Cenozoic Convergence Between The North Eastern Pamir And The South Tianshan: Insights From Structural Features And Sedimentary Records

The collision and the subsequent convergence between the Indian and the Eurasian Plate in the Cenozoic caused extensive intracontinental deformation in Central Asia,making the tens to hundreds of kilometers of northward indentation of the Pamir salient,accompanied with large-scale crustal thickening and denudation.It also caused the South Tianshan reactivation with significant tectonic uplift,leading to the convergence between the Pamir and the South Tianshan.During this convergence process,intense interactive deformation occurred in the Pamir-South Tianshan convergence zone,and widespread deposition of up to several kilometers of thick Cenozoic terrestrial strata were deposited in the Pamir-South Tianshan convergence zone and the southwestern Tarim Basin.The deformation characteristics of the convergence zone and the Cenozoic sediments record the deformation and evolution of the northeastern Pamir and the critical information of its Cenozoic convergence with the South Tianshan.Therefore,studying the initial deformation time and the temporal and spatial distribution of deformation in the convergence zone,and the analysis of sedimentary records are useful to understand the expansion of northeastern Pamir and the collision between the Pamir and the South Tianshan.Based on the interpretation and analysis of seismic profiles in the Pamir-South Tianshan convergence zone and the balanced cross section of three of them,combined with the sedimentary evolution analysis of the growth strata in the Wupoer piggyback basin and the provenance analysis of the late Cenozoic sediments in the North limb of the Qiate anticline and the well Wuqia 1,this study investigates the structural deformation characteristics of the Pamir-South Tianshan convergence zone and the structuralsedimentary response to the Pamir-South Tianshan convergence.The following conclusions have been drawn:1?Through field investigation,seismic interpretation,and balanced cross section,this study suggests that the convergence zone can be divided into three segments,i.e.,The Bashibulake structural belt,the Kelatuo structural belt and the Atushi structural belt from west to east.Each segment has different stratal distributions and tectonic deformation styles.The structural deformation patterns of those segments vary greatly.The Bashibulak tectonic segment in the west shows a superimposed feature,with the Pamir thrust system overlaping with the South Tianshan thrust system.The middle Kelatuo segment shows an ongoing collision feature,where the structural overlapping does not occur.In the eastern Atushi segment,the Pamir and the South Tianshan thrust systems individually thrust northward and southward,respectively,which are separated by the Kashi depression in between.The Mingyaole and Mushi anticlines in the convergence zone are both controlled by the Pamir thrust system.Combined with the balanced cross sections,the initial deformation time of the Pamir thrust system is late Miocene,and the initial reactivation of the South Tianshan thrust belt is late Oligocene to early Miocene.The time of interaction between these two is from late Miocene to early Pliocene.2?The Wupoer Piggyback Basin records the late Cenozoic expansion of the Pamir salient and its collision with the South Tianshan.There were two regional unconformities in the Wupoer piggyback basin;one corresponded to the unconformity between the Atushi Formation and the underlying Miocene.The other was the unconformity between the Quaternary Xiyu Formation conglomerate and the underlying growth strata.The onset of the piggyback basin was in the latest Miocene to Pliocene(?5.8-4.5Ma).According to the stratigraphic characters,the growth strata were separated into 6 units(G1-G6).There were three main depocenters in the piggyback basin during the deposition of growth strata G1 and G2 in the basin,which were controlled by the three segments of the Pamir Frontal Thrust(PFT)and the paleotopography.During the deposition of the growth strata G3 and G4,previous two depocenters in the north intergraded to one due to westward increasing displacement.During the deposition of the growth strata G5 and G6,a united depocenter formed in the basin,which migrated eastward compared with those in previous stages.It was further confirmed the collision between the Pamir salient and the South Tianshan made the deformation transmitted eastward since Quaternary.We speculate the distance between the Pamir salient and the South Tianshan was comparatively close in the late Pliocene.Meanwhile,the northward motion of the Pamir salient continued,which made the deformation transmit eastward.3?Detrital zircon provenance analysis of the Late Miocene Pakabulake Formation in the north limb of the Qiate anticline and the Miocene Keziluoyi Formation in the Well Wuqa 1 better defined the late Cenozoic evolution of the Pamir-South Tianshan convergence zone.Zircon U-Pb ages of four detrital zircon samples from the Miocene Pakabulake Formation in the north limb of the Qiate anticline range from ?161 to 2983 Ma,with primary age peaks of ?274-301 Ma and ?425.6-470 Ma,and a secondary age peaks of ?812-890 Ma.A tiny amount of Triassic zircons of ?217-250 Ma are also found in the samples.Two detrital zircon samples from the Miocene Kiziluyi Formation of the Well Wuqia 1 show a U-Pb age range of ?228-2816 Ma,with primary age peaks of ?260-280 Ma and ?449-470 Ma,and a second age peak of ?810-860 Ma.The samples also contain a tiny amount of Triassic zircons of ?228-250 Ma.This indicates that the Permian and Paleozoic bedrock of the South Tianshan has been the primary provenance of the north limb of the Qiate anticline since the Miocene(22.1Ma),and the Wulagen paleo-uplift near the north limb of the Qiate anticline also provided a small amount of provenance,while although the provenance of the North Pamir can reach the area,its influence is limited.Based on previous detrital zircon provenance analysis in the Wuheshalu section,the provenance from the Pamir reached the front of the South Tianshan with the activity of the north Pamir thrust system in the early Miocene.The provenance of the South Tianshan could also reach the Pamir because of the activity of the South Tianshan thrust system.Due to the continuing northward thrusting of the North Pamir thrust system and the continuing southward thrusting of the South Tianshan thrust system,substantial uplift occurred in the west of the convergence zone,and the deposits source from the Pamir and the South Tianshan began to separate.Based on the above analysis,the convergent process between the Pamir and the South Tianshan has been established.From the late Miocene to the early Pliocene,the north Pamir thrust system and the South Tianshan thrust system interacted.Since the Pliocene,continued convergence between the Pamir and the South Tianshan caused substantial uplift in the west.The interactive deformation of the convergence zone gradually transferred eastward to the Tarim Basin,leading to the eastward migration of the depositional center of the Wupoer piggyback basin.Since the Quaternary,the north Pamir thrust system has overthrust to the northeast and controlled the formation of the Mushi anticline and the Mingyaole anticline,while the South Tianshan thrust system has overthrust to the south and controlled the formation of the Atushi fold and thrust belt.