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Geophysical investigations of Tasmania at multiple scales

Eshaghi, E ORCID: 0000-0002-6821-7340 2017 , 'Geophysical investigations of Tasmania at multiple scales', PhD thesis, University of Tasmania.

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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 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.

Item Type: Thesis - PhD
Authors/Creators:Eshaghi, E
Keywords: geophysics, Tasmania, modeling, geological features
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Copyright 2017 the Author

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