Structural Characteristics Andtectonic Evolution Of The Central Segment Of The Qiangtang Accretionary Complex In Yadanarea,South Qiangtang,Northern Tibet

The research area is located in the central segment of western Qiangtang and the tectonic location is between Longmucuo-Shuanghu Suture Zone and Bangonghu-Nujiang Suture Zone.The accretionary complex formed by the Indosinian accretionary orogensis is widely distributed in the research area.Previous studies on the material composition,formation and evolution of accretionary complexes have been carried out in detail.However,precisely research on structural deformation of accretionary complexes and the exhumation process of accretionary complexes are relatively insufficient.Therefore,this paper chooses the typical Yadan area where the accretionary complex develops to carry out precisely material composition,structural deformation research.Its purpose is to find out the material composition,structural deformation pattern and deformation sequence of the accretionary complex,and then to explore the formation and evolution of Paleo-Tethyan Ocean.Guided by the theory of plate tectonics and accretionary orogeny,this paper applies the theory and method of structural analysis to research the tectonic deformation of Yadan area accretionary complex and its tectonic significance by material composition of accretionary complex-magmatic activity-structual deformation-Geochronology and Geochemistry.Through large scale mapping and structural profile,The accretionary complex mainly experienced three stages of tectonic deformation events.The first phase of deformation?D₁?occurred in the Late Triassic,related to the subduction of Paleo-Tethys Ocean and developed a penetrative foliation S₁which completelyreplacefoliation S₀.The mass and the shallow metamorphic matrix showdextral ductility shear and shape the structural traces of the E-W direction throughout the region.The second phase?D₂?is related to the collision between the north and south Qiangtang.Under the horizontal compression of the N-S direction,the buckle fold are formed that causes the foliation replacement of the Middle Permian Longge Formation limestone which is not involved in the subduction.Meanwhile the accretionary complex form axial plane cleavage.The third?D₃?tectonic deformation event is affected by Cenozoic India-Eurasia collision.D₃develop at least three stages of tectonic deformation events.Research area was affected by the second stage E-W extrusion,forming east-west horizontal normal folds or recumbent folds.Some areas are superimposed to form large-scale dome structures and small-scale crenulation lineation.Typical detachment fault contact between the accretionary complex and the Middle Permian Longge Formation limestone,which is not involved to the subduction.Phengite ⁴⁰Ar-³⁹Ardating on four samples yield plateau ages of roughly 224Ma,whichindicate the exhumation.The age of detrital zircon?750⁸50Ma?shows that provenances is from south Qiangtang which should be the northern edge of Gondwana.The two-phase growth zone of garnet indicates that there is stagnation or even resubduction during exhumation.The blocks in the accretionary complex contain the Mesozoic amphibolite.And their geochemical data indicate that protolithis are OIB-Type basalt with strongly contamination from the crust.Amphibolite is produced by plate rollback during subduction of Bangonghu-Nujiang Ocean and exhumate to the surface under the tectonic setting of partial extension and regional compression of the South Qiangtang.Finally,Amphibolite are involved in the accretionary complex and undergo later deformation together.