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