Shear zone
This article may be too technical for most readers to understand.(August 2012) |
In geology, a shear zone is a thin zone within the
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.
General introduction
A shear zone is a zone of strong deformation (with a high
Shear zones form a continuum of geological structures, ranging from brittle shear zones (or
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.
- 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.[2] 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.
Rocks produced in shear zones
The deformations in shear zones are responsible for the development of characteristic fabrics and mineral assemblages reflecting the reigning pressure–temperature (pT) conditions, flow type, movement sense, and deformation history. Shear zones are therefore very important structures for unravelling the history of a specific terrane.
Starting at the Earth's surface, the following rock types are usually encountered in a shear zone:
- uncohesive fault rocks. Examples being fault gouge, fault breccia, and foliated gouge.
- cohesive fault rocks like crush breccias and cataclasites (protocataclasite, cataclasite, and ultracataclasite).
- glassy pseudotachylites.
Both fault gouge and cataclasites are due to
- foliated mylonites (phyllonites).
- striped gneiss.
Mylonites start to occur with the onset of semibrittle behaviour in the alternating zone characterised by
Sense of shear
The sense of shear in a shear zone (
Indicators
The main macroscopic indicators are striations (
En echelon tension gash arrays (or extensional veins), characteristic of ductile-brittle shear zones, and sheath folds can also be valuable macroscopic shear-sense indicators.
Microscopic indicators consist of the following structures:
- asymmetric folds.
- foliations.
- imbrications.
- Crystallographic preferred orientation(CPO).
- mantled and winged porphyroclasts. Well-known examples are theta (Θ)-objects and phi (Φ)-porphyroclasts, as well as sigma (σ)- and delta (δ)-winged objects.
- mica fish (foliation fish).
- pressure shadows
- pull-aparts.
- quarter structures.
- shear band cleavages.
- step-over sites.
Width of shear zones and resulting displacements
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.
Strain softening and ductility
Because shear zones are characterised by the localisation of strain, some form of strain softening must occur, in order for the affected host material to deform more plastically. The softening can be brought about by the following phenomena:
- grain-size reductions.
- geometric softening.
- reaction softening.
- fluid-related softening.
Furthermore, for a material to become more ductile (quasi-plastic) and undergo continuous deformation (flow) without fracturing, the following deformation mechanisms (on a grain scale) have to be taken into account:
- diffusion creep (various types).
- dislocation creep (various types).
- dynamic recrystallization
- pressure solution processes.
- grain-boundary sliding (superplasticity) and grain-boundary area reduction.
Occurrence and examples of shear zones
Due to their deep penetration, shear zones are found in all metamorphic facies. Brittle shear zones are more or less ubiquitous in the upper crust. Ductile shear zones start at greenschist facies conditions and are therefore restricted to metamorphic terranes.
Shear zones can occur in the following
- transcurrent setting – steep to vertical:
- strike-slip zones.
- transform faults.
- compressive setting – low-angle
- recumbent fold nappes (at the base of).
- subduction zones.
- thrust sheets (at the base of).
- extensional setting – low-angle
Shear zones are dependent neither on rock type nor on geological age. Most often they are not isolated in their occurrence, but commonly form fractal-scaled, linked up, anastomosing networks which reflect in their arrangement the underlying dominant sense of movement of the terrane at that time.
Some good examples of shear zones of the strike-slip type are the
Importance
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.
See also
Literature
- Passchier CW & Trouw RAJ. (1996). Microtectonics. Springer. ISBN 3-540-58713-6
- Ramsay JG & Huber MI. (1987). The Techniques of Modern Structural Geology. Volume 2: Folds and Fractures. Academic Press. ISBN 0-12-576902-4
- Scholz CH. (2002). The mechanics of earthquakes and faulting. Cambridge University Press. ISBN 0-521-65540-4
References
- ISBN 0-12-576902-4
- ISBN 0-521-65540-4