Upper Palaeozoic-mesozoic Structural Characteristics And Tectonic Evolution Of The West Part Of South Qiangtang Basin

The Bangong-Nujiang structral zone and the Longmu co–shuanghu structural zone and SouthQiangtang Basin which is between them are extremely important area in Qinghai-Tibet Plateau.They retain important information about the ancient, middle and Neo-Tethys evolution. Throughthe field mapping of regional geological survey at1:50000and tectonic analysis at1:10000atDuoma, Chaizhuri around Ritu and Rena co area of Gaize, and the work of indoor data analysis,microscopic observation,the structural stages of Paleozoic and Mesozoic strata in these threeregions were divided according to the degree of metamorphism and deformation. Compared withthe same strata in the area of Mayigangri, the tectonic deformation stages in Duoma were dividedinto three stages. The first stage(D1) is characterized by intensively transposition of structures andflexible deformation. Projection statistical results of S1cleavage indicate the presence of North toSouth direction of compressive stress in the study area. The metamorphism degree of secondstage(D2) of is lower and the deformation is weaker. S2cleavage projection statistical results showthe North to South direction compression effect. The third tectonic movement is brittle deformation,making early stratification and cleavage wrinkles and experienced progressive deformation.The Mesozoic strata in the area of Rena Co, Gaize at least experienced contractive tectonicmovement two times before Cenozoic tectonic movement (D3). Influenced by coaxial progressivedeformation, the spatial distributions of the strata were controlled by superimposed folds thatresulted in the occurrence of the strata repeatedly in multiple directions. The stress direction of thefirst stage was proved to be north-south by the work of tectonic analysis in the areas of NiangrongCo and Lagana. The tectonic deformation of the second stage (D2) is ductile-brittle and the resultsof occurrence of S2cleavage and lineation showed in D2stage this study area experienced nearlynorth-south compression. The third stage (D3) was mainly in the form of disturbance of Cenozoictectonic movement.Shamuluo Formation around Chaizhuri of Ritu which located in the south of Bangonghu ophiolitemelange experienced two north-south contractive motions (D1and D2) and followed with a thrustprocess from south to north (D3). The original bedding and foliation S1were strongly deformed bycutting and replacement by the second stage cleavage (S2).S2cleavages are penetrative, and thestereographic projection of S2indicated the presence of the compression in NNE-SSW direction inthis study area.The result of Compared the feathers of tectonic deformation with the area of Mayigangri showedthat the four regions had similar deformation characteristics in the formation of same age. TheCarboniferous strata in north part of south Qiangtang all experienced one stage of flexibledeformation (D1) in early Permian. In D2stage of movement, same type rock with same age indifferent area had similar metamorphism and deformation.D2shows the characteristics of ductile tofragile.In D3stage of movement, four area all bore D3movement in late Jurassic. D3similar to D2,the same type rock with same age but in different area have the same metamorphism anddeformation.In the late Phase of Yanshan, Carboniferous strata and Mesozoic intrusive rocks in the area ofMayigangri were presented in the way of large boudins or structural lens with east-west orientationdistribution by the north-south extrusion force. Different scales of folds could be found in thestrata in South Qiangtang Basin, Most of these axial planes of folds were nearly east-west strike,making the Triassic and Jurassic strata occur repeatedly. Bangong-Nujiang suture zone and shiquanhe–Laguo co ophiolite belt had characteristics of accretionary complex. Based onaccretionary complex patterns and regional survey data get the result of the Neo-Tethys subductedto north,and Oceanic trench backwards to south, the South Qiangtang accrete to south.