Permo-Triassic structural evolution of the Shiwandashan and Youjiang structural belts, South China

- The Shiwandashan belt underwent NW-SE shortening in the Permian to Triassic.
- The Permo-Triassic NW-SE shortening was related to the Paleo-Pacific subduction.
- The Youjiang structural belt underwent NE-SW shortening in the Triassic.
- The Triassic NE-SW shortening was related to the Indochina-South China collision.

We conducted field mapping coupled with radiometric dating across the Shiwandashan and Youjiang structural belts (SWSB and YJSB), to investigate how southwest South China evolved and to better understand its links to plate boundary dynamics during the Late Permian to Middle Triassic. Our results reveal an episodic tectono-magmatic evolutionary history of the SWSB and YJSB. The SWSB underwent significant NW-SE shortening punctuated by ∼250-240 Ma S-type pluton emplacement during the Late Permian to Middle Triassic; the shortening was expressed by thin-skinned NW-verging thrusts and folds, and conjugate sets of ∼N-trending sinistral and ∼E-trending dextral faults. The NW-SE shortening overlapped with, and was succeeded by Triassic NE-SW shortening in the YJSB. The NE-SW shortening was expressed by NE-verging thrusts and folds, which documented a northeastward propagation of foreland deformation. The NE-verging folds overprinted older NW-verging folds, forming superimposed folds at the juncture of the YJSB and SWSB in the Long'an area. Our results, combined with regional considerations, support a model of the NW-SE shortening as an Andean-type orogeny that developed in response to westward subduction of the Paleo-Pacific Plate, and the NE-SW shortening as a product of the Indochina-South China collision. The subduction of the Paleo-Pacific plate assisted in westward motion of the South China oceanic lithosphere, which may have facilitated the closure of the Paleo-Tethys ocean and subsequent collisions of South China with North China and Indochina.