HR: 1340h
AN: V43D-1635    [Abstracts]
TI: Seismic Imaging of the Mantle Under the Western-Pacific arc and Back-arc Regions
AU: * Zhao, D
EM: zhao@aob.geophys.tohoku.ac.jp
AF: Tohoku University, Department of Geophysics, Sendai, 980-8578, Japan
AU: Umino, N
EM: umino@aob.geophys.tohoku.ac.jp
AF: Tohoku University, Department of Geophysics, Sendai, 980-8578, Japan
AU: Hasegawa, A
EM: hasegawa@aob.geophys.tohoku.ac.jp
AF: Tohoku University, Department of Geophysics, Sendai, 980-8578, Japan
AB: We have used multi-scale seismic tomography to determine the detailed 3-D structure of the crust and mantle under the Western-Pacific arc and back-arc regions. The subducting Pacific and Philippine Sea (PHS) slabs are imaged clearly from their entering the mantle at the oceanic trenches to their reaching the mantle transition zone and finally to the core-mantle boundary (CMB). Recent local tomography of Northeast Japan has imaged the shallow portion of the slab from the Japan Trench down to about 200 km depth under Japan Sea. The 3-D Vp and Vs structures of the forearc region under the Pacific Ocean are constrained by accurately located hypocenters with sP depth phases. Strong lateral heterogeneity is revealed along the main thrust zone under the forearc region, and there is a good correlation between the heterogeneity and the distribution of large thrust earthquakes. A similar feature is found in the forearc region of the Kyushu subduction zone. A joint inversion of local and teleseismic data imaged the subducting Pacific slab down to 670 km depth under the Japan Islands and the Japan Sea. The PHS slab is detected down to 500 km depth under SW Japan. A mantle upwelling is found under SW Japan that rises from about 400 km depth right above the Pacific slab up to the PHS slab. Continuous slow anomalies are revealed under the volcanic front in the Ryukyu arc. The Eurasian lithosphere is found to subduct down to about 300 km depth under South Taiwan. Regional and global tomography revealed the Pacific slab that is stagnant in the mantle transition zone under Eastern China. The stagnant slab may have played an important role in the formation of the intraplate volcanoes in NE Asia. Global tomography shows pieces of fast anomalies in the middle and lower mantle as well as in the D" layer above the CMB, suggesting that the stagnant slab finally collapses down to the lower mantle and CMB as a result of very large gravitational instability from phase transitions. Prominent slow anomalies are also revealed in the mantle under the subducting slabs, which may represent either mantle plumes or upwelling flows associated with the deep subduction of the slabs.
UR: http://www.aob.geophys.tohoku.ac.jp
DE: 8104 Continental margins: convergent
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8180 Tomography (6982, 7270)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting