Geophysical study of the Solomon islands plate convergence

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The Solomon island arc area is a complex plate convergence Zone. On the northern side is the North Solomon Trench at which the Pacific plate with the Ontong Java plataeau seems to be in collision with the arc system. Another trench on the southern side is the San Cristobal Trench, where the Indo-Australian plate and the Solomon Sea plate are presently subducting beneath the Solomon arc.We have carried out a Japan-U.S.joint geophysical survey in the Solomon Sea region using the R/V Maurice Ewing equipped with a 130-ch multi-channel seismic reflection survey system and a 140-liter airgun array during the period of October 17 to November 19,1995. The Japanese side brought 18 digital compact ocean bottom seismographs (OBS's) which were placed along a 464-km long transect of the whole arc system. Airgun shooting was made for more than 4,000 km long covering the whole Solomon Islands region. Thus, we have succeeded in acquiring data to obtain the whole crustal structure and to test different collision models of the Ontong Java plateau. Preliminary look into the data shows that the subduction of the Ontong Java plateau seems to be ongoing despite the general notion that the present subduction is only from the southern side. Also, it is more likely that the plateau is mostly subducting rather than wedging upward onto the arc. This suggests that the fate of an oceanic plateau is such that it is mostly recycled into the mantle and does not significantly contribute to the increase of continental volume.