Structural Deformation And Structural Evolution Characteristics In The Northern Of Bogda Mountains

Bogda Mountain belongs to the eastern branch of the northern Tianshan Mountains and the northern branch of the eastern Tianshan Mountains,located in the southern part of the Central Asian orogenic belt.Although there have been preliminary studies on the tectonic deformation of the Bogda Mountain,these studies are still very limited and there are many controversies,especially for the geometric structure,activity mode,evolution history,etc.,there is still a lack of systematic research.Based on a large number of field geological observations and geophysical data interpretation,this paper analyzes the geometric characteristics of the Bogda Mountain fault system,the kinematic characteristics of the fault development and its temporal and spatial variation laws,to define the formation mode and evolution law of the fault system.The stress field characteristics and dynamic mechanism developed by the fault system during the period,summarize the tectonic evolution history of the Bogda Mountains since the Late Carboniferous.The paper also focuses on the structural differences between the eastern and western parts of Bogda Mountain and the regional dynamics mechanism that caused this difference.In this field work,70 effective stress field data were collected,based on the cutting relationship between different orientation fractures in the field outcrop,the different orientations of the same section and the fault relationship activities of different natures,and the fracture development characteristics of strata in different ages.Combined with the dynamics of regional dynamics,the fractures of different periods are screened,and the data are analyzed and processed to obtain the stress field characteristics of different periods in Bogda Mountain.In this work,five stages of compressive stress field and three stages of strike-slip stress field were determined.The regional tectonic analysis shows that the Bogda fault system mainly experienced 9 stages:the brittle-ductile shear-slip activity in the early(late Carboniferous),the weak extension activity in the Early Permian period,and the extrusion activity in the Late Permian period.Weak stretching in the Early Middle Triassic,squeezing activities in the Late Triassic,weak stretching in the Early Middle Jurassic period,intense squeezing in the Late Jurassic period,Cretaceous-Paleogene weak squeezing and strong extrusion since the New Age.The Bogda Mountain fault system originated from the arc-arc collision collage process between the Halic Island arc and the Great South Lake island arc in the middle of the Late Carboniferous,and then in the collage process of the Kazakhstan plate during the Late Permian period.Suffered from the SEE direction,it was subjected to NNE-direction extrusion due to the collision of the Mongolian-Siberian plate during the Late Jurassic period.During the Cenozoic period,influenced by the remote dynamics of the Indian Ocean plate striking the Eurasian plate,the Bogda Mountain showed near-SN extrusion.The difference in stress transmission caused different tectonic deformation features in the eastern and western parts of Bogda.