東北本州弧新生界の変形相とその指標  [in Japanese] Deformation facies and its indicator of the Cenozoic system in the Northeast Honshu Arc, Japan  [in Japanese]

Since the geometrical feature of a folded surface is closely related to mechanism and condition of folding which depend on ductility at folding, the deformation facies of fold can be represented by any kind of indicator corresponding with the geometry of the folded surface. Interlimb angle of fold is a most useful indicator, especially for describing and discussing the deformation facies of the Cenozoic folds in the Northeast Honshu Arc. Taking the middle Miocene Onnagawa Formation and its equivalents as a key horizon, in which marine hard shle facies predominates, ninety-four folds found in twenty-two geological profiles are described and measured. Three deformation facies are discriminated as a result; the low, the moderate, and the high ductility facies, which are indicated by the interlimb angle of over 90°, 90°to 60°and below 60°, respectively. The low ductility facies is most extensively distributed in the Northeast Honshu Arc, whereas the distribution area of the high dictility facies is limited to narrow zones together with the moderate ductility facies around it; one is the oil field along the coastal region of the Sea of Japan where the folds are trending in the direction from north by northeast to south by southwest, and the other is in the inland basins where the folds are developed in the direction from north by northwest to south by southeast, or from north to south. These high ductility zones, which would have been evolved in these directions since the Miocene, reveal the tectogenesis of the Northeast Honshu Arc and the Sea of Japan.