HR: 0800h
AN: T41D-1250    [Abstracts]
TI: Crustal growth of oceanic island arc inferred from seismic structure of Mariana arc-backarc system
AU: * Takahashi, N
EM: narumi@jamstec.go.jp
AF: Japan Agency for Earth-Marine Science and Technology, 2-15, Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Kodaira, S
EM: kodaira@jamstec.go.jp
AF: Japan Agency for Earth-Marine Science and Technology, 2-15, Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Ito, A
EM: iaki@jamstec.go.jp
AF: Japan Agency for Earth-Marine Science and Technology, 2-15, Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Klemperer, S L
EM: sklemp@stanford.edu
AF: Dept Geophysics, Stanford University, Mitchell Bldg, Stanford, CA 94305-2215 United States
AU: Kaneda, Y
EM: kaneday@jamstec.go.jp
AF: Japan Agency for Earth-Marine Science and Technology, 2-15, Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Suyehiro, K
EM: suyehiro@jamstec.go.jp
AF: Japan Agency for Earth-Marine Science and Technology, 2-15, Natsushima-cho, Yokosuka, 237-0061 Japan
AB: The Izu-Ogasawara-Marina arc (IBM arc) is one of the typical oceanic island arcs and it has developed repeating magmatic arc volcanisms and backarc spreading since Eocene. Because tectonics of the IBM arc is relatively simple and does not include collisions between the arc and a continent, it is one of best targets to research crustal growth. In 2003, wide-angle seismic survey using 106 ocean bottom seismographs had been carried out as a part of Margin program in collaboration between US and Japan in Mariana region. The seismic line runs from a serpentine diaper near the trench to Parece Vela basin through the Mariana arc, the Marina trough and the West Mariana ridge. We present the characteristics of the seismic structure of the Mariana arc-backarc system and discuss the crustal growth process by comparison with a structure of the northern Izu-Ogasawara arc. Main structural characteristics of the Mariana arc-backarc system are (1) variation of the crustal thickness (Mariana arc: 20 km, West Mariana ridge: 17 km, Mariana trough and Parece Vela basin: 6 km), (2) distribution of an andesitic middle crust with about P-wave velocity of 6 km/s, (3) variation of P-wave velocity in the middle crust (4) velocity anomalies of the lower crust in transition area between the arc and the backarc, (5) thickening of the lower crust under the Mariana trough axis and (6) slow mantle velocities under the Mariana arc, Mariana trough axis and the West Mariana ridge. Above characteristics from (1) to (4) are common to the seismic structure of the northern Izu-Ogasawara arc. In particular, the vertical P-wave velocity gradients of the middle crust under the forearc in both regions tend to become large rather than those under the arc. Main differences of seismic structures between both regions are the velocity gradients and an existence of a thin transition layer between the middle and lower crust. These differences and similarities of the velocity gradient might originate the age and indicate a difference of a crustal differentiation relating each tectonic stage.
DE: 9355 Pacific Ocean
DE: 7220 Oceanic crust
DE: 8105 Continental margins and sedimentary basins
DE: 7205 Continental crust (1242)
DE: 3025 Marine seismics (0935)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting