Brimpaen belt segments that appear to have rotated separately during D4. Fault 08 extends west to the Mosquito Creek Fault and, like the Jalur Fault, probably originated as an antithetic, sinistral transtensional oblique-slip fault. Early-D4 transtensional movement phases of the Mosquito Creek Fault were probably also directly related to formation of the Jalur Fault, via transfer faults such as Fault 08, since the Mosquito Creek Fault appears to bound the western flank of the entire Jalur Rift, serving to separate clockwise-rotated crustal elements within the rift interior from unrotated crustal elements that are preserved just to the west.

3.3.4.6 Marathon Fault

The Marathon Fault is the name given to a system of low- to moderately dipping faults that link to form an undulating decollement that is interpreted to underlie much of the ?Late Ordovician-Silurian Grampians Group, separating it from underlying Cambrian bedrock. Although the Marathon Fault system at the base of the Grampians Group succession is not well exposed (the ‘best’ exposures are north of Dunkeld, and in the Black Range; Cayley & Taylor, 1997a; Morand et al., 2003), the fault was intersected in drill hole STAVELY02. Here the Marathon Fault presents as a shallow-dipping interface between moderately east-dipping Grampians Group and the Mount Stavely Volcanic Complex. The fault zone in STAVELY02 (Figure 2.18) is strongly oxidised and sheared, with a damage zone in Grampians Group several metres thick preserving slip-planes with fault striations and fault steps that constrain normal (extensional) fault-displacements. The upper 4-5 m of Mount Stavely Volcanic Complex rocks in the fault footwall are also strongly sheared and oxidised. These characteristics suggest substantial top-to-east displacements occurred along this splay of the Marathon Fault.

In most cases, movement along splays of the Marathon Fault appear to have had the effect of removing stratigraphy, consequently the fault splays are interpreted as extensional structures. They are among the last to deform the Grampians Group, are intruded by Early Devonian granites – themselves a likely product of extension and decompression melting – and are therefore considered to be D4 in age.

Marathon Fault splays persistently cut up-section wherever recognised within Grampians Group strata. This characteristic likely relates to the relative lack of favourable, laterally persistent stratigraphic horizons at lower levels within the Grampians Group along which larger, simpler, lower-angle detachment could occur. The Marathon Fault system was superimposed over a Grampians Group stratigraphy that had already been subject to substantial shortening and thickening, including formation of the Wartook and Geerak synclines, structural complexity that further diminished the likelihood that laterally persistent detachment horizons were available for Marathon Fault propagation (Cayley & Taylor, 1997). The lack of preferred horizons for Marathon Fault propagation means that the geometries of individual Marathon Fault splays within the Grampians Group likely bear direct allegiance to parent structures in underlying Cambrian bedrock. Different splays of the Marathon Fault system locally display overlapping deformation fronts which has resulted in locally complex geometries within Grampians Group (e.g. Asses Ears Range, southern Victoria Range).

Splays of the Marathon Fault system have been interpreted wherever the down-dip projection of Grampians Group stratigraphy instead passes into Cambrian bedrock (for example at Brimpaen; Cayley & Taylor, 1997a: and at Mount Dundas; Morand et al., 2003), and where apparent stratigraphic thicknesses within the hangingwall succession exceed the space available as implied by potential field data (for example the Big Cord Splay beneath the Victoria Range, and in the central Grampians Ranges west of Halls Gap). Splays from the Marathon Fault decollement appear to also extend up into the interior of the Grampians Group succession in places, where they locally juxtapose rock-packages of profoundly different orientations – for example the Cranage Splay west of Halls Gap.

Beneath the main Grampians Ranges, interpretations of regional magnetic and gravity data acquired since 1997 suggest that a much greater thickness of Grampians Group thickness exists than originally interpreted by Cayley & Taylor (1997a). Based on this new data, we now interpret Marathon Fault splays here to plunge to considerable depth to form a structural basin we name the Jalur Rift (see Figure 3.9). Cross sections through the Grampians Ranges have been revised in accordance with the new structural understanding of the Cambrian bedrock to show a more complex basal fault geometry than previously assumed, with consequent changes to Grampians Group thicknesses that range from around 1 km above a ‘basement high’ between rejoining splays of the Escondida Fault, to possibly more than 5 km thickness in structural deeps beneath the Mount William Range, beneath the Asses Ears region, and beneath the Mount Difficult Range. This new knowledge is captured in the STAVELY 3D model.

Elsewhere, structural contouring of the Marathon Fault and drilling indicate that most Grampians Group true thicknesses are less than 100 m, for example in the Black Range region (Cayley & Taylor, 1997c), and south of Mount Stavely in drill hole STAVELY02.

Geometrical arguments suggest that the Marathon Fault system links down-dip into structures developed within the Cambrian bedrock during D4, including the Escondida, Golton and Mehuse faults, the Moyston fault to the east (Cayley & Taylor, 1997a; 2001), the Latani, (Morand et al., 2003), Henty, Muirfoot and Cherrypool faults in the west, and into bounding faults of the Jalur Rift such as the Victoria Valley South and North faults, the Mosquito Creek Fault and the Jalur Fault beneath the Grampians Ranges.

Lateral displacements across Marathon Fault splays can be measured where stratigraphy and/or structure can be matched directly across the fault traces (e.g. scissor-fault restoration of Grampians Group strata across the curved Cranage Splay), or where relative differences in fault-slip magnitude are apparent on steep D4 strike-slip faults that cut both Grampians Group and underlying Cambrian bedrock. For the latter example, at the Griffin Fireline in the Serra Range, a difference in magnitude of several kilometres exists in the lateral offset apparent across the D4 Yarrack Fault, between the underlying Cambrian bedrock (interpreted from regional magnetic imagery of the lateral offset of magnetic Cambrian strata; see Figure 3.68) and the overlying Grampians Group (measured in outcrop as the lesser amplitude of deflection of strata in the Serra Range across the underlying fault trace position). This difference reveals differential slip on the intervening Marathon Fault plane of kilometres scale. This differential sub-horizontal D4 slip served to limit upward propagation of the steep Yarrack Fault into the Grampians Group cover during D4. This same characteristic is apparent for the Escondida Fault along much of its length within the Grampians Ranges, and also for the Muirfoot and Mosquito Creek faults.

In a new style of structural interpretation developed for the Grampians Ranges during this project, scissored Marathon Fault splay displacements confined to a sub-horizontal plane are used to reconcile the strikingly different curvatures of sub-vertically plunging oroclines in fault splay hangingwalls versus kink-rotated bedrock in fault splay footwalls (see Figure 3.73), interpreted specifically for:

1: the Cranage Splay to explain the juxtaposition of oroclinally-curved east-west striking Grampians Group mapped in the southern Wartook Syncline in the splay hangingwall against and above the north-south linearly-striking Grampians Group that crops out continuously and is mapped in detail in the western Serra Range in the splay footwall,

2: the Mafeking Splay to explain the offset of highly curved Grampians Group strata mapped within the Mafeking Orocline in the splay hangingwall, from underlying linear Cambrian strata of the Elliot and Narrapumelap belts and unconformably overlying Grampians Group strata with an intact basal unconformity in the Willaura Syncline, mapped and imaged in potential field data in the splay footwall,

3: the Big Cord Splay to explain the juxtaposition of the highly warped Grampians Group mapped in the Big Cord Orocline in the northern Victoria Range in the splay hangingwall against the directly underlying, unwarped Cambrian bedrock including the Grampians ’Deeps’ Belt that clearly visible in potential field data in the splay footwall, and

4: in the Brimpaen region to explain how a Marathon Fault splay cuts off folded Grampians Group stratigraphy in the Asses Ears Range to the south (Cayley & Taylor, 1997a; 1997b), juxtaposing it against Cambrian strata to the north, and

5: in several other less well-constrained instances, including in the Black Range, northern Mount Difficult Range, and in the southern Serra Range.

All these relationships share complementary geometries that can be related directly to dextral transtension involving soft-linked clockwise hangingwall and footwall rotations. This implies that all splays of the Marathon Fault system are linked, expressions of a single bedrock-hosted dextral transtensional, rotational fault system that developed along the Cambrian bedrock-Grampians Group interface during D4 to accommodate the different structural responses of these two rock sequences during a single deformation event.

The D4 dextral-transtensional Marathon Fault system appears to have propagated into the region from the southeast, as a footwall splay-fault, mega-kink and rift network related to localised dextral transtensional opening of a releasing bend in the reactivated Moyston Fault (see Section 4.1.6 – Analysis of D4 and D3 retrodeformation results). Deformation was bedrock-driven, and the differences in structural style between the Cambrian bedrock and the Grampians Group can be attributed to their different age and degree of lithification at the time of the D4 deformation – the Cambrian bedrock was approximately 100 million years old and responded to D4 with a brittle deformation mode.

The Grampians Group was young and poorly lithified and responded to D4 with a ductile deformation mode. The differences in the two deformation modes were accommodated across the Marathon Fault system to form the present-day complex distribution of warped Grampians Group stratigraphy. Grampians Group appears to have been laterally continuous across most of the Grampians-Stavely Zone at the time of D4, so that portions of Grampians Group strata actively rotating and translating above D4 bedrock rifts and kinks likely impinged against adjacent static Grampians Group strata, driving the Marathon Fault detachments. Much Grampians Group has eroded since the Early Devonian, so that the nature of the internal confining geometries that gave rise to the observed structures is speculative.

Despite the apparent complexity, most structures in the Grampians Group can be related back to a simple theoretical block diagram involving bedrock-driven brittle dextral transtensional faulting and dextral mega-kinking, with ductile Grampians Group deformation lagging behind deformation of the Cambrian bedrock (Figure 3.73). The Grampians Group structures that are well exposed and can be mapped in detail are an important constraint on the structural history of the directly underlying and adjacent Cambrian bedrock that are not exposed. This realisation formed a critical component of the retrodeformation sequence for D4 that can be applied to the entire Grampians-Stavely Zone (see Section 4.1 – D4 and D3 retrodeformation testing).

In some places within the main Grampians Ranges, specifically north of Wartook, beneath the northern part of the Wartook Syncline, and at Willaura, the lowermost known stratigraphy of the Grampians Group appears largely intact and no basal fault is required to explain local stratigraphic relationships. The basal Grampians Group unconformity is exposed at Willaura (Stuart-Smith & Black, 1999) and in a low road cutting south of Balmoral (in the Glenelg Zone; Morand et al., 2003). We interpret these regions to represent places where splays of the Marathon Fault system, if present, likely passed above, or below the unconformity.

3.3.4.7 Mehuse Fault

The Mehuse Fault is inferred to bound the west side of the Dryden Belt along its entire length (Cayley & Taylor, 1996, 1997a, 2000a, 2001). The Mehuse Fault is not exposed but its position and geometry can be determined accurately using potential field data. The Mehuse Fault juxtaposes highly magnetic, dense igneous rocks of the Mount Stavely Volcanic Complex to the east against non-magnetic Glenthompson Sandstone to the west. The Mehuse Fault appears to dip east, and in several locations, truncates the base of the west-dipping and west-facing Dryden Belt. It appears to have offset the D1a Dryden Fault (Cayley & Taylor, 2001) suggesting a D2-D3 reverse movement history.

Modelling of potential field data (Cayley & Taylor, 2001) and deep seismic reflection data (Korsch et al., 2002) show the Mehuse Fault to dip east. The Mehuse Fault is imaged in deep seismic lines 97AGS-V1 and 09GA-AR1 as weak discontinuous east-dipping reflections, which bound the western side of a slightly more reflective rock body correlated with the Mount Dryden Belt (Cayley et al., 2011b). These reflections can be traced down dip to ca. 3.4 s TWT (ca. 10 km), converging on the Moyston Fault and linking into this structure below ca. CDP 4500. These relationships and the close-parallelism of the Mehuse Fault to the adjacent Moyston Fault along its length, suggests that the Mehuse Fault formed as a footwall splay to the (reactivated) Moyston Fault. The degree of offset of the Dryden Belt across the Mehuse Fault is unknown, but is sufficient to have completely obscured, in potential field data, any down-dip continuation of the Dryden Belt in the Mehuse Fault footwall. This implies an overthrust magnitude of kilometres, consistent with reflective bodies visible in the interpreted footwall of the Mehuse Fault in deep seismic reflection line 97GA-V1. No Grampians Group is known to the east of the Mehuse Fault, suggesting vertical uplift of kilometre-scale across the Mehuse Fault during D2-D3, with removal of all Grampians Group from the uplifted side by erosion.

Beneath the Murray Basin, 9 km northwest of Dadswells Bridge, the Mehuse Fault, and the adjacent Dryden Belt, appears to be offset by D4 splay faults that link east to the Wildcat/South Faults, and west to the Golton Fault. North of Antwerp, the position of the Mehuse Fault and the Dryden Belt both appear to be truncated and imbricated across a series of younger sinistral D4 strike-slip faults – the largest of these are the Tullyvea and Babatchio faults (Section 3.3.5.21 – Tullyvea Fault and Section 3.3.5.3 – Babatchio Fault).

South of Wickliffe the Mehuse Fault trace appears to merge with the Golton Fault trace. Further south, the two faults may merge into, or lie parallel to the Moyston Fault in a region obscured beneath Newer Volcanic Group.

3.3.4.8 Mosquito Creek Fault

The Mosquito Creek Fault is an Early Devonian subvertical, north-trending strike-slip fault that preserves evidence of considerable (~17 km) late sinistral displacement where it cuts and offsets Grampians Group stratigraphy and structures in the Victoria Range (Taylor & Cayley, 1997; Cayley & Taylor, 1997a), juxtaposing them laterally against Cambrian bedrock. The Mosquito Creek Fault belongs to a group of strike-slip faults that were among the last major structures to deform the Grampians Group. Regional magnetic data suggests that the Mosquito Creek Fault trace passes just west of Glenisla Crossing, truncates or merges with the northern end of the Cherrypool Fault, truncates the western end of the Dollin Fault and the Brimpaen Belt and continues north where it is apparently truncated obliquely by the Escondida Fault south of McKenzie Creek (see Figure 3.64).

To the south, the Mosquito Creek Fault passes west of Mount Sturgeon, and apparently juxtaposes Grampians Group to the east laterally against Cambrian metasediments encountered in drill holes such as Jennawarra 1 (ID 310348, MGA 54 611580 5823420) and Linlithgow 1 (ID 75743, MGA 54 608284 5818497).

Although the movement history of the portion of the Mosquito Creek Fault exposed within the Victoria Range is sinistral, geometrical arguments arising from retrodeformation of the D4 fault network (see Figure 4.6) suggest that the overall movement along northern extensions to the Mosquito Creek Fault west of Brimpaen during D4 were most likely to be dextral. The change in net displacement sense along this composite structure coincides with a region of significant coeval rifting that formed the main structural basin (Jalur Rift) into which the main Grampians Ranges stratigraphy subsided during D4. Overprinting criteria based on a reconstructed fault network configuration is consistent with the notion that early dextral strike-slip displacement – with the northern end of the Mosquito Creek Fault likely continuous with the dextral Mackenzie River Fault – was overprinted by subsequent sinistral displacements associated with clockwise block-rotations of Cambrian bedrock beneath the Grampians Ranges, and with dextral movements along the Escondida Fault. The northern end of the Mosquito Creek Fault may have been continuous with the sinistral Olive Fault at this time, but these structures are now also offset dextrally across the Escondida Fault. The sinistral phases of Mosquito Fault displacement appear – as for the sinistral Latani, Olive and possibly Dollin faults – to have an antithetic relationship to the Escondida, Golton and Henty faults. Alternatively, the sinistral displacements recorded across the Mosquito Creek Fault in the southern Victoria Range may be of D5 age. We correlate D5 deformation with the Mid-Devonian Tabberabberan Orogeny, which is widely associated with sinistral transpression. The Mosquito Creek Fault appears to cut or link into the western end of the Yarrack Fault. Overprinting relationships with the Victoria Valley Batholith are unclear.

3.3.5 District-scale D3 and D4 faults

Individual D3-D4 district-scale faults are described below, arranged in alphabetical order. As for the regional-scale D3-D4 faults, district-scale D3-D4 faults are grouped because of ambiguities associated with attribution of a definitive D3 age to many faults. Because of this, the relative ages and timing constraints of individual faults are discussed on a case-by-case basis.

Several new district-scale D3-D4 faults have been identified as part of the new interpretation and development of the STAVELY 3D model and are described and named for the first time here. Numerous other D3-D4 faults of minor extent remain unnamed. The geometries, age and interpreted movement histories of unnamed faults that are included in the STAVELY 3D model are provided in Appendix 6 – Fault summary table.

3.3.5.1 Ashens Fault (new name)

The northeasterly-trending Ashens Fault is entirely concealed beneath the Murray Basin. The Ashens Fault is interpreted from regional potential field data, and from the alignment of a series of igneous intrusions of inferred Early Devonian age. The fault trace cuts obliquely across the Golton, Mehuse, Moyston and Coongee faults, and extends east into the Stawell Zone. The lack of lateral offsets of the Ashens Fault across these structures suggests that it is relatively late, although the inferred Early Devonian coincident intrusions suggest movement in the Early Devonian, i.e. during D4. The dip direction of the Ashens Fault is poorly constrained, however a concentration of Early Devonian intrusions between the Ashens and Winjallok/Plantaganet faults suggests that these fault systems may be conjugate extensional structures, with the Ashens Fault having an overall southeasterly dip. This is how the fault is presented in the STAVELY 3D model.

3.3.5.2 Angip Fault (new name)

The Angip Fault is a composite fault-system concealed beneath the Murray Basin, interpreted from potential field data. The northwestern portion of the Angip Fault is interpreted as a dextral strike-slip fault that splays from the hangingwall side of the interpreted Moyston Fault position east of Underbool, and cuts obliquely southeast across Moornambool Metamorphic Complex rocks to intersect, offset, and strike-slip imbricate the Coongee Fault just south of Angip. The amount of dextral offset across this portion of the Angip Fault is approximately 35 km. The parallelism of the Angip Fault to the east-dipping Moyston Fault position, and the oblique truncation across the west-dipping Coongee Fault suggests that this portion of the Angip Fault dips east.

Farther south, the Angip fault system may diverge, with dextral strike-slip displacement components partitioned into a reactivated Coongee Fault, and dip-slip components partitioned into an orthogonal curved fault that strikes east towards Donald and offsets magnetic stratigraphy in the interior of the Stawell Zone with an apparent dextral sense. This portion of the Angip Fault is the locus of several small intrusions interpreted from geophysical data, and probably developed as a north-dipping extensional fault. This fault appears to link farther east into extensional and strike-slip structures associated with the Culgoa Shear Zone (Cayley & Musgrave, in review), also intruded by Early Devonian granites. Overprinting criteria and the presence of small intrusions interpreted to be Early Devonian along parts of its length suggest the Angip Fault is a D4 structure.

3.3.5.3 Babatchio Fault (new name)

The Babatchio Fault is interpreted from regional magnetic data as a hangingwall splay of the Golton Fault, along which approximately 44 km of dextral displacement occurred during D4. The Babatchio Fault sits between two subparallel, northerly-trending linear magnetic belts of interpreted Dryden North Belt (the Hindmarsh and Jeparit belt segments) that appear to have formed by dextral strike-slip imbrication. Relationships to the Golton Fault imply an easterly dip for the Babatchio Fault. The overall offset of these belt segments, and of the Babatchio Fault, from the Dryden North Belt is sinistral, but this offset is attributed to the Tullyvea Fault.

The southern end of the Babatchio Fault truncates the southern end of the eastern (Jeparit) belt where it merges into the Golton Fault approximately 5 km west of Antwerp. Farther north, near the Wyperfield National Park, the northern end of the Babatchio Fault truncates the northern end of the western belt (the Hindmarsh Belt, intersected farther south in drill hole STAVELY16) to merge farther north with the Golton Fault, which continues north beyond the northern margin of STAVELY.

3.3.5.4 Barbican Fault and Cattle Camp Fault

The Barbican and Cattle Camp faults are late, north-dipping faults mapped within the Grampians Group (Cayley & Taylor, 1997a). We now recognise these structures as part of a single northeast-trending fault system that coincides with the long-axis of the Victoria Valley Batholith, which was emplaced in the core of the Jalur Rift, and, farther east in the Stawell Zone, with the White Rabbit intrusion and northwestern flank of the Stawell Granite (Cayley & Taylor, 2001). This fault may have initiated as a D4 extensional structure that, with the Victoria Valley Faults, facilitated intrusion of all these granite bodies. The last displacements along the Barbican and Cattle Camp portions of the fault appear to be reverse, but timing is unconstrained and could reflect reactivation during the Tabberabberan (D5) or even Kanimblan orogenies. The Tabberabberan Orogeny is characterised by transpressional deformation.

3.3.5.5 Cherrypool Fault

The Cherrypool Fault was originally interpreted to explain offsets in rock types encountered in mineral exploration drill holes near the Black Range, and a zone of hydrothermal (potassic) alteration aligned along the Glenelg River near Cherrypool (Cayley & Taylor, 1997a). New company aeromagnetic data has refined the position of the Cherrypool Fault (see Figure 3.39). The Cherrypool Fault is now interpreted to truncate the northern end of the Muline Fault System (Glenisla Belt) to its east and link north into the Mosquito Creek Fault, and truncate the southern end of the Mouchong Fault System (Black Range Belt) to its west. The southern end of the Cherrypool Fault is itself truncated by the D4 Henty Fault System.

Offset across the Cherrypool Fault has been reinterpreted as sinistral, part of a conjugate strike-slip fault network active in the Latest Silurian-Early Devonian (D3-D4). Overprinting criteria suggests that the Cherrypool Fault may be a D3 structure. Prior to being cut and offset by the D4 Henty Fault, the Cherrypool Fault may have been continuous with the Latani Fault, another sinistral strike-slip fault that is cut by the Henty Fault (see Figure 4.7). The Cherrypool Fault is interpreted to be subvertical in orientation. A splay fault from its western side corresponds with the course of the Glenelg River at the northern end of the Rocklands Reservoir. It appears to die out as a small, northwest-aligned en-échelon extensional basin developed within the interior of the Black Range Belt. This basin has locally downthrown a greater thickness of Grampians Group cover rocks in the vicinity of the Eclipse (McRaes) Prospect.

3.3.5.6 Curtis Fault (new name)

The east-northeast trending Curtis Fault cuts and offsets the Moyston Fault near Moyston, cuts across the Moornambool Metamorphic Complex and extends east to link up to structures that controlled the intrusion of the Early Devonian Dunneworthy Granite within the Stawell Zone. The Curtis Fault is not well exposed, but is interpreted to have controlled the intrusion of the elongate Early Devonian Curtis Diorite that extends east from the northeastern flank of the Mount Ararat Granodiorite (Cayley & Taylor, 2001), and an unnamed, buried intrusion approximately 3.5 km north of Ararat.

The dip and movement history of the Curtis Fault is poorly constrained. Assuming an association between extension and magmatism, the concentration of magmatism at Mount Cole and Mount Ararat to the south of the Curtis Fault suggests that the southern side is downthrown. In an extensional setting this implies a southerly dip to the Curtis Fault.

3.3.5.7 Dimboola Fault (new name)

The north-trending Dimboola Fault is entirely buried beneath the Murray Basin. Regional magnetic data suggests a 4 km dextral offset of Dimboola Belt stratigraphy across the Dimboola Fault just east of Dimboola. The offset of magnetic stratigraphy is coincident with a change to lower gravity to the east, interpreted to reflect a thicker succession of relatively low-density Grampians Group. The sharp change in gravity across the Dimboola Fault is interpreted to reflect a steep – subvertical – fault dip. The Dimboola Fault lies subparallel to the Mackenzie River Fault to its east, has a similar overall geometry, appears to offset Grampians Group, and is therefore interpreted to be of D4 age and origin. The Dimboola Fault appears to link south into the West Wail Fault and north into the Golton and Glenlee faults. These faults appear to have experienced dextral strike-slip and transtensional movements during D4, and are considered to be elements of a single dextral strike-slip fault system – the Dimboola Duplex. 3.3.5.8 Dollin Fault (new name)

The Dollin Fault is a concealed, east-northeast trending, steeply-dipping fault that disrupts Cambrian bedrock within the Brimpaen Belt in the vicinity of Brimpaen near the northern Grampians Ranges. It is interpreted from magnetic data to occupy a local region of low magnetic intensity (Figure 3.20). The Dollin Fault appears to be confined between the Mosquito Creek and Escondida faults, and therefore likely shares the D4 age of these structures. The geometry of the Dollin Kink just to the north of the fault trace suggests formation as a sinistral drag-fold response to sinistral displacement across the Dollin Fault. The 3 km amplitude of the Dollin Kink constrains a minimum lateral displacement of 3 km to the Dollin Fault.

3.3.5.9 Escondida East splay fault

A buried rejoining splay of the Escondida Fault is interpreted from a change in gravity a short distance to the east of the main Escondida Fault trace, where it is buried beneath the main Grampians Ranges. Lower gravity to the east is interpreted to reflect a greater thickness of Grampians Group, with the thickness change attributed to a step in basement depth across the fault-splay. The position of the Escondida East splay fault coincides with the Early Devonian Epacris Hills Granite (Cayley & Taylor, 1997a) with the implication that the granite may have intruded up along the fault plane.

3.3.5.10 Glenlee Fault (new name)

The linear, northwest trending Glenlee Fault is concealed beneath the Murray Basin. It is interpreted from regional magnetic data as a fault that cuts across the northern Dimboola Belt. The Glenlee Fault diverges from the Golton Fault footwall a short distance north of Arkona, a region of local complexity where imbricate splays – the Tullyvea and Babatchio faults – branch from the Golton Fault hangingwall. Segments of the northern Dryden Belt, and of the Golton Fault, appear to have been imbricated adjacent to the southern Glenlee Fault by approximately 11 km sinistrally. This imbrication may have been along part of the Tullyvea Fault.

Along its southern portion the Glenlee Fault appears to juxtapose Grampians Group to the south against various imbricated fault slices of Dryden Belt to the north, including rocks intersected in drill hole STAVELY16. Farther west, the Glenlee Fault passes through Glenlee, where it bounds the southern margin of a local structural deep – the Lorquon Rift – that appears to contain a thick succession of Grampians Group and so is interpreted to be of D4 age. The Lorquon Rift is also bound by the Lorquon and Golton faults. The Glenlee Fault may terminate against the Lorquon West Fault in the vicinity of Woorak West, or may continue northwest to link into the northern Escondida Fault. The dip of the Glenlee Fault is poorly constrained, but its oblique linkage with the Golton Fault system implies a similar northeast-dip, and this is how the fault is presented in the STAVELY 3D model.

The complex combination of overprinting criteria, the linear trace of the Glenlee Fault, and the cross-cutting relationships with the West Wail, Babatchio and Tullyvea faults suggest a late D4 age for the Glenlee Fault. Differing rock-juxtapositions along its length suggest a combination of Golton and West Wail fault offset influences, but with dextral and transtensional displacements predominant across it, and of larger magnitude in the south.

3.3.5.11 Grass Flat Fault (new name)

The Grass Flat Fault is a northeast-trending fault concealed beneath the Murray Basin interpreted from potential field data to bound the southern flank of the coeval Duchembegarra pluton. This tonalitic pluton is Early Devonian (drill hole VIMP1), the same as the interpreted (D4) age of the fault. The Grass Flat Fault is bound at its eastern end by the Escondida Fault and at its western end by the Henty Fault, and probably formed in response to differential movements along those bounding strike-slip structures during D4. The pluton and the Grass Flat Fault are apparently offset, with sinistral sense, across the Muirfoot Fault. The Grass Flat Fault is interpreted as a north-dipping, north-side-down, extensional fault, with a vertical throw of over a kilometre indicated by the thickness of the granite to its north. The Grass Flat Fault is probably a conjugate structure to the Winian East Fault that bounds the north side of the Cooac Rift that hosts the Duchembegarra Tonalite (see figure 3.9). The Grass Flat Fault is modelled as a subordinate structure to the Winian East Fault because this was the only relationship that allowed for down-dip structural linkage of this group of D4 structures into other major D4 structures that are mapped within STAVELY.

3.3.5.12 Hopkins River Fault (new name)

The Hopkins River Fault is a northeast-trending fault of limited, apparently dip-slip displacement that cuts obliquely across the Elliot, Narrapumelap and Stavely belts, and links south into the Nareeb Fault near Chatsworth. Despite limited displacement the trace of the Hopkins River Fault is obvious as it coincides with a pronounced step in topographic relief, with a few tens of metres uplift along its western-side. In the current compressional crustal stress regime, this uplift polarity suggests a westerly dip for the Hopkins River Fault. Uplifted Brighton Group strand-line deposits to the west of the Hopkins River Fault have been eroded to expose underlying Cambrian bedrock. Rocks of the Newer Volcanic Group are largely confined to the east of the fault. Near Chatsworth House, a small remnant lava-flow is confined within an abandoned palaeo-tributary to the ancestral Hopkins River that was cut into the western river bank (MGA 54 641308 5810367). These relationships suggest that topographic relief along the fault scarp was present and eroding at the time of eruption of this part of the Newer Volcanic Group.

The Hopkins River Fault appears to be compartmentalised into offset segments that are separated across the Escondida Fault and across splays of the Yarrack Fault. Offsets here are interpreted as a consequence of compartmentalisation between pre-existing fault-splays, not as a consequence of later movements along the Yarrack and Escondida faults. Cross-cutting relationships with the Nareeb and Escondida faults suggest that the Hopkins River Fault is likely to have developed relatively late in D4, in the Early Devonian, after movements on the Escondida Fault had ceased and the Mafeking Megakink had fully formed. The northeast-trend of the Hopkins River Fault suggests that it possibly originated as either an extensional fault of limited displacement (if late in D4) or as a compressional fault of limited displacement (if formed during D5). Farther northeast the Hopkins River Fault position is hidden beneath Newer Volcanic Group, but it may link to east-trending D4 transtensional faults interpreted to be associated with intrusion of Early Devonian granites at Dunneworthy and Mount Cole.

3.3.5.13 Latani Fault

The Latani Fault is a northerly-trending subvertical fault recognised south of deep seismic reflection line 09GA-SD1 in the vicinity of Balmoral (Morand et al., 2003). The Latani Fault has deformed the Grampians Group, so is Late Silurian in age or younger. In the south, it forms the western limit to outcrop of the Grampians Group, implying a limited amount of relative west-side uplift with erosion of Grampian Group on the upthrown side. Farther north, near Kanagulk, where it cuts across outcrops of Grampians Group, subhorizontally plunging lineations on subsidiary fault surfaces indicate strike-slip displacement along it. Oblique folds and lineations imply considerable sinistral strike-slip, and lesser west-side up, displacements. South of Balmoral, in the vicinity of Yarramyljup Creek, the Latani Fault converges with, and cuts the Yarramyljup Fault, and possibly links into the Henty West Fault, a relationship obscured by south-dipping normal faults. The Latani Fault truncates the western end of Grampians Group at Mount Dundas and Log Hut Creek, passing south beneath younger rocks. North of Balmoral, the Latani Fault appears to merge with, and be truncated by, the Henty Fault, implying a D3 or early D4 age (see Figure 4.5). With D4 dextral offset of the Henty Fault taken into account, the Latani Fault appears to have been continuous with the Cherrypool Fault pre-late D4.

3.3.5.14 Log Hut Fault

The Log Hut Fault is a subvertical northwest-trending structure that juxtaposes subvertical Grampians Group (see Figure 3.4C) against high-grade metamorphic rocks of the Glenelg Zone in Log Hut Creek, a short distance west of the Yarramyljup Fault (Simpson & Woodfull, 1994; Morand et al., 2003). Subhorizontal intersections between various sets of cataclasite bands developed in Grampians Group sandstone adjacent to the Log Hut Fault suggest a predominant dip-slip displacement. The extent of the Log Hut Fault is not clear – to its west, it appears to be truncated by the Latani Fault. Farther east it may be offset across the Wannon Fault and link farther east into the Yarramyljup Fault, in a region entirely concealed beneath Rocklands Volcanic Group.

3.3.5.15 Lorquon Fault (new name)

The curved, northeast-trending Lorquon Fault is entirely concealed beneath the Murray Basin. The Lorquon Fault is interpreted from potential field data to sharply bound the northwestern margin of a triangular structural low also bound by the Glenlee and Golton faults – this is the Lorquon Rift. The gravity low of the Lorquon Rift is attributed to several kilometres of interpreted Grampians Group fill and a D4 age. The Lorquon Fault is truncated on its western side by the Glenlee Fault near Woorak West, and on its eastern side by the Golton Fault near Lake Hindmarsh. The geophysical data indicates a moderate to steep southeasterly dip for the Lorquon Fault.

The northwestern flank of the Lorquon Fault is characterised by high-frequency TMI consistent with near-surface magnetic strata of the Dimboola Belt and an apparent absence of overlying Grampians Group. The east-side-down displacement indicated for the Lorquon Fault suggests an extensional origin. The fault geometry and relationship to Grampians Group is consistent with the Lorquon Fault developing as an extensional fault conjugate to dextral movements on the bounding Glenlee and Babatchio faults during D4.

3.3.5.16 Lorquon West Fault (new name)

The Lorquon West Fault is a buried, northerly trending, steeply-dipping fault interpreted to splay from the hangingwall side of the Escondida Fault beneath the Murray Basin northeast of Nhill. It links north into the Golton Fault near the Wyperfield National Park boundary, approximately 13 km northwest of Perenna. The Lorquon West Fault appears to have caused relatively minor (approximately 2-3 km) sinistral strike-slip offsets of curved magnetic stratigraphy within the northern Dimboola Belt, and locally forms the eastern margin to an overlying sub-rift interpreted to be filled with at least a few hundred metres of Grampians Group fill east of Netherby. The association of the Lorquon West Fault with the Escondida and Golton faults, and with a sub-rift containing Grampians Group indicates movement during D4. It can be considered to have developed antithetic to dextral displacements on the Escondida and Golton faults.

3.3.5.17 Mackenzie River Fault

At Horsham, the northerly-trending, subvertical Mackenzie River Fault separates Grampians Group to the east from Cambrian volcanic rocks of the Dimboola Belt to the west. Grampians Group to the east of the fault position is exposed in a quarry at Mckenzie Creek, and in several old excavations adjacent to the Henty Highway near the intersection of A Smiths Road. Cambrian bedrock to the west of the fault position is intersected in a number of drill holes including drill holes STAVELY09, STAVELY10 and STAVELY12; see Figure 3.29). The Mackenzie River Fault truncates the Grampians Group, consequently the movement must be D3-D4 and of post- Mid-Silurian age.

Regional magnetic data and deep seismic reflection imagery (Korsch et al., 2002) suggest that Grampians Group, several kilometres thick, occurs just to the east of this steep structure, implying considerable west-side-up movement. However, since all the stratigraphy, and the basal Grampians Group appears to dip east across this area, the vertical offset magnitude could be apparent. Dextral strike-slip displacement of 18 km across the Mackenzie River Fault superimposed onto an already east-dipping sequence can equally account for the geometry and apparent vertical offsets observed, can simultaneously explain the large lateral offset of the buried eastern margin of the Dimboola Belt across it as imaged in regional potential field data, and is a better fit for the predominantly strike-slip displacements of other north-trending faults observed in this region.

Regional magnetic data makes it clear that the southern end of the Mackenzie River Fault, and any connectivity with fault extensions farther south along-strike, is abruptly truncated and cross-cut by a narrow, northwest-trending fault slice of foliated and fractured ultramafic-mafic rocks that occupy the hangingwall of the Escondida Fault (intersected in drill hole STAVELY10; see Figure 3.64). Since this movement appears to be of D4 age, the Mackenzie River Fault is interpreted as D4, part of the Dimboola Duplex.

3.3.5.18 Muirfoot Fault

The Muirfoot Fault (Spencer-Jones, 1965; Cayley & Taylor, 1997a, 1997c) is a sinistral, subvertical strike-slip fault which cuts across Grampians Group in the western Black Range. The Muirfoot Fault extends down into the underlying Cambrian bedrock, and can be traced north using magnetic data beyond the limits of Grampians Group outcrop in the Black Range, where it is seen to separate and sinistrally-displace the Black Range and Black Range West / Mitre belts by a minimum of 7 km (see Figure 3.16). The amplitude of the adjacent Connangorach Fold contributes a couple of kilometres to this offset estimate.

Farther north still, the Muirfoot Fault trace appears to pass just east of Mount Arapiles. Steeply east-dipping splays from the Muirfoot Fault may control the major subvertical northwest-trending cliff-lines developed in Grampians Group quartz arenite at Mount Arapiles (Figure 3.66).

Where the Muirfoot Fault trace is crossed by deep seismic reflection line 09GA-SD1, the fault truncates a set of steeply west-dipping reflectors that appear to correspond to Stavely Arc igneous rocks within the Black Range Belt. Although the Muirfoot Fault here appears to dip west near-surface, we interpret the fault to dip east overall, and to link into other faults of the D3-D4 system in the mid-crust.

At its southern end the Muirfoot Fault is cut and offset by the east-west trending D4 Henty Fault. The Muirfoot Fault is therefore likely to be of D3 age, part of a family of sinistral fault segments that include the Latani and Cherrypool faults.

3.3.5.19 Nareeb Fault (new name)

The Nareeb Fault is a subvertical dextral strike-slip fault that cuts obliquely across the southern Grampians-Stavely Zone. It is especially obvious in regional magnetic data where it offsets three dykes of the Chatsworth Basalt northwest of Chatsworth, with a dextral offset of approximately 1.6 km. Magnetic data also implies small dextral offsets of the Bunnugal and Boonawah belts across the interpreted position of the Nareeb Fault where buried beneath Newer Volcanic Group cover. Gravity data shows the Nareeb Fault to dextrally offset the Stavely Belt by a similar magnitude south of drill hole STAVELY02. At MGA 54 645434 5808375, west-trending crush zones and vein arrays mapped in outcrop are developed subparallel to the trend of the Nareeb Fault. The Nareeb Fault is considered to be part of a D4 strike-slip fault system that includes the Yarrack Fault. The Narreeb Fault appears to link west into the Mosquito Creek Fault near the southern end of the Serra Range, and trends southeast towards the inferred position of the Moyston Fault beneath Otway Basin cover. It appears to be subvertical at surface, but is interpreted to shallow in dip magnitude to dip northeast and link into the Yarrack Fault at depth.

The apparently northwest-dipping Hopkins River Fault also cuts across dykes of the Chatsworth Basalt and appears to terminate at a high angle against the Nareeb Fault. The opposing dip of these two faults suggests that they may be conjugates.

3.3.5.20 Olive Fault

The north-trending Olive Fault bounds the northwestern margin of the Mount Difficult Range in the northern Grampians Ranges, and separates Grampians Group exposure at Mount Zero from the main Grampians Ranges (Cayley & Taylor, 1997a, 1997b). This indicates a post-Silurian, possibly D3-D4 age for the Olive Fault. The Olive Fault is not exposed, but Grampians Group adjacent to the fault plane on Smith Road is highly fractured with numerous cataclastic gouge veins. The asymmetry of plunging en-échelon folds truncated along the eastern side of the Olive Fault is consistent with formation in response to sinistral shear. The straightness of the Olive Fault trace implies a steep, possibly subvertical dip.

The Olive Fault strikes north towards the buried Golton and Mehuse fault positions, and is interpreted to be truncated against these D3-D4 structures. The southern end of the Olive Fault is intruded by the Mackenzie River Granite, proving a pre-400 Ma age constraint and obscuring links with other faults. The Olive Fault is interpreted to have been truncated by the Escondida Fault during its D4 reactivation prior to intrusion of the Mackenzie River Granite. Prior to D4 dextral offset across the Escondida Fault, the Olive Fault may have been continuous with the northern Mosquito Creek Fault, which is also subvertical, also preserves evidence of sinistral fault displacements, and is of similar relative age.

Figure 3.66 Mount Arapiles, Muirfoot Fault splay plane A) Looking southwest from the Natimuk-Frances Road at the northeast-facing >100 m high ‘Watchtower Face’ (MGA 54 574440 5932540). Summit fire-tower for scale. This part of the cliff lies directly along the alignment of the D3-D4 Muirfoot Fault, a subvertical sinistral strike-slip structure that displaces Grampians Group in the Black Ranges 46 km to the southeast (Cayley & Taylor, 1997c) and offsets the Black Range Belt from the Black Range West / Mitre Belt, visible in regional magnetic and deep seismic reflection data. The Watchtower Face is remarkably planar, smooth and polished, with open, subhorizontally-pitching undulations at metre to tens of metres scale. The Watchtower Faces are interpreted as a fault-plane related to the Muirfoot Fault. The polished surface is interpreted as a slickenside, the undulations representing slickenlines with sub-horizontal pitches defining strike-slip displacement on the fault, in accord with what is already known for the Muirfoot Fault in the Black Range. B) Oblique side-on view (location in A), looking upwards and southwards at ‘The Watchtower’ and at the large-amplitude fault-plane undulations that form the ‘North Watchtower Face’. ‘The Watchtower’ at left is one erosional remnant of the Muirfoot Fault splay hangingwall that stands slightly proud of the rest of the cliff. Subsidiary rejoining fault splays extend out from the main fault on both the hangingwall (white arrows) and footwall sides, to enclose large, lens-shaped blocks of fault-breccia. The main fault plane (marked by black arrows) dips approximately 75° towards the northeast on average, according with overall Muirfoot Fault dip in the upper crust where imaged in deep seismic reflection data further south along strike in deep seismic reflection line 09GA-SD1. Many other subparallel faults that cut Mount Arapiles (and the adjacent ‘Mitre Peak’) are probably splays of the Muirfoot Fault.

3.3.5.21 Tullyvea Fault (new name)

The Tullyvea Fault is a concealed, northerly trending fault interpreted from magnetic data. It branches from the hangingwall side of the Golton and Babatchio faults near Antwerp and links northeast into the Moyston Fault near Jeparit, and into a complex, poorly understood region with magnetic character gradational between the Grampians-Stavely Zone (linear belts of magnetic rocks with similarity to the Dryden Belt west of Rainbow) and the western Stawell Zone (Moornambool Metamorphic Complex – complex disrupted magnetic character east of Rainbow, reminiscent of the disrupted accretionary complex rocks exposed west of Ararat).

Displacement across the Tullyvea Fault is currently interpreted to be dextral overall, but of relatively minor magnitude. The southern end of the Tullyevea Fault appears to sinistrally offset the Hindmarsh and Jeparit belt segments, and the Golton and Babatchio faults, from the northern end of contiguous Dryden North Belt by about 11 km. These movements are attributed to D3 and/or D4

Larger scale strike-slip duplication of segments of the Dryden Belt across the Tullyvea Fault than was used for the D4 retrodeformation discussed in Section 4.1 – D4 and D3 retrodeformation testing is possible, and could account for the additional width of magnetic stratigraphy that exists between Rainbow and Underbool/Boinka farther north. In this instance, the Tullyvea Fault and related faults may have a large dextral strike-slip component. In either interpretation, the Tullyvea Fault appears to cut across the Moyston Fault and segments of the Dryden Belt and is therefore attributed to D4. Future work that constrains the affinity of rocks in this region may change interpretation of the movement history of the Tullyvea Fault.

Figure 3.67 Oblique view of the Bunnugal Belt volume (green), showing its northern termination across the north-dipping D4 Victoria Valley South Fault, and the conjugate relationship between the Victoria Valley South Fault and the subsidiary southeast-dipping Victoria Valley North Fault. The Victoria Valley South Fault also bounds the southern end of the rotated Grampians ‘Deeps’ Belt. The Victoria Valley North Fault cuts and offsets the interior of the Grampians ‘Deeps’ Belt.

3.3.5.22 Victoria Valley South Fault, Victoria Valley North Fault (new name)

The Victoria Valley South Fault and Victoria Valley North Fault are an inferred, northeast-trending extensional fault system buried beneath the Grampians Ranges. The trace and northwest dip for the Victoria Valley faults are interpreted from potential field data. The Victoria Valley South Fault trace lies parallel to the southern margin of the Victoria Valley Batholith, and the Victoria Valley North Fault fault trace lies parallel to the northern margin. The Victoria Valley faults appear to be link-structures between the bounding Mosquito Creek (west) and Escondida faults against which they terminate at high strike-angles.

The Victoria Valley faults both appear to be components of a family of D4 faults associated with formation of the D4 Jalur Rift, a D4 structural basin that developed in the Early Devonian and into which previously deformed parts of the Grampians Group allochthon collapsed along basal detachment faults grouped collectively as the Marathon Fault (see Figure 3.9 and Section 3.3.4.6 – Marathon Fault). Jalur Rift formation appears to have been superimposed over previously continuous D1a Cambrian volcanic belts, which were segmented, separated and clockwise block-rotated during D4 (see Figure 4.4). The coincidence of the Victoria Valley fault positions with the southern and northern flanks of the Victoria Valley Batholith suggests that they both also partly controlled granite emplacement.

The Victoria Valley South Fault appears to be a key structure that bounds the southern margin of the Jalur Rift. It lies parallel to the Marathon Fault splay that bounds the eastern flank of the Serra Range, and dips moderately northwest towards the position of the Victoria Valley North Fault trace (see Figure 2.16). Both faults are likely to be extensional. Potential field data show that the Victoria Valley South Fault truncates the southern margin of the buried Grampians ‘Deeps’ Belt where it is partly intruded by the Victoria Valley Batholith. The Victoria Valley South Fault also truncates the northern end of the Bunnugal Belt where it is buried beneath the western flank of the Serra Range. These truncations suggest that considerable displacement occurred across this fault during D4 (Figure 3.67).

The overall westerly dip of the Bunnugal Belt south of the Victoria Valley South Fault trace contrasts with the overall northeasterly-dip that is apparent for the Grampians ‘Deeps’ Belt to the north of the fault trace. We interpret this difference to reflect clockwise rotation of the Grampians ‘Deeps’ Belt in the core of the Jalur Rift during D4. The implication is that the Victoria South Valley Fault initiated as an asymmetric rift structure that segmented and separated the Bunnugal and Elliot belts early in D4 (see Section 3.4.4 – Mafeking Megakink), and evolved into a transtensional fault against which para-allochthonous crustal elements in the core of the Jalur Rift – including the rotated Grampians ‘Deeps’ Belt – were emplaced late in D4. The overall setting of this fault is consistent with formation in response to dextral transtension compartmentalised within the Jalur Rift, bounded between the Mosquito Creek, Escondida and Golton faults.

Potential field data show that the Victoria Valley North Fault partly underlies the northern Victoria Range, and cuts across the middle of the Grampians ‘Deeps’ Belt near Big Cord, sinistrally offsetting – by approximately 4 km – the portion of that belt that underlies the Victoria Range from the portion of that belt that appears partly intruded by the Victoria Valley Batholith. Unlike the Victoria Valley South Fault, no relative rotations or fundamental rift-disruptions of Cambrian bedrock are apparent across the fault trace. The dip of the Victoria Valley North Fault is poorly constrained, but the alignment of the Victoria Valley Batholith along the southern flank of the fault implicates this structure as a control on batholith emplacement, so that the fault may dip southeast, towards the rift centre and the intrusion, and towards the position of the Victoria Valley South Fault.

Possible relationships between the northwest-dipping Victoria Valley South Fault and southeast-dipping Victoria Valley North Fault were investigated during construction of the STAVELY 3D model. The initial working assumption was that the Victoria Valley North Fault may be the primary structure, with the Victoria South Valley Fault representing a secondary hangingwall splay. This proved impossible to sustain geometrically – the largest breaks in Cambrian stratigraphy occur across the position of the Victoria Valley South Fault, which demand this fault to have the greater depth-extent. Therefore, the Victoria Valley South Fault appears to be the primary structure since this fault can be modelled to link down-dip into both the Escondida Fault and Mosquito Creek Fault, with the Victoria Valley North Fault representing a subsidiary splay structure consistent with the more limited offsets of Cambrian bedrock and structure observed across it (see Figure 3.67 and Appendix 2 – Stavely Serial Section 4). Both Victoria Valley Fault traces lie parallel to the north-dipping Barbican Fault and north-dipping Cattle Camp Fault, which lie directly along-strike from one another and are also aligned along the long-axis of the Victoria Valley Batholith. These faults are probably all related; the minor thrust-displacements on the Barbican and Cattle Camp Faults may be due to later reactivation (D5).

3.3.5.23 Wannon Fault (new name)

The concealed northeast-trending Wannon Fault is interpreted to extend from Wannon, west of Hamilton, northeast to Woohlpooer. The Wannon Fault is interpreted to truncate Grampians Group in the eastern end of the Dundas Range near Cavendish and cut into folded Grampians Group in the Victoria Range near Woohlpooer. It is therefore regarded to be of D4 age. Displacement on the Wannon Fault appears significant, because its trace position coincides with an abrupt change in the apparent strike of the D1a Yarramyljup Fault, which locally trends southeast on the north side of the Wannon Fault, and south on the south side of the Wannon Fault. This reorientation is of the exact same style and magnitude as seen for the abrupt strike change that separates the northwest-trending Tyar Belt from the northerly trending Grampians ‘West’ Belt across the position of trace of the Wannon Fault at its northern end. Dextral offset accompanying a localised anticlockwise displacement across the Wannon Fault plane is implicated as a common origin for both these reorientations.

At its southwestern end, the Wannon Fault appears to link into, or be truncated by, the Latani Fault, which lies a considerable distance into the Glenelg Zone interior. Its northwestern end appears to be truncated against the Mosquito Creek Fault. No offset continuations to this fault are known. Instead, it appears to be confined between these flanking D3-D4 structures. The dip of the Wannon Fault is unknown, but is interpreted to be towards the southeast based on an interpreted transtensional origin coupled with the fact that a greater thickness of Grampians Group seems to be preserved to the south of the fault, as indicated by historical drill hole intersections. A southerly dip also allows for linkage at depth into the Latani and Mosquito Creek faults both of which also appear to dip east at depth.

While both drilling and gravity data indicate that Grampians Group is thicker to the southeast of the Wannon Fault, Rocklands Volcanic Group appears to be evenly distributed on either side. This suggests that dip-slip displacement across the Wannon Fault is equal in age to – or slightly younger than – the Grampians Group, but is older than the Rocklands Volcanic Group. The Wannon Fault trace appears coincident with a series of gravity lows, which are interpreted to be associated with underlying intrusions within a possible caldera complex of inferred Early Devonian age. These relationships suggest that the Wannon Fault may have developed in the Early Devonian (D3-D4) as a southeast-dipping normal and/or transtensional fault, creating a structural rift – the Rocklands Rift (see Figure 3.9) into and along which Early Devonian magmatic complexes were subsequently emplaced.

3.3.5.24 West Wail Fault (new name)

The West Wail Fault is entirely buried beneath the Murray Basin. The West Wail Fault position has been interpreted from trends in regional magnetic data that correspond with different sequences within the Dimboola Belt. The presence of a fault separating different Cambrian rock-types within the Dimboola Belt is supported by recent drill intersections of sheared ultramafic rocks in STAVELY10 and mafic rocks in STAVELY09 on either side of the interpreted fault position.

In the south near Horsham, the West Wail Fault appears to represent a hangingwall splay of the Escondida Fault, as it lies subparallel to that structure between Horsham and the Dimboola Fault. The West Wail Fault appears to truncate the southern end of the Mackenzie River and Dimboola faults, and is therefore an Early Devonian D4 structure. North of the Dimboola Fault, the West Wail Fault appears to diverge east from the Escondida Fault, cutting across the Dimboola Belt interior with a similar dextral offset-character to the adjacent Dimboola Fault, until truncated at its northern end by the Glenlee Fault a short distance northwest of Arkona. The similarities in overall structural style and geometry to bounding D4 faults suggest that the West Wail Fault is part of the D4 dextral imbricate fault system – the Dimboola Duplex. As for other faults within this complex, the West Wail Fault is interpreted to dip northeast becoming east dipping towards its northern end.

3.3.5.25 Winian East Fault (new name)

The Winian East Fault is a concealed, northeast-trending fault interpreted from potential field data that bounds the northern flank of the Duchembegarra pluton. The Winian East Fault is cut at its eastern end by the Escondida Fault and at its western end by the Henty Fault. These faults all appear to be of similar age, suggesting that the Winian East Fault formed in response to compartmentalised extension in the rock-panel between these large, bounding D4 strike-slip faults. The Duchembearra Tonalite is intersected in drill hole VIMP1 is dated at 404 Ma (Maher et al., 1997), and this is taken as the age of the whole batholith. The Winian East Fault is one of a number of subparallel structures that collectively define the northern rift flank of the Duchembegarra batholith – other related structures are buried beneath the batholith and only apparent as offsets in gravity interpreted to reflect different pluton thicknesses and/or compositions juxtaposed across the faults. The Winian East Fault is interpreted as a south-side-down, steep south-dipping extensional fault. The Winian East Fault probably formed coeval with the Grass Flat Fault. The Winian East Fault appears to be the master structure that links south into the Henty Fault and Escondida Fault systems at depth. The Grass Flat Fault appears to be a subordinate, conjugate splay of opposite dip.

The coincidence of the Winian East and Grass Flat Fault positions with the southern and northern flanks of the Duchembegarra batholith, suggests that these faults controlled batholith emplacement during D4 and are probably intruded by the batholith. In all respects including orientation the Winian East and Grass Flat faults are comparable to the Victoria Valley faults. They are considered to be separated parts of a single, regional scale transtensional fault and magmatic system.

3.3.5.26 Woorndoo Fault

The Woorndoo Fault was originally inferred to be parallel to Salt Creek on the basis of tilted and overturned Grampians Group strata to its west (Spencer-Jones, 1965). We now interpret the Woorndoo Fault to separate Mount Stavely Volcanic Complex exposed south of Lake Bolac (Buckland, 1981; Ramsay, 1981) from Grampians Group that crops out farther west.

The greater thickness of Grampians Group to the west of the Woordoo Fault suggests that the fault may have developed as a west-side-down extensional fault in the Siluro-Devonian (D3-D4). Recent and ongoing west-side-up displacement across the inferred fault position has formed a gently west-dipping tilt-block of Grampians Group and Brighton Group, bound between the Woorndoo and Hopkins River faults. In the present compressional stress-field (see Sandiford et al., 2004), this displacement implies reverse reactivations and a westerly dip for the Woorndoo Fault. Thus, the Woorndoo Fault does not appear to be a direct correlate of the east-dipping Golton Fault that bounds Grampians Group further north.

The Woorndoo Fault may have originated in the Siluro-Devonian as an extensional structure akin to splays of the Marathon Fault, coeval with, and subparallel to, the Hopkins River Fault. Minor reverse faults, dipping approximately 60° southeast in overturned sandstone beds of the Grampians Group exposed on the southern shore of Lake Alexander (Stuart-Smith & Black, 1999) may have developed antithetic to movements along the Woorndoo Fault, or may be a consequence of later (D5) reverse reactivations of it.

3.3.5.27 Yarrack Fault (new name)

The Yarrack Fault is a subvertical, northwest-trending dextral strike slip fault that is exposed in creeks south of Glenthompson as a series of anastomosing shear zones containing dextral shear fabrics, northeast-dipping foliations, and west-dipping tension veins (e.g. at MGA 54 642174 5828402, where the fault cuts through Glenthompson Sandstone). Farther southeast, a splay of the Yarrack Fault cuts across and truncates the northern margin of the Buckeran Diorite, with parts of this intrusion offset right-laterally (Figure 2.15). Fault-planes developed in the Buckeran Diorite (exposed at MGA 54 645235 5824153) show moderately northwest-plunging slickenside striations on west-dipping fault planes with dextral offsets of magmatic features (e.g. aplite veinlets), and conjugate steep northeast-striking tension fractures, consistent with dextral transtensional oblique-slip fault displacements.

The magnitude of displacement across the Yarrack Fault can be estimated from the amplitude of the Yarrack Orocline, where it has reoriented deformed Glenthompson Sandstone south of Glenthompson (Section 3.4.7 – Yarrack Orocline), and from the size of lateral offsets in the Bunnugal and Stavely belts visible in regional magnetic data. Where the Yarrack Fault cuts the Bunnugal Belt north of Strathmore, magnetic trends in Cambrian volcanic stratigraphy have been laterally offset in a dextral sense by approximately 6 km (Figure 3.68). Nearby, the Yarrack Fault is expressed in overlying Grampians Group of the southern Serra Range as a dextral drag-fold (± fault?) with dextral lateral offset of approximately 1 km at Griffon Gap. Displacement on the Yarrack Fault apparently diminishes westwards so that where the fault cuts across the Victoria Valley at Chimney Pot Gap the dextral lateral offset is approximately 400 m (measured as offset of the Geerak Syncline and Mosquito Creek Fault).

Farther east, the Yarrack Fault breaks into a horsetail of splay faults which cut across, and offset the southern extent of the Stavely Belt (Figure 3.69). Offsets visible in regional magnetic data here are predominantly dextral, with splays linking farther east into the Escondida and Moyston faults, and south into the Nareeb Fault. Minor antithetic sinistral structures are also evident in regional magnetic data, and together with offsets on the Yarrack Fault splays, have led to a broadening of the Stavely Belt to approximately 5 km in this area.

The Yuppeckiar Diorite (Cayley et al., in prep) north of Glenthompson is interpreted to be truncated along its western side by a splay from the Yarrack Fault. The irregular shape of the Yuppeckiar Diorite is coincident with the position of F4 fold axes mapped in adjacent poly-deformed Glenthompson Sandstone adjacent to the Yarrack Orocline, and so is interpreted as a consequence of D4 folding of this intrusion (Figure 3.68). The Yarrack Fault cannot be traced west of the Mosquito Creek Fault, and therefore seems to link into it. At its eastern end, the Yarrack Fault splays appear to extend as far as the Moyston Fault. Although always steep to subvertical where mapped at surface, the Yarrack Fault splays are most likely to shallow to northeasterly dips, facilitating linkage into the enclosing Mosquito Creek and Moyston faults. The expression of the Yarrack Fault within the Grampians Group constrains its age to post-Late Silurian (D4). The differences in magnitude of lateral displacement across this fault in the Cambrian bedrock and in the overlying Grampians Group can be attributed to differential movements accommodated across intervening coeval Marathon Fault detachments (see Section 3.3.4.6 – Marathon Fault). The Victoria Valley Batholith shows no offset from the Yarrack Fault, indicating that it’s intrusion in the Early Devonian post-dated displacement across the Yarrack Fault.

3.3.6 Post Devonian faults

The only post-Devonian faults included in the STAVELY 3D model are of Late Mesozoic age, related to Otway Basin rifting that occurred during Australia – Antarctica separation. These faults disrupt Mesozoic-Cenozoic stratigraphy of the Otway Basin, and serve to bound the northern margin of the Otway Basin. The Glenthompson and Tarrington faults are the largest, and are named and described separately. A series of smaller unnamed faults (C2, C3, 32, 33, 76, 80) are provided in Appendix 6 – Fault summary table. The relationships between the post-Devonian faults and key older structures that were probably reactivated in the Late Mesozoic are depicted in Figure 3.57.

3.3.6.1 Glenthompson Fault

The east-west trending Glenthompson Fault passes through Strathmore, Glenthompson and north of Mount Stavely, with sub-parallel splays extending east from Stavely and west towards Dunkeld and possibly beyond, where it is buried beneath Newer Volcanic Group. In the vicinity of Glenthompson, the Glenthompson Fault has experienced Recent and ongoing south-side uplift, expressed in the landscape. This has resulted in pooling of Quaternary sheet-flood alluvium to the north of the fault trace, and uplift and erosion of Neogene Brighton Group sediments to expose Cambrian bedrock to the south. In the current compressional in-situ crustal stress regime (Sandiford et al. 2004), this uplift polarity is consistent with a southerly dip for the Glenthompson Fault. Therefore, the Glenthompson Fault dips towards the Otway Basin with a geometry that is consistent with formation as an extensional fault related to the northern margin of the Otway Basin. The current reverse displacement across the Glenthompson Fault is interpreted as minor fault inversion. A short splay of the Glenthompson Fault 4 km to the north cuts across the Yuppeckiar Diorite, and has experienced north-side uplift. This suggests a northerly dip to the splay, conjugate to the main Glenthompson Fault, but the behaviour of this minor fault at depth is not known.

3.3.6.2 Tarrington Fault

The Tarrington Fault is a concealed, south-side-down Otway Basin-bounding fault that runs west-northwest through Hamilton. The position of the fault is constrained by drill holes that encounter pre-Devonian bedrock (Rocklands Volcanic Group, Grampians Group and Cambrian rocks) at shallow depths in Hamilton and further north, from drill holes that penetrate several hundreds of metres of Otway Basin stratigraphy to the south of Hamilton and Tarrington (e.g. petroleum exploration wells Moyne Falls 1 and Hawkesdale 1 that intersect Cambrian bedrock at 939 m and 1740 m depth respectively. The Tarrington Fault is interpreted to dip south.

The position of the Tarrington Fault beneath Newer Volcanic Group cover is defined in gravity data by a sharp boundary to a prominent gravity low south of the fault trace, and defined in regional magnetic data by a sharp boundary to a prominent magnetic high to the south of the fault trace. The magnetic high is shown by drilling to represent extensive Older Volcanics lava flows incorporated within the Otway Basin succession. The Tarrington Fault represents a significant boundary for mineral exploration, since south of the fault lie considerable additional thicknesses of Otway Basin stratigraphy and Newer Volcanic Group which conceal prospectivity Cambrian stratigraphy of the Stavely Arc.

The Tarrington Fault appears to lose displacement towards the east/southeast, with Cambrian bedrock rising to shallow depths to the south of the fault trace near Caramut. Here, displacement on the Tarrington Fault appears to have stepped south into equivalent south-dipping extensional structures that continue further east (e.g. Fault number 32 in Figure 3.57). This stepping of sub-basin subsidence along the northern Otway Basin margin appears to coincide with the interpreted position of the buried Boonawah Belt. This implicates local reactivation of segments of the D1a Boonawah Belt bounding faults as Otway Basin transfer structures that served separate different sub-basins developed along the northern Otway Basin margin. Northeast-striking D4 faults may also have reactivated during formation of the Otway Basin to help define the sub-basins.

3.4 Modelled folds

While the overall extent and size of D1a folds in STAVELY remains quite poorly known, panels of consistently dipping and facing strata mapped both west and east of Stavely and intersected in mineral exploration drill holes in the Black Range suggest that the influence of regional-scale D1a folds is quite minor. Instead, D1a deformation and uptilting of the Cambrian bedrock appears to have been fault-dominated. No D1a folds are represented in the STAVELY 3D model. The STAVELY 3D model is too coarse in scale to allow for internal subdivision of Glenthompson Sandstone, Mount Stavely Volcanic Complex, or Nargoon Group strata or internal structure.

D1a folds tend to trend northwest subparallel to the main D1a (thrust) faults. Mapped D1a fold closures are typically of chevron style, with subhorizontal fold plunges consistent with formation in response to crustal shortening (Figure 3.70 ). D1a folds are typically associated with a stylolitic to slaty S1a cleavage that lies axial plane to fold hinges (Figure 3.71). D1a folds have been mapped in Glenthompson Sandstone, Nargoon Group and in the Mount Stavely Volcanic Complex in the southeast and west of STAVELY. Reversals of facing direction in Cambrian strata intersected in mineral exploration drill holes elsewhere in STAVELY may be due to the presence of D1a fold closures. The consistently low dip of bedding in Glenthompson Sandstone intersected in drill hole STAVELY11 is unusual in the context of the typically steep dip of bedding in this formation elsewhere in STAVELY, and so may represent the crest of a D1a fold hinge.

The subhorizontal plunges typical of most D1a folds means that their limbs share similar strike trends. This characteristic makes it difficult to recognise fold closures in potential field data, even where they may be of large enough scale (amplitude and/or wavelength) to be captured in a regional scale 3D model. No D1a folds were recognised in deep seismic reflection data, probably because the fold closures are too small or too tight. Figure 3.68 Tilt and band pass filtered regional magnetic image of the D4 Yarrack Fault, with the Bunnugal Belt highlighted to show the nature of its disruption and dextral strike-slip offset across the Yarrack Fault. Mapped bedding trends in Glenthompson Sandstone and Grampians Group (west of the Marathon Fault) are depicted in orange. The Yarrack Fault cuts across D1a structures, D1b granites and D3 structures in the Grampians Group, but is intruded by Early Devonian granites. This constrains a D4 age for the Yarrack Fault and related drag fold structures. East-west bedding trends in Glenthompson Sandstone south of Glenthompson define the disrupted northern limb of the Yarrack Orocline, a kilometres-scale clockwise drag fold superimposed on steeply-dipping D1a structures adjacent to the Yarrack Fault trace (Cayley et al., in prep). The stereonet depicts poles to Glenthompson Sandstone bedding measured in the orocline and beyond the orocline limits (outside the figure, south of the Bushy Creek Granite near Chatsworth – for clarity, complicating effects of D1a folds are removed by excluding minority east- and south-facing D1a fold limb bedding orientation poles). The stereoplot shows that the Yarrack Orocline reorients D1a bedding strike through approximately 90° without changing the overall D1a dip-magnitude range. This indicates a subvertical orientation for the Yarrack Orocline axis, consistent with strike-slip displacement on the adjacent Yarrack Fault. This is a similar orientation to other D4 oroclinal drag folds and megakinks mapped in STAVELY. A drag-fold of smaller amplitude but similar geometry to the Yarrack Orocline and the Bunnugal Belt offset occurs in Grampians Group strata of the Serra Range directly above the buried Yarrack Fault position – the smaller amplitude of the Grampians Group-hosted fold compared to the Yarrack Orocline in Cambrian bedrock testifies to a significant component of Yarrack Fault strike-slip displacement accommodated by coeval lateral displacement across the intervening Marathon Fault detachment. Magnetic data indicates that the Yuppeckiar Diorite is folded and segmented by the Yarrack Fault, as is the Late Cambrian Buckeran Diorite further south (see Figures 2.15, 3.69), where the Yarrack Fault breaks into a horsetail of splay faults which cut and displace the Stavely Belt to link into the Escondida and Moyston faults.

Figure 3.69 Yarrack fault splays Tilt and band pass filtered magnetics image showing deformation by the Yarrack Fault and associated splay faults (thick lines).

Figure 3.70 Mesoscopic, upright D1a northeast-verging fold couple deforming Glenthompson Sandstone turbidites, exposed in a low cutting on Spittle Road (MGA 54 648408, 5834098). This outcrop is part of a D1a fault-slice of Cambrian metasediment incorporated into the interior of the Narrapumelap Belt. The D1a fold-style is typical, and is accompanied by a typical slaty (in mud and silt lithologies) to spaced stylolitic (in sandstone lithologies) S1a cleavage that is seen to fan around the D1a fold closures at a high angle to bedding, as seen just to the right of the hammer. Larger wavelength (>100 m) D1a fold closures in Glenthompson Sandstone are implied elsewhere in STAVELY by dip and facing-direction reversals, for example: in road cuttings along the Glenelg Highway east of Glenthompson, along Williamsons Road, in gullies adjacent to the Stavely Belt, and in tributaries of the Hopkins River west of Chatsworth.

Figure 3.71 Looking south at west-southwest dipping amalgamated sandstone beds in Glenthompson Sandstone, showing a pervasive spaced stylolitic S1a cleavage (parallel to hammer handle) that is developed at an oblique strike-angle to bedding (arrowed), a geometry that implies an overall westerly stratigraphic facing here, confirmed by graded bedding in adjacent outcrops, exposed on the bank of Bushy Creek (MGA 54 642782, 5816546). No folds of sufficient scale to be modelled are known to be associated with D1b, or D2. Some small-scale folds in altered Mount Stavely Volcanic Complex appear to be related to, or to overprint, alteration associated with the intrusion of Late Cambrian porphyry stocks, and so may be of D1b or possibly D2-D3 age., (e.g. Figure 3.72)

Figure 3.72 Open folds in Mount Stavely Volcanic Complex tuff, intersected in mineral exploration drill hole SNDD3 (130.5m downhole). Drill hole SNDD3 is located at the Thursday’s Gossan prospect. The folds overprint laminated quartz veins and pyrite-sericite alteration related to the adjacent Thursday’s Gossan porphyry system, and are therefore likely related to D1b or to subsequent D2-D3 deformations. D3 folds in STAVELY appear confined to the Grampians Group. D3 folds are large, open structures with sub-horizontally plunging axes, including the Wartook and Geerak synclines, and possibly the Willaura Syncline (Spencer-Jones, 1965; Cayley & Taylor, 1997a). The subhorizontal plunges of these folds is consistent with formation in response to crustal shortening. Their relative D3 timing is based on overprinting criteria – the Wartook Syncline is overprinted by subvertically-plunging D4 warp folds. Although well exposed, mapped in detail, and important for constraining the structural history of the underlying bedrock (see Figure 2.16) none of the D3 folds developed in the Grampians Group have been included in the STAVELY 3D model. The scale of the STAVELY 3D model is too coarse to distinguish the internal stratigraphic layering within the Grampians Group, which the folds deform.

The subvertical plunges of all the D4 folds is consistent with formation in a strike-slip-dominated strain regime. Although very well exposed and a critical timing and tectonic constraint to D4 folds developed in the underlying Cambrian bedrock (see Section 4.1 – D4 and D3 retrodeformation testing), the subvertically-plunging dextral D4 warps developed in the Grampians Group – the Mafeking, Cranage and Big Cord oroclines, and the Asses Ears ‘Anticline’ (see Figure 3.73; Cayley & Taylor, 1997a) – have not been included in the STAVELY 3D model. Likewise, subvertically-plunging D4 folds developed entirely within Glenthompson Sandstone – for example the large dextral Yarrack Orocline (see Figure 3.68) – have not been represented in the STAVELY 3D model. This is because no stratigraphic or structural subdivisions within the Glenthompson Sandstone or within the Grampians Group that can display the effects of the oroclinal folding have been captured at the scale of the STAVELY 3D model.

Although not modelled, the subvertical orientations of oroclines developed in the Grampians Group and Glenthompson Sandstone are a critical constraint for establishing a subhorizontal plane of movement for D4 at a regional scale, and for justifying a plan-view retrodeformation of D4 and D3 structures using potential field data at regional scale. In addition to the Mafeking Orocline which is described with the Mafeking Megakink below, the orientations of the rotation-poles of the Cranage Orocline in the northern Grampians Ranges, and the Big Cord Orocline in the northern Victoria Range, can be estimated from the intersections of bedding planes measured around the full extent of these oroclines.

The orientation of the Cranage Orocline can be estimated by plotting the orientation of bedding within both limbs of the Wartook Syncline where it has been refolded by the orocline. The Wartook Syncline retains its bilateral symmetry through more than 130° of clockwise rotation around the orocline. Both limbs lose dip-magnitude equally where they have been reoriented to strike east-west, suggesting that orocline development was accompanied by a component of ‘unfolding’ of the pre-existing syncline. The symmetry of the Wartook Syncline defines an average pole of rotation for the Cranage Orocline that plunges subvertically (Figure 3.73). The subvertical pole of rotation for the Cranage Orocline is further demonstrated by Wartook Syncline axes calculated at different positions along the length of the syncline from the orientations of bedding in opposing fold limbs – Wartook Syncline axis plunge magnitudes remain close to subhorizontal throughout, even though the fold plunge azimuth changes by approximately 130° across the width of the Cranage Orocline. This indicates that refolding of the Wartook Syncline occurred about a subvertically plunging pole of rotation. This passes a basic ‘orocline fold test’ for the Cranage Orocline.

Grampians Group bedding in the northern Victoria Range exhibits a change in strike of nearly 180° across the width of the Big Cord Orocline, with a range of bedding dip magnitudes that are similar throughout, defining a subvertical clockwise rotation pole for the orocline (approximately 78° – 85° towards 310°; Figure 3.73). This rotation pole can be used as a proxy to define the rotation experienced by the block of Cambrian bedrock within the Jalur Rift that includes the Grampians ‘Deeps’ Belt and the adjacent Brimpaen Belt. It is closely comparable to that measured for the Mafeking Megakink.

Subvertical D4 folds that deform different volcanic belts of the Stavely Arc, and the D1a faults that bound them, have been captured in the STAVELY 3D model where they are of sufficient scale for the entire belt boundary orientations and/or dips to have been folded. These are mentioned explicitly in the relevant fold descriptions below.

Figure 3.73 Schematic block diagram that depicts the context of D4 oroclines developed within Grampians Group (in blue) above subhorizontal D4 scissor faults (fault-slip planes depicted in green with footwall lateral transport directions indicated by black arrows on diagram and on map) that separate Grampians Group from coeval clockwise megakinks and dextral strike-slip faults developing in directly underlying Cambrian bedrock (in pink). The context of the evolution of the oroclines above megakinks is shown in Figure 4.3. Geometry and D4 timing of oroclines and related faults constrains the context of D4 structures preserved in underlying Cambrian bedrock. Structural data (from Cayley & Taylor, 1997b and Taylor & Cayley, 1997) confirms D4 Grampians Group oroclines as sub-vertically-plunging, superimposed over strata already folded and tilted during D3. Subvertical orocline axis plunges are indicated by a variable strike but unchanging dip-magnitude range in Grampians Group bedding refolded about oroclines. The Cranage Orocline refolds the southern Wartook Syncline. Bedding poles in the western (blue) and eastern (pink) syncline limbs both show evidence of clockwise strike rotation through approximately 90° with no overall change to dip-magnitude range. The Wartook Syncline axis maintains an unchanging subhorizontal plunge along its length, confirming a sub-vertical pole of rotation for the Cranage Orocline (in green) that refolds it. The Mafeking Orocline refolds gently-dipping strata with no overall change in dip-magnitude range, showing evidence of clockwise strike-rotation through more than 90° about a subvertical rotation-pole. This orocline directly overlies the Mafeking Megakink. The Big Cord Orocline refolds gently dipping strata in the northern Victoria Range with no overall change to dip-magnitude range, with evidence for clockwise strike-rotation through nearly 180° about a subvertical rotation-pole to form the southern orocline limb. This orocline directly overlies the rotated Grampians Deeps Belt in the core of the Jalur Rift. Subvertical poles of rotation constrained for all three D4 oroclines confirm a sub-horizontal movement plane for differential displacements between Grampians Group and underlying Cambrian strata during D4. The schematic block diagram explains the origin and context of the D4 geometries mapped in and around the margins of the Grampians Ranges, including: the dextral Escondida Q and Golton W faults; subhorizontal D4 scissor faults that separate block rotated and/or laterally translated bedrock from overlying detached and partially-rotated Grampians Group in the Mafeking Splay near Mafeking E; Marathon Fault splay near Brimpaen T; the Marathon Fault splay near Dunkeld Y; the Big Cord Splay in the northern Victoria Range U. The schematic diagram also explains the origin and context of enigmatic Marathon Fault splays that cut into the Grampians Group interior to separate variably rotated and translated strata across the Cranage Splay R and in the northern Grampians I. Lateral bedrock translations related to D3 north-south sinistral displacements and D4 northwest-southeast dextral transtensional displacements explain the context of Marathon Fault splays that separate Grampians Group strata from underlying Cambrian bedrock in the Black Range. Some Grampians Group strata apparently did not detach from the underlying Cambrian bedrock during formation of the D4 Mafeking Megakink and Orocline, instead rotating along with the Cambrian bedrock in the middle limb of the megakink (arrowed), so that the original basal unconformity between the Cambrian bedrock and ?Late Ordovician-Early Silurian Grampians Group is locally preserved intact as beneath the Willaura Sandstone O (Stuart-Smith & Black, 1999) – see also Figure 4.4.

Figure 3.74 Perspective 3D view looking north at Black Range region inversion models with TMI (RTP) magnetic image, showing strike-slip faults and associated drag folds. See Figure 2.35 for location.

3.4.1 Bepcha Fold

The Bepcha Fold is an open convex-west drag fold that affects the southern 5 km of the Black Range Belt in the Black Range (see Figures 3.74, 4.5). The Bepcha Fold refolds the Mouchong Fault System, the east and west faults of which enclose the Black Range Belt. The Bepcha Fold has an amplitude of approximately 1.5 km and apparently formed as a result of around 45° of anticlockwise drag folding of Cambrian rocks, about a subvertical to steeply northeast-plunging axis, adjacent to the D3 Cherrypool Fault (see Section 3.3.5.5 – Cherrypool Fault). The Bepcha Fold coincides with local overturning of the prospective Black Range Belt to a steep northeasterly dip. The Cherrypool Fault orthogonally truncates the southern end of the Bepcha Fold and offsets it from the truncated northern end of the Glenisla Belt with 19 km of sinistral displacement. This overprinting relationship implies a D3 age for the Bepcha Fold.

3.4.2 Connangorach Fold

The Connangorach Fold is an open, convex-west drag fold entirely concealed beneath the Murray Basin, but apparent in regional magnetic data where it affects the southern end of the Black Range West/Mitre Belt approximately 9 km southeast of Toolondo (see Figure 3.74). The Connangorach Fold has an amplitude of approximately1.5 km and apparently formed as a result of around 25° anticlockwise open drag fold-induced rotation, about a subvertical axis, adjacent to the interpreted position of the Muirfoot Fault (see Section 3.3.5.18 – Muirfoot Fault). The Connangorach Fold appears terminated against the Muirfoot Fault. Its geometry is consistent with the sinistral strike-slip displacement mapped across the Muirfoot Fault where it is exposed south along strike in the Grampians Group (Cayley & Taylor, 1997c). We interpret growth of the Connarngorach Fold as a direct consequence of sinistral movement along the Muirfoot Fault during D3 – D4.

3.4.3 Dollin Kink

The Dollin Kink is interpreted as an angular convex-west kink-type fold with an interlimb angle of approximately 75° that refolds rocks of the Mount Stavely Volcanic Complex north of Brimpaen. The Dollin Kink has been mapped using regional magnetic data (Figure 3.20 and Figure 3.74). The Dollin Kink has an amplitude of nearly 3 km, and appears to have formed as a result of sinistral drag folding of Cambrian rocks, about a subvertical axis, adjacent to the Dollin Fault. The Dollin Fault is a concealed east-west trending fault segment that separates magnetic stratigraphy within the Brimpaen Belt (see Section 3.3.5.8 – Dollin Fault). The size of the kink suggests at least several kilometres of sinistral displacement or rotation occurred across the Dollin Fault, prior to it being truncated by the Mosquito Creek and Escondida faults. The Dollin Kink is interpreted to be of D3 – D4 age. The Dollin Kink is confined to the interior of the Brimpaen Belt, and so is not discriminated in the STAVELY 3D model.

3.4.4 Mafeking Megakink

The Mafeking Megakink is an angular, Z-shaped dextral megakink with fault-displaced kink-axes developed in Cambrian bedrock in the southeast of STAVELY (Figure 3.44). The Mafeking Megakink is superimposed over the D1a volcanic (fault) belts of Stavely Arc stratigraphy. The Mafeking Megakink directly underlies the smoothly-curved D4 Mafeking Orocline within the Grampians Group with a coincident western axis, geometry and polarity, but with a different (kink) morphology (see Figure 2.16). The D4 age of the Mafeking Orocline constrains the D4 age of the underlying Mafeking Megakink. The central megakink limb is linear and contains northwest-southeast striking Stavely Arc stratigraphy including the Elliot and Narrapumelap belts– these are readily apparent in potential field data. The Mafeking Megakink is defined in outcrop by large-scale changes in the predominant dip-direction and stratigraphic facing-direction of bedding in Glenthompson Sandstone measured across it – bedding dip is mapped as predominantly to the northeast within the central megakink limb, and predominantly to the west in both of the enclosing megakink limbs.

The moderate northeast dip of interlayered volcanic belts of the Stavely Arc and Glenthompson Sandstone in the central limb is imaged as a distinctive multi-layered package in deep seismic reflection line 09GA-AR1. The subhorizontal dip of the sequence as imaged in line 09GA-AR1 is apparent – the deep seismic reflection line intersects the middle limb of the megakink at a low strike-angle. Deep seismic reflection data (09GA-AR1) shows the central limb of the megakink to be bound at depth by the Escondida Fault. This relationship is corroborated at surface by mapping and regional magnetic data which both show the Escondida Fault to occupy the western axis of the megakink. Regional magnetic and deep seismic reflection data show that the eastern megakink axis is occupied by the Golton Fault. Regional magnetic data and deep seismic reflection data show both these faults to laterally displace the megakink hinges with a dextral shear sense.

The preserved geometries and overprinting criteria indicate that Mafeking Megakink formation initiated early in D4 as asymmetric rifting in the centre of STAVELY, superimposed at a high angle over Cambrian bedrock that had previously been thrust stacked during D1a. Rotational rifting along northeast-southwest oriented D4 tear-faults began to break the Cambrian bedrock into large, internally coherent, partially to completely separated, fault blocks that combined to form the floor of the Jalur Rift. Overall rift-translations during D4 appears to have been directed southeastwards towards the position of the Moyston Fault, implicating the transtensional reactivation of this major structure as a driver of this deformation.

The most southerly block within the Jalur Rift was trapezoidal, measured approximately 26 km long and 25 km wide, and eventually became the central limb of the Mafeking Megakink. It appears to have initiated early in D4 as a partially-separated rift-block. The separated portion of this block contains the Elliot Belt that appears to have rift-separated from the northern end of the Bunnugal Belt and from the southern end of the Grampians ‘Deeps’ Belt early in D4, along tear-structures such as the Victoria Valley South Fault. The portion of the block that appears to have retained lateral continuity during early D4 contains the Narrapumelap Belt segment which, rather than rifting from the northern end of the Stavely Belt and southern end of the Dryden Belt, appears to have begun to buckle, rotating about megakink axes located near the northern end of the Stavely Belt and the southern end of the Dryden Belt. Our interpretation is that these buckled parts of the block lay beyond the tip lines of early tear structures developing within the Jalur Rift core (see Section 4.1.1 – Mafeking Megakink retrodeformation).

The different strike and facing directions within the central limb indicate that it rotated clockwise through approximately 140° relative to the adjacent Stavely and Dryden belts as the Mafeking Megakink grew progressively in amplitude during D4. Despite the markedly different dip-directions of the Narrapumelap and Stavely belts within and adjacent to the Mafeking Megakink, the directly measured and modelled (from regional magnetic data) dip magnitudes for the top 1-2 km of these volcanic belts are similar. This indicates that clockwise rotation of the central limb of the megakink occurred around a sub-vertically oriented fold axis (see Figure 3.44). This interpretation is corroborated by the orientation of bedding measured in directly overlying Grampians Group, where the range of dip magnitudes that occur across the width of the contemporaneous Mafeking Orocline remain near-constant through a more than 90° change in dip-direction, consistent with clockwise rotation about an averaged orocline pole that plunges 80°-85° towards 030° (see Figure 3.73).

Mafeking Megakink formation was apparently followed by dextral strike-slip fault-rupture through the positions of both megakink axes, so that the central limb of the Mafeking Megakink became completely separated and laterally displaced from its enclosing limbs late in D4. Today, regional magnetic data shows that the southwestern and northeastern ends of the Narrapumelap Belt are offset dextrally from the ends of the Stavely and Dryden belts across, respectively, the Escondida and Golton faults. This relationship is mirrored in overlying Grampians Group with the western end of the Mafeking Orocline cut by the Thermopylae and Escondida faults. The slight clockwise curvature of the northern end of the Stavely Belt apparent in potential field data is interpreted as D4 drag folding that accompanied the dextral fault rupture and displacement of the Mafeking Megakink. The amplitude of the Mafeking Megakink together with the fault offsets of its axes combine to give approximately 60+ km of southerly lateral translation of the eastern limb of the Mafeking Megakink (east of the Golton Fault), relative to the western limb (west of the Escondida Fault) during D4 (see Figure 4.4).

Farther west in the core of the Jalur Rift, additional large trapezoidal blocks of Cambrian bedrock that contain the Grampians ‘Deeps’ and Brimpaen belts also appear to have undergone approximately 140° of clockwise rotation during D4. In contrast to the Mafeking Megakink, these additional blocks of Cambrian bedrock appear to have become completely separated from one another and from surrounding Cambrian bedrock early in D4, with their relative rotations and translations accommodated across bounding faults including the Mosquito Creek, Escondida and Dollin faults, Fault 08, and the Victoria Valley South Fault. As for the Mafeking Megakink, the D4 timing and large magnitude of clockwise rotations about sub-vertical axes experienced by these blocks of Cambrian bedrock is constrained by structures developed in directly overlying Grampians Group – the Cranage and Big Cord oroclines (see Figures 2.16, 3.73).

The rotated central limb of the Mafeking Megakink cannot be traced north beyond the Grampians Ranges – the adjacent Dimboola Belt shows no evidence of similar clockwise rotations. Instead the Mafeking Megakink appears to terminate beneath the Grampians Ranges, possibly separated from the Dimboola Duplex farther north by a related, large strike-slip to transtensional D4 fault that links the Escondida and Golton faults – the Jalur Fault (see Figure 2.32) .

The smoothly-curved morphology of D4 oroclinal fold structures developed in the overlying Grampians Group contrasts with the angular morphology of the D4 megakinks and block rotations preserved in the directly underlying Cambrian bedrock. The tens-of-kilometre scale of relative lateral displacements that result from these strikingly different fold morphologies require the Grampians Group to be locally separated from the underlying Cambrian bedrock by low-angle detachment faults – these structures are parts of the Marathon Fault system (Cayley & Taylor, 1997a). Scissor-style fault-separations across different splays of the Marathon Fault system are required to account for differences in magnitude of rotations preserved in the Grampians Group versus the consistently larger-angle rotations apparent in underlying Cambrian bedrock (Figure 4.3). The differences indicate that Grampians Group deformation lagged Cambrian bedrock deformation, consistent with current theories of basement-driven basin deformation.

Small parts of the unconformably overlying Grampians Group appear to have rotated together with the Cambrian bedrock. The best-known example is the sequence of the Grampians Group that crops out north of Wickliffe. This sequence overlies the central limb of the Mafeking Megakink with an intact unconformity (Spencer-Jones, 1965; Stuart-Smith & Black, 1999), so that no local differential lateral displacement relative to the directly underlying Cambrian bedrock during D4 can be interpreted.

3.4.5 Tyar Fold

The Tyar Fold is a tight, J-shaped, steeply-plunging fold that affects the northwest 4.5 km of the Tyar Belt, and refolds the Tyar Faults north and south that bound that belt. The Tyar Fold is interpreted from regional magnetic data, which clearly images the fold in magnetic volcanic rocks of the Tyar Belt (see Figure 3.74). The Tyar Fold has an amplitude of approximately 2 km, and apparently formed as a result of approximately 90° of clockwise drag folding of Cambrian bedrock, about a sub-vertical axis, adjacent to (south of) the Henty Fault. The clockwise asymmetry of the Tyar Fold is consistent with formation in response to dextral fault-drag. The Henty Fault is a D4 structure that orthogonally truncates the Tyar Belt, cuts Grampians Group, and offsets the Tyar Belt from the truncated southern end of the Glenisla Belt with 21 km of dextral displacement (see Section 3.3.4.3 – Henty Fault). Because of this relationship, the Tyar Fold is also interpreted as a D4 structure. The Tyar Fold is cut off by the Henty Fault at surface, and therefore the fold is likely to extend to depth in the footwall of this major structure (see Figure 3.13). The geometry of the Tyar Fold hinge at depth is unconstrained. It may have a curved plunge that shadows the Henty Fault, or it may remain largely subvertical, in which case its northeast limb and amplitude will widen with depth, as presented in the STAVELY 3D model.

3.4.6 Wallup Fold

A few kilometres south of Wallup, regional magnetic data reveals a buried, near-isoclinal J-shaped fold of over 8 km apparent amplitude developed within magnetic Moornambool Metamorphic Complex stratigraphy adjacent to the Moyston Fault (Figure 3.59). The magnetic stratigraphy is interpreted to be Carrolls Amphibolite or equivalent. The asymmetry of the Wallup Fold suggests a drag-induced formation in response to dextral shear. The Wallup Fold appears truncated against the Moyston Fault, and therefore most likely formed in the Moyston Fault hangingwall in response to dextral movements along it. The dextral sense of displacement implied by the form of the Wallup Fold is inconsistent with the persistently sinistral D1-D3 strain-history preserved in the Moyston Fault hangingwall succession, and so is equated to reactivation of the fault during D4. The amplitude of the Wallup Fold implies a minimum of 8 km of local dextral strike-slip displacement across the Moyston Fault during D4.

Stratigraphy within the Moornambool Metamorphic Complex is not differentiated within the STAVELY 3D model, and so the Wallup Fold is not captured by the model.

3.4.7 Yarrack Orocline A complex, Z-shaped drag fold, disrupted across the Yarrack Fault, has been formed by D4 strike-slip induced rotations of Cambrian bedrock in the vicinity of Glenthompson (Cayley et al., in prep.). The Yarrack Orocline is clearly imaged in regional magnetic data where magnetic rocks of the Bunnugal Belt have undergone clockwise rotational deflections of approximately 45° where offset across the Yarrack Fault north of Strathmore. Rotations associated with faulting here have formed steeply-plunging D4 drag folds with up to 1.5 km amplitude.

The magnitude of right-lateral displacement across the Yarrack Fault in the Cambrian bedrock here is 5.5-6 km, given by offsets of Bunnugal Belt stratigraphy (see Figure 3.68). The magnitude of right-lateral offset of directly overlying Grampians Group stratigraphy in the Serra Range at the Griffin Fireline, across the buried Yarrack Fault position, is considerably smaller at approximately 1.5 km. This difference reflects accommodation of a portion of the lateral bedrock displacement across the Yarrack Fault as subhorizontal slip along splays of the Marathon Fault that underlie the Grampians Group at this location – as for other structures, Grampians Group deformation lagged deformation of the underlying Cambrian bedrock.

Farther west, the Yarrack Fault is intruded by the Victoria Valley Batholith, but reappears as a northwest-trending fault of diminished displacement that cuts the Victoria Range at Chimney Pot Gap, with approximately 400 m right-lateral offsets of the traces of the Mosquito Creek Fault and Geerak Syncline here.

The reoriented, fragmented limb of the Yarrack Orocline is well exposed in a network of creeks south of Glenthompson. Here bedding within the Glenthompson Sandstone and some D1a structures have been curved to locally strike east-west in the core of the Yarrak Orocline, adjacent to the Yarrack Fault. West of the Yarrack Fault trace, northerly-trending, steeply-dipping and D1a-folded Glenthompson Sandstone has undergone clockwise rotational deflections of approximately 90° about a subvertical axis, to build a J-shaped drag fold of over 4 km apparent amplitude (see Figure 3.68; note only data from comparable west- and north-facing D1a fold limbs is plotted for simplicity). The eastern end of this part of the Yarrack Orocline is truncated at high angle against a splay of the Yarrack Fault.

East of the Yarrack Fault folds related to the Yarrack Orocline are incoherent, a complex series of poorly-exposed subordinate kink folds with axes of steep but variable plunge that link obliquely into an anastomosing array of dextral strike-slip fault splays that together comprise the Yarrack Fault. The change in fold character across the Yarrack Fault implies a linked origin – we interpret the Yarrack Orocline as a subvertically plunging Z-shaped drag fold formed in response to at least 6 km of dextral strike-slip displacement across the Yarrack Fault.

Kink folds related to the Yarrack Orocline may also have overprinted the Yuppeckiar Diorite (Cayley et al., in prep.). This is a possible explanation for the complex shape of this elongate intrusion, which is quite unlike any other intrusions in the region. The Yuppeckiar Diorite has not been radiometrically dated, but these overprinting relationships imply a pre-Devonian (i.e. Cambrian, likely D1b) age.

Stratigraphy and structures within the Glenthompson Sandstone and Grampians Group are not differentiated within the STAVELY 3D model and so these parts of the Yarrack Orocline are not captured. The Yarrack Orocline is only apparent in the STAVELY 3D model where it has locally refolded the Bunnugal Belt adjacent to its offset across the Yarrack Fault.

4. Discussion

4.1 D4 and D3 retrodeformation testing

Construction of the STAVELY 3D model was accompanied by a plan-view palinspastic (area-balanced) retrodeformation of D4 structures (and D3 – where observed in Cambrian bedrock) mapped in the Cambrian bedrock. This was undertaken for a wider region of western Victoria that encompasses all of STAVELY (Figure 4.1) to test if the larger D4 (and D3) structures could be accounted for systematically within the STAVELY 3D model volume, and to establish the possible affects of D4 and D3 on the distribution of, and correlations between, different volcanic belts of the Stavely Arc. The large lateral displacements indicated for many of the D4 and D3 strike-slip faults, the truncations of the ends of some volcanic belts against D4 and D3 faults, and the large magnitudes of rotations of Cambrian bedrock strata that occur within some of the D4 megakinks and oroclinal folds, and in the Jalur Rift all indicate that the overall shape of the Grampians-Stavely Zone changed considerably during D3-D4, so that much of the present-day complexity seen in Cambrian structures – particularly the diversity of orientation of volcanic (fault) belts apparently developed during D1a – might be apparent. The regional-scale D4 retrodeformation is constrained in part by the age, magnitude and geometry of D4 structures mapped in the Grampians Group (Cayley & Taylor, 1997a).

Understanding the magnitude and geometry of D3 and D4 effects at the regional scale is critical for developing a predictive capacity for the present-day distribution of base and precious metal mineral occurrences throughout the region, since overprinting criteria and geochronology both indicate that the Cambrian mineral occurrences currently known were formed pre-D3.

For example, understanding the effects of subsequent faulting is critical for interpreting fault-offsets that have deformed Cambrian-aged mineral occurrences. These are poorly exposed and diamond drilling will be an important part of future mineral exploration in the region. Interpretation of fault-offsets in diamond drill core will, in many cases, be dependent on downscaling regional structural (stress) constraints presented in the STAVELY 3D model.

As another example, the potential exists for D3 and especially D4 structures to be mistaken for primary Cambrian ‘cross-arc’ features – for example transform structures – and thus have potential to miss-direct prospectivity analyses and mineral exploration targeting. A discussion of the workflows -adopted to unravel the effects of D4 and to discriminate between D2, D1 and primary arc features in the field and in diamond drill core is provided in Duncan et al. (in prep).

One of the main aims of the regional geological interpretation and STAVELY 3D model construction is to understand the origin and geometry of D1 structures, since they were the first to deform the Stavely Arc and are apparently associated with magmatism and mineralisation. This is considered important as only a few volcanic belts of the Stavely Arc are (poorly) exposed, and known mineral prospects and historical mineral exploration efforts have largely occurred within and immediately adjacent to these few belts (Cairns et al., 2018). Establishing direct correlations of the exposed and buried volcanic belts of the Stavely Arc has potential to extend the demonstrated prospectivity along strike, including under cover, and thus greatly expand the mineral exploration search-space.

A plan-view regional-scale palinspastic retrodeformation of D4-D3 was possible because the D4 and D3 events in STAVELY are observed to have involved large-scale strike-slip dominated fault translations and large-magnitude bulk-rotations about sub-vertical axes, all characteristics consistent with crustal blocks movements confined largely to the sub-horizontal plane. This approach was constrained by the subvertical orientation of the axes of rotation of regional-scale D4 oroclines in both Grampians Group cover (see Figure 3.73), and underlying Cambrian bedrock (see Figures 3.44, 3.68), and the predominance of subhorizontal (strike-slip) displacement vectors directly measured for major D3 and D4 faults such as Mosquito Creek, Muirfoot, Latani, Escondida and Yarrack faults. Most D4 structures that deform Grampians Group are steeply-dipping and/or plunging, and are observed to soft-link directly downwards into D4 structures of similar geometry and movement history in underlying Cambrian bedrock. D4 structures in Cambrian bedrock are clearly superimposed over D1a and D1b structures, and so provide overprinting relationships that confirm that the Grampians-Stavely Zone has experienced a very complex, compound structural history. Most D3 structures in the Grampians Group are low-angle and appear to be ‘thin-skinned’; it is hard to resolve any direct linkage into underlying Cambrian bedrock. Only where D3 structures are steep in attitude is such linkage established (e.g. Cherrypool, Muirfoot, Latani faults). Clockwise block rotations of both Cambrian bedrock and Grampians Group strata during D4 are generally confined to an area beneath the main Grampians Ranges which we have named the Jalur Rift. These rotations have profoundly changed the in-plan shape of this part of STAVELY, and so retrodeformation of these structures is a key constraint to retrodeformation of the surrounding D4 structures. With a subhorizontal plane of movement tightly constrained by outcrop, D4 and D3 retrodeformation can be extended laterally beyond these limits utilising plan-view potential field datasets, particularly regional magnetic data. The retrodeformation sequences and reconstructions presented here are based on regional magnetic data imagery (Figure 4.2). There is an initial assumption that Cambrian bedrock segments enclosed between the various D4 and D3 faults did not experience significant change to their surface areas during D3 and D4, other than where clear rift-openings are constrained (e.g. Jalur Rift), so that the overall surface area of the Grampians-Stavely Zone remains similar throughout the retrodeformation, for reasons outlined below.

Figure 4.1 Regional magnetic first vertical derivative pseudocolour image of western Victoria, showing the location of the reference region that includes STAVELY, depicted in Figures 3.9, 4.2, 4.13, 4.15, 4.17, and used for the D3-D4 retrodeformation.

Figure 4.2 Schematic present-day geology of the reference region (see Figure 4.1) that encompasses STAVELY and an additional northern part of the Grampians-Stavely Zone, depicting A) Regional tilt and band pass filtered magnetic data image with faults colour-coded by interpreted movement-sense, with locations of deep seismic reflection lines that inform the down-dip geometries and extents of these structures depicted in white, and the locations of inset areas that detail the D3-D4 retrodeformation process in Figures 4.6 and 4.7 (left) and Figure 4.4 (right) and B) the distribution of all Stavely Arc D1a volcanic belt segments and major (named) faults that occur between the Moyston and Yarramyljup faults, with all post-Cambrian cover and intrusive rocks omitted for clarity.

The thickness of coeval Grampians Group tectonostratigraphy and/or volume of Early Devonian magmatism provide direct constraints on the magnitude of rifting of Cambrian bedrock associated with D4 (e.g. Jalur, Lorquon, Cooac, Rocklands rifts; see Figure 3.9). The sub-kilometre to few-kilometre magnitude of D4 rift-related subsidence is relatively small compared to the tens-of-kilometres+ magnitudes of D4 and D3 strike-slip translations and rotations. The widespread preservation of Grampians Group cover along the length and width of the Grampians-Stavely Zone indicates that limited uplift of Cambrian bedrock and erosion was associated with either D3 or D4. These observations both indicate that strike-slip displacements were predominant over dip-slip displacements in Cambrian bedrock during D3 and D4 across the length and width of STAVELY.

With these characteristics, the assumption of the maintenance of near-constant surface-area for the Cambrian bedrock within the Grampians-Stavely Zone throughout D3-D4 is justified. The preferred D3-D4 retrodeformation scenario presented in this report was developed by iteration, with the magnitudes of plan-view overlaps (i.e. rifting) or gaps (i.e. shortening) between all the different segments of Cambrian bedrock bound between D3 and D4 fault segments minimised throughout all increments of retrodeformation, except where such overlaps were permitted within the cores of the larger recognised rifts (particularly, the Jalur Rift).

4.1.1 Mafeking Megakink retrodeformation

Retrodeformation of D4 for the entire Grampians-Stavely Zone is underpinned by the Z-shaped Mafeking Megakink and related structures, and by the fault network that segments the volcanic belts of the Black Range. The Mafeking Megakink is the most well-constrained large D4 structure because it is overlain directly by the D4 Mafeking Orocline, which exposes clockwise rotation and fault-truncation of Grampians Group tectonostratigraphy in the southeastern Grampians Ranges that can be attributed to D4 and can be used to directly constrain the timing and mechanics of retrodeformation of the underlying megakink.

East-west trending strata in the southern limb of the Mafeking Orocline is coincident in position and orientation with the underlying central limb of the Mafeking Megakink (Figure 4.3), which includes the Elliot and Narrapumelap belts. The western end of the Mafeking Orocline is juxtaposed abruptly against northerly-trending Grampians Group strata across the Thermopylae and Escondida faults. This juxtaposition directly overlies a similarly high-angle juxtaposition of Cambrian strata within the central limb of the Mafeking Megakink against the northerly-trending Stavely Belt across the Escondida Fault, apparent in regional magnetic and deep seismic reflection data and, further south, in outcrop.

These commonalities suggest that growth of the Mafeking Megakink in Cambrian bedrock during D4 involved clockwise rotation of the central limb through more than approximately 140° relative to the enclosing limbs, followed by dextral strike-slip fault disruption of the megakink hinges. The magnitude of strike-slip disruption of the megakink hinges is constrained by correlation of the Stavely and Dryden belts with the Narrapumelap Belt, supported by similar relative positions of these volcanic belts to adjacent volcanic belts during D1a time, and by matching of key stratigraphic marker horizons, particularly the occurrence of fault-slices of Williamsons Road Serpentinite within the interior of both the Stavely and Narrapumelap belts.

Mafeking Megakink formation apparently imparted comparable clockwise rotations and fault offsets into the directly overlying Grampians Group cover to form the Mafeking Orocline and Thermopylae and Escondida faults. The differences in fold-form between the Mafeking Megakink and the Mafeking Orocline represent the contrast between a brittle mode of D4 deformation in old Cambrian bedrock and a ductile mode of D4 deformation in younger, semi-lithified Grampians Group. The differences in rotation-magnitude between the Mafeking Megakink and the Mafeking Orocline are accommodated by scissor-style displacements across an intervening splay of the Marathon Fault system (Figure 4.3 and Figure 2.16).

The Mafeking Megakink can be retrodeformed relative to other Cambrian bedrock in STAVELY (Figure 4.4) because the geology of the western flank of the megakink, including the Stavely Belt, is simple and appears to have remained relatively stable with respect to Gondwana crust further west since Cambrian time. It thus forms a static foundation pinned to Gondwana, against which lateral movements of Cambrian bedrock during D4 can be referenced. Cambrian bedrock here can be traced westwards into the Glenelg Zone and into South Australia with little evidence of subsequent rotations or significant (>10-15 km) lateral offsets – offsets across the D4 Yarrack and Nareeb faults are limited and can be restored with confidence by matching offset segments of the Bunnugal Belt in regional magnetic data. Offsets across the D3-D4 Mosquito Creek and Latani faults are limited and can be restored by matching offsets of Grampians Group and of the Yarramyljup Fault. With this stable foundation, and with the amplitude and magnitude of rotation of the central Mafeking Megakink limb clearly apparent, the magnitude of southerly relative strike-slip translation of the eastern limb of the Mafeking Megakink (containing the Dryden Belt and the Moyston Fault footwall) relative to Gondwana during D4 can be calculated and is substantial – more than 65 km.

Figure 4.3 The D4 Mafeking Megakink, showing restored (left) and present-day (right) tilt and band pass filtered regional magnetic data of the Stavely/ Bunnugal, Narrapumelap/Elliot and Dryden belts, and restored (left) and present-day (right, but with strike-slip offsets of the megakink hinges omitted for clarity) schematic block-diagrams that depict how the once linear and combined D1a fault slices of the Stavely Arc (in pink) unconformably overlain by Grampians Group (in blue) came to be refolded and segmented into the Mafeking Megakink during D4, with Grampians Group strata partly detached along sub-horizontal scissor faults (in green) to form smoothly-curved oroclinal folds. The subhorizontal nature of the D4 kink-translations of Cambrian bedrock are constrained by the sub-vertical pole of rotation established for the Mafeking Megakink axis (green), defined by the large strike-rotation at constant dip-magnitude for the Stavely Belt (overall belt orientation measured (dark purple) and modelled from magnetic data (pale purple) versus the Narrapumelap Belt (overall belt orientation measured (dark green) and modelled from magnetic data (pale green). The lower-dipping pole and plane of the Narrapumelap Belt (in aqua) relates to the at-depth dip-estimate calculated from deep seismic reflection data which confirms a listric shape to the D1a belt that is unrelated to Mafeking Megakink axis orientation (see Figure 3.45). The subhorizontal nature of the D4 kink-translations are further corroborated by the sub-vertical poles of rotation established for the coeval D4 Mafeking Orocline in the overlying Grampians Group (structural data from Taylor & Cayley, 1997). See Figure 3.73 for additional structural data and an explanation of numbered scissor fault geometries.

Figure 4.4 Mafeking Megakink reconstruction sequence. A five-step plan-view retrodeformation sequence for D4 structures in the Mafeking sub-region (see Figure 4.1 for location) for the period ~405 to ~399 Ma, applied to a tilt and band pass filtered magnetic image. The retrodeformation reunites the Stavely, Narrapumelap and Dryden belts, and the Bunnugal and Elliot belts and related rocks into two simple, subparallel north-trending belts in a configuration that is interpreted to reflect the geometry at the end of Cambrian (D1) deformations. Post-Cambrian cover is generally thin and non-magnetic, with most of the magnetic signatures in this region attributable to Cambrian bedrock (thicker Grampians Group within the Grampians Ranges and near Woorndoo (shaded in blue) causes local attenuation of magnetic signatures from underlying Cambrian rocks). A subhorizontal plane of retrodeformation is constrained by the subhorizontal fault movements mapped on major D4 faults such as the Escondida (see Figure 2.14), Yarrack (see Figure 3.68) and Marathon (see Figure 2.18) faults, and by the subvertical plunges indicated for associated D4 kink and drag folds such as the Mafeking Megakink (see Figure 3.44), Yarrack Orocline (see Figure 3.68), and for the Mafeking and Cranage oroclines in the overlying Grampians Group (see Figure 3.73). The location of this sub-region relative to adjacent parts of the greater Grampians-Stavely Zone and the volcanic belts within it (volcanic belt colours match Figure 3.9) is depicted by shading in the bottom maps for each increment of the retrodeformation process. The position of a portion of deep seismic reflection line 09GA-AR1 is shown as the thicker black line. Only one retrodeformation scenario is permissible for the Mafeking Megakink: 1: lateral translation retrodeformation of late dextral strike-slip megakink hinge offsets across the Escondida and Golton faults preceded by: 2: approximately 150° of rotational retrodeformation of the central megakink limb within a sub-horizontal plane to reunite the Narrapumelap Belt with the Stavely and Dryden belts, and the Elliot Belt with the Bunnugal and Grampians ‘Deeps’ belts. Retrodeformation is consistent with a dextral transtensional finite strain ellipse with an initial orientation as depicted, with northwest-trending dextral strike-slip faults, northeast-trending antithetic sinistral strike-slip faults, and northeast-trending extensional faults predominant. Megakinking involved early rift-initiations west of (Jalur Rift, Victoria Valley South Fault, Willaura Rift) and east of the central megakink limb. The vast scale and rotational character of Jalur Rift opening implied by this reconstruction (arrows) demands infilling by collapse and lateral translation of adjacent crust, so that Mafeking Megakink formation is interpreted as the driver for segmentation, rotation and translation of northern parts of the Bunnugal Belt to become the Grampians ‘Deeps’ and Brimpaen belts (Figures 3.41, 3.20, 3.21), for Dimboola Duplex initiation and translation (Figure 3.15), and for coeval fault-segmentation and translation of the geology within the adjacent Black Range (Figure 3.13). The present-day position of D4 rifts inferred to the east of the central Mafeking Megakink limb is mostly occupied by the Moyston Fault, but D4 rifting here explains the local preservation of thick Grampians Group strata in the Woorndoo region and the presence and movement history of the east-dipping transtensional Marathon Fault splay intersected in drill hole STAVELY02 (Figure 2.18). Megakink retrodeformation indicates a transtensional, scissor-fault origin for the coeval sub-horizontal Marathon Fault detachment fault splays that separate overlying Grampians Group from the Cambrian bedrock, and above which the Grampians Group became refolded by smoothly-curving oroclinal folds (see Figure 4.3). The large magnitude of clockwise rotation indicated by the interlimb angles of the Mafeking Megakink are corroborated by the magnitudes of curvature of overlying oroclinal folds, and by direction changes of over 90° in Grampians Group palaeocurrent data measured in the western (Mirranatwa and Woorndoo palaeocurrents) versus central (Willaura palaeocurrents) limbs. The latter stages of megakink tightening are expected to have been associated with local transpression, particularly in the interior of the Willaura Rift – this may be represented by overthrusting across the Golton Fault, and related upturned of strata in The Terraces east of Halls Gap (see Figure 3.65). The area-balance of each increment of the retrodeformation process for this sub-region was tested for consistency and area-balance against the greater Grampians-Stavely Zone (shaded regions in wider zone maps at bottom), including the Black Range sub-region (see Figure 4.6). The before D4-D3 and after D4-D3 shapes of the Mafeking region and of the greater project area are depicted in Figure 4.13. Palaeocurrent data from George (1994) and Stuart-Smith & Black (1999).

Figure 4.5 Oblique view of TMI pseudocolour image of the Black Range, showing the near surface traces of major Cambrian (D1a, unnamed in red) faults within the Cambrian volcanic belts, and the Siluro-Devonian (D3-D4, named in black) faults that segment and offset them. The positions of major D3-D4 folds and the interpreted positions of Early Devonian granite intrusions are also shown. The tabular blocks that extend to depth in the Tyar (west), Black Range West/Mitre and Black Range (central) and Glenisla (east) belts depict the results of dip-modelling of multiple magnetic profiles constructed across each of the volcanic belts at right-angles to their local strike (see Figures 2.35, 3.17). The variations in dip between the different model blocks highlight an overall westerly-dip to the volcanic belts , with portions locally overturned to dip steeply east adjacent to the fault-truncated and drag-folded ends – the dip-modelling results help inform the construction of the volcanic belt volumes in the STAVELY 3D model (see Figure 3.12).

Figure 4.6 A six-step plan-view retrodeformation sequence for D4 (and D3?) structures in the Black Range sub-region (see Figure 4.1 for location), for the period ~405 to ~399 Ma, applied to a tilt and band pass filtered magnetic image. Post-Cambrian cover is thin and generally non-magnetic with many of the magnetic signatures in this region attributable to Cambrian bedrock (except for a veneer of magnetic Rocklands Volcanic Group adjacent to the Latani Fault). A subhorizontal plane of retrodeformation is constrained by the widespread extent of thin Grampians Group cover across the width of the subregion, by the subhorizontal fault movements mapped on major D4 and D3 faults such as the Muirfoot (Cayley & Taylor, 1997a) and Latani (Morand et al., 2003) faults, and by subvertical plunges indicated for associated D4 and D3 drag folds such as the Tyar and Beptcha folds. Retrodeformation accounts for all the D1a belt termination and offsets mapped within this area. The location of this sub-region relative to adjacent parts and belts in the greater Grampians-Stavely Zone is depicted by shading in the bottom maps for each increment of the retrodeformation process (belt colours match Figure 3.9). Two retrodeformation scenarios are permissible: 1: D4 dextral strike-slip fault (black) and fold retrodeformation followed by D3 sinistral strike-slip fault (red) and fold retrodeformation and 2: quasi-simultaneous retrodeformation of D4 dextral and sinistral faults and unfolding of folds, assuming conjugate formation within a dextral transtensional stress regime, superimposed over a pre-existing D3 sinistral regime (depicted). Late dextral displacements appear to have involved lateral displacement of the whole region north of the Henty Fault southeast towards the core of the developing Jalur Rift. The area-balance of each increment of the retrodeformation process for this sub-region was tested for consistency against the greater Grampians-Stavely Zone, including the Mafeking Megakink (see Figure 4.4). The before D4-D3 and after D4-D3 shapes of the Black Range region and of the greater project area are depicted in Figure 4.7 and Figure 4.13.

Figure 4.7 Present day (left) and pre-D4-D3 plan views of geology in the Black Range (see Figure 4.1 for location), showing the positions of major D4 and D3 faults and folds. Retrodeformation is consistent with a dextral transtensional finite strain ellipse with an initial orientation as depicted, rotating clockwise through time with northwest-trending dextral strike-slip faults, northeast-trending antithetic sinistral strike-slip faults and northeast-trending extensional faults predominant. The position of a part of deep seismic reflection line 09GA-SD1 is depicted in white, highlighting the potential magnitude of sinistral offset across the Muirfoot Fault.

Figure 4.8 Locations of overturned Glenthompson Sandstone and Stavely Arc strata mapped in and east of the Dryden Belt along the Moyston Fault footwall, superimposed on tilt and band pass filtered regional magnetic data. The positions of some D4 normal faults interpreted to be associated with the overturning are also depicted.

Figure 4.9 Moderately (~42°) southeast-plunging synform in Glenthompson Sandstone, exposed adjacent to the Lake Lonsdale spillway (MGA 54 641000 5900675). The fold has typical D1a morphology, associated with S1a spaced slaty to stylolitic cleavage. Grading in turbidite bedding indicates that both limbs face downwards (younging directions indicated by ‘Y’s), so that the structure is a synformal anticline. This fold was most likely formed as an upright D1a anticline (as for most other D1a folds recognised across STAVELY, e.g. see Figure 3.70) but has been subsequently overturned. Overturning was not associated with the formation of additional penetrative fabrics, a characteristic also seen in other regions of completely overturned but otherwise undeformed Cambrian strata mapped along the Moyston Fault footwall (e.g. Figure 2.20). This suggests that overturning along the fault footwall was by rigid block-rotations (e.g. Figure 4.11), probably within a transtensional stress-field. Overturning is attributed to D4 based on overprinting criteria.

Figure 4.10 Oblique view looking southwest at south-dipping D4 extensional faults modelled as an en-échelon array along the Moyston Fault footwall (translucent blue mesh). The faults link west towards the Mehuse (in yellow) and Golton faults, cutting and locally overturning intervening Cambrian strata, including Glenthompson Sandstone and volcanics of the Stavely Arc in the Dryden Belt. The locations of key outcrops discussed in the text are shown. See also Figure 4.8. Figure 4.11 Schematic block diagram illustrating the D4 dextral transtensional deformation sequence that segmented and overturned parts of the Glenthompson Sandstone and D1a structures including parts of the Dryden Belt (in purple). Cambrian bedrock was already upended prior to D4 (top), a steeply west-dipping west-facing D1a Dryden Belt enclosed by fault slices of predominantly west-dipping (indicated by dip symbols) and west-facing (indicated by Y symbols) Glenthompson Sandstone with occasional upright D1a folds. This geometry had formed during D1a overthrusting of the western flank of the main Stavely Arc edifice (in green, below the Stavely Base Fault). The sequence was bound along its eastern flank by the east-dipping D1a Moyston Fault, thrust across the eastern flank of the main Stavely Arc edifice at depth. Moyston Fault reactivation during D2 and possibly D3 had formed footwall splays, including the east-dipping Mehuse Fault which cuts the west-dipping D1a succession to displace the upper Dryden Belt westwards. This represents the starting configuration for subsequent reworking during D4 (the positions of incipient D4 faults are marked by dashed lines). Moyston Fault reactivation during D4 was dextral transtensional (centre). A finite strain ellipse for south-east directed dextral transtension predicts formation of south-east dipping, northeast-trending D4 extensional faults, as mapped along the Moyston Fault footwall, regularly spaced in en-échelon fashion (see Figure 4.8), including the Jalur and Ashens faults and Faults 16, 85, 86, 87, 18, 88 and 84. Faults 16 and 18 are illustrated as examples. Most D4 faults cut across and displace the Dryden Belt to link with the Mehuse and/or Golton faults (see Figure 4.10). These extensional faults are predicted to be listric in profile, reducing in dip-magnitude with depth, so that hangingwall blocks underwent roll-over rotations during faulting. D4 roll-over rotations were superimposed on strata already steeply-dipping courtesy of previous deformations (mostly D1a), meaning that only a minor component of D4 roll-over rotation is needed to explain how Cambrian strata and D1a structures are locally overturned and downward-facing adjacent to D4 structures. The roll-over effects from successive adjacent D4 faults is cumulative, so that roll-over rotations attributed to younger D4 faults (e.g. Fault 16 in the diagram) sum to preceding rollover effects of older D4 faults in their hangingwalls (e.g. Fault 18 in the diagram). This model predicts how almost complete overturning of Cambrian bedrock and related D1a structures in the Moyston Fault footwall is possible (e.g. Figure 2.20 and Figure 3.53A). Western Victoria has been eroded by several kilometres since D4 (bottom). D4 subsidence and southeasterly lateral translation from structural levels now eroded away explains the structural context of discrete offset blocks of Stavely Arc volcanics such as Mount Asler (Cayley & Taylor, 2000c, 2001). Similar relationships are implied south of Moyston, where mineral exploration drill holes have encountered fault-blocks of Stavely Arc volcanics in positions east of the coherent Dryden Belt (e.g. Cayley & Taylor, 2000b).

4.1.2 Jalur Rift retrodeformation

North of Mafeking, along the strike of the D4 Escondida Fault, an additional clockwise warp of similar scale, orientation and magnitude to the Mafeking Orocline is developed in Grampians Group, superimposed over the southern parts of the Wartook Syncline. This is the Cranage Orocline (see Figure 2.16). By direct comparison, clockwise rotations of Cambrian bedrock directly underlying the west limb of the Cranage Orocline during D4 are implicated in its formation. This area lies within the core of the Jalur Rift (see Figure 3.9), and includes the Brimpaen Belt. Similarly, the adjacent Big Cord Orocline in the northern Victoria Range overlies Cambrian bedrock containing the Grampians ‘Deeps’ Belt, suggesting comparable clockwise rotations in this block of bedrock during D4 also.

Retrodeformation of all the D4 Jalur Rift rotations, within the limits dictated by the distributions of Cambrian bedrock and structure and by overlying Grampians Group structure, profoundly changes the plan-view appearance of a large portion of central and eastern Grampians-Stavely Zone that is bound between the Mosquito Creek and Golton faults. The retrodeformed Cambrian bedrock here becomes much narrower and simpler east-west, and much longer and more strike-continuous north-south. This revised shape constrains, in turn, the range of retrodeformations viable for D4 structures within adjacent parts of STAVELY.

4.1.3 ‘Crab Nebula’ retrodeformation

Sinistral and dextral strike-slip faults segment and laterally offset Grampians Group by many kilometres in and adjacent to the Black Range (Cayley & Taylor, 1997a), and also bound and offset the terminated ends of the underlying Tyar, Black Range, Black Range West/Mitre, Glenisla and Grampians ‘West’ volcanic belts of the Stavely Arc (the radially-shaped ‘Crab Nebula’; see Section 3.2.5 – Stavely Arc). The consistent offset of sinistral strike-slip faults (e.g. Muirfoot and Cherrypool faults) by dextral strike-slip faults (e.g. Henty Fault) here establishes additional overprinting criteria that implicates D3 and, subsequently and/or simultaneously, D4 in the local fault-offsetting, refolding and reorienting of the Cambrian rocks here. Drag-fold asymmetries developed in the Cambrian bedrock adjacent to the strike-slip faults further constrain the offset-senses apparent for the D3 and D4 faults. The similar contained stratigraphy and overall westerly dips indicated for these belts (Figure 4.5) suggests all are segments of a single parent volcanic belt.

Retrodeformation of the effects of D4 and then D3 here (Figure 4.6) involves progressively ‘unrotating’ and ‘un-offsetting’ displacements across the various faults to see if it is possible to account for all the (D1a) volcanic belt end terminations and offsets in a way that remains consistent with the displacement senses of the faults that bound them, and to re-establish pre-D4 and pre-D3 positions and orientations for Cambrian bedrock without violating area- and volume-balancing laws (e.g. Elliott, 1983; Gibbs, 1983). This retrodeformation is constrained along its eastern and southern sides by the adjacent Jalur Rift retrodeformation into which the D4 Henty Fault links, and along its western side by the Yarramyljup Fault, which shows minimal evidence of local rotation or disruption since the Cambrian. The southern end of the D3 Cherrypool Fault is truncated by the D4 Henty Fault, which also appears to truncate the southern end of the Glenisla Belt (Figure 3.39). Dextral offset across the Henty Fault is constrained by the clockwise Tyar Fold superimposed over the northwestern end of the Tyar Belt. The magnitude of displacement across the Henty Fault is suggested by the approximate 19 km lateral offset of the Tyar and Glenisla belts. The ends of both belts terminate on opposite sides of the Henty Fault and so can be matched across the restored fault trace.

With dextral offset across the Henty Fault restored, the D3 Cherrypool and Muirfoot faults become aligned with the sinistral Latani Fault (Morand et al., 2003). Constraints on the magnitude of retrodeformation of sinistral displacements across the Latani and Cherrypool and Muirfoot faults include the offset of the Black Range Belt from the Black Range West/Mitre Belt, and the 20 km offset of the southern end of the Black Range Belt which is overprinted by the anticlockwise Bepcha Fold (Figure 4.5) from the northern end of the Glenisla Belt.

The best fit retrodeformation of D4 and D3 fault-offsets in the Black Range unites all the disjointed volcanic belts of the Stavely Arc (i.e. the ‘Crab Nebula’) into a single, linear, strike-persistent, prospective, D1a fault-belt (Figure 4.7). The retrodeformation restores D4 rotations and dextral displacements along the Henty Fault to reunite segments of D3 sinistral strike-slip faults for retrodeformation. As for the Jalur Rift and Mafeking Megakink, retrodeformed Cambrian bedrock in the Black Range is narrower and simpler east-west, and longer and more strike-continuous north-south by a similar magnitude. Based on this interpretation the Black Range Belt, which hosts the Eclipse (McRaes) Prospect amongst several others, shares its mineral prospectivity directly (along strike) with the Black Range West/Mitre Belt to the north, with the Glenisla and Tyar belts and, farther afield, with the Grampians ‘West’ and Boonawah belts that extend south along the western margin of the Victoria Range and farther south. In conjunction with the Jalur Rift, this retrodeformation places tight constraints on the range of retrodeformation options that are viable for the northern part of STAVELY.

4.1.4 Dimboola Duplex retrodeformation

The effects of D4 and D3 on the Dimboola and Dryden North belts that are mapped in outcrop (south of Horsham and in the northern Grampians Ranges) and in drilling, and in potential field and deep seismic reflection data, provide the primary constraints on D4-D3 retrodeformation of the northern half of STAVELY. D4 fault timing here is constrained by the observed juxtaposition of Grampians Group against Cambrian bedrock across faults such as the D4 Mackenzie River Fault where it cuts the Dimboola Belt. The movement senses used for retrodeformation are constrained by extrapolation from adjacent Grampians Group exposures and from the interpretation of fault geometries and apparent offsets revealed across key faults in regional magnetic and deep seismic reflection data; for example, dextral strike-slip offsets across the Mackenzie River Fault.

The lateral extents of D4 and D3 faults are interpreted from analysis of their effects on adjacent volcanic belts of the Stavely Arc – for example, the lateral offsets observed across the trace of the Mackenzie River Fault within the Dimboola Belt do not extend across to the Dryden North Belt or its projected along-strike position, or across to the hangingwall strata adjacent to the Escondida Fault. This indicates that the Mackenzie River Fault is confined between the Escondida and Golton and/or Mehuse faults.

The maintenance of overall northerly-trends within Dimboola Belt strata, across a whole series of subparallel D4 faults that are arranged en-échelon in plan and cannot be traced beyond the Escondida Fault in the west and the Golton and/or Mehuse faults in the east indicates that D4 here was fault-dominated rather than fold-dominated, and that faulting was compartmentalised and of a strike-slip duplex style – this is the basis for definition of the Dimboola Duplex. The best-fit D4 and D3 retrodeformation of the Dimboola Duplex shortens the strike-length of the Dimboola Belt host considerably and widens it in places.

Retrodeformation of dextral displacements across the Escondida Fault and related splays move much of the southern Dimboola Belt considerably farther north relative to Gondwana. Retrodeformation of dextral displacements across the Golton Fault (magnitude constrained by the amplitude and offset of the Mafeking Megakink) and related splays such as the Babatchio and Tullyvea faults result in a greater strike-length for the Dryden North Belt, which is moved northwards relative to the Dimboola Belt, but the overall in-plan relationships between these volcanic belts are retained (see Figure 4.12).

4.1.5 Retrodeformation of locally overturned Cambrian strata in the Moyston Fault footwall

Kilometre-scale blocks of overturned Cambrian bedrock are aligned along the Moyston Fault footwall, mapped in outcrop between Moyston and Mount Drummond (Cayley & Taylor, 2001; Figure 4.8). The rocks affected are predominantly Glenthompson Sandstone, which occurs in a series of northeast-striking fault-bounded blocks.

Dip magnitudes of bedding in the Glenthompson Sandstone are generally low, but sedimentary structures preserved within (e.g. graded bedding in turbidites; see Figure 2.20 and Figure 3.53A) show bedding to be completely overturned. The large (kilometre) scale and high-degree of overturing seems incompatible with the otherwise undeformed appearance of these rocks – cleavage is weak, and few folds are developed other than near Lake Lonsdale where occasional chevron-style fold closures look similar in morphology to upright D1a folds mapped elsewhere in the Grampians-Stavely Zone, but are locally steeply-plunging and downward-facing (Figure 4.9).

Blocks of overturned Glenthompson Sandstone are locally juxtaposed against fault blocks of Mount Stavely Volcanic Complex, such as at Mount Asler (see Figure 4.8). These blocks of Stavely Arc rocks are isolated from the more continuous volcanic belts of Stavely Arc farther west (e.g. the Dryden Belt), and so are very atypical for the overall zone.

The distribution and northeast-dipping strike of the blocks of overturned strata is aligned along the footwall of the Moyston Fault, and so suggest formation as an en-échelon array associated with movements along the Moyston Fault. Cayley & Taylor (2001) proposed rigid block-fault overturning of cold, Cambrian footwall rocks in response to sinistral-oblique overthrusting across the Moyston Fault during the Benambran Orogeny (D2) to explain them. Although such a scenario is consistent with the geometries observed, the degree of overturning required to explain the observed geometries is extreme – equivalent to isoclinal recumbent folding, for which other compelling evidence is completely lacking.

New mapping of the northeast-trending faults that bound the overturned blocks in STAVELY has served to further clarify overprinting relationships with adjacent structures in the eastern Grampians-Stavely Zone, and constrains a younger (D4) age for the block overturning and the faults that bound it.

In particular, late north to northeast-trending, steeply east to southeast-dipping faults that cut across and offset the Dryden Belt at and adjacent to Mount Dryden (Buckland 1987; Cayley & Taylor, 2000a, 2001) can be traced beyond the limits of the belt in regional magnetic data (Figure 4.8 and Figure 4.10) and are clearly shown to be continuous with the faults that bound the blocks of overturned strata at Mount Drummond, Mount Asler and Lake Lonsdale. These faults all link directly into the Moyston Fault footwall. These faults are observed to cut across the D1a Dryden Fault and either link into, or cut, the D3 Mehuse Fault or, more likely, the D4 Golton Fault. The faults must therefore be of D3 or, more likely, D4 age. They are of the same geometry as, and lie in the hangingwall of, the larger D4 Ashens Fault, along which intrusions were emplaced in the Early Devonian.

The apparent width of Dryden Belt stratigraphy increases eastwards across the positions of Fault 16 and Fault 18 (see Figure 4.8) which, given the upwards-widening geometry of this part of the Dryden Belt, implies an east-side down displacement across these structures. This is an extensional movement sense. Fault 18 additionally serves to separate the west-dipping upright part of the Dryden Belt at Mount Dryden, from locally steep east-dipping and overturned parts of the Dryden Belt exposed south along-strike at McMurtrie Hill.

New mapping has now identified additional faults of this geometry and likely D4 timing spaced along the entire exposed length of the Moyston Fault footwall. These are, from south to north, faults 84, 88, 18, 87, 86, 85 and fault 16 in the STAVELY 3D model (Appendix 6 – Fault summary table; Figure 4.10). They form an extensive en-échelon array that separate a series of blocks of overturned Cambrian bedrock.

The greater regional constraints on the stress history and structural setting for D4 now provided by the retrodeformation suggest an alternative explanation for the block-overturning of the Cambrian bedrock – hangingwall block-rotations along listric D4 extensional faults developed as an en-échelon array along the Moyston Fault footwall as the Moyston Fault underwent dextral oblique transtensional reactivation (Figure 4.11). The northeast to north alignment of these faults is consistent with extensional fault geometries expected for the D4 strain ellipse (see Section 4.5.1 – A scalable structural template for STAVELY through time).

In this style of interpretation D4 block-rotation was superimposed obliquely onto Cambrian bedrock that was already steeply dipping courtesy of D1a thrusting (original D1a orientations are still preserved at Mount Dryden and southeast of Mount Stavely), so that anything more than 25° of subsequent extensional dip-slip (roll-over) rotation on southeast-dipping D4 faults would be sufficient to impart an overturned geometry to Cambrian strata within enclosed D4 fault blocks. The faults form an en-échelon array, and the footwall propagation mode expected for the formation of such an array means that the rotations from later-developed faults would be superimposed onto the rotations of previously formed faults in the hangingwall, so that the total magnitude of dip overturn of strata observed in places such as west of Moyston and at Mount Drummond might sum to over 90° across just a few of the observed faults. This is a simpler explanation for the observed geometries, and is consistent with the minimal penetrative deformation seen within the overturned fault blocks. This Cambrian strata was fully lithified and at a high crustal level at the time of D4, and was thus likely overturned while in a brittle deformation regime. Pre-existing D1a structures including F1a folds were preserved intact, but were wholesale reoriented into downward facing attitudes as mapped at Lake Lonsdale. Isolated blocks of Stavely Arc rocks such as Mount Asler can be simply explained as D4 fault blocks dropped from higher (i.e. more easterly) parts of the adjacent west-dipping Dryden Belt that have subsequently eroded away.

Dextral transtension is consistent with evidence of late clockwise plan-view rotations of some Moyston Fault segments, for example the localised southeast dip of the Moyston Fault plane exposed adjacent to Lennox Lane, south of Moyston (Cayley & Taylor, 2001).

The scale of D4 overturning in the Moyston Fault footwall is too small to be depicted in the regional-scale retrodeformation, other than with the assumption that the Cambrian bedrock here was probably slightly narrower pre-D4.

4.1.6 Analysis of D4 and D3 retrodeformation results

With the retrodeformation completed, the initial D3-D4 structural system configuration within the STAVELY bedrock appears greatly simplified, characterised by southeast-directed translation along predominantly northerly-trending, east and southeast-dipping dextral structures (yellow structures in Figure 4.12). Southeast-directed dextral transtension is implicated for D3-D4 overall. Since this translation direction is towards the position of the Moyston Fault, and the Moyston Fault itself and its hangingwall region preserves evidence of late dextral strike-slip displacement across it (e.g. Wallup Fold, overturned strata, see Figures 4.8; Miller et al., 2006) dextral trans-tensional reactivation of the Moyston Fault due to southeast displacement of the Stawell Zone during D4 is implicated as the local control on D4, and most likely previously D3, in the adjacent Grampians-Stavely Zone.

The influence of D4 and D3 is observed to diminish progressively towards the west across the width of the Grampians-Stavely Zone, reinforcing this interpretation. Overall, the retrodeformation of D3 and D4 structures in the Grampians-Stavely Zone indicate that the greater Stawell Zone moved 100-115 km southwards overall relative to Gondwana during the Late Silurian-Early Devonian.

D4 normal faults with southeast dips, and D4 and D3 strike-slip faults with easterly overall dips are the only faults with geometries able to link directly into the Moyston Fault footwall at depth. Thus, D4 and D3 faults with easterly dips are likely to be primary structures that persist to the greatest depth and with the greatest lateral subsurface extent. The largest D4 faults with directly mapped or imaged (by deep seismic reflection data) geometries in STAVELY are the Escondida, Golton, Henty, Henty West, Mosquito Creek, Jalur, Lorquon, and Winian East faults, and all appear to be east-dipping.

The largest of the D4 faults, such as the Golton and Escondida faults are strike-persistent, and form a common linkage between D4 folds such as the Mafeking Orocline and Mafeking Megakink in the south, D4 rifts such as the Jalur Rift in the centre, and D4 fault networks such as the Dimboola Duplex in the north. The Escondida Fault occupies the western axis of the Mafeking Megakink and is the western master fault into which all the strike-slip duplex faults, including large structures such as the Henty Fault System, link. The Golton Fault occupies the eastern axis of the Mafeking Megakink and is the eastern master fault into which all the Dimboola Duplex faults link in the east. This commonality indicates that the Mafeking Megakink, the Jalur Rift and the Dimboola Duplex and Henty Fault System are simply different deformation modes developed within a single strike-slip transtensional system. The east-dip of the D4 structures indicates that dextral transtensional displacement originated from the east, and from the Moyston Fault.

The Henty, Escondida and Golton faults all converge towards one portion of the Moyston Fault (today, near Lake Bolac; considerably further north when retrodeformed; see Figure 4.13), implicating this portion of the Moyston Fault trace as the key to understanding the development and style of the largest D3 and D4 structures in STAVELY. This portion of the Moyston Fault strikes northeast, and is the southern portion of a localised convex-east bulge that is apparent in the retrodeformed Moyston Fault trace (Figure 4.13). The bulge is likely an arbitrary legacy of earlier (D1-D2) movements on the Moyston Fault (eg. the result of differential D1a shortening across a Cambrian ‘transfer structure’; see Section 4.4.3 – Understanding the form and distribution of potential transfer structures), but its geometry is a classic ‘releasing-bend’ shape for dextral transtensional shear, which is the nature of reactivation of the Moyston Fault during D4.

With such a geometry, localisation of sizeable oblique rotational pull-apart rifts along this portion of the Moyston Fault footwall are expected for D4 dextral transtension. Such rifts would fill by the lateral rotational southeast-directed collapse of footwall Stavely Arc material. This is an explanation for the mode of initiation of the Mafeking Megakink adjacent to the Moyston Fault trace. As dextral transtension progressed, localised oblique rotations would transition to strike-slip translations, also directed towards this portion of the Moyston Fault. This explains the origin of more disrupted rotations and scissor-style rifts preserved within the Jalur Rift, and the cascade of D4 effects that extend northwards into the Grampians-Stavely Zone interior as the Dimboola Duplex, Henty Fault System, and related structures.

Northwest-dipping D4 normal faults and westerly-dipping strike-slip faults are, in most cases, likely to be conjugate, secondary structures that are expected to sole-out against the primary D4 normal faults. Such inferences form a set of geometrical rules that helped inform the STAVELY 3D model construction – for example to infer primary-secondary relationships – at depth – between the Winian East and Grass Flat faults. Where local geometries require different primary-secondary relationships – for example between the Victoria Valley South Fault and the Victoria Valley North Fault, these are revealed to be subsidiary to even larger east-dipping structures, in this example the Mosquito Creek and Escondida faults which are considered to be the primary structures that enclose both of the Victoria Valley faults.

The success of a regional-scale palinspastic retrodeformation of D4 effects demonstrates the viability of a dextral-transtensional strain ellipse with a northwest-southeast–oriented long axis (sigma 3) to explain the origin, context, and diversity of all D4 structures spanning the Grampians-Stavely Zone (Figure 4.13). This scenario gives predictive capacity about the expected movement histories of D4 faults, and their depth-extents and interrelationships, dependent on their orientation in 3D space. It provides a methodology for a systematic interpretation of the D4 structural system in places between points of geological control, and formed the basis of the methodology used for construction of the D4 fault framework modelled at the crustal-scale. The regional-scale retrodeformation has provided constraints on the likely magnitude of strike-slip displacements on many D4 and D3 faults, which allows estimations of the magnitude and polarity of out-of-plane displacements across these faults there they intersect serial cross section locations at low-strike angles. This was a critical step in construction of crustal-scale cross sections used to construct the STAVELY 3D model, since no single cross section could be balanced without reference to the geology of adjacent cross sections.

With the effects of D4 restored, the new configuration for a pre-D4 Grampians-Stavely Zone emerges, in which it is possible to evaluate the nature of earlier episodes of deformation, including the limited effects of D3 within the Cambrian bedrock, and particularly the effects of D1a and D1b events associated with cratonisation of the Stavely Arc and related rocks.

D3 structures in the Cambrian bedrock include sinistral fault-offsets demonstrated across the Cherrypool, Latani and Muirfoot faults in the Black Range, and suggestions of a previous oblique-thrust history of D3 movement along the Escondida Fault. Once restored, these structures form a simple north-trending network aligned along the length of the Grampians-Stavely Zone that appears to dip east towards the position of the Moyston Fault at crustal scale, suggesting that D3 structures within STAVELY may also have developed as footwall splays of a Moyston Fault reactivated during D3 sinistral transpression. It is possible that the main elements of the D4 structural system may have exploited a pre-existing D3 structural system.

With the effects of D4 and D3 removed, it becomes apparent that D1 in the Cambrian bedrock of the mid-upper crust of the Grampians-Stavely Zone is relatively simple. South and west of the Grampians Ranges, the D4-D3 restoration of geological mapping, deep seismic reflection data and dip-modelling of potential field data show D1a within the Grampians-Stavely Zone to have been manifest as a homoclinally west-dipping, east-verging thrust system that imbricates fault-repeats of the Stavely Arc and underlying (and overlying) Cambrian sedimentary rocks.

The retrodeformed D1a thrust faults are west-dipping, and control four subparallel belts of Stavely Arc strata –from west-to-east the Boonawah-Grampians ‘West’-Tyar-Glenisla-Black Range-Black Range West/Mitre Belt, the Bunnugal-Elliot-Grampians ‘Deeps’-Brimpaen Belt, the Caramut Belt, and the Stavely-Narrapumelap-Dryden-Dryden North Belt. Their overall geometry would have been similar to that still preserved in the southwest of STAVELY where the effects of D3 and D4 are minimal (see Figure 3.11). These rocks of the Stavely Arc are thrust-intercalated with belts of west-dipping and facing Glenthompson Sandstone.

The overall retrodeformed D1a fault system here appears consistent with a typical imbricate thrust system where successive thrusts propagate from the footwall side. South of the Grampians Ranges, all the volcanic belts of the Stavely Arc were apparently thrust eastwards towards surface during D1a, over the western flank of a basement-high of highly seismically-reflective rocks that are likely to be of mafic composition and may also be a part of the primary (autochthonous) Stavely Arc. The basement high is visible in deep seismic reflection line 97GA V1 and 09GA-AR1 and appears to have acted as a footwall buttress, against and over-which the D1a thrust system has propagated. Where mapping, deep seismic reflection data and forward modelling of potential field data suggests that the thickness of Cambrian metasediments within the D1a thrust sheets exceeds what is considered reasonable for the Glenthompson Sandstone, additional D1a thrust-imbrication is inferred, either within the Glenthompson Sandstone (as described for the area between the Stavely and Bunnugal belts, or by invoking overthrusting of the Stavely Arc crest and in-situ Nargoon Group by Glenthompson Sandstone, as inferred across the Stavely Base Fault for the southeast parts of the Stavely Arc.

The eastern flank of the basement-high was apparently also overthrust at this time, by the Stawell Zone, across the east-dipping Moyston Fault (Cayley & Taylor, 2001; Korsch et al., 2002; Miller et al., 2005), but there is little evidence of D1a structures of this geometry developing in the south of the Grampians-Stavely Zone. Deep seismic reflection line 09GA-AR1 reveals why this may be the case – the eastern Grampians-Stavely Zone is underlain by a large upward-convex volume of highly reflective material, which we interpret to represent autochthonous Stavely Arc. The position and persistence of this buttress further north in the Grampians-Stavely Zone can also be interpreted from the lateral persistence of the Dryden North Belt that overlies it. This massive feature may have acted as a structural buttress that defined the eastern edge of the Grampians-Stavely Zone, and served to limit the propagation of thrusts west into the interior of the zone during D1a.

With the effects of D4 and D3 removed, the Cambrian bedrock of STAVELY is revealed to exhibit one major along-strike internal complexity – the loss northwards of two of the west-dipping D1a imbricate fault slices of the Stavely Arc (containing the Caramut and Bunnugal-Elliot-Grampians ‘Deeps’-Brimpaen belts), to be replaced along-strike in the north by a single large east-dipping D1a fault slice that contains the Dimboola Belt. The east-dipping Dimboola Belt lies in the hangingwall of the Escondida Fault, which locally appears to have been active during D1a and dips towards the position of the Moyston Fault. The fault-slice that contains the Dimboola Belt appears to widen northwards and is east-dipping at the crustal-scale. In a pre-D3-D4 retrodeformed STAVELY, D1a to the north of the Grampians Ranges is apparently manifest as a doubly-vergent thrust system.

A critical link to the simpler D1a geology further south is provided by the strike-persistent Dryden North Belt. The magnetic and gravity character of the west-dipping and facing stratigraphy mapped at Mount Dryden persists as the Dryden Belt and Dryden Belt North into the northernmost reaches of STAVELY. The consistency of the potential field character is taken to indicate consistency of structural style and geometry. The implication is that the bounding D1a thrust faults that enclosed the Dryden and Dryden North belts during D1a – the Stavely Base and Dryden West faults – must also maintain an overall west-dip and east-vergence from the far south into the far north of STAVELY, albeit locally offset across later structures such as the Mehuse and Golton faults. This demonstration of strike-persistence of at least one part of the east-vergent D1a imbricate thrust system is critical for understanding the structural significance of the east-dipping parts of the system that are confined to the area north of the Grampians Ranges.

Whilst it is tempting to interpret a direct linkage between the D1a portion of the Escondida Fault that lies to the north of the Grampians Ranges and the Moyston Fault that was also active during D1a, such an interpretation is inconsistent with the persistence – into the same area – of the intervening west-dipping D1a fault system that hosts the Dryden North Belt. The near-surface geometries of the west-dipping Dryden West (and Stavely Base Fault) and east-dipping Escondida Fault imply that they must converge and interact down-dip, independent of the Moyston Fault. Down-dip convergence in coeval thrust systems is a hallmark of a pop-up structure, or of ‘back-thrust’ behaviour. The precise inter-relationship between these two sets of oppositely-dipping faults during D1a at depth is unknown, and is ambiguous in the deep seismic reflection data. The Dryden and Stavely Base fault system is the easternmost of the series of imbricate west-dipping thrusts developed south of the Grampians Ranges, and therefore may therefore be the youngest. The greater strike persistence of the Stavely-Narrapumelap-Dryden-Dryden North Belt compared to the Dimboola Belt suggests that, despite the larger width of magnetic stratigraphy exposed within the Dimboola Belt, it is the former volcanic belt that is the larger, primary structure, so that the faults may have developed together as a doubly-vergent pop-up, with the Escondida Fault representing a backthrust from the Dryden West / Stavely Base Fault system –this is the D1a geometry incorporated into the STAVELY 3D model. Alternatively, it remains possible that the D1a component of movement along the Escondida Fault is older than the Dryden and Stavely Base faults, and linked east into the Moyston Fault at D1a time, but has been truncated subsequently by west-dipping structures.

The change from the simple east-vergent imbricate D1a thrust system to the south, to the more complex pop-up-style thrust systems to the north occurs in Cambrian bedrock now concealed beneath the present position of the Grampians Ranges in the area now occupied by the Jalur Rift, and is discussed in more detail below. Such changes in geometry within a coeval structural system normally require accommodation across transfer structures, and this key understanding informs interpretation of structures in Cambrian bedrock concealed beneath this younger cover, and informs construction of deeper levels of the STAVELY 3D model.

Figure 4.12 Schematic plan view of the D3-D4 retrodeformation applied to the same tilt and band pass filtered regional magnetic data image and D3-D4 fault network as Figure 4.2. This is a constrained interpretation of the pre-D3-D4 configuration of STAVELY. The retrodeformation shows how it is possible to account for the strike-slip displacements across all the major D4 and D3 faults mapped across the entire central Grampians-Stavely Zone to produce a revised and greatly simplified Cambrian bedrock (D1a) configuration that: (A) is area-balanced throughout the retrodeformation process (with one minor but unavoidable qualification outlined below) and (B): is consistent with a dextral transtensional finite strain ellipse that is generalised for the Early Devonian across Victoria (Cayley & Musgrave, in prep.) and indicated for the major D4 faults in STAVELY, with northwest-trending dextral strike-slip faults, north-trending antithetic sinistral strike-slip faults and northeast-trending extensional faults predominant, and: (C): allows for direct and greatly simplified correlation between volcanic belts , which become reunited into four main D1 fault slice groupings (coloured). Restoration involved identification and matching of offset D1a volcanic belt terminations across known D3 and D4 faults (including those expressed in overlying Grampians Group), using the Glenelg Zone margin as a reference boundary. Restoration assumed near-constant volume, apart from sites of Early Devonian rifting and granite intrusion where a component of extension during D4 is assumed. Retrodeformation of dip-slip components of transtensional (and transpressional) faults (for example those centred in the core of the Jalur, Lorquon, Cooac and Rocklands rifts – see Figure 3.9), and dip-rotations (for example dip-rotations adjacent to D4 folds in segments of the ‘Crab Nebula’ see Figure 3.17) – necessitates local removals (or additions) of surface-area from the final retrodeformation – but these are of insignificant magnitude at Zone-scale. Note the offset, segmented, repeated, and locally reversed (about D4 features such as the Mafeking Megakink where retrodeformation involves rotation) positions of segments of the major deep seismic reflection transects in white, an indication of the magnitude of the complicating effects of D3 and D4 on interpretation of the Cambrian strata imaged by these transects.

Figure 4.13 Stepwise area-balanced D4- D3 plan-view retrodeformation of the reference region Cambrian geology from the present day (at left) to pre-Silurian configuration (at right). The effect of the retrodeformation is to displace the northeast corner of the reference region northwest by 114 km, and to significantly reshape the Grampians-Stavely Zone interior while retaining a near-constant total surface-area overall (a small surface area reduction is due to the closure of identified D4 rifts during retrodeformation). The retrodeformed Grampians-Stavely Zone shape dictates the shape of the retrodeformed Moyston Fault trace, and of the retrodeformed Stawell Zone. The lower 98 km magnitude of lateral displacement indicated for the southwest corner of the reference region represents a 16 km component of dextral strike-slip displacement accumulated across the Angip, Coongee and Curtis faults and other structures within the Stawell Zone during D4. The retrodeformation reveals that the current geometry of D4 structures within the Grampians-Stavely Zone can be attributed to dextral transtensional displacement largely localised across the Moyston Fault plane. A sizeable oblique pull-apart rift appears to have formed in one portion of the Moyston Fault footwall, localised on a northeasterly-trending ‘releasing bend’ that is apparent in the retrodeformed shape of the Moyston Fault trace. The releasing-bend shape was likely an arbitrary legacy of earlier (D1-D2) Moyston Fault movements. Stavely Arc crust collapsed laterally and rotationally southeastwards into the footwall pull-apart rift during D4 to first form the Mafeking Megakink and the Jalur Rift. As transtensional D4 deformation continued, its effects cascaded northwards into the Grampians-Stavely Zone interior as the Dimboola Duplex and related structures.

4.2 D2 retrodeformation testing

D2 timing relates to the Benambran Orogeny of Late Ordovician time. D2 is only expressed in the Grampians-Stavely Zone as initiation of the Grampians Basin, presumably in response to limited reverse reactivation of the adjacent Moyston Fault. Where the effects of the Benambran Orogeny are strongly expressed farther east in the Lachlan Fold Belt it is characterised by strong east-west directed crustal shortening and thickening (e.g. VandenBerg et al., 2000; Gray et al., 2006, Cayley et al., 2011a). D2 in STAVELY is therefore likely to have involved reverse dip-slip fault movements. Plan-view potential field datasets are not appropriate to illustrate or constrain the retrodeformation of such structures. Section-view geophysics is available as deep seismic reflection lines 97GA-V2, 97GA V1 and 09GA-AR1 and 09GA-SD1, but the magnitude of possible displacements along the Moyston Fault during D2 are very poorly constrained and probably relatively minor at the regional scale – in effect the eastern margin of the Grampians-Stavely Zone was likely thrust west a few kilometres to slightly narrow the Grampians-Stavely Zone overall. Because of this a retrodeformation of D2 has not been presented.

4.3 D1 retrodeformation testing

D1 timing relates to the Delamerian Orogeny of Late Cambrian time. D1a in particular is a profound deformation event in the Grampians-Stavely Zone that converted an undeformed subaqueous and still active Mid-Late Cambrian Stavely Arc into uplifted bedrock. D1a involved massive (at least 50 percent) crustal shortening within the Grampians-Stavely Zone, with concomitant shortening of the adjacent back-arc to form the Glenelg Zone to the west, the whole suprasubduction zone region translating and shortening some hundreds of kilometres approximately westwards towards Gondwana. The Stavely Arc terrane and flanking terranes were tilted and thickened across a succession of thrust faults that appear to have propagated out of the back-arc, and combined to form an imbricate stack containing the series of volcanic (fault) belts of the Stavely Arc that comprise large areas of the bedrock within STAVELY.

Plan-view potential field datasets (e.g. regional magnetic data) are not appropriate to illustrate or constrain the retrodeformation of such structures. Section-view geophysics is appropriate and available as deep seismic reflection lines 97GA-V2, 97GA V1 and 09GA-AR1 and 09GA-SD1, but undertaking a full palinspastic retrodeformation of the effects of D1 was beyond the scope of this project. A schematic retrodeformation of the effects of D1 across western Victoria and incorporating the Grampians-Stavely Zone is presented as a schematic east-west cross section sequence in Figure 4.18, and in Figures 4.17 (1 – 3), and shows our estimate of the nature and degree of deformation imposed upon the entire Stavely Arc suprasubduction zone system in the Late Cambrian.

4.4 Implications for the architecture of the Stavely Arc through time

The structural history of STAVELY is complex and spans a wide range of geological time and events. Regional (e.g. Cayley & Musgrave, in prep) and local (e.g. Cayley & Taylor, 1997a, 2001; Miller et al., 2001, 2004, 2005, 2006) constraints have revealed that the structural architecture has evolved in response to two main stress regimes, imposed consecutively. To develop a predictive capacity for the present-day distribution of mineral systems within STAVELY it is critical to understand the effects of D4 on D1a structures to reconstruct:

1. The original configuration of the Stavely Arc when it was active in the Cambrian,

2. The configuration of the Stavely Arc following D1a when it deformed and uplifted against the east Gondwana margin, and

3. The configuration of the Stavely Arc at the time it was host to the last, mineralised pulse of magmatism in the Late Cambrian (D1b).

Although there is relatively limited surface expression, each of the three reconstructed fault belts of Stavely Arc rocks host mineral occurrences where exposed at surface. An important implication of this is that all (post-D4) volcanic belts may be prospective for arc-related mineral systems. However, it should be noted that mineral prospectivity may be localised to discrete regions, as seen in modern arc systems of comparable scale.

4.4.1 Original configuration of the Stavely Arc

Constraints on the original configuration of the Stavely Arc prior to D1 deformation are provided by the structural restoration described above, deep crustal seismic reflection data, and interpretation of the regional magnetic character of the restored northern Dimboola Belt.

As described above, volcanic (fault) belts of Stavely Arc rocks exposed at, or near, the present-data surface apparently emanate from a large, approximately triangular reflective body imaged by deep seismic reflection data in the middle and lower crust beneath the southern parts of the Grampians-Stavely Zone (Figure 2.22). This reflective body is interpreted to represent autochthonous parts of the buried Stavely Arc edifice, and suggests that the original Stavely Arc may have been linear, northerly-trending system well over 100 km wide and with over 10 km of vertical extent. Geometrical arguments indicate that this autochthonous body is likely to extend northwards to underlie the footwall of the Moyston Fault into the far reaches of the Grampians-Stavely Zone, but not be directly equivalent to the broad belt of igneous rocks in the northern parts of the Dimboola Belt.

The retrodeformed Dimboola Belt north of Horsham is likely to be para-allochthonous as it occurs west of the D1a Dryden North Belt, but the northernmost reaches of the Dimboola Belt are very wide and symmetrical in shape, and so the retrodeformed belt here may preserve original pre-D1a geometries. With the effects of D4 and D3 strike-slip segmentations and offsets removed, a series of large (40-90 km long, and 25-33 km wide), spaced, elliptical magnetic features can be interpreted within the Dimboola Belt (Figure 4.14). These may represent clusters of individual Cambrian magmatic complexes and are centred, from south to north, beneath Pimpinio, Perenna, the Big Desert, and Berrook (Figure 4.1).

The symmetrical, weakly deformed appearance of Stavely Arc rocks in the northern Dimboola Belt suggests three possibilities: 1: D1a fault (thrust) slices of the Stavely Arc have been eroded to expose the underlying in situ crust, or 2: the D1a deformation diminishes in intensity northwards within the Grampians-Stavely Zone, so that few D1a fault slices ever existed here, and the northern Dimboola Belt is the least deformed or 3: Stavely Arc igneous activity in the northern Dimboola Belt outlasted, and thus overprinted, the effects of D1a. In the latter two cases the northern parts of the Dimboola Belt may be highly prospective for D1b-related (mineralised) intrusions and related systems. Additional research is required to further test these possibilities.

Away from the effects of the D1a Escondida Fault, these retrodeformed elliptical bodies have outward radial-dips that can be modelled from potential field data. Westerly dips are predominant along undisrupted southwestern flanks, northeast dips are predominant along the northeastern flanks. These elliptical shapes may represent the eroded stumps of overlapping igneous complexes that combined to form the Stavely Arc. Their regular spacing of 65-85 km apart is typical of volcanic centre spacings in modern magmatic arc systems (e.g. Tamura et al., 2002), and has been related to slab-driven mantle wedge flow (eg. Lee & Wada, 2017).

Further south, such features appear to plunge to greater depths within the crust, so that the only igneous rocks exposed at surface are within the narrow linear D1a volcanic (fault) belts. Applying a volcanic centre spacing of 75-80 km for the restored southern parts of the Grampians-Stavely Zone predicts additional buried magmatic centres equivalent to those imaged in the Dimboola Belt beneath the Grampians Ranges, beneath Glenthompson, and beneath the Otway Basin close to Warrnambool. These may be the source of the triangular reflective packages imaged by deep seismic reflection data, and the origin of the fault belts of Mount Stavely Volcanic Complex now thrust to surface in the vicinity.

4.4.2 Influence of deformation on Cambrian intrusives including porphyries

The Lalkaldarno Porphyry and related intrusions dated at around 500 Ma (Lewis et al., 2015; Lewis et al., 2016; Schofield et al., 2018 Section 2.2 – 2.2 New geochronology constraints on the development and duration of the Stavely Arc), such as the informally-named Victor Porphyry at the Thursday’s Gossan Prospect, appear to have intruded and stitched a fault succession near Mount Stavely that contains fault slices of Williamsons Road Serpentinite lying within the interior of the Stavely Belt thrust sequence. This relationship implies that porphyry intrusion post-dated D1a thrust-imbrication and tilting of the Stavely Arc succession. Consequently, porphyry systems that intruded late in the Cambrian deformation history probably suffered little tilting or deformation during the Delamerian Orogeny. Their intrusion is interpreted to coincide with a change to sinistral transtension late in the Delamerian Orogeny, termed D1b. It is expected that mineralised intrusions emplaced during D1b would have exploited a combination of pre-existing D1a structures and cross-cutting D1b transtensional structures during their ascent through the crust. Thus the inter-plays that occur between D1a and D1b structures are likely to be an important consideration for directing exploration for intrusion-related mineral systems. No explicit D1b structures are included in the STAVELY 3D model, but a range of likely orientations and movement histories can be predicted from the stress-history estimated for D1b (see Figure 4.17-2). The Late Cambrian porphyries were subject to D4 deformation in the Early Devonian and so are expected to have been segmented by subvertical strike-slip, and low-angle dip-slip faults (as experienced by the Grampians Group).

Figure 4.14 Retrodeformed regional magnetic tilt and band pass filtered pseudocolour image of the northern Moornambool Metamorphic Complex, showing the pre-D4-D3 geometry of the northern Dimboola and Dryden North belts. With D4 and D3 fault displacements restored, the elongate domal nature and concentric, outward dipping stratigraphic layering in the northern Dimboola Belt is clearly apparent. Concentric magnetic stratigraphy surrounding two separate centres may reflect a sequence of igneous episodes in in-situ spaced magmatic centres of the Stavely Arc, subsequently flattened and elongated during D1.

4.4.3 Understanding the form and distribution of potential transfer structures

The intersections of ‘transfer’ structures – conjugate strike-slip fault networks that typically develop at normal or oblique angles to the principle axis of magmatism within active arc environments in compression – are often cited as preferred foci of mineralised magmatic intrusions introduced during episodes of extension (e.g. Cooke et al., 2005). Consequently, the identification of such structures is often prioritised during mineral exploration of ancient arc systems.

A key challenge inherent in understanding ancient arc terranes is taking into correct account the effects of subsequent deformation events, since younger faults misinterpreted as ‘transfer’ structures will misdirect mineral exploration efforts. This is an important consideration in STAVELY where D4 deformation has caused complex dismemberment and reorientation of Cambrian bedrock, for example in the Elliot Belt (Section 3.2.5.10 – Elliot Belt). The oblique orientation of these structures, which may be potentially confused with, or misinterpreted as, Cambrian ‘transfer’ structures is unrelated to the original geometry or structure of the active Stavely Arc.

The original axis of the active Stavely Arc probably trended northwest along the eastern Gondwana margin, inboard of the Gondwana-Palaeopacific plate boundary, across which sinistral-oblique convergence continued throughout the Cambrian (Cayley, 2011). In this style of convergence sinistral transfer structures developed during the early stages of the shortening of the arc in the Late Cambrian Delamerian Orogeny (D1a) would have been subvertical, trending west-northwest at an oblique angle to the arc axis, and would have been predominant over conjugate dextral structures of more westerly trend.

Transfer structures coeval with arc-magmatism are typically difficult to recognise in ancient terranes. This is because magmatism inherently tends to ‘burn out’ the very structures that control its emplacement. In addition, ancient arc terranes tend to be strongly deformed, with later deformations serving to obscure the context of early structures. Transfer structures often form as accommodating structures that separate evolving structural systems of differing character. Abrupt lateral changes in magmatic history, age, alignment and/or structural character are also often associated with transfer structures in modern arcs. Relicts of such characteristics can sometimes be preserved and discriminated in ancient arcs, including in the Stavely Arc.

Retrodeformation of D3 and D4 structures in STAVELY reveals a significant along-strike change in D1a structural style, with the southern end of the east-dipping Dimboola Belt appearing to change south into the northern end of the west-dipping restored Bunnugal-Elliot-Grampians ‘Deeps’-Brimpaen belt (Figure 4.15A). This abrupt change in structural transport direction during D1 and/or D2 within STAVELY requires separation across a transform fault, and so suggests the possibility of an early transfer structure in this position, active prior to, and during, D1a. A shallower structural level for the Stavely Arc to the north of this position versus the south is indicated by the much broader width of coherent arc stratigraphy exposed in the Dimboola Belt, and in extensions to this belt that extend north of STAVELY.

The structural style of D4 deformation also changes abruptly across this area, with block rotation and rifting to the south (the Mafeking Megakink and the Jalur Rift) changing northwards to strike-slip imbrication within the Dimboola Belt (Figure 4.15B).

All these changes imply the presence of a major pre-existing transfer structure, active prior-to and during D1a to influence Stavely Arc magmatism, to accommodate opposite thrust directions in the Dimboola and Bunnugal belts, and across which different style of structure developed during D4. The strike-persistence of the retrodeformed west-dipping D1a Stavely – Narrapumelap – Dryden – Dryden North belt along the eastern edge of STAVELY suggests that the transfer structure position may have been overthrust late in D1a. Reoriented fragments of former transfer structures would be expected to be incorporated into subsequent thrust-fault slices in such a scenario, and occasional oblique ‘cross’ faults confined within the Dryden Belt may be examples of this (Figure 4.16). Such relict ‘cross fault’ segments are expected to occur within many of the volcanic belts.

Possible D1a ‘transfer’ structure fragments may be identified by recognising changes in geology and/or its geophysical character along strike. Because transfer faults active during orogenesis typically develop to accommodate differential shortening between adjacent regions, they retain key features that make their recognition possible even in ancient terranes. The Stavely Belt contains considerable internal structural complexity, with disordered stratigraphy, and infaulted slivers of exotic material (see Section 3.2.5.16 – Stavely Belt). This complexity can be traced virtually unchanged through the Mafeking Megakink hinge (Narrapumelap Belt) and into the southern Dryden Belt, where regional magnetic data reveals a series of parallel fault slices of alternating magnetic and non-magnetic character. This pattern changes abruptly along strike between Barton and Jallukar, across a curved cross-fault that strikes orthogonally to the volcanic sequence and belt-bounding faults (Figure 4.16). North of this cross-fault the Stavely Arc stratigraphy within the Dryden Belt is structurally much simpler (Cayley & Taylor, 2001).

The profound disruptions in geology that are seen across this cross-fault within the Dryden Belt cannot be traced beyond the faulted margins of the belt into adjacent Cambrian geology. To the east the Moyston Fault shows no sign of disruption. To the west, magnetic trends within Glenthompson Sandstone likewise shows little disruption. This suggests that the cross-fault formed prior to the bounding Dryden and Mehuse faults, which indicate a pre- to syn-D1a timing for the structure. It is therefore a candidate for interpretation as a fragment of a former Cambrian transfer structure, possibly active during Stavely Arc magmatism and during D1a, and serving to separate domains of locally more intense shortening to the south, from domains of locally lesser shortening to the north.

In the northern part of STAVELY, along-strike changes in regional magnetic character that do not coincide with younger subvertical faults may reflect the influence of former transfer structures. Subsequent lateral offsets across structures such as the Escondida and Golton faults mean that it is highly unlikely that such structures will preserve any continuity today.

Figure 4.15 D1a thrust belts in the Grampians Stavely Zone, including arc-oblique D1 transfer structures that explain Stavely Arc thrust-system vergence reversals. A) Proposed restored geometry of the Stavely Arc at D1a culmination at ~500Ma. The arc flanks were overthrust from both fore- and back-arc sides by large thrust faults with mirrored geometry – the Moyston and Yarramyljup faults. The Stavely Arc was additionally internally imbricated by multiple thrusts. These also show a somewhat mirrored geometry. West-dipping thrusts are inferred to propagate from the footwall of the Yarramyljup Fault, and are predominant in the south. East-dipping thrusts (e.g. D1 Escondida Fault) are inferred to propagate from either the footwall of the Moyston Fault or as backthrusts from west-dipping thrusts such as the Dryden Fault, and are predominant in the north. The along-strike change in thrust fault dip predominance implies an intervening oblique transfer structure active during D1a to facilitate opposing thrust-transport directions. The distribution of restored fault belts suggests the transfer structure cut across the Stavely Arc axis at an oblique angle (red line). The proposed geometry matches the predicted position for sinistral transfer or cross-arc structures developed during sinistral transpression. Transfer or cross-arc structures are implicated as conduits for upwards fluid flow, and therefore as the loci of mineralisation. B) Proposed geometry of the Stavely Arc at the end of D4 at ~400Ma. Dextral-transtensional reactivation of the Moyston Fault precipitated transtensional segmentation of the adjacent Stavely Arc footwall. The initial, proximal response to D4 of extensional mega-kinking (Mafeking Megakink) appears to evolve with time into rifting (Jalur Rift) and then into dextral strike-slip faulting (the Dimboola Duplex within the Dimboola Belt and the Henty Fault system). This transition in structural style appears coincide with the position indicated for the D1a transfer structure, and therefore may reflect its reactivation, possibly as structures such as the Jalur Fault.

Figure 4.16 Possible transfer fault segment within the Dryden Belt. Shaded tilt and band pass filtered regional magnetic image, including key Dryden Belt locations. A major change in internal structural complexity occurs between Jallukar and Barton, where two or three west-dipping D1a strike faults within the belt at Barton are apparently truncated by an east-west trending north-dipping structure that is confined within the belt boundaries, with a narrower and simpler Dryden Belt sequence continuing north past Jallukar. With differential D1a shortening exhibited on either side, the east-west trending structure may be interpreted as a rotated remnant of a D1a subvertical ‘transform’. Other faults that cut the Dryden Belt margins and adjacent stratigraphy are interpreted as of younger (D4) age.

4.4.4 Influence of deformation on Devonian intrusive rocks

Extension and lithospheric thinning associated with the tectonic mode switch during D4 possibly caused the Early Devonian intrusive magmatism and related volcanic rocks in STAVELY. Magmatism would have been emplaced into extensional structures which, in a dextral transtensional stress regime, would be aligned northeast. This explains the northeast trend of the largest of the extensional faults interpreted to belong to the Marathon Fault and Victoria Valley Fault groups that segment the Grampians Group, and of other granites north of Mount Arapiles. Northeast elongation is therefore a possible indicator of Devonian-aged intrusions under cover, and for extensions of the Marathon Fault system that may locally bound the Grampians Group under cover.

4.5 Strain (Stress) history mapping

Systematic mapping of fault geometries, fault movements and fault movement histories, referenced against key exposed structures, has enabled the construction of a detailed strain-ellipse history that graphically illustrates a succession of differing strain conditions imposed onto the rocks of STAVELY (Figure 4.17). Each strain condition was formed in response to a differing imposed stress condition, with notable variations apparent that allow for clear discrimination between the stress and strain conditions that prevailed for D1a, D1b, and D2-D5 for STAVELY, spanning the Late Cambrian – Middle Devonian. The value of this strain history in 3D model construction lies in its ability to provide a systematic methodology for placing different fault movement histories into a wider tectonic context, particularly fault movement-senses (normal, reverse, strike-slip, dextral, sinistral etc.). Knowledge of the wider context allows structures that share common ages but different geometries to be grouped. Groups of mapped and interpreted faults can then be directly related to buried faults interpreted from geophysics or by other means. Where structures of similar age share orientations and overprinting criteria (relative age) they can be inferred to have experienced similar movement histories that are consistent with the strain-ellipse history. Such predictions inform interpretation of fault geometries and fault inter-relationships at regional scale, particularly at depth, and in locations where existing datasets do not provide direct control.

Figure 4.17 Stress history sequence for STAVELY (dashed borders depict present day area configuration for reference, based on the area of Figure 3.9; shaded areas depict past area configurations), showing the interpreted plan-view structural evolution from ~505 – 500 Ma (D1a) to ~ 380 Ma (D5).

Figure 4.17-1: ~505-~503 Ma – mid-D1a. The active Stavely Arc underwent sinistral transpressive crustal shortening along the east Gondwana margin during a phase of subduction trench-advance (black open arrows), the arc becoming deformed into a northerly-trending imbricate thrust-fault system. East of the Stavely Arc, the proto-Stawell Zone was thickened and uplifted as an accretionary wedge, thrust over the eastern flank of the arc along the Moyston Fault. West of the Stavely Arc, the proto-Glenelg Zone also experienced east-west shortening and thrusting – this zone is depicted as a static reference for the strain reconstruction. The principal shortening direction (1) for this time is constrained by the northerly trend and subhorizontal plunge of F1a fold axes in the Grampians-Stavely Zone, by northerly trending subvertical S1a slaty cleavage, and by stretching lineations (see Figure 3.58B) in high-grade D1 mylonites of the doubly-vergent Moornambool Metamorphic Complex in the Moyston Fault hangingwall (Q; Cayley & Taylor, 2001; Miller et al., 2005, 2006). The locations of key towns are also retrodeformed.

Figure 4.17-2: ~503-~500Ma – D1a culmination. Stavely Arc magmatism was in hiatus as shortening within the same D1a finite stress field reached its maximum. This is the end of crustal shortening associated with the Delamerian Orogeny. Retrodeformation of subsequent structures indicates that the Grampians-Stavely Zone was considerably narrower at this time compared to today, (depicted by shading relative to the static Glenelg Zone reference). Retrodeformed locations of key towns are also depicted.

Figure 4.17-3: ~500-~498 Ma – D1b. The end of Stavely Arc activity coincides with the onset of Macquarie Arc magmatism in a position outboard of Gondwana, implying an eastward jump in the position of convergence along the Gondwana-Palaeopacific plate boundary at ~498-495 Ma (Cayley, 2011). This jump is manifest in STAVELY as a tectonic mode switch to sinistral transtension, with translation directed towards the new Macquarie Arc position (see Figure 4.8 (pp 182) in Schofield et al., 2018). This switch appears manifest as transtensional reactivation of several structures in STAVELY (in bold), with associated formation of north-aligned rifts (in pink). Decompression of the Stavely Arc associated with transtension may have triggered generation of the final pulse of arc-related magmatism – Late Cambrian granites and mineralised porphyries. These are clustered and north-aligned, implying intrusion was into active distributed D1b rift centres. The magnitude of D1b fault and rift displacements is probably too small to represent at the scale of STAVELY – the Grampians-Stavely Zone may have widened slightly overall.

Figure 4.17-4: ~440 – 410Ma – D2, D3. The structural history of the Grampians-Stavely Zone between ~495 and ~440 Ma is poorly known – substantial erosion is required to explain the Cambrian crustal level that unconformably underlies the next-youngest geological event: Grampians Group deposition. A prolonged deformation and uplift/subsidence hiatus indicated in the Stawell Zone (Stavely Arc accretionary wedge), in the Stavely Zone, and in the Glenelg zone (Stavely Arc back arc) between ~495 and ~450 Ma demonstrates that local subduction beneath the Stavely Arc had completely ceased by ~490Ma and did not recommence. Localised Ordovician ‘A’-type magmatism (eg. Padthaway Ridge) is attributed to rift-controlled decompression melting in the active Ordovician Macquarie Arc back-arc. Grampians Group deposition (shaded) may have begun in the Late Ordovician, and was certainly substantially complete by the Late Silurian (Cayley & Taylor, 1997a; Retallack, 2009). Grampians Group is only known to the west of the Moyston Fault as a westwards-thinning succession (depicted by shaded arrows) along the length of the fault, implicating a foreland-basin depositional setting, consistent with footwall sag in response to reverse Moyston Fault reactivation. This event is attributed to D2, of Benambran Orogeny age. Reverse reactivation of the Moyston Fault at this time is consistent with structural data from west-dipping 440 Ma-aged convergent structures in the Stawell Mine (Q; Miller et al., 2006). D3 structures are those that overprint Grampians Group. The predominance of thrust and sinistral strike-slip components to some D3 faults (e.g. Grampians thrust faults; Latani Fault) implies that the transpressive D2 stress-field persisted into D3 (time-equivalent to the Bindian Orogeny). The main faults reactivated during D2 and D3 appear to have been the Moyston, Mehuse and possibly proto-Escondida faults. The magnitude of D2 and D3 fault displacements is probably too small to represent at project scale although the Grampians-Stavely Zone may have narrowed slightly overall.

Figure 4.17-5: ~405-395 Ma – D4. Deformation of the Grampians-Stavely Zone into virtually its present-day configuration. A large magnitude of southeast-directed dextral transtension in the Early Devonian (time-equivalent to the end of the Bindian Orogeny) reactivated the Moyston Fault, forming footwall fault splays that tore pre-existing D1, D2 and D3 Grampians-Stavely Zone structures apart. Predominantly dextral strike-slip faults segmented and locally rotated the Cambrian-aged volcanic belts of the Stavely Arc into the Mafeking Megakink, Dimboola Duplex and related structures. The Grampians Group was segmented, with portions collapsed into new structural basins such as the Jalur Rift. The D4 stress-history evolved through time, as shown by coeval structures at the Stawell Mine (Q; Miller et al., 2006). Decompression associated with transtension apparently triggered new melt generation leading to widespread granite intrusion and related eruption of the Rocklands Volcanic Group. These intrusions are clustered and north-east aligned, implying intrusion was into active distributed D4 rift centres.

Figure 4.17-6: ~395-380 Ma – D5. Minor reactivation of some Grampians-Stavely Zone structures during west-directed sinistral transpression – time equivalent to the Tabberabberan Orogeny. Major structures that appear to have reactivated at this time are the Mosquito Creek Fault and Golton Fault, which both preserve evidence of late sinistral – transpressional displacements. The Barbican and Cattle Camp thrust faults (Cayley & Taylor, 1997) may also have formed at this time. Early Devonian granites were jointed. The D5 stress-history is expressed in the Stawell Mine as late west-directed thrust faults (Q; Miller et al., 2006). Since stress is a scale-invariant vector, the stress – and strain – history insights gained from regional-scale mapping and interpretation of geophysical datasets and model-construction undertaken during this project can be downscaled to district, prospect and even drill hole-scale to inform on interpretations of fault movement history of intersected structures that have dismembered poorly-exposed mineralisation systems, based on their relative ages and orientations.

4.5.1 A scalable structural template for STAVELY through time

Cambrian D1a deformation is interpreted to reflect the culmination of collision, south of Victoria, of the microcontinent Vandieland into the subduction zone active along the east Gondwana margin (Cayley, 2011; Cayley et al., 2011b; Figure 4.18). Collision is interpreted to have shortened, and then extinguished, the Delamerian subduction system (Cayley, 2015). This deformation apparently involved oblique-sinistral convergence, so that a finite strain ellipse for sinistral transpression aligned to match a west-northwest-east-southeast principle shortening direction can be used to describe the types of D1a structures that are expected to have formed, at all scales.

The period that closely followed the culmination of D1a in the Late Cambrian apparently involved an abrupt change in regional stress regime from sinistral transpression to sinistral transtension (D1b). This is interpreted to be the result of a ‘tectonic mode switch’ that accompanied an abrupt shift in Gondwana-Palaeopacific plate boundary position to a more oceanward position (Cayley, 2015). This switch possibly triggered the pulse of Late Cambrian magmatism recorded in the rock record as the Bushy Creek Igneous Complex and associated porphyries. Therefore, a finite strain ellipse for D1b sinistral transtension, aligned to match a southwest-northeast principle extension direction can be used to describe the types of post-D1a Cambrian structures that might have accompanied, and controlled Late Cambrian granite and porphyry intrusion.

Extensional structures are implicated in the delivery of magmas towards surface and, in such D1b a sinistral transtensional stress regime, extensional structures would be aligned west-northwest. The regional geodynamics suggest that most D1b faults and rifts probably dipped towards the east and towards the location of ongoing plate convergence beneath the Macquarie Arc. This may explain the elongate northerly-trends of the Buckeran Diorite and Lalkaldarno Porphyry, of late dykes intruding larger Cambrian plutons (e.g. aplite into Bushy Creek Granodiorite) and may be a structural template that allows for the age interpretation of Cambrian intrusions under cover. For example, the intrusions along the western side of the Muirfoot Fault are aligned parallel to the D1b extension direction within the Black Range West/Mitre Belt, an orientation that implicates them as part of the Late Cambrian magmatic pulse. Similarly, the northwest elongation of buried intrusions interpreted east of Winian East may distinguish this intrusion as Cambrian and distinct from the larger north-east aligned intrusions to the south and west that are dated as Early Devonian (e.g. in VIMP1; Maher et al., 1997). Gravity lows of complex shape beneath the Rocklands Volcanic Group east of Balmoral are more difficult to interpret. It is possible that these could reflect northwest-oriented Cambrian intrusions, northeast-oriented Early Devonian granites, or reflect local sub-basins with a greater thickness of Grampians Group sediments.

Analysis of convergent structures in the Grampians Group and underground at Stawell (Miller et al., 2006) show that sinistral transpression continued to be intermittently important for the Grampians-Stavely Zone at different times in the Ordovician (D2) and Silurian (D3), pre-D4. Towards the end of the Silurian, the Grampians Group itself appears to have been shortened and thickened by D3 thrusting and folding, with northwest-directed sinistral transpression implicated (Wilson et al., 1992; Cayley & Taylor, 1997a; Miller et al., 2001), so that a finite strain ellipse for D3 sinistral transpression aligned to match a west-northwest to east-southeast principle shortening direction can be used to describe the types of D3 structures that are expected to have formed, at all scales. These events appear related to early far-field effects of the Bindian Orogeny.

A second important change in the finite strain ellipse for western Victoria occurred in the Early Devonian at around 400 Ma, with an abrupt mode switch in strain to dextral transtension (D4). This is the main D4 event in the Grampians-Stavely Zone, and appears related to Tasmanian geodynamics (Cayley, 2015) which precipitated dramatic dextral transtensional reactivation of the Moyston Fault. In consequence, extensional collapse of the Moyston Fault footwall led to the formation or reactivation of several footwall splay faults including the Golton and especially Escondida faults, as dextral transtensional/strike slip faults, megakinks and strike-slip duplexes.

The finite strain ellipse for late D4 dextral transtension predicts northwest-trending dextral strike-slip faults, northeast-trending extensional faults, and subordinate north-trending antithetic (sinistral) cross-faults and west-northwest-trending thrust faults. This is the stress-regime in which the Grampians Group and Stavely Arc (and any associated mineralisation) became dismembered along strike-slip faults and related oroclinal folds and kinks.

The practical aspects of recognising structures that have deformed the Grampians-Stavely Zone, including STAVELY, are provided in Duncan et al. (in prep).

Figure 4.18 Geodynamic section series for STAVELY. Sections are schematic, depicting geology between Kangaroo Island (west) – Padthaway – Balmoral – Ararat – Castlemaine – Heathcote – Avanel (east) Series presents a geodynamic scenario for Neoproterozoic – Devonian western Victoria, incorporating scenario’s proposed by Foden et al., 2006, Cayley, 2011; Cayley et al., 2011a; Cayley et al., 2011b; Gibson et al., 2015, Cayley et al., in prep. 1: Model commences in Neoproterozoic, with established Adelaide Rift System, including hyper-extended passive margin with Sub Continental Lithospheric Mantle (SCLM – e.g. Hummocks Serpentinite) exhumed to sea-floor, outboard rifted microcontinental ribbon(s), and an eastward transition to the Palaeo-Pacific Ocean. Site of future spontaneous nucleation of subduction at the continent-ocean boundary is marked. 2: Early Cambrian (pre-D1a). Oblique-sinistral subduction commenced along the Continent-Ocean Boundary, probably propagated into STAVELY from farther south. Stavely Arc initiation involved the eruption of juvenile mafic boninitic arc-magmas and opening of a back-arc basin (Kanmantoo Trough) with associated rift-magmatism (Truro Volcanics). This period marked the onset of the Delamerian Orogenic cycle. 3: Subduction matures (pre-D1a). Stavely Arc magmatism evolves to intermediate calc-alkaline compositions. Trench is in long-term retreat, with a low-relief, ‘Japan-Style’ arc and an extending back-arc basin locally experiencing decompression melting and associated high-temperature metamorphism and deformation and intrusion of subduction-related melts (Glenelg River Metamorphic Complex – GRMC; Kemp 2003). 4: D1a deformation. Slab-shallowing, possibly a consequence of the oblique approach of a buoyant exotic microcontinent (Vandieland) south along-strike, leads to trench advance and shortening and thickening of the upper plate. This is the time of stress history 1 and 2 (see Figure 4.17). The Stavely Arc is deformed, converted to ‘Andean-Style’ with suppressed arc magmatism. This event forms the major Cambrian convergent structures of western Victoria and is the culmination of the D1a (Delamerian) deformation event. The Kanmantoo Trough is inverted, with metamorphic components – GRMC – folded and imbricated. The Stavely Arc is fault-imbricated. A thick, cool accretionary wedge (proto-Stawell Zone) adjacent to a type-1 backstop (Moyston Fault) builds upon the shallowed Palaeo-Pacific slab accumulating its own, highly complex and polydeformed, structural history. Farther east, the outer parts of the wedge remain undeformed and in an open oceanic setting (proto-Bendigo Zone). 5: D1b deformation. Collision of the Vandieland microcontinent into the subduction zone south of Victoria (e.g. Cayley, 2011) congests the subduction zone, and stalls local plate convergence, extinguishing the Stavely Arc. Plate rearrangement leads to the development of a new subduction zone farther outboard – the Macquarie Arc. This new subduction zone releases convergent stress within the Delamerian Orogen, causing a tectonic mode switch to sinistral transtension and triggering a pulse of pent-up arc-magmatism derived from the fertile mantle wedge (Cayley, 2015). This is the time of stress history 3 (see Figure 4.17). This is the porphyry-intrusion event. 6: D2 deformation. Throughout the Ordovician, the Vandieland microcontinent is drawn northwards towards the Macquarie Arc trench, eventually arriving into the plane of the depicted cross-section at around 440 Ma. During this time, oblique convergence between Vandieland and Gondwana shortens and imbricates the intervening Bendigo Zone trapped plate segment into a fold-and-thrust belt, piggy-backing the Stawell Zone in the process (Cayley et al., 2011a). This is the time of stress history 4 (see Figure 4.17). Orogenic gold deposits form at this time. Rejuvenation of Stawell Zone shortening reactivates the Moyston Fault to form the Grampians Basin as a foreland basin in the fault footwall – this is D2 in STAVELY, which eventually evolves into D3. 7: At the end of the Silurian, the Grampians Group is structurally thickened during the culmination of complex transpression (D3), followed by a discrete period of D4 dextral transtension, expressed in the Cambrian bedrock of the Grampians-Stavely Zone as transtensional reactivation of the Moyston Fault and related formation of the angular, dextral Mafeking Megakink, rifts such as the Jalur Rift, and strike-slip fault duplexes. This is the time of stress history 5 (see Figure 4.17). Transtension precipitated a pulse of decompression melting preserved in the rock record as large granite bodies and associated voluminous pyroclastics and other volcanics (Rocklands Volcanic Group) intruded into rift centres. This event brings to a close the complex tectonic evolution of the region. D5 is a minor subsequent event (not illustrated separately).

4.5.2 Summary

The volcanic belts exposed within the interior of STAVELY appear to represent relatively narrow (~2-5 km), steeply west-dipping fault-slices of the middle to upper Stavely Arc stratigraphy, thrust to surface over the western flank and crest of a large arc edifice that is deeply buried in the south. To the north, the main thrust slice that contains the Stavely-Narrapumelap-Dryden-Dryden North and Dimboola belts appears to thicken to become the predominant fault-slice of Stavely Arc stratigraphy that underlies the Murray Basin – the internal structure of the northernmost parts of this belt appears to preserve the eroded stumps of primary igneous structures that we relate to Stavely Arc formation.

Although the Grampians-Stavely Zone is internally structurally complex, its overthrust boundaries are relatively simple and subparallel in strike, and serve to form the limits to the region most prospective for minerals systems related to the Stavely Arc.

The underlying autochthonous magmatic Stavely Arc edifice, and a possible continental ribbon foundation of Proterozoic crystalline basement upon which it rests, appears to have acted as a structural buttress that has controlled the imbricate thrust style of D1a deformation within the Grampians-Stavely Zone, and has deflected the more intense Cambrian deformation in the adjacent Glenelg and particularly Stawell zones. Post-Cambrian deformations (D2, D3, D4) have involved deformations largely confined to the horizontal plane, so that the upper levels of the Stavely Arc system that survived D1 deformation have been able to persist to the present day. The post-Silurian tectonic stability of the Grampians-Stavely Zone means that Grampians Group and other younger cover rocks have not been completely removed by subsequent uplift and erosion.

The high degree of structural complexity within the Grampians-Stavely Zone is a consequence of two main orogenic events. The first of these is represented by the Late Cambrian Delamerian Orogeny, the last phases of which are expressed within the Grampians-Stavely Zone as D1a structures formed as a consequence of east-west oblique shortening. The second significant deformation event, a late part of the Bindian Orogeny, is expressed within the Grampians-Stavely Zone as D4, and includes complex Siluro-Devonian deformation of the Grampians Group (Cayley & Taylor, 1997a; Miller et al., 2001) and, in the underlying Cambrian bedrock, complex second-generation transtensional structures that have dismembered, offset, refolded and tilted Cambrian structures to varying degrees.

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Appendix 1 STAVELY 3D model export

STAVELY 3D model formats

The STAVELY 3D model has been exported in five separate formats intended to facilitate importing into as many software packages as possible. There is no one format that can facilitate transfer of geoscience data to all software packages. For example, DXFs can be exported as points, curves and surfaces, but not voxets (block models). Therefore, many of the formats described below, only enable export of components of the geoscience data contained in the original GOCAD® project. The following headings correspond to the folder structure in Appendix 1 of the STAVELY 3D model release.

DXF

The DXF file format was independently tested with the successful importation of points, curves and surfaces into Leapfrog, SurpacTM, Vulcan, and Micromine, and we believe it to be the most portable file format for geoscience software packages. The DXF file format enables import of all the surfaces which are the primary focus of the STAVELY 3D model. Note that it is not possible to export DXF files as voxets (block models). Therefore, the DXF folder is restricted to points, curves and surfaces.

GOCAD®

All objects within the STAVELY 3D model have been exported using the GOCAD® native format (.vs, .pl, .ts, .vo etc). The advantage of this format is that the export files are all text files which can be manipulated and should allow import into any software package.

Geoscience ANALYST Project

Geoscience ANALYST is a free 3D visualisation software package which enables users to import, visualise, annotate, save and distribute 3D models and data (http://www.mirageoscience.com/our-products/software-product/geoscience-analyst). The STAVELY 3D model has been exported in Geoscience ANALYST format, which enables users who do not have access to commercial 3D software to visualise and interrogate the model. Geoscience ANALYST also has functionality to import a large variety of other geoscience formats, which effectively facilitates visualisation with company in-house datasets.

The other advantage of the Geoscience ANALYST file format (geoh5) is that it is based on free and open source HDF5 (https://gist.github.com/jincandescent/06a3bd4e0e54360ad191). Because this format is public, and has a list of other advantages (see link above) the file format could become a useful exchange format for industry.

GOCAD Project

The STAVELY 3D model was built using GOCAD®, and the original GOCAD® project has been included here. The GOCAD® project contains all components of the STAVELY 3D model.

STAVELY 3D voxets (block model)

It is problematic to import GOCAD® voxets (block models) into other software packages. Voxets (block models) cannot be exported as a DXF and the native format (.vo) is not always user friendly. We considered that the STAVELY voxet (block model) was an important component for visualising the STAVELY 3D model and wanted to provide another option – a text file – for this to be imported into other software packages.

The STAVELY voxet (block model) is made up of a large number of cells which have been divided into regions. Each region is intended to represent a different geological unit. To be able to visualise these regions when the voxet (block model) is imported into another software package, they need to be identified. Therefore, each region in the STAVELY voxet (block model) was given a numeric identifier (see Microsoft® Excel table in the “3D block model” folder). The STAVELY voxet (block model) was then converted into a set of points (retaining the numeric identifier), and exported as a .csv file. The .csv file can be converted back into a block model with the accompanying table of parameters, and the regions (which enable visualisation of the various geological units) can be initialised using the identifier associated with each point. The STAVELY voxet (block model) is large (14 million cells), however an import using this workflow has been tested successfully.

A1.2 3D Model Contents

The table below shows a list of all the objects within the STAVELY 3D model and what format these objects have been exported as. Pale blue highlight indicates groups of objects.

GOCAD® Geoscience GOCAD® DXF .csv Project ANALYST

Points Towns     Drillholes    

Curves Belt interpretations     Fault interpretations     Constraining curves    Border Coastline Structural Zones     Stavely-Sections     Stavely Project area     Roads     Intrusives    

Surfaces Model surfaces Intrusives     Faults     Magnetic inversion bodies     Base arc surf     Base Kanmantoo Group surf     Moho surf     Murray bsmt mag bore     Newer volcanics base     rocklands volcanics base     Strat otway basement seebase     top arc surf     Zone54 SRTM gda94mga54 90m cut   

Voxets Model voxet 1 stavely block model    

Dataset images Gravity cba267 1200dpi    Gravity cba267-HP15000 1200dpi    Gravity cba267-HP30000 1200dpi    Gravity HP30km-RTP intensity 1200dpi    Magnetics-filtered Tilt-BP 1200dpi    Magnetics-TMI 1200dpi    Radiometrics RGB 1200dpi    Seamless-geology 250k 1200dpi    SRTM 1200dpi   

Other Gravity forward models    Cross section voxets    Stavely section    Seismic sections  

Appendix 2 Geological cross sections

Appendix 2 contains the individual serial geological cross sections used to construct the STAVELY 3D model. The serial geological cross sections have been georeferenced and are available within the GOCAD® project.

Appendix 3 Forward model sections

Appendix 3 contains the forward models of the individual serial cross sections used to construct the STAVELY 3D model. The forward models of the individual serial cross sections have been georeferenced and are available within the GOCAD® Stavely Project.

Appendix 4 Geological units

The following is a summary of the geological units within STAVELY. Geological units are presented in chronological order. Some of the descriptions have been adapted from Schofield et al. (2018) with additional detail and references relevant to the STAVELY 3D model.

Upper mantle

Upper mantle rocks (below the Moho) in western Victoria are modelled as Proterozoic peridotite. These rocks are likely direct equivalents of the olivine-orthopyroxene-chrome spinel mantle rocks that have been faulted up into the directly overlying crust as the Hummocks Serpentinite (see below) and as related unnamed pyroxenites and metamorphic derivatives mapped in the Glenelg Zone.

Peridotites in western Victoria apparently range in composition from pyroxenite (typical density 3.1 – 3.6 g/cm3) as exposed in some Glenelg Zone fault zones (Morand et al., 2003), to dunite (typical density 2.84 – 2.85 g/cm3) as seen in olivine-dominated peridotite xenoliths carried to surface from the upper mantle by nearby Newer Volcanic Group eruptions (e.g. Mt Shadwell, near Mortlake in southern STAVELY).

The average density used for upper mantle rocks in the STAVELY 3D model is 3.25 g/cm3. This reflects the likely composition that is equivalent to the Hummocks Serpentinite, in combination with the other end-member peridotite compositions known to occur beneath the region.

The Proterozoic age assigned to the upper mantle beneath STAVELY is based on field relationships which suggest a Neoproterozoic age for the Hummocks Serpentinite in the western Glenelg Zone, and on geophysical evidence for a possible old microcontinental ribbon that forms the lower crust of the Grampians-Stavely Zone, the foundation upon which the Stavely Arc was formed. This buried ribbon of continental crust apparently rifted from the eastern edge of Paleoproterozoic – Mesoproterozoic ‘Gawler Craton’ crust along the east-dipping Apsley Fault in the Neoproterozoic, probably during Rodinia breakup.

Hummocks Serpentinite

The Hummocks Serpentinite (Wells, 1956) crops out in several places in the western, lower-grade parts of the Glenelg Zone as a fibrous to platy ultramafic rock dominated mineralogically by antigorite with subordinate chrysotile veins. Magnetite and magnesite give the rock a very high magnetic susceptibility. These low-grade metamorphic minerals pseudomorph a coarse-grained cumulate texture, interpreted to have originally been olivine, orthopyroxene and chrome spinel (Turner et al., 1993). Within the Glenelg River Metamorphic Complex, metamorphosed ultramafic rocks are magnesian talc-chlorite schist with a lower magnetic susceptibility. Pyroxenite also occurs locally.

Occurrences of ultramafic Hummocks Serpentinite and related ultramafic rocks (e.g. talc schist, pyroxenite; see Morand et al., 2003) in fault-zones within the Glenelg Zone are incorporated into serial cross sections 6, 7 and 8 used to construct the STAVELY 3D model (see Appendix 2 – Geological cross sections). Hummocks Serpentinite density values modelled range from 2.79 – 3.0 g/cm3, consistent with the higher-end range of densities of variously serpentinised or otherwise metamorphosed Late Precambrian-Early Paleozoic ultramafic rocks measured in Victoria. The use of higher-end density values is appropriate because the high reflectivity of these rocks imaged in deep seismic reflection data (e.g. Line 09GA-SD1) indicates average density values that are significantly higher than adjacent Glenelg Zone Metamorphic Complex rocks (pelitic schist, gneiss and granite with a measured and modelled density range of 2.69-2.87 g/cm3).

Serpentinite and related ultramafic rocks that are geochemically and petrologically similar to the Hummocks Serpentinite occur as exotic rocks hosted by fault zones within volcanic belts of the Stavely Arc in the Grampians-Stavely Zone, for example: Williamsons Road Serpentinite in the Williamsons Fault in the Stavely Belt, serpentinite in the Escondida Fault in the Dimboola Belt (intersected by stratigraphic drill holes STAVELY10 and VIMP6), serpentinite and peridotite in the Boonawah Belt (e.g. mineral exploration drill hole PRC-04), serpentinite at the base of the Dryden Belt (Fryingpan Prospect), serpentinite in the Dryden North Belt (stratigraphic drill hole VIMP6). These occurrences appear to lie stratigraphically beneath the Stavely Arc succession (e.g. in the Dimboola Belt, and in the Dryden Belt), or have been thrust into the interior of the arc succession during D1a. These slices are interpreted as Hummocks Serpentinite or equivalent. Highly conductive magnetotelluric anomalies at the base of the Stavely Arc edifice in the mid crust are coincident with a more reflective seismic package and suggest these mafic-ultramafic rocks might be widespread at depth across STAVELY (Robertson et al., 2015).

Serpentinites and related rocks that occur in the Grampians-Stavely Zone are not discriminated as separate volumes in the STAVELY 3D model – most occurrences of serpentinite are too small to be captured at the scale of the model. They are included in the ‘MSVC mafic’ units in the serial cross sections, and incorporated into the generalised Mount Stavely Volcanic Complex volumes in the STAVELY 3D model.

The maximum age of the Hummocks Serpentinite is not well constrained, but a minimum age of Neoproterozoic is implied by nearby metasedimentary rocks that are intruded by dykes and sills dated at approximately 650Ma (Morand & Fanning, 2006, 2009). Although local field relationships are now obscured by the effects of the subsequent Delamerian Orogeny, we consider the Neoproterozoic metasedimentary rocks to have been deposited upon a substrate that included Hummocks Serpentinite equivalents, since the Hummocks Serpentinite has a peridotitic geochemistry consistent with a lithospheric mantle origin, and so most likely represented the local basement to basin sedimentation. This implied age constraint is consistent with Sm/Nd geochronology directly obtained for the Hummocks Serpentinite at its type locality, however the Sm/Nd age of approximately 700 Ma (Turner et al., 1993) is a model age and so has unquantified uncertainty (possibly ± 200 Ma; Foden pers. comm. in Morand et al., 2003).

The Hummocks Serpentinite has been interpreted to represent lithospheric mantle exhumed by hyperextension in the Neoproterozoic related to Rodinia breakup and opening of the Palaeo-Pacific Ocean (Gibson et al., 2015). Such an origin explains the widespread distribution of similar rocks within fault zones that span the Glenelg Zone (Morand et al., 2003), that can be traced down to the mid-crust in deep seismic reflection data (Cayley et al., 2011b), and also occurs in fault zones within the Grampians-Stavely Zone – e.g. the Williamsons Road Serpentinite (Buckland, 1987; Stuart-Smith & Black, 1999).

Whether all the serpentinites represent fragments of exhumed mantle (as interpreted for the Hummocks Serpentinite; Turner et al., 1993; Gibson et al., 2015) or are residual ultramafic cumulate magma chambers associated with the Cambrian subduction (e.g. VandenBerg et al., 2000; Crawford et al., 2003) is yet to be definitively established. However, investigation of serpentinised ultramafic rocks in the Grampians-Stavely Zone undertaken as part of the Stavely Project showed that all rocks examined are petrographically similar (Bailey et al., 2016). Analyses of primary chrome spinels from these rocks show highly refractory compositions (e.g. high Cr#), reflecting a depleted mantle origin (Bailey et al., 2016). Therefore, we consider it possible that all these serpentinites, in both the Glenelg and Grampians-Stavely zones, are part of the same exhumed hyperextended mantle unit that has been segmented into several discrete slices during subsequent thrusting associated with the Delamerian Orogeny.

Retrodeformation of D4 structures in STAVELY (see Section 4.1 – D4 and D3 retrodeformation testing) suggests that many of the disparate serpentinite occurrences in STAVELY may represent segments of originally more continuous and strike-persistent D1a fault-slices. Structural restorations show that serpentinites in the Stavely, Narrapumelap and Dryden belts may have been contiguous prior to D4 (see Figure 4.3). Similarly, serpentinites within the Glenisla Belt may correlate along-strike with serpentinites in the Boonawah Belt further south.

Unaffiliated unreflective rocks

The Proterozoic age and rifted continental character assigned to an elongated, upward-convex ribbon of unreflective crust buried deeply beneath the Stavely Arc in STAVELY is based on its uniform unreflective appearance in deep seismic reflection data (a characteristic of sialic crystalline rocks), its density (Murphy et al. 2006), its geometry, its tectonostrati-graphic position, and the continental geochemistry of the overlying Stavely Arc which demands a continental margin setting for the arc in the Cambrian.

The western side of this buried continental ribbon is directly underlain by the Apsley Fault (see Section 3.3.1.1 – Apsley Fault), the footwall of which comprises highly reflective rocks that thicken westwards and upwards, passing into unreflective crust beneath the greater Glenelg Zone farther west that is comparable in appearance and context to the crust beneath the Stavely Arc (Cayley et al., 2011b). The rocks that form the footwall to the Apsley Fault lie deeply buried beneath the Glenelg Zone, but are visible in deep seismic reflection data and can be traced near-continuously towards a surface position the Fleurieu Peninsula in South Australia, where inliers of Paleoproterozoic crust are exposed in the interior of the Adelaide fold and thrust belt. The Gawler Craton is directly observed to unconformably underlie the Adelaidean succession in South Australia, and the upper parts of the Adelaidean succession have now been traced into the Glenelg Zone of western Victoria. For these reasons, we interpret the crust in the Apsley Fault footwall, and beneath the Stavely Arc as Paleoproterozoic – Mesoproterozoic ‘Gawler Craton’ continental crust.

The Apsley Fault has the appearance of a partially-inverted extensional fault, which suggests that the buried unreflective crust beneath the Stavely Arc may represent a rift-fragment pulled off the top of the ‘Gawler Craton’ crust that forms the Apsley Fault footwall. The apparent Neoproterozoic age and mantle character of intervening peridotite rocks exposed in the Glenelg Zone (Hummocks Serpentinite) suggests that this rifting was of lithospheric scale and occurred in the Neoproterozoic. Hyper-extension related to Neoproterozoic Rodinia breakup is therefore implicated (Gibson et al., 2015).

Continental breakups often result in microcontinental ribbons, and this is the interpretation we favour for the unreflective rock packages beneath the Stavely Arc edifice. The lower crust of the Grampians-Stavely Zone is likely to share the same rock properties as the uppermost crust lying in the footwall to the Apsley Fault further west in such a scenario. Petrophysical properties can thus be estimated from direct examination of exposed Paleoproterozoic-Mesoproterozoic Gawler Craton rocks in South Australia. An assumed average density of 3.1 g/cm3 has been used for unaffiliated unreflective rock packages in the STAVELY 3D model.

Unaffiliated highly reflective rocks

The rocks that form the footwall to the Apsley Fault lie deeply buried beneath the Glenelg Zone and beneath the western Grampians-Stavely Zone, but are visible in deep seismic reflection data and can be traced westwards towards a near-surface position the Fleurieu Peninsula in South Australia, where inliers of Paleoproterozoic crust are exposed in the interior of the Adelaide fold and thrust belt. The basal few kilometres thickness of this package is highly reflective, and can be traced in the footwall of the Apsley Fault beneath STAVELY. The higher reflectivity reflects a significantly higher velocity (i.e. density) for these rocks compared to exposed sialic Gawler Craton rocks. Increased density indicates a more mafic chemistry, likely comparable to the known mafic-ultramafic igneous successions of similar reflectivity that occur within fault-slices in the overlying Glenelg Zone. A range of density values (2.85-3.1 g/cm3) have been used to model the highly reflective rock packages that occupy the footwall of the Apsley Fault in STAVELY.

Glenelg Zone

The Glenelg River Metamorphic Complex (Wells, 1956) occupies parts of the Glenelg Zone in southwestern Victoria, and extends northwest beneath the Murray Basin before re-emerging hundreds of kilometres northwest in the Palmer-Springton region of South Australia (Sandiford et al., 1990). The Glenelg Zone, and the Glenelg River Metamorphic Complex within it, bounds the western flank of the Grampians-Stavely Zone along the Yarramyljup Fault. Thus, the Glenelg Zone defines the western flank of the STAVELY 3D model.

The Yarramyljup Fault is regarded as the effective western edge of the search-space for Cambrian arc-hosted mineral systems. Thus, the Glenelg Zone geology has not been internally differentiated within the STAVELY 3D model, other than fault-slices of ultramafic rocks (correlates of the Hummocks Serpentinite – see below) which have been incorporated into some of the serial cross sections. The Glenelg Zone was modelled using a range of averaged density values (2.69 – 2.87 g/cm3) for the rock successions.

The Yarramyljup Fault dips west, so that Grampians-Stavely Zone rocks extend west in the fault footwall, beneath overthrust Glenelg Zone rocks. The STAVELY 3D overlaps the surface position of the eastern Glenelg Zone boundary to illustrate this relationship.

Rock units in the Glenelg River Metamorphic Complex are derived from a suite of sedimentary and igneous rocks, including Proterozoic sediments and igneous rocks (including the Hummocks Serpentinite), and Early-Mid Cambrian sedimentary (Normanville Group and Kanmantoo Group) and igneous (Truro Volcanics; see Morand et al., 2003) rocks. These units can all be observed in outcrop in the low-grade (greenschist) parts of the Glenelg Zone near the South Australian border.

Metamorphosed correlates in the Glenelg River Metamorphic Complex include pelitic, quartzo-feldspathic and psammitic schist, calc-silicate, amphibolite, talc schist and, in the highest-grade parts of the complex, migmatite and diatexite. The Glenelg Zone is intruded throughout by abundant granite plutons, ranging from Middle-Cambrian – Early Ordovician in age. All these rocks are very distinctive in outcrop, and are easily recognised in drill core.

Low-pressure, high-temperature style of metamorphism (peaking at ~0.4-0.6 GPa and ~650-660 °C; Kemp, 2003; Morand et al (2003 ,.affected the Glenelg River Metamorphic Complex in the Middle Cambrian. The high temperature gradient is characteristic of a supra-subduction zone setting (e.g. Miller et al., 2005). Such a setting for the Glenelg Zone is further corroborated by: (1) an abrupt change in palaeogeography in the Early Cambrian from a long-standing shoaling passive margin succession (Adelaide Rift succession – including the Heysen Supergroup; Preiss, 1982) to an abruptly deepened basin succession with rejuvenated mafic magmatism (Stansbury Basin, marked by eruption of the Truro Volcanics) and: (2) minor boninitic/sanukitoid melt components with unambiguous supra-subduction zone geochemistry incorporated into some Middle Cambrian granites within the Glenelg River Metamorphic Complex interior (e.g. Kemp, 2003, 2004; Foden et al., 2006). For these reasons, we interpret the Glenelg Zone as the deformed remnants of a Neoproterozoic-Cambrian passive margin that evolved into a back-arc basin, beginning in the Early Cambrian. At that time, the back-arc basin lay some distance to the west of the main Stavely Arc complex (see Schofield et al., 2018). Subsequent crustal shortening in the Late Cambrian (D1a) internally thrust-imbricated both the arc and back-arc successions, with the imbricated back-arc overthrusting the western flank of the Stavely Arc along the Yarramyljup Fault at the culmination of D1a.

The Yarramyljup Fault juxtaposes some of the highest-grade rocks of the Glenelg River Metamorphic Complex rocks against low-grade (sub-greenschist) Cambrian rocks of the Grampians-Stavely Zone (e.g. Nargoon Group). This relationship indicates that movement along the Yarramyljup Fault during D1a Delamerian orogenesis outlasted and overprinted peak metamorphism, and that the Glenelg Zone experienced considerably more uplift and erosion during and post D1a.

Stawell Zone

Moornambool Metamorphic Complex

The Moornambool Metamorphic Complex (Cayley &Taylor, 2001) is a wedge of fault-intermixed metamorphosed Cambrian turbidites (Saint Arnaud Group, metamorphosed to biotite, muscovite-sillimanite/muscovite-garnet-staurolite schists: Lexington Schist; Good Morning Bill Schist) and mafic volcanic rocks (Magdala Basalt, metamorphosed to hornblende-garnet-diopside bearing Carrolls Amphibolite) that comprises the western margin of the Stawell Zone, forming a region that is approximately 15 km wide at surface, but likely narrows in width with depth. The Stawell Zone, and the Moornambool Metamorphic Complex within it, bounds the eastern flank of the Grampians-Stavely Zone along the Moyston Fault. Thus, the Moornambool Metamorphic Complex defines the eastern flank of the STAVELY 3D model.

The Moyston Fault dips moderately to steeply east, so that Grampians-Stavely Zone rocks extend east in the fault footwall, beneath overthrust Moornambool Metamorphic Complex rocks. The STAVELY 3D model overlaps the surface position of the Moyston Fault to illustrate this relationship.

The Moyston Fault is regarded as the effective eastern edge of the exploration search space for Cambrian arc-related mineral systems – the rocks further east have allochthonous accretionary wedge characteristics. Thus, the Moornambool Metamorphic Complex has not been internally differentiated within the STAVELY 3D model. The Stawell Zone volume was modelled using averaged density (2.68 – 2.91 g/cm3) and magnetic susceptibility values for the rock successions exposed within it (summarised individually below). Further east, the Moornambool Metamorphic Complex passes into low-grade metasediments (turbidites) of the Saint Arnaud Group across the Coongee Fault (Cayley & Taylor, 2001). Saint Arnaud Group turbidites of lower density than their metamorphosed equivalents are modelled in serial cross sections 5 and 6.

The Moornambool Metamorphic Complex consists of greenschist to amphibolite grade metamorphic rocks in a tectonic melange. Mylonite to cataclasite textures are ubiquitous, reflecting high grades of metamorphism combined with strong deformation adjacent to major thrust faults that were active during and after metamorphism. The highest grade and most deformed rocks lie adjacent to the Moyston Fault (Cayley & Taylor, 2001). All these rocks are very distinctive in outcrop, and are easily recognised in drill core (e.g. STAVELY01). The age of metamorphism in the Moornambool Metamorphic Complex is not directly dated, but the unroofing and cooling of the complex is broadly defined by Ar/Ar cooling ages from approximately 508-490 Ma in metamorphic minerals within the complex (Miller et al., 2005; Phillips et al., 2012). These Cambrian ages indicate that western Stawell Zone orogenesis was coeval with Stavely Arc magmatism and D1 (Delamerian) deformation (Miller et al., 2005), cooling thereafter.

The pressures accompanying metamorphism are the highest seen in the Tasmanides, with maximum temperatures and pressures of approximately 600°C and 0.7-0.8 GPa (Radojkovic, 1989; Phillips et al., 2002) in the Moyston Fault hangingwall (Cayley & Taylor, 2001). This is a ‘Barrovian’ style of high-grade metamorphism characteristic of an uplifted, colder, amagmatic accretionary wedge setting (e.g. Miller et al., 2005).

Such a setting for the Stawell Zone is further corroborated by: (1) oceanic Cambrian constituent rocks, comprising a combination of MORB (and BABB and intraoceanic arc) mafic basalts and related ultramafic rocks overlain by deep marine black shale and turbidites: (2) the melange-style of deformation within the Moornambool Metamorphic Complex, which is characteristic of the progressive off-scraping that occurs in the lower levels of an accretionary prism above a subducting ocean slab, and: (3) the wedge-geometry of Cambrian faults developed within the Moornambool Metamorphic Complex and between the complex and the Stavely Arc. This matches the geometry of the most-common ‘type 1’ backstop fault systems that develop at the interface between an arc and its related accretionary wedge (e.g. Taylor & Cayley, 2000; Cayley & Taylor, 2001; Cayley et al., 2011a).

For these reasons, we interpret the Moornambool Metamorphic Complex as a Cambrian accretionary wedge, developed along the eastern, outboard flank of the Stavely Arc when it was active in the Cambrian (see Cayley et al., 2011a; Schofield et al., 2018). Although it probably formed progressively throughout the evolution of the arc, Moornambool Metamorphic Complex growth possibly accelerated when the Stavely Arc became shortened and imbricated during the Late Cambrian (D1a) crustal shortening event, which we relate to a period of trench advance, likely associated with flat-slab subduction. The thickened accretionary wedge overthrust the eastern flank of the Stavely Arc along the Moyston Fault at the culmination of D1a.

The metamorphic conditions of the highest-grade Moornambool Metamorphic Complex rocks exposed in the Moyston Fault hangingwall reflect exhumation from depths of 20-25 km (Miller et al., 2005) and constrain a large (~15-20 km) vertical (east-over-west) displacement across the Moyston Fault plane during D1a.

The Moyston Fault juxtaposes the highest grade Moornambool Metamorphic Complex rocks against low-grade (sub-greenschist) Cambrian rocks of the Grampians-Stavely Zone (e.g. Glenthompson Sandstone, Mount Stavely Volcanic Complex). This relationship indicates that movement along the Moyston Fault during D1a (Delamerian orogenesis) outlasted and overprinted peak metamorphism, and that the Moornambool Metamorphic Complex experienced considerably more uplift and erosion during and post D1a.

Magdala Basalt (Carrolls Amphibolite)

Tholeiitic lavas and mafic volcanogenic sedimentary rocks of the Magdala Basalt, and high-grade metamorphic derivatives (Deenicull Schist, Carrolls Amphibolite), are confined to the Stawell Zone, and are exposed within the Moornambool Metamorphic Complex. They are fault-intercalated with pelitic Saint Arnaud Group and pelitic metamorphic rocks. These rocks have not been discriminated in the STAVELY 3D model, since they are located outside the Grampians-Stavely Zone, but their geophysical properties have been combined with pelitic rocks to form a Moornambool Metamorphic Complex volume in the STAVELY 3D model.

The Magdala Basalt comprises massive and pillowed basaltic lavas, volcaniclastic interflow sediments and minor chert deposited within a submarine sea floor setting (Cayley & Taylor, 2001; Crawford et al., 2003). Some of these rocks are geochemically similar to basalts emplaced in a back-arc setting (Crawford et al., 2003; Squire et al., 2006). Others have a typical MORB geochemistry.

The age of the Magdala Basalt is poorly constrained. A high uncertainty Pb/Pb model age obtained for the Magdala Basalt yields an estimated age for extraction from the mantle of 518 ± 52 Ma (Crawford et al., 2003). Compositionally-similar basalts in the Heathcote Volcanic Group in central Victoria (Crawford and Keays, 1978; Crawford, 1982; Crawford and Keays, 1987; VandenBerg et al., 2000; Crawford et al., 2003; Squire et al., 2006) have a well-constrained Lower Cambrian age from trilobite fossils (VandenBerg et al., 2000).

Saint Arnaud Group (Lexington Schist, Good Morning Bill Schist)

Low-grade Saint Arnaud Group metasediments occur east of the Moornambool Metamorphic Complex, and are modelled with average density values of 2.68 – 2.79 g/cm3 in serial cross sections 5 and 6. The petrophysical characteristics of high-grade metamorphic derivatives of the Saint Arnaud Group within the Moornambool Metamorphic Complex are combined with the fault-intercalated Carrolls Amphibolite to produce a generalised volume for the Moornambool Metamorphic Complex in the STAVELY 3D model.

The Saint Arnaud Group consists of thick, unfossiliferous, turbidite sequences of sandstone and mudstone and minor black shale that were deposited in a deep marine environment, probably on an igneous oceanic substrate of Magdala Basalt. The Saint Arnaud Group is interpreted to be time-equivalent to the Glenthompson Sandstone in the Grampians-Stavely Zone (VandenBerg et al., 2000).

The Cambrian age of the Saint Arnaud Group is broadly constrained by the underlying Magdala Basalt, and by its apparent incorporation into the Moornambool Metamorphic Complex, which was metamorphosed at approximately 500 Ma (Miller et al., 2005).

Most of the Saint Arnaud Group has Gondwanan provenance, in common with most other early Paleozoic sequences in the Tasmanides, although some material was apparently derived locally from the adjacent Moornambool Metamorphic Complex (e.g. the Concongella Gritstone; Cayley & Taylor, 2001; Cayley et al., 2011a). Such sedimentary recycling is a characteristic of sediment packages accumulating in a submarine accretionary wedge setting.

The high-grade metamorphic derivatives of the Saint Arnaud Group include the quartz-biotite dominant Lexington Schist and the quartz-muscovite-garnet dominant Good Morning Bill Schist. Metamorphosed black shale is altered to the quartz-biotite-graphite dominant Rhymney Schist (Cayley & Taylor, 2001).

Grampians-Stavely Zone

Kanmantoo Group (Glenthompson Sandstone) Glenthompson Sandstone occurs as several separate fault-bounded ‘panels’ of steeply dipping, folded and faulted clastic rocks that separate the different volcanic (fault) belts of the Stavely Arc (Figure 1.3). Glenthompson Sandstone sediments are mostly non-magnetic (e.g. see Skladzien et al., 2016a), and so can be clearly distinguished from the magnetic rocks of the volcanic belts in magnetic data, even where deeply buried beneath younger rocks. The enormous stratigraphic thicknesses of Glenthompson Sandstone measured in some places, for example in the ‘panel’ of homoclinally west-dipping and facing stratigraphy that lies between the Stavely and Bunnugal belts, is apparent. The uniform low regional metamorphic grade here is not consistent with the more than 10 km of apparent stratigraphic thickness. Instead there is evidence of considerable internal imbrication of Glenthompson Sandstone stratigraphy along cryptic bedding-parallel faults that were active during D1a (Cayley et al., in prep).

The Glenthompson Sandstone is modelled as discreet simple volumes within the STAVELY 3D model. The density of the Glenthompson Sandstone is variable, dependent on local sandstone (less dense) / siltstone, mudstone (more dense) ratios, and on the presence of other rock types within the unit. Modelled density values range from 2.68 – 2.85 g/cm3, with the upper end of the range likely reflecting the effects of local contact metamorphism by adjacent intrusive rocks, and the inclusion of a component of local, thin denser mafic igneous rocks (Chatsworth Basalt – see below) into some volumes, and contact metamorphism associated with those.

D1a faults that imbricate the Glenthompson Sandstone internally are not captured in the STAVELY 3D model. Only the D1a faults that separate large volumes of generalised Glenthompson Sandstone from large volumes of generalised Mount Stavely Volcanic Complex and/or other rocks, and D4 faults that deform all pre-Early Devonian rocks are modelled.

The recognition that part or all of the Glenthompson Sandstone may predate deposition of the Mount Stavely Volcanic Complex opens the possibility that the Stavely Arc may overlie Glenthompson Sandstone in addition to a Precambrian continental substrate. The reasons for this interpretation are outlined further below, and mean that parts of the unreflective basement to the Stavely Arc, imaged in the lower crust in deep seismic reflection data, are modelled as volumes of Glenthompson Sandstone, overlying Precambrian crystalline rocks.

In addition to the large panels of Glenthompson Sandstone between the volcanic belts, numerous smaller (kilometre-scale) lenticular bodies of Glenthompson Sandstone are incorporated into the interior of the volcanic belts themselves, most notably within the Stavely and Narrapumelap belts. These juxtapositions appear to be as fault slices formed during subsequent tectonic activity, since many of the Glenthompson Sandstone bodies within the volcanic belts appear totally unrelated and unmixed with the enclosing volcanic rocks. These fault-slices have not been captured by the STAVELY 3D model because they are too small at the model scale.

The Glenthompson Sandstone is a deep marine turbidite sequence comprising interlayered, terrigenous thinly- to thickly bedded micaceous quartz arenite and quartz lithic sandstone and siltstone, with minor interbeds of shale and pelitic mudstone and thin calcareous beds. Constituents of the Glenthompson Sandstone include graphitic slate, chert and cleaved sandstone in the Black Range area. Preserved sedimentary features indicate marine deposition in a submarine mid-fan environment below storm wave base (Stuart-Smith & Black, 1999).

While deformed Glenthompson Sandstone located between the Stavely and Bunnugal belts is intruded by the Bushy Creek Granodiorite and Buckeran Diorite, both of which are Late Cambrian (~502-498 Ma; Lewis et al., 2016) and therefore provide a minimum age constraint on Glenthompson Sandstone deposition and deformation, the absolute age of the Glenthompson Sandstone is unknown. It has previously been assumed to have been rapidly deposited following cessation of Stavely Arc magmatism (VandenBerg et al., 2000; Crawford et al., 2003) , however the sedimentary characteristics are inconsistent with synorogenic deposition during D1a, and the Glenthompson Sandstone generally lacks evidence of inheritance from the Stavely Arc, despite being deposited in the same region.

Detailed mapping has constrained the stratigraphic facing directions within different packages of Glenthompson Sandstone, and has led to the assumed timing of the Glenthompson Sandstone to be revised (Cayley et al., in prep). The panel of Glenthompson Sandstone east of the Stavely Belt, and south of the Escondida Fault, faces west, and the uppermost parts of this belt can be observed to transition upwards into proximal volcaniclastic sediments containing angular clasts of porphyritic dacite and andesitic detritus (e.g. see Figure 2.3B in Schofield et al., 2018). These relationships suggest that the upper parts of the Glenthompson Sandstone were coeval with Stavely Arc volcanism, and that the bulk of the formation may in fact be older than the Stavely Arc. This interpretation therefore includes the Glenthompson Sandstone as a formation within the Kanmantoo Group.

The youngest detrital zircons within the Glenthompson Sandstone are provenance clusters at ~ 590-540 Ma (Fanning & Morand, 2002; Lewis et al., 2015; Lewis et al., 2016; Maas & Taylor, in prep.). Most of the Glenthompson Sandstone thus shares zircon inheritance characteristics with the Kanmantoo Group farther west (Ireland et al., 2002). Both are likely to have been sourced from farther afield, possibly from eroding Precambrian source terranes in Antarctica (Ireland et al., 1998). Chatsworth Basalt

Occurrences of Chatsworth Basalt are too thin to be discriminated in the STAVELY 3D model, but they are incorporated into the Glenthompson Sandstone volumes within which they occur, where the basalt and associated contact metamorphism probably contribute to the slightly higher apparent overall density modelled for Glenthompson Sandstone in places.

The Chatsworth Basalt is a fine-grained basalt to microgabbro (Cayley et al., in prep.) that occurs as narrow (~100 m) northerly-trending, steeply-dipping dykes, sills, and some potential lava flows, enclosed primarily within the steeply-dipping panel of Glenthompson Sandstone lying between the Stavely and Bunnugal belts. They were first described and sampled by Stuart-Smith & Black (1999), and subsequently by Cayley et al. (in prep.). These rocks are moderately magnetic and so can be easily traced in regional magnetic data.

Although the Chatsworth Basalt has been locally recrystallised (Stuart-Smith & Black, 1999), probably during contact metamorphism associated with later granite intrusion, microgabbroic textures in some rocks are considered to be primary, reflecting slow cooling at subvolcanic depths (Cayley et al., in prep.). The sills intrude sub-parallel to bedding in the enclosing Glenthompson Sandstone and have themselves imparted narrow contact metamorphic aureoles over the host rocks which, away from younger granite intrusions, are generally of very low metamorphic grade. The Chatsworth Basalt appears to have been locally folded and fault-imbricated along with the Glenthompson Sandstone during D1a, and the deformed succession is intruded by the late Cambrian Bushy Creek Granodiorite (~502-498 Ma; Lewis et al., 2016).

The geochemistry of the Chatsworth Basalt is unlike known igneous rocks of the Stavely Arc, and is compositionally similar to MORB with a lack of a clear subduction signature (Stuart-Smith & Black, 1999; Taylor et al., 2015; see Schofield et al., 2018). Their composition is similar to MORB-like mafic dykes in South Australia, for which a 510 ± 2 Ma age has been obtained (Liu & Fleming, 1990; Chen & Liu, 1996). This suggests that both the Chatsworth Basalt and the surrounding Glenthompson Sandstone which host it are older than the nearby Mount Stavely Volcanic Complex.

Nargoon Group

Recognition that the Glenthompson Sandstone is most likely older than the Mount Stavely Volcanic Group means that the Nargoon Group represents a separate phase of post-arc sedimentation (Taylor et al., 2015). Nargoon Group is therefore modelled as a discrete volume in the STAVELY 3D model. Although younger, the Nargoon Group is similar, and modelled density values of 2.70-2.82 g/cm3 are comparable to the Glenthompson Sandstone.

Nargoon Group is now recognised as confined to the uppermost tectonostratigraphic position preserved within Cambrian rocks in the Grampians-Stavely Zone, the region west of the Boonawah-Grampians West-Tyar-Glenisla-Black Range-Black Range West/Mitre belts and east of the Yarramyljup Fault in the west of STAVELY.

In the western Grampians-Stavely Zone, at the type locality, Nargoon Group consists of clastic sediments and displays sedimentary features such as low angle cross-bedding, suggestive of shallow water deposition (Morand et al., 2003). Near Balmoral, Nargoon Group sediments are subvertically-dipping, folded and weakly cleaved, and unconformably underlie flat-lying Grampians Group stratigraphy. This indicates that the Nargoon Group was deformed prior to deposition of the Grampians Group, most likely during the Cambrian (Morand et al., 2003).

X-ray diffraction analysis (McKnight & Taylor, in prep.) of select drill core and chips from the Grampians-Stavely Zone and HyLoggerTM hyperspectral analysis of stratigraphic drill holes within STAVELY (Thomas et al., 2015) shows the Nargoon Group to contain abundant albite and chlorite together with detrital amphibole and garnet. This composition differs notably from the micaceous Glenthompson Sandstone. The higher proportion of ferromagnesian mineralogy in the Nargoon Group is also reflected in Cr contents, with whole rock Cr ranging from 768-1370 ppm in Nargoon Group (e.g. drill holes STAVELY05 and STAVELY14), compared to much lower Cr concentrations (49-167 ppm) in Glenthompson Sandstone (e.g. drill hole STAVELY17; see McAlpine et al., 2017). Similarly, handheld portable XRF Cr analyses for the Nargoon Group are much higher than for the Glenthompson Sandstone (2780-860 ppm versus 50-115 ppm; D. Taylor pers. comm., 2017). Serpentinites exposed in faults across the region are all characterised by high-Cr chrome spinel (e.g. Bailey et al., 2016), and erosion of serpentinite fault slices exposed during the Delamerian Orogeny could explain the high Cr content of the Nargoon Group sediments. This correlation suggests that the Nargoon Group is a syn-orogenic sedimentary sequence deposited and deformed in the final stages of the Delamerian Orogeny, as originally defined.

Subvertical D4 strike-slip faults that segment volcanic (fault) belts of upturned Stavely Arc rocks in the Black Range also segment Nargoon Group lying stratigraphically above the Mount Stavely Volcanic Complex, and Glenthompson Sandstone lying stratigraphically below the Mount Stavely Volcanic Complex, and locally juxtapose the Cambrian metasediments. The best exposed Nargoon Group-Glenthompson Sandstone juxtaposition is across the sinistral Cherrypool Fault. Other juxtapositions occur across the Muirfoot and Henty faults. These juxtapositions are captured in serial cross sections 6-8.

North of Mitre, the volcanic rocks of the Black Range West/Mitre Belt appear to pinch out within the Mouchong Fault System. Further north, Nargoon Group appears to be juxtaposed against Glenthompson Sandstone with the intervening Mount Stavely Volcanic Complex locally absent at the current level of exposure. This relationship is captured and modelled in serial cross section 10. In serial cross section 11 this relationship is overthrust by the Yarramyljup Fault, and lies outside of the STAVELY 3D model.

Mount Stavely Volcanic Complex

The Mount Stavely Volcanic Complex comprises a faulted succession of Mid-Late Cambrian mafic, intermediate and felsic igneous rocks. The succession is dominated by eruptive rocks; volcanic breccia, tuff, lava flows and high-level sills. Volcanogenic sediments and black shale and chert incorporated within the succession show that these mostly erupted into a submarine environment. Some of the succession hosts VHMS (Cu-Pb-Zn-Ag-Au) mineralisation. The Mount Stavely Volcanic Complex includes arc-related intrusive rocks, such as felsic granite and felsic porphyry stocks, some of which host Cu-Mo-Au mineralisation and hydrothermal alteration (e.g. Thursday’s Gossan Prospect), and so are modelled and described separately as Late Cambrian intrusives in the STAVELY 3D model.

The geochemistry of the Mount Stavely Volcanic Complex ranges from mafic-boninitic to intermediate-felsic calc-alkaline, and possesses key geochemical fingerprints of a continental subduction origin (see Schofield et al., 2018). The Mount Stavely Volcanic Complex is therefore regarded as part of the Stavely Arc. The Stavely Arc is imaged in deep seismic reflection data as extending to depth, where it is widespread beneath the greater Grampians-Stavely Zone. Given the known prospectivity modelling the geometry, distribution and inter-relationships of the Mount Stavely Volcanic Complex has been a focus of the STAVELY 3D model.

The interpreted, generalised distributions of ultramafic-mafic dominated (denser) versus intermediate-felsic dominated (less-dense) stratigraphic components are presented in the serial crustal-scale cross sections. These are included in the model to allow for higher-resolution forward-modelling reconciliation against available geophysical datasets (mainly gravity). Ultramafic-mafic components are modelled with a density range of 2.71 – 3.05 g/cm3, while intermediate-felsic components are modelled with a density range of 2.62 – 2.90 g/cm3. The degree of overlap in density range reflects the need for generalisation at the regional (crustal) scale of the modelling.

Serpentinised ultramafic rocks with markedly different geochemistry occur as thin fault-slices within the volcanic complex – e.g. Williamsons Road Serpentinite. These rocks are regarded as exotic to the Mount Stavely Volcanic Complex, appear to have been faulted into the succession during Cambrian regional deformation (D1a), and are described separately (see Hummocks Serpentinite above). The ultramafic rocks are not discriminated as separate volumes within the STAVELY 3D model, because they are too small to be resolved.

Originally defined just within the Stavely Belt (Buckland, 1987), more recent mapping (e.g. Cayley & Taylor, 1997a; 2001) and stratigraphic drilling (Schofield et al., 2018), interpretation of geophysical data, and mineral exploration drilling now confirm that parts of the exact same Mount Stavely Volcanic Complex succession of arc-related igneous rocks occur in a number of narrow, elongate separate, steeply-dipping fault-bounded volcanic belts that span the length and the breadth of the Grampians-Stavely Zone, with very little change in character.

Rocks of the Mount Stavely Volcanic Complex show a range of magnetic susceptibilities, but the most magnetic are the mafic and intermediate basalt-andesite units of the succession. These rocks are also considerably denser than surrounding Cambrian sedimentary stratigraphy (e.g. Glenthompson Sandstone, Nargoon Group). The contrasting geophysical properties of constituent rocks has allowed for internal stratigraphy and structures within the various volcanic belts of the Stavely Arc to be interpreted in regional magnetic and gravity data. At a larger scale, the extents of the various volcanic belts, and the locations of later faults that cross-cut and segment them (e.g. D4) can be interpreted with confidence even where they are deeply buried beneath younger cover rocks (e.g. Grampians Group).

The volcanic belts are linear and relatively narrow, with sharply defined flanks that are predominantly fault-bounded (D1a faults). This characteristic allows for dip-modelling of the overall volcanic belts, and/or magnetic components within them. Selected results of dip-modelling are presented in Section 3.2.5, and are an important constraint used to constrain the subsurface geometry of different volcanic belts in the STAVELY 3D model.

The distribution of Mount Stavely Volcanic Complex stratigraphy within most of the volcanic belts is internally complex. In addition to the inherent nature of a volcanic landscape most of the belts appear to have experienced strong internal deformation and stratigraphic disruption during D1a. In places where faults are minimal and facing directions can be established (e.g. in the Dryden Belt), the overall stratigraphic character seems to reflect early eruption of mafic boninitic rocks, with an upward evolution into more intermediate-felsic calc-alkaline rocks. This overall pattern is also indicated in buried successions that have been tested by drilling, such as the southern Dimboola Belt.

The lowest stratigraphic successions in most volcanic belts seem to be serpentinised ultramafic rocks that are not related to the Stavely Arc, and may represent parts of a pre-arc basement upon which the Stavely Arc was built. Fault slices of Glenthompson Sandstone and other exotic rocks (e.g. Williamsons Road Serpentinite, black shale) have been incorporated into the interiors of most of the volcanic belts, and locally even dominate volumetrically. The scale of these disruptions is too small to be captured at the scale of the STAVELY 3D model.

The Mount Stavely Volcanic Complex is dominantly coherent and fragmental andesitic to dacitic volcanic rocks (Buckland, 1987). Exposed rocks of the Mount Stavely Volcanic Complex are known from the Stavely, Narrapumelap, Dryden, Bunnugal, Elliot, Glenisla and Black Range belts. Other volcanic belts containing, or likely to contain, rocks of the Mount Stavely Volcanic Complex are clearly imaged in regional magnetic data (Figure 1.3 and Figure 2.5), parts of which have been tested by stratigraphic drilling as part of the Stavely Project.

Several formations are recognised within the Mount Stavely Volcanic Complex, the type localities of which are all located within the Stavely Belt. These include the Fairview Andesite Breccia, Nanapundah Tuff, Towanway Tuff, and the Narrapumelap Road Dacite Member. Other units included in the Mount Stavely Volcanic Complex (e.g. Williamsons Road Serpentinite, Glenronald Shale Member) are likely unrelated to the volcanic succession, and are rather considered to be structurally emplaced. Stratigraphic subdivision of the Mount Stavely Volcanic Complex is outlined in detail by Buckland (1987), Crawford et al. (1996) and Stuart-Smith & Black (1999).

In the Stavely Belt, the Mount Stavely Volcanic Complex comprises intermediate to felsic, dominantly medium-K calc-alkaline lava, breccia, interbedded subaqueous tuffaceous and volcaniclastic rocks (Figure 3.49). The Fairview Andesite Breccia is a pale to dark green, polymictic volcanic breccia, matrix supported with large (up to 30 cm across), angular clasts of andesite and dacite (Figure 3.49A) in a fine-grained to microcrystalline matrix of volcaniclastic material (Buckland, 1987). The morphology and composition of most of the formation is consistent with autobrecciation during volcanic flow (Stuart-Smith & Black, 1999), although some parts display characteristics consistent with deposition as subaqueous debris flows/slumps.

Numerous thick, massive andesite lava flows are interlayered within the breccia; the largest mapped south of the Glenelg Highway (Buckland, 1987). The andesite is generally porphyritic and non-vesicular. Contacts with the enclosing breccia range from primary to faulted. Dark green-grey, magnetic clinopyroxene-phyric amygdaloidal andesite lava crops out on the southern bank of the Hopkins River, and forms a discrete flow layer that can be traced north and south for several kilometres in regional magnetic data (Stuart-Smith & Black, 1999). Rare hornblende-bearing andesite lavas also occur. Subordinate basaltic lava flows are also interbedded with the breccia, including a thin basalt lava flow near the Junction Prospect.

The Glenronald Shale Member is a laminated black, pyritic carbonaceous shale with associated fine grey volcanic siltstone and black chert up to 13 m thick. Much of its lithology is consistent with deposition in an anoxic, sediment-starved, deep marine environment that appears quite at odds with the massive volcanic stratigraphy that now encloses it. The volcaniclastic siltstone contains abundant mineral fragments of quartz, sericitic feldspar, biotite and chlorite (Buckland & Ramsay, 1982).

The Nanapundah Tuff is best developed north of Mount Stavely, where it has a maximum thickness of 800 m near the type locality (Buckland, 1987). It comprises an andesitic sandstone, with crystal fragments set in a groundmass of fine volcanic material altered to a felted low-grade chlorite-sericite metamorphic mineral assemblage.

The Towanway Tuff is a well-bedded sequence of dacitic volcaniclastic sandstone, and contains a coherent dacite lava flow differentiated as the Narrapumelap Road Dacite Member. Maximum thickness for the Towanway Tuff appears to be about 2000 m, although this is a very approximate estimate that does not account for the possibility of structural duplication or truncation. It comprises rhyolitic to dacitic, lithic lapilli tuff, crystal tuff and volcaniclastic sandstone. The rocks range from thinly bedded, laminated cherty crystal tuff, to fine-grained laminated lapilli tuff, to medium-bedded, coarse-grained volcaniclastic sandstone. Volcanic clasts within the tuff are mostly dacitic, with occasional andesite. Beds are commonly graded, with basal small-scale scours and other characteristics of submarine, turbidite deposition.

The Narrapumelap Road Dacite Member crops out south of the Stavely railway siding in the north, and in the Hopkins River Valley near Yarrack Road (the type locality) in the south. It is up to 100 m thick (Buckland, 1987), and consists of porphyritic plagioclase, quartz ± clinopyroxene ± hornblende-phyric dacite with a fine, microcrystalline groundmass.

Mafic, possibly rift-related, tholeiitic rocks are a minor component of the Mount Stavely Volcanic Complex. Irregularly shaped, porphyritic diorite, granodiorite and tonalite intrusions within the succession appear to post-date early (D1a) deformation. A range of geodynamic models have been proposed to explain the origin of the Mount Stavely Volcanic Complex. These are discussed in detail in Schofield et al. (2018), Section 4.1 – Geodynamic synthesis and implications for the geological evolution of STAVELY.

Similar rocks to the Stavely Belt are exposed in the Dryden Belt, including andesitic and dacitic lavas, volcanic conglomerates and sandstones, and dioritic intrusives (Cayley & Taylor, 2001). Also included within the Dryden Belt are a group of low-Ti andesitic and dacitic lavas and high-level intrusions (Crawford et al., 2003). The rocks exposed in the Dryden Belt are considered to be equivalent to the Mount Stavely Volcanic Complex by Cayley & Taylor (1997a, 2001). The Stavely and Dryden belts are linked by the Narrapumelap and Elliot belts, both of which also contain similar rocks.

Stavely Arc rocks exposed in the Black Range, west of the Grampians Ranges, crop out poorly, with most information coming from mineral exploration drilling. Volcanic rocks in the Black Range are mafic to felsic, with intermediate to felsic volcanics dominating the eastern side of the Black Range Belt (Cayley & Taylor, 1997c). Similar geochemical features suggest that the volcanic rocks in the Black Range may be correlatives of those in the Dryden Belt (Maher et al., 1997; Crawford et al., 2003; see Schofield et al. 2018, Section 2.6 – Geochemistry of the Stavely Arc).

A range of comparable rock-types occur in all the other volcanic belts in STAVELY, and so all are probably fault-repeats of the same basic stratigraphic units (e.g. Crawford, 1982; Buckland, 1987; McArthur, 1990; Cayley & Taylor, 1997a; Direen, 1999; Cayley & Taylor, 2001). These narrow, steeply-dipping volcanic (fault) belts are all interpreted to be thrust slices of a single igneous complex, carved from the flanks and crest of an underlying primary Stavely Arc edifice during D1a (Delamerian) deformation.

Trace element patterns for the exposed volcanic rocks are similar and show a typical ‘subduction signature’ found in igneous rocks from magmatic arcs (e.g. see Tatsumi & Eggins, 1995), including low HFSE, enrichment in LILE and LREE over HFSE, and negative Nb and Ti anomalies. Few Nd isotope data have been published for the exposed volcanic belts. Whelan et al. (2007) reported Nd 495 Ma values of between -2.38 and +4.35 for volcanic rocks in the Stavely Belt. The Dryden Belt, except for distinctive low-Ti rocks described below, typically shows element abundances intermediate between the Black Range and the Stavely belts (Crawford et al., (2003; see also Maher et al., 1997; Direen, 1999; Schofield et al., 2018).

The most geochemically distinctive rocks in the exposed volcanic belts are low-Ti andesites and dacites in the Dryden Belt. These low-Ti rocks show markedly different geochemical characteristics to the majority of calc-alkaline rocks, and overlap with those of modern boninites (although with largely lower MgO; see Le Bas, 2000). Altered, ultramafic to mafic glassy lavas with boninitic characteristics also occur just east of the Dryden Belt in the Fryingpan Prospect (Cayley & Taylor, 2001). Correlatives of the low-Ti rocks in the Dryden Belt are intersected under cover in the Dimboola (VIMP8) and Black Range (VIMP3) belts (Maher et al., 1997), in the Brimpaen Belt (STAVELY04) and in the Grampians ‘West’ Belt (e.g. water bore Woolphoer 6) supporting the notion that such rocks occur across multiple volcanic belts, and thus form an integral part of a single Stavely Arc succession.

Mafic to felsic volcanic rocks with calc-alkaline geochemical affinities (according to the classification of Kuno, 1968) were intersected in drill holes STAVELY02, STAVELY07, STAVELY12 and STAVELY16, in the Stavely, Brimpaen, Dimboola and Dryden North (Hindmarsh) belts respectively. These rocks are predominately calc-alkalic to alkali-calcic according to the classification scheme of Frost et al. (2001; Figure 2.35). Lithologies vary in detail (see Schofield et al., 2015b) but in summary comprise coarse basaltic volcaniclastics and volcanic breccia with intervals of coherent basalt and dolerite (STAVELY02), coherent and fragmental andesites and dacites (STAVELY07 and STAVELY12), and coarse polymictic volcanic breccia with large clasts of andesite and lesser basalt (STAVELY16).

Volcanic (basalt) and intrusive (quartz-gabbro to quartz-diorite) rocks in the Dimboola Belt (STAVELY09), and in the Boonawah Belt (PRC-05) show consistently depleted geochemistry that distinguishes them from other igneous associations in STAVELY. They are weakly calc-alkaline according to the classification of Kuno (1968; Figure 2.37) but border on tholeiitic compositions. Trace element patterns are depleted relative to N-MORB. They have characteristics that suggest relatively small degrees of crustal contamination, or that the crustal material itself was isotopically juvenile. Tholeiitic (basalt and dolerite and quartz dolerites (PRC-01, PRC-03) in the Boonawah Belt have compositions comparable to N-MORB, similar to basalt from drill hole VIMP9 (Dryden North Belt) which also has a tholeiitic bulk composition.

Intrusions

Intrusive igneous rocks belonging to three distinct episodes of magmatism are recognised throughout STAVELY: Middle Cambrian associated with Stavely Arc growth (dated at ~510 Ma, pre-D1a); a pulse of mineralised magmatism in the Late Cambrian associated with Stavely Arc termination (dated at ~500 Ma, accompanying D1b, including mineralised porphyries); and in the Early Devonian associated with later Lachlan orogenesis (D4) ( dated ~405 Ma) and accompanying eruption of the Rocklands Volcanic Group.

Middle Cambrian intrusive rocks recognised and dated so far (Lewis et al., 2015, Lewis et al., 2016) are all fragments encountered in mineral exploration drill holes, fault-intercalated with other units of the Mount Stavely Volcanic Complex. These are therefore interpreted to be allochthonous, but are likely to be part of the Stavely Arc succession and have experienced the effects of D1a disruption. These granites are too small to be included separately in the STAVELY 3D model.

Late Cambrian and Early Devonian granites, diorites and related rocks are included in the STAVELY 3D model where they are large enough to form discrete volumes (larger than the 500m3 voxel cell size), and where they are considered to be of mineral exploration significance (e.g. prospective Late Cambrian porphyry stocks). Intrusive rocks are modelled with a density range of 2.5 – 2.76 g/cm3, reflecting a range of compositions from quartz diorite (e.g. Late Cambrian Buckeran Diorite) to granodiorite (e.g. Bushy Creek Granodiorite), to granite (e.g. Victoria Valley Granite)

While distinguishing Cambrian from younger intrusive igneous rocks is difficult without geochronological data owing to similar overall geophysical responses, a structural (stress) history developed for STAVELY provides a structural template that characterises the very different stress-fields into which Late Cambrian and Early Devonian granites intruded (see Section 4.5.1 – A scalable structural template for STAVELY through time).

This difference predicts different pluton geometries. Autochthonous Late Cambrian intrusions are expected to be elongated northwest-southeast, orthogonal to the maximum extension direction indicated for contemporaneous Late Cambrian D1b sinistral transtension. A dated example is the Buckeran Diorite near Chatsworth. Autochthonous Early Devonian (granite) intrusions are expected to be elongated northeast-southwest orthogonal to the maximum extension direction indicated for the contemporaneous Early Devonian D4 dextral transtension. Dated examples are the Victoria Valley Batholith, and the Duchembegarra Tonalite (which extends beneath Mt Arapiles). These very different shapes and alignments have been used to tentatively assign ages to undated granites, including intrusions concealed beneath the Rocklands Volcanic Group and beneath the Murray and Otway basins. Intrusive rocks interpreted to be associated with D1b have proven prospectivity for Cambrian porphyry mineralisation styles. Magmatic-hydrothermal gold and molybdenum is hosted by some D4 intrusive rocks (e.g. Mafeking).

Overprinting criteria also serve to distinguish between the D1b and D4 episodes of intrusive activity. Late Cambrian granites are seen to be cut and offset by D4 structures, including in potential field geophysics. Early Devonian granites are seen to overprint and ‘stitch’ D4 structures, with patterns that are obvious in potential field data.

Late Cambrian intrusions

Most of the interpreted D1b Cambrian intrusive rocks are buried beneath younger cover (e.g. northerly-elongated intrusions in the Black Range West/Mitre Belt) and are known only from drill hole intersections and/or interpreted from potential field data using overprinting criteria. However, some examples do crop out and have been dated radiometrically at ~495 – 505 Ma. Just west of the Stavely Belt, the Bushy Creek Granodiorite and Buckeran Diorite and related intrusions form a northerly-aligned train of Late Cambrian intrusions that are grouped together and referred to as the Bushy Creek Igneous Complex by Whelan et al. (2007). The geometry of the Bushy Creek Igneous Complex is consistent with emplacement during D1b. They intrude and ‘stitch’ upturned Glenthompson Sandstone, constraining the age of host rock regional deformation to pre-D1b. These D1b intrusions have been locally segmented (Buckeran) and folded (Yuppeckiar) by D4 dextral strike-slip faults and folds related to the Yarrack Fault.

Geochemical features of the Bushy Creek Igneous Complex suggest similarities with subduction-related magmatism, and so may share a direct genetic link with the Mount Stavely Volcanic Complex, although petrogenesis of the Bushy Creek Igneous Complex is complicated by mixing with a more evolved, crustal component (Whelan et al., 2007). Several porphyries of dominantly tonalitic or dacitic composition are known in the south of STAVELY. One porphyry at the Thursday’s Gossan Prospect is dated at approximately 505-500 Ma (Lewis et al., 2015; Lewis et al., 2016; Schofield et al., 2018), and is part of the Lalkaldarno Porphyry group that are also elongated north-south, and therefore consistent with emplacement during D1b. The Late Cambrian porphyries intrude upturned and faulted rocks of the Mount Stavely Volcanic Complex in the Stavely Belt, constraining the age of deformation of the Stavely Belt to pre-D1b.

Porphyries have also been noted outside of the Stavely Belt, intruding the Buckeran Diorite (Lexington Prospect) and Glenthompson Sandstone (Junction Prospect; andesitic to dacitic porphyry dykes in STAVELY17 (Anderson, 1993; Taylor et al., 2014; Schofield et al., 2015a; Taylor et al., 2015; Skladzien et al., 2016b; Schofield et al., 2018).

Similar porphyries have been drill tested farther north in STAVELY, including in the Brimpaen Belt (STAVELY06, STAVELY07), and are interpreted from potential field data in the other volcanic belts of the Stavely Arc. The porphyry intersected in drill hole STAVELY06 is a quartz diorite with a medium-K, calc-alkaline composition. N-MORB-normalised multi-element diagrams and REE patterns are very similar to those for the Buckeran Diorite (see Schofield et al., 2018). Phenocrystic plagioclase is often granophyrically intergrown with quartz, suggesting shallow-level emplacement and high crustal levels of preservation that are comparable to those seen in the Stavely Belt.

Some of the volcanic belts of the Stavely Arc interpreted to contain Late Cambrian D1b granites and porphyries appear to have experienced subsequent block-rotations during D4 (e.g. the Narrapumelap and Elliot belts in the interior limb of the Mafeking Megakink; the Tyar Belt; the Brimpaen Belt). These volcanic belts are allochthonous, and so porphyries and other intrusive rocks emplaced into them during D1b are unlikely to retain their original orientations.

Early Devonian intrusions

Early Devonian intrusions in STAVELY include the large Victoria Valley Complex that intrudes the Grampians Group, and other plutons farther north (e.g. Duchembegarra Tonalite, which extends beneath Mt Arapiles). The geometry of these granites range from semi-circular (e.g. Mafeking, Honeysuckle) to markedly elongate along a northeast-southwest axis (e.g. Victoria Valley). The exposed granites exhibit textures indicating high-level intrusion (Cayley & Taylor, 1997a; Hergt et al., 2007). Close compositional similarities between the granites and the Rocklands Volcanic Group further west suggest that some of the magmas made it to the surface and erupted as lavas and ignimbrites.

The Victoria Valley Complex shares remarkably similar geochemical and isotopic characteristics with the Bushy Creek Igneous Complex that is 100 million years older (Hergt et al., 2007; Whelan et al., 2007). This suggests that a subduction-modified source persisted beneath STAVELY for at least approximately 100 Ma after the cessation of arc magmatism (Hergt et al., 2007), a feature which may have implications for Devonian intrusion-related mineral systems (see Schofield et al., 2018 Section 4.2.2.2 – Post-Cambrian intrusion-related mineral systems).

Apatite fission track data collected from the Early Devonian granites yield apparent ages of 300-340 Ma (Foster & Gleadow, 1992). The population of fission tracks is unimodal and full length (13-14 m), indicating that the Early Devonian granites cooled below the partial annealing temperature (60 °C) at the time of the apparent ages of 300-340 Ma and have not been buried since (Foster & Gleadow, 1992). For a typical geothermal gradient of 25 °C/km, this 60 °C constraint places the Devonian granites within 2 km of the surface. This recording of long-term shallow depth is consistent with high-level emplacement rather than rapid exhumation.

Cover rocks to Cambrian bedrock

Grampians Group

The oldest cover unit overlying Cambrian bedrock is the ?Late Ordovician- to Silurian-aged Grampians Group (see Figure 2.16; Spencer-Jones, 1965; Cayley & Taylor, 1997a; Miller et al., 2001; Retallack, 2009). Grampians Group is one of the thickest cover units, with an apparent stratigraphic thickness of over 3.7 km (Cayley & Taylor, 1997a; Stuart-Smith & Black, 1999). Modelling of new gravity data shows that the Grampians Group attains deformed thicknesses of well over 4 km in some places – notably in the deepest parts of the Jalur Rift beneath the main Grampians Ranges (see Figure 3.9), adjacent to the McKenzie Fault east of Horsham, in the core of the Lorquon Rift buried beneath the Murray Basin east of Netherby (see Figure 3.9), and possibly east of Log Hut Creek northwest of Hamilton. In such places, thick Grampians Group likely presents an insurmountable barrier to the exploration of prospective Cambrian bedrock.

The widespread distribution of Grampians Group at surface and in the subsurface (Figure 3.6) is not as large an impediment to mineral exploration of underlying Cambrian bedrock as first appearances suggest. Although extensive, the Grampians Group has experienced a complex structural history and is structurally thin-skinned (Cayley & Taylor, 1997a). One consequence of this is that the Grampians Group is often separated from underlying Cambrian bedrock by low-angle detachment faults – part of the D4 Marathon Fault system (see Section 3.3.4.6 – Marathon Fault). Where the Marathon Fault system lies close to surface, overlying Grampians Group is a relatively thin veneer that could be successfully explored through – for example in the vicinity of the Black Range for the Tyar, Glenisla, Black Range, and Black Range West/Mitre belts, and south of Chatsworth, for the southern Stavely Belt (thin Grampians Group and the basal detachment fault penetrated by drill hole STAVELY02).

The current research has established the nature of the structural linkage that exists between D4 structures developed and mapped in the Grampians Group cover, and D4 structures now recognised and mapped in the underlying Cambrian bedrock. Additional, higher resolution gravity data allows for more accurate modelling of cover thicknesses. Thus, the nature, geometry, movement history, depth and extent of the Marathon Fault system is now better understood, better constraining estimations of the true thickness of Grampians Group cover expected in different areas.

The purpose of the Grampians Group volume included in the STAVELY 3D model is to depict the extent of the unit, including beneath Murray and Otway Basin cover, and to highlight regions where the unit is expected to be too thick to be explored through. The regional scale of the STAVELY 3D model (and voxel size of 500 m3) limits the ability of the model to capture the finer details of Grampians Group true thickness where it is thin – however these details can be extrapolated from the more detailed serial cross sections

Grampians Group internal stratigraphy is not captured in the generalised STAVELY 3D model volume. Such detail cannot be meaningfully discriminated at the model scale, and is not considered important for mineral exploration of the Cambrian bedrock. Details of the stratigraphic variation are presented in Cayley & Taylor, 1997, and in updated and revised form in Figure 2.12, and in detailed cross sections included as mock-ups in the STAVELY 3D model. These are adapted and updated from previously published serial cross sections (Cayley & Taylor, 1997a; 2001; Morand et al., 2003). Serial cross section locations are depicted in Figure 3.5.

The Grampians Group volume is modelled using density values that range from 2.6 – 2.76 g/cm3, with a median density of approximately 2.65 g/cm3. This reflects the dominant quartz-arenite sandstone constituent rocks, with subordinate conglomerate and mudstone of higher density distributed throughout the multilayer sequence.

Grampians Group is a fluvial to shallow marine red bed mudstone sequence sandwiched between two siliciclastic sandstone sequences. The interior of the Grampians Group succession appears to have been partially repeated through thrust duplexing, so that the sequence now presents as a thick, regularly interlayered sandstone-mudstone succession. Many of the sandstones are quarzitic and are very resistant to erosion. The quartzite forms the dominant landscape of large topographic strike ridges rising up to 500 m higher in elevation than the surrounding almost flat landscape. The intervening valleys are generally developed on more easily eroded red-bed mudstone. The maximum preserved demonstrated stratigraphic thickness is about 3700 m in the centre of the main Grampians Ranges, although the true vertical thickness is highly variable.

An unconformable relationship between the Grampians Group and the Cambrian bedrock is exposed near Willaura and near Balmoral. Drill hole STAVELY02 (south of Mount Stavely) and mineral exploration drilling in the Black Range (e.g. ACBR016) have penetrated through Grampians Group and into Cambrian bedrock in less than 100 m. Although the nature of the contact between the Cambrian bedrock and overlying Grampians Group is variable (i.e. fault-related versus unconformable), these drilling intersections show that prospective bedrock occurs at relatively shallow depths beneath at least some parts of the Grampians Group.

The Grampians Group sequence contains significant structural complexity (Cayley & Taylor, 1997a; Miller et al., 2001). In addition to possible fault duplication the succession has been folded by long-wavelength, open, low-plunge folds (e.g. D3 Wartook Syncline) that are also consistent with formation during simple orthogonal crustal shortening during D3. The D3 structures are overprinted by subvertical strike-slip faults (e.g. Escondida, Mosquito Creek, Muirfood, Henty faults), large, subvertically-plunging, south, south-east and east-convex warp folds (Mafeking, Cranage, Big Cord oroclines, Asses Ears ‘Anticline’) consistent with formation in response to clockwise rotations, and low angle detachment faults (various splays of the Marathon Fault system), collectively consistent with dextral transtensional deformation of the Grampians Group during D4 (Figure 3.73). D3 and D4 structures in the Grampians Group are overprinted by Early Devonian (~410-400 Ma) granites, which provide a tight upper age constraint.

The structural complexities are well exposed in the Grampians Ranges, and provide a structural template that can be used to constrain retrodeformation of D4 structures now recognised in the underlying Cambrian bedrock (see Section 4.1 – D4 and D3 retrodeformation testing). These complexities also make it impossible to estimate local true vertical thicknesses of Grampians Group cover from simple down-dip projections of exposed strata. Instead, the best guide to Grampians Group true vertical thickness, and thus whether prospective bedrock occurs at explorable depths, comes from potential field data, active and/or passive seismic techniques, and from drilling.

Reinterpretation of the Grampians Group distribution using improved regional geophysical coverage since initial mapping has allowed for improved estimation of the true vertical thickness of the Grampians Group. Beneath parts of the main Grampians Ranges, some areas are reinterpreted as considerably thicker than interpreted by Cayley & Taylor (1997a). However, in the north, south and west of STAVELY, the Grampians Group appears to be thin, certainly thinner than in the main Grampians Ranges, with high frequency magnetic features in the bedrock still visible.

In addition to drill hole control on true vertical thickness of Grampians Group in STAVELY, a geophysical constraint exists about 70 km to the north. Seismic line MEMV96-09 images three sub-horizontal reflectors above the interpreted Cambrian bedrock interface, which corresponds well with depth to magnetic basement estimates from McLean (2010) (Figure 2.24). The three reflectors can be correlated with stratigraphy encountered in Gunamalary 2, located 9 km to the northwest. Gunamalary 2 intersects Murray Basin stratigraphy from surface to a depth of 479 m, Permian stratigraphy (Urana Formation) from 479 m to 576 m, and terminates in Grampians Group stratigraphy at 717.6 m. Seismic line MEMV96-09 indicates a total thickness of Grampians Group of about 250 m, and a depth to Cambrian bedrock of approximately 825 m.

Attenuated high-frequency signals in regional magnetic data in the northwest of STAVELY are comparable with the magnetic signature constrained for seismic line MEMV96-09 and indicate that a similar thickness of total cover (approximately 825 m), probably including Permian rocks and Grampians Group, extends to the south near Netherby. Further east, Grampians Group and Permian reflectors apparently thin. In seismic line MEMV96-09 both units may be absent east of CDP 1400, east of which Murray Basin sediments appear to directly overlie Cambrian bedrock at an approximate depth of 500 m (Figure 2.24). This thinning of cover accords with decreasing attenuation of high-frequency magnetic signals in regional magnetic data. Nearby drill hole STAVELY16 and mineral exploration drill hole WI-1-6 both pass from Murray Basin stratigraphy straight into Cambrian volcanic rocks at depths of 300 m and 359 m respectively.

Rocklands Volcanic Group

Felsic volcanic rocks of the Early Devonian Rocklands Volcanic Group occur in the southwest of STAVELY where they unconformably overlie the Grampians Group and Cambrian bedrock. They are geochemically (Simpson, 1997; Hergt et al., 2007) and temporally (Fanning, 1991; Hergt et al., 2007; Lewis et al., 2016) similar to Early Devonian felsic intrusives of the Victoria Valley Complex. The Rocklands Volcanic Group occupies an area of approximately 1500 km2, confined to a region approximately bounded by Hamilton in the south, the Yarramyljup Fault in the west, the Black Range in the north and the Victoria Range in the east. The northern and southern margins of the Rocklands Volcanic Group are unconformably overlain by Murray Basin sediments and basalts of the Newer Volcanic Group, respectively.

The Rocklands Volcanic Group is too thin (around its margins) and/or too unconstrained in thickness (in the unit interior) to be modelled as a volume in the STAVELY 3D model, or in the serial cross sections. Estimations of maximum true vertical thickness vary from a few tens of metres to a few hundreds of metres, less than the 500 m3 voxel size in the STAVELY 3D model. Instead the interpreted base position of the Rocklands Volcanic Group has been included in the STAVELY 3D model as a surface.

Although regarded as a cover succession to the prospective Cambrian bedrock, the Rocklands Volcanic Group host several gold and base metal prospects and related hydrothermal alteration systems of Early Devonian age, and may represent a mineral exploration target.

The Rocklands Volcanic Group stratigraphy is sub-horizontal with a maximum exposed true stratigraphic thickness of 250 m, comprised mostly of thick, densely welded ignimbrites (Simpson, 1997). The Rocklands Volcanic Group has been intersected by several mineral exploration drill holes and water bores, some of which penetrate the underlying Cambrian bedrock and/or Grampians Group (e.g. Woohlpooer 6), demonstrating that, at least around the margins, the volcanics form flat-lying sheets generally less than 100 m thick and more typically only a few tens of metres thick.

The Rocklands Volcanic Group was erupted onto a land surface of deformed Grampians Group and Cambrian bedrock (Simpson & Woodfull, 1994; Cayley & Taylor, 1997a). Regional potential field data image circular-shaped bodies lying beneath the Rocklands Volcanic Group, which may represent related (possibly parental) intrusions (Figure 2.9).

It is possible that the Rocklands Volcanic Group may be locally thicker in its interior, and may represent the upper parts of Early Devonian caldera or rift complex (the Rocklands Rift; see Figure 3.9) complexes developed beneath, for example, regions such as Mount Mackersey and the circular ‘Honeysuckle anomaly’ that apparently intrudes the Glenisla and Tyar belts south of Cherrypool. The formation of caldera complexes is generally associated with collapse, and such an interpretation can explain the significantly lower elevation of the Rocklands Volcanic Group compared to the surrounding older ranges of Grampians Group stratigraphy and potential granitic source rocks (Victoria Valley). The lower elevation of the Rocklands Volcanic Group was previously attributed to eruption of the volcanics into a Devonian-aged valley eroded through the Grampians Group (Morand et al., 2003).

The upper parts of caldera complexes are prospective for epithermal mineral systems, which may provide a context for the known gold and base metals prospects hosted by the Rocklands Volcanic Group. The undeformed and near flat-lying nature of the preserved Rocklands Volcanic Group and AFTA data (Foster & Gleadow, 1992) confirm that the Grampians-Stavely Zone has remained near the surface and undeformed since the Devonian.

Urana Formation

Small amounts of Permian fluvial and foraminifera-bearing marine glacial sediments are documented across western Victoria. Exposed Permian rocks are included in the Bacchus Marsh Formation (O’Brien et al., 2003). The Permian rocks often contain distinctive bluey-green mudstone and striated or faceted pebbles. Beds of pebbly polymictic sandstone containing quartz and granite clasts are channelised (fluvial and/or marine) facies. There are also poorly-sorted open framework conglomerates of diamictite representing glacial till and marine dropstone environments.

In the north of STAVELY, a 174 m thick interval of Permian stratigraphy was intersected by drill hole Warraquil 3 within the Netherby Trough, a narrow (10-20 km wide) northwest trending infrabasin that underlies the Murray Basin 50 km northwest of Dimboola. North of STAVELY, a 370 m thick interval of Permian stratigraphy was intersected by drill hole Gunamalary 2 (Figure 2.24). Both occurrences are assigned to the Urana Formation, which is equivalent to the Bacchus Marsh Formation.

Although locally as thick as the overlying Murray Basin, estimating the distribution of Permian rocks beneath younger cover using geophysics is difficult, since they appear geophysically similar to Grampians Group. Because of this uncertainty, and the apparent discontinuous nature, no surface representing the distribution of Urana and Bacchus Marsh Formations has been included in the STAVELY 3D model.

Otway Basin

The Otway Basin fringes the southern edge of STAVELY (Figure 3.1). The Otway Basin contains a Cretaceous sedimentary and igneous succession (Otway and Sherbrook groups) associated with continental rifting between Australia and Antarctica, overlain by a Cenozoic sedimentary succession that is attributed to thermal sag that followed continental breakup (Wangerrip, Nirranda, Heytesbury and Brighton groups). Where the Otway Basin succession is locally thick it can present an impenetrable barrier to mineral exploration. Although thicker, more diverse and complex than the Murray Basin to the north, the Otway Basin remains too thin in STAVELY to be captured as a model volume. Instead, the base of the Otway Basin is included as a surface in the STAVELY 3D model.

Good descriptions of the basin development and complete stratigraphy are available in Woollands & Wong (2001), Duddy (2003) and Holdgate & Gallagher (2003), with recent updating of stratigraphic nomenclature in VandenBerg (2009). The Otway Basin stratigraphy known in STAVELY is summarised in Schofield et al., 2018.

Only small portions of the northern margin of the Otway Basin are present in STAVELY where it is almost completely covered by the Newer Volcanic Group (see Figure 3.1). Most of the Otway Basin units are only known from drilling, for example: STAVELY01; mineral exploration drill holes PRC-5, PRC-6, PRC-7; petroleum exploration holes Moyne Falls 1, Hawkesdale 1 and Garvoc 1 near the southern boundary of STAVELY, which pass through at least 1000 m of typical Otway Group, including intervals of Older Volcanic Group basalt (e.g. Hamilton South 3) before intersecting Cambrian bedrock (see Figure 3.2).

The only exposed Otway Basin unit in STAVELY is the youngest unit, the Brighton Group. The Brighton Group is the time equivalent to, and continuous with, the Loxton Sand in the Murray Basin (Morand et al., 2003). The Brighton Group is an extensive sand sheet about 10-30 m thick and represents a high-energy beach environment related to the late Cenozoic marine incursion. It is associated with a distinctive landform, a series of fossilised beach dunes (strandlines) created when the sea progressively retreated during the Miocene-Pliocene. These strandlines form an irregularly spaced set of linear ridges 10-20 m high that can contain coarse-grained heavy mineral sand deposits (Olshina & van Kann, 2012).

The best exposure of the Brighton Group in STAVELY is east of the Hopkins River around Woorndoo, where the exposed and intact sand sheet displays the strandline ridges. Near Glenthompson, a more dissected continuation of this sheet is preserved on an upthrown block of Cambrian bedrock.

Regional magnetic data show that the Brighton Group extends underneath the Newer Volcanic Group. The repetitive variation of magnetic intensity in the Newer Volcanic Group reflects varying basalt thickness over dune crests (e.g., near Hamilton, Figure 2.5). Drill control shows lava up to 20 m thicker in the interdune swales whilst the adjacent dunes crests are more thinly covered to occasionally even emergent.

Unlike the unconsolidated Cenozoic sediments, sediments of the Otway Group have been lithified. Otway Group rocks are a hard green-grey rock that can have a distinctive speckled appearance and can be confused with low-grade Cambrian bedrock. It consists of thick to thinly bedded arkose sandstone sometimes flecked with coal (the river channel and river flood deposits). There are also lesser amounts of dark interbedded mudstone and thin coal layers (the overbank deposits). There is a significant component of mafic volcaniclastic material that is often broken down into a clayey matrix, resulting in a green colouration.

Murray Basin

The Murray Basin is a Paleocene to Recent intracratonic sag basin occurring in the north of STAVELY, and covers large portions of Victoria, South Australia and New South Wales. The Murray Basin onlaps against the Paleozoic bedrock and conceals a significant portion of the Stavely Arc in the northwest of STAVELY. The top of the Murray Basin is the current flat landscape surface observed today. The undulating basal contact slopes gently away to the northwest at a dip of about three degrees (McLean, 2010), with lateral thickness variations that are gradual and attributable to sag, rather than abrupt across faults.

In the north of STAVELY, the thickness of the Murray Basin thickness reaches 250 m (e.g. groundwater bores Babatchio 20141 and Hindmarsh 20020) to 300 m (e.g. STAVELY16). Mineral exploration drill hole WI-1-6 nearby intersects 369 m of Murray Basin stratigraphy.

Farther north, seismic reflection line MEMV96-09, and petroleum drill hole Gunamalary 2 constrain Murray Basin thickness at 479 m, overlying Permian stratigraphy (Urana Formation) and Grampians Group. East of CDP 1400 in seismic line MEMV96-09 Murray Basin sediments appear to directly overlie Cambrian bedrock at an approximate depth of 500 m (Figure 2.24).

The Murray Basin succession remains too thin in STAVELY to be captured as a model volume, or forward modelled in the serial cross sections. Instead, the base of the Murray Basin is included as a surface.

The Murray Basin contains a high degree of local complexity and hosts many stratigraphic units and variations. The entire regional framework, including good unit descriptions, is contained in the synthesis volume of Brown & Stephenson (1991). The Victorian portion is detailed in Lawrence (1975) and Macumber (1991) with a brief overview in Holdgate & Gallagher (2003). In Victoria the stratigraphic sequence represents an initial fluvial system (Renmark Group) overlain by a transgressive shallow marine carbonate system (Murray Group) that was in turn overlain by a regressive beach to fluvial system (Wunghu Group) as the sea retreated.

Neotectonic fault activity may have created local basement highs within the Murray Basin but generally the drill hole density is too low to pick fault offset from a valley and ridge topography.

Newer Volcanic Group

The Newer Volcanic Province, consisting of basaltic rocks of the Miocene to Holocene Newer Volcanic Group, covers an area of 15 000 km2 in western Victoria. It represents a large intraplate basalt province formed from hundreds of small eruptions and flows, most dated between 2-4 Ma (Gray & McDougall, 2009). Flows of this age have a well-developed clay soil profile several metres thick although fresh rock is exposed in patches of stony ground and in creek beds. The basalts are glassy with small amounts of olivine, pyroxene or plagioclase crystals and often vesicular. Basalt geochemistry straddles alkaline to tholeiitic compositions (Price et al., 2003; Gray & McDougall, 2009; Boyce et al., 2014).

The Newer Volcanic Group is found in the southern half of STAVELY, where it commonly crops out at the surface. Early lava flows tend to be confined to pre-existing river valleys. Later lava flows tend to overtop the confining valleys, spreading out laterally to create wide plains of relatively thin lava flows. Thus, much of the Newer Volcanic Group cover comprising this plain is only 20 m thick, overlying Otway Basin rocks and in places directly on Paleozoic bedrock.

In the west of STAVELY, the large basalt plain covering the Dunkeld to Hamilton area is the result of overflow of the ancestral Wannon River, and is locally thicker. Around Wickliffe to Chatsworth, the lava flows remained confined within the ancestral Hopkins River to produce a narrow valley flow about 30 m thick alongside the current river. Near Lake Bolac another volcanic plain about 50-80 m thick has developed over rocks of the Otway Basin where the ancestral Salt Creek has been overtopped (Raiber & Webb, 2008).

Where the lava plain has drowned the pre-existing strandline topography of the Brighton Group, lava thickness changes from almost nothing above buried dune crests to more than 20 m above the buried swales, imparting a distinctive repetitive variation in the TMI response of the unit.

Appendix 5 Deformation history summary

Delamerian Orogeny – D1

The Delamerian Orogeny (D1) is expressed within the Grampians-Stavely Zone as two consecutive events.

D1a transpressional deformation of the Grampians-Stavely Zone

The first major deformation event to affect the Stavely Arc was east-west crustal shortening towards the end of the Delamerian Orogeny (herein termed D1a – terminology that is specific to the Grampians-Stavely Zone and is not equivalent to D1 in adjacent zones which have their own, more complex, structural nomenclature reflecting the polydeformation of high-grade metamorphic rocks (e.g. Wilson et al., 1992). D1a deformation within the Grampians-Stavely Zone appears to have been a relatively short-lived event, superimposed over rocks dated at approximately 510-503 Ma (Lewis et al., 2015, Lewis et al., 2016, Schofield et al., 2018). D1a shortening precedes a pulse of transtension (D1b) associated with intrusion of porphyries and granites at approximately 502-495 Ma (e.g. Foster et al., 1996; Stuart-Smith & Black, 1997; Morand et al., 2003). The D1a event therefore coincides with the last contractional phases of the Delamerian Orogeny mapped in the Glenelg Zone and in South Australia. Deformation timing interpreted from Ar/Ar cooling ages of metamorphic minerals within the Moornambool Metamorphic Complex of the Stawell Zone in the Moyston Fault hangingwall (Miller et al., 2005) only provide an upper age constraint for the D1 – equivalent polydeformation event in that zone (locally referred to as D1-D3).

Structures assigned to D1a within the Grampians-Stavely Zone include a family of predominantly north-northwest-trending, ductile and ductile-brittle structures – upright, sub-horizontally plunging, mesoscopic-scale folds, a sub-vertical slaty cleavage that is typically best developed in the vicinity of F1a fold closures, and spaced, northwest-striking high-angle (60-80°) strongly-emergent thrust-faults that bound the volcanic belts of the Stavely Arc – the Mount Stavely Volcanic Complex – and structurally interleave them with fault-belts of Kanmantoo Group (Glenthompson Sandstone) and Nargoon Group metasedimentary rocks.

Glenthompson Sandstone within the Grampians-Stavely Zone is mostly homoclinally dipping, but is deformed by occasional upright, gently to moderately- plunging, generally east-vergent chevron-style F1a folds (e.g. Figure 3.70; also at MGA 54 648408 5834098 and MGA 54 647555 5829253). Mount Stavely Volcanic Complex strata is also mostly homoclinally dipping but with occasional upright kink-style F1a folds. Anastomosing arrays of thin sinistral shear zones occur locally, and these are also attributed to D1a (e.g. at MGA 54 645140 5834908, at MGA 54 640583 5829254, at MGA 54 639713 5829593 and MGA 54 639655 5829609).

D1a folds and faults are associated with quartz veins and a weak, steeply-dipping, spaced stylolitic to slaty cleavage in pelitic rocks. S1a cleavage strikes subparallel to north-south fault trends, is developed axial-planar to F1a folds, and is the main mesoscopic expression of penetrative strain within the Cambrian rocks, other than in regions of soft-sediment deformation. S1a cleavage apparently formed by a combination of local dissolution of quartz and growth of fine felted chlorite and sericite and other low-temperature minerals, consistent with low temperature prograde (clockwise P-T-t path) regional metamorphism and crustal shortening at very shallow depths (Stafford et al., 2000).

D1a faults are most apparent where they juxtapose contrasting stratigraphy, for example fault slices of Glenthompson Sandstone and Williamsons Road Serpentinite within the interior of the Mount Stavely Volcanic Complex in the Stavely Belt, and as the major faults that bound the volcanic belts of the Stavely Arc. North-south trending faults also occur within stratigraphic packages, for example in steeply-dipping Glenthompson Sandstone west of the Stavely Belt where dip-slip faults appear to truncate and offset dykes of the Chatsworth Basalt and associated contact metamorphism. Imbrication along thrust faults here helps explain the large apparent thickness (>10 km) of uniformly west-dipping and facing sedimentary rocks, but precise fault locations are difficult to identify away from the dykes due to monotonous stratigraphy, poor outcrop, and lack of biostratigraphic control.

D1a crustal shortening apparently interrupted Stavely Arc magmatism, uplifted and tilted the volcano-sedimentary sequences, and closed the Kanmantoo Basin farther west to form the Glenelg Zone in the process. The Glenelg Zone apparently overthrust the western flank of the Stavely Arc along the Yarramyljup Fault (Gibson & Nihill, 1992; Morand et al., 2003) while the accretionary Stawell Zone apparently overthrust the eastern flank of the Stavely Arc along the Moyston Fault (Taylor & Cayley, 2000; Cayley et al., 2011a). This accretionary wedge represents the Early Cambrian part of the Palaeo-Pacific oceanic plate accreted onto the Stavely Arc in the Late Cambrian (e.g. Wilson et al., 1992; Miller et al., 2005). Kinematic indicators along the Moyston Fault demonstrate that the early movements along it were in response to sinistral transpression (Cayley & Taylor, 2001), and this is consistent with Cambrian East-Gondwana geodynamics (e.g. Cayley, 2011) and with the overall form and orientation of D1a structures within the Grampians-Stavely Zone.

Thus, D1a is implicated as the event that converted a low-relief, active magmatic arc system into uplifted Andean-style continental crust.

D1b transtensional deformation of the Grampians-Stavely Zone and associated magmatism

The second significant deformation event in STAVELY – D1b – is marked more by the voluminous magmatism associated with it than by structures mappable at regional scales. Magmatic complexes dated as Late Cambrian intruded along northerly-trending alignments that are interpreted to be cryptic ?transtensional structures/rift complexes that formed subsequent to, or reactivated, pre-existing transpressive D1a structures. This event is of mineral exploration significance because porphyry stocks with demonstrated Cu-Au-Mo mineralisation are associated with D1b. Some have intruded along pre-existing D1a structures (e.g. Williamsons Fault within the Stavely Belt), indicating that D1a structures were apparently reactivated and dilated during D1b.

Benambran Orogeny – D2

The main expression of the Benambran Orogeny (D2) in the Grampians-Stavely Zone is as basin-formation. Possibly beginning in the Late Ordovician, a narrow, linear, strike-persistent basin appears to have formed along the western, footwall side of the Moyston Fault. This is the Grampians Basin. The basin-deep appears to have persisted into Late Silurian, to accumulate over 3 km stratigraphic thickness of terrestrial-shallow marine sediment as the Grampians Group. The simple, elongate configuration of the Grampians Basin matches the concept of a foreland basin, a local footwall downwarp formed in response to crustal downloading associated with reverse reactivation of the Moyston Fault (Cayley et al., 2011a). This interpretation is consistent with the timing and sinistral-transpressive stress-history of fault-reactivation and associated quartz-gold mineralisation documented in the hangingwall of the Moyston Fault (Stawell Zone, event D4c in Miller et al., 2006). There is little evidence for internal syn-sedimentary deformation within the Grampians Group succession. For example, most stratigraphic thicknesses and lithofacies remain laterally constant over long distances (Spencer-Jones, 1965). This suggest that the parts of the succession most proximal to the active basin-bounding faults have been eroded or are buried.

Bindian Orogeny – D3 and D4

A small volume of felsic intrusions (dykes) introduced into the Stawell region at approximately 413 Ma and into Willaura at 412.6 ± 3.5 Ma (Willaura Rhyolite; Stuart-Smith & Black, 1999) implies a brief period of transtension, followed by dextral transpression at approximately 410 Ma (Miller et al., 2006). The transient 412-413 Ma magmatic event was introduced into Grampians Group sediments while they were still unlithified (Stuart-Smith & Black, 1999). Although early D4 transpression has not been discriminated within the Grampians-Stavely Zone, this event possibly heralds a dramatic longer-term change in the regional stress-field – a tectonic mode switch to dextral transtension, D4, with structures developed across the Lachlan Fold Belt (Cayley & Musgrave, in review).

A period of deformation in the Siluro-Devonian – referred to as D3 and D4 in this report (for a discussion of complexities within the Bindian Orogeny, refer to Cayley & Taylor, 1997a, VandenBerg et al., 2000; Cayley, 2015; Cayley & Musgrave, in review) – imparted complex thrust and fold (D3) and transtensional fault and fold (D4) structures on Grampians Group sediments, and reworked and reoriented D1a structures in the underlying bedrock. The D3 and D4 deformation events both post-dated deposition of the upper – Ludlow – parts of Grampians Group and pre-dated and/or accompanied igneous activity at approximately 405 – 400Ma. D3 is only recognised within the Grampians Group and a few bounding structures (e.g. the Mehuse Fault). The effects of D4 are widespread within the Grampians-Stavely Zone, recognised in the Grampians Group and in the underlying Cambrian bedrock. Structures formed during D3 and particularly D4 appear to link east into the Moyston Fault footwall at depth, and may have formed in response to reactivations of that major zone-bounding structure (e.g. Cayley & Taylor 1997a, 2001; Korsch et al., 2002; Morand et al., 2003). The effects of D3 and D4 deformation lose intensity progressively towards the west.

Where the effects of D4 deformation on Cambrian bedrock within the Grampians-Stavely zone appears minimal, for example in the Dimboola Belt north of Netherby, the effects of D4 deformation in the overlying Silurian Grampians Group also appears minimal. For example, Grampians Group imaged in deep seismic reflection line MEMV96-09 appears thin, regionally flat-lying, and structurally simple (see Figure 2.24).

Where Grampians-Stavely Zone Cambrian bedrock transitions laterally to greater D4 structural complexity, for example in the vicinity of the main Grampians Ranges, the greater complexity is matched by greater structural complexity in the overlying Grampians Group. This link emphasises the high degree of structural linkage between the Cambrian bedrock and Grampians Group cover, and the huge role that D3 and D4 deformation played in delivering the complex southern parts of the Grampians-Stavely Zone to its present-day configuration of dispersed, multiply reorientated Stavely Arc fragments.

D3 and D4 faults and folds tend to be associated with brittle and semi-brittle (cataclastic) structures, indicative of deformation at very shallow crustal levels. D4 structures mapped in Cambrian rocks typically lack evidence of penetrative strain and include: brittle fault and joint arrays often with prominent fault-plane artefacts (lineations, steps, fractures), distributed anastomosing and conjugate arrays of micro-faults and fractures, kink band arrays and cataclastic shear bands and, within larger strike-slip fault zones discrete wide and/or anastomosing deformation zones that exhibit a mix of cataclastic fault melange, stylolitic cleavage, brittle faults, joints and kink-folds. The best exposed examples of the latter style are for the D4 Escondida Fault, exposed in creeks near Sheepwash Road east of Mount Stavely, and for the D4 Yarrack Fault exposed in creeks south of Glenthompson.

D3 and D4 structures mapped within the overlying Grampians Group are associated with significant penetrative strain, including open folds, spaced arrays of cataclastic veinlets, distributed transgranular and intergranular grain-scale strain (Cayley & Taylor, 2001). This indicates that Grampians Group exhibited a more plastic, ductile response to D3 and especially D4 compared to the underlying bedrock.

These differences in behaviour are consistent with the age and burial history differences – Cambrian bedrock was approximately 90 million years old during D3 and D4 and is likely to have been lithified by that time. In contrast, D3 terminated Grampians Group deposition, and the oldest Grampians Group at that time would have been approximately 30 million years old or less with thicknesses less than 3.5 km and would not been fully lithified. D4 occurred shortly afterwards. The difference in rheology between old Cambrian bedrock and Grampians Group cover is reflected in D3 and D4 folds with kink-morphologies in Cambrian bedrock, whereas D3 and D4 folds in the Grampians Group have smoothly-curving morphologies. Because of differences if structural style, D3-D4 faults in and kinks in Cambrian bedrock are soft-linked to more widely-dispersed fault and fold structures in the Grampians Group.

A distinct family of faults within the Grampians-Stavely Zone, including interpreted D3 thrust-faults such as the Mehuse Fault, and faults with more complex structural histories, such as the Escondida (D1a and D4), Marathon and Golton faults (D4), and Mosquito Creek (D3 and D4), Mackenzie River, Henty (D4), Muirfoot, Cherrypool and Latani (D3-D4) faults, and related folds and megakinks such as the subvertically-plunging Mafeking Megakink, Yarrack Orocline, Tyar and Bepcha folds and warps (D4), all deform the Grampians Group cover succession as well as the underlying Cambrian bedrock, demonstrating that structural linkage existed between the cover and underlying bedrock during D3 and especially D4.

Typical D3 structures in Grampians Group include bedding-parallel dip-slip faults. These may be thrust faults as they locally exhibit structural duplication (Cayley & Taylor, 1997a, 2001). However, the homogeneity of the sequence, along with a paucity of fossils within the succession, makes demonstrating larger-scale stratigraphic repetition problematic. Repetition of pedostratigraphy reported within Grampians Group stratigraphy (Retallack, 2009) supports widespread structural repetition on bedding-parallel faults in the Grampians Ranges. For additional information on these faults refer to Cayley & Taylor 1997a, 2001). The large open Wartook Syncline (Spencer-Jones, 1965) folds the bedding parallel faults, but appears to be a late contractional structure which we include in D3.

Typical D4 structures in Grampians Group include late subvertical strike-slip faults and associated large subvertically plunging warp folds. Overprinting criteria show that D4 structures overprint D3 structures. Large, open, subvertically-plunging D4 warps refold the trend of the D3 Wartook Syncline (Cranage Orocline), part of the lower stratigraphy of the Mount William Range at Mafeking (Mafeking Orocline), and D3 fault-imbricated stratigraphy within the Victoria Range (Big Cord Orocline). In each instance, D4 warping involved clockwise rotations around subvertical axes through more than 90°. These D4 structures imply commensurate – or larger – rotations in the directly underlying Cambrian bedrock during D4.

In many places, Grampians Group strata appears to be underlain by a detachment, the D4 Marathon Fault. This complex, composite structure separates Grampians Group from the underlying Cambrian bedrock. It comprises numerous separate splays that show a variety of dips, often gently to moderately dipping. A splay of this fault was intersected in drill hole STAVELY02. The Marathon Fault seems to have formed in response to D4 dextral transtension, particularly in the vicinity of the Jalur Rift and Mafeking Megakink. See Cayley & Taylor (1997a) for details of the complex range of structures developed within the Grampians Group.

Where structures of D3 and D4 age can be traced away from the Grampians Ranges by field mapping and/or potential field data they can be shown to have significantly reworked the Cambrian bedrock. Faults assigned to D3 in the Cambrian bedrock include the Mehuse Fault. The Mehuse Fault cuts and offsets the western flank of the Dryden Belt with a reverse sense of displacement. The sinistral strike-slip Latani, Cherrypool and Muirfoot fault are also assigned to D3. The Latani Fault cuts across the Yarramyljup Fault, while the Muirfoot and Cherrypool faults sinistrally offset D1a fault belts of Stavely Arc strata in the Black Range.

Faults assigned to D4 in the underlying Cambrian bedrock include reactivation and southerly extensions to the large east-dipping Escondida Fault, and the associated development of a complex network of large predominantly dextral and subvertical strike-slip faults, Golton, Mosquito Creek, Henty and Henty West faults.

Where D3 and especially D4 faults can be mapped in the underlying Cambrian bedrock, they typically coincide with abrupt, drag-folded fault-terminations of volcanic belts (e.g., the Tyar Belt folded about the Tyar Hinge adjacent to its truncation by the D4 Henty Fault; the southern end of the Black Range Belt folded about the Bepcha Fold adjacent to D3 Cherrypool Fault truncation and; the Stavely Belt truncated against the Escondida Fault beneath Grampians Group west of Mafeking). D3 and D4 faults are associated with larger-scale drag-folds of Cambrian D1a structures about subvertical axes, such as the D4 Yarrack Fault south of Glenthompson associated with the Yarrack Fold, and the D3 Latani Fault and unnamed splay faults, associated with local anticlockwise rotation of a portion of the Yarramyljup Fault near Mount Dundas. The larger-displacement D4 faults typically coincide with juxtapositions of thick Grampians Group against Cambrian bedrock (e.g. across the McKenzie Creek Fault just south of Horsham).

The surface positions of some of the subvertical faults correspond to the positions of steep fault-breaks imaged in deep seismic reflection data. These breaks are imaged as shallowing to easterly dips at depth, suggesting linkage with the Moyston Fault footwall (Cayley et al., 2011b).

D4 structures developed in the underlying Cambrian bedrock show a south to north change in overall structural style from fold-dominated – the Mafeking Megakink and Yarrack Orocline in the southern Grampians-Stavely Zone; to rift-dominated – the northeast-trending Jalur Rift beneath the Grampians Ranges into which a large portion of the deformed Grampians Group appears to have collapsed to form the main Grampians Ranges; to fault-dominated – the dextral strike-slip Dimboola Duplex in the north of the zone.

Additional D4 structural basins that contain remnants of deformed Grampians Group are distributed along the length of the central part of the Grampians-Stavely Zone in an en-échelon fashion. These basins are also bound by D4 faults and include Log -Hut Creek (Morand et al., 2003), Woorndoo and D4 fault-bounded rifts buried beneath the Murray Basin in the vicinity of Glenlee and Lorquon – each associated with a distinct local gravity low, interpreted to indicate significant thicknesses of Grampians Group strata. The D4 rifts are discreet structures that overprint the earlier, broader and simpler D2 Grampians Basin.

The D4 rifting episode was associated with a pulse of granitic magmatism. Magmatism is concentrated into D4 rift centres and shares the predominant north-eastern elongation of the rifts. For example, the core of the Jalur Rift is occupied by the elongate, northeast- trending Victoria Valley Batholith and related intrusions at Mackenzie Creek, Stoney Creek and Mafeking. These Early Devonian granites are all dated at around 400 Ma old. The Cooac Rift between Grass Flat and Winiam East (see Figure 3.9) is filled with a magmatic complex that lies buried beneath the Murray Basin sediments. The Duchembegarra Granite within the Cooac Rift is dated at 404 ± 6 Ma (drill hole VIMP1; Maher et al., 1997). The southern parts of this magnetic complex underlie and contact-metamorphose Grampians Group at Mount Arapiles (Cayley & Taylor, 1997a).

The close spatial association of D4 structural basins and faults with D4 magmatism suggests a genetic link, and the possibility of rift-associated decompression as a trigger for mantle melting and associated A-type crustal magmatism. The granites locally intrude and therefore overprint the D4 structures developed in the Grampians Group, and hence provide a tight upper age constraint to D4. The close spatial and temporal link between D4 dextral transtension and Early Devonian magmatism, and the strong preferential alignment of many Early Devonian plutons implicates northeast-trending transtensional and extensional D4 structures as the controls on final granite emplacement (Cayley & Musgrave, in review).

All Early Devonian granites in the Grampians-Stavely Zone display porphyritic and other textures indicative of intrusion to sub-volcanic levels (Cayley & Taylor, 1997a). The granites are geochemically similar to the Rocklands Volcanic Group which are interpreted as the eruptive phase of the magmatic event. The Rocklands Volcanic Group unconformably overlie deformed Grampians Group (Simpson & Woodfall 1994; Morand et al., 2003), and may represent the upper parts of a caldera complex developed north of Cavendish and east of Gatum.

Tabberabberan Orogeny – D5

The Middle Devonian Tabberabberan Orogeny (D5)post-dates intrusion of the Early Devonian granites and eruption of the related Rocklands Volcanic Group.Structures that can be attributed to the Tabberabberan Orogeny include subvertical joint arrays that cross-cut D4 structures in the Grampians Group without deflection, and are also developed in Early Devonian granites (Spencer-Jones, 1965) and, locally, in the Rocklands Volcanic Group.

Fault movements proven to be associated with D5 are limited, because of a lack of clear timing constraints. Late sinistral strike-slip movements along the Mosquito Creek Fault documented in the Victoria Range (Cayley & Taylor, 1997a) clearly truncate and overprint D4 oroclinal bending in the Victoria Range (Big Cord Orocline), and has a movement sense that appears to be inconsistent with D4. Overprinting relationships with the Victoria Valley Granite are unclear. We attribute this movement, and associated folding in the Victoria Range, to the Tabberabberan Orogeny. Final movements along the Cattle Camp Fault overprint strike-slip deformation associated with the Mosquito Creek Fault, and so may also be Tabberabberan. The Cattle Camp Fault appears to link east into the Barbican Fault in the northern Mt William Range.

Post-Tabberabberan Deformation

The limited expression of post-Devonian deformation within the Grampians-Stavely Zone is illustrated by the widespread preservation of relatively thin, undeformed Early Devonian ignimbrite and lava flows of the Rocklands Volcanic Group (Simpson, 1997; Morand et al., 2003) and in unusually old fission-track cooling ages for the zone which constrain limited post-Devonian uplift (Foster & Gleadow, 1992).

North of the Grampians Ranges, the Netherby Trough beneath the Murray Basin is coincident with the Dimboola Belt and, in addition to Grampians Group, contains Permian fluvio-glacial strata. This suggests intracratonic sag episodes in the Permian, possibly in response to limited reactivations of pre-existing bedrock structures.

South of the Grampians Ranges, extensional faults and transfer structures have been identified which relate to formation of the Otway Basin in the Cretaceous, these trend east-west and northeast-southwest. Some of these structures are currently active and are undergoing inversion within the current crustal stress-regime with uplift expressed in the topography (see Cayley et al, in prep).

Appendix 6 Fault summary table

See accompanying file: Appendix_6 STAVELY_3D_Model_Fault_Summary_Table.pdf