Triassic Sedimentary Filling And Tectonic Evolution In Bayan Har Basin

The Triassic sedimentation in the Bayan Har Basin,Tibet Plateau,preserves direct records about the tectonic evolution of the Paleo-Tethyan Ocean.Detailed studies of sedimentary filling,material provenance and structural architecture are of great significance for reconstructing the Triassic evolution of the Paleo-Tethys Ocean in the Tibet Plateau.Based on field investigations,this thesis basically focuses on the Triassic sections in the Hoh Xil,Budongquan-Zhiduo and Maqin-Ganzi areas.Through comprehensively petrological,sedimentological,structural geological,geochemical,detrital zircon U-Pb chronological studies,a series of analyses have been carried out on sedimentary filling,material provenance and structural architecture of the Triassic sediments in the Bayan Har Basin.These research results help to establish a tectonic evolution model and preliminarily draw the following new insights.1.The Triassic sediments in the Bayan Har Basin are composed of a set of thick clastic strata.The sedimentary facies in the Hoh Xil area are characterized by neritic facies,bathyal(abyssal)facies,abyssal facies and neritic(transition)facies.In the Budongquan-Zhiduo area,they mainly consist abyssal facies,neritic facies,bathyal(abyssal)facies,abyssal facies and neritic(transition)facies.Investigated Triassic sandstone samples in this thesis plot into the provenance area of collisional orogenic belt in the Dickinson sandstone clastics triangle diagram and show a continental island arc and Andean continental margin affinity in the geochemical element tectonic environment diagram.These features indicate that the Bayan Har Basin could be located in the subduction-related setting of the Paleo-Tethyan Ocean,which might be caused by the collision orogenesis between the East Kunlun continental margin arc and the North Qiangtang passive continental margin.2.Geochemical analyses suggest that the East Kunlun Orogenic Belt should be the main source area of the Triassic detritus in the Bayan Har Basin.According to paleocurrent and detrital zircon age distribution results,the basin is considered to have predominantly four provenances.In the Muztag-Bukadaban-Budongquan area,the paleocurrent is dominated by SE and SSE orientations.Detrital zircon age distribution shows inconspicuous Precambrian and ca.345-300 Ma peaks,indicating the substances were mainly derived from the East Kunlun Orogenic Belt.In the Maduo-Dari area,the paleocurrent is mainly SE and SW direction.Zircon of detritus display similar patterns to these in the East Kunlun and West Qinling,suggesting the provenance of detritus was mainly sourced from the East Kunlun and West Qinling.In the Zoige-Songpan-Litang area,the paleocurrent is mainly SW-oriented,implying the material in this area might be basically came from the western Yangtze Block.Besides,the paleocurrent in the Zhiduo-Yushu-Ganzi area is mainly NW direction.Lower to Middle Triassic sedimentary rock samples show a dominant detrital zircon peak age of ca.1110-820 Ma,whereas Upper Triassic sandstone samples exhibit an obvious peak of ca.1100-900 Ma as well as an inconspicuous peak of ca.2149-1750 Ma,indicating the detritus were mainly derived from the Qiangtang Terrane and Yidun island arc.3.According to the aeromagnetic characteristics,the Bayan Har Basin,together with adjacent areas,could be divided into five magnetic units,including the Tarim block(southern segment),the Qiangtang Block,the Yangtze Block(western segment),the Qingling-QilianKunlun Orogenic Belt and the Bayan Har Basin.In the east segment of the Bayan Har Basin(eastern margin of the Songpan-Ganzi Terrane),these aeromagnetic data are shown as NEoriented high positive anomaly zones,which are similar in intensity and shape to those in the West Qingling area.In the west-central segment of the Bayan Har Basin,it shows weakly positive anomaly,suggesting the magnetic basement in this region might be dominant by the Proterozoic units.Whereas,in the Hoh Xil-Budongquan-Yushu region,the aeromagnetic features show a block-like weak magnetic field with variations of positive to negative aeromagnetic values,suggesting the magnetic basement beneath might be the Mesoproterozoic to Neoproterozoic metamorphic rocks.4.Based on sedimentary filling and structural features,along with regional tectonic evolution,the Bayan Har Basin could be divided into two stages of basin evolution,including the Late Permian to Early Triassic remnant ocean basin and the Middle Triassic to Late Triassic peripheral foreland basin.Where after,the Triassic System in the Bayan Har Basin have experienced a series of deformation during the subsequent Indosinian,Yanshanian and Himalayan orogeny.Specifically,the structural deformations during the early Indosinian time were basically featured with tight folds,sparse but well-developed ductile shear and intense schistosity zones,while the structural styles during the late Indosinian time were mainly characterized by thrust nappes,highly-developed ductile shear zones and fold structures.In addition,the Yanshanian structural styles were characterized by sparsely-developed faults with open folds,whereas the Himalayan styles were mainly intermittent and differential uplifts.