Variation in crustal structure along the Kyushu-Palau Ridge at 15-21 degrees N on the Philippine Sea plate based on seismic refraction profiles

We acquired coincident wide-angle and multi-channel seismic reflection data along four profiles perpendicular to the Kyushu-Palau Ridge (KPR) between 15 degrees N and 20 degrees N on the Philippine Sea plate. The crustal thickness beneath the KPR, which is a remnant arc created in the Late Eocene, varies along the strike from 8 to approximately 20 km and is always thicker than the adjacent oceanic crust of the West Philippine Basin to the west and the Parece Vela Basin to the east. The thickest crust among the four profiles, which is primarily due to a thickening of the lower crust, is found where the KPR adjoins Oki-no-Tori-Shima Island. There is no clear evidence of the thick (>5 km) middle crustal layer with a P-wave velocity of 6.0-6.5 km/s that has been inferred beneath the conjugate rifted counterpart of the Izu-Ogasawara(Bonin)-Mariana Island-arc. Our results suggest that the crust of the KPR at 15-21 degrees N represents a less mature island arc crust relative to that further north along the ridge where a mid-crustal layer of 6 km/s has been reported.