What is a fracture?
Strictly speaking, a fracture is any planar or sub-planar discontinuity that is very narrow in one dimension compared to the other two and forms as a result of external (e.g. tectonic) or internal (thermal or residual) stress. Fractures are discontinuities in displacement and mechanical properties where rocks or minerals are broken, and reduction or loss of cohesion characterizes most fractures. They are often described as surfaces, but at some scale there is always a thickness involved. Fractures can be separated into shear fractures (slip surfaces) and opening or extension fractures (joints, fissures and veins). In addition, closing or contraction fractures can be defined.
 |
| Three types of fracture. |
 |
| The orientation of various fracture types with respect to the principal stresses. |
Fractures are very narrow zones, often thought of as surfaces, associated with discontinuities in displacement and mechanical properties (strength or stiffness).
A shear fracture or slip surface is a fracture along which the relative movement is parallel to the fracture. The term shear fracture is used for fractures with small (mm- to dm-scale) displacements, while the term fault is more commonly restricted to discontinuities with larger offset. The term slip surface is used for fractures with fracture-parallel movements regardless of the amount of displacement and is consistent with the traditional use of the term fault. Fractures are commonly referred to as cracks in material science and rock mechanics oriented literature.
Extension fractures are fractures that show extension perpendicular to the walls. Joints have little or no macroscopically detectable displacement, but close examination reveals that most joints have a minute extensional displacement across the joint surfaces, and therefore they are classified as true extension fractures. Extension fractures are filled with gas, fluids, magma or minerals. When filled with air or fluid we use the term fissure. Mineral-filled extension fractures are called veins, while magma-filled fractures are classified as dikes. Joints, veins and fissures are all referred to as extension fractures.
Contractional planar features (anticracks) have contractional displacements across them and are filled with immobile residue from the host rock. Stylolites are compactional structures characterized by very irregular, rather than planar, surfaces. Some geologists now regard stylolites as contraction fractures or closing fractures, as they nicely define one of three end-members in a complete kinematic fracture framework together with shear and extension fractures. Such structures are known as anticracks in the engineering-oriented literature. Rock mechanics experiments carried out at various differential stresses and confining pressures set a convenient stage for studying aspects of fracture formation.
 |
| Experimental deformation structures that develop under extension and contraction. Initial elastic deformation is seen for all cases, while ductility increases with temperature (T) and confining pressure (Pc). YP, yield point. |
Similarly, numerical modeling has added greatly to our understanding of fracture growth, particularly the field called linear elastic fracture mechanics. In the field of fracture mechanics it is common to classify the displacement field of fractures or cracks into three different modes. Mode I is the opening (extension) mode where displacement is perpendicular to the walls of the crack. Mode II (sliding mode) represents slip (shear) perpendicular to the edge and Mode III (tearing mode) involves slip parallel to the edge of the crack. Modes II and III occur along different parts of the same shear fracture and it may therefore be confusing to talk about Mode II and Mode III cracks as individual fractures. Combinations of shear (Mode II or III) fractures and tension (Mode I) fractures are called hybrid cracks or fractures. Furthermore, the term Mode IV (closing mode) is sometimes used for contractional features such as stylolites. The mode of displacement on fractures is an important parameter, for instance when fluid flow through rocks is an issue.
 |
| Mode I, II, III and IV fractures. |
Extension fractures and tensile fractures
Extension fractures ideally develop perpendicular to s3 and thus contain the intermediate and maximum principal stresses (2y ¼ 0 ). In terms of strain, they develop perpendicular to the stretching direction under tensile conditions, and parallel to the compression axis during compression tests. Because of the small strains associated with most extension fractures, stress and strain axes more or less coincide.
Joints are the most common type of extension fracture at or near the surface of the Earth and involve very small strains. Fissures are extension fractures that are more open than joints, and are characteristic of the uppermost few hundred meters of the solid crust, where they may be up to several kilometers long.
 |
| Fissures formed in Thingvellir, Iceland, along the rift axis between the Eurasian and Laurentian plates. The fissures are open extension fractures in basalt, but the vertical displacement (right-hand side down) indicates a connection with underlying faults. |
Extension fractures are typical for deformation under low or no confining pressure, and form at low differential stress. If extension fractures form under conditions where at least one of the stress axes is tensile, then such fractures are true tensile fractures. Such conditions are generally found near the surface where negative values of s3 are more likely. They can also occur deeper in the lithosphere, where high fluid pressure reduces the effective stress. Many other joints are probably related to unloading and cooling of rocks.
Shear fractures
Shear fractures show fracture-parallel slip and typically develop at 20–30 to s1, as seen from numerous experiments under confined compression. Such experiments also show that they commonly develop in conjugate pairs, bisected by s1. Shear fractures develop under temperatures and confining pressures corresponding to the upper part of the crust. They can also form near the brittle–plastic transition, where they tend to grow into wider bands or zones of cataclastic flow. Such shear factures result in strain patterns otherwise typical for plastic deformation.
While extension fractures open perpendicular to s3, shear fractures are oblique to s3 by an angle that depends mostly on rock properties and state of stress.
Brittle and plastic deformation show different stress-strain curves (blue versus red curves): the more ductile the deformation, the greater the amount of plastic deformation prior to fracturing. It is also interesting to note the relationship between confining pressure (depth) and strain regime (contractional or extensional). The experimental data indicate that the brittle-plastic transition occurs at higher confining pressure under extension than under contraction.
