Apatite Fission Track Thermochronological Study In Cenozoic Basins In The Eastern Tibetan Plateau

The ultimate formation of the Tibetan Plateau is attributed to the collisional collage of Paleozoic-Mesozoic multiple terranes,island arcs and microcontinental blocks and the critical Cenozoic collisional compression of the Indian and Eurasian plates.Along with a range of tectonic deformation,magmatic activities and elevation uplift,the development of the Tibetan Plateau shaped a unique the Third Pole landscape and profoundly influenced the climate,environmental changes and biodiversity evolution in Eurasia area.Therefore,the formation and evolution of the Tibetan Plateau,including tectonic mechanisms,topographic evolution,and climatic effects,has been the focus and frontier of geoscientific research.A large number of studies have generated substantial geological evidence,although the core problems of the magnitude,timing,process,and mechanism of uplift and expansion of the Tibetan Plateau have not been widely acknowledged,in which the timing of the formation and evolution of the eastern Tibetan Plateau are crucial to understanding the eventual formation of the entire plateau.The eastern Tibetan Plateau,a transitional belt from east-west to north-south tectonics accompanying massive strike-slip and thrust faulting systems,potassic magmatic rocks and successive sedimentary basins(Shanglaxiu,Xialaxiu,Nangqian,Gonjo and Markam basins),is a key region for studying the tectonic deformation of the plateau as well as the processes and mechanisms of plateau expansion to the east and southeast.The serious of basins controlling by major strike-slip and thrust faulting systems maintain thick layers of Cenozoic sediments and record information on regional tectonic deformation.Because of relatively lower closure-temperature(60～120℃),apatite fission track low-temperature thermochronology is one of the important methods to study tectonic deformation with the capability preserving exhumation information on shallow crust.Therefore,this paper presents a systematic thermochronology of apatite fission tracks in the sediments and volcanic rocks of the Cenozoic basins(Shanglaxiu,Xialaxiu,Nangqian,Leiwuqi and Markam basins)in the eastern Tibetan Plateau,combined with sedimentary evolution analysis and previous research results,to reveal significant Cretaceous-Paleogene exhumation and tectonic deformation events of the eastern Tibetan Plateau,and to provide more lowtemperature thermochronological evidence for tectonic deformation and mechanisms studies.The following innovative understand and conclusions were obtained:(1)The range of central ages of twenty-nine detrital apatite fission track samples are 38.6～144.0 Ma(mainly between 47.6 and 102.2 Ma),and the decomposed ages are concentrated in 38.6～117.0 Ma,which can be divided into three dominant decomposed age: static peak P3(117.0～85.6 Ma),moving peak P2(63.7～44.3 Ma),and static peak P1(38.4～37.7 Ma).The static peak age of 117.0～85.6 Ma reflects the collision between the Qiangtang and Lhasa terrane and the subsequent rapid exhumation of the eastern Qiangtang terrane.The moving peak age of 63.7～43.3 Ma indicates that the collision between the Indian and Eurasian blocks triggered rapid uplift and exhumation the of eastern Qiangtang terrane,with an exhumation rate of 260～570 m/Ma;during which the eastern Tibetan Plateau underwent rotation,deformation and uplift of various micro-terranes causing the initial shaping of the tectonic configuration of the region.The static peak age P1(38.4～37.7 Ma)and the complete annealed sandstone fission track age together reveal a strong compressional and tectonic exhumation event in the eastern Qiangtang terrane during 38.4～30.8 Ma in response to the intra-continental subduction of the Asian plate.The apatite fission track low-temperature thermochronological results combined with the sedimentary evolution analysis of the serious basins manifest that the eastern Tibetan Plateau has experienced at least three major tectonic deformation and exhumation events,the late early Cretaceous-late Cretaceous,the Paleocene-Early Eocene,the late Eocene-early Oligocene,respectively.(2)The central ages of four apatite fission track samples of igneous rock are in the range of 33.7～38.2 Ma,indicating eruption timing of potassic-ultrapotassic volcanic rocks in the Markam area of the eastern Qiangtang terrane,combined with stratigraphic contact relationships and previous studies together suggest a late Eocene-early Oligocene age for the stratigraphic deposits of the Markam Basin.