Late Paleogene reactivation of the Sinnyeong Fault in the Gyeongsang Basin, SE Korea and its tectonic implications

Abstract:

The WNW-trending Gaeum Fault System (GFS) in the Cretaceous Gyeongsang Basin can give important clues for understanding the crustal deformation history in the SE Korea during the late Cretaceous to Paleogene. This study focuses on the Sinnyeong Fault, one of the most conspicuous strands among the GFS, based on the detailed field observations. Its main movement is interpreted as sinistral strike-slip with small reverse component, although it could have experienced other faultings with difference senses before/after this movement. An intriguing feature of the fault is the asymmetry of damage zones on both sides of the fault core. Sedimentary strata of the southern damage zone are much more folded and deformed than those of the northern damage zone, and the southern damage zone width is much wider than the northern one. The NW-trending en-echelon folds are also exclusively localized in the southern periphery along the fault with a continuous width less than 30 m, indicating a NE-SW compressional stress regime. The folds are interpreted to have formed coevally with or immediately before the sinistral movement of the fault, because they only occur in the nearby area of the fault and are sharply cut by the fault core. The asymmetric damage zone probably resulted from the concentration of regional compressional stress along pre-existing WNW-trending weak zone, such as densely populated fracture zone, within a relatively stable intraplate region. In the Gyeongsang Basin, the NNE-trending Yangsan Fault System dextrally cuts through the granites of ca. 48 Ma and the GFS sinistrally cuts through the granites of ca. 52 Ma. These similar ages as well as the geometries and kinematics of the two fault systems support that they acted as a great conjugate fault system under the NE-SW compressional stress regime. In addition, many previous studies have mentioned that the regional crustal uplift, folding, and reactivation of inherited structures occurred due to a tectonic inversion in East Asia during the late Eocene to Oligocene. It is thus concluded that the most conspicuous two fault systems in the Gyeongsang Basin were reactivated simultaneously as a conjugate fault system along the suitably orientated pre-existing crustal weaknesses under the NE-SW compressional stress regime during the late Paleogene.