Study On The Characteristics Of Basement Faults And Their Control On Sedimentary Cover Structures In Eastern Sichuan

Eastern Sichuan refers to the area between the Huayingshan fault and the Qiyueshan fault in the east of the Sichuan Basin.The basement was formed during the Pre-Sinian and is a geological body with significant heterogeneity.Sinian-Quaternary strata are developed in the sedimentary cover,with strong deformation.In the plan view,it is an arc protruding to the northwest,and it is characterized by the development of ejective folds in the profile.The basement faults refer to the deep-cutting faults developed in the basement,which can cut into the sedimentary cover upward and control the structural evolution of the cover.Studying the characteristics of basement faults and their control on the sedimentary cover structures can deepen the understanding of the structural evolution in eastern Sichuan and provide some guidance for the natural gas exploration in eastern Sichuan.Therefore,this study uses the latest geological data,geophysical data and previous research results to analyze the basement structure,basement faults distribution,evolution of basement fault activity and their control on the structural evolution of sedimentary cover in the eastern Sichuan structural belt.The following major achievements and understandings have been achieved:1.About the basement structure,it is recognized that the basement in eastern Sichuan can be divided into three secondary structural units from SW to NE.They differ in density and magnetism,and are separated by two NW-SE-trending basement faults(Shuangmiao-Luotian fault in the north and Linshui-Fuling fault in the south)from SW to NE.The middle unit is characterized by high density,high magnetism and magmatic rock,with relatively strong structure.Its two sides are low density and low magnetism units with relatively weak structure.2.About the basement fault distribution,nine basement faults are determined and divided into three orders.Among them,are two first-order,four second-order and three third-order basement faults.According to the trend,they can be divided into three groups: SW-NE-trending,NW-SE-trending and S-N-trending,and they divide the basement into rhombic or polygonal fault blocks.3.About the evolution of basement fault activity:(1)The multiphase activities of basement faults in geologic history are found,and active basement faults in each phase include NE-trending and NW-trending faults.The basement faults activities were the weakest during the Early-Middle Ordovician and Early-Middle Permian,and they were stronger in other phases.(2)It is found that most of the basement faults undergone a reversal process.For example,Changshou-Nanchuan fault and Fuling-Yunyang fault were normal faults during the Sinian-Middle Triassic and reversed during the Late Triassic;The Qiyueshan fault was a reverse fault during the Late Ordovician-Carboniferous and Late TriassicQuaternary,and a normal fault in other phases.(3)It is found that except the Changshou-Nanchuan fault and the Fuling-Yunyang fault,other basement faults were characterized by segmental activity.For example,during the Sinian,the northern segement of Huayingshan fault was active and the southern segement was inactive;During the Early-Middle Ordovician,the southern segment of Huayingshan fault was active and the northern segment was inactive.4.About the basement faults control on the sedimentary cover structures:(1)It is found that the development,migration and extinction of the XuanhanKaijiang paleo-uplift,Shizhu paleo-uplift,Kaijiang-Luzhou paleo-uplift depressions distributed around the paleo-uplifts were jointly controlled by multiple active basement faults.For example,during the early Sinian,the Fuling-Yunyang fault and the northern segement of Huayingshan fault had opposite dips and were normal faults.Therefore their common footwall uplifted as a horst and formed the Xuanhan-Kaijiang paleouplift.(2)It is proposed that the sedimentary cover in the eastern Sichuan structural belt can be divided into three secondary structural units from SW to NE,corresponding to three fault blocks in the basement,and their corresponding boundaries are two NW-SEtrending basement faults(Shuangmiao-Luotian fault in the north and Linshui-Fuling fault in the south).When affected by the compression from the southeast side of the basin,the relatively strong basement fault block in the middle undergo structural displacement and moved to the northwest.The relatively weak basement fault blocks on both sides undergo deformation,releasing part of the stress,resulting in relatively weak structural displacement and small northwest displacement.At the same time,the Shuangmiao-Luotian fault and Linshui-Fuling fault fault were strike-slip faults,so the current arc-shaped structural characteristics were formed.