Why doesn’t one single, solitary structural discontinuity form and cut across a laboratory test specimen or an outcrop, rather than forming a network? Why isn’t the San Andreas Fault just a single, continuous strand? Why are echelon arrays formed by the different structure types, such as joints, faults, or deformation bands?

Planar breaks in rock are one of the most spectacular, fascinating, and important features in structural geology. Joints control the course of river systems, the extrusion of lava flows and fire fountains, and modulate groundwater flow. Joints and faults are associated with bending of rock strata to form spectacular folds as seen in orogenic belts from British Columbia to Iran, as well as seismogenic deformation of continental and oceanic lithospheres. Anticracks akin to stylolites accommodate significant volumetric strain in the fluid-saturated crust. Deformation bands are pervasive in soft sediments and in porous rocks such as sandstones and carbonates, providing nuclei for fault formation on the continents. Faults also form the boundaries of the large tectonic plates that produce earthquakes—and related phenomena such as mudslides in densely populated regions such as San Francisco, California—in response to tectonic forces and heat transport deep within the Earth. Faults, joints, and deformation bands have been recognized on other planets, satellites, and/or asteroids within our Solar System, attesting to their continuing intrigue and importance to planetary structural geology and tectonics.

This chapter is devoted to geologic structural discontinuities that accommodate displacements perpendicular to their surfaces, including opening-mode fractures such as cracks, joints, veins, and dikes and closing-mode structures referred to as anticracks (Table 4.1). Opening-mode structures (mode-I, Fig. 1.16) are one of the most common types of geologic structural discontinuity. Cracks are defined as sharp planar to curviplanar surfaces of opening-mode displacement discontinuity (Table 1.1). During the process of crack growth, crack walls first were created, then were moved apart normal to the fracture trace to provide a slot-like opening in the rock. Frequently the crack is filled by mineral precipitates from hydrothermal solutions, such as quartz or calcite (producing veins), crystallized magma (producing igneous dikes and sills), or even petroleum or natural gas. Near the Earth’s surface cracks or joints are often found gaping without any infilling solids (Fig. 4.1); these produce the fracture permeability necessary for efficient transport of groundwater, natural gas, and other fluids. Cracks form interesting and aesthetically pleasing patterns; these joint sets and echelon arrays contain information on the growth history of the cracks and, in turn, the brittle deformation of the host strata. Joint patterns can also provide clues to the geomorphologic and tectonic development of a region. In rock engineering, joints and other types of fractures divide an outcrop into an assemblage called a rock mass (e.g., Hoek and Brown, 1980; Chapter 3).

Geologic fracture mechanics (GFM) can be thought of as an interdisciplinary field combining approaches from engineering, materials science, and geology. It includes Linear Elastic Fracture Mechanics (LEFM) but relaxes some of the assumptions that are required for LEFM to apply to geologic structural discontinuities (i.e., fractures and deformation bands). LEFM is widely regarded as the most simple and restrictive special case of fracture mechanics (see discussions by Latzko, 1979; Kanninen and Popelar, 1985, p. 13; and Anderson, 1995, p. 117). Upon close examination, it may be seen that many of the predictions of LEFM do not match geologic observations as well as might be desired, suggesting the need for a more general approach that includes material from chemistry (to better consider diagenesis (Fig. 9.1) and subcritical fracture propagation) and plasticity (to better represent near-tip processes). Elements of some of these approaches are described in this chapter.

This chapter provides a synopsis of the use of 2-D (two-dimensional) stress in rock deformation. First, we’ll look at how to apply these simple equations to problems in Coulomb frictional sliding along surfaces in rock. Then we’ll introduce two other very useful, but somewhat more involved, failure criteria for rocks. By the end of the chapter you should be able to start with the stresses on a small piece of intact rock, know how to deal with either a simple Coulomb slip surface or a crack, and then apply this to understand the field-scale characteristics of large-scale fractured outcrops (Fig. 3.1).

Rheology is the study of flow or, more generally, the response of a material like rock to imposed stresses or strains (e.g., Johnson, 1970, pp. 13–22; Weijermars, 1997, p. 13; Karato, 2008). In this chapter we first review some aspects of experimental rock deformation that are relevant to the simplest and perhaps most widely used rheologic model for rocks, that of an elastic material. We’ll then examine the terminology of deformation and strain that flows from the corpus of laboratory studies of rock deformation.

Deformation bands are a common and important type of tabular geologic structural discontinuity that results from strain localization in porous granular rocks (e.g., Aydin et al., 2006; Fossen et al., 2007). First recognized in sandstone (e.g., Aydin, 1978; Aydin and Johnson, 1978; Hill, 1989), they were subsequently identified in other porous rock types including carbonate grainstones (Tondi et al., 2006), nonwelded tuffs (Wilson et al., 2003; Evans and Bradbury, 2004), chalk (Wennberg et al., 2013), and even sedimentary sequences on Mars (Okubo et al., 2009). Similar structures, sometimes called Lüders’ bands (e.g., Friedman and Logan, 1973; Olsson, 2000), have been noted and investigated in engineering materials such as polystyrene plastic (Argon et al., 1968; Bowden and Raha, 1970; Kramer, 1974) and mild steel (Nadai, 1950, p. 279) long before deformation bands were recognized as such in rocks (Aydin, 1978).

Faults are an efficient mechanism for allowing large strains to accumulate in the upper lithosphere of the Earth and other planetary bodies. In general, faults are arrays of 3-D surfaces along which large shear offsets (fault-parallel displacements) have been accommodated by frictional sliding (see Chapter 1). Faults can redirect the flow of subsurface fluids (either channeling or restricting the flow), modify the transmission of seismic waves, and create spectacular surface topography—through steady or destructive slip events—that have attracted mankind over millennia for the associated resources and natural beauty.

This chapter lays out a simplified roadmap to the main aspects of fracture mechanics that are important for geologic structural discontinuities. Fracture mechanics is a branch of engineering that nevertheless provides tools, concepts, and a mechanical context for interpreting the major characteristics and patterns of geologic discontinuities that are seen in the field (e.g., Rudnicki, 1980; Pollard and Segall, 1987; Engelder et al., 1993; Pollard and Fletcher, 2005; Segall, 2010; Gudmundsson, 2011).