HR: 0800h
AN: T41C-0699    [Abstracts]
TI: Distribution and nature of the crust-mantle transition layer deduced from amplitude modeling of wide-angle seismic data along the Izu-Bonin island arc
AU: * Sato, T
EM: tsato@jamstec.go.jp
AF: IFREE, JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan
AU: Kodaira, S
EM: kodaira@jamstec.go.jp
AF: IFREE, JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan
AU: Takahashi, N
EM: narumi@jamstec.go.jp
AF: IFREE, JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan
AU: Miura, S
EM: miuras@jamstec.go.jp
AF: IFREE, JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan
AU: Kaneda, Y
EM: kaneday@jamstec.go.jp
AF: IFREE, JAMSTEC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001, Japan
AB: The Izu-Bonin island arc formed by subducting of the Pacific plate beneath the Philippine Sea plate is a region of the crustal growth. This arc is divided into the northern and the southern part by the Sofu-gan tectonic line, due to the difference of the geological and geophysical characters (Yuasa, 1985). In the seismic velocity structure along the volcanic front in the northern Izu-Bonin island arc, it is clarified that this arc has not only the middle and the lower crust but also the 7.2-7.6 km/s layer (crust-mantle transition layer) underlying the lower crust (Kodaira et al., 2007, accepted). However, since this velocity structure along the volcanic front in this arc is calculated by the tomography method, the nature of the crust-mantle transition layer and uppermost mantle is unknown. To understand the nature of the crust-mantle transition layer in the Izu-Bonin island arc on the crustal growth, it is also important to know the seismic reflectivity at the top and bottom of this transition layer in this arc. In this study, we clarify the distribution of the seismic reflectivity at the top and bottom of the crust-mantle transition layer along the volcanic front in this arc using the velocity contrast values at these reflectors estimated by the amplitude modeling of wide-angle data. In 2004 and 2005, seismic refraction/reflection surveys using ocean bottom seismographs (OBSs) and controlled sources were conducted along the volcanic front in the Izu-Bonin island arc from the Sagami Bay to Kaitoku Seamount (Kodaira et al., accepted). In record sections of several OBSs, not only the first arrived phases but also later phases reflected from interfaces in the crust and uppermost mantle can be observed. The velocity contrast values at the top and bottom of this transition layer were estimated from the comparison of the observed and synthetic wave forms computed by a finite difference wave propagation program code "e3d" (Larsen and Grieger, 1998). Along the northern Izu-Bonin island arc, the top of the crust-mantle transition layer has a velocity contrast value of about 0.4 km/s ranging from Kurose hall to Kayo Seamount. On the other hand, the velocity contrast of the bottom of the crust-mantle transition layer is weaker southward along the northern Izu-Bonin island arc. From these results of this study and the petrological study (e.g., Tatsumi, 2000), this transition layer along the northern Izu-Bonin island arc is interpreted as the mixture material of the mafic restites and olivine cumulate during the crustal growth. In addition, the variation of the velocity contrast at the bottom of this transition layer along the northern Izu-Bonin island arc implies that this transition layer has more olivine cumulate southward along this arc. Besides, record sections of some OBSs are characterized by fuzzy reflections from the top and bottom of the crust-mantle transition layer looking southwards bounded on the Sofu- gan tectonic line, suggesting that the character of the crust-mantle transition layer of the northern Izu-Bonin island arc differs from that of the southern arc.
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 7205 Continental crust (1219)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8185 Volcanic arcs
DE: 9355 Pacific Ocean
SC: Tectonophysics [T]
MN: 2007 Fall Meeting