Detrital Apatite Fission Track Thermochronology Research Of Paleogene In Nangqian Basin,eastern Tibetan Plateau

The collision between the India plate and Eurasian plate caused the uplift of the Tibetan Plateau to be one of the most important geological events in the earth history nearly 100 Ma,which resulted a series of magmatic activities,folds,thrust-slip faults and formation of unique geological and forms on Tibetan Plateau.Not only the uplift of the Tibetan Plateau has changed the tectonic and geomorphic characteristic and its surrounding areas,but has profound in the climate and ecological environment of Tibet-plateau and that is a hot spot in the earth science community.One of the most scientific problems of the time,process and mechanism of the uplift of the Tibetan Plateau,but the understanding of these key problems has not been unified.If these problems are resolved,it is necessary to obtain more evidences of the spatial-temporal evolution of tectonic uplift in the key areas of the Tibetan Plateau through various scientific research methods.Nangqian basin in the eastern Tibetan Plateau is one of the typical basins,which is located at the transition of the east-west and south-north tectonic belts.The thick Paleogene strata preserved in the basin not only record the sedimentary evolution information of the basin,but also record the tectonic uplift and exhumation process of the surrounding orogenic belts.Extensive field geological survey and measured section,this paper reveals the major tectonic deformation events and basin evolution process in Nangqian Basin,mainly through the Eocene fission track thermochronology of detrital apatite in Nangqian Basin,combined with the sedimentary tectonic evolution of Nangqian Basin and previous research results.It provides low temperature thermochronology evidence for the study of early Cenozoic tectonic deformation in eastern Tibetan Plateau.This paper mainly obtains the following understandings and conclusions:(1)There are “static peak” of 117-110 Ma and “moving peak” of 63.7-38.4 Ma in the fitting age of clastic apatite fission track of paleogene in Nangqian basin.The “static peak” of 117-110 Ma indicates that the eastern Qiangtang terrane experienced rapid structural exhumation at the end of early Cretaceous,which may reflect the deformation exhumation signal caused by the collision between Lhasa and Qiangtang terrane.The “moving peak” of 63.7-38.4 Ma indicates the rapid stripping signal of the quasi-synchronous tectonic response caused by the collision of the Indian and Eurasian plates in the Early Cenozoic,and the rate is about 0.3-0.6 mm/yr.(2)The fission track age of unannealed detrital apatite controls the sedimentary age of nangqian Basin.Combined with the sedimentation rate,the sedimentary age of Gongo Formation is about 60 Ma at the bottom and 29 Ma at the top.(3)The Nangqian Basin experienced four stages of sedimentary-tectonic evolution in the early Cenozoic: 60-52 Ma,The initial collision between The Indian and Eurasian plates led to the transformation of the Nangqian area from an early denudation area to a small catchment basin,and the basin formed and accepted the conglomerate clastic deposits of the diluvial fan in the lower member of Gongo Formation.Around 52 Ma,Large-scale strike-slip faulting turned the basin into a pull-apart basin,forming a wide lacustrine environment of fine detrital rock intercalated with limestone in the middle section of Gongo Formation from 52 Ma to 38 Ma.38-35 Ma,Regional tectonic activity transformed the area from strike-slip pull-apart to strike-slip compression,and the basin uplifted and the water became shallow,forming mudstone intercalated gypsum deposits in salt-lake environment.Since 35 Ma,the basin has been gradually uplifted and deposited large alluvial fan conglomerate and sandstone assemblages in the upper member of Gongo Formation,and finally entered the stage of post-basin evolution.