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Geophysical Investigations of Tasmania at Multiple Scales

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Geophysical Investigations of Tasmania at Multiple Scales Esmaeil Eshaghi School of Physical Sciences (Earth Sciences) Submitted in fulfilment of the requirements for the degree of Doctor of Philosophy University of Tasmania- Australia March, 2017 i Declaration of originality This thesis contains no material which has been accepted for a degree or diploma by the University or any other institution, except by way of background information and duly acknowledged in the thesis, and to the best of my knowledge and belief no material previously published or written by another person except where due acknowledgement is made in the text of the thesis, nor does the thesis contain any material that infringes copyright. Authority of access This thesis may be made available for loan and limited copying and communication in accordance with the Copyright Act 1968. Statement regarding published work contained in thesis Chapters 4, 5 and 6 may contain copyright material and access to the material may need to be sought from the respective journal. Esmaeil Eshaghi February, 2016 ii Statement of Co-Authorship The following people and institutions contributed to the work undertaken as part of this thesis: Esmaeil Eshaghi, School of Physical Sciences – Earth Sciences = Candidate Anya Reading, University of Tasmania = Author 1 Michael Roach, University of Tasmania = Author 2 Mark Duffett, Mineral Resources Tasmania = Author 3 Daniel Bombardieri, Mineral Resources Tasmania = Author 4 Matthew Cracknell, University of Tasmania = Author 5 Stephen Kuhn, University of Tasmania = Author 6 John Everard, Mineral Resources Tasmania = Author 7 Grace Cumming, Mineral Resources Tasmania = Author 8 Paper 1, The onshore and offshore tectonic structure of Tasmania from 3D gravity modelling: Chapter 4 comprises material to be reformatted for submission to Australian Journal of Earth Sciences (AJES). The Candidate was the primary author and with Author 1, Author 2 and Author 3 contributing to the development of research ideas. The Candidate performed data processing, modelling and interpretation. Author 4, Author 5 and Author 6 contributed to the assessment and interpretation of results and presentation. Paper 2, Major granite bodies beneath prospective regions of West Tasmania: geometry redefined by potential field modelling: Chapter 5 comprises a manuscript to be submitted to Ore Geology Reviews. The Candidate was the primary author with Author 1, Author 2, Author 3 and Author 4 contributing to the development of research ideas and modelling. The Candidate performed model construction, modelling and interpretation. Author 5 contributed to the discussion of research findings and presentation. Paper 3, 3D inverse modelling of the prospective Heazlewood-Luina-Waratah region combining improved geological mapping and constraints frompetrophysics: Chapter 6 comprises a manuscript to be submitted to the journal, Interpretation. The Candidate was the primary author and with Author 1, Author 2 and Author 3 contributing to the development of research ideas. The Candidate performedmodel construction, modelling and interpretation. Author 4 and Author 5 contributed to modelling and presentation. Author 7 and Author 8 contributed to model construction and geological interpretation. Sigood Anya M. Reading John M. Dickey Supervisor Head of School School of Physical Sciences (Earth Sciences) School of Physical Sciences University of Tasmania Universityfasmania }, Date: (1/ 2- l/ ,.. l -- J2J-{V _N'/ ?- -- ·-·-··-� ( L_ ____ _ iv Abstract Tasmania, part of east Australia, is notable for its diverse geology. It contains rocks ranging in age from Mesoproterozoic to Cainozoic. Multiple orogenic events associated with granite intrusions have affected Tasmania during the Neoproterozoic, Cambrian and Devonian. Therefore, the tectonic structure of Tasmania is complex and many aspects of its evolution are the subject of current geological debate. The multiple orogenic and intrusive events have produced environments for mineralisation and Tasmania is prospective for a wide range of economic deposits. In this study, geophysical data, geological information and an improved petrophysical database have been used to investigate Tasmania at multiple scales including a sub continental scale study, a regional scale study, a local scale investigation and a prospect scale study. Deep tectonic boundaries and major geological features have been investigated using modelling techniques (e.g. 2D and 3D inversions of potential field data) to facilitate a better understanding of the tectonic evolution and present day geological structures of Tasmania. Modelling of local and prospect scale studies was performed using 3D inversion of gravity and magnetic data upon a geologically constrained initial model. Petrophysical properties (i.e. density and magnetic susceptibility) of major subsurface units are characterised in this research and used to better constrain properties during modelling. At the sub continental scale across east Australia, the Curie Point Depth (CPD) is estimated using spectral analysis of magnetic data with a low resolution of 100 × 100 km across east Australia and 50 × 50 km across Tasmania. The interpreted CPD is relatively deep across north of the Delamerian, Thomson and Lachlan Orogens and shallower throughout regions associated with Cainozoic volcanism and in the northern part of Queensland. While the CPD and Moho depth determined from seismic data generally correlate, the CPD is dominantly deeper than Moho across the Thomson Orogen and north of the Lachlan Orogen. Tasmania is characterised by CPDs ranging from ~25 km to ~40 km that correlate with seismic Moho depth reasonably well. At the regional scale study, gravity derived Moho depth has been investigated throughout onshore and offshore Tasmania. At this scale, Moho depth determined from seismic data has been reinvestigated based on 3D modelling of well distributed onshore and offshore gravity Bouguer anomaly with a resolution of 1 km × 1 km. The modelled gravity Moho depth is v inferred to be generally deeper than seismic Moho depth across onshore Tasmania. In addition, the gravity derived Moho depth map delineates ocean-ward crustal thinning and relatively shallow Moho depth across Bass Strait compatible with failed rifting in the Cretaceous. At the local scale of investigation, the geometry of granites and major geological features is refined within West Tasmania, at a 500 m × 500 m resolution, with a focus on major subsurface units to improve previous models and identify new prospective regions. A new sub-surface granite body is inferred that underlies much of the eastern region of Rocky Cape Group outcrop. This interpreted intrusion may be either Neoproterozoic or Devonian. The subsurface geometry of the known Devonian Granites in western Tasmania was also refined using both geometry and property inversions. At the prospect scale study, the Heazlewood-Luina-Waratah region, which hosts a series of significant deposits, is investigated, at 250 m × 250 m resolution, to provide a platform to facilitate further refinement and opportunities for discovery in future research. Using this model, the geometry of the Meredith Batholith and ultramafic complexes were refined, resulting in the identification of three regions prospective for mineralisation including: 1) northeast of the Waratah region associated with a newly identified granite cupolas, 2) above the Bells Syncline associated with high magnetic intensity, and lithologically prospective for skarn mineralisation, and 3) across the recently re-mapped ultramafic complexes linking the Heazlewood and Mt Stewart ultramafic complexes. vi Acknowledgement I would like to express my utmost gratitude toward all who have assisted me during my PhD, aiding me with their worthwhile and friendly advice and comment. Firstly, I’d like to deeply thank my primary supervisor, Anya Reading. Thank you so so much, Anya, for trusting me and giving the chance to an unknown candidate to commence his PhD under your supervision and continuous support during the last 3.5 years. You have kept supporting me especially during the first year of this PhD when I was frustrated with lots of unfamiliar ideas and not certain whether I can get through this challenge or not. Furthermore, you never gave up on encouraging me, tolerating my English language and providing me scientific feedback. I’d also like to acknowledge my other supervisors, Michael Roach and Mark Duffett. Thank you Michael for your significant ideas which firstly looked scary and unresolvable, but helped me to improve the quality of my outcomes. Mark, you allowed me to feel free to contact you whenever I had any questions and then provided me your advice and excellent support. Thank you and thank you and again thank you Anya, Michael and Mark. Special thanks must go to Daniel Bombardieri for instructing me on using GOCAD and inversion workflow. Thanks also to Matthew Cracknell for his support on bringing up new ideas and revising my written pieces. I would like to acknowledge the help of Stephen Kuhn as my officemate with regular conversations and sharing ideas about the PhD, global warming and other topics. In addition, I would also like to thank people from Earth Science and CODES, especially the geophysics group, Deborah Macklin, Ian Little, Sasha Stepanov, Ron Berry, Alexander Cuison, Jodi Fox, Jacob Mulder, Jeff Steadman and Selina Wu and also Joanne Whittaker from Institute for Marine and Antarctic Studies (IMAS). Hey Auntie Jane, I have
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