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The Tectonic Characteristics And Tectonic Evolution Of The Bangong- Nujiang Suture Zone In Gaize Area

Posted on:2016-02-16Degree:MasterType:Thesis
Country:ChinaCandidate:C QinFull Text:PDF
GTID:2180330461455578Subject:Structural geology
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Bangong- Nujiang suture zone is a vital internal geological boundary of Qinghai-Tibet plateau, separating the Qiangtang block and Lhasa block and retaining important information related to the evolution of Tethys. Currently fierce debates exist in key tectonic issues about Bangong- Nujiang oceanic basin, such as the basin nature, the evolution model, subduction polarity and closing mechanisms,etc, which directly affect the understanding of the tectonic evolution along Bangong- Nujiang suture zone. By geological mapping(scale: 1: 50,000, 1: 10,000) and detailed analysis of outcrop structure, we divide the structural deformation to several stages. Based on the existing geological survey and related research and the principles of petrology & geochemistry, isotopic chronology as the main line, methods of geology, geochemistry, geochronology and other disciplines combined as research methods, we identified the formation age and the tectonic environment of the volcanic and igneous rocks in Gerze area, the central-western part of Bangong-Nujiang suture zone, then determined the existing limitation and evaluation characteristics in different stages of the ocean basin. Preliminary studies indicate that:(1) Suffering the long-term and complex evolution of opening and closing of Bangong-Nujiang Ocean, the superposition of volcanic rift basin in post-collision period and strong tectonic transformation due to India- Asia continental collision, the study area possess a structure pattern with blocks separated, belts in north and south, segmentations in east and west and layered structure vertically. The study area is on the south side of the Bangong- Double Lake- Nujiang- Changning docking belt(I level tectonic unit) of the late Paleozoic continental Qiangtang- Sanjiang orogenic system in the Pan-China plate. II level tectonic units is divided by the Dagenong-Zhachaya – Nibu fault, the northern part belongs to the South Qiangtang hyperplasia arc basin(II), while southern part belongs to the Bangong- Nujiang oceanic junction zone.(2) Study area experienced 3 major tectonic deformation stages: the deformation of 1st phase(D1) relates to the contemporaneous orogenic extrusion events as Bangonghu- Nujiang suture zone, mainly developing E-W trend structures belts under compression force from S-N direction(including NW-SE), and marked by the development of penetrative S1 foliation. The deformation of 2nd phase relates to the N-S trend sinistrogyration compressional event in late orogenic period near squeeze NS compressional event related to the left, marked by the S1 sinistral shear foliation in ophiolite, asymmetric vertical folds in Shamuluo group(J3K1s) and inclined folds, forming a regional axial plane cleavage S2. The deformation of 3rd phase(D3) relates to the N-S directional dextrorotational compression event in the post-orogenic period, marked by S2 dextral shear zone foliation in ophiolite, right rotary shear foliation in Sewa formation, local right-lateral asymmetry plunging vertical fold under the background of right-handed strike-slip fault in Riganpeicuo group, superimposed broad folds in E-W direction and nearly S-N direction and deflection of the axial direction to south, forming axial plane cleavage S2 in Sewa group.(3) According to the characteristics of survey area in stratigraphy, petrology, geochemistry, geochronology and the analysis of tectonic record, make clear that the region has experienced four evolutionary stages: ① Bangong- Nujiang ocean expansion phase(P-T1), according to the research results the class I high pressure eclogite, the cleavage time of Bangong- Nujiang oceanic basin at least advance to the Late Permian- Triassic; ② Bangong- Nujiang ocean subduction and high pressure metamorphic rocks exhumation stage(T2-J2), by studying results of class II high pressure eclogite, Bangong- Nujiang oceanic basin has begun to dive at least in the early Jurassic, lower limit of the dive time is the early Jurassic; ③ Bangong-Nujiang oceanic closure and collision orogenic stage(J3-K1); ④Post-collision stage(K1-present), through analysis of two volcanic rocks isotope U-Pb ages, the weighted average ages are 106.7 ± 0.7 Ma and 106.0 ± 0.8 Ma, during the late period of Early Cretaceous, the tectonic setting is rift inside of continent, represented the volcanic rocks formed in the stage of continental extension after the collision of the ocean basin.
Keywords/Search Tags:Bangong-Nujiang suture, gaize area, Structural deformation, Fold, Tectonic evolution
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