THE STRUCTURAL AND METAMORPHIC HISTORY OF THE WILMOT AND FRANKLAND RANGES, SOUTH - WEST TASMANIA. by CLIVE ANTHONY BOULTER, B.Sc. (Hons) VOLUME ONE TEXT submitted in partial fulfilment of the requirement for the degree of Doctor of Philosophy. UNIVERSITY OF TASMANIA HOBART I agree that, this thesis may be available for loan and copying, immediately after its acceptance by the Universit f Tasmania (January 1978) (Signed) —Cep Date .76 ...... A4k.'78 Except as stated herein this thesis contains no material which has been accepted for the award of any other degree or diploma in any university, and, to the best of my knowledge and belief, this thesis contains no copy or paraphrase of material previously published or written by another person, except when due reference is made in the text of the thesis. C. BOULTER "Those who wish for satisfactory foundations of facts on which to build their theories, must even be content to take their hammers in their hands, and having strapped on their knapsacks, to seek in the field of nature, the facts for themselves". Haughton, 1856. THE STRUCTURAL AND METAMORPHIC HISTORY OF THE WILMOT AND FRANKLAND RANGES, SOUTHWEST TASMANIA. ABSTRACT Tidally dominated, shallow-shelf sea conditions are indicated for the deposition of much of the late Precambrian sequence of the Frankland and Wilmot Ranges, Southwest Tasmania. Predominantly pelitic units may reflect tidal-flat or deltaic situations, and detritus in marine quartz sand horizons was derived from an environment of prevailing aeolian conditions perhaps by transgression. Overprinting proves five cleavage-forming deformation events followed by at least two conjugate kink sets and a regional 'fault- drag' rotation of all structural surfaces. The major geometrical features are the result of D and D whilst D and D locally 1 4 2 3 produce macroscopic folds; all D to D 1 5 events are essentially coaxial. Major D 4 folds are generally tight upright structures with wavelengths and amplitudes of about four kilometres and they rotate all earlier events. Gravitational gliding was responsible for the emplacdment of F 1 structures which were subsequently, in the main, only slightly modified by further overriding during the same event. D folds root to the west and southwest and though 1 F show the same direction of overthrusting it is uncertain whether 2 D /D is a continuous event. D /D show orthotectonic characteristics 1 2 1 2 whilst D is of a paratectonic style. D and the later conjugate 4 5 kink bands are minor in scale. The regional rotation of deformations before and including those of D 5 and the kinks is considered to have been caused by pre-Ordovician transcurrent movement on the Lake Edgar Fault. Quartz arenite, in D first deformed by plastic 1' deformation in hydrolytically weakened diagenetic quartz overgrowth but soon stress difference, in the grain-supported arenite, was taken up by intragranular plasticity of the detrital grains. Structural grains become apparent in outcrop at less than 10% shortening and good cleavages require less than 20% shortening. Penetrative fabrics developed in pelitic rocks in D l . All phases are extremely heterogeneous, with unstrained zones surviving to the present day. Post-D cleavages usually involve microfolding of earlier fabrics, 1 pressure dissolution and/or intracrystalline plasticity of quartz and mica. The relative importance of each mechanism is dependent on the pre-existing fabric and mica content of the rock. Investigations of strain in quartz arenite revealed a need to measure sedimentary fabrics from non-orogenic areas to provide a sound basis for work in deformed material where initial marker ratios and orientations were variable. Methods chosen must also allow for an independent check on the validity of two-dimensional strain ratios. The geometry of deformed cross-bedding shows that flexural slip was important in the formation of major F 1 structures which were modified by an average 25% flattening. As sedimentary structures are commonly modified or mimicked by deformation, care in interpretation is emphasised. Soft sedimentary, pre-tectonic clastic dykes are often planar and sub-parallel to cleavages where both structures are at an angle to bedding. Ready convergence of sedimentary and tectonic elements is thus demonstrated and the use of the approximate parallelism of dykes and cleavage to support tectonic dewatering is considered unsound. LIST OF CONTENTS Page CHAPTER 1 INTRODUCTION 1.1: Nature of the Study 1 1.2: Location and Access 3 1.3: Geomorphology 5 1.4: Previous Literature 9 1.5: Conventions 12 1.6: Acknowledgements 14 CHAPTER 2 ROCK TYPES 2.1: Metasediments - General Considerations 17 2.2: Metasedimentary Lithologies 18 2.2: i) The quartz arenite association 18 2.2: ii) The mudstone association 21 2.2: iii) Meta-carbonate rocks 26 2.3: Environmental Interpretation of the Meta-sediments 27 2.4: Environmental Conclusions 32 2.5: Meta - Igneous Rocks 33 CHAPTER 3 MINOR STRUCTURES, MICROFABRIC AND STRUCTURAL SEQUENCE 3.1: Establishment of Structural Sequence 34 3.2: Minor Folds and Boudinage of the First Deformation Event 37 3.2: i) Quartzite structures 37 3.2: ii) Interlayered Quartzite/Phyllite structures 39 3.3: D Microfabric 40 1 iv Page 3.4: Minor Structures of the Second Deformation Event 47 3.4: i) Quartzite structures 47 3.4: ii) Interlayered Quartzite/Phyllite structures 48 3.5: D Microfabric 49 2 3.6: Minor Structures of the Third Deformation Event 58 3.7: D Microfabric 58 3 3.8: Minor Structures of the Fourth Deformation Event 61 3.9: D Microfabric 63 4 3.10: Minor Structures and Microfabric of the Fifth Deformation Event 66 CHAPTER 4 MAJOR FOLD GEOMETRY AND SUB - AREA DESCRIPTIONS 4.1: Nature of Structural Domainc 68 4.2: Sub-area Descriptions 69 4.2: i) Sub-area 1: Gordon Dam to Serpentine Dam 69 4.2: ii) Sub-areas 2, 5: Mt. Sprent to Koruna Peak including the Central Wilmots 74 4.2: iii) Sub-areas 3, 4: The Bell, Detached Peak, and The Starfish 78 4.2: iv) Sub-area 6: Southern Wilmots 81 4.2: v) Sub-area 7: Tribulation Ridge 84 4.2: vi) Sub-area 8: Coronation Peak - Double Peak 86 4.2: vii) Sub-area 8: Tombstone Hill 89 4.2: viii) Sub-area 9: Redtop Peak to Cleft Peak 91 4.2: ix) Sub-areas 10, 11: Greycap, Frankland Saddle, Frankland Peak, Secheron Peak, Terminal Peak 95 4.3: Post D Minor Structures 103 5 4.4: Discontinuous Structures 105 Page 4.5: Synthesis of Major Structures 107 CHAPTER 5 MINERALOGY AND PRESSURE/TEMPERATURE CONDITIONS DURING DEFORMATION 5.1: Introduction 110 5.2: Textural Analysis 111 5.3: Metamorphism of the Metasedimentary Rocks 114 5.4: Metabasites and Related Rocks 119 5.5: Discussion of Pressure Temperature Conditions 124 CHAPTER 6 THE DEFORMATION OF SEDIMENTARY STRUCTURES 6.1: General Considerations 128 6.2: The Tectonic Dewatering Hypothesis 129 6.3: Clastic Dykes and Tectonic Structures in the Frankland/Wilmot Area 131 6.3: i) Tectonic Structures at Locality One 131 6.3: ii) Relations of Clastic Dykes to Cleavages at Locality One 132 6.3: iii) Tectonic Structures and Relations to Clastic Dykes at Locality Two 134 6.3: iv) Discussion of time of formation of the clastic dykes and the present form of the dykes 136 6.3: v) The Tectonic Dewatering Hypothesis in the light of this and other Recent Studies 139 6.4: Primary Deformation of Cross-bedding and its Simulation of Tectonic Folds 144 6.5: Tectonic Deformation of Cross-bedding and Mechanisms of Formation of Large-scale D folds 147 1 CHAPTER 7 THE MEASUREMENT OF STRAIN 7.1: Outline of the Strain Analysis Project and Objectives 152 7.2: Published Methods for the Analysis of Strain with Complex Initial Fabrics 153 vi Page 7.2: i) Discussion of the fundamental assumptions in the strain analysis methods 157 7.2: ii) The Elliott Method 158 7.3: Sedimentary Distributions on Elliott Plots 161 7.4: Problems of Strain Analysis with the Elliott Method 167 7.5: Modifications to the Elliott Technique 169 7.6: A Pilot Project to Test the Modified Elliott Approach - Strain Analysis of a Cleaved Oosparite Affected by One Deformation 170 7.6: i) Discussion of the constrictional tectonic strain values of the oolite markers 175 7.6: ii) Conclusions from the application of the Elliott Method 175 7.7: Strain Analyeis within the Frankland Range Quartz Arenite 176 7.7: i) Structural position of the specimens 176 7.7: ii) Working methods and practical problems 177 7.7: iii) Elliott plots and the assessment of strain 178 7.7: iv) Heterogeneous deformation and deformation mechanisms in D 184 1 7.7: v) Exceptionally low strain states in tight D 1 folds 185 7.8: Strain Analysis in the Quartzite using Three Sections Perpendicular to XY 188 7.9: Summary and Conclusions on Strain Analysis Methods 196 CHAPTER 8 SUMMARY AND CONCLUSIONS 198 8.1: Sedimentation 198 8.2: Nature of the D Event 1 199 8.3: The Metamorphic Climax 204 8.4: Structural Evolution Post the 206 Metamorphic Climax 8.5: Microfabric Summary 209 8.6: Fabric Strain 211 vii Page 8.7: Structural Correlation in Southwest 213 Tasmania 8.8: Observations on Some General Aspects of Tasmanian 215 Precambrian Geology REFERENCES 218 APPENDIX 239 Rock Specimen Collection 1 CHAPTER ONE INTRODUCTION 1.1: Nature of the Study The study was primarily a structural characterisation of the 'metamorphosed Precambrian' rocks of the Frankland and Wilmot Ranges, Southwest Tasmania (see Figure 1.1). Basic features such as the structural sequence .j and geometryt and metamorphic conditions were to be studied with follow up investigations on aspects requiring more detailed analysis.