PALEOMAGNETIC EVIDENCE FOR BLOCK ROTATIONS IN CENTRAL HOKKAIDO-SOUTH SAKHALIN, NORTHEAST ASIA

Abstract

The Eocene to Lower Miocene strata in the Bykov region of south Sakhalin Island, Far East Russia, show a mean paleomagnetic direction of Dec. = 28.7°, Inc. = 55.7°, α95=6.7, and N=22, and a paleomagnetic pole at 66.3°N, 245.7°E, and A95=8.8° which shows that south Sakhalin rotated 25°±15° clockwise with respect to Northeast Asia since the Early Miocene. This pole agrees with that of the Eocene to Middle Miocene sedimentary rocks in central Hokkaido, Japan. It suggests that both south Sakhalin and central Hokkaido have rotated clockwise with respect to Northeast Asia since the Early Miocene. These clockwise rotations can be interpreted as a vertical axis rotation of the blocks distributed, in the north–south direction, more than 1000 km along the dextral Hokkaido–Sakhalin Shear Zone. Considering the development of an en-echelon structure of the basins in the shear zone, we recognized five blocks, each of which had rotated since the Early Miocene. We conclude that this dextral shear is responsible not only for these block rotations but also for the opening of the Kuril Basin and the Japan Sea, and that the shear motion was caused by oblique convergence between the Eurasian and the Okhotsk plates since the Early Miocene.