Geometry and kinematics of the Ocheon Fault System along the boundary between the Miocene Pohang and Janggi basins, SE Korea, and its tectonic implications

摘要

Detailed geological mapping and observations of various structural elements were made in order to determine the geometry and kinematics of the Ocheon Fault System (OFS) along the boundary between the Early Miocene Janggi and the Middle Miocene Pohang basins, SE Korea, and to reveal its roles on the basin evolutions. The OFS is a NE-trending relayed fault system composed of a number of NE or NNE-trending normal-slip and sinistral-normal oblique-slip faults, and has a scissor fault geometry decreasing in vertical offset southwestward. The constituent faults created independent grabens or half-grabens on the hanging-walls for the deposition of the Early or Middle Miocene strata. The OFS was initially the northwestern border fault of the Janggi Basin which acted as normal faults by the WNW-ESE tensional stress associated with the NNW-directed dextral simple shear caused by the East Sea opening. Afterwards, it experienced clockwise rotation with change of slip sense from normal-slip to sinistral-normal oblique-slip in response to the progressive dextral simple shear. At about 17 Ma, the shear stress propagating westward was released rapidly by the dextral strike-slip faulting of the NNW-trending Yeonil Tectonic Line (YTL) and the normal faulting of the NNE-trending western border faults of the Pohang Basin. At that time, the depocenter suddenly migrated northward and the depositional environment also changed rapidly from terrestrial to marine due to dramatic subsidence of the Pohang Basin. The Pohang Basin is interpreted to be a pull-apart basin extended at releasing bend/overstep between two PDZs (Principal Displacement Zones), i. e., the YTL and probably the East Korea Fault. The OFS was also reactivated as the eastern border faults of the Pohang Basin. In contrast to the western border faults, the OFS was rotated clockwise and could not be linked with the YTL because of its scissor fault geometry. Our results suggest that the NNW-trending regional dextral shear stress persisted for a considerable period of time in SE Korea during the East Sea opening, supporting the pull-apart opening of the East Sea rather than the fan-shaped opening. Most of the previous studies advocating the pull-apart opening emphasize the role of the NNE-trending strikeslip faults, like the Yangsan fault and OFS, as PDZs. In contrast, this study suggests that the NNE-trending faults in SE Korea acted as major normal faults at releasing bends or stepovers in the NNW-trending dextral fault system during the East Sea opening.