HR: 1340h
AN: T33B-0529    [Abstracts]
TI: The Upwelling Flow at the Triple Junction in Central Japan Inferred from Seismic Tomography and its Implications for Arc Magmatism
AU: * Nakajima, J
EM: nakajima@aob.geophys.tohoku.ac.jp
AF: RCPEV, Graduate School of Science, Tohoku University, Aramaki-Aza-Aoba, Aoba-ku, Sendai, 980-8578 Japan
AU: Hasegawa, A
EM: hasegawa@aob.geophys.tohoku.ac.jp
AF: RCPEV, Graduate School of Science, Tohoku University, Aramaki-Aza-Aoba, Aoba-ku, Sendai, 980-8578 Japan
AB: The Pacific plate subducts beneath eastern Japan on the North American (or Ohotsuku) plate at the Japan and Kurile trenches and produces a long volcanic chain from Hokkaido to Tohoku without large gaps. Our recent studies through seismic tomography have revealed the distinct low-velocity zone in the mantle wedge beneath Tohoku and Hokkaido, northeastern Japan, sub-parallel to the down-dip direction of the Pacific slab (Nakajima et al., 2001; Nakajima and Hasegawa, 2005). The zone is considered to be the upwelling flow induced by the viscous coupling between the subducting and overriding plates (Hasegawa and Nakajima, 2004). The location of the volcanic front agrees well in space with regions where the upwelling flow reaches the Moho and, therefore, we infer that the upwelling flow is the main source of arc magmas and plays a crucial role in forming arc volcanoes. The volcanic chain beneath Kanoto, Japan, where the Philippine Sea plate subducts to the north between the Pacific and the overriding continental plates, shows a different feature and deflects toward the back-arc region. A numerical simulation (Iwamori, 2000) showed that the shift of the volcanic chain to the west is due to the anomalous thermal structure resulting from the subduction of the Philippine Sea plate. We conducted a seismic tomography beneath the central part of Japan to reveal the heterogeneous structure in the mantle wedge. Obtained results show the presence of the prominent low-velocity zone inclined in the mantle wedge even beneath the central part of Japan. The low-velocity zone shifts gradually to the west corresponding to the shift of the volcanic chain, which again indicates that the upwelling flow controls the location of arc volcanoes. Our results further imply the origin of the high strain rate zone, the Niigata-Kobe Tectonic Zone (Sagiya et al., 2000), is closely associated with the upwelling flow in the mantle wedge.
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8180 Tomography (6982, 7270)
DE: 8185 Volcanic arcs
SC: Tectonophysics [T]
MN: Fall Meeting 2005