Composition, Structural Characteristics And Evolution Of Back-Longmenshan Orogen (North Section) In The Northwest Margin Of Yangtze Block

Longmenshan intra-continent compound orogenic belt, an inland orogenic belt with NE orientation, is located at the northwestern margin of Yangtze block and is surrounded by Bikou block, southern Qinling orogenic belt, Songpan-Ganzi orogenic belt, western Sichuan foreland basin and Hannan-Micangshan tectonic belt and constitutes the eastern margin zone of Qinghai-Tibetan plateau, which is convergent and combining zone of different blocks and kinds of orogenic belt through various mechanisms and directions and exhibits complex tectonic images and matching history. Comprehensive studies have given in this thesis on material composition, formation age, formation environment, deformation characteristics and deformation sequence of back-Longmenshan orogenic belt in the northwest margin of Yangtze block by the compound study methods of structural geology, petrology, rock geochemistry, isotopic geochronology and geophysics and a few major new developments and preliminary cognitions are acquired as follows:1.Material composition studies of back-Longmenshan orogenic beltThe basement formation in early-middle stage of Neoproterozoic is characterized by volcanic erupting and subsequent magma intruding, and in which island-arc volcanic rock of Tongmuliang Group represents the production of Rodinia super-continent's convergence while volcanic rock of Liujiaping Group, Jiaoziding granite and Datan granite imply the primary divergence of Rodinia super-continent. The evolution process, the stage from extension and division-collapse during Nanhua period-early Paleozoic to evolution of passive continental margin during late Paleozoic-middle Triassic, is mainly of oceanic faces for sediment, which can be divided into following sediment systems:division-stable stage of Nanhua-Sinian periods, extension and division-collapse stage of Cambrian-Silurian periods, passive continental margin of Devonian period-middle Triassic epoch and a few of volcanic rocks of Maotazi intra-continent rift of early Ordovician epoch are included. The intra-continent orogenic evolution of late Triassic epoch-Cenozoic casts post-collision granites in its north of Bikou block while intra-continent sediments in its south fore-Longmenshan orogenic belt and south margin of Bikou block. Therefore, back-Longmenshan orogenic belt undertakes the process of extensional detachment, thrust nappe and strike-slip shear afterwards on the basis of intra-continent subduction previously.2.Tectonic characteristics studies of back-Longmenshan orogenic beltBack-Longmenshan orogenic belt has undertaken poly-cycle and multi-type tectonic deformation and tectonic superposition. After detailed structure analysis, it is considered that the deformations can be divided into 5 systems as follows:intra-continent subduction deformation under the mechanism of compression-contraction in Late Triassic epoch, detachment deformation and ductile shear deformation along the strata corresponding extensional effect in Early Jurassic epoch, thrust nappe deformation resulting from compression-contraction in Early-Middle Jurassic epoch, sinistral strike-slip ductile shear deformation from Late Jurassic to Late-Middle Cenozoic and dextral strike-slip shear deformation in Middle-Late Cenozoic, in which deformations extensional detachment is obvious in Jiaoziding Dome and Wugongkou of southern margin of Bikou block in Back-Longmenshan orogenic belt and is weaker to the east. Chronology studies on the deformed rocks of extension detachment orogen suggests that the extension detachment is active in about 193-165Ma, which is formed under the extensional background of Crustal balancing effect after intra-continent subduction orogenesis.3.Forming and evolution studies of Back-Longmenshan orogenic beltBack-Longmenshan orogenic belt has mainly undertaken 3 evolutional stages since Neoproterozoic, such as the basement formation of Early-Middle Neoproterozoic, from extension and division-collapse of Nanhua epoch-Early Paleozoic to passive continental margin evolution in Late Paleozoic-Middle Triassic epoch and intra-continent orogenic evolution in Late Triassic epoch-Cenozoic. The third stage can be divided into 5 secondary stage furtherly like intra-continent subduction orogenesis in Late Triassic epoch, extensional detachment in Early-Middle Jurassic epoch, thrust nappe in Late Jurassic epoch-Early Cretaceous epoch, sinistral strike-slip in Late Cretaceous epoch-Early-Middle Cenozoic and dextral strike-slip in Middle-late Cenozoic. From which it can be summarized that Longmenshan orogenic belt is an intra-continent compound orogen which has close relationship to the tectonic evolution of terranes adjacent to it both in south and north and mainly results from the interaction of Qinling orogenic belt, Yangtze block and Songpan-Ganzi orogenic belt and has complex tectonic movement.