Quantitative Study Of Mesozoic Contractional Tectonic Deformation In The Western Segment Of Daqingshan, Inner Mongolia

The western segement of Daqingshan in Inner Mongolia is located on the northern margin of North China Craton,and lie in the western segement of the Yinshan-Yanshan intraplate orogenic belt.The Mesozoic thrusts in the western segement of the Daqingshan have been studied for nearly a century.But the combination and expansion of the thrust system,especially the shallow crustal shortening caused by thrusts in the western segement of the Daqingshan have not been revealed clearly.This paper takes the thrust system as the research object in the western segement of Daqingshan.Through the large-scale regional geological mapping,detailed structural observations of the outcrops and routes,the drawing and restoration of the balanced cross-sections,based on the structural characteristics,the structural styles and the age of the thrusts and period of tectonic evolution in the western segement of Daqingshan,the crustal shortening caused by the thrust system can be quantitatively analyzed and evaluated.These studies helps us fully understand the characteristics of Mesozoic contractional deformation,the intraplate contractional deformation performance and the remote effect caused by plate convergence in the northern margin of North China Craton.The main results and understanding are as follows:(1)The structural pattern of the thrust system and the geometric and kinematic characteristics of the main thrusts in the western segement in the Daqingshan were determined.There are three types of combined style named imbricate fan,triangle and local duplex.(2)According to the relationship between the thrusts and the age of the thrusts,the expansions of the Mesozoic thrusts in the western segement of Daqingshan is out-of-sequence.(3)There are three stages of contractional deformations during Mesozoic in the western segement of Daqingshan.The first stage occurred in the Triassic,Tianlinbei thrust developed under N-S contraction.The second stage occurred in the late period of Early Jurassic,Mentougou thrust developed under N-S contraction.The third stage occurred in the Middle Jurassic-Lower Cretaceous,Jidengwan thrust,Gongshanwan thrust,Ximawan thrust and Shamujia thrust developed under N-S contraction.(4)According to the styles and expansions of thrusts,through the drawing and restoration of the balance section,the crustal shortening caused by the Mesozoic contractional deformation was quantitatively evaluated.During Mesozoic,the minimum Triassic north-south crustal shortening in the western segement of Daqingshan is 8.1 %,and the minimum Jurassic-Early Cretaceous north-south crustal shortening in the western segement of Daqingshan is 21.2 %.