HR: 16:00h
AN: T44A-01 INVITED     [Abstracts]
TI: A New Insight Into Mantle Flow Beneath the NE Japan And Kurile Arcs Inferred From Comprehensive Seismological Observations
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
AU: Hori, S
EM: hori@aob.geophys.tohoku.ac.jp
AF: RCPEV, Graduate School of Science, Tohoku University, Aramaki-Aza-Aoba, Aoba-ku, Sendai, 980-8578 Japan
AU: Shimizu, J
EM: shimizu@aob.geophys.tohoku.ac.jp
AF: RCPEV, Graduate School of Science, Tohoku University, Aramaki-Aza-Aoba, Aoba-ku, Sendai, 980-8578 Japan
AB: The northeastern (NE) Japan arc is one of the most investigated arcs in the world through various seismological approaches, and recent comprehensive observations have provided crucial constraints on mantle dynamics. Here we introduce our recent results and propose a model for the mantle flow with implication for arc magmatism. Seismic tomography studies have revealed an inclined low-velocity zone in the mantle wedge of NE Japan (Tohoku) and southern Kurile (Hokkaido) down to depths of ~150 km [Zhao et al., 1992; Nakajima et al., 2001; Nakajima and Hasegawa, 2005]. The zone is considered to be an upwelling flow portion of the secondary convection mechanically induced by slab subduction [Hasegawa et al., 1991; Hasegawa and Nakajima, 2004]. Shear-wave splitting observed in Tohoku and Hokkaido [Nakajima and Hasegawa, 2004; Shimizu, 2004] suggests that the upwelling flow is generated sub-parallel to the maximum dip-direction of the subducting slab, not to the relative plate motion. For NE Japan, a quantitative analysis of the inclined low-velocity zone shows that the upwelling flow contains melt inclusions with volume fractions of 0.1-3 % [Nakajima et al., 2005], suggesting that the upwelling flow is the main source of arc magmas. Interestingly, the inclined low-velocity zone (upwelling flow) shows an along-arc variation in its S-wave velocity reduction rate: very low velocity regions periodically occur about every 80 km along the strike of the arc. Clustering of Quaternary volcanoes and topographic highs at the surface are located immediately above these very low-velocity areas in the mantle wedge, and low-frequency microearthquakes, perhaps caused by rapid movements of fluids in the lower crust, also occur right above them [Hasegawa and Nakajima, 2004]. These observations suggest that the upwelling flow plays an important role on arc magmatism and the 3D magma supply system is working beneath NE Japan.
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8185 Volcanic arcs
SC: Tectonophysics [T]
MN: Fall Meeting 2005