Study On Structural Deformation Of Mianlue Tectonic Zone

Mianlue Tectonic Zone is located in the Southern Qinling Orogenic Belt.It is a plate suture zone formed by the northward subduction of Mianlue ocean in Indosinian and the collision between the two continental blocks in North and South China.Mianlue Tectonic Zone is a composite tectonic belt formed by superimposing Mes-Cenozoic intracontinental deformation on the basis of Indosinian plate suture zone.Mianlue Tectonic Zone has formed different types of rock assemblages,different scales and different types of tectonic traces in its long evolution process.Based on these information,the process of collision and amalgamation between the North China Plate and the Yangtze Plate and the characteristics of intracontinental deformation can be well recovered,and then the formation and evolution of the Qinling Orogenic Belt can be discussed.In this paper,through detailed structural analysis and paleostress inversion,it is revealed that Mianlue Tectonic Zone experienced three stages of structural deformation:(1)the D₁ stage was NW-SE compression,characterized by vertical-horizontal folds,near-horizontal wrinkle lineation,and high angle reverse faults,which were formed in the collision between North China and Yangtze in Early-Middle Triassic.(2)The D₂ stage is NE-SW compression,mainly developed left strike-slip shear deformation,superimposed on the early deformation structure track.The nearly horizontal stretching lineation of east-west direction is generally developed in the structural belt,and locally developed vertical folds,formed in the Late Triassic to Middle Jurassic,the Qinling Orogenic Belt transformed into intracontinental deformation,large-scale left strike-slip occurred,The blocks on the east and west sides of Ningshan fault zone are trending East-West respectively Sliding on both sides.(3)The D₃ stage is N-S compression.Under the multi-directional convergence structure system from late Jurassic to Cretaceous,Mianlue Tectonic Zone was compressed in the north-south direction,forming a series of conjugate shear faults,cutting the early structural deformation.Dabashan Mountain and Western Qinling on the north side were thrust to the south,The northern margin of Yangtze wedge into Qinling Orogenic Belt along Micangshan area,forming a southward protruding Dabashan arc thrust nappe tectonic zone,Wudu-Zhouqu arc structural belt in the Western Qinling and a series of NE and SW trending conjugate shear faults.Based on the results of the third phase tectonic deformation obtained in this study and the previous research,the structural evolution of the Mianlue Tectonic Zone is discussed.In the Early to Middle Triassic,the Mianlue Ocean closed and the North China Plate collided with the Yangtze Plate;in the Late Triassic,the collision ended and a large-scale left lateral strike slip occurred in the region,accompanied by a large number of magmatic rocks.Since the late Jurassic,the north and south blocks were strongly compacted with each other,which resulted in the strong contraction and narrowing of the middle part of the Qinling Orogenic Belt.