The initiation and development of fractures in rocks is the key part of many problems from academic to industrial, such as faulting, folding, rock mass engineering, reservoir characterization, etc. Conventional ways of evaluating the fracture historical deformations depend on the geologists' visual interpretation of indicating structures such as fault striations, fault steps, plumose structures, etc. on the fracture surface produced by previous deformations, and hence suffer from problems like subjectivity and the absence of obvious indicating structures. In this study, we propose a quantitative method to derive historical shear deformations of rock fractures from digital outcrop models (DOMs) based on the analysis of effects of fault striations and fault steps on the shear strength parameter of the fracture surface. A theoretical model that combines effects of fault striations, fault steps and isotropic base shear strength is fitted to the shear strength parameter. The amount of fault striations and fault steps and their occurrences are estimated, and the historical shear deformations can be inferred. The validity and the effectiveness of the proposed method was proved by testing it on a constructed fracture surface with idealized striations and a fracture surface with clear fault steps. The application of this method on an example outcrop shows an intuitive idea of how the rock mass was deformed and that the distribution, occurrence and mode of new fractures are strictly controlled by preexisting fractures, and hence emphasizes the importance of preexisting fractures in modeling the development of fracture systems.

No abstract

The knowledge of the strain/stress field evolution in time is fundamental to the understanding of the earth dynamic system. Based on the principle that past tectonic stress should have left traces in the rocks, geologists have been trying to determine the paleostress history from evidence found in rocks for decades. Recent development of techniques for automatic extraction of fracture surfaces from digital outcrop models and estimation of historical shear deformation on rock fractures provide an efficient way of quantitatively acquiring large amounts of high quality fracture/fault slip data from outcrops. Unlike traditional paleostress inversion methods whose data is manually collected in the field, the new techniques provide much more detailed information about the strain of the outcrop and a good opportunity to develop quantitative methods for deciphering more realistic paleostrains. In this study, instead of fitting the slip data from several fractures to calculate the overall strain tensor, the local strain tensor is calculated for slip on each fracture from the outcrop. Then the local strain tensors are grouped into populations corresponding to different strain events using a clustering analysis technique. Theoretical advantages of this new method over the traditional ones are discussed. The applications on outcrops in the eastern Tian Shan area give a clear picture of the late Cenozoic paleostrain variation over space and time, and also throw light on the cause for the change in the strain regime in time, the fracture development patterns and the distribution of shear displacements in fracture networks.

The knowledge of the strain/stress field evolution in time is fundamental to the understanding of the earth dynamic system. Based on the principle that past tectonic stress should have left traces in the rocks, geologists have been trying to determine the paleostress history from evidence found in rocks for decades. Recent development of techniques for automatic extraction of fracture surfaces from digital outcrop models and estimation of historical shear deformation on rock fractures provide an efficient way of quantitatively acquiring large amounts of high quality fracture/fault slip data from outcrops. Unlike traditional paleostress inversion methods whose data is manually collected in the field, the new techniques provide much more detailed information about the strain of the outcrop and a good opportunity to develop quantitative methods for deciphering more realistic paleostrains. In this study, instead of fitting the slip data from several fractures to calculate the overall strain tensor, the local strain tensor is calculated for slip on each fracture from the outcrop. Then the local strain tensors are grouped into populations corresponding to different strain events using a clustering analysis technique. Theoretical advantages of this new method over the traditional ones are discussed. The applications on outcrops in the eastern Tian Shan area give a clear picture of the late Cenozoic paleostrain variation over space and time, and also throw light on the cause for the change in the strain regime in time, the fracture development ★ Code available from: https://github.com/EricAlex/structrock.