Quaternary volcanic front at the junction of the South-west Japan Arc and the Ryukyu Arc

The Hohi volcanic zone, a 70 by 40 km volcanic graben on Kyushu Island at the junction of the South-west Japan Are and the Ryukyu Are, has been formed since 6 Ma to the present. Quaternary volcanism, producing centers along a NE-SW trending line, the volcanic front, started at about 1.5 Ma. This volcanic front crosses the E-W alignment that was built by the Pliocene volcanoes of the Hohi volcanic zone. The Quaternary volcanic front is parallel to the depth contours of the present deep-seismic plane, and is perpendicular to the current direction of subduction of the Philippine Sea plate. The age of formation of the volcanic front coincides with the time when several other tectonic events occurred, such as the counter-clockwise shift from NNW to WNW in relative convergence direction of the Philippine Sea plate. The volcanic rocks erupted after 1.5 Ma are more widely distributed, with a larger total volume in the denser seismic zone than in the aseismic zone. This suggests that the volcanism on the volcanic front is closely related to seismicity at about 120 km depth. The active seismic area may have accelerated to generate a partially molten zone beyond the dragged hydrated peridotite layer above the downgoing slab in comparison to the aseismic area. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.