Tomographic evidence for the subducting oceanic crust and forearc mantle serpentinization under Kyushu, Japan |
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| Authors: | Shaohong Xia Dapeng Zhao Xuelin Qiu |
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| Institution: | aDepartment of Geophysics, Tohoku University, Sendai 980-8578, Japan;bCAS Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China |
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| Abstract: | We determined high-resolution three-dimensional P- and S-wave velocity (Vp, Vs) structures beneath Kyushu in Southwest Japan using 177,500 P and 174,025 S wave arrival times from 8515 local earthquakes. A Poisson's ratio structure was derived from the obtained Vp and Vs values. Our results show that significant low-Vp, low-Vs and high Poisson's ratio zones are extensively distributed along the volcanic front in the uppermost mantle, which extend and dip toward the back-arc side in the mantle wedge. In the crust, low-Vp, low-Vs and high Poisson's ratio anomalies exist beneath the active volcanoes. The subducting Philippine Sea slab is clearly imaged as a high-Vp, high-Vs and low Poisson's ratio zone from the Nankai Trough to the back-arc. A thin low-velocity zone is detected above the subducting Philippine Sea slab in the mantle wedge, and earthquakes in the upper mantle are distributed along the transition zone between this thin low-velocity zone and the high-velocity Philippine Sea slab, which may imply that oceanic crust exists on the top of the slab and the forearc mantle wedge is serpentinized due to the slab dehydration. The seismic velocity of the subducting oceanic crust with basaltic or gabbroic composition is lower than that of the mantle according to the previous studies. The serpentinization process could also dramatically reduce the seismic velocity in the forearc mantle wedge. |
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| Keywords: | Serpentinization Subducting oceanic crust Philippine Sea slab Seismic tomography |
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