Volcanic history and tectonics of the Southwest Japan Arc

Remarkable changes in volcanism and tectonism have occurred in a synchronous manner since 1.5-2 Ma at the junction of the Southwest Japan Arc and the Ryukyu Are. Although extensive volcanism occurred in Kyushu before 2Ma, the subduction-related volcanism star ted at ca 1.5 Ma, forming a NE-SW trend volcanic front, preceded by significant changes in whole-rock chemistry and mode of eruptions at ca 2 Ma. The Median Tectonic Line has intensified dextral motion since 2 Ma, with a northward shift of its active trace of as much as 10 km, accompanied by the formation of rhomboidal basins in Central Kyushu. Crustal rotation and incipient rifting has also occurred in South Kyushu and the northern Okinawa Trough over the past 2 million years. We emphasize that the commencement age of these events coincides with that of the transition to the westward convergence of the Philippine Sea plate, which we interpret as a primary cause of these synchronous episodes. We assume that the shift in subduction direction led to an increase of fluid component contamination fi om subducted oceanic slab, which then produced island-are type volcanism along the volcanic front. Accelerated trench retreat along the Ryukyu Trench may have caused rifting and crustal rotation in the northern Ryukyu Arc.