日本海南部〜九州周辺の新生代後期テクトニクス

Abstract

Late Cenozoic tectonic history around the southern part of the Japan Sea and Kyushu is reconstructed on the basis of offshore reflection seismic survey and borehole stratigraphy together with geologic information on land. Three tectonic episodes are identified in the study area as follows;(1) Paleomagnetic data suggest that western half of the Japan Sea was generated through Early to Middle Miocene rifting and clockwise rotation of southwest Japan. Seismic profiles around the southern margin of the Japan Sea show extensional grabens filled with late Early Miocene sediments. Coeval deformation in the Fukue Basin off northwestern Kyushu, which is located around rotation pivot of the rifted sliver of southwest Japan, is characterized by right-lateral leaky wrenching along a NNE-SSW structural trend.(2) Southern margin of the Japan Sea suffered extensive inversion around the end of Miocene, probably caused by resumption of subduction of the Philippine Sea Plate. ENE-WSW trending folds (San'in Folded Zone) converge into Tsushima Island at the western end of southwest Japan. Shortening of the southern part of the Japan Sea was accommodated by left-lateral movement along the Tsushima-Goto Fault on the western margin of the back-arc basin. In a sharp contrast, Goto-nada and Amakusa-nada Seas, and central Kyushu began to subside with many extensional features in the same period, which may be linked to mantle upwelling.(3) In Quaternary, study area is divided into three tectonic domains: Inner Zone of southwest Japan is characterized by simple shear deformation caused by right-lateral movements on the Median Tectonic Line (MTL). Southern part of the Japan Sea is undulated under E-W compressive stress. Central Kyushu continued to subside alternatively controlled by pull-apart basin formation on the MTL and mantle upwelling.