Figure 1.
Boudinaged Quartz vein(with strain fringe) showing sinistral shear sense. boudinagejpg|thumb|Boudinaged Jasperoid in sheared basalt Fortnum Gold Mine Australia Quartz (from German) is the most abundant Mineral in the Earth 's Continental crust (although Feldspar is more common in This article is about the geologic usage for human physiology usage see the articles Laterality. Starlight Pit, Fortnum Gold Mine, Western Australia.

Study of geological shear is related to the study of structural geology, rock microstructure or rock texture and fault mechanics. Structural geology is the study of the three dimensional distribution of rock bodies and their planar or folded surfaces and their internal fabrics Rock microstructure includes the texture of a rock and the small scale rock structures This page is intended to be a list of rock textural and morphological terms In Geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement

Shear is the response of a rock to deformation usually by compressive stress and forms particular textures. In Materials science, deformation is a change in the shape or size of an object due to an applied force. Compressive stress is the stress applied to materials resulting in their compaction (decrease of volume Shear can be homogeneous or non-homogeneous, and may be pure shear or simple shear. Simple Shear is a special case of Deformation of a fluid where only one component of Velocity vectors has a non-zero value \ V_x=f(xy

The process of shearing generally occurs within brittle-ductile and ductile rocks. Ductility is a mechanical property used to describe the extent to which materials can be deformed plastically or "stretched" into "wires" without Within purely brittle rocks, compressive stress results in fracturing and simple faulting. In Geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement

Rocks

Rocks typical of shear zones include mylonite, cataclasite, S-tectonite and L-tectonite, pseudotachylite, certain breccias and highly foliated versions of the wall rocks. Mylonite is a fine-grained compact rock produced by dynamic crystallization of the constituent minerals resulting in a Cataclasite is a metamorphic rock that is formed by mechanical Shear stress during faulting. Tectonites are rocks whose fabric reflects the history of their deformation or rocks with fabric that clearly displays coordinated geometric features that indicate continuous Tectonites are rocks whose fabric reflects the history of their deformation or rocks with fabric that clearly displays coordinated geometric features that indicate continuous Pseudotachylite is a fault rock that has the appearance of the Basaltic Glass, Tachylyte. Breccia (ˈbrɛtʃiə ˈbrɛʃiə breach is a rock composed of angular fragments of several Minerals or rocks in a matrix, that is a cementing material

Shear zone

Asymmetric shear in basalt, Labouchere mine, Glengarry Basin, WA. Shear asymmetry is dextral, pen for scale.

A shear zone or shear is a wide zone of distributed shearing in rock. Typically this is a type of fault but it may be difficult to place a distinct fault plane into the shear zone. In Geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement Shear zones may form zones of much more intense foliation, deformation, and folding. Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic In Materials science, deformation is a change in the shape or size of an object due to an applied force. See also Folding The term fold is used in Geology when one or a stack of originally flat and planar surfaces such as Sedimentary

Many shear zones host ore deposits as they are a focus for hydrothermal flow through orogenic belts. An ore is a volume of rock containing components or Minerals in a mode of occurrence that renders it valuable for mining Orogeny (Greek for "mountain generating" is the process of natural Mountain building and may be studied as a tectonic structural event as a geographical event and They may often show some form of retrograde metamorphism from a peak metamorphic assemblage and are commonly metasomatised. Metasomatism is the chemical alteration of a rock by Hydrothermal and other fluids

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, similar to fault systems.

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 compressive stress by movement on foliation planes. Compressive stress is the stress applied to materials resulting in their compaction (decrease of volume

Shearing at ductile conditions may occur by , and dislocation creep within minerals, by fracturing of minerals and growth of sub-grain boundaries, as well as by lattice glide, particularly on platy minerals, especially micas.

Mylonites are essentially ductile shear zones. Mylonite is a fine-grained compact rock produced by dynamic crystallization of the constituent minerals resulting in a

Microstructures of shear zones

Typical example of dextral shear foliation in an L-S tectonite, with pencil pointing in direction of shear sense. Note the sinusoidal nature of the shear foliation.

During the initiation of shearing, a penetrative planar foliation is first formed within the rock mass. Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic This manifests as realignment of textural features, growth and realignment of micas and growth of new minerals. The word "mica" is thought to be derived from the Latin word la micare, "glitteren" in reference to the brilliant appearance of this mineral (especially

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 assymmetric 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 foliaions begin to break down into a rodding lineation or a stretch lineation. Such rocks are known as L-tectonites.

Stretched pebble conglomerate L-tectonite illustrating a stretch lineation within a shear zone, Glengarry Basin, Australia. Pronounced assymmetric shearing has stretched the conglomerate pebbles into elongate cigar shaped rods.

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 caused by the alignment of micas or platy minerals. Geology the term fabric describes the spatial and geometric configuration of all the elements that make up a particular rock. The word "mica" is thought to be derived from the Latin word la micare, "glitteren" in reference to the brilliant appearance of this mineral (especially 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. Mylonite is a fine-grained compact rock produced by dynamic crystallization of the constituent minerals resulting in a

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 boudins (Figure 1)
• asymmetric folds

Transpression

Transpression regimes are formed during oblique collision of tectonic plates and during non-orthogonal subduction. In Geology, a vein is a finite volume within a rock, having a distinct shape filled with Crystals of one or more Minerals which were precipitated A porphyroclast is a Clast or Mineral fragment in a Metamorphic rock, surrounded by a Groundmass of finer grained Crystals Porphyroclasts boudinagejpg|thumb|Boudinaged Jasperoid in sheared basalt Fortnum Gold Mine Australia In Geology, a subduction zone is an area on Earth where two tectonic plates meet and move towards one another with one sliding underneath the other Typically a mixture of oblique-slip thrust faults and strike-slip or transform faults are formed. A thrust fault is a type of fault, or break in the Earth's crust with resulting movement of each side against the other in which a lower stratigraphic position is pushed up Microstructural evidence of transpressional regimes can be rodding lineations, mylonites, augen-structured gneisses, mica fish and so on. Lineations in Structural geology are linear structural features within rocks Mylonite is a fine-grained compact rock produced by dynamic crystallization of the constituent minerals resulting in a Gneiss (ˈnaɪs is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from preexisting formations that were originally

A typical example of a transpression regime is the Alpine Fault zone of New Zealand, where the oblique subduction of the Pacific Plate under the Indo-Australian Plate is converted to oblique strike-slip movement. New Zealand is an Island country in the south-western Pacific Ocean comprising two main landmasses (the North Island and the South Island The Pacific Plate is an oceanic Tectonic plate beneath the Pacific Ocean. The Indo-Australian Plate is a major Tectonic plate that includes the continent of Australia and surrounding Ocean, and extends northwest to include the Here, the orogenic belt attains a trapezoidal shape dominated by oblique splay faults, steeply-dipping recumbent nappes and fault-bend folds. Orogeny (Greek for "mountain generating" is the process of natural Mountain building and may be studied as a tectonic structural event as a geographical event and In Geology, a nappe is a large sheetlike body of rock that has been moved more than 2 km (1

The Alpine Schist of New Zealand is characterised by heavily crenulated and sheared phyllite. Crenulation is a texture formed in Metamorphic rocks such as Phyllite, Schist and some Gneiss by two or more stress directions resulting Phyllite is a type of foliated Metamorphic rock primarily composed of Quartz, Sericite Mica, and chlorite; the rock represents It s being pushed up at the rate of 8 to 10 mm per year, and the area is prone to large earthquakes with a south block up and west oblique sense of movement. An earthquake is the result of a sudden release of energy in the Earth 's crust that creates Seismic waves Earthquakes are recorded with a Seismometer

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. In Geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement Microstructural evidence of transtension includes rodding or stretching lineations, stretched porphyroblasts, mylonites, etc. Lineations in Structural geology are linear structural features within rocks A porphyroblast is a large Mineral Crystal in a Metamorphic rock which has grown within the finer grained groundmass.

See also

• Convergent boundary
• Crenulation
• Fault (geology)
• Foliation (geology)
• Rock microstructure
• Sense of shear indicators: dextral and sinistral

References

Diagrams and definitions of shear