Stress Field Transformation And Dynamic Mechanism In The Middle Part Of Lhasa Block During Late Cenozoic

Qinghai-Tibet Plateau has been a hotspot of global geological studies,and the Lhasa block is located in the hinterland of it,thus the collision of the Indian Plate and Eurasian Plate has significant response on Lhasa block.The Rift system which is consist of seven north-south rifts and pre-existing east-west strike-slip fault system in the middle of Lhasa block have intense activity since the Late Cenozoic,which is the good entry point study of the dynamic mechanism for the region,and Whereby which can inversion the dynamic verification mechanism of the research area and the growth mode conversion of the Tibetan Plateau.Based on comprehensive field geological survey,we recourse to regional fracture and focus on measurement and statistics the geometry and kinetic data of structural phenomena in the intersection of the slip fault zone and the rift system in southern Tibet.And combining the method of ESR dating and U-series dating,we have researched the tectonic stress transformation process in the middle of Lhasa Block since late Cenozoic.There are three results as follow:(1)By the study of wings occurrence measurement statistics in Dang Xiong –Yang Bajing basin and using the stereographic projection analysis show that the direction of the two tectonic stress that formed the folds is both nearly NS in the middle of Lhasa Block during late Cenozoic.And direction of the late stage stress is clockwise around 9 ° to the direction of early stage.(2)Through the study of triangle elevation in western margin of Ya dong –Gu Lu Rift and dislocation aqueous offset measurement statistics of strike-slip fault system between Namco and Shigatse are not change on linear correlation uniform,which shows that these active faults were gradually to move and the tectonic stress formed these faults had developed by stage.(3)By the study of fault scratches and conjugate joints measurement statistics and then making the stress circle and rose graphic,we find that the direction of maximum principal stress are EW and NS.And combined the method of U-series dating and ESR dating,we get the result that the direction of tectonic stress have changed from East-west stretch to North-south in the middle of Lhasa Block since the late Miocene.There is a close relationship between with the tectonic geomorphology evolution of Lhasa Block and the tectonic stress field evolution convert,the tectonic stress of Lhasa Block gradually changed from NS compressive stress to EW tensile stress since late Cenozoic,and combined with the three construction phase in the district which are north-south compression followed by stretching from east to west,and then mainly for large-scale expansion development of southern Tibet rift system since the late Pleistocene.The power source of the three phases described above are all come from the process of Indian Plate crash dive into Eurasian Plate,which let tectonic stress of Lhasa Block that within Eurasian Plate decomposition and gradually deflected in the direction of East-west for the confining of the Pacific Plate from the East.