Kinematic interpretation of layer-parallel shearing structures in Danyang area and its tectonic implication

Layer-parallel shearing has been reported along the Taebaek-type Joseon Supergroup, which is distributed between the Okdong Fault and the Gakdong Thrust, which are the major tectonic lines along the Okcheon Belt. We conducted a study to understand the kinematics of these structures. We identified several shear indicators through field surveys in the Danyang area, providing information on the sense of shear and the vorticity analysis using C' foliations. As a result, a number of C ' foliations, indicating right-lateral sense, were observed in the Myobong Formation, Hwajeol Formation, and Dumugol Formation, which are phyllosilicate rich layers. In contrast, there are also some shear folds indicating left-lateral shear, which deformed the right-lateral C' foliations, suggesting that it occurred after the right-lateral shear. The mean kinematic vorticity number (W-m) derived from the right-lateral C' foliations, range from 0.738 to 0.813 in the Myobong Formation near the Okdong Fault, indicating prevalent simple shear. In contrast, they range from 0.170 to 0.466 in the Hwajeol and Dumugol Formations farther from the Okdong Fault, indicating predominance of pure shear. This may indicate stress change and strain partitioning between the Okdong fault. The kinematics and changes of the W-m across the Okdong fault indicate that they are strongly associated with the movement of the Okdong fault, and the minimum instantaneous stretching axes (ISA(3)) and the strain partitioning indicates that the Okdong fault experienced transpressional stress (WNW-ESE compression) condition. In addition, the paleo-convergence angle based on Wm was rotated clockwise from ENE-WSW toward the WNW-ESE with distance from the Okdong Fault, which may suggest a kinematic relation with the Gakdong Thrust.