TECTONIC ENVIRONMENT OF THE LATE CENOZOIC LARGE IGNEOUS PROVINCE BENEATH THE WEST ANTARCTIC ICE SHEET

The West Antarctic Ice Sheet (WAIS) flows through the volcanically active West Antarctic rift system and extended over the Ross Sea continental shelf at the last glacial maximum. Active subglacial volcanism and a vast (>1 x 10⁶km³)extent of subglacial volcanic structures have been interpreted throughout the rift from aerogeophysical surveys over central West Antarctica in the 1990s from the CASERTZ and SOAR program combined with results over the WAIS from 1958-1964 UW aeromagnetic profiles and 1978-79 SPRI-USGS coincident aeromagnetic and radar ice thickness profiles.

Modeling of magnetic anomalies constrained by radar ice sounding shows volcanic sources at the base of the ice throughout large areas, whose subglacially erupted hyaloclastite edifices appear to have been removed by moving ice, as in Iceland. This implies but cannot prove that most of the late Cenozoic volcanism occurred since the existence of the WAIS about 10 Ma despite ages as great as 30 Ma of some volcanic exposures. However, the inferred ages of the subglacial volcanic rocks are highly uncertain. Surface wave tomography shows that the uppermost mantle beneath the West Antarctic rift and extending across much of West Antarctica has slow velocities. Temperature profiles inferred from the tomographic images are consistent with lithospheric rejuvenation that has occurred within the past 25 m.y. but which may be quiescent at present. The late Cretaceous West Antarctic erosion surface has been offset along multiple N-S and E-W oriented faults, producing the Marie Byrd Land dome, with a structural relief of about 3 km. A close relationship exists between these fault offsets and two spatial-temporal patterns of volcanic activity, suggesting that the displacements are post-25 Ma in age.

A mantle plume or lower lithospheric extension beneath the rift area of West Antarctica have previously been suggested as possible causes for this late Cenozoic volcanism. Magma production from plumes either as calculated by Westaway (1993), about 0.8 x 10⁶km³/m.y., or the order of magnitude lower value of Turner et al. (1994), about 0.1 x 10⁶km³/m.y., would accommodate the formation of the >1 x 10⁶km³ West Antarctic continental flood(?) basalts (or their subglacial equivalent) in a 1-10 m.y. period.