[SIT21-P03] 地質学的証拠が示す日本海のプル・アパート拡大
キーワード：日本海、プル・アパート、対馬－五島断層
Back-arc basin opening is generally considered to be due to a regional stress field caused by shallow mantle processes such as slab-pull, corner flow and extrusion. Opposite to these hypotheses, the Japan Sea is considered to be opened by plume-like mantle upwelling. The double-door opening model based on paleomagnetic declinations observed in Japan led researchers to this hypothesis. Various scale of rotation, however, could have caused paleomagnetic declination. Paleomagnetic declinations therefore, could not be regarded as an independent constraint of the opening tectonics of the Japan Sea. Examinations based on geological relationships between Japan and marginal areas are needed to discuss the opening tectonics of the Japan Sea.
There is only one Precambrian Belt, the Oki-Hida Belt, is developed in Japan. On the other hand, three Precambrian systems, the Yeongnam Massif, Gyeonggi Massif and Nangrim Massif, are distributed from south to north in the Korean Peninsula. Therefore the interpretation that the Oki-Hidea regards as the extension of the Yeongnam is the northern maximum of the paleo-position of Japan before the Japan Sea opening. Sedimentary basins subsided from the late Cretaceous to early Miocene are developed from northern Kyushu to the East China Sea. These sedimentary basins are characterized by the development of faults oriented to NW – SE to NNW –SSE directions, which indicates that northern Kyushu would not significantly rotated during the Japan Sea opening. Alignments of pre-Tertiary systems oriented ENE-WSW also continues from northeastern Kyushu to the western Chugoku district of Honshu, which indicates that Honshu would not rotated with respect the East China Sea. These observations indicate that the Japan Sea would have opened with a pull-apart matter.
At the west of northern Kyushu, the Tsushima-Goto Fault, the tectonic domain boundary between the Japan Sea and the East China Sea strikes with an NNE-SSW direction. The Tsushima-Goto extends as the western margin fault of the Tsushima Basin at the east of Korean Peninsula. Geological structure developed along the fault, such as rapid sedimentation of the Nojima Group in northwestern Kyushu, indicates that the Tsushima-Goto would have carried out strike-slip activities during the Japan Sea opening. The metamorphic folded belt developed along the southern margin of the Tertiary sedimentary basins is disconnected by the Tsushima-Goto and the Goto Canyon. These observations indicate that the Tsushima-Goto would have been the western master fault of the Japan Sea opening. In northeast Japan arc, a sinistral alignment of faults originally formed in Paleogene is developed. The activities of the Kuroko rift and the Joban forearc basin indicate that sinistral strike-slip movements would have occurred there during the Japan Sea opening. The Japan Trench terminates the sinistral fault system, which indicates that the trench would have been the eastern master fault of the Japan Sea opening.
In conclusion, body of geological evidence indicates that the Japan Sea would have opened as a pull-apart mater. The Tsushima-Goto Fault and the Japan Trench would have been the western and eastern master faults. The body of evidence also indicates that paleomagnetic declinations observed Japan would have been results of some kind of block rotations, such as ball bearing-type rotation, rather than coherent arc rotations. Plume-like active mantle upwelling therefore is not required to explain the opening tectonics of the Japan Sea.
There is only one Precambrian Belt, the Oki-Hida Belt, is developed in Japan. On the other hand, three Precambrian systems, the Yeongnam Massif, Gyeonggi Massif and Nangrim Massif, are distributed from south to north in the Korean Peninsula. Therefore the interpretation that the Oki-Hidea regards as the extension of the Yeongnam is the northern maximum of the paleo-position of Japan before the Japan Sea opening. Sedimentary basins subsided from the late Cretaceous to early Miocene are developed from northern Kyushu to the East China Sea. These sedimentary basins are characterized by the development of faults oriented to NW – SE to NNW –SSE directions, which indicates that northern Kyushu would not significantly rotated during the Japan Sea opening. Alignments of pre-Tertiary systems oriented ENE-WSW also continues from northeastern Kyushu to the western Chugoku district of Honshu, which indicates that Honshu would not rotated with respect the East China Sea. These observations indicate that the Japan Sea would have opened with a pull-apart matter.
At the west of northern Kyushu, the Tsushima-Goto Fault, the tectonic domain boundary between the Japan Sea and the East China Sea strikes with an NNE-SSW direction. The Tsushima-Goto extends as the western margin fault of the Tsushima Basin at the east of Korean Peninsula. Geological structure developed along the fault, such as rapid sedimentation of the Nojima Group in northwestern Kyushu, indicates that the Tsushima-Goto would have carried out strike-slip activities during the Japan Sea opening. The metamorphic folded belt developed along the southern margin of the Tertiary sedimentary basins is disconnected by the Tsushima-Goto and the Goto Canyon. These observations indicate that the Tsushima-Goto would have been the western master fault of the Japan Sea opening. In northeast Japan arc, a sinistral alignment of faults originally formed in Paleogene is developed. The activities of the Kuroko rift and the Joban forearc basin indicate that sinistral strike-slip movements would have occurred there during the Japan Sea opening. The Japan Trench terminates the sinistral fault system, which indicates that the trench would have been the eastern master fault of the Japan Sea opening.
In conclusion, body of geological evidence indicates that the Japan Sea would have opened as a pull-apart mater. The Tsushima-Goto Fault and the Japan Trench would have been the western and eastern master faults. The body of evidence also indicates that paleomagnetic declinations observed Japan would have been results of some kind of block rotations, such as ball bearing-type rotation, rather than coherent arc rotations. Plume-like active mantle upwelling therefore is not required to explain the opening tectonics of the Japan Sea.