GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 1.3 132 Wilkes Subglacial Basin case study The Wilkes Subglacial Basin in East Antarctica hosts one of the largest marine-based and hen- ce potentially more unstable sectors of the East Antarctic Ice Sheet (EAIS). Predicting the past, present and future behaviour of this key sector of the EAIS requires that we improve our under- standing of the crustal and lithospheric cradle on which it flows. This is particularly important in order to quantify geothermal heat flux heterogeneity in the region and its influence on subglacial hydrology a prime target of 4D Antarctica - as mentioned above.    The WSB stretches for almost 1600 km from the Southern Ocean towards South Pole. Like many intracratonic basins, it is a long-lived and enigmatic geological feature, which originated and evol- ved in different tectonic settings. A wide basin formed in the WSB likely in a distal back arc basin setting, likely in response to a retreating West Antarctic active margin from Permo-Triassic times. Jurassic extension followed leading to the emplacement at shallow crustal levels of part of a huge flood basalt province that extends from South Africa to Australia.  The region was then affected by upper crustal extension and transtension of poorly constrained, but possibly late Mesozoic to Ceno- zoic age, producing narrow graben-like features. These grabens were then glacially overdeepened and this created remarkably deep basins (up to 2.1. km bsl) considering their location in cratonic continental crust. These basins are of key importance for ice sheet dynamics as they presently steer enhanced glacial flow in the Matusevich, Cook and Ninnis glacial catchments. Here we present our enhanced geophysical imaging and preliminary modelling efforts in the WSB region performed within the 4D Antarctica project of ESA. We exploit a combination of recent airborne radar and aeromagnetic data compilations and crustal and lithosphere thickness estimates from both satellite and airborne gravity and passive seismic constraints to initiate the development of new integrated geophysical models for the region. To help constrain the starting models, including depth to basement beneath the Permian to Jurassic cover rocks, we applied a variety of depth to magnetic and gravity source estimation approaches from both line and gridded datasets. A remarkable result is the huge difference between recent satellite gravity estimates of crustal thickness and sparse seismological imaging of crustal thickness. To address these discrepancies we also examine different scenarios for isostatic compensation of Rock Equivalent Topography and intra- crustal loads, as a function of variable effective elastic thickness (Te) across theWSB and its flanks. Our models suggest that in order to reconcile such large differences in crustal thickness estimates from gravity and seismology an anomalously dense lower to mid crustal layer is required. It resembles in terms of bulk density and architecture structures observed over uplifted deep crustal Paleoprotero- zoic domains imaged by higher resolution geophysical data in formerly adjacent Australia.   Our study also reveals a major lithospheric-scale boundary along the northeastern margin of the WSB, separating the Ross Orogen from a cryptic and composite Precambrian Wilkes Terrane. At the onset of enhanced flow for the central Cook ice stream, we image a Precambrian base- ment high with a felsic bulk composition. We propose, based on the similarity in potential field signatures, that it represents buried late Paleoproterozoic to Mesoproterozoic igneous basement as exposed in South Australia, where notably it is associated with high GHF (80-120 mW/m 2 ), primarily caused by anomalously radiogenic granitoids. This suggests that such rocks may exist also in the WSB region and may cause extensive areas of enhanced GHF. We hypothesise that the differences in basement depth and metasediment/sediment thickness, coupled with differences in intracrustal heat production and crust and lithosphere thickness give rise to significantly greater heterogeneity in GHF beneath different sectors of the WSB than previously resolved. Finally, we conclude that the next step to try and quantify such GHF heterogeneity in the WSB is to develop a suite of probabilistic thermal models informed by the new knowledge in crust and lithosphere composition and architecture derived from our initial 4D Antarctica study.