Geologic structures and basin formation tectonics in and around Toyama Trough, the Sea of Japan

Abstract

Syn-rift and post-rift geological structures in the Toyama Trough and adjacent areas of the southeastern margin of the Sea of Japan were interpreted from 2D and 3D seismic reflection data and other published data. The trends of the fault systems differ between the western and eastern parts of the study area, which are divided by N-S trending transcurrent faults along the eastern margin of the Toyama Trough. These faults extend to the Kashiwazaki-Choshi Tectonic Line, the border between the Southwest and Northeast Japan arcs. The western part of the study area is characterized by the superposition and intersection of six syn-rift fault systems with different trends, resulting from successive changes in the stress field during the multi-stage opening of the Sea of Japan. During the early rift stage (43-21 Ma) in the northern Toyama Trough, WNW-ESE trending rift basins formed, followed by NNW-SSE trending rift basins. During 21-18 Ma, ENE-WSW trending normal faults formed parallel to the Southwest Japan Arc. During the late rift stage (18-15 Ma), NNE-SSW trending en echelon pull-apart basins developed in the southern Toyama Trough and in the Northern Fossa Magna, related to right lateral divergent wrench deformation. The Toyama Trough is interpreted to be a composite rift basin in which the crustal thickness decreases from south to north.

After 15 Ma, the study area experienced compressional tectonics. Onlap unconformities and E-W trending reverse faults formed during the Middle Miocene (15-13 Ma) and Late Miocene (10-6 Ma). Since 6 Ma, successive changes in the direction and magnitude of compressional stress have resulted in the differentiation of uplifted and subsided areas related to the inversion of reverse faults. The post-rift structures and present course of the Toyama Deep-Sea Channel are strongly controlled by the older syn-rift structures.