The Structural Deformation And Cause Analysis Of The Curved Thrust Nappe Structural Belt In The Western Qinling Mountains

The West Qinling orogenic belt is located in the northeast of the Tibet Plateau.It is the key position of structural transition between the E-W striking Central Orogenic System and the N-S striking Sichuan and Yunnan-Helan Tectonic Belt and it is an important area for the study of the tectonic and intracontinental tectonic deformation of the mainland of China.The main tectonic framework of the West Qinling Orogenic Belt is a series of giant thrust nappe structures with prominent southward curve protruding.The multi-stage tectonic deformation in the orogenic belt makes it an ideal area for our study of the deformation mechanism of the West Qinling Mountains curved structural belt.In this paper,the author has consulted a large number of domestic and foreign literatures on the research progress of curved structural belts.Based on the analysis and arrangement of the latest research results of curved structural belts and the comprehensive collection of the field structural elements and deformation characteristics of the West Qinling Mountains curved structural belt,the author has made clear the tectonic deformation period of the curved belt by analyzing the geometry and kinematics of the Western Qinling Mountains curved structural belt.On the basis of comprehensive collection of fault data,the author retrieved the paleo stress field of each tectonic period in the West Qinling Mountains curved structural belt by using fault slip vector analyzing.Combining the above study and combining with the regional geological background,the author discusses the formation mechanism and tectonic evolution of the West Qinling Mountains curved structural belt.Based on the field analysis of the structural deformation of the arc structural belt,combined with the regional geological background,the author believes that the Western Qinling Mountains curved thrust nappe structural belt has undergone three stages of tectonic evolution: In the first stage,occured the thrust nappe and folds were occured under the action of the paleostress field in the near NE-SW direction;In the second stage,the sinistral shear action was occured in the region under the continuous action of the paleostress field in the near NE-SW direction.A series of sinistral slip faults were developed in the Western Qinling Mountains,and early folds were transformed,and West Qinling Mountains was thrust to the Songpan-Ganzi terrane.In this stage,the West Qinling orogenic belt was curved under the blocking of the Bikou terrane and the influencing of the Mianxian-Lueyang sinistral slip fault.The stress field in the local area changed from NE-SW direction to SEE-NWW direction.In the third phase,the ancient stress field in the NNE-SSW direction superimposed and reformed the West Qinling Mountains curved structural belt.It caused a late strike slip faults reconstruced early structures and the development of wide folds and brittle faults since the Jurassic strata.