Late Triassic successive amalgamation between the South China and North China blocks: Insights from structural analysis and magnetic fabrics study of the Bikou Terrane and its adjacent area, northwestern Yangtze block, central China

The Bikou Terrane, located at the conjunction of the Longmenshan fold-thrust belt and the west Qinling orogenic belt in centeral China, was involved in the Late Triassic collision between the South China and North China blocks. The Bikou Terrane has preserved crucial information on structural geometry and kinematics of Triassic tectonics, and is therefore of great importance for reconstructing the Paleo-Tethyan evolutionary history. However, multi-phase tectonic events of the Bikou Terrane are unsettled. This work presents detailed structural analysis based on both the field and laboratory works, which reveals three phases of deformation events in Bikou and its adjacent areas, including top-to-the-SW shearing related to SW-ward thrusting (DI) mainly to the north of the Bikou Terrane, top-to-the-NNW shearing related to NNW-ward thrusting (DII) in the Bikou Terrane, and strike-slip faulting (DIII) locally developed in the northern Bikou Terrane. Anisotropy of magnetic susceptibility (AMS) study and related structural analysis not only support the multiphase deformation but also reveal a gradual transition from the DII-re-lated magnetic fabrics to the DIII-related magnetic fabrics in the Bikou Terrane. Integrating published geochronological data, it is constrained that DI occurred at ca. 237-225 Ma, DII occurred at ca. 224-219 Ma, and DIII possibly occurred during the Early Cretaceous. Based on regional tectonics, the DI event corresponds to the collision between the South Qinling block and the Bikou Terrane, and the DII event reflects the intracontinental amalgamation between the Bikou Terrane and the Yangtze block, which indicates a Late Triassic successive amalgamation from the North China block to the South China block. Intracontinental adjustment represented by the strike-slip (DIII event) occurred after the final amalgamation between the North China and South China blocks. By applying AMS on deciphering structural geometry and multiphase deformation, our study suggests that AMS is a useful tool for structural analysis.