Mathematics – Logic
Scientific paper
Apr 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....3328c&link_type=abstract
EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #3328
Mathematics
Logic
Scientific paper
The ca. 120 Ma Ontong Java Plateau (OJP), the most voluminous large igneous province (LIP) on Earth, encompasses approximately 57 million cubic km of crust in the western Pacific Ocean. OJP defies explanation by extant plume models, and cannot be linked to any hotspot track or currently active hotspot. The arrival of a hot plume at the base of oceanic lithosphere, accompanied by voluminous decompression melting, should have resulted in a combination of buoyancy and crustal growth capable of maintaining OJP above sea level. Yet all OJP basalts sampled from obducted Solomon Islands sections or drilled erupted well below sea level. Furthermore, plateaus within oceanic lithosphere should subside via either thermal conduction or continuous viscous spreading of the anomalous mantle material, but paleoenvironments interpreted from OJP sediment show that the OJP subsided either very little or erratically. A cylindrical, approximately 300 km deep, low velocity root is centered beneath OJP's thickest crust. Although its slow shear wave velocities could indicate a thermal anomaly of up to 700 degrees K, high enough to create continued volcanism, OJP shows no evidence of active or recent volcanism, so this keel probably represents a chemical heterogeneity. Shear wave splitting suggests that ambient Pacific asthenosphere flows around the root, implying that it is rheologically strong and rigidly coupled to OJP's crust. Key geophysical and geodynamic results are thus at odds with a plume model for OJP's origin, and an extraterrestrial impact model for OJP, together with extensive neighboring deep ocean basalts in the Nauru, Pigafetta, and East Mariana basins that also formed at ca. 120 Ma, seems much more consistent with existing data and results. A bolide approximately 50 km in diameter would create a crater about 500 km across and 150 km deep, accounting for the volume of OJP's crust and mantle root. Pacific lithosphere would be deformed within a 1000 km radius of the center of impact, and the instantaneous unroofing of mantle 150 km or more deep would instigate extensive decompression melting instantaneously. Melt would flow into the crater both from below and laterally, and would also exploit concentric and radial fractures beyond the crater, resulting in emplacement of the Nauru, East Mariana, and Pigafetta Basin flood basalts, and perhaps near-simultaneous seafloor spreading in the Nova-Canton Trough. Refractory 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. We speculate that a regional tectonic event extended the southeastern portion of OJP at ca. 90 Ma, allowing for decompression melting of mantle rock formed from the ca. 120 Ma impact. Finally, Aptian time is marked by major global events that include onset of the Cretaceous normal magnetic polarity superchron, the Selli oceanic anoxic event, and worldwide radiogenic isotopic excursions that suggest a rapid change from terrigenous-dominated values to mantle-dominated values in marine sediments (Sr and Hf). A major bolide impact could have caused or contributed to such phenomena.
Coffin Millard
Ingle Stephanie
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