Permian And Jurassic Structural Deformation Features Of Western South Qiangtang Terrane,Tibet And Its Significance

The evolution of the Bangong Co-Nujiang Ocean has attracted much more attention all the time. There are many different opinions about its subduction polarity, and most of the predecessors discussed it from petrology, geochemistry and other aspects, while lack of the researches of tectonic deformation. South Qiangtang basin is the pivotal area to study the early structural evolution of the Qinghai-tibet plateau, which retains the important information about the evolution of paleo-tethy,meso-tethy, and nco-tethy ocean. In this paper, guided by the structural analysis theory, the works such as field mapping at a large scale and tectonic measured section are done to study the structural style and deformation phases of the Sewa formation in the research area. Comprehensive studies have shown that the Jurassic Sewa formation of the two areas both develops tight three-level complex fold and superimposed gentle-open fold in late period. The earlier fold axis and slaty cleavage are nearly east-west trending, which is product of S-N trending compression. In Renacuo region, there is angular unconformity between the early Cretaceous volcanic rock of Meiriqiecuo formation and Jurassic Sewa formation which has deformationed, and tight fold tectonic style of Sewa formation didn’t form in volcanic rock of Meiriqiecuo formation, which can infer tight fold formed before the early Cretaceous. Combined with tectonic background in the study area, we can infer that early tight three-level complex fold of Jurassic Sewa formation is the product of northward subduction of Bangong Co-Nujiang Ocean in the study area, and the superimposed gentle-open fold at late stage is related to the collision between Southern Qiangtang block and Lhasa block.Furthermore, this paper counts the structural information in detail about the Rituzhapu area in south Qiangtang by measuring and drawing the lithology-structure section, including the occurrence of bedding, cleavage, small axial surface of fold and hinges, etc, and further studies the Late Palaeozoic structural deformation sequence in the research area. The integrated study indicates that the Late Palaeozoic structure in the research area develops three stages structural deformation: D1 phase is the shear deformation under the toughness condition, forming the SN trending penetrative cleavage S1, which is inferred to be related with the subduction of the paleo-tethy ocean along the trench, and is the result of the shear deformation of NE-SE direction; D2 stage deformation is the gentle-open upright plunging third level complex fold with nearly EW trending fold axis on the basis that S1/S0 is the deformation plane, which is inferred to be connected with the extinction of paleo-tethy ocean; D3 stage deformation forms the gentle-open fold, stacking obliquely with a large angle on D2 stage fold and forming the dome-basin superposed fold type, which is inferred to be associated with the escape structure of Qiangtang block of the Qinghai-tibet plateau. This paper offers the structural evidence on the evolution of the South Qiangtang in the Qinghai-tibet plateau.