MAGMA DISTRIBUTION IN ISLAND-ARC MANTLE IN 3 DIMENSIONS

Distribution of partial melt (magma) in the upper mantle is investigat ed in detail from the seismic tomography of the mantle wedge beneath n ortheastern Japan. The comparison of the fine velocity structure with the laboratory velocity data on a partially-molten mantle rock yields estimates of three-dimensional distribution of melt. The results indic ate that the cause of island are volcanism in northeastern Japan is th e upwelling of hot mantle materials (volcanic plumes) from beneath. Th e source of volcanic magma is diapir-like melting regions localized wi thin the volcanic plumes. Extensive volcanic activity at the volcanic front is due to the presence of the vast melting zones right beneath t he Moho discontinuity. Those melting zones may cause melting of lower crustal materials and produce felsic magma. Melt stays underneath the Moho, where crystallization fractionation may proceed. Melt exists at greater depths in the back-are region, which may cause across-are vari ations of chemical compositions of the volcanic rocks in northeastern Japan. We suggest that magma migration in the ductile lower crust may cause low-frequency microearthquakes, and magma penetration into the b rittle upper crust may produce mid-crustal S-wave reflectors.