Tomographic evidence for the mantle upwelling beneath southwestern Japan and its implications for arc magmatism

The seismotectonics and volcanotectonics of southwestern (SW) Japan have been affected by the subduction of the old Pacific and the young Philippine Sea slabs. We have attempted to determine the three-dimensional P- and S-wave velocity structures of the upper mantle in SW Japan by using a large amount of high-quality arrival-time data recorded by the dense seismograph network on the Japanese Islands to understand the tectonic framework of SW Japan. Our results show the existence of a large low-velocity anomaly below the Philippine Sea slab in the Chugoku and Shikoku districts at least down to depths of 300 km. We infer that the low-velocity zone is an upwelling flow from depth. Two shallower low-velocity zones derived from this low-velocity zone are detected. One is toward the Quaternary volcanoes in the Chugoku district and the other is toward the uppermost mantle beneath the Kii Peninsula. The former suggests that the magma for the Quaternary volcanoes in the Chugoku district is attributable to the upwelling flow in the upper mantle. This idea supports the geochemical and petrological observations that the erupted alkalic basalt in the Chugoku district should have originated from a more primitive or enriched mantle. The latter is responsible for the absence of a high-velocity anomaly corresponding to the subducted Philippine Sea slab beneath the Kii Peninsula. The Philippine Sea slab could have been more hydrated locally beneath the Kii Peninsula than that in the surrounding regions by the interaction with the mantle upwelling. Consequently, dehydration reactions could be involved during the progressive metamorphism accompanied by intensive intraslab seismicity, lowering the seismic velocity in the slab. Aqueous fluids thus released to the overriding plate might be a possible source of mantle-type helium and trigger a long-term swarm activity in the crust of the overriding plate. Our results indicate the importance of the heterogeneous structure below the Philippine Sea slab with regard to the tectonics of SW Japan.