University of Tasmania
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Geoelectric structure of the Tasmanian lithosphere from multi-scale magnetotelluric data

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posted on 2023-05-28, 12:33 authored by Ostersen, TCH
The magnetotelluric (MT) method is a powerful geophysical technique capable of imaging geo electric structure at depths ranging from the uppermost crust to the asthenospheric mantle. It exploits electrical resistivity as a key physical property that varies over several orders of magnitude in Earth materials and is dependent upon mineralogical and textural phenomena. Three dimensional (3D) resistivity models and two dimensional (2D) resistivity sections there fore yield geological information that would otherwise be inaccessible. This thesis presents an investigation of the geoelectric structure of the Tasmanian lithosphere driven by three separate newly collected or newly compiled MT data sets. While some of the first geoelectric studies in Australia took place in Tasmania, this is the first study that covers the entire island state. Tasmania has a plate tectonic history that brings together contrasting terrains with subsequent orogenesis, volcanism, sedimentation and intrusive events. The new geoelectric data sets therefore have the potential to advance knowledge of these diverse influences to benefit of both the fundamental understanding of tectonic evolution and practical, pre-competitive resources exploration. Long period MT data were collected in the course of a major field program across Tasmania, and on Flinders Island in Bass Strait, as part of the national Australian Lithospheric Archi tecture Magnetotelluric Project (AusLAMP) initiative. This deployment yielded a regionally spaced data set with periods ranging 5 s to 16 000 s. Induction arrow patterns in AusLAMP data agree with earlier MT studies in highlighting a region of low resistivity coincident with, and extending southward from the Tamar River in northern Tasmania. Phase tensor ellipses plotted for the MT sites reveal stark contrasts in geoelectric structure between East Tasmania terrain (ETT) and West Tasmania terrain (WTT), with the ETT giving low minimum phase angles indicative of higher resistivities throughout the lithosphere. By contrast, the WTT is characterised by highly variable phase angles reflecting a heterogeneous resistivity structure Three dimensional inverse modelling of the AusLAMP data set yielded a regional-scale 3D resistivity model spanning the upper-crust through to lithospheric mantle depth range (‚Äöv†¬¿10 km to ‚Äöv†¬¿100 km). The model reveals anomalously low resistivities along major terrain boundaries in the mid- to upper-crust likely resulting from conductive phases and pore fluids in shear zones. Highly resistive zones in the upper-crust correlate with the distribution of voluminous Devonian and Cambrian granitoid intrusive rocks. At deeper levels in the model, in the mid- to lower crust, clear differences in the electrical structure of the geologically distinct ETT and older WTT emerge. The ETT is uniformly resistive, likely reflecting depletion of conductive phases following voluminous granitic melt extraction, while the WTT is electrically heterogeneous. Elevated resistivity in the ETT continues to the lithospheric mantle and is interpreted as the result of mantle geochemistry depleted of volatiles through similar processes. Elevated lithospheric mantle conductivities imaged beneath central and northwestern Tasmania in the WTT are interpreted as being due to subduction-related mantle metasomatism potentially associated with intrusion of ubiquitous Jurassic dolerites. In addition to the long period MT surveys, two broadband MT traverses were acquired for shallower investigations of the lithosphere beneath western and north western Tasmania. These surveys transected major crustal boundaries between Precambrian Tyennan basement and the highly metalliferous Cambrian to Devonian Dundas-Fossey Trough. Two dimensional inverse modelling of the traverses broadly agree with regional 3D modelling derived from AusLAMP MT data, and bring the geoelectric structure of the upper crust in these regions into sharper focus. In the west transect, upper crustal rocks within the Precambrian basement are considerably less resistive than younger Dundas-Fossey rocks and preserve internal heterogeneity potentially imaging relict metamorphic fluid pathways. The highly resistive Dundas-Fossey Trough contains small-scale low resistivity structure coincident with crustal-scale faults observed at surface. Low resistivities in the western part of the west transect are interpreted to image serpentinised Cambrian ultramafic rocks. Inverse modelling of the north transect was ultimately not carried out due to the severity of power line noise and consequent insensitivity of the data to upper crustal structure. ix of the AusLAMP long period MT sites were deployed in central eastern Tasmania sur rounding the Lemont geothermal prospect, a region of anomalously high surface heat flow that was the subject of exploration for geothermal energy by KUTh Energy Ltd between 2007 and 2011. A 255-site broadband MT data set was acquired by KUTh Energy Ltd at Lemont between 2008 and 2010 which culminated in the inversion of a 3D resistivity model of the area. The newly acquired long period MT data, together with the existing broadband data, presented an opportunity to re-model the Lemont MT data set. This modelling exercise improved on the previous 3D resistivity model by increasing the horizontal resolution of the model space, and incorporating the AusLAMP 3D model as a priori model structure. Together with different inversion approaches to static shift effects in the data, these improvements yielded significant differences in subsurface resistivity structure relative to the previous model, with the updated model containing more detail. The distribution of low resistivity zones in the updated model agrees with inferred subsurface fault structures thought to represent porous conduits for fluids radiogenically heated by underlying Devonian granitoids. The updated model fits and improves the conceptual model of the Lemont geothermal resource, with the ability to target exploration drill holes with greater confidence.


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