TECTONICS OF AN ARC ARC JUNCTION - AN EXAMPLE FROM KYUSHU ISLAND AT THE JUNCTION OF THE SOUTHWEST JAPAN ARC AND THE RYUKYU ARC

Three distinctive geologic events occurred synchronously during the last 6 m.y. on Kyushu Island, lying at the junction of the Southwest Japan Arc and the Ryukyu Arc. These events included: (1) dextral-fault displacement along a 100 km section of the Median Tectonic Line; (2) formation of a rectangular volcano-tectonic depression 70 lan long and 40 km wide; and (3) approximately 30-degrees of counterclockwise crustal rotation. We propose an integrated tectonic model that explains these three tectonic features in terms of subduction of the Philippine Sea plate, which resumed at around 6 Ma after a halt of more than 5 m.y. Oblique subduction of the Philippine Sea plate beneath the Southwest Japan Arc detached a fore-arc sliver that was displaced along a dextral fault, the Median Tectonic Line, north of the Nankai Trough. A volcano-tectonic depression, the Hohi volcanic zone, was formed by the pull-apart structure on the western margin of the Median Tectonic Line. By contrast, normal subduction of the Philippine Sea plate under the Ryukyu Arc caused back-arc spreading, observed as crustal extension in the northern Okinawa Trough. The crustal extension caused the counterclockwise rotation for the northernmost Ryukyu Arc including southern Kyushu.