Geological Features In Zanzong Co Area,North Tibet

The study area is located in the Zanzong Co area in the middle segment of the Bangong-Nujiang Suture Zone,stretching across the northern edge of the Bangong-Nujiang Suture Zone and the southern edge of the South-Qiangtang terrane.The study of Mesozoic orogeny in the study area is of great significance to understand the evolution and the formation of the Tethys and the Qinghai-Tibetan Plateau.However,previous studies in this area mainly started from a certain sedimentary system or magmatic event in one direction,and there is also a lack of systematic structure analysis in this area.Therefore,this dissertation chooses the Zanzong Co area as the study area,through the comprehensive analysis of sedimentation-magma-structure,to explore the sedimentary record,magma response and structral deformation as well as their developments and connections in the process of collision and orogeny of the Cretaceous between the Lhasa and Qiangtang terranes in the middle segment of the Bangong-Nujiang Suture Zone.Guided by plate tectonic theory,this dissertation uses sedimentary facies analysis,geochemical analysis and structure analysis theory to combine sedimentary-magma-tectonics closely,and combines sedimentary-magma events by interlayer volcanic rocks,then makes a comprehensive analysis of the material presence of sedimentation-magma and the material transformation of tectonic movements with the complementary analysis system of material basis and tectonic deformation as an outline.Based on the re-determination of the continental red beds by means of sedimentary facies analysis and interlayer volcanic dating,the molasse formation of the Cretaceous Jingzhushan Formation in the study area was newly identified and the latest age of the Lhasa-Qiangtang collision was restricted to the late Early Cretaceous of about 115 Ma.Through the geochemical analysis of the early Cretaceous interlayer volcanic rocks,it is inferred that it is the result of delamination effect under the condition of the thickened lower crust,representing the magma response of the early uplift of the Qinghai-Tibetan Plateau during the middle Cretaceous period,and indicating that the collisional orogenic events of the Lhasa-Qiangtang are of great significance to the early evolution of the plateau.Through detailed field investigation and chronologic and geochemical analysis,a set of Muddijiangya bimodal volcanic rocks about 72 Ma in the late Cretaceous was identified,which was the end of Lasa-Qiangtang collision orogenic process and marked the post-orogenic extension evolution stage.Through the detailed structural analysis,the structural patterns and structural hierarchy of each geological body in the study area are identified,and the four-stage tectonic deformation is identified.Based on the analysis of sediment-magma-structure coupling,we propose that the orogeny in the middle segment of the Bangong-Nujiang Suture Zone has the characteristics of accretion-collision second-order composite orogeny from the aspects of material basis and tectonic reconstruction.