Study on the Extension Tectonics of Northeast Honshu Arc Just Before 15 Ma.

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1) Half-grabens were active in Northeast Japan in the period from 23 Ma to 14 Ma. They trend NNW to NNE, and have a width of about 10 km. The grabens were filled by thick and coarse clastic sediments.2) Newly investigated grabens in the Oguni area Yamagata Prefecture and the Ani area of Akita Prefecture have nearly same characteristics as others in Northeast Japan.3) The analysis of the metalliferous veins suggests that one of the middle to Late Miocene Principal stress trendeds ENE-WSW and was weakly compressional and another was NNW-SSE in trend and tensional. This stress field suggests that not only Southwest Japan but also Northeast Japan were suffered by the subduction of the Philippine Sea Plate.4) The early Miocene half-grabens in Northeast Japan are west side down in the Pacific side and east side down in Japan Sea side. Our new sand box experiments testified that the normal faults with fault plane dipping toward the high strain region are selectively grown, and that the degree of the asymmetry depends positively on the magnitude of the strain gradient. These results suggest that there was a zone of maximum tensional strain along the Ou backbone region during the early Miocene.5) The laws of convergence rate of plates (Otsuki, 1989) and the Paleomagnetic data from the Philippine Sea Plate suggest that not only Southwest Japan but also Northeast Japan suffered the subduction of the Philippine Sea Plate, and that the development of the early Miocene half-grabens and the Japan Sea opening were due to the slow northward motion of the Philippine Sea Plate during the period from ca. 24 Ma to ca. 12 Ma.