Stuctural Evolution And Uplift History Of Dazhuoma Region In Eastern Qiangtang Basin

Qiangtang Basin is located in the central part of the Tibet Plateau.It is important to clarify its uplift history for understanding the formation and evolution of the entire Tibet Plateau.Basing on the study of previous uplift history of Qiangtang Basin,two distinct uplift stages which areLate Mesozoic and Cenozoic,have been identified.However,there is still much controversy about the uplift process,the main uplift period and when the Qiangtang Basin rose to its present height since the late Mesozoic..In order to analyze these key scientific problems,this paper chooses Dazhuma region as the research object.On the basis of detailed geological survey in the field,firstly,the geometric and kinematic characteristics of tectonic deformation in this area are restored through the balanced crosssectiom restoration technique.At the same time,sandstone samples of Jurassic and Triassic strata are taken.Subsequently,using apatite fission track and(U-Th)/He low-temperature thermochronology method to abtain the age and related data,we can conduct the inversion of thermal history to reflect the uplift history of the region.Furthermore,we can restrict the uplift and tectonic deformation history of the Qiangtang Basin.Regional geological survey results show that Late Triassic,Middle-Late Jurassic and Late Cretaceous strata are largely exposed.A large number of Middle Jurassic and Late Triassic strata were pushed on to the Late Cretaceous Abushan Formation strata.The klippe structure can be seen in the front of the thrust belt.In addition,it is widely seen that the Late Cretaceous Abushan Formation strata are in angular unconformity with Jurassic and Triassic strata.Field investigation of surface structures and balanced profiles show that the surface structures in this area are a series of thrust structures from north to south,and a small number of recoil faults have been developed.Three stages of faults have been determined in this area.According to the field geological characteristics,there are three thrust belts from north to south: Dazhuoma thrust belt,Qixiangcuo thrust belt and Zagazangbu thrust belt.There are obvious folds and faults in the thrust belt and among the thrust belts.The long-distance nappe caused by low-angle thrust faults is an important effect of the formation shortening in this area.At the same time,basing on the balanced crosssectiom restoration technique,the deformation reduction rate before Cenozoic is 7% and that since Cenozoic is 48%.The results of low temperature thermochronology experiments and simulation analysis show that there are two distinct stages of tectonic uplift and denudation in this area: the first stage is 120-80 Ma,which may be related to the closure of the Middle Tethyan Ocean and the collision of the Lhasa block with the Qiangtang block,and is characterized by the firststage of faults and folds;the second stage is 55-38 Ma,which may be related to the closure of the New Tethyan Ocean caused by the convergence of Indian plate and Eurasia plate,and is characterized by the long-distance thrust nappe structure and large crustal shortening in the study area.