Shear zone

# Shear zone

Discussion

Encyclopedia
A shear zone is a very important structural
Structural geology
Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation in the rocks, and...

discontinuity surface in the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain
Deformation (mechanics)
Deformation in continuum mechanics is the transformation of a body from a reference configuration to a current configuration. A configuration is a set containing the positions of all particles of the body...

into planar or curviplanar high-strain zones. Intervening (crustal) blocks stay relatively unaffected by the deformation. Due to the shearing
Shearing (physics)
Shearing in continuum mechanics refers to the occurrence of a shear strain, which is a deformation of a material substance in which parallel internal surfaces slide past one another. It is induced by a shear stress in the material...

motion of the surrounding more rigid medium, a rotation
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

al, non co-axial component can be induced in the shear zone. Because the discontinuity surface usually passes through a wide depth-range, a great variety of different rock types with their characteristic structures are produced.

## General introduction

A shear zone is a zone of strong deformation (with a high strain rate) surrounded by rocks with a lower state of finite strain
Deformation (mechanics)
Deformation in continuum mechanics is the transformation of a body from a reference configuration to a current configuration. A configuration is a set containing the positions of all particles of the body...

. 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
Fracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...

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
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

reacts plastically long before feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

s do). Thus differences in lithology
Lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...

, grain size, and preexisting fabrics determine a different rheological
Rheology
Rheology is the study of the flow of matter, primarily in the liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force....

response. Yet other, purely physical factors, influence the changeover depth as well, including:
• geothermal gradient, i.e. ambient temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

.
• 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 earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...

s nucleate, is tied to the brittle domain, the schizosphere. Below an intervening alternating zone, there is the plastosphere. In the seismogenic layer
Seismogenic layer
The seismogenic layer is the range of depths within the crust or lithosphere over which most earthquakes are initiated. Typically in continental crust this is in the uppermost 15 km...

, 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
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...

assemblages reflecting the reigning pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

(pT) conditions, flow type, movement sense, and deformation history. Shear zones are therefore very important structures for unravelling the history of a specific terrane
Terrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...

.

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 gouge
Fault gouge is an unconsolidated tectonite with a very small grain size. Fault gouge has no cohesion, it is normally an unconsolidated rock type, unless cementation took place at a later stage...

, fault breccia
Fault breccia
Fault breccia, or tectonic breccia, is a breccia that was formed by tectonic forces....

, and foliated gouge.
• cohesive fault rocks like crush breccias and cataclasite
Cataclasite
Cataclasite is a type of cataclastic rock that is formed by fracturing and comminution during faulting. It is normally cohesive and non-foliated, consisting of angular clasts in a finer-grained matrix.- Types of cataclasite :...

s (protocataclasite, cataclasite, and ultracataclasite).
• glassy pseudotachylite
Pseudotachylite
Pseudotachylite is a fault rock that has the appearance of the basaltic glass, tachylyte. It is dark in color and has a glassy appearance. However, the glass has normally been completely devitrified into very fine-grained material with radial and concentric clusters of crystals...

s.

Both fault gouge and cataclasites are due to abrasive wear on brittle, seismogenic faults.
• foliated mylonite
Mylonite
Mylonite is a fine-grained, compact rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. It is classified as a metamorphic rock...

s (phyllonites).
• striped gneiss.

Mylonites start to occur with the onset of semibrittle behaviour in the alternating zone characterised by adhesive wear. Pseudotachylites can still be encountered here. By passing into greenschist facies conditions, the pseudotachylites disappear and only different types of mylonites persist. Striped gneisses are high-grade mylonites and occur at the very bottom of ductile shear zones.

## Sense of shear

The sense of shear in a shear zone (dextral, sinistral
Sinistral
Sinistral and dextral are scientific terms that describe chirality or relative direction in a number of disciplines.The terms are derived from the Latin words for “left” and “right” ....

, reverse or normal) can be deduced by macroscopic structures and by a plethora of microtectonic indicators.

### Indicators

The main macroscopic indicators are striations (slickensides), slickenfibers, and stretching– or mineral lineations. They indicate the direction of movement. With the aid of offset markers such as displaced layering and dykes
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...

, or the deflection (bending) of layering/foliation into a shear zone, one can additionally determine the sense of shear.

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 fold
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...

s.
• foliations.
• imbrications.
• Crystallographic preferred orientation
Texture (crystalline)
In materials science, texture is the distribution of crystallographic orientations of a polycrystalline sample. A sample in which these orientations are fully random is said to have no texture. If the crystallographic orientations are not random, but have some preferred orientation, then the...

(CPO).
• mantled and winged porphyroclast
Porphyroclast
thumb|350px|right|A [[mylonite]] showing a number of porphyroclasts: a clear red [[garnet]] left in the picture while smaller white [[feldspar]] porphyroclasts can be found all over...

s. Well-known examples are theta (Θ)-objects and phi (Φ)-porphyroclasts, as well as sigma (σ)- and delta (δ)-winged objects.
• mica fish
Mica fish
Mica fish are lenticular, elongate lozenge, parallelogram-shaped, or lens-shaped single mica crystals that are often used as shear sense indicators. They commonly occur in micaceous quartzitic mylonites. They characteristically lie with their longest dimension at a small angle to the mylonitic...

(foliation fish).
• 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 mechanism
Deformation mechanism
In structural geology, metallurgy and materials science, deformation mechanisms refer to the various mechanisms at the grain scale that are responsible for accommodating large plastic strains in rocks, metals and other materials.-Mechanisms:...

s (on a grain scale) have to be taken into account:
• diffusion creep
Diffusion creep
Diffusion creep refers to the deformation of crystalline solids by the diffusion of vacancies through their crystal lattice. Diffusion creep results in plastic deformation rather than brittle failure of the material....

(various types).
• dislocation creep (various types).
• dynamic recrystallization
Dynamic recrystallization
Dynamic recrystallization is a type of recrystallization process, found within the field of metallurgy. In dynamic recrystallization, as opposed to static recrystallization, the nucleation and growth of new grains occurs during deformation rather than afterwards as part of a separate heat...

• pressure solution
Pressure solution
In structural geology and diagenesis, pressure solution or pressure dissolution is a deformation mechanism that involves the dissolution of minerals at grain-to-grain contacts into an aqueous pore fluid in areas of relatively high stress and either deposition in regions of relatively low stress...

processes.
• grain-boundary sliding (superplasticity
Superplasticity
In materials science, superplasticity is a state in which solid crystalline material is deformed well beyond its usual breaking point, usually over about 200% during tensile deformation. Such a state is usually achieved at high homologous temperature, typically half the absolute melting point. ...

) and grain-boundary area reduction.

## Occurrence and examples of shear zones

Due to their deep penetration, shear zones are found in all metamorphic facies
Metamorphic facies
The metamorphic facies are groups of mineral compositions in metamorphic rocks, that are typical for a certain field in pressure-temperature space...

. Brittle shear zones are more or less ubiquitous in the upper crust. Ductile shear zones start at greenschist facies conditions and are therefore bound to metamorphic terranes.

Shear zones can occur in the following geotectonic settings:
• 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
• metamorphic core complex
Metamorphic core complex
Metamorphic core complexes are exposures of deep crust exhumed in association with largely amagmatic extension. They form, and are exhumed, through relatively fast transport of middle and lower continental crust to the Earth's surface...

detachments
Detachment fault
Detachment faulting is associated with large-scale extensional tectonics. Detachment faults often have very large displacements and juxtapose unmetamorphosed hanging walls against medium to high-grade metamorphic footwalls that are called metamorphic core complexes...

.

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
Fractal
A fractal has been defined as "a rough or fragmented geometric shape that can be split into parts, each of which is a reduced-size copy of the whole," a property called self-similarity...

-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 South Armorican Shear Zone and the North Armorican Shear Zone in Brittany
Brittany
Brittany is a cultural and administrative region in the north-west of France. Previously a kingdom and then a duchy, Brittany was united to the Kingdom of France in 1532 as a province. Brittany has also been referred to as Less, Lesser or Little Britain...

, the North Anatolian Fault Zone in Turkey
Turkey
Turkey , known officially as the Republic of Turkey , is a Eurasian country located in Western Asia and in East Thrace in Southeastern Europe...

, and the Dead Sea Fault
The Dead Sea Transform fault system, also sometimes referred to as the Dead Sea Rift, is a geologic fault which runs from the Maras Triple Junction to the northern end of the Red Sea Rift...

in Israel
Israel
The State of Israel is a parliamentary republic located in the Middle East, along the eastern shore of the Mediterranean Sea...

. Shear zones of the transform type are the San Andreas Fault
San Andreas Fault
The San Andreas Fault is a continental strike-slip fault that runs a length of roughly through California in the United States. The fault's motion is right-lateral strike-slip...

in California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

, and the Alpine Fault
Alpine Fault
The Alpine Fault is a geological fault, more specifically known as a right-lateral strike-slip fault, that runs almost the entire length of New Zealand's South Island. It forms a transform boundary between the Pacific Plate and the Indo-Australian Plate. Earthquakes along the fault, and the...

in New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

. A shear zone of the thrust type is the Moine Thrust in northwestern Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

. An example for the subduction zone setting is the Japan Median Tectonic Line
Japan Median Tectonic Line
, also Median Tectonic Line , is Japan's longest fault system. It connects with the Itoigawa-Shizuoka Tectonic Line , the left blue line on the map, and the Fossa Magna, the pink shaded region....

. Detachment fault related shear zones can be found in southeastern California, e.g. the Whipple Mountain Detachment Fault. An example of a huge anastomosing shear-zone is the Borborema Shear Zone
Borborema Plateau
The Borborema Plateau is a plateau in northeastern Brazil which extends across the states of Pernambuco, Paraíba, and Rio Grande do Norte....

in Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

.

## 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
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

circulating dissolved ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s. This can bring about metasomatism
Metasomatism
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...

in the host rocks and even re-fertilise mantle material.

Shear zones can host economically viable mineralization
Mineralization (geology)
In geology, mineralization is the hydrothermal deposition of economically important metals in the formation of ore bodies or "lodes".The first scientific studies of this process took place in Cornwall, United Kingdom by J.W.Henwood FRS and later by R.W...

s, examples being important gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

deposits in Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...

terranes.

## 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