Study On The Characteristics Of In-Situ Stress Field In The Southern Tibetan Plateau And Its Arc Structural Belt In Southeastern Margin

Tibetan Plateau is the main battlefield for the development of western China.As the region with the most abundant hydropower resources and the weakest transportation infrastructure,a large number of engineering construction are facing complex geological environment problems such as high ground stress and high earthquake intensity.Therefore,it is of great scientific significance to study the characteristics of tectonic stress field in Tibetan Plateau.In this paper,the focal mechanism solution inversion method and the statistical analysis method of existing in-situ stress test data are used to systematically analyze the regional tectonic stress field of the south and southeastern margin of the Tibetan Plateau,and the initial in-situ stress field of the project area of Nepal's Upper Arun Hydropower Station on the southeastern margin of the plateau is analyzed by three-dimensional finite element regression analysis method.The main research results are as follows:(1)The P and T axis analysis of the focal mechanism solution and the inversion analysis of the tectonic stress field show that the tectonic stress field has significant regional characteristics,and the tectonic principal compression stress orientation is radiative from the south of Tibetan Plateau to the east.Specifically,the tectonic principal compression stress orientation is from the near NS direction of the orogenic belt area to the NE direction of Tibet area,and continuously to the SE direction in Sichuan-Yunnan area.The stress structure types in different regions are quite different,and closely related to the action forms of active structural fault zones and secondary blocks.(2)Based on the statistical analysis of the measured stress data in the study area,the distribution characteristics of dominant orientation of the maximum horizontal principal stress,and the variation characteristics of horizontal principal stress value and lateral pressure coefficient with depth were obtained.The results show that the orientation and magnitude of the stress field of shallow rock masses in the crust have regional differences.The surface value of the maximum horizontal principal stress in Tibet region is smaller than that in Sichuan-Yunnan region,while the gradient value is larger than that in Sichuan-Yunnan region,and the convergence value of the lateral pressure coefficient with depth is also greater than that in Sichuan-Yunnan region.Moreover,the maximum horizontal principal stress in Tibet region is always dominant.In summary,the stress fields in the southern and southeastern margin of the plateau have the characteristics of "shallow weak and deep strong" and "shallow strong and deep weak" respectively.(3)The results of in-situ stress test and 3D in-situ stress field inversion in Nepal's Upper Arun engineering area show that the maximum horizontal principal stress isdominant and in NE direction,which is different from the regional tectonic principal compression stress near NS direction,because the shallow surface stress field in the engineering area is mainly controlled by local geological structure and topography.