HR: 0800h
AN: C51A-0068    [Abstracts]
TI: Question of Ages of Cenozoic Volcanic Centers Inferred Beneath the West Antarctic Ice Sheet (WAIS) in the West Antarctic Rift System (WR) from Coincident Aeromagnetic and Radar Ice Sounding Surveys
AU: * Behrendt, J C
EM: behrendj@colorado.edu
AF: INSTAAR, University of Colorado, Boulder, CO 80309-0450, United States
AU: * Behrendt, J C
EM: behrendj@colorado.edu
AF: U.S. Geological Survey, Federal Center, Denver, CO 80225, United States
AU: Finn, C A
EM: cfinn@usgs.gov
AF: U.S. Geological Survey, Federal Center, Denver, CO 80225, United States
AU: Blankenship, D D
EM: blank@utig.ig.utexas.edu
AF: Institute of Geophysics, University of Texas, Austin, TX 78759-8345, United States
AB: The recently acquired radar ice sounding surveys (Holt, et al., 2006) extending the 1990s Central West Antarctica (CWA) aerogeophysical survey to the Amundsen and Bellingshausen sea coasts allows us to revise a thought experiment reported by Behrendt et al., 1991 from very limited bed elevation data. Were the ice of the WAIS flowing through the WR to be compressed to the density of crustal rock, almost all of the area beneath the WAIS would be at or above sea level, much >1 km elevation. There are only about 10-20% of the very deep areas (such as the Bentley subglacial trench and the Byrd Subglacial Basin) filled with 3-4-km thick ice that would be well below sea level. The age of the 5-7-km high rift shoulder bounding the asymmetric WR from northern Victoria Land through the Horlick Mountains (where it diverges from the Transantarctic Mountains) to the Ellsworth Mountains has been reported as old as Cretaceous. Volcanic exposures associated with the West Antarctic rift system in the present WAIS area extend at least to 34 Ma and the West Antarctic ice sheet has flowed through the rift possibly as far back in time as 25 Ma. Active volcanism has been reported for the WR at only a few widely scattered locations, so speculations about present volcanic activity beneath the WAIS are quite uncertain, and it is probably quite rare. The Central West Antarctic aeromagnetic and radar ice sounding survey carried out in the 1990s revealed about 1000 "volcanic centers" characterized by 100-1000 nT shallow source magnetic anomalies, at least 400 of which have associated bed topography. About 80% of these show relief <200 m and have been interpreted as smoothed off as they were erupted (injected) into the moving WAIS. Several kilometer-thick highly magnetic sources are required to fit these anomalies requiring high remanent magnetizations in the present field direction. We interpreted these sources as subvolcanic intrusions which must be younger than about 100 Ma because the Antarctic plate has been in its approximately present position since that time. Eighteen anomalies have >600 bed relief and were interpreted as erupted subaerially at a time when the WAIS was absent. At least one of these subaerially erupted peaks (Mt. Resnik, having 2 km bed relief) was erupted through a magnetic reversal. About 100 "volcanic" anomalies show reversed magnetic polarization indicating these must be at least as old as the Brunes-Matayama reversal at about 780 Ka. Essentially no volcanic rocks or detritus has been reported from the few drill holes that have penetrated the WAIS, although some have speculated, from the presence of smectite recovered from rock cores into the Ross Sea continental shelf, that this mineral has resulted from alteration of volcanic rock erupted beneath the WAIS. We consider the absence of volcanic samples from beneath the WAIS is not evidence of their absence. This seems particularly true considering the long time of the apparently coincident volcanism beneath the WAIS, possibly as great as 25 Ma, and the relatively brief age of the ice presently comprising the WAIS, about 200 Ka at most (e.g. perhaps the bulk of the volcanic centers are >10 Ma). Because none of the volcanic rocks or subvolcanic intrusions inferred to underlie the "volcanic centers" marked by high amplitude anomalies and low relief bed topography has been directly sampled, the question of their age cannot be answered.
DE: 0726 Ice sheets
DE: 0925 Magnetic and electrical methods (5109)
DE: 1605 Abrupt/rapid climate change (4901, 8408)
DE: 8178 Tectonics and magmatism
DE: 9310 Antarctica (4207)
SC: Cryosphere [C]
MN: 2007 Fall Meeting