Earth Planets Space, Vol. 59 (No. 6), pp. e17-e20, 2007E-LETTER
Azusa Nishizawa1, Kentaro Kaneda1, Yasutaka Katagiri1, and Junzo Kasahara2
1Hydrographic and Oceanographic Department, Japan Coast Guard, Tokyo 104-0045, Japan
2Japan Continental Shelf Survey Co. Ltd., Tokyo 104-0031, Japan
(Received January 6, 2007; Revised May 20, 2007; Accepted May 24, 2007; Online published June 19, 2007)
We acquired coincident wide-angle and multi-channel seismic reflection data along four profiles perpendicular to the Kyushu-Palau Ridge (KPR) between 15°N and 20°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°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.
Key words: Kyushu-Palau Ridge, Oki-no-Tori-Shima island, West Philippine Basin, seismic refraction, crustal structure.