Compilation of seismic structural models of the Kyushu-Palau Ridge, paleo-island arc in the Philippine Sea plate, at 13-30 N

The Kyushu-Palau Ridge (KPR) is a 2600 km long bathymetric high extending north-south at the center of the Philippine Sea plate. The origin of the KPR is regarded as a remnant of the proto Izu-Ogasawara (Bonin)-Mariana (IBM) Island arc that was separated by backarc spreading of the Shikoku and Parece Vela Basins in the late Eocene. The extensive seismic explorations were implemented to grasp the spatial distribution of the arc crust of the KPR in 2004-2008 under the Japanese Continental Shelf Survey Project. We carried out 27 seismic reflection and refraction profiles across the ridge between 13 and 30 N and one along the ridge in the northernmost part. We deployed ocean bottom seismographs (OBSs) as a receiver at an average interval of 5 km along each line. A tuned airgun array with a volume of 8,040 cubic inches (132 liters) or a non-tuned airgun array with a volume of 6,000 cubic inches (98 liters) was shot at an interval of 200 m (90 sec) for the wide-angle seismic profiles. Multichannel reflection data using 480 ch. or 240 ch. hydrophone streamer were also collected on the coincident lines. We obtained P-wave velocity models using tomographic inversion, forward modeling with two-dimensional ray tracing and comparison with synthetic seismograms. The maximum crustal thickness for each profile across the KPR varies from 8 to 23 km among the seismic lines. The KPR crusts are roughly thicker in the north than those in the south and are always thicker than the neighboring backarc basin oceanic crusts of the West Philippine Basin to the west and of the Shikoku and Parece Vela Basins to the east. The thick crust is mainly attributed to the lower crust with P-wave velocity of 6.8-7.2 km/s. Pn velocities just beneath the KPR are less than 8 km/s, often accompanying with rather high Vp of 7.2 km/s at the base of the crust. Reflection signals observed in far offsets along several lines suggest some reflectors exist at the depths 23-40 km beneath the KPR. The crustal structure of the eastern transition from the KPR to the backarc basins of the Shikoku or Parece Vela Basins is characterized by a thinner curst and slightly higher Pn velocity compared with those of a typical oceanic basin, which may relate to the rifting, breakup and early separation of the proto-island arc. On the other hand, the crustal models of the western edge of the KPR show large variations among the seismic lines. This is because the tectonic settings of the western side are different from north to south along the KPR, such as the Daito Ridges as paleo-island arcs and intra-arc basins in the north, and the West Philippine Basin as a backarc basin and the CBF Rift as the spreading center of the West Philippine Basin in the south.