Mesozoic–Cenozoic tectonic relationships between the Kuqa subbasin and Tian Shan, northwest China: constraints from depositional records

Abstract

Stratigraphic profiles in the Kuqa subbasin, located along the southern flank of the Tian Shan, northwest China, display similar Mesozoic–Cenozoic evolutionary sequences of compositional maturity exist in conglomerate clasts, sandstone framework grains and detrital heavy minerals. The similarity of these time-series profiles resulted from orogenesis and intracontinental deformation of the provenance area, the (ancestral) Tian Shan. Mineralogical maturities of conglomerates and sandstones increase from Triassic to Middle Jurassic and decrease from Upper Jurassic–Lower Cretaceous to Tertiary. This trend parallels a similar decrease in maturity in detrital heavy minerals. Zircon, rutile and tourmaline dominate in Middle–Upper Triassic to Middle Jurassic sandstone, whereas unstable hornblende and pyroxene dominate the heavy mineral suites in the Tertiary, especially in Miocene and Pliocene strata. These compositional trends are coeval with gradual southward migration of alluvial depositional fronts and depositional depocenters from Cretaceous to Miocene. Since the Pliocene, large-scale southward migration of alluvial depositional systems occurred while depositional depocenters migrated northward. Based on the Mesozoic–Cenozoic depositional records, the tectonic evolution of the Tian Shan–Kuqa subbasin may be divided into five phases. (1) In Early Triassic, following compression and uplift since Permian, gravelly braided river-alluvial plain depositional systems developing in the Kuqa subbasin, characterized by sands of mixed orogenic provenance type derived from the ancestral Tian Shan. (2) From Middle Triassic to Middle Jurassic, sediment composition of the Kuqa subbasin reflects arc orogen and mixed orogenic sand types, with extensive planation of the ancestral Tian Shan and the development of relatively stable coal-bearing depositional environments. (3) Renewed compression-uplift of the Tian Shan occurred in Late Mesozoic, especially in Cretaceous, with most detrital modes of the Kuqa subbasin indicating provenance attributes akin to the South Tian Shan. (4) Further intensive compression-uplift and, probably, southward (basinward) propagation of the South Tian Shan occurred during Miocene; and (5) supracrustal gravity gliding and collapse occurred in the Tian Shan–Kuqa subbasin probably due to intensive deep compression, with rejuvenated uplift and probable southward (basinward) structural propagation of the South Tian Shan developing since Pliocene. Observed changes in sediment composition and depositional environment since Pliocene time were also controlled by frequent and abrupt climate changes in addition to intensive uplift of the Tian Shan. Moreover, all depositional records in the Kuqa subbasin seem to reflect west to east differences in provenance along the Tian Shan, probably indicating greater exhumation depth in the western Tian Shan than that in the eastern Tian Shan since Early Cretaceous.