Structural Characteristics And Dynamic Evolution Of The Western Segment Of The Qiangtang Accretionary Complex In Gangma Co Area,qiangtang Basin,northern Tibet

The study area is located in the west of South Qiangtang terrane between the Longmu Co-Shuanghu suture zone and the Bangong-Nujiang suture zone,which is the key area for the study of Tethys ocean evolution.This area is also the Duobuzha polymetallic ore concentration area,which is included in the national integrated exploration area.The spatial and temporal structure and evolution of the study area are of great geological significance for the geotectonic division,polymetallic exploration and Tethys study in China.Previous studies on this area mainly focused on the exhumation mechanism of Accretive complex,late Paleozoic paleontology,sedimentary paleogeography and magmatic evolution,while the study on Late Triassic accretionary complex and the structural deformation sequence and evolution model of late Paleozoic shallow metamorphic strata not involved in deep subduction were relatively weak.Therefore,this paper focus on the western part of South Qiangtang terrane as the research area to unveil its tectonic and dynamic evolution.Guided by the theory of plate tectonics and accretion orogeny,using the theory and method of structural analysis,and relies on such projects as "Qiangtang-East Tibet terrane framework and collision orogeny in the central part of the Qinghai-Tibet Plateau" and "Rongma 1:50,000 regional geological survey in Tibet",this paper uses main field research methods such as sediment-magma-structure-chronology large-scale profile and mapping in key sections,combines comprehensive test means to study the tectonic deformation and dynamic evolution in the western part of South Qiangtang terrane.The study of the structure has shown that the Gangma Co mélange exhibits four phases of deformation.The D1 ductile top-to-the-north structures forms in prograde setting with the peak metamorphic condition of 1.95Gpa/510?.The D2 structures are characterized by top-to-the-south reversed shear and related folds,formed in a retrograde setting.The D3 flattening structures are related to the top-to-the-south detachment.The D4 structures are top-to-the-south imbricate faults.The 40Ar-39 Ar analysis of muscovite indicates that the high-pressure rocks in the mélange were exhumed at 212.9±1.1 Ma and 210.7±1.4 Ma.The structures of the Carboniferous strata in Rutog area show the characters of fold and thrust belt.The 40Ar-39 Ar analysis of sericite may indicate that the fold and thrust belt were formed before 189.9±3.2 Ma.Here,we propose that the HP-bearing mélange was formed by southern subductionin a divergent double subduction setting.During the subduction reversal,deeply subducted HP-bearing mélange was exhumed by the pull up and progressive pull away of the southern slab to the north.The detachment fault and a stable two-layer structure consisting of the upper passive margin strata and lower mélange thus formed.In the final collision setting of the north Qiangtang and south Qiangtang,a previous detachment was reactivated and turned into a thrust fault to expose the mélange in the hinterland,and the foreland area formed a fold and thrust belt.