Low-temperature Thermochronological Constraints On The Exhumation History Of The North Qiangtang Basin And The Central Uplift Since The Late Triassic

The uplifting mechanism and process of the Tibetan Plateau(TP)have always been the focus of geoscience research.The model of TP uplift varies from whole scale uplift to progressive growth,through the studies of sedimentology and stratigraphy,structural geology,magmatism,and paleoaltimetry.The most studies indicate that the growth of the TP is mainly caused by the shortening and thickening of the crust after the IndiaAsia collision.However,it is still controversial whether the plateau has a certain embryonic form prior to the India-Asia collision.In particular,whether the collision between Lhasa terrane(LT)and Qiangtang terrane(QT)has an impact on the growth of the TP.There is no doubt that the LT and QT recorded a lot of important information in the process of collision,which became an important breach to solve this problem.Previous studies focused on the LT,while the QT is a bit less.In recent years,the lowtemperature thermochronology has become a very mature means to restore the history of orogenic belt exhumation.This thesis aims at the QT and attempts to explore its early exhumation by means of low-temperature thermochronology(we will verify the exhumation process of the source area by the provenance analysis of the sink area),and then to limit the early growth of TP.The QT central uplift and Puruogangri in the North QT have higher altitude,which certainly record more comprehensive exhumation information.In this thesis,zircon and apatite(U-Th)/ he analyses of the granites collected from the two places were carried out.The modeling results identify three important exhumation events.(1)Exhumation from the Middle-Late Jurassic to the Early Cretaceous in the central uplift.The exhumation rate is 200-250 m /Ma.This exhumation event occurred under the subduction of the Bangong-Nujiang ocean ridge.It is the earliest exhumation event recognized by the TP.(2)Exhumation of the central uplift from the Late Cretaceous to the Early Paleogene,which is caused by the collision between QT and LT.The exhumation rate is 40-70 m / Ma.Long-term slow and stable denudation rate is the main feature of modern plateau growth.The extensive continental deposits in the late Cretaceous indicate that the QT has reached above sea level.Combined with the shortening of the crust and the discovery of ultrapotash volcanic rocks,it shows that the QT had a certain height prior to the India-Asia collision.It forms the prototype of the plateau,representing the initial growth stage of the TP.(3)The Cenozoic exhumation events in the North QT.It mainly occurred in 40-30 Ma,and the exhumation rate reached 2-2.5 km /Ma under the action of Tanggula thrust fault at 40 Ma,which transported a large number of material sources for the Hoh Xil basin,representing the stage of northward expansion of the TP.