東北日本上部マントルの部分溶融域のマッピング : 島弧におけるマグマの上昇

概要

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From the seismic velocity structure of northeastern Japan, we have mapped the regions of partial melt in the mantle wedge beneath the island arc. Massive partial melting, that supplies magma to the crust and the volcanoes, is found to exist beneath the volcanic front and downwards to the west. Extensive melting occurs at 〜90 km, 〜65 km and 〜40 km depths, and most extensive beneath the Moho at the volcanic front (up to 3 vol. % partial melt). The melt staying underneath the Moho may cause melting of lower-crustal materials and produce felsic magma. Fractional crystallization may also proceed in the melting regions at the volcanic front. It is observed that mid-crustal S-wave reflectors exist in between the partial melting and volcanoes, and that low-frequency microearthquakes occur right above the melting regions. We suggest that magma movement in the ductile lower-crust causes low-frequency microearthquakes, and that magma penetration from the ductile lower-crust to the brittle upper-crust produces mid-crustal reflectors.
1996-11-01