Tectonically controlled multiple stages of deformation along the Yangsan Fault Zone, SE Korea, since Late Cretaceous

Abstract

We describe the structural characteristics and tectonic evolution of the NNE–SSW-striking Yangsan Fault, Korea. The surface trace of the fault extends for over 170 km on land, displaying ∼20–30 km of dextral offset. It transects mainly Mesozoic–Cenozoic sedimentary and igneous rocks. Our field observations suggest that the style of deformation within the fault core is controlled by the ductility of the protoliths. Where sedimentary rocks have been faulted, fault cores tend to be relatively wide, having hosted multiple events. Internal fault core structures are characterized by networks of multiple anastomosing strands of fault gouge and breccia, which enclose lenses of fractured protolith. However, where igneous (crystalline) rocks have been faulted, slip tends to be much more localized within narrow fault cores comprising cataclastic rocks. Combining our new data with previous research results, we infer that four main movement phases have occurred on the fault: (1) Late Cretaceous sinistral slip faulting with a component of extensional deformation, under a regime of NW–SE compression; (2) late Paleogene intense dextral slip faulting, which occurred under a regional NE–SW compressional regime; (3) middle Miocene weak sinistral slip faulting, during NNW–SSE compression; and (4) local Quaternary dextral slip faulting with a reverse component, under ENE–WSW to E-W compression. The most intense phase of deformation appears to have occurred during the late Paleogene due to a major reorganization of plate motion in and around the NW Pacific area. Our findings highlight that the fault has been a preferred zone of weakness for episodic reactivations having significant implications for the changing tectonic environments of East Asia from Late Cretaceous to Quaternary, and that its segments underwent distinguishable deformations at each phase probably due to their geometrical features and fault rock properties.