Structural Characteristics And Tectonic Evolution Of The Central Part Of Longmu Co-shuanghu Suture

The Longmu Co-Shuanghu suture, which is located in the Central Qiangtang, isan extremely important Indosinian tectonic zone in the Tibetan Plateau that separatesthe South Qiangtang accretionary complex belt and the North Qiangtang terrane. Thissuture retains important information regarding the evolution of the Palaeo-TethysOcean and its deformations after Jurassic reflect a portion of the closure of the Meso-and Neo-Tethys ocean.1:50000and1:10000geological mappings indicate that theaccretionary complex belt on th south of the suture formed during the northwestwardsubduction of the Paleo-Tethys Ocean in Triassic and tectonically linked with Nierong,Jiayuqiao and Jitang metamorphic complexes in east Tibet and Changning-Mengliancomplex in west Yunnan. Generally, the above metamorphic complexes constitute alarge-scale Indosinian accretionary orogen in Tibetan Plateau. Based on structuralanalyses on both macroscopic and microscopic scales, it’s concluded that theaccretionary complex mainly experienced three stages of deformation and two stagesof regional metamorphism. The earliest deformation(D1) bearing dextral shearexhibites intense rheology and shaped the general structural style, formed the regionalE-W-and N-S-trending tectonic belts and developed a penetrative foliation S1. The D1deformation is resulted from the subduction of the Paleo-Tethys Ocean dated at244-202Ma. Around200Ma，The Paleo-Tethys Ocean closed and the subduction andaccretion event finished. Under the convergence background, the accretionary complexexperienced the second contractional deformation(D2) and formed flexural folds, smallthrust faults and an axial crenulation foliation S2overpinting S1, reflecting that theaccretionary complex experienced a subsequent compressional deformation whenexhumed onto an shallower tectonic level. The last deformation(D3) recorded by nearlyN-S-trending buckle folds and nonpenetrative axial cleavage S3is deduced to beassociated with an contraction deformation of Qiangtang basin in Pliocene.After the Jurassic, the South Qiangtang accretionary complex belt and the NorthQiangtang terrane merged into one unified terrane which exhibits intraplate deformation. The North Qiangtang terrane experienced Late Yanshanian, and Cenozoicdeformations on shallow-superficial level resulting from the closure of the Meso-andNeo-Tethys Oceans. The structural mapping of the Permian-Triassic structural layerindicates that the Mesozoic structural style was dominated by Late Yanshanianhorizontal contractional deformation characterised by NW-SE trending thrust faultsand flexural-slip folds. These folds and thrusts were superimposed on Indosinianbuckle folds. Structural analysis on deformations of the Neogene structural layerindicates that there were two transitions in the tectonic evolution of North Qiangtangterrane dividing the Cenozoic deformation into three stages. In episode I (50-40Ma to8-4Ma), the crust experienced large-scale N-S horizontal shortening and verticalthickening showing by buckle folds and thrust faults. In episode II (8-4Ma to86.9Ka), E-W passive extension formed conjugate strike-slip fault systems and pull-apartbasins. During this episode, the Qiangtang basin was extruded eastwards, and a numberof superficial N-S trending buckle folds were formed. In episode III (86.9Ka to thepresent), the stress field was reversed, and the Qiangtang basin experienced E-W activeextension, forming conjugate normal faults.