Research On Tectonic Deformation In Bitu Area Of Eastern Tibet

Sanjiang Tethys orogenic belt is located in the eastern part of the ancient Tethys tectonic domain,namely the intersection of Gondwana continent and Laurasia continent.It belongs to a tectonic domain with the most complex crustal structure and the most types of orogenic belts in the world.As the most typical developing region of the global Tethys structure in mainland China,the belt has undergone a complex and complete evolutionary history.The Bitu area is located in the southeastern section of the Bangong Lake-Nujiang suture zone in the Sanjiang Tethys orogenic belt,the Dingqing-Bitu-Nujiang suture zone,which is clamped in the Gondwana-Gongdish-Chaotun block and Panhuaxia Between the Changdu and Simao blocks in the mainland,they experienced intense stretching,extrusion,collision,settlement and uplift,and the geological structure is extremely complicated.In this paper,through the analysis of the characteristics of the small structures developed in the Bitu area,combined with the mechanics and dating data,the prehistoric geological studies of the study area will be used to stage the tectonic evolution of the Bitu area.The study area is mainly developed in the Carboniferous,Permian,Triassic and Quaternary strata.The faults mainly developed NE-SW to Naozhong fault,The material of the fracture zone is composed of a lens belt(sandstone,quartz vein,granite),a shard zone,a granule zone and a fault mud zone;NW-SE to Polang fault,The fracture zone is mainly composed of breccia belt,broken grain belt and limestone treatment belt;NW-SE to Yuqu River fault,NW-SE to Baita fault,and secondary faults in multiple directions.In addition,large nappes are found in the middle of the study area.In general,the tectonic evolution of the bitu region can be divided into the following five stages: In the first stage,the compression movement after the Late Jurassic,the direction of the principal compressive stress is E-W direction,which is mainly represented by the mylonite belt developed by the toughness deformation of the granite,and goes to the NE-SE,NW-SE and N-S groups;In the second stage,the extrusion movement after the formation of the Late Jurassic mylonite belt,the direction of the principal compressive stress is near N-S direction,which is mainly represented by the near E-W intensive cleavage zone that cuts through the mylonite belt;The third stage: presumed to be the compression movement of the Yanshanian period,the direction of the principal compressive stress is near E-W direction,which is mainly represented by the folds and cleavage folds in the surrounding area of the nappe structure,and the fold hub is oriented near the N-S direction;The fourth stage: the squeezing movement of the Pleistocene,the principal stress direction is the near E-W direction,which is mainly represented by the reverse fault formed by the thrust movement in the study area;In the fifth stage,the regional tension movement after the formation of the Pleistocene thrust fault is mainly manifested by the NW-SE and NE-SW normal faults developed in the study area,as well as the graben,horst and other normal fault combinations.