東北本州弧中部地域の新生代テクトニクス  [in Japanese] Neogene tectonics of the central part of Northeast Honshu Arc  [in Japanese]

Neogene tectonics in the east-west transection of the central part of the Northeast Honshu Arc (N38°-N39°) is summarized based on stratigraphy and structural features. The tectonic process is divided into five stages. In the Early Miocene (-15 Ma), block tilting occurred and half grabens were formed in the backarc region under an EW-trending extensional stress condition. In the early Middle Miocene (15-12 Ma) a large quantity of basalt erupted in the rift zone which may have been formed relevant to the opening of the Japan Sea. Thereafter, the major part of the arc subsided and the abrupt transgression occurred. During this period, the orientation of horizontal maximum compressive stress (σ_<Hmax>) axis was northeast-southwest. This orientation was subparallel to that of the rift zone. The Japan Sea coast area of the backarc region continued to subside slowly during the late Middle Miocene (12 Ma-10 Ma). In Late Miocene to the Early Pliocene (10-3 Ma), the presentbackbone regionbegantouplift and the onlandcauldronswarms were formed. On the other hand, the backarc region had subsided continuously. The stress field in this period was the same as that in the early Middle Miocene. The magnitude of the deviator stress may have been small. At the Late Pliocene, the mode of tectonics has been changed. The stress field was replaced by the east-west trending strong compression. North-south trending folds and thrusts were developed along the margin of the Ou Backbone Ranges and the Dewa Hill. The beginning of thrust-faulting differes across the arc. It was 2.4 Ma in the western margin of the Dewa Hill, but was 1.5 Ma in the eastern margin. The marginal thrusts in the Ou Backbone Ranges were formed after 0.5 Ma. Such a compressional tectonics has been kept on until now.