新生代東北本州弧のジオテクトニクスの研究に関する幾つかのコメント  [in Japanese] Several comments on the geotectonics of the Northeast Honshu Arc  [in Japanese]

The Neogene geotectonics in the Northeast Honshu Arc is summerized below with several comments for the convenience of future reference. The tectonic movements, once called the Oshima orogeny, or the subduction which was responsible for the deformation of the Paleozoic and Mesozoic sediments in the Northeast Honshu Arc continued from the Jurassic into the early Cretaceous. A preparatory stage of separation from the Asiatic Continent started immediately after this activities. It can be attributed that the thinning and stretching of the thick crust formed after the Oshima orogeny or the relevant subduction were carried out through vigorous silicic igneous activities accompanied with empiacement of granites. This stage may be regarded as the pre-rifting stage. The rifting stage may be indicated by the initial volcanism of intermediate nature (propyrite) in the Green Tuff Region at earliest ivliocene and followed by heavy accumulation of silicic green tuffs. Subsidence of half graben which is induced by the rifting of crust in deeper part facilitated presumably the sedimentary facies change from coarse debris flow or deltaic sediments to shallow marine sediments both in time and space. Then, it may have been followed again by the stage of steady subsidence which indicated by a thick accumulation of monotonous siliceous or hard shale accompanied with extrusion of basalt and dolerite in the Dewa Hill region. Extrusion of basalt and dolerite marks the rifting stage. Change of stress condition from tensional to compressional seems to commenced just after the deposition of the stagnant type black shale facies. Uplift of the Back-bone Range was completed through the volcanic activities accompanied with caldron collapse through the Late Miocene to the Plio-Pleistocene. As the studies on the tectonics of the Dewa Hill region and the coastal region of the Sea of Japan from the Pliocene to the Holocene has been made clear by several authors, already, no comments on this problem are needed here.