Fracture patterns in successive folding in the western Sichuan basin, China

Abstract

A detailed field survey of fractures was carried out in the Pingluoba and adjacent anticlines in the western Sichuan basin to discuss fracture development during progressive folding, including the relative timing of fracture formation, the successive deformation in a rock layer, the distribution of fracture sets in folds, and the variation of fractures along with the subsequent deformation of rock layers. Based on a comparison of the diverse fracture patterns in folded strata at different stages of fold growth (horizontal, low-angle dipping, high-angle dipping, and vertical layers), a model of fracture development related to fold evolution was built up. It is suggested that the strata were initially under uniform compression, and two sets of planar conjugate shear fractures were formed, the orientations of which would vary along with the subsequent folding of strata. Then, during progressive folding, the deformation of rheologically hard layers was controlled by tangential longitudinal strain, inducing the development of a set of tensile fractures that struck parallel to the fold axis. Layer parallel shear was predominant in the soft layers, and two sets of bedding-oblique dextral shear fractures intersecting with the bedding at angles of 25–40° and 160° were developed. In the progressive folding process, the shear fractures were developed later than the tensile fractures. A polar stereonet of fractures was generated and applied to improve the efficiency and accuracy of fracture identification in the field, providing precise evidence for the exact analysis of fracture patterns and rock deformation.