Glacial removal of late Cenozoic subglacially emplaced volcanic edifices by the West Antarctic ice sheet

Abstract

Local maxima of the horizontal gradient of pseudogravity from closely spaced aeromagnetic surveys over the Ross Sea, northwestern Ross Ice Shelf, and the West Antarctic ice sheet, reveal a linear magnetic rift fabric and numerous subcircular, high-amplitude anomalies. Most of these anomalies have sources that probably resulted from late Cenozoic volcanism. Some of these volcanic structures penetrate the Neogene sediments beneath the deglaciated continental shelf and are present at the base of the present grounded ice sheet and beneath the ice shelf. Geophysical data indicate two or three youthful volcanic edifices at widely separated areas beneath the sea and ice cover in the West Antarctic rift system. In contrast, we suggest glacial removal of edifices of volcanic sources of many more anomalies. Magnetic models, controlled by marine seismic reflection and radar ice-sounding data, allow us to infer that glacial removal of the associated late Cenozoic volcanic edifices (probably debris, comprising pillow breccias, and hyaloclastites) has occurred essentially concomitantly with their subglacial eruption. “Removal” of unconsolidated volcanic debris erupted beneath the ice is probably a more appropriate term than “erosion,” given its fragmented, ice-contact origin. The exposed volcanoes may have been protected from erosion by the surrounding ice sheet because of more competent rock or high elevation above the ice sheet. Glacial removal may be the general case; exposed late Cenozoic volcanic peaks and outcrops, consisting primarily of flows, which erupted during Antarctic glacial conditions since (Approx.)30 Ma, may be the exceptions. The volume of the exposed volcanoes is small in contrast to the much greater volume (>106 km3) of late Cenozoic magmatic rock remaining at volcanic centers beneath the continental shelf, Ross Ice Shelf, and West Antarctic ice sheet.