33-8 Characteristics of Thinned Crust and Magnitude of Lithospheric Extension in the West Antarctic Rift System

Session: Going with the Shear - New Insights into Lithospheric Extensional and Strike-Slip Systems (Posters)

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During the middle Cretaceous Period, the Pacific active margin of Gondwana underwent rapid lithospheric extension/transtension over a vast region spanning the Ross Embayment, Marie Byrd Land (MBL), and Zealandia. Metamorphic core complexes (MCCs) developed in MBL and southern Zealandia, as one of the expressions of intracontinental tectonism ca. 115-90 Ma that preceded break-up across Gondwana’s convergent margin. In addition to the few MCCs exposed, other MCCs may have subsided within extended lithosphere or are concealed beneath the continental ice sheet. Much of the thinned continental crust of the West Antarctic Rift System (WARS) has subsided to depths from 500 to 2500 m below sea level.  Shallowest depths below sea level coincide with prominent, broad bedrock ranges such as the Mid-Shelf and Central Highs, which show evidence of subaerial exposure in the Oligocene. Steep magnetic and gravity gradients suggest fault control for the highs and intervening basins, which contain up to ≥3700m of sediment. The highs show evidence of glacial valleys opening the possibility that fault block mountains hosted an alpine ice cap that expanded to form the West Antarctic Ice Sheet.  

For the Antarctic region covered by ice sheet and ocean, β-factor, the ratio of initial crustal thickness to final crustal thickness, provides a means to quantify the extent and magnitude of thinning. We calculate the distribution of β-factors for the 550km² Ross Ice Shelf (RIS) region, over which we obtained airborne magnetics and gravity data on a 10-km flightline spacing. For this region we used the magnetic anomaly data to calculate basement topography (bathymetry), in order to determine the position of the top of the crust. Our calculation used a uniform 38 km for initial crustal thickness (see 10.1080/08120090701615691). The presentday crustal thickness is provided by a continent-wide Moho model determined from seismic surface wave observations that define the base of the crust (10.1002/2014JB011332).

We find that β-factor ranges from 1.6-1.9 in the eastern and southern part of the RIS sector and part of the Mid-Shelf High. β values for the western RIS are 2.0-2.3. The change in degree of crustal extension coincides with a geophysical boundary between the West Antarctic greywacke-granite province and the 'cratonic margin' crust of East Antarctica that occurs within the Mid-Shelf/Central Highs. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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