Variations In Sedimentology,Structure Of The Eastern And Western Xigaze Forearc Basin And Their Tectonic Implications

Plate tectonics is the key theory to understand the self-organizing(evolution of the ocean-continent)system of the earth and the coupling effect between the crust and mantle.The subduction of the oceanic lithosphere and collision between the continents are the two main principal processes of plate tectonics.The study of northward subduction of the Neo-Tethys and collision between India and Asia since the Mesozoic provides a good example to understand these processes of the plate tectonics.The Xigaze fore-arc basin,located within the India-Asia collision zone in the South Tibet,has preserved key information of the subduction of Neo-Tethys and the collision between India and Asia.The research of structure deformation and burial and exhumation of Xigaze fore-arc basin contributes to reconstruct the process of subduction and collision,and it’s essential for understanding the geological evolution process of the Tibetan Plateau.In this thesis,we focus on the Xigaze fore-arc basin,and conduct a detailed field geological survey,sedimentology,structural geology,isotope chronology,and lowtemperature thermochronology to reveal the process of deposition,structural deformation,burial and exhumation of the basin.Several main conclusions are as follows:(1)The provenance of the basin shows difference between the eastern and western parts of the basin.The probability of older detrital materials in the eastern basin is higher than that of the western basin,indicating variation of erosion in the source area,Lhasa terrane,which may be caused by laterally differential paleo-topography of the Lhasa terrane.Additionally,according to the detrial zircon U-Pb analysis,we found the youngest sedimentary rocks(～49 Ma)within the accretionary mélange at the south margin of the Xigaze basin,which extends the syn-collisional sedimentary records along the Indus-Yarlung suture zone.(2)The structural deformation characteristics and shortening(>～40-60%)between the eastern and western parts of the basin have no large differentiation.The deformation of the basin is dominated by wide folds with local tight folds.Due to activity of the Great Counter Thrust(GCT),it makes strata overturned at the southernmost of the basin.The strata have been shortened about ～60-80% in Lazi area.(3)The exhumation history shows variations along the E-W strike of the basin.The southernmost part of the west basin underwent an early phase of the Eocene rapid exhumation and this is related to early-stage collision between the Indian and Asian plates.It’s much later than the record(～90-80 Ma)at the same position of the eastern basin.There is a prevalent Miocene cooling/exhumation episode through the entire basin and the mélange,influenced by the activity of the north-vergent GCT.The sedimentation of the basin last until in the latest Eocene or earliest Oligocene time.The western part mélange of probably caused by the superposed thrust slices reseting underlied sedimentary sequences during the activity of the GCT.After then,these underlied sequences were exposed to the surface by the Yarlung River incision.(4)In the early stage(～110-80 Ma),the laterally different provenances of the forearc sequences may be influenced by the inherited topographic features of the Lhasa terrane caused by the oblique convergence along the Bangong-Nujiang suture.During the Cenozoic,lateral difference of provenance and exhumation history of the syncollisional sequences may mainly be controlled by the oblique close of the Neo-Tethys and the lateral variations geometry of the under-thrusting Indian slab.