Counterclockwise rotation of Northeast Japan: Paleomagnetic evidence for regional extent and timing of rotation

Abstract

Early to Late Miocene welded tuffs have been sampled from northeast Honshu Island and the Oshima Peninsula of Hokkaido Island, Japan for paleomagnetic study. Characteristic directions with a high unblocking temperature component above 560°C are isolated from 24 sites. Westerly declinations ( D = −1° ∼ −91°) are identified in welded tuffs older than 17 Ma from these areas. When combined with previous paleomagnetic data from welded tuffs, the Early Miocene paleomagnetic poles derived from northeast Honshu Island and the Oshima Peninsula are statistically identical and indicate that northeast Honshu Island and the Oshima Peninsula were subjected to counterclockwise rotation as a single tectonic block. These results establish that the rotating block of Northeast Japan extends north to the Oshima Peninsula to the latitude 43°N. The dimension of this rotating block is of the order of 500 km. The paleomagnetic data provide a best curve for the rotation process, with the counterclockwise rotation reaching a climax at 15.0 Ma and a net amount of rotation of 46.4°. The 500 km long Northeast Japan block was rotated counterclockwise through more than 45° and accompanied a clockwise rotation of the 600 km long Southwest Japan block at about 15 Ma.