The Tectonic Deformation Characteristics And Provenance Analysis Of Bayan Har Mountains

The Hoh Xil-Bayan Har Triassic sedimentary basin, whih is located in the northeast of Qinghai-Tibet Plateau hinderland, is tectonically bounded by Muztag-South of eastern Kunlun-A’nyemaqen in the north, by Xijir Ulan-Yushu-Jinsha River fault zone in the south, and by Longmenshan nappe structure zone in the east. It’s shape likes a huge inverted delta. In addition to parts of west and east outcropping the Permian System, other parts of the exposed Jurassic-Quatermary, most areas of the Bayan Har tectonic belt were covered by thick Triassic, so to contain a lot of geological information.Thus it became a mysterious region of the country, and has attracted great attention of scholars at home and abroad, known as ―China‖ Bermuda.In this paper, based on the Republic-Yushu section, the auther has carried on the research of tectonic deformation characteristics for the Bayan Har mountains. The study area from north to south is divided into four tectonic deformation zoning, respectively tight fold thrust zone, broad and gentle fold thrust belt and leant plunging fold thrust zone. And on this basis, combining with field phenomenon and indoor analysis, stage-3 major tectonic deformation are identified, namely subduction extrusion deformation, dextral strike-slip shear deformation and brittle faulting structure. ①Subduction extrusion deformation is characterized by closed fold and thrust, strike-slip fault. With the matching the regional subduction and convergence between the Yangtze plate and North China plate including the Qaidam plate in the middle-late Triassic, it established the main NWW-SEE fold-thrust structural framework in the Bayan Har mountains. ②Dextral strike-slip shear deformation is characterized by plunging vertical fold and cleavage function which were overlaid on the above early schistosity plane, probably due to NE oblique subduction of the Bayan Har block and Yangtze Plate. ③ Brittle faulting structure is characterized by developmental brittle normal faults and can match rapid uplift of the Qinghai-Tibet Plateau. Deformation characteristics show that a series of broad and gentle folds were cut off by brittle fracture. On this basis, detailed comparisons have carried out about deposition of the sub-zone, clastic composition, palaeocurrent, detrital zircon age information and so on. According to the results, this three deformation zoning show certain regularity or similarity on the horizon, composition change and zircon age spectrum. Just the structure looks different. Therefore, we believe that they are different tectonic assemblage style under the same tectonism. Regional subduction and convergence between the Yangtze plate and North China plate(including the Qaidam plate) in the middle-late Triassic may be the main tectonism that lay major NWW-SEE fold-thrust tectonic framework in the Bayan Har Mountain Group. And then the sinistral strike-slip deformation and late fault activity just play a fine-tuning role in the tectonic framework.On the basis of comprehensive study on the predecessors’ research for paleocurrent, combined with statistical analysis of sandstone detrital composition, the author find that the Bayan Har Basin has the change rule of ophiolitic melange-turbidite-deep sea basin from edge to center. The basinal terrigenous fragments in the north and middle of the basin were dominantly derived from two important orogens. One is called Kunlun orogen which was the main provenance, the other is called Qinling orogen. Part of provenance is from the Qaidam plate. There exist divergent paleocurrent in the Bayan Har basin which the overall direction is north east to south west, in accordance with the style of remnant-ocean basin paleocurrents. The Bayan Har Basin had the geography pattern of ocean alternating with continent type when the first lithologic member in the Bayan Har Mountain Group deposited. The sea basin reached to the maximum and the sedimentary center had the trend to the north passage in second and third lithologic member. The sea basin was gradually narrowing in fourth lithologic member, showing the fading trend from east to west and with the development of two sedimentary center.