Crustal Structure of the Southern Kyushu-Palau Ridge, the Other Half of the Proto Izu-Bonin-Mariana Island Arc

The Kyushu-Palau Ridge (KPR) is a bathymetric high extending north-south direction at the center of the Philippine Sea and considered as a remnant of a proto Izu-Bonin-Mariana island arc separated by the backarc spreading of the Shikoku and Parece Vela (Oki-no-Tori-Shima) Basin. We conducted a wide-angle and multi-channel seismic experiment to investigate variation in crustal structures along the KPR. The experiment consisted of four seismic lines that were selected to represent the variations in seafloor topography of the southern KPR. All of the profiles cross the KPR perpendicularly and their profile lengths range from 175 to 375 km. The controlled seismic source was a tuned array of 36 airguns with a total volume of 8,040 inch3. In the wide-angle seismic survey, we shot the airgun array at an interval of 200 m (90 s) for each line. We used 200 ocean bottom seismographs (OBS) at an interval of 5 km as receivers. The travel time and amplitude data obtained by the OBSs were modeled by a tomographic inversion and two-dimensional ray tracing. The maximum crustal thickness beneath the KPR varies from 14 to 20 km according to the profile and the crust is significantly thicker than those of the both sides of each profile, that is, the oceanic crusts of the West Philippine Basin to the west and of the Parece Vela Basin to the east. The thickest crust among the four profiles is found in the region where the KPR connects with the Oki-no-Tori-Shima island. The thicker crust beneath the KPR is mainly due to a fat lower crust. The thick (> 5 km) middle crust with P wave velocity of 6.0-6.3 km/s that characterizes the northern Izu-Bonin island-arc crust does not exist so clearly in our profiles. This may relate to the fact that the southern KPR is deeper in seawater and does not have a mature island arc crust compared with the northern KPR where existence of a 6 km/s layer has been reported previously.