[SIT32-P07] Evidence for the strike-slip opening of the Japan Sea
キーワード：日本海、拡大様式、ストライクスリップ
The opening tectonics of the Japan Sea is important for our understanding of the evolution and dynamics of East Asia. An appropriate model of the opening tectonics of the Japan Sea is the double door opening model based on paleomagnetic declinations observed in the Japan Arcs. The model assumes that SW Japan arc would have 40-60° clockwise rotated and NE Japan would have 40° anticlockwise rotated during the Japan Sea opening. Structural features of the Japan Arcs, the Japan Sea and their vicinities indicate that the Japan Sea would have opened south-southwestward as a strike-slip matter using the Tsushima-Goto Fault, Sakhalin-Central Hokkaido and the Japan Trench as master faults.
Faults oriented to an NNE-SSW direction formed in the Cretaceous are developed both in north Kyushu and SE Korea. Faults oriented to NNW-SSE to NW-SE directions are developed in the Paleogene north Kyushu sedimentary basin and the Paleogene East China Sea Shelf Basin. Pre-Paleogene strata continues with same direction from north Kyushu to the western Chugoku district. The Tsushima-Goto Fault striking between the Japan Sea and the East China Sea disconnects the north Kyushu sedimentary basin and the East China Sea Shelf basin. These structural features indicate that north Kyushu-the western Chugoku district would have migrated along the Tsushima-Goto Fault during the Japan Sea opening.
Echelon faults oriented to an NNW-SSE direction are developed in NE Japan. Rapid subsidence was occurred during the Japan Sea opening at the bending points of the NNW-SSE faults where the faults turn anticlockwise their directions from north to south. The eccentrically-subsidence indicates that the NE Japan would have carried out sinistral deformation using the Japan Trench as the master fault. Along Sakhalin-central Hokkaido, the boundary between the JS and the Okhotsk Sea, the Cretaceous Yezo Supergroup characterizing flysch is distributed as a narrow band. Dextral structures developed there in early Miocene, which would have been caused by the extending of the Kuril Basin. The Japan Sea is composed of basins and continental fragments bounded by faults oriented to NNE-SSW to N-S directions and faults oriented to an E-W direction.
These lines of evidence indicate that the Japan Sea would have opened as a strike-slip matter rather than the double-door matter. Paleomagnetic declinations observed in Japan would be the results of rotations of small faulted-blocks rather than those of coherent arcs. The brittle/ ductile transition zone in the crust beneath Japan would have been shallow under the high geothermal condition, which are represented by the submarine green tuff volcanism during the Japan Sea opening. The shallow brittle/ ductile transition would have made block rotation easy.
Faults oriented to an NNE-SSW direction formed in the Cretaceous are developed both in north Kyushu and SE Korea. Faults oriented to NNW-SSE to NW-SE directions are developed in the Paleogene north Kyushu sedimentary basin and the Paleogene East China Sea Shelf Basin. Pre-Paleogene strata continues with same direction from north Kyushu to the western Chugoku district. The Tsushima-Goto Fault striking between the Japan Sea and the East China Sea disconnects the north Kyushu sedimentary basin and the East China Sea Shelf basin. These structural features indicate that north Kyushu-the western Chugoku district would have migrated along the Tsushima-Goto Fault during the Japan Sea opening.
Echelon faults oriented to an NNW-SSE direction are developed in NE Japan. Rapid subsidence was occurred during the Japan Sea opening at the bending points of the NNW-SSE faults where the faults turn anticlockwise their directions from north to south. The eccentrically-subsidence indicates that the NE Japan would have carried out sinistral deformation using the Japan Trench as the master fault. Along Sakhalin-central Hokkaido, the boundary between the JS and the Okhotsk Sea, the Cretaceous Yezo Supergroup characterizing flysch is distributed as a narrow band. Dextral structures developed there in early Miocene, which would have been caused by the extending of the Kuril Basin. The Japan Sea is composed of basins and continental fragments bounded by faults oriented to NNE-SSW to N-S directions and faults oriented to an E-W direction.
These lines of evidence indicate that the Japan Sea would have opened as a strike-slip matter rather than the double-door matter. Paleomagnetic declinations observed in Japan would be the results of rotations of small faulted-blocks rather than those of coherent arcs. The brittle/ ductile transition zone in the crust beneath Japan would have been shallow under the high geothermal condition, which are represented by the submarine green tuff volcanism during the Japan Sea opening. The shallow brittle/ ductile transition would have made block rotation easy.