日本海東縁の秋田沖新礁における多段階の反転テクトニクス

抄録

The Dewa Bank Chain located in the Eastern Japan Sea Mobile Belt in the offshore Akita, consisting of several banks namely Oga Mukaise Bank, Shinguri Bank, and Tobishima Bank, is considered to be formed since 200 to 300 m.y. before by tectonic inversion reactivating some early Miocene normal faults relating to Japan Sea rifting. The compressional stress causing inversion trends E-W, whereas the extensional stress in the early Miocene relating rifting of Japan Sea had trended NNE-SSW direction. Such overprint of deformations by different tectonic stress fields causes formation of complex and diverse geologic structures, and the relationship between new and old structures are remained unclear.

Recently, the author carried out seismic stratigraphic analysis and structural interpretation of 3D seismic data acquired in 2014 by METI（Ministr y of Economy, Trade and Industry）at offshore Akita and Yamagata prefectures. As a result of the study, Shingri Bank was revealed to be formed at least in the two stages, early inversion stage and late inversion stage, and this mechanism is found to be explained by the difference of old and new stress directions. In the early inversion stage, some NNE-SSW trending and WNW dipping normal faults formed syn-rift stage in the early Miocene has been reactivated within the compressed zone. The reactivated reverse faults and uplifting hanging wall blocks have been arranged N-S trended en echelon. Most of those reverse faults became inactive before the late inversion stage. In the late inversion stage, the reverse faults and uplifting blocks have been squeezed within the compressed zone, and converged into single, N-S trending reverse fault and uplifting block. Such deformation process can be explained by a newly proposed multistage inversion tectonics model under oblique stress conditions.