ANTIPODAL HOTSPOTS, LARGE IGNEOUS PROVINCES, AND SEA-LEVEL CHANGES: VESTIGES OF OCEANIC LARGE-BODY IMPACTS?

Of 45 prominent or ‘primary’ hotspots, found in most hotspot compilations, 22 (49%) form antipodal pairs within conservative drift limits (≤20 mm/yr) supporting a model of impact-induced hotspot volcanism at, and potentially greater flood-basalt volcanism antipodal to, oceanic large-body impact sites. All but 4 of the remaining primary hotspots have volcanic centers near their antipodes. The available ages, or estimated minimum age ranges, for both hotspots of an antipodal pair tend to be similar (≤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 (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. Because of possibly higher seismic efficiencies for oceanic impacts compared to continental ones (De Carli et al., this session), continents might have acted as shields to the formation of antipodal hotspot pairs. Numerical models indicate that large oceanic impacts (~10-km-diameter bolide) penetrate well into the upper mantle (~40-km depth), eject mostly water or water vapor from the transient crater, and generate megatsunami (~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. Such impacts and their associated megatsunamis, therefore, might have been responsible for rapid eustatic changes in sea level and abrupt changes in the isotopic composition of sea water 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 possibly initiated the Cretaceous/Tertiary transition at ~68-67 Ma. Phanerozoic mass extinction events, therefore, might have resulted from catastrophic impact-generated megatsunami in a dominantly oceanic hemisphere and vast quantities of noxious volcanic gases in a dominantly continental hemisphere.