Geophysical investigations of Tasmania at multiple scales

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 ‚àöv= 100 km across east Australia and 50 ‚àöv= 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 ‚àöv= 1 km. The modelled gravity Moho depth is 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 ‚àöv= 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 ‚àöv= 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.