Joints at high angles to normal fault strike: an explanation using 3-D numerical models of fault-perturbed stress fields☆
Introduction
Much of the understanding of joint orientations around normal faults follows from Anderson (1951), who indicated that opening mode cracks (specifically, dikes) should form parallel to the strikes of normal faults (perpendicular to the least compressive remote stress) (Fig. 1a). Although this assumption provides a rationale for predicting fault orientations in extending crust, it does not incorporate the stress perturbation induced by growing faults and the consequences of this for joint growth close to faults (Fig. 1b). The fact that faults perturb the surrounding stress field during slip events (Pollard and Segall, 1987; Barton and Zoback, 1994) has important consequences for the orientations of secondary structures such as smaller faults, joints, veins, and solution surfaces.
Geological observations have established that secondary structures can be induced in regions of increased stress around both opening mode (mode I) cracks, such as dikes (Delaney and Pollard, 1981; Delaney et al., 1986; Pollard and Segall, 1987; Rogers and Bird, 1987; Baer and Reches, 1991; Kattenhorn and Watkeys, 1995) and sliding mode (modes II and III) cracks, such as faults (Lajtai, 1969; Segall and Pollard, 1980, 1983; Rispoli, 1981; Cruikshank et al., 1991; Rawnsley et al., 1992; Homberg et al., 1997; Ohlmacher and Aydin, 1997; Willemse et al., 1997; Martel and Boger, 1998; Vermilye and Scholz, 1998). Laboratory experiments that record acoustic emission events in a sample undergoing shear failure also reveal microcrack development ahead of a propagating fault tip (Reches and Lockner, 1994).
If sufficient field evidence exists to suggest a genetic relationship between normal faults and spatially associated joints, joint orientations can perhaps be explained in the context of the fault-perturbed stress field. Joints act as paleostress indicators in rock and are assumed in this study to be mode I fractures that form perpendicular to the least compressive stress. Previous work on stress field heterogeneities around faults follows predominantly from field observations or modeling of either strike-slip or thrust faults (Barton and Zoback, 1994; Bürgmann et al., 1994; Roering et al., 1997). Our investigation focuses on the mechanics of stress perturbations by normal faulting and resultant joint development.
This study is motivated by observations of joints striking at high angles to normal faults in Arches National Park, Utah, in contrast to the predictions of Anderson (1951). First, the field example will be described to establish the connection between normal faults and joints. Second, we present three-dimensional mechanical analyses of the perturbed stress field in the vicinity of mechanically interacting, discontinuous normal faults. We then compare the numerical model results with field examples to demonstrate a similarity between model predictions and what is observed in nature, so as to provide a mechanical rationale for the field observations. Finally, we address implications of our model results for the analysis of deformation around normal faults in general and provide examples of applications.
Section snippets
Faults and joints in sandstone
Exceptional surface exposure in many regions in the Colorado Plateau physiographic province of the USA, such as Arches National Park in southeastern Utah (Fig. 2), provides ideal locations for the study of joints and faults in sandstone. Prominent exposure of Jurassic sandstones in this region has inspired a number of investigations of fault and joint characteristics (locations shown in Fig. 2). Dyer (1983, 1988) investigated systematic joint domains on both flanks of the Salt Valley anticline.
Numerical modeling
Field observations at Rocky Mesa are consistent with joint development being associated with faulting. We use numerical models to examine the stress field around idealized normal faults in order to investigate how joints might form in a fault-perturbed stress field. The 3-D numerical method addresses the effects of fault tipline shape and mechanical interaction of fault segments upon the numerical solution to the perturbed stress field. Using the underlying assumption that joints are opening
Discussion
Detailed field observations indicate that joints striking at high angles to normal faults at Rocky Mesa are related to the faulting. The state of stress during joint growth included fault-perpendicular tension induced by bending, lithostatic stresses, perturbation of the stress field by normal fault slip, regional tectonic stresses during and subsequent to the Laramide orogeny, and possibly internal fluid pressures in joints. Any hypothesis for the history of joint growth should reconcile this
Conclusions
Observations of normal fault and joint relationships in Arches National Park imply a genetic relationship between faulting and resultant joint development at high angles to fault strike. A numerical analysis of stress fields around normal faults at depth indicates that slip events perturb the stress field in the near-tip region (<1% of the fault length) in a complex and highly variable manner. At the upper tips of faults, high stress magnitudes result in joints that form with identical strikes
Supplementary data
Acknowledgements
Funding was provided by the U.S. Department of Energy, Grant No. DEFG03-94ER14462, and the Rock Fracture Project of Stanford University. We benefited from insightful discussions with Juliet Crider, Michele Cooke, Manuel Willemse, and Laurent Maerten. Sincere thanks to Thomas Roznovsky for providing assistance with field mapping. We acknowledge the National Park Service for their support of scientific research in the parks, particularly Karen McKinlay-Jones at Arches National Park. Finally, we
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2022, Journal of Structural GeologyCitation Excerpt :As fractures form in these damage zones, lithology, bed thickness, mechanical stratigraphy, pre-existing fractures, and the presence or absence of inter-layer slip impact their development (e.g., Price, 1959; Couples and Lewis, 1998; Cooke and Underwood, 2001; Nelson, 2001; Bergbauer and Pollard, 2004; Smart et al., 2009; McGinnis et al., 2015; Surpless and Wigginton, 2020). As normal fault segments propagate, the stress field adjacent to isolated or interacting faults is perturbed relative to the remote tectonic stress field, resulting in a range of possible fracture orientations and patterns (e.g., Price and Cosgrove, 1990; Rives et al., 1992; Kattenhorn et al., 2000). Based on numerical modeling and field data, Kattenhorn et al. (2000) showed that fractures can grow at high angles to a normal fault plane near the lateral tips of normal faults, and they also demonstrated that a perturbed stress field extends furthest from fault segments proximal the lateral fault tipline and within relay zones between normal fault segments.
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Supplemental 3-D perspective color images of numerical model results may be viewed online at http://veo.elsevier.nl/sg/publish/831
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Present address: Department of Geology and Geological Engineering, University of Idaho, Moscow, ID 83844, USA.