Abstract: A Tectonic Synthesis of Tertiary Back-Arc Basins around Southwest Japan

Yasuto Itoh

An interpretation is presented for the Tertiary tectonic evolution of offshore basins in the western part of the Japan Sea and East China Sea on the basis of geological and paleomagnetic data and seismic surveys. Rifting of the Japan Sea in the early Miocene caused rapid subsidence in back-arc basins along southwestern Japan and right wrench deformation of the Fukue basin in the East China Sea. Sedimentary rocks in the Japan Sea back-arc basins are underlain by voluminous syn-rift igneous rocks. Normal faults formed within the Fukue basin because of a releasing bend along a major transcurrent fault. In the middle Miocene, southwestern Japan was deformed by crustal indenters on the subducting oceanic plate, and by arnalgamation with northeastern Japan. Concurrently, the back-arc basins of southwestern Japan were broadly folded and faulted by ENE-WSW compression. Young, buoyant lithosphere of the Philippine Sea plate (Shikoku basin) began to subduct under southwestern Japan around the end of the Miocene. Strong tectonic coupling along the convergent margin resulted in remarkable N-S shortening of southwestern Japan, as indicated by a prominent unconformity in the back-arc basins and as left wrenching between the Japan Sea and the East China Sea. The study area has been the locus for alkaline volcanism related to mantle upwelling during the Quaternary and is characterized by high heat flow.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994