Strain Analysis Based On Fracture Fabrics And Its Application In Folds In Clastic Rock Zone

Focusing on the topic of quantitative analysis of strata strain in brittle-ductile deformation, the fracture patterns of a NE striking fold belt in the western Sichuan foreland basin, China, were analyzed based on detailed field fracture measurement according to the method of fracture fabrics analysis. Based on the precise understanding of the patterns and mechanism interpretations of fractures in anticlines, we took the phenomenon of the episodic development of fractures as evidence and realized the quantitative calculation of strata strain in brittle-ductile folding deformation.Compared to operating methods in previous studies, the field measuring and analyzing method based on fracture fabrics in this study have many advantages. The field fracture survey supporting system is an integrated operation including measuring complex fractures in folded strata, creating a polar stereonet of fracture fabrics, analyzing the deformation mode of the fractures, and fracture divided period coordination. Based on the corresponding relationship between fractures in the outcrops and their poles in the fracture fabrics, the identification of fractures was more convenient and accurate and field observation was more targeted. Therefore with the system the efficiency and accuracy of field work could be improved. The statistics analysis of fracture fabrics is more accurate, comprehensive and intuitive, which effectively improves the precision of strain analysis and provide basis for the quantitative analysis of strata strain in brittle-ductile deformation.With this method the survey of fracture patterns in different stages of fold evolution were accomplished. It was found that the plane conjugate shear fractures, tensile fractures parallel to fold axis, and stratabound shear fractures caused by layer parallel shear were mainly developed, and a model of the relationship between fracture development and successive folding deformation was established. According to the comparison of fracture patterns at different sampling sites, the position in folds (deformation modes), dip angle of strata (deformation degree), thickness of strata and lithology were considered as the principal controlling factors on fracture patterns and densities in the study area.Based on comparison of fracture fabrics in the successive folding process, it was discovered that a second failure often occurred in the continuous increase of strain after the rock was firstly cracked, that was, the episodic development of fractures. Based on the orientation variation of the fractures and the dip angle of strata when failure occurred, the longitudinal strain between the two episodes was equally about30%in both pure shear and simple shear deformation. According to related discussion, it was inferred that a large ductile deformation happened before the rocks were cracked under burial conditions. After the development of stratabound shear fractures, the layer parallel shear of the folded strata was through the sliding of blocks along fractures and associated shear deformation near sliding faces, rather than uniform simple shear. The episodic development of fractures proposed a new model of fracture development and stress-strain relationship after rock failure, which could be used to trace complex brittle-ductile deformation process in the field. Moreover, the decomposition of fracture fabrics provided a new method for the relative timing of fracture formation in areas where no clear abutting relationships could be observed. According to analysis of fracture fabrics and re-examination from fracture features, it was proved fracture sets of two periods were developed in the study area, and the direction of stress field of each period was discussed. Based on fracture evidence and finite element simulation it was discovered that pre-existing fault and anticline would induce a rotation to the direction of a stress which was oblique to them. It was also found under the effect of pre-existing fractures, the stress direction in blocks between fractures would change diversely, and the non-systematic fractures were developed.