Magnetic Stratigraphy Of The Paleogene Jianchuan Basin On The Southeastern Margin Of The Qinghai-Tibet Plateau

The collision and convergence of the Indian and Eurasian plates triggered the crustal deformation of the Tibetan plateau,which controlled the formation and evolution of the Cenozoic sedimentary basins in and around the plateau.These Cenozoic sedimentary basins are generally situated in an arcuate belt that extends into the central plateau(e.g.Nangqian-Yushu Basin,Gonjo Basin,and Jianchuan Basin).These continental sequences have recorded the important information of large-scale compression and thrust,crustal shortening,and early uplift of the Tibetan plateau.Jianchuan Basin is located in the southeastern margin of the Tibetan plateau.It has been the focus of studying the Cenozoic paleoenvironment,paleoelevation,and tectonic evolution of that part of Tibet.Several studies have been conducted in Jianchuan Basin.However,the stratigraphic age of the Jianchuan Basin is still controversial.Based on early geological survey reports,six main formations are defined in the Jianchuan Basin.From bottom to top,these are the Paleocene Yunlong and Guolang formations,Eocene Baoxiangsi formation,Oligocene Jinsichang formation,Miocene Shuanghe formation,and Pliocene Jianchuan formation.The Upper Shuanghe and Jianchuan formations have been reassigned to Eocene,but robust age constraints on the lower Yunlong and Guolang formations are still lacking.Here,we studied on the Xinsong section(26.45°N,99.78°E)in the Jianchuan Basin for magnetostratigraphy research.A total of 1036 oriented paleomagnetic core samples were collected.In the laboratory,thin section observation,rock magnetism,and stepwise thermal demagnetization were conducted.Combined with published biostratigraphic and isotopic dating results,our results show polarity sequences of the Xinsong section can be well correlated with the Geomagnetic Polarity Time Scale(GPTS)between ?62 and ca.39 Ma.According to the age of the Xinsong section,we calculated the sedimentary accumulation rate.It is suggested that the sedimentary accumulation rate was Ca.15 cm/kyr from 50 to 42 Ma and Ca.24 cm/kyr from 42 to39 Ma.The change in depositional rates is consistent with the change of depositional environments in the Jianchuan Basin.Compared with the sedimentary and tectonic evolution of Nangqian-Yushu Basin,Gonjo Basin and Jianchuan Basin,it indicated that Jianchuan Basin may be a compressional sedimentary basin controlled by boundary thrust faulting.