Two-Phase Rifting in the East Asian Continental Margin: Southwest Japan Rotated Twice

Rifting is one of the most important tectonic phenomena in continental margins. There are many rifted basins in the East Asian continental margin. Some of these basins form marginal seas with oceanic crust and the others form continental rift basins. The Japanese Islands have been recognized as continental slivers rifted from the Asian continent, associated with the opening of the Japan Sea. As a consequence of the rifting, Southwest Japan experienced clockwise tectonic rotation. The bulk of the clockwise rotation of Southwest Japan occurred during the Middle Miocene. On the one hand, Early Miocene paleomagnetic directions from Southwest Japan show declinations of about 40 to 50° , showing the clockwise rotation of Southwest Japan during the opening of the Japan Sea. Late Cretaceous directions from Southwest Japan, on the other hand, show larger declination values of about 70° . An unknown clockwise tectonic rotation may have superimposed on Cretaceous paleomagnetic declinations of Southwest Japan. The possible clockwise rotation may be attributed to a rifting event in East China to the west of the Korean Peninsula, because Early Tertiary rifted basins have recently been postulated in that region (e.g. Bohai Basin), and because Southwest Japan was flanked along the eastern side of the Korean Peninsula during the Cretaceous. The rift basin formation is considered to have caused the tectonic rotation of an area incorporating the Korean Peninsula and Southwest Japan. The Cretaceous paleomagnetic pole for the Korean Peninsula is rotated clockwise by 23° with respect to that for the North China Block, suggesting a rotational motion of Korea after the Cretaceous. This clockwise rotation is observed from the whole of the Korean Peninsula. The occurrence of the rotation is further confirmed by comparison of paleomagnetic poles of an older age between the two regions: Triassic data also record the consistent clockwise rotation of the Korean Peninsula (15° ). Therefore the Korean Peninsula probably rotated clockwise during the Early Tertiary. A two-phase rifting occurred in the East Asian continental margin. This is characterized by a first phase (Early Tertiary) that has caused the clockwise rotation of an area incorporating the Korean Peninsula and Southwest Japan, followed by a second phase (Middle Miocene) that has resulted in the clockwise rotation of Southwest Japan.