HR: 1330h
AN: V12B-0576    [PDF]
TI: Impact Origin for the Greater Ontong Java Plateau? Geophysical and Geodynamic Evidence.
AU: * Coffin, M F
EM: mcoffin@ori.u-tokyo.ac.jp
AF: Ocean Research Inst., University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo, 164-8639 Japan
AU: Ingle, S
EM: single@geo.titech.ac.jp
AF: Dept. Earth and Planetary Sci., Tokyo Inst. Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
AB: The $\sim$120 Ma Greater Ontong Java Plateau (OJP), Earth's most voluminous large igneous province (LIP), encompasses $\sim$5.7 x 10$^{7}$ km$^{3}$ of crust in the west Pacific Ocean. OJP defies explanation by extant plume models, and cannot be easily linked to any hotspot track. The arrival and decompression melting of a hot plume at the base of oceanic lithosphere, should have resulted in buoyancy and crustal growth capable of maintaining OJP above sea level. Yet all sampled OJP basalts erupted below sea level. Plateaus within oceanic lithosphere should subside via either thermal conduction or continuous viscous spreading, but interpreted paleoenvironments show that total OJP subsidence was much less than that of other oceanic LIPs or normal oceanic crust. A cylindrical, $\sim$300 km deep, low velocity root is centered beneath OJP's thickest crust. Its slow shear wave velocities could indicate a thermal anomaly sufficient to cause continued volcanism, but OJP shows no evidence of recent volcanism. Thus key geophysical and geodynamic results are at odds with a plume model for OJP's origin, and an extraterrestrial impact model seems much more consistent with existing data and results. A bolide $\sim$20 km in diameter, impacting young oceanic lithosphere would instantaneously remove $>$60 km of overlying crust and mantle, and extensive decompression melting would follow. Melt would flow into the crater, and would also exploit concentric and radial fractures beyond the crater, resulting in emplacement of the plateau and nearby ocean basin flood basalts. Anomalous mantle would fill space created by decompression melt beneath and proximal to the crater, creating a low-velocity mantle root rigidly coupled to OJP crust. In the impact scenario, OJP formed under conditions of isostatic equilibrium, and thus neither rose above sea level nor subsided as would typical oceanic lithosphere. Aptian time is marked by major global events that include onset of the Cretaceous normal magnetic polarity superchron, the Selli oceanic anoxic event, marine faunal extinctions and worldwide radiogenic isotopic excursions (Sr and Hf) in marine sediments. A major bolide impact could have caused or contributed to such phenomena.
DE: 0930 Oceanic structures
DE: 1213 Earth's interior--dynamics (8115, 8120)
DE: 6022 Impact phenomena
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8450 Planetary volcanism (5480)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting