STRUCTURAL CHARACTERISTICS AND TECTONIC EVOLUTION OF THE YANGSAN FAULT, KOREAN PENINSULA

The Yangsan fault running to NNE direction is one of the major faults in the Korean peninsula. Several different opinions with regard to the movement pattern of the fault have been proposed up to now. In order to clarify structural characteristics and evolution of the Yangsan fault, the geological structures are analyzed and the paleostresses are reconstructed based on the fault-slip data measured along the fault. And, the results are also compared with the tectonic history around the Korean peninsula. Interpretation based on the bedding attitude of sedimentary rocks distributed around the fault and the geometrical analysis of the small faults developed along the fault implies that the Yangsan fault is not a monophase deformed fault but the polyphase deformed fault which has been experienced several times of deformation. Six tectonic events related to the movement of the Yangsan fault are reconstructed based on the analyzing results of about 1,300 striated faults, 300 tension gashes, and 210 relative chronologies measured along the fault. The results of the age dating on the igneous rocks around the fault and the fault gouges along the fault indicate that the Yangsan fault was firstly initiated in the Paleocene, and it has also been reactivated during Tertiary to Quaternary age.

By comparing with six sequential tectonic events and the tectonic history around the Korean peninsula such as the Bulguksa Disturbance, the opening of the East Sea (Sea of Japan) and the rotation of Japan the evolution of the Yangsan fault could be interpreted as follows. The initial feature of the Yangsan fault was formed under NE-SW compression during in the Paleocene. In next stage, the fault experienced a right-lateral strike-slip movement continuously by the NW-SE extension. The movement had been continued until the late Miocene age, which was the most active faulting period. In this period, the Yangsan fault was subjected to a right-lateral movement. During the late Miocene, a left-lateral movement was followed by subsequent NW-SE compression. In the Pliocene, N-S compression and E-W extension activated on the Yangsan fault. In the last stage, the fault has been reactivated by a right-lateral movement since Quaternary under E-W compression that can be correlated to the current tectonic stress around the Korean peninsula.