Reconstructing the tectonics and paleogeography during the ocean-land transition of the "Sanjiang"orogenic belt in southwest China

The " Sanjiang " orogenic belt in the southwest contains the lost Tethys Ocean and the "arc-basin system" of the continental margins on both sides.Based on the principle of tectonic facies and basin analysis, lithofacies characterization, palaeogeography restoration, palaeostructure reconstruction and oceanic and continental transformation are carried out for the sedimentary geological bodies in the "Sanjiang" orogenic belt. Before the Middle Permian, the Tethys Ocean continued to expand, and the two continental margins of Gondwana and Eurasia were stretched and split apart, forming the detachment block(continental margin platform)and continental margin rift basin, which constituted the "multi-island ocean" structural-geographic pattern. During the Middle Permian, the Tethys oceanic plate began to subduct northward, and the Dongdashan-Lincang line of the Eurasian continental margin formed an accretive front arc.The front arc turned into a backarc stretching environment on the continental side, which continued to maintain the paleogeographic pattern of "platform and basin". During the Late Permian to Early Triassic, the Tethys oceanic plate continued to subduct northward, and the oceanic crust of Jinshajiang-Alao Mountain and Lancang River back-arc basins also subducted westward and eastward under the Changdu -Simao block, making them turn into bidirectional back-arc basins. And the Zhongza-Zhongdian block, Ganzi - Litang Ocean also maintained the stretching environment.During the Late Triassic, the oceanic crust of Tethys Ocean, Lancang River and Jinshajiang-Ailaoshan back-arc basin continued to subduct until it closed.The Changdu - Simao block turns into a backarc bidirectional foreland basin, and the Zhongzan - Zhongdian block turns into a peripheral foreland basin.The eastern part of the Zhongzan-Zhongdian block was subducted by the ocean basin behind the Ganzi - Litang arc, forming the arc-basin system.In the Late Triassic, the Tethys oceanic plate began to submerge southwestward, forming the Gangdis-Tengchong marginal island arc.The eastern part of the Baoshan block was a marginal island arc in the early Late Triassic and turned into a backarc foreland basin in the Late Triassic.In the west, peripheral foreland basins are formed.Thus, during the subduction stage of the oceanic plate, the tectono-geographic pattern of the Gondwana and Eurasian continental margins and the two-level "multi-arc basin" on the detachment block were formed.