Crustal properties and heat flow in Victoria Land and Wilkes Subglacial Basin, Antarctica – forward and inverse modelling of gravity and magnetic data coupled with petrological measurements

This thesis utilises inverse and forward modelling of geophysical data to investigate the subglacial geology and influences on geothermal heat flow in northern Victoria Land and in Wilkes Subglacial Basin, a region of East Antarctica adjacent to the Transantarctic Mountains (TAM) that is comprehensively covered by ice. The TAM separate the warmer lithosphere of the Cretaceous-Tertiary West Antarctic Rift System and the colder and older provinces of East Antarctica. Low velocity zones beneath the TAM imaged in recent seismological studies have been interpreted as warm low-density mantle material, suggesting a strong contribution of thermal support to the uplift of the TAM. For the first of the three main studies that comprise this thesis (Chapter 2), I present new Curie Point Depth (CPD) and geothermal heat flow (GHF) maps of the northern TAM and adjacent Wilkes Subglacial Basin (WSB) based exclusively on high resolution magnetic airborne measurements. I find shallow CPD and high GHF beneath the northern TAM, reinforcing the hypothesis of thermal support of the topography of the mountain range. Additionally, the study demonstrates that limiting spectral analysis to areas with a high density of aeromagnetic measurements increases the resolution of CPD estimates, revealing localised shallow CPD and associated high heat flow. The recovered CPD and GHF estimates show a good agreement to sparse in situ GHF measurements and the location of active volcanoes. Subglacial geology influences geothermal heat flow. However, due to the lack of geological information most GHF models do not account for a heterogeneous crust since direct geological information in Antarctica is limited to ice-free regions along the coast, high mountain ranges or isolated nunataks. Therefore, indirect methods are required to interpret subglacial geology and heterogeneities in crustal properties. In Chapter 3 I present a 3D crustal model of density and susceptibility distribution in WSB and TAM region based on joint inversion of airborne gravity and magnetic data. This model reveals a large body located in the interior of WSB interpreted as a batholithic structure, as well as a linear dense body at the margin of the adjacent Terre Adélie Craton. Density and susceptibility relationships are used to interpret petrophysical properties and permit the reconstruction of those crustal bodies. The petrophysical relationships indicate that the postulated batholith is granitic in composition. Emplacement of a large volume of granite batholiths can potentially elevate local geothermal heat flow significantly. Finally, I present a new conceptual tectonic model for this region based on the inversion results. Chapter 4 addresses the validation of geophysical modelling to constrain hidden subglacial geology. I present a new rock property catalogue containing density and susceptibility measurements on 320 rock samples from northern Victoria Land. This catalogue is used to assess the reliability of local and regional scale inverse results. I compare those measurements to the inverted values. The close correspondence between inverted and measured rock properties allows us to predict locations of rock types where currently such information is missing. The presented studies in this thesis demonstrate that subglacial geology matter for geothermal heat flow. Future heat flow models need to account for heterogeneities in subglacial geology and crustal properties in terms of radiogenic heat production and thermal conductivity. To obtain such information the broader geophysical community needs to increase their efforts of imaging geology hidden beneath the ice in Antarctica. This thesis provides evidence that joint inversion of gravity and magnetic airborne data is a powerful tool to infer subglacial geology and crustal properties. Furthermore, this thesis provides a workflow that can be followed for joint inversion studies in other areas of Antarctica where sufficient airborne data exists.

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