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Structural discontinuity surface in the Earth's crust and upper mantle

In geology, a shear zone is a thin zone within the Earth's crust or upper mantle that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shear zone takes the form of a fracture called a fault. In the lower crust and mantle, the extreme conditions of pressure and temperature make the rock ductile. That is, the rock is capable of slowly deforming without fracture, like hot metal being worked by a blacksmith. Here the shear zone is a wider zone, in which the ductile rock has slowly flowed to accommodate the relative motion of the rock walls on either side. Because shear zones are found across a wide depth range, a great variety of different rock types with their characteristic structures are associated with shear zones.

A shear zone is a zone of strong deformation (with a high strain rate) surrounded by rocks with a lower state of finite strain. It is characterised by a length to width ratio of more than 5:1. Shear zones form a continuum of geological structures, ranging from brittle shear zones (or faults) via brittle–ductile shear zones (or semibrittle shear zones), ductile–brittle to ductile shear zones. In brittle shear zones, the deformation is concentrated in a narrow fracture surface separating the wall rocks, whereas in a ductile shear zone the deformation is spread out through a wider zone, the deformation state varying continuously from wall to wall. Between these end members, there are intermediate types of brittle–ductile (semibrittle) and ductile–brittle shear zones that can combine these geometric features in different proportions. This continuum found in the structural geometries of shear zones reflects the different deformation mechanisms reigning in the crust, i. e. the changeover from brittle (fracturing) at or near the surface to ductile (flow) deformation with increasing depth. By passing through the brittle–semibrittle transition the ductile response to deformation is starting to set in. This transition is not tied to a specific depth, but rather occurs over a certain depth range the so called alternating zone, where brittle fracturing and plastic flow coexist. The main reason for this is found in the usually heteromineral composition of rocks, with different minerals showing different responses to applied stresses (for instance, under stress quartz reacts plastically long before feldspars do). Thus differences in lithology, grain size, and preexisting fabrics determine a different rheological response. Yet other, purely physical factors, influence the changeover depth as well, including:
geothermal gradient, i. e. ambient temperature. confinement pressure and fluid pressure. bulk strain rate. stress field orientation. In Scholz's model for a quartzo feldspathic crust (with a geotherm taken from Southern California), the brittle–semibrittle transition starts at about 11 km depth with an ambient temperature of 300 °C. The underlying alternating zone then extends to roughly 16 km depth with a temperature of about 360 °C. Below approximately 16 km depth, only ductile shear zones are found. The seismogenic zone, in which earthquakes nucleate, is tied to the brittle domain, the schizosphere. Below an intervening alternating zone, there is the plastosphere. In the seismogenic layer, which occurs below an upper stability transition related to an upper seismicity cutoff (situated usually at about 4–5 km depth), true cataclasites start to appear. The seismogenic layer then yields to the alternating zone at 11 km depth. Yet big earthquakes can rupture both up to the surface and well into the alternating zone, sometimes even into the plastosphere.

The sense of shear in a shear zone (dextral, sinistral, reverse or normal) can be deduced by macroscopic structures and by a plethora of microtectonic indicators.

The width of individual shear zones stretches from the grain scale to the kilometer scale. Crustal scale shear zones (megashears) can become 10 km wide and consequently show very large displacements from tens to hundreds of kilometers. Brittle shear zones (faults) usually widen with depth and with an increase in displacements.

The importance of shear zones lies in the fact that they are major zones of weakness in the Earth's crust, sometimes extending into the upper mantle. They can be very long lived features and commonly show evidence of several overprinting stages of activity. Material can be transported upwards or downwards in them, the most important one being water circulating dissolved ions. This can bring about metasomatism in the host rocks and even re fertilise mantle material. Shear zones can host economically viable mineralizations, examples being important gold deposits in Precambrian terranes.