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

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. 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 20 Ma Eurasian mean palaeopole. The deflection, -28.1° ± 14.0°, is explained by a counterclockwise rotation of the Tsushima Islands relative to Eurasia after the igneous activity around 15 Ma. 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 15 Ma. -from Authors