Uplift, exhumation, and deformation in the Chinese Tian Shan

The terranes composing the basement of the Tian Shan were originally sutured together during two collisions in Late Devonian-Early Carboniferous and Late Carboniferous- Early Permian time. Since then, the range has repeatedly been uplifted and structurally reactivated, apparently as a result of the collision of island arcs and continental blocks with the southern margin of Asia far to the south of the range. Evidence for these deformational episodes is recorded in the sedimentary histories of the Junggar and Tarim foreland basins to the north and south of the range and by the cooling and exhumation histories of rocks in the interior of the range. Reconnaissance apatite fission-track cooling ages from the Chinese part of the range cluster in three general time periods, latest Paleozoic, late Mesozoic, and late Cenozoic. Latest Paleozoic cooling is recorded at Aksu (east of Kalpin) on the southern flank of the range, at two areas in the central Tian Shan block along the Dushanzi-Kuqa Highway, and by detrital apatites at Kuqa that retain fission-track ages of their sediment source areas. Available ⁴⁰Ar/³⁹Ar cooling ages from the range also cluster within this time interval, with very few younger ages. These cooling ages may record exhumation and deformation caused by the second basement suturing collision between the Tarim-central Tian Shan composite block and the north Tian Shan. Apatite data from three areas record late Mesozoic cooling, at Kuqa on the southern flank of the range and at two areas in the central Tian Shan block. Sedimentary sections in the Junggar and Tarim foreland basins contain major unconformities, thick intervals of alluvial conglomerate, and increased subsidence rates between about 140 and 100 Ma. These data may reflect deformation and uplift induced by collision of the Lhasa block with the southern margin of Asia in latest Jurassic-Early Cretaceous time. Large Jurassic intermontane basins are preserved within the interior of the Tian Shan and in conjunction with the fission-track data suggest that the late Mesozoic Tian Shan was subdivided into a complex of generally east-west-trending, structurally controlled subranges and basins. Apatite data from five areas record major late Cenozoic cooling, at sites in the basin-vergent thrust belts on the northern and southern margins of the range, and along the north Tian Shan fault system in the interior of the range. The thrust belts and fault system have been sites of active shortening and exhumation since at least ca. 25 Ma, apparently induced by the collision of the Indian subcontinent with the southern margin of Asia. On the basis of regional relations, the north Tian Shan fault system is likely an important active right-lateral transpressional structure that has reactivated the north Tian Shan-central Tian Shan suture zone. In general, most of the Chinese Tian Shan appears to have been exhumed only limited amounts through Mesozoic and Cenozoic time. Within our sampling areas, only limited areas along the north Tian Shan fault zone and in parts of the rangemargin thrust belts were exhumed more than &3 km during the Cenozoic India-Asia collision. Modern intermontane basins are present within the Tian Shan and help divide it into a number of subranges, much like the late Mesozoic Tian Shan. This modern physiography likely reflects in part reactivation of pre-Cenozoic structural trends.