Paleomagnetic study of the Tsushima Islands, southern margin of the Japan Sea.

Abstract

Paleomagnetic study of Oligocene and Miocene rocks from the Tsushima Islands, the southern margin of the Japan Sea, has revealed considerable deflection of the mean paleomagnetic direction compared with the Eurasian mean pole. Samples were collected from 48 sites of the late Oligocene to Early Miocene Taishu Group and Middle Miocene igneous intrusions with radiometric dates around 15Ma. After demagnetization experiments, 14 sites from six stratigraphic units finally yielded resolvable stable magnetization components. An overall-mean direction of D: -22.4°, I: 44.7°, and α95: 11.7° was obtained. The mean declination was significantly deflected from that of the expected direction calculated from the 20Ma Eurasian mean paleopole. The deflection, -28.1°±14.0°, is explained by a counterclockwise rotation of the Tsushima Islands relative to Eurasia after the igneous activity around 15Ma. This result suggests that the Tsushima Islands were not included in the tectonic regime of the Southwest Japan block, which rotated clockwise during the formation of the Japan Sea at about 15Ma.