Late Permian to Middle Triassic sedimentary records in Shiwandashan Basin: Implication for the Indosinian Yunkai Orogenic Belt, South China

The Shiwandashan Basin is an elongate region of the Late Permian-Middle Triassic potential sediment accommodation that formed on continental crust between the Yunkai Orogen and the adjacent Yangtze Craton Block, South China. Several terms such as remanent marine trough, structural window, rift and pull-apart basin have been proposed. Studies of the Late Permian-Middle Triassic sedimentary sequences and the tectonics lithofacies and paleogeography of the Shiwandashan Basin indicate that the Late Permian-Middle Triassic succession is composed of thick, coarse molasse deposits and fine clastics, organized into coarsening-upward and fining-upward stratigraphic sequences. The Shiwandashan Basin went through the process of growth and disappearance from molasse to turbidite deposition and finally to volcanic eruption during Late Permian-Middle Triassic. To some extent, the facies evolution reflects the tectonic controls on the sedimentation of the foreland basin. The coarsening-upward sequences (hundreds to thousands of metres scale), occurring immediately adjacent to the thrust front, demonstrate that important episodic thrusting-orogenesis and piedmont flexuring-subsidence with low velocity took place in the northwestern part of the Yunkai Orogen, whereas fining-upward sequences are the indicator of continued deformation with high thrust velocity. The following four facts have been demonstrated: (1) a foreland basin existed in Late Permian-Middle Triassic. (2) The development of the foreland basin was controlled by the evolution the Yunkai Orogen. (3) The basin and mountains continuously changed and migrated. (4) The Shiwandashan Basin was a key demarcation between an active region in southeastern South China (Cathaysian Block) and a relatively stable region in northwestern (Yangtze Block). It was a convergent boundary in Late Permian-Middle Triassic and its tectonic nature changed in the wake of the tectonic evolution of the surrounding area.