SUBSURFACE MODELLING OF THE GILMORE FAULT ZONE: IMPLICATIONS FOR LACHLAN TECTONIC RECONSTRUCTIONS Deepika Venkataramani Supervisors: Dr David Boutelier, Dr Robert Musgrave, Dr Alistair Hack & Prof Bill Collins Thesis submitted in partial fulfilment of the requirements for the Master of Philosophy (MPhil) degree, School of Environment and Life Sciences, University of Newcastle March 2017 DECLARATION The work presented in this thesis is entirely the result of original research conducted by the author unless otherwise acknowledged. This work has not been accepted for the award of any other degree or qualification at any other university or tertiary institution. Deepika Venkataramani March 2017 i ABSTRACT This study considers the tectonic evolution of the Lachlan Orogen by modelling the subsurface morphology of the Gilmore Fault Zone (GFZ). The GFZ marks a distinct geophysical contrast between (high gravity, low magnetic intensity) high-grade metamorphic rocks found in the Wagga Metamorphic Belt (WMB) to the west, and the (low gravity, uniformly high magnetic intensity) low-grade metavolcanic rocks found in the Macquarie Arc and Silurian rift basins to the east. Subsurface structure around the GFZ in the vicinity of Barmedman (34°8'33.94"S 147°23'11.39"E) has been inverted by iterative 2.5D potential-field modelling of gravity and magnetics, constrained by pre-existing reflection seismic profiles, potential-field interpretations by previous workers, and physical properties data collected on representative lithologies. My findings show that the surface structure mapped as the Gilmore Fault is an east- dipping, shallow thrust fault, and does not correspond to the major crustal ‘suture’ envisaged in regional tectonic studies. This shallow east-dipping fault should be renamed the Barmedman fault. The Barmedman Fault flake (as opposed to the GFZ) is curved, and terminates abruptly to the north, indicating the Barmedman Fault flake is the base of a series of thrust flakes imposed on the pre-existing main fault in the GFZ. The GFZ is best described as a steep west-dipping fault zone constituting the eastern flank of the Silurian Tumut Trough. I conclude that the modelled structure of the GFZ is not consistent with the terrane accretion model since the GFZ does not mark a suture between significantly different geological units. The modelled structure of the GFZ shows evidence of multiple contractional and extensional events which are the main characteristics of the accretionary orogen tectonic model. However, steep faults are observed as well, indicating that significant lateral slip, a characteristic of the orocline tectonic model, is mechanically possible. ii ACKNOWLEDGEMENTS I thank Su God for all that I am allowed to be and for all the blessings. I acknowledge the Aboriginal custodians of the land in the region that I studied, the Elders, past, present and future, and Country itself. I OFFER MY HUMBLE GRATITUDE TO ALL THOSE WHO HAVE HELPED ME ‘ACHIEVE’ UP TO THIS POINT. I acknowledge the funding provided by the Geological Survey of NSW. I would especially like to thank Phil Gilmore, John Greenfield, Rosemary Hegarty, Jaime Robinson and Mark Eastlake for their encouragement and eagerness to help me solve problems and make discoveries. I acknowledge and thank Astrid Carlton for her multiple emails, phone calls to technical support, patience and help in processing the magnetic field data. I would also like to thank Linda Stenning for helping me process and correct the collected gravity data. Thank you to my academic SUPERvisors for pointing me in the right direction, for their multidisciplinary guidance in helping me fine tune my research and sharpen my logic like a well-crafted Katana. I thank David Boutelier for his calm, precise guidance on understanding tectonic concepts and for helping me to organise myself as a researcher. I thank Bill Collins for always setting aside time to address key concepts and improve my understanding. Thank you for your time, patience and guidance. Thank you Alistair Hack for remembering that I wanted to get involved in Geophysics and for bringing this research topic to my attention. I would have missed out on this wonderful opportunity if it weren’t for you! Finally I thank Bob Musgrave for his altruistic desire to teach and help me learn. Thank you for always encouraging me to exhibit my work and for your support at conferences. Thank you for helping me grow my knowledge in the field of Geophysics. Every conversation with you is an opportunity to expand my knowledge. I appreciate you all. No word is powerful enough to convey the debt of gratitude to my divine parents. Thank you for setting my feet upon the path of knowledge and teaching me that there is no end, that learning goes on throughout life but for also imbuing me with the strength and confidence to tackle any obstacle in my way. Venkat, thank you for your well-placed proverbs and the endless support only a dad like you can give. Thank you Amrita for having unstinting confidence in me, because of this I will always have confidence in myself. To Anu, I thank you for teaching me, “To strive, to seek, to find, and not to yield.”(Alfred Lord Tennyson) iii Table of Contents DECLARATION....................................................................................................................... i ABSTRACT ........................................................................................................................... ii ACKNOWLEDGEMENTS ....................................................................................................... iii TABLE OF CONTENTS........................................................................................................... iv LIST OF FIGURES ................................................................................................................. vi LIST OF PLATES……………………………………………………………………………………………………………………..…vii LIST OF TABLES .................................................................................................................. vii CHAPTER 1: INTRODUCTION ................................................................................................. 1 1.1 THE SOUTHERN TASMANIDES OF EASTERN AUSTRALIA .............................................. 1 1.2 AIMS ........................................................................................................................ 3 1.3 OBJECTIVES .............................................................................................................. 5 CHAPTER 2: GEOLOGICAL SETTING........................................................................................ 8 2.1.1 Orogenic Events......................................................................................................11 2.1.2 The Macquarie Arc..................................................................................................12 2.1.3 The Gilmore Fault Zone ...........................................................................................12 2.2 EXISTING TECTONIC MODELS .........................................................................................16 2.2.1 The Lachlan Orogen Formed By Suspect Terrane Accretion .......................................16 2.2.2 The Lachlan Orogen Formed As An Extensional Accretionary Orogen .........................16 2.2.3 The Lachlan Orogen Was Subjected To Subduction Flip/ Polarity Reversal ..................18 2.2.4 The Lachlan Orogen Achieved Its Current Form By Oroclinal Bending.........................18 2.2.5 Extrusion of the WMB: ............................................................................................20 CHAPTER 3: METHODS AND DATA ACQUISITION...................................................................21 3.1 PREVIOUS WORK ...........................................................................................................22 3.2 REFLECTION SEISMIC DATA ............................................................................................22 3.3 GRIDDED POTENTIAL FIELD DATA ...................................................................................24 3.4 COLLECTED TMI AND MAGNETIC SUSCEPTIBILTY .............................................................25 3.4.1 Magnetic/Solar Storms And CMEs............................................................................25 3.4.2 Buckshot Gravel......................................................................................................28 3.4.3 High Frequency Power Lines ....................................................................................28 3.4.4 Problems With Processing/ Despiking Data...............................................................28 3.5 COLLECTED GRAVITY .....................................................................................................28 3.6 PETROPHYSICAL PROPERTY DATA ...................................................................................31 3.6.1 Petrophysical Properties Analysis.............................................................................34 3.6.2 Remanence ............................................................................................................37 iv 3.7 GEOLOGICALLY CONSTRAINED INVERSION BY ITERATIVE FORWARD MODELLING .............39 CHAPTER 4: GEOPHYSICAL MODELLING RESULTS AND GEOLOGICAL INTERPRETATION ...........42 4.1 REFLECTION SEISMIC INTERPRETATION ..........................................................................42 4.2 GEOLOGICAL INTERPRETATION .....................................................................................43