HR: 0800h
AN: V51B-0581    [Abstracts]
TI: Antipodal Hotspots and Bipolar Catastrophes: Were Oceanic Large-Body Impacts to Blame?
AU: * Hagstrum, J T
EM: jhag@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 937, Menlo Park, CA 94025 United States
AB: One aspect of the hotspot distribution that has received little attention is its antipodal character. Of 45 "primary" hotspots, found in most hotspot compilations, 22 (49%) form antipodal pairs within conservative drift limits ($\leq$20 mm/yr). All but 4 of the remaining primary hotspots have volcanic centers near their antipodes. In addition, the available ages, or estimated minimum age ranges, for both hotspots of an antipodal pair tend to be similar ($\leq$10 Myr difference) or overlap. Monte Carlo analyses indicate that the primary antipodal hotspot pairs and their ages are not due to chance at the $>$99.9% confidence level ({\it p}$<$0.001). All hotspot pairs include at least one oceanic hotspot, and these are consistently opposite those hotspots related to large igneous provinces and continental volcanism. A model of hotspot formation is proposed in which minor volcanism is induced at, and lithospheric fracturing and flood-basalt volcanism is caused by focused seismic energy antipodal to, oceanic large-body impacts. Because continental impacts have low seismic efficiencies ($\sim$10$^{-4}$), continents possibly acted as shields to the formation of antipodal hotspot pairs. Published numerical models indicate that large oceanic impacts ($\sim$10-km-diameter bolide) penetrate well into the upper mantle ($\sim$40-km depth), eject mostly water or water vapor from the transient crater, and generate megatsunami ($\sim$4 km initial height) capable of coastal stratigraphic effects on a global scale. Impact-generated megatsunami, consequently, are expected to leave the most prominent and widespread record of large oceanic impacts, and might have been responsible for apparent rapid eustatic changes in sea level and abrupt changes in the isotopic composition of seawater in the geologic past. Moreover, large oceanic impacts during the Late Permian were perhaps the principal cause of end-Kazanian and end-Tatarian flood basalt eruptions, apparent regressive-transgressive shifts in sea level, and pulses in extinction rates making up the Permian/Triassic transition, and might have initiated the Cretaceous/Tertiary transition at $\sim$68-67 Ma. Phanerozoic mass extinction events, therefore, might have been the result of catastrophic megatsunami in a dominantly oceanic hemisphere and vast quantities of noxious volcanic gases in a dominantly continental one.
DE: 8450 Planetary volcanism (5480)
DE: 5420 Impact phenomena (includes cratering)
DE: 4556 Sea level variations
DE: 4564 Tsunamis and storm surges
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting