Historical shear deformation of rock fractures derived from digital outcrop models and its implications on the development of fracture systems

Abstract

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.

Quantitatively deciphering paleostrain from digital outcrops model and its application in the eastern Tian Shan, China

“…The fracture/fault slip data is derived from digital outcrops using method proposed by Wang et al (2019). Basically, this method analyzes the anisotropy of the fracture surface morphology caused by slip displacement on fracture surfaces that are automatically extracted from the digital outcrop, and then estimate the slip direction, as well as the relative amount of slip indicators (fault striations and steps).…”

“…The relative amount of slip indicators may also serve as a good measure of the quality of the fracture/fault slip data, which according to Hippolyte et al (2012), has a primary influence on the quality of the stress inversion results. For a more rigorous and detailed description of this method please see Wang et al (2019), here we mainly discuss its quality assessment. Usually thousands of fracture surfaces, hence fracture/fault slip data, can be extracted from just one single outcrop, and much more detailed information about the strain of the outcrop can be provided by this large amounts of data than traditional paleostress inversion methods whose data is manually collected in the field.…”

“…The detailed information about the strain of the outcrop also shows that some of the assumptions of the traditional paleostress inversion methods are problematic as will be discussed later in the application examples. Wang et al (2019) defined the terms "quasi striations" and "quasi steps" to refer to slip indicators that are causing similar anisotropy of the fracture surface morphology as the traditional fault striations and fault steps do, whether they can be obviously seen on the fracture surface or not. A parameter * / that describes the contribution of fracture surface morphology on the shear strength was proposed by Grasselli et al (2002).…”

“…Other researchers who recently have called for a cautious and critical attitude are complaining about the lack of parameters for assessing the quality of the field data as well as the paleostress inversion results (e.g., Sperner and Zweigel, 2010;Hippolyte et al, 2012;Lacombe, 2012;Simón, 2019). Now there may be a chance to solve the above problems, benefiting from recent development of techniques for automatic extraction of fracture surfaces from digital outcrop models (Wang et al, 2017) and estimation of historical shear deformation on rock fractures (Wang et al, 2019), which provide an efficient way of quantitatively acquiring large amounts of high quality fracture/fault slip data from outcrops. Usually thousands of fracture/fault slip data can be extracted from just one single outcrop.…”

Quantitatively Deciphering Paleostrain From Digital Outcrops Model and its Application in the Eastern Tian Shan, China

“…Other researchers who recently have called for a cautious and critical attitude are complaining about the lack of parameters for assessing the quality of the field data as well as the paleostress inversion results (e.g., Hippolyte et al, 2012;Lacombe, 2012;Simón, 2019;Sperner & Zweigel, 2010). Now, there may be a chance to solve the above problems, benefiting from recent development of techniques for automatic extraction of fracture surfaces from digital outcrop models (Wang et al, 2017) and estimation of historical shear deformation on rock fractures (Wang et al, 2019), which provide an efficient way of quantitatively acquiring large amounts of high-quality fracture/fault slip data from outcrops. Usually, thousands of fracture/fault slip data can be extracted from just one single outcrop.…”