Precedente internacional y nacional

The questions posed in “Section 1.4: major research questions to be addressed” were pursued in chapters 2, 4, 5 and 6 and the responses are summarised in this section. Correlations between tectonic boundaries (i.e. Moho and CPD) using different geophysical data and methods have been investigated in Chapters 2 and 4. In addition, subsurface geology was better constrained using a multi-disciplinary approach and 3D potential field modelling across West Tasmania (local scale, Chapter 5) and the Heazlewood-Luina-Waratah region (prospect scale, Chapter 6) to address the questions.

7.2.1.On a sub continental scale, does the Curie Point Depth (CPD) determined from spectral analysis of magnetic data correlate with Moho depth

determined from seismic data throughout east Australia? What do any disparities between these depth surfaces tell us about the overall context of tectonic structure and evolution?

Chapter 2 showed a sub continental scale CPD model, with a resolution of 100 km × 100 km across east Australia and 50 × 50 km across Tasmania, that improved knowledge in comparison to previous studies. Relationships between the CPD and seismic Moho depth across east Australia indicate that these boundaries correlate to some extent. The range of differences between these two boundaries is usually within the expected uncertainties of models. Nevertheless, Proterozoic basements and northern parts of the Thomson and Lachlan Orogens indicate deeper CPDs than the seismic Moho depth. This could be because of presence of features associated with low thermal gradient (i.e. Precambrian terranes). The other scenario is that upper parts of the lithosphere are magnetised and the Moho boundary is not necessarily a magnetic boundary. The estimated CPDs are predominantly deeper than expected from geothermal gradients estimated from the OZTemp model. This paradox between estimated and expected CPDs could be due to the uncertainty in large scale spectral analysis of aeromagnetic data and/or the incomplete understanding of the thermal regime and undergoing terranes associated with low geothermal gradient at depths deeper than 5 km.

Tasmania generally exhibits shallower CPD ranges compared to the average CPD values across east Australia indicating the possibility of more recent active tectonics associated with a higher average geothermal gradient. In detail, northeast Tasmania shows a shallower CPD range compared to western parts of Tasmania which would be consistent with the presence of younger terranes and tectonic evolution of this region.

7.2.2. Does the Moho depth defined by gravity data correlate with the Moho depth defined by seismic data for the Tasmania region? What do any disparities between these depth surfaces tell us about the evolution of Tasmania?

Chapter 4 examined a 3D regional scale model, with a resolution of 1000 mE × 1000 mN × 500 mZ, to improve knowledge about the gravity constrained Moho depth compared to the previous seismic Moho depth across onshore and offshore Tasmania. The refined gravity derived Moho depth can assist to better reconcile tectonic structures throughout Tasmania. Findings in this chapters show that the gravity Moho depth does not correlate with the seismic constrained Moho model. The gravity model shows that the study area is characterised by the continental crust without the presence of the (purely) oceanic crust. Compared to the seismic

Moho depth, the gravity Moho depth is generally deeper across onshore Tasmania. Northeast Tasmania is characterised by very deep Moho depths (up to 40 km), unlike the seismic model. This is likely due to the crustal thickening across this region during Tabberabberan. The gravity constrained Moho depth delineates the crustal thinning across the Bass Basin and beyond the unstretched continental crust limit.

7.2.3.Is it possible to better constrain subsurface geology in the region of West Tasmania using a multi-disciplinary approach and 3D potential field modelling? How might any improvements aid the understanding of tectonic processes and help to direct the activities of mineral explorers?

The local scale model in Chapter 5, with a resolution of 500 mE × 500 mN × 200 mZ, has improved knowledge into distribution of major geological features (e.g. Devonian Granites) using a multi-disciplinary approach and 3D potential field modelling across West Tasmania. In this chapter, I found that the Devonian Granites have shallower depths between the Heemskirk and Meredith Granites and are deeper toward west of the Rocky Cape Group compared to the initial model. In addition, the refined model shows the maximum thickness of Housetop Granites is < 6 km with likely thick (1—2 km thickness) and highly magnetised ultramafic complexes at depth throughout the Housetop region. This model also identifies two granitic intrusions: 1) a new granitic body across east of the Rocky Cape Group, with minimum depth of > 3 km, with a possible emplacement age of either Neoproterozoic or Devonian, and 2) a non-magnetic, low density Cambrian Granite component in south of the MRV following 2D models presented by Leaman (2003). The newly identified granitic intrusion across east of the Rocky Cape Group suggests a wider distribution of either Neoproterozoic or Devonian Granites across West Tasmania. Therefore, the implication of the newly identified granitic body in the east of the Rocky Cape Group is to suggest a greater extent of the Neoproterozoic or Devonian tectonic events across Tasmania. If this granitic unit has a Neoproterozoic origin, it would improve our insights into the possible extent of Wickham Orogeny and its impact across northwest Tasmania. The newly identified granitic body to the west of the Rocky Cape Group has a minimum depth of deeper than 3 km and would be challenging to drill directly, whereas the likely Cambrian Granite component in south of the MRV has an estimated burial depth of ~1 km and can be accessed by a deep drill hole for a direct sampling for petrophysical characterisation and determining the exact age of the component via geochronology.

To direct the activities of mineral explorers, the refined geometry across West Tasmania suggests the presence of possible deep mineralisation (e.g. Ag, Pb-Zn, Fe, W skarn) across the Housetop region, similar to the Heazlewood UC. In addition, the Heemskirk and Meredith Granites are linked at shallower depths, compared to previous models, and that this is compatible with known and potential mineralisation between these two components. Hence, the local scale model emphasises that the area between the Heemskirk and Meredith Granites units could be a target for further future exploration activities.

7.2.4.Is it possible to make similar improvements on a local and prospect scale and how might these improvements help to direct the activities of mineral explorers?

Chapter 6 showed a prospect scale model, with a resolution of 200 mE × 200 mN × 100 mZ, providing insights into distribution and geometry of major units across the Heazlewood-Luina- Waratah region. The model improved constraints on the geometry of the Meredith Batholith and exhibited recently observed ultramafic complexes linking the Heazlewood River and Mt Stewart UCs. The refined geometry of batholiths indicates that the northern part of this component has a steeply dipping boundary at depth with likely granite cupolas at shallow depths (less than 2 km depth) toward northeast of the study area. Property inversions of the model show that the Heazlewood River UC are characterised by a high internal variation of density and magnetic susceptibility values. These complexes are also characterised by high average density values. In addition, geometry and property inversions of the contact aureole of the Meredith Batholith and Mt Stewart UC show these units have a maximum depth extent of < 2 km with variable density and magnetic properties. This model identifies three regions with potential for mineralisation for future explorations at a deposition scale which are discussed in Section 7.3.