LARGE SCALE CONTINENTAL EXTENSION IN WEST ANTARCTICA

One of the largest and least known active rift provinces on Earth is the West Antarctic Rift System (WARS), the broad region of extended continental crust, 750–1000 km wide, that underlies the Ross Embayment and West Antarctic ice sheet. The province today varies in crustal thickness from 22 to ~30 km, with crustal stretching factors of β = 1.3 to 1.8, and is underlain by warm mantle with low seismic velocities. Major broad extension occurred as little as 20 million years in the middle Cretaceous, with ~600 km of translation in the south and ≥1000 km in the north, based on paleomagnetic work. One extensive exposure of granulite grade gneisses and granite, the Fosdick Mountains, forms a dome exhumed upon a detachment fault and rapidly cooled at circa 100 Ma. Also in this time interval, mylonites developed in the Cape Colbeck shear zone and a detachment system was active on the Ross Sea’s Central High. On the whole, however, the crustal structures responsible for large intracontinental extension have been poorly known in respect to location, dimension, kinematics and timing, because they are obscured by ice sheets, ice shelves, and ocean.

The disadvantageous situation is being addressed through a new aerogeophysics acquisition over a vast under-explored sector of the rift, the Ross Ice Shelf. The multi-institution, multidisciplinary program in 2015 to 2017 is called ROSETTA-Ice, for Ross Ocean and ice Shelf Environment, and Tectonic setting Through Aerogeophysical surveys and modeling. The investigation of crustal geology combines new gravity data with magnetics to model bathymetry, then delineate sediments, bedrock geological units, and faults beneath the Ross Ice Shelf. The results are being used to interpret the locations and characteristics of faults and tectonic basins, and thereby achieve a greater understanding of Mesozoic large-scale extension across the West Antarctic rift system, active volcanism from Oligocene to present, and ongoing narrow extensional to wrench deformation in the province.

This contribution aims to provide an overview of the tectonic evolution of the West Antarctic extensional province, reexamine ‘legacy’ geophysical data within the context of 21^st century datasets, and introduce new hypotheses about large scale continental extension in Antarctica, based on analysis of ROSETTA-Ice data.