Magma Distribution in Island Arc Mantle in Three Dimensions.

Abstract

Distribution of partial melt (magma) in the upper mantle is investigated in detail from the seismic tomography of the mantle wedge beneath northeastern Japan. The comparison of the fine velocity structure with the laboratory velocity data on a partiall-ymolten mantle rock yields estimates of three-dimensional distribution of melt. The results indicate that the cause of island arc volcanism in northeastern Japan is the upwelling of hot mantle materials (volcanic plumes) from beneath. The source of volcanic magma is diapir-like melting regions localized within the volcanic plumes. Extensive volcanic activity at the volcanic front is due to the presence of the vast melting zones right beneath the 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-arc region, which may cause across-arc variations 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 brittle upper crust may produce mid-crustal S-wave reflectors.