GLACIAL REMOVAL OF LATE CENOZOIC SUBGLACIALLY EMPLACED VOLCANIC EDIFICES BY THE WEST ANTARCTIC ICE-SHEET

Local maxima of the horizontal gradient of pseudogravity from closely spaced aeromagnetic surveys over the Ross Sea, northwestern Ross Ice S helf, and the West Antarctic ice sheet, reveal a linear magnetic rift fabric and numerous subcircular, high-amplitude anomalies, Most of the se anomalies have sources that probably resulted from late Cenozoic vo lcanism, Some of these volcanic structures penetrate the Neogene sedim ents beneath the deglaciated continental shelf and are present at the base of the present grounded ice sheet and beneath the ice shelf, Geop hysical data indicate two or three youthful volcanic edifices at widel y separated areas beneath the sea and ice cover in the West Antarctic rift system. In contrast, we suggest glacial removal of edifices of vo lcanic sources of many more anomalies, Magnetic models, controlled by marine seismic reflection and radar ice-sounding data, allow us to inf er that glacial removal of the associated late Cenozoic volcanic edifi ces (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 fragm ented, ice-contact origin, The exposed volcanoes may have been protect ed from erosion by the surrounding ice sheet because of more competent rock or high elevation above the ice sheet, Glacial removal may be th e general case; exposed late Cenozoic volcanic peaks and outcrops, con sisting primarily of flows, which erupted during Antarctic glacial con ditions since similar to 30 Ma, may be the exceptions, The volume of t he exposed volcanoes is small in contrast to the much greater volume ( >10(6) km(3)) of late Cenozoic magmatic rock remaining at volcanic cen ters beneath the continental shelf, Ross Ice Shelf, and West Antarctic ice sheet.