Research On Fault Activity In Three-dimensional Continuous Area Of Jizhong Depression In Cenozoic

Three-dimensional continuous areas lie in the middle of Jizhong Depression,including Langgu sag,Baxian sag and Raoyang sag,with rich oil and gas resources.Faults in research areas are divided into four levels,of which there are 4 first-grade faults and 23 second-grade faults. On the plane,these faults combination styles mainly have parallel styles,en-echelon styles, oblique crossing styles and brush styles; on the profile, the faults combination styles mainly have ladder-like, “Y”-like, graben-like and horst-like.Through the analysis of the structural sections and the stretching calculation,such conclution as follows can be achieved. The north of the study area stretches stronger than the middle part and the south during Es4 period.In Es3 period,each part of the study area acts very strong and reaches the maximum amount of stretch,then reduces in Es2 period.From Es1 period to Ed period,the north of the study area is exposed to denudation,the stretching activity of the middle Baxian sag gets stronger than the north and the south because Niudong fault acts strong.At the end of this period,stretch activity decreases.In the Neogene,tectonic activity is weak and the stretch activity of the Baxian sag is a little stronger than the north and the south.According to the analysis of fault active strength and fault growth index,in Ek-Es4 period,the fault activity intensity has the characteristic of strong-acting north and weak-acting south.In Es3-Es2 period,the numbers of active faults are obviously growing and structure acts intensively,especially in Es3 period.Influenced by Daxing and Niudong fault,Fault activity intensity in Langgu sag and Baxian sag is strong in the west and weak in the east,while strong in the east and weak in the west in Raoyang sag because of Maxi fault.In Es1-Ed period,Langgu sag is highly denuded,Niudong fault acts intensively,active faults slightly increase and activity intensity drops.After Neogene, the overall tectonic activity is weak in the study area,only third-grade and fourth-grade faults develop and structure tends to be stable.Main faults’ activity intensity in the research area is studied by fault throw method. Daxing fault and Niudong fault are characterized with segmentation and activity properties of each segment are different.Maxi fault,Boye fault,Renxi fault and Wangju fault show intensive activity in the Es3 period,Da Wangzhuang fault and Xinzhen fault are active in the Ek-Es4 period,and Hejian fault acts strong in the Es1-Ed period.The intensity of fault activity shows obvious difference in the study area.The intensity and regularity of fault activity are different in different part of one fault.In the same profile,every fault has different formation time and activity intensity. Boundary faults are mainly formed in the Paleogene,controlling the formation and development of the depression.The second-grade and third grade faults are mainly formed in the Es3,which are controlled by main-fault activity intensity.After Neogene, only third-grade and fourth-grade faults develop.