The Thermal History And Uplift-Exhumation Process Of The East Kunlun Orogenic Belt During Meso-Cenozoic Time

The East Kunlun Orogenic Belt?E-KOB?,in the northern margin of the Tibet Plateau,not only recorded a long and complex orogenic process before late Triassic,which is associated with the evolution of Proto-and Paleo-Tethys Oceans,but also experienced strongly intracontinental deformation and rapid exhumation and uplift as the consequence of the collision and convergence between India and Asia,making it one of the focal region for geologist for a long time.However,it still remains poorly constrained till now as to the initial time of the significant uplift and exhumation of the E-KOB during Cenozoic time,the attribution of the closure of the Kunlun Ocean and the intracontinental evolution to the uplift and exhumation process in the Mesozoic,as well as whether or not a heterogeneous uplift and exhumation occurred in different part of the E-KOB during the Meso-Cenozoic time.The thermochronology,which could simultaneously record the information of time,temperature and depth during the exhumation of rocks,is an important method to the research on the uplift and exhumation history of the orogen.Aiming at the above issues,combined with previous studies on the sedimentary and defamation of the E-KOB and its adjacent areas,the ⁴⁰Ar/³⁹Ar dating,as well as the fission track analysis,of the bedrocks and sedimentary rocks from different localities of the E-KOB,has been carried out in this dissertation,in order to recover the long-term thermal history and uplift and exhumation process of the E-KOB during Meso-Cenozoic time.The main achievements of this thesis are as follows.?1?Based on the results of the ⁴⁰Ar/³⁹Ar dating of the muscovite,biotite and K-feldspar of the bedrocks from Qiamntagh,Kaimuqi and Xiangride areas,it is suggested that both the northern and central E-KOB has experienced a rapid cooling process during late Permian to late Triassic.Integrated detrital zircon U-Pb and muscovite ⁴⁰Ar/³⁹Ar dating of the Lower Triassic Hongshuichuan Formaiton and the Upper Triassic Bayanhar Group at the Tatuo and Budongquan areas respectively,indicates that they were mainly derived from the E-KOB.Additionally,the E-KOB was identified as the major source for the thick Triassic sequence in the Songpan-Ganzi Terrane as indicated by numerous previous provenance studies.Hence,it is implied that the northern and central E-KOB has experienced a rapid uplift and exhumation during late Permian to late Triassic,making the basement rocks and granitoid rocks exposed to the surface.The southern E-KOB still received marine deposit until early Triassic,leading to the burial related heating process of the Zhiyu pluton at early Mesozoic,and significantly uplift since late Triassic.The rapid uplift and exhumation process at the early time of Mesozoic was related with the continuous subduction and closure of the Kunlun Ocean.?2?The bedrock samples from the Qiamntagh,Kaimuqi and Xiangride areas experienced a long and slow cooling and exhumation process during middle-late Mesozoic to early Cenozoic,and stayed in the partial annealing zone of apatite fission track.The thermochronology results of both this and previous studies have showed that,bedrock samples from different localities of the E-KOB recorded a large range of apatite and zircon fission track and zircon?U-Th?/He ages varying from middle-late Mesozoic to early Cenozoic.The provenance study of the lower Jurassic Yangqu Formation at eh Tatuo area based on the integrated detrital zircon U-Pb and muscovite ⁴⁰Ar/³⁹Ar dating results suggests that it was primarily sourced from the E-KOB.In combination with previous provenance analysis of the Mesozoic to early Cenozoic strata in the adjacent areas of the E-KOB,it is suggested that the E-KOB experienced a continuous slow unroofing process and provided detritus for the different regions in the central and northern Tibet Plateau during the middle-late Mesozoic to early Cenozoic,implying a relatively stabled tectonic setting during this time.Moreover,summary of both our newly obtained and published⁴⁰Ar/³⁹Ar data from the E-KOB has illustrated that the effect of the ductile shearing activity of the Kunlun Fault during late Jurassic to early Cretaceous,which was associated with the accretion and collision of the Lhasa block,was limited in the southern margin of the E-KOB.?3?A rapid cooling and exhumation event since late Oligocene to middle Miocene?ca.30²0Ma?was revealed by the bedrock samples from the Qiamntagh,Kaimuqi and Xiangride areas.The detrital apatite fission track dating of the Cenozoic strata in the Huatiaoshan area was indicative of a continuously rapid exhumation of the E-KOB during Miocene to Pliocene.Coupled with the studies of sedimentology and detrital thermochronology of the Kumukol,Hol Xil and Qaidam Basin,the E-KOB was absence of significant positive topography in the early Cenozoic as the result of the prolonged denudation and unroofing at the early stage,and not dramatically uplift as a whole until late Oligocene to middle Miocene,which contributed to the distinct changes in the deposition area,depocenter,paleocurrent,heavy mineral assemblage and composition,and basin evolution of the three Cenozoic basins mentioned above.Besides,a differential uplift and exhumation process in south-north direction was inferred by the apparent difference of the K-feldspar ⁴⁰Ar/³⁹Ar age spectrum of bedrocks from different regions of the E-KOB and their corresponding thermal history,as well as the spatial variation of the thermochronological ages of the bedrock samples from Kaimuqi and Xiangride area.And the heterogeneous uplift and exhumation was obviously related with the thrust fault activities in the E-KOB.From the temporal view of point,the initial of the rapid uplift and exhumation of the E-KOB during Cenozoic is coeval with the regional thrust fault activities?such as Qimantagh,Nothern and Southern East Kunlun and Eastern Qaidam thrust belts?.Therefore,the rapid uplift and exhumation of the E-KOB since late Oligocene to middle Miocene was associated with the large-scale thrusting activities and the consequent crust shortening,corresponding to the continuous convergence between India and Asia.