Shear (geology)

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Boudinaged_quartz_vein_(with_strain_fringe)_showing_''Fault_(geology).html" "title="quartz.html" ;"title="Boudinaged quartz">Boudinaged quartz vein (with strain fringe) showing ''Fault (geology)">sinistral Sinistral and dextral, in some scientific fields, are the two types of chirality ("handedness") or relative direction. The terms are derived from the Latin words for "left" (''sinister'') and "right" (''dexter''). Other disciplines use different ...

shear sense'', Starlight Pit, Fortnum Gold Mine, Western Australia In geology, shear is the response of a rock to

deformation Deformation can refer to: * Deformation (engineering), changes in an object's shape or form due to the application of a force or forces. ** Deformation (physics), such changes considered and analyzed as displacements of continuum bodies. * Defor ...

usually by

compressive stress In long, slender structural elements — such as columns or truss bars — an increase of compressive force ''F'' leads to structural failure due to buckling at lower stress than the compressive strength. Compressive stress has stress units (fo ...

and forms particular textures. Shear can be homogeneous or non-homogeneous, and may be pure shear or simple shear. Study of geological shear is related to the study of structural geology,

rock microstructure Rock microstructure includes the texture and small-scale structures of a rock. The words ''texture'' and ''microstructure'' are interchangeable, with the latter preferred in modern geological literature. However, ''texture'' is still acceptable ...

or rock texture and

fault mechanics Fault mechanics is a field of study that investigates the behavior of geologic faults. Behind every good earthquake is some weak rock. Whether the rock remains weak becomes an important point in determining the potential for bigger earthquakes. ...

. The process of shearing occurs within

brittle A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Br ...

, brittle-ductile, and

ductile Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

rocks. Within purely brittle rocks, compressive stress results in

fracturing Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...

and simple

faulting In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

Rocks

Rocks typical of shear zones include

mylonite Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a ...

, cataclasite, S-tectonite and L-tectonite, pseudotachylite, certain

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

s and highly foliated versions of the

wall rock Wall rock is the rock that constitutes the wall of an area undergoing geologic activity. Examples are the rock along the neck of a volcano, on the edge of a pluton that is being emplaced, along a fault plane, enclosing a mineral deposit, or where a ...

Shear zone

A shear zone is a tabular to sheetlike, planar or curviplanar zone composed of rocks that are more highly strained than rocks adjacent to the zone. Typically this is a type of fault, but it may be difficult to place a distinct fault plane into the shear zone. Shear zones may form zones of much more intense

foliation In mathematics (differential geometry), a foliation is an equivalence relation on an ''n''-manifold, the equivalence classes being connected, injectively immersed submanifolds, all of the same dimension ''p'', modeled on the decomposition of ...

, and

folding Fold, folding or foldable may refer to: Arts, entertainment, and media * ''Fold'' (album), the debut release by Australian rock band Epicure * Fold (poker), in the game of poker, to discard one's hand and forfeit interest in the current pot *Abov ...

. En echelon veins or fractures may be observed within shear zones. Many shear zones host

ore Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 Apr ...

deposits as they are a focus for hydrothermal flow through

orogenic belts Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted t ...

. They may often show some form of retrograde metamorphism from a peak metamorphic assemblage and are commonly metasomatised. Shear zones can be only inches wide, or up to several kilometres wide. Often, due to their structural control and presence at the edges of tectonic blocks, shear zones are mappable units and form important discontinuities to separate terranes. As such, many large and long shear zones are named, identical to fault systems. When the horizontal displacement of this faulting can be measured in the tens or hundreds of kilometers of length, the fault is referred to as a megashear. Megashears often indicate the edges of ancient tectonic plates. American Geological Institutebr>Glossary of Geology
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Mechanisms of shearing

The mechanisms of shearing depend on the pressure and temperature of the rock and on the rate of shear which the rock is subjected to. The response of the rock to these conditions determines how it accommodates the deformation. Shear zones which occur in more brittle rheological conditions (cooler, less confining pressure) or at high rates of strain, tend to fail by brittle failure; breaking of minerals, which are ground up into a breccia with a ''milled'' texture. Shear zones which occur under brittle-ductile conditions can accommodate much deformation by enacting a series of mechanisms which rely less on fracture of the rock and occur within the minerals and the mineral lattices themselves. Shear zones accommodate

by movement on foliation planes. Shearing at ductile conditions may occur by fracturing of minerals and growth of sub-grain boundaries, as well as by ''lattice glide''. This occurs particularly on platy minerals, especially micas.

Mylonite Mylonite is a fine-grained, compact metamorphic rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. Mylonites can have many different mineralogical compositions; it is a ...

s are essentially ductile shear zones.

Microstructures of shear zones

During the initiation of shearing, a penetrative planar

is first formed within the rock mass. This manifests as realignment of textural features, growth and realignment of micas and growth of new minerals. The incipient shear foliation typically forms normal to the direction of principal shortening, and is diagnostic of the direction of shortening. In symmetric shortening, objects flatten on this shear foliation much the same way that a round ball of treacle flattens with gravity. Within asymmetric shear zones, the behavior of an object undergoing shortening is analogous to the ball of treacle being smeared as it flattens, generally into an ellipse. Within shear zones with pronounced displacements a shear foliation may form at a shallow angle to the gross plane of the shear zone. This foliation ideally manifests as a sinusoidal set of foliations formed at a shallow angle to the main shear foliation, and which curve into the main shear foliation. Such rocks are known as L-S tectonites. If the rock mass begins to undergo large degrees of lateral movement, the strain ellipse lengthens into a cigar shaped volume. At this point shear foliations begin to break down into a rodding lineation or a stretch lineation. Such rocks are known as L-tectonites.

Ductile shear microstructures

Very distinctive textures form as a consequence of ductile shear. An important group of microstructures observed in ductile shear zones are S-planes, C-planes and C' planes. *S-planes or ''schistosité'' planes are generally defined by a planar

fabric Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not th ...

caused by the alignment of micas or platy minerals. Define the flattened long-axis of the strain ellipse. *C-planes or ''cisaillement'' planes form parallel to the shear zone boundary. The angle between the C and S planes is always acute, and defines the shear sense. Generally, the lower the C-S angle the greater the strain. *The C' planes, also known as shear bands and secondary shear fabrics, are commonly observed in strongly foliated mylonites especially phyllonites, and form at an angle of about 20 degrees to the S-plane. The sense of shear shown by both S-C and S-C' structures matches that of the shear zone in which they are found. Other microstructures which can give sense of shear include: * sigmoidal veins * mica fish * rotated porphyroclasts * asymmetric

boudin Boudin () are various kinds of sausage in French, Luxembourgish, Belgian, Swiss, Québécois, Acadian, Aostan, Louisiana Creole, and Cajun cuisine. Etymology The Anglo-Norman word meant 'sausage', 'blood sausage' or 'entrails' in general. ...

s (Figure 1) * asymmetric folds

Transpression

Transpression regimes are formed during oblique collision of tectonic plates and during non-orthogonal subduction. Typically a mixture of oblique-slip thrust faults and strike-slip or transform faults are formed. Microstructural evidence of transpressional regimes can be rodding lineations,

s, augen-structured

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...

es, mica fish and so on. A typical example of a transpression regime is the Alpine Fault zone of

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...

, where the oblique subduction of the Pacific Plate under the Indo-Australian Plate is converted to oblique strike-slip movement. Here, the

orogenic belt An orogenic belt, or orogen, is a zone of Earth's crust affected by orogeny. An orogenic belt develops when a continental plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological processes collec ...

attains a trapezoidal shape dominated by oblique

splay fault Splay may refer to: *Splay, a verb meaning slant, slope or spread outwards *Splay (physiology), the difference between urine threshold and saturation *Splay (Japanese band), a J-pop band from Osaka *Splay Networks, a Sweden-headquartered group of ...

s, steeply-dipping recumbent

nappe In geology, a nappe or thrust sheet is a large sheetlike body of rock that has been moved more than or above a thrust fault from its original position. Nappes form in compressional tectonic settings like continental collision zones or on the o ...

s and fault-bend folds. The Alpine Schist of New Zealand is characterised by heavily crenulated and sheared

phyllite Phyllite ( ) is a type of foliated metamorphic rock created from slate that is further metamorphosed so that very fine grained white mica achieves a preferred orientation.Stephen Marshak ''Essentials of Geology'', 3rd ed. It is primarily compo ...

. It is being pushed up at the rate of 8 to 10 mm per year, and the area is prone to large

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

s with a south block up and west oblique sense of movement.

Transtension

Transtension regimes are oblique tensional environments. Oblique, normal geologic fault and detachment faults in rift zones are the typical structural manifestations of transtension conditions. Microstructural evidence of transtension includes rodding or stretching lineations, stretched

porphyroblast A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained matrix. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape. The most common porphyrobl ...

s, mylonites, etc.

References

Diagrams and definitions of shear
(

Wayback Machine The Wayback Machine is a digital archive of the World Wide Web founded by the Internet Archive, a nonprofit based in San Francisco, California. Created in 1996 and launched to the public in 2001, it allows the user to go "back in time" and see ...

), by

University of the West of England The University of the West of England (also known as UWE Bristol) is a public research university, located in and around Bristol, England. The institution was know as the Bristol Polytechnic in 1970; it received university status in 1992 and ...

, Bristol. Archive copy incomplete, 12/31/2012. {{Structural geology Structural geology Geological processes Deformation (mechanics)