Magnetic Stratigraphy And Tectonic Evolution Of Cenozoic Seru Formation In Leiwuqi Area,eastern Tibet

The collision and subduction of the Indian plate to the Eurasian plate in the Early Cenozoic led to the formation of a large scale Qinghai-Tibet Plateau.The collision caused a large scale tectonic shortening of the Qinghai-Tibet Plateau from south to north,resulting in the thickening of the crust and the formation of a large scale strike-slip fault zone in the southeast of the Plateau and the crustal material in the middle and east escaping southeast along the strike-slip fault.Eastern Qinghai-Tibet plateau is the transition area of the Qinghai-Tibet plateau by the east-west tectonic belt into north and south to tectonic belt,along the Qiangtang-Hengduan mountains-Jinsha river-Red river large arc structural belt of thrust nappe tectonics,strike-slip fault zone and the concomitant Cenozoic continental basin and large-scale Cenozoic potassic magmatic belt,the area is the study of the Indian plate and Eurasian plate collision,Eastward and southeastward expansion mechanisms of the plateau and key areas of climate evolution.The Leiwuqi area in eastern Tibet is located at the turning point of the Qiangtang-Hengduan mountains-Jinshajiang-Red river large arc tectonic belt in the eastern Tibetan Plateau.The Cenozoic strata are distributed in the area,which records the information of Cenozoic basin and climate evolution as well as regional tectonic activities.However,due to the factors of high altitude,bad environment and inconvenient transportation,the geological research of Leiwuqi area is relatively low at present.In this paper,based on the high-density and high-precision magnetostratigraphic studies of the Cenozoic Seru Formation system in the Leiwuqi area,combined with the control of the detrital zircon U-Pb age and the macro-age control of the plant and sporopollen assemblages,a preliminarily established Cenozoic age of the area is established.Seru Formation sedimentary and magnetic stratigraphic chronology,through the analysis of stratigraphic contact relationship,sedimentary characteristics,sedimentary evolution,as well as changes in magnetic declination and deposition rate in different periods,combined with previous research results,the major structures in the Seru Formation sedimentary period were discussed.This event provides key stratigraphic age data for further in-depth research on Cenozoic sedimentary-tectonic-climate in this area.Through the research,this paper mainly achieves the following results and understandings:(1)The Cenozoic magnetic stratigraphic sequence of Seru Formation in Leiwuqi area,Eastern Tibet has been established(38.3-<15.9 Ma).The stratigraphic age of the lower member of Seru Formation is 38.3-26.4 Ma,and the main magnetic bearing mineral is hematite.The age of the upper member is 26.4-<15.9 Ma,and magnetite is the main magnetic-carrying mineral.(2)The Cenozoic Seru Formation in Leiwuqi area experienced three stages of sedimentary evolution: In the early stage(38.3-37.0 Ma),the purplish red coarse clastic rocks accumulated rapidly in the near source diluvial fan,in the middle stage(37.0-26.4 Ma),in the river/flood plain environment,and in the late stage(26.4-15.9Ma),in the lake-swamp environment,the coal-bearing fine clastic rocks deposited rapidly.(3)During 30.6-26.0 Ma,a 16.1° clockwise rotation occurred around the eastern tectonic node of the Qinghai-Tibet Plateau in the Leiwuqi area,representing a major tectonic event(tectonic uplift and strike slip)in the eastern Qinghai-Tibet Plateau.