抄録
Recent paleomagnetic surveys revealed that Northeast Japan was rotated counter-clockwise about 20° during the middle Miocene opening of the Japan Sea, whereas Southwest Japan clockwise about 45°. A reconstruction of the pre-Miocene Japanese Islands is attempted on the basis of the following main premises. 1) The bending of zonal geologic units in the Central Japan was caused by the collision of the Izu-Ogasawara Ridge after the opening of the Japan Sea. 2) The Yamato Bank and several plateaus in the Japan Sea are regarded as continental fragments and there were no marginal basins before the opening. 3) Northeast Japan and Central Hokkaido are geologically continuous to West Hokkaido and Sakhalin, respectively. Each pair is treated as a single crustal block. The reconstruction gives consistent explanations for such geologic features as the change of volcanic front, distributions of Paleogene coal-bearing strata and Cretaceous to Paleogene subduction complex and felsic volcanic rocks, continuity of geotectonic units mainly consisting of the Jurassic subduction complex, and tectonic lines. A most significant corollary in this reconstruction is that Central Hokkaido was adjoined to Northeast Japan.
