HR: 1340h
AN: T13B-1368    [Abstracts]
TI: Seismic Structure of the Daito Ridges on the Northwestern End of the Philippine Sea Plate
AU: * Nishizawa, A
EM: azusa@jodc.go.jp
AF: Oceanogr. and Hydrogr. Dept., JCG, 5-3-1 Tsukiji Chuo-ku, Tokyo, 104-0045 Japan
AU: Kaneda, K
EM: kentaro-kaneda@kaiho.mlit.go.jp
AF: Oceanogr. and Hydrogr. Dept., JCG, 5-3-1 Tsukiji Chuo-ku, Tokyo, 104-0045 Japan
AU: Katagiri, Y
EM: yasutaka-katagiri@kaiho.mlit.go.jp
AF: Oceanogr. and Hydrogr. Dept., JCG, 5-3-1 Tsukiji Chuo-ku, Tokyo, 104-0045 Japan
AU: Ohara, Y
EM: ohara@jodc.go.jp
AF: Oceanogr. and Hydrogr. Dept., JCG, 5-3-1 Tsukiji Chuo-ku, Tokyo, 104-0045 Japan
AU: Takahashi, N
EM: narumi@jamstec.go.jp
AF: JAMSTEC, 3173-25 Showa-cho Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Kodaira, S
EM: kodaira@jamstec.go.jp
AF: JAMSTEC, 3173-25 Showa-cho Kanazawa-ku, Yokohama, 236-0001 Japan
AU: Tsuru, T
EM: tsurut@jamstec.go.jp
AF: JAMSTEC, 3173-25 Showa-cho Kanazawa-ku, Yokohama, 236-0001 Japan
AB: The northwestern end of the Philippine Sea plate is characterized by several bathymetric highs, the Daito Ridges. We conducted two wide angle seismic and multi-channel seismic (MCS) profilings across the Oki-Daito Ridge, a member of the Daito Ridges in May_|July 2004, which provided the first images of the deep structure in this region and important clue to origin of these bathymetric highs. In this seismic experiment, the controlled sources were a tuned array of 36 airguns with a total volume of 8,040 inch$^{3}$ and two explosives. We shot the airgun array at an interval of 90 s, 100 m along EW 240 km and NS 660 km long lines. We used 251 ocean bottom seismographs (OBS) at an interval of 3-5 km, and a 6 km long, 480-channel hydrophone streamer as receivers. Both of the OBS and MCS data were of good quality. Clear PmP signals and reflections from the bottom of the lower crust were detected in many OBS records. These data were modeled by a tomographic inversion and two-dimensional ray tracing. Preliminary results shows the upper crust of the Oki-Daito Ridge has P wave velocity of 3.5-6.5 km/s with a thickness of about 7 km and the lower crust is 6.5-7.0 km/s with 20 km thickness. The total crustal thickness exceeds 25 km, which is thicker than the island arc curst obtained at the northern part of the Izu-Bonin arc. Several previous studies suggest that the origin of the Oki-Daito Ridge is a remnant island arc. The obtained crustal model, however, is similar to that of the southern Kerguelen Plateau (Indian Ocean), one of the largest oceanic plateaus in the world, that is a representative large igneous province (LIP), rather than that of the remnant island arc.
DE: 8015 Local crustal structure
DE: 7218 Lithosphere and upper mantle
DE: 3025 Marine seismics (0935)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting