Polyphase Tectonic Events and Cenozoic Basin‐Range Coupling in the Tianshan Belt, Northwestern China

Abstract  Studies show that the Tianshan orogenic belt was built in the late stage of the Paleozoic, as evidenced by the Permian red molasses and foreland basins, which are distributed in parallel with the Tianshan belt, indicating that an intense folding and uplifting event took place. During the Triassic, this orogenic belt was strongly eroded, and basins were further developed. Starting from the Jurassic, a within‐plate regional extension occurred, forming a series of Jurassic‐Paleogene extensional basins in the peneplaned Tianshan region. Since the Neogene, a collision event between the Indian and the Eurasian plates that took place on the southern side of the Tianshan belt has caused a strong intra‐continental orogeny, which is characterized by thrusting and folding. Extremely thick coarse conglomerate and sandy conglomerate of the Xiyu Formation of Neogene System were accumulated unconformably on the Tianshan piedmont. Studies have revealed that the strong compression caused by the Indian‐Eurasian collision had a profound influence over the orogenic belt in the hinterland, and Mesozoic‐Cenozoic brittle deformed structures superposed on the ductile deformed Paleozoic rocks. The Mesozoic extensional basins were converted into Cenozoic compressional basins. The deformation in the basins is featured by step thrusts and fault‐related folds. Statistics of joints show that the principal compressive stress since the Neogene is in a N‐S direction. Meanwhile, owing to the underthrusting of the basin toward the orogenic belt, the Paleozoic strata were thrust on the Meso‐Cenozoic rocks as tectonic slices, revealing distinct kinematic features in different geologic units. The basin‐range coupling zones are characterized by intensive compression, folding and thrusting, accompanied by local sub‐E‐W‐trending strike‐slip faults. In the Tianshan region, Cenozoic thrusting is the most common basin‐range coupling mode. The folding and faulting of Mesozoic sedimentary rocks, spontaneous combustion of Jurassic coal layers and formation of sintered rocks, the Cenozoic earthquakes and active faulting, and the unique mosaic pattern of basin‐range framework of Xinjiang are all products of tectonism since the Neogene.