The 3D Crustal Structure of Wilkes Subglacial Basin and Transantarctic Mountains in East Antarctica - inferred from Joint Inversion of airborne gravity and magnetic data.

Heterogeneities in subglacial geology and crustal properties can play a major role in determining the boundary conditions at the crucial interface between the solid earth and the cryosphere in Antarctica. Geothermal heat flow, a parameter closely tied to regional geology, can particularly influence the behaviour of the overriding icesheet. However, direct geological samples which could inform understanding of heat flow and other geological parameters are limited to ice free regions along the coast, high mountain ranges or isolated nunataks, while the origin of geological material transported by glaciers themselves is often ambiguous. Geophysical joint inversion of gravity and magnetic data can therefore play a key role in constraining the geological and crustal properties of the rocks hidden beneath the ice.

We present a 3D crustal model of Wilkes Subglacial Basin and Transantarctic Mountains based on joint inversion of airborne gravity and magnetic data using the “Variation of Information” inversion algorithm incorporated in the software JIF3D. The applied “Variation of Information” technique enforces a coupling in the objective function between inverted susceptibility and density distribution during the inversion. The objective function is minimized iteratively until a reasonable mismatch between observed and inverted data is reached. The coupling ensures that identical geometries in the inverted density and susceptibility distribution are found relating to shared gravity and magnetic sources. This technique provides an enhanced inversion result for interpreting subglacial geology since inverted geometries relate to both petrophysical quantities compared to separately inverting for density and susceptibility distributions.

Our model reveals a large body located in the interior of Wilkes Subglacial Basin interpreted as a batholithic intrusive structure, as well as a linear dense body at the margin of the Terre Adélie Craton. Density and susceptibility relationships of the anomalous bodies, together with their shapes in 3-dimensions are used to inform the composition and the origin of these crustal bodies. Comparing the density and susceptibility values recovered by our inversion with measurements on Antarctic rock samples indicates that the postulated batholitic intrusion is granitic in composition, but distinct from the Granite Harbour Igneous Complex described previously in the Transantarctic Mountains area. Emplacement of such a large volume of intrusive granite can potentially elevate local geothermal heat flow significantly, due to relative concentration of radiogenic heat producing elements such as U, K and Th. Finally, we present a new conceptual tectonic model for the region based on the inversion results, which includes passive margin development ~670 Ma, with the emplacement of the batholith intrusion before the Granite Harbour Suite.