In this paper, the Japan Sea opening and accompanied large scale ductile flow in continental lithosphere, a big diastrophism which occured in Miocene, in proto-Japan arc are investigated. The essence of continental rifting is the thinning of continental lithosphere by ductile flow in rocks. Therefore, rigid-plate tectonics which has been believed so far is no longer justified in the case of continental extension. Instead, we should take into account realistic flow laws for contstituent rocks of continental lithosphere to analyze continental deformation. In areas of high geothermal gradients such as continental arc, it is shown by computer simulation that ductile stretching in the S lithosphere responsible for continental extension occurs quite easily under reasonable intraplate stresses less than 200 bars. Such a continental extension is accelerated and completed (continental breakup) only within a few Ma, which is in good agreement with observed duration of back-arc rifting such as that in Japan and Italy (opening of the Tyrrhenian Sea). A preliminary investigation of crustal deformation in the forearc region of southwestern Japan is described. At the time of the Japan Sea opening, continental extension occurred and small scale rifted basins formed along the MTL and in the forearc region in southwestern Japan. However, at the culmination of the Japan Sea opening contemporaneous with the clockwise rotation of southwestern Japan (15 Ma, Otofuji and Matsuda, 1984), the forearc region was under a strong N-NNE tending compressional stress field, and was consequently shortened. Possibe causes for such forearc shortening associated with back-arc extension are (1) friction between subducting plate and overlying (arc) plate, and (2) the effect that overlying plate is draged toward trench by the secondary flow caused by plate subduction.

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The Philippine Sea has been formed through the episodic backarc rifting- and spreading. Newest and active backarc rifting was found along the volcanic front of the Izu-Bonin Arc. We presented newly obtained data concerning the Sumisu Rift, a backarc rift of the Izu-Bonin Arc to understand the Tohoku Arc in the Miocene related to the opening of the Japan Sea. The Sumisu Rift is situated near behind the volcanic front between the Sumisujima and Torishima islands, including the sedimentary basins with about 2000m deep. This rift has been formed by a normal faulting with a N-S trend since about 2 Ma. In the rift, a backarc volcanism dominated by basalt (BABB) with a small amount of dacite and rhyolite has been continued after the rift initiation and formed small bodies of volcanoes. The sedimentation in the rift basin is dominated by hemipelagite and arc-derived ashes, especially thick pumice deposits after 0.2 Ma and the burying of the basin has rapidly proceeded.

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The Plio-Pleistocene in the middle reaches of the Shinano river near Niigata-Nagano prefectural boundary consists of andesitic lava and pyroclastic rocks, and volcanic conglomerate with intercalated beds of conglomerate, sand, and silt. Although it mainly consists of volcanic materials and rarely yields fossils, the stratigraphy is able to correlate to the Uonuma Group in the Uonuma Hills, mainly composed of terrestrial elastics (Uonu ma Hills Collaborative Research Group, 1983), on the basis of several key tephras and fission track age data. Based on the lithofacies, it is divided into the Amamizuyama, Kamigo and Tomaru Formations, and into the Higashinosawa, Shikumi and Kusso Formations, respectively in the left side and the right side of the Shinano River, in ascending order. The Amamizuyama and Higashinosawa Formations mainly consist of alternated beds of sand and silt, with aphyric andesite lava and dacite pumiceous tuff, and are correlated to the Lowermost and Lower Formations of the Uonuma Group. The Kamigo and Shikumi Formations consisting of volcanic mud flow deposit, alternated bed of sand and silt, dacitic tuff and aphyric andesite to dacite lava, are correlated to the Middle Formation of the Uonuma Group. The Tomaru and Kusso Formations mainly consisting of volcanic mud flow deposit, which derived from the Torikabto valcano to the south, are correlated to the Upper Formation of the Uonuma Group. Volcanic rocks belong to tholeiitic rock series except for them of the Tomaru and Kusso Formations, which are calk-alkali rock series.

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