DETAILED SUBDUCTION STRUCTURE ACROSS THE EASTERN NANKAI TROUGH OBTAINED FROM OCEAN-BOTTOM SEISMOGRAPHIC PROFILES

To investigate the deep crustal structure of the Philippine Sea Plate at its northern margin, we performed a seismic refraction and wide-ang le reflection survey around the eastern Nankai Trough by using an ocea n bottom seismograph array with an air gun source. We derived a crusta l structure model across the trough, from the Shikoku Basin to the con tinental slope, that explains not only our seismic data but also previ ously published gravity data. The origin of the Zenisu Ridge, a conspi cuous topographic high along the oceanward slope of the Nankai Trough, is still a matter of argument. There has been some controversy as to whether the igneous activity of the Izu-Qgasawara Are or the seafloor spreading of the Shikoku Basin is responsible for the formation of the ridge. Although the crustal thickness beneath the ridge is between 8 and 11 km, slightly thicker than typical oceanic crust, our structure model clearly indicates that the Zenisu Ridge has an oceanic crust and its structure is very similar to that of the Shikoku Basin. Beneath t he south flank of the Zenisu Ridge, the Moho shows an offset of 5 km i n depth. This offset may correlate with the recently proposed nascent subduction boundary and subduction-collision tectonics of this area. T he velocity structure beneath the continental slope appears characteri stic for a well-developed accretionary wedge bounded by the continenta l upper crust of the Japan Island Are to the northwest, and subducting oceanic crust which can be traced beneath the accretionary wedge and the continental upper crust.