Abstract
The thermal structure of the upper mantle beneath the Japan arc is investigated from seismic and laboratory velocity data. Comparisons of seismic velocity structure with laboratory velocity data yield estimates of temperature and degree of partial melt. The diagrams for determining temperature and melt fraction are presented by combining upper mantle velocity structure with elastic-wave velocity in mantle rocks at high pressure and temperature. Employing the velocity structure in northeastern Japan, we obtain up to 2vol.% melt and 1270°C at 40km depth, 1vol.% melt and 1360°C at 65km depth, and 1vol.% melt and 1460°C at 90km depth. The amount of melt is larger and the area of melting is broader for more extensive low velocity regions in the mantle wedge right beneath the volcanic front. We also expect partial melting and temperatures higher than the mantle solidus beneath volcanoes in central Japan and in Kyushu, such as Mt. Asama, Mt. Fuji and Mt. Unzen. It is noted that these high temperature regions are localized in the mantle wedge beneath the volcanoes and downwards to the west. Localized plume-like upwellings (volcanic plumes) from deep mantle appear to produce magmatism and high heat flow in the Japan arc.
