Fracture Patterns During The Cenozoic Fault-propagation Folding Process:Insights From The Clastic Rock Of Eastern Kuqa Depression

Based on the field geological investigation,this paper has improved and expanded the method of extracting structural fracture information in foreland thrust belt,and achieved the goal of obtaining fracture surface from the outcrop of 3D laser scanning data and calculating the attitude,and identifying the target scale of fractures and calculate the surface density.Combined with the research methods of geology,geometry,statistical analysis and finite element simulation,the development law,formation mechanism,distribution characteristics and corresponding control factors of the fractures in the folding process of kuqa foreland thrust belt are systematically studied.The folding development and regularity of kuqa foreland thrust belt are studied.We have proposed that the large number of fault propagation folds of Eastern Kuqa can be divided into two different stages according to the forming process.The East Qiulitag box-like anticline and its associated fractures is the early stage of fault propagation folds,whereas the Tugeerming and Aidong anticline near the Tianshan mountain are the late stage of fault propagation folds,and some small back thrust fault and fold will appear at the two wings.The characteristics of structural fractures and the mechanism of the different fracture sets,are analyzed to explore the fracture development pattern in the fault propagation fold early stage.By comparing the fracture characteristics and corresponding stress background at different positions of fold,it is believed that there are mainly three types of stress zones,extension zones,transition zones and shortening zones in the entire box-like anticline.In addition,the fracture pattern in the folding process of Eastern Qiulitag fold are built.In the early stage of folding,the horizontal stratum develops small-scale thrust faults what are consistent with the axial plane due to the compression of the stress field at that time.As the compression process continues and some gap begins to appear between strata,a set of fracture which is parallel to the extrusion stress and perpendicular to the folding axial plane are developed.In the middle stage of the folding process,the tensile stress appears in the outer arc due to he curvature increase,so a set of tensile fracture which are parallel to the axial plane forms by the tangential longitudinal strain.The fault activity is strong at this time,so a large number of fault-related fractures are developed near the fault.At the same time,loads of shear fractures and tensile fractures are developed due to interlayer slip.In the later stage of folding,the stress field of the whole fold changes,and there will be some special types of fractures in the high and steep strata.The fracture characteristics in the late stage of fault propagation fold are analyzed,including the back-thrust positions in the front and rear wings,and the vertical strata in the front wing,and the model of fracture development is explored.We have found that the tugeerming anticline which are formed by the back-thrust fault near the rear wing of the fold,fractures at the hanging wall are more developed than that at the footwall under the strong effect of faults.For aidong anticline which are formed under back-thrust fault at the front wing of the fold,the shear fracture perpendicular to the axial plane and the tensile shear fracture parallel to the axial plane are mostly developed,and some tilted fractures are also common.In addition,we also have found a set of horizontal fractures in the front wing of the fault-propagated folds in the kuqa foreland thrust belt,which intersect with the stratigraphic surface at a high angle.They can be the R' fracture due to the simple interlayer slip,and the outer arc tension of the folds can also lead to the formation.