Paleomagnetism of Tertiary rocks on the Korean Peninsula: Tectonic implications for the opening of the East Sea (Sea of Japan)

Two different distributions of declinations with dual polarities are found in the Tertiary rocks of the Yeonil Basin and Yangnam Basin along the Yangsan Fault in the southeastern part of the Korean Peninsula. These are northward declinations in the Middle Miocene Yeonil Group and strong eastward declinations in the Early Miocene Yangbuk Group and in the Eocene Volcanic Rocks. This is interpreted by invoking (1) the Yangsan Fault as a dextral wrench fault system by spreading in the Uleung Basin of the East Sea (Sea of Japan), and (2) a clockwise vertical rotation caused by the movement of the fault at around 17.3 Ma just prior to the sedimentation of the Yeonil Group. The timing of the deflection found in Korea is distinguished from that in southeastern and northwestern Japan at around 15 Ma. Two stages of spreading are thus constrained in the East Sea area: parallel at 23-15 Ma and pivotal at around 15 Ma. Crustal thinning also occurred. A reliable pole position (84.9°N, 292.6°E, d(p)/d(m) = 3.3°/4.8°) for the Miocene (about 15 Ma) in Korea is obtained from the Yeonil Group.