Mesozoic-Cenozoic Tectonic Evolution And Range-Basin Coupling Of The South Tian Shan

The Tian Shan Orogen,a Paleozoic orogenic belt in central Asia,underwent multi-phase tectonic activation dominated by north-south compression after the Early Mesozoic,especially the far-field effect of the Cenozoic India-Asia collision.However,there are few reports about the tectonic deformation and onset of the Triassic intracontinental deformation in the Tianshan area,and exactly when the Cenozoic compressive deformation in the southern Tianshan orogenic belt was initiated is controversial.This structural context is key to further understand the intracontinental deformation history,especially the early construction of the Eurasia continent during the Early Mesozoic.The extremely thick Meso-Cenozoic sediments in the Kuqa Depression along the southern Tian Shan range record the basin-mountain coupling process of the South Tian Shan and Kuqa Depression,and it is natural to use these Meso-Cenozoic strata to decipher the southern Tianshan's uplift history.In this study,detailed structural investigations,stratigraphic analysis,and detrital zircon U-Pb dating of the Meso-Cenozoic sediments were conducted in the Kuqa Depression with the purpose of reconstructing the Meso-Cenozoic uplift history of the South Tian Shan range.The age data of the 12 samples from the Meso-Cenozoic sediments in the Kuqa Depression presented show that the detrital U–Pb zircon ages range between 145 and 3336 Ma.These ages can be statistically divided into four main groups: 210–250 Ma(sub-peak at 226 Ma),260–350 Ma(sub-peak at 290 Ma),360–460 Ma(sub-peak at 410 Ma),and 500–3336 Ma(sub-peak at 886 Ma).It was established based on the morphological characteristics of the 12 samples,internal crystal textures,and Th/U ratios,that the magmatic zircons were dominant(95%)in the samples.These age groups indicate that the dominant clastic provenance for the Kuqa depression since the latest Paleozoic was the Tian Shan area.The summary of the detrital zircon ages of the six Cenozoic samples in this study and the 16 Mesozoic samples from previous studies indicate that the source area of the Kuqa Depression moved southward from Central Tian Shan–South Tian Shan to South Tian Shan from the Mesozoic to the Cenozoic.Variations in zircon U-Pb age distribution within 12 samples collected in the Kuqa Depression suggest that the depositional evolution of the Kuqa Depression underwent six different phases,which were,Early Triassic–Middle Triassic;Middle Jurassic–Early Cretaceous;Cretaceous–Early Cenozoic;Late Eocene–Oligocene;Miocene–Middle Miocene,and Miocene–Pliocene.Multiple sets of growth strata was identified from the Growth strata profile analysis in Kuqa depression:(1)The Middle–Late Triassic Kelamayi growth strata was identified in the Southern margin of the South Tian Shan,and the youngest zircon age of two tuffaceous sandstone samples from the bottom and top of the Kelamayi growth strata yielded zircon U–Pb ages of 223.4±3.1 Ma and 215.5±2.9 Ma,respectively,which indicates the growth structure was active between ca.223 Ma and ca.215 Ma.(2)The Eocene Kumugeliemu growth strata are identified in the Kumugeliemu syncline in the central Kuqa depression;(3)The growth stratigraphic sequence of Oligocene Jidike Formation and Middle Miocene Kuqa Formation growth strata was studied in the north limb of the Kumugeliemu syncline;(4)The continuous growth stratigraphic sequence of the Pliocene Kuqa Formation–Pleistocene Xiyu Formations is identified from in the different fold-thrust belts in the Kuqa depression.Combined with growth strata,detrital zircon U-Pb source changes,depositional unconformity,previous paleomagnetic cooling and low-temperature chronology data,the basin-mountain coupling process is reasonably studied.(1)In the Middle-Late Triassic,Tian shan area underwent a phase difference uplift provide new source for Kuqa depression,and syn-tectonic growth strata(223-215 Ma)growth in the fault zone on the southern margin of the south;(2)The Late Triassic to Early Middle Jurassic was a stable sediment source and a major coal-forming stage,reflecting that the Tianshan area was in a tectonic quiet period during this stage.(3)The change of sediment source from the Middle Jurassic to the Early Cretaceous in the Kuqa depression and the unconformity between the early Cretaceous "wall" conglomerate and the Late Jurassic indicated that the Late Jurassic–early Cretaceous compressional deformation in the Tian Shan area.(4)The Eocene Kumugeliemu growth strata in the central Kuqa depression and the provenance change recorded the initiation of Cenozoic intracontinental deformation in the South Tianshan Mountains(?50Ma).(5)Provenance changes from Oligocene to Middle Miocene and two growth stratigraphic sequences(?36Ma and ?13Ma)in Kuqa depression indicate the two stages tectonic uplift in Tian Shan region.(6)The continuous growth stratigraphic sequence of Pliocene Kuqa Formation and Pleistocene Xiyu Formation,widely developed in several fold belts of the Kuqa depression,indicates that the Cenozoic geomorphic pattern of the South Tianshan Mountains in the Kuqa depression was formed since the Pliocene(?6.5Ma).The multistage intracontinental deformation of the Tianshan orogenic belt since Mesozoic is mainly derived from the convergence events in the southern margin of the Tarim block,especially the far-field effect of the India-Eurasia collision since Cenozoic,which resulted in the uplift of Tian Shan mountain to present topography.