SHALLOW SOURCE MAGNETIC ANOMALIES OBSERVED OVER THE WEST ANTARCTIC ICE SHEET (WAIS)

Study of a block of coincident aeromagnetic and radar ice-sounding data acquired in the 1990s over the central area of the WAIS reveal about 1000 50->1000-nT magnetic anomalies, whose steep gradients indicate their sources essentially at the glacier bed. The flight elevation was 1 km and the ice is 2-3 km thick, so these magnetic amplitudes are unusually high. The anomalies are caused by subvolcanic intrusions of interpreted late Cenozoic age. About 400 of these anomalies (conservatively selected) have topographic expression as revealed by the 20-m contour interval glacier bed elevation map. More than 80% of these topographic "edifices" have <200 m relief. Obviously this low relief, even if extremely magnetic, could not produce the observed overlying anomalies at the 3-4 km observation elevation above them. We interpret this result as an indication of removal of volcanic edifices comprising hyaloclastite, pillow breccia, and other volcanic debris injected into the moving ice during eruption as has been discussed by Behrendt et al. in several papers. Although the aeromagnetic and radar ice-sounding survey over the WAIS has been discussed in these papers, the significance of the high magnetizations and thickness of the subvolcanic intrusions beneath the relatively low amplitude bed topography has been insufficiently emphasized. Only several-kilometer-thick, extremely magnetic volcanic centers (or other extremely magnetic intrusions) could produce these anomalies - NOT several hundred meter-thick volcanic flows or sills. Several of the anomalies have been modeled; any feasible models require 1-2 km thick bodies having apparent susceptibilities as great 0.3 SI, which almost certainly is the result of a very high remanent component (consistent with values reported for late Cenozoic volcanic exposures in West Antarctica). Moreover, the models show that these sources are magnetized in the present magnetic field direction. Because the Antarctic plate has been essentially stationary since 100 Ma the sources could be that old and still have present field direction. However, Jurassic age intrusive and extrusive tholeiitic rocks associated with initial rifting of Gondwanaland would be unlikely as sources because they would be expected to produce anomalies incompatible with the present field direction, for the high Q rocks beneath the WAIS apparently required.