Miocene clockwise rotation of Southwest Japan

Abstract

During the Miocene, Southwest and Northeast Japan rotated clockwise and counterclockwise, respectively, associated with opening of the back-arc basins of the Japan Sea. Kinematic models proposed in the mid-1980s suggest that the differential rotation of the two island arc slivers occurred simultaneously at ~15 Ma, over a period of ~1-2 Myr, and this view still has a strong influence on geotectonic studies of Japan. However, recent progress in biostratigraphic and radiometric dating has provided new data indicating that the clockwise rotation of Southwest Japan occurred a few millions of years earlier than was previously suggested. In addition, reinvestigation of middle Miocene (15-14 Ma) igneous rocks that have a northeast-directed remanent magnetization direction suggests that this direction represents a geologically instantaneous paleomagnetic record and should therefore not be used for tectonic analysis. Here we review paleomagnetic and geochronological data for Southwest Japan published during the last quarter century with the aim of revising the timing and amount of clockwise rotation. Results show that clockwise rotation occurred at 18-16 Ma. The maximum amount of rotation relative to the tectonically stable part of the Asian continent is estimated to be 41.7°±5.4°, based on the assumption that the main part of Southwest Japan rotated as a rigid block. The angular velocity of rotation was about ≥21°/Myr. These findings are incorporated into a model for the Japan Sea involving two stages of opening as follows: (1) during the earlier stage (Eocene-Oligocene), Southwest Japan was rifted from the eastern margin of the Asian continent with little or no rotation; and (2) during the later stage (Miocene, 18-16 Ma), Southwest Japan migrated to its present position via ~40° of clockwise rotation.