ALBERT

Description: 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. © 1988, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.

Description: Detailed correlation and chronology of the Neogene and Quaternary marine sediments in the Japanese island arc area have been established by the application of magnetostrati-graphic methods supplemented by microbiostratigraphic data. The sedimentation rates of several sedimentary sequences show a similar pattern among the studied areas, and two drastic synchronous changes in the rates of sedimentation are recognized. Thus the Japanese Neogene and Quaternary can be divided into three major time intervals, named I, II, and III in increasing age. The boundaries between these three intervals are 4.7 mybp (base of the Gilbert Epoch; magnetic anomaly 3) and 10.4 mybp (Epoch 9; magnetic anomaly 5). The geographic distribution of the land area during the time interval I and II was similar to the present; however, in the time interval III, it is completely different but similar to the present Bonin-Mariana arc area. It has been documented by Hawaiian hot spots and spreading features on the East Pacific Rise that the plate motion in the Pacific Ocean area has also changed drastically. The time interval I is the period of high rate of sedimentation (several hundreds cm/1,000 years) and moderately increasing plate motion; the time interval II extremely low rate of sedimentation (less than several cm/1,000 years) and slow plate motion, and at the same time land areas were expanded; the time interval III moderate rate of sedimentation (several tens cm/1,000 years) and high rate of plate motion, and land areas were reduced. These drastic changes can be explained by the “cyclic evolutionary model” originally proposed by Kanamori, and Forsyth and Uyeda’s slab-pulling driving force of the oceanic plate motion as follows. The drastic change from the time interval III to II is ascribable to detachment of the down-going slab from the ocean plate. The reduction in plate motion may also be triggered by the detachment, which releases the ocean plate from the down pulling force. The high rate of sedimentation in the time interval I is resulted from the steepening in the topographic relief and the increase in the amplitude of tectonic deformation, which should be related with the horizontal com-pressional stress caused by the coupling between the continental and oceanic lithospheres. © 1978, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.