2.2 Terminology: Types of Fractures
The generic term, fracture, refers to any surface produced by brittle deformation. Basic fracture types are presented in Figure 10 and field examples are provided in Figure 11 and Figure 12. A fracture is any planar or sub-planar (gently wavy or irregular) discontinuity characterized by reduced or lost cohesion; in this type of deformation, rocks or minerals are broken. The two basic types of deformation producing fractures are opening and shearing; there is a third type that results of the combination of the two basic types.
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Opening or extension fractures show displacement perpendicular to the fracture surfaces (Figure 10a). Joint is the term usually used for extension fractures whose movement can be detected only by close examination. Extension fractures can be infilled by minerals (Figure 11) or by rocks; the latter is the result of magma cooling. In the former case, the extension joint is called a vein, and in the latter, a dike. The term fissure is also used in the literature, but typically for open joints (Fossen, 2016).
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Shear fractures have displacement of the slip type, that is, the movement is parallel to the fracture walls (Figure 10b and Figure 12). Fossen (2016), page 121, states that ‘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 offsets.’
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Hybrid fractures combine both types of displacement; that is, sliding (shear) and opening (extension).
In fracture mechanics, the displacement/propagation of fractures is classified into Modes I, II, and III (Figure 10). In Mode I, opening (extension) occurs; the displacement is perpendicular to the fracture surfaces (Figure 10a). In Modes II (sliding mode) and III (tearing mode), the displacement is parallel to the fracture surface. This shear can be perpendicular (Mode II) or parallel (Mode III) to the edge of the fracture (Figure 10b) and occur along different parts of the same shear fracture (Fossen, 2016).
Figure 10 – The term fracture is generic for any discontinuity formed by brittle deformation. There are two basic types of fractures: a) joints (extension fractures) which can be infilled or barren; and, b) faults (shear fractures) with slip parallel to fracture surface. The marker (dotted line) indicates the presence (b) or the absence (a) of shear. In fracture mechanics fractures are classified into Modes I, II and III.
Figure 11 – Examples of veins (i.e., infilled joints that are formed by the opening mode perpendicular to a tensile stress. The infillings themselves are a demonstration of the opening mode, as it is needed to make room for the vein minerals. a) Massive granite with black infilling. b) Limestone pebble with fractures infilled by calcite. c) Two fracture faces in gneiss. The fracture to the left is gray and has no infilling, whereas the one to the right is brown and has an infilling of brown and white minerals. d) A detail of the brownish fracture face and of the brown and white infilled minerals. Just beside the infilling, the rock changes its color, likely due to chemical reaction with hot and saline water from which the infilled minerals were crystalized (photographs: Amélia Fernandes; c-d: Barra Mansa, Rio de Janeiro, Brazil).
Figure 12 – Example of shear fracture cutting through sandstone bedding. In this case, the shear fracture is a normal fault (as discussed in Section 3.1). The offset is deduced from the relative displacement of the orange/red layers, and the sense of shear is shown by the arrows (photograph: Ricardo Hirata, nearby Tegucigalpa, Honduras).