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Blastomylonite: A mylonite with extensive recrystallization.
Boudin: Elongated structure that may form during shear due to differences in ductility of the minerals of a rock. A boudin is made when a less ductile layer stretches and then breaks as the surrounding rock deforms ductilely. It comes from the French word for sausage, referring to the shape the boudins often take (Winter, 2001).
Brittle deformation: Deformation involving a ‘through-going discontinuity’ in the rock (Snoke et al., 1998). It includes tensile cracking, shear fracturing and frictional sliding. Brittle deformation occurs when the stresses on a rock exceed the failure strength of the rock, causing a loss of cohesion.
The classification of a rock as brittle or ductile depends on scale. For example, cataclastic flow can be considered macroscopically ductile or microscopically brittle (Snoke et al., 1998).
Cataclasis: The brittle fracturing, shearing, and grinding of rocks. The process includes grain rotation (Engelder, 1974). It is typically associated with grain size reduction and increasing average roundness of particles due to fracturing at stress concentrations on the corners of angular grains (Lin, 1999).
Cataclasite: A fine-grained, cohesive fault rock that generally forms at shallow depths in the crust, dominantly by brittle deformation processes such as microcracking and abrasion. Cataclasites can have foliation, developed through cataclastic flow (Chester et al., 1985).
Different authors define cataclasites more specifically in different ways. Sibson (1977) defined cataclasites as cohesive (at time of formation), random-fabric fault rocks comprising of over 10% matrix (defining matrix as being made up of grains smaller than 0.1mm). He applied the prefixes proto- and ultra- to describe the proportion of matrix in the rock as follows:
Protocataclasite: A cataclasite with 10-50% matrix
Cataclasite: A cataclasite with 50-90% matrix.
Ultracataclasite: A cataclasite with 90-100% matrix.
Woodcock and Mort (2008) defined cataclasites as cohesive (at the present outcrop), non-foliated fault rocks consisting of less than 30% clasts over 2 mm in diameter. In their classification scheme, protocataclasites range from 0-50% matrix, and the prefix meso- may be applied to cataclasites with 50-90% matrix.
A rock that fits the description of a cataclasite but has highly angular clasts (indicating minimal abrasion) may be termed a microbreccia.
Cataclastic flow: The macroscopically ductile movement of rock resulting from microscopically brittle fracturing and sliding. Cataclastic flows display complex transitions in rheology but can sometimes be approximated as viscous fluids.
Clast: Fragments of rock surrounded by a finer-grained matrix.
In Woodcock and Mort’s (2008) classification scheme, clasts are defined differently for breccias and cataclasites.
Of a breccia: Fragments of at least 2 mm in size.
Of a cataclasite or mylonite: Fragments of at least 0.1 mm in size (i.e. any grains visible to the naked eye).
Cohesive: Not “capable of being broken into component granules with fingers or with the aid of a pen knife” (Brodie et al., 2007).
Synonym: Coherent.
Crush Breccia: A cohesive fault rock consisting of 0-10% matrix, defined by Sibson (1977), with matrix being defined as grains not visible to the naked eye (<0.1mm). Crush breccias are subdivided based on clast size. Crush breccia: Dominantly fragments larger than 5 mm. Fine crush breccia: Dominantly fragments between 1 mm and 5 mm. Crush microbreccia: Dominantly fragments smaller than 1 mm. Some more recently published fault rock classification schemes (Killick, 2003; Woodcock and Mort, 2008) have chosen to omit the crush breccia category, and instead include rocks of that description under fault breccia and cataclasite definitions. Damage zone: The region encompassing a system of subsidiary brittle deformation structures around the fault core. These structures may include small faults, veins, fractures, cleavage, and folds (Caine et al., 1996). Ductile deformation: “Continuous and homogeneously distributed” deformation. It can involve numerous mechanisms ranging from macroscopic cataclastic flow to crystal-plastic processes (Snoke et al., 1998). Ductile deformation occurs when the stresses on a rock cause permanent strain, without the loss of cohesion. The classification of a rock as brittle or ductile depends on scale. For example, cataclastic flow can be considered macroscopically ductile or microscopically brittle (Snoke et al., 1998). Fault: A discrete shear fracture along which there has been displacement. For more information: http://en.wikipedia.org/wiki/Fault_(geology) Fault Breccia: A coarse-grained fault rock, formed by brittle deformation processes. Breccias are defined more specifically in different ways by different authors. It consists of angular rock clasts, typically poorly sorted but may be graded or grain-size laminated. Grain size distribution is typically large, and most naming conventions rely on the coarsest grain size present (e.g. megabreccia, microbreccia). Inter-clast porosity may be filled with particulate matrix, or mineral cement, or may be empty. Higgins (1971) and Sibson (1977) both defined fault breccias as incohesive (at time of formation) fault rocks consisting of at least 30% visible grains (grains > 0.1mm).
Woodcock and Mort (2008) modified that definition by setting a larger minimum clast size, defining breccias as fault rocks consisting of at least 30% clasts over 2 mm in diameter, and removing incohesion as a criterion. These authors then subdivided breccias into crackle, mosaic, and chaotic, by the area taken up by clasts and how well the clasts fit together (amount of clast rotation). These properties are directly related (Mort and Woodcock, 2008).
Crackle breccia: A breccia consisting of at least 75% clasts over 2 mm in diameter. The clasts have undergone less than 10° of rotation.
Mosaic breccia: A breccia consisting of 60-75% clasts over 2 mm in diameter. The clasts have undergone 10-20° of rotation.
Chaotic breccia: A breccia consisting of 30-60% clasts over 2 mm in diameter. The clasts have undergone over 20° of rotation.
Fault core: The part of a fault zone that accommodates the most slip; this is an area of high deformation and it contains the principal slip zone(s). Within the fault core, any pre-existing structures, both sedimentary and tectonic, have been completely overprinted (Billi et al., 2003). The fault core is surrounded by a damage zone.
Fault gouge: A fine-grained, incohesive fault rock, generally forming at shallow depths in the crust by brittle deformation processes such as microcracking and abrasion. Fault gouge is defined differently by different authors.
Sibson (1977) defines fault gouge as an incohesive (at the time of formation), random-fabric fault rock consisting of less than 30% visible fragments.
Woodcock and Mort (2008) define fault gouge as an incohesive (at the present outcrop) fault rock consisting of less than 30% clasts over 2 mm in diameter.
Fault gouge is defined by Brodie et al. (2007) as a ‘clay-rich incohesive cataclasite’.
Fault rock: A rock associated with a fault or shear zone. Can form under brittle conditions at shallow depths in the Earth’s crust, or under plastic conditions deeper in the Earth’s crust or upper mantle (Snoke et al., 1998).
Foliation: Penetrative fabric showing planar alignment of grains. Foliation gives the appearance of ductile continuous flow and is characteristic of mylonites, but can also be seen in cataclasites and fault gouge. The process may produce layering by mineral, texture, or fabric segregation (Chester, 1985).
Synonyms: flow structure, fluxion structure.
Incohesive: “Capable of being broken into component granules with fingers or with the aid of a pen knife.”
Synonyms: “Incoherent, friable” (Brodie et al., 2007).
Matrix: Groundmass of fine-grained material, often surrounding larger fragments.
In Woodcock and Mort’s (2008) classification scheme, matrix is defined differently for breccias and cataclasites.
Of a breccia: grains less than 2 mm.
Of a cataclasite or mylonite: grains less than 0.1 mm (i.e. not visible to the naked eye).
Microbreccia: A cohesive fault rock made up of over 30% fragments larger than 2 mm (Higgins, 1971). This term is not used in more recent classification schemes (ex. Sibson, 1977; Killick, 2003; Woodcock and Mort, 2008).
The term microbreccia has also been used to describe fine-grained rocks with highly angular clasts, indicating that clasts have not undergone much abrasion. Their rounded-clast equivalent may be termed a cataclasite if the clasts are of the appropriate size, depending on the classification scheme.
Mylonite: A fine-grained, cohesive foliated fault rock formed by ductile deformation. Mylonites generally form at depth, through plastic processes such as dynamic recrystallization, that tend to result in a reduced grain size. Polymineralic mylonites may also include porphyroclasts of the more resistant minerals within the fluxion structure of the ductilely deformed minerals.
Sibson (1977) defined mylonites as cohesive, foliated fault rocks consisting of over 10% matrix. The prefixes proto- and ultra- are applied to describe the proportion of matrix in the rock.
Protomylonite: A mylonite with 10-50% matrix.
Mylonite: A mylonite with 50-90% matrix.
Ultramylonite: A mylonite with 90-100% matrix.
Woodcock and Mort (2008) defined mylonites as being foliated fault rocks consisting of less than 30% grains larger than 2 mm. In their classification scheme, protomylonites range from 0-50% matrix, with matrix defined as grains smaller than 0.1mm. The prefix meso- may be applied to mylonites with between 50% and 90% matrix.
Mylonites were originally thought to have been formed by brittle processes, and were described as “microscopic pressure-breccias with fluxion structure in which the interstitial dusty, siliceous, and kaolinitic paste has only crystallised in part” (Lapworth, 1885). The term mylonite, which comes from mylon, the Greek word for mill, implies that the rocks formed by the milling down of original rock. However, it is now understood that mylonites form at elevated temperatures and pressures, characteristic of ductile conditions, and involve recrystallization (Hatcher, 1978).
Porphyroclast: Larger grains in a matrix of broken up material. Porphyroclasts are initially larger in the protolith or made of more resistant minerals, so they do not get crushed as quickly as the surrounding material.
Synonym: survivor grains.
Principal slip zone (PSZ): The plane along which the most slip is accommodated during a single earthquake event. It is within the fault core, surrounded by the damage zone.
Synonyms: principal slip surface (PSS), principal slip plane (PSP).
Pseudotachylyte: Frictional melt formed along a fault, typically recognized by the presence of glass or devitrified glass (Sibson, 1977). It generally has the appearance of dark, intrusive veins with a quench texture (Shand, 1916). The network of veins can be the cementing matrix between clasts of breccias (Lin, 2007). Seismic slip rates are necessary to form pseudotachylytes, so they are manifestations of past earthquakes. They are rare in the geologic record, most likely due to their poor preservation potential (Kirkpatrick and Rowe, 2013). The term pseudotachylyte also refers to impact melt glass (Shand, 1916).
Alternate spelling: Pseudotachylite.
Shear Zone: The high-strain zone where adjacent parallel surfaces slide or flow past each other.
For more information: http://en.wikipedia.org/wiki/Shear_zone
Survivor Grains: Isolated clasts separated by a finer-grained matrix (Cowan, 2003), that have escaped grain fracturing (Cladouhos, 1999).
Synonyms: relict grains, refractory grains, porphyroclasts.
Billi, A., Salvini, F., Storti, F., 2003. The damage zone-fault core transition in carbonate rocks: implications for fault growth, structure and permeability. Journal of Structural Geology 25, 1779-1794.
Brodie, K., Fettes, D., Harte, B., 2007. Structural terms including fault rock terms. In Metamorphic Rocks: a Classification and Glossary of Terms (eds D. Fettes & J. Desmonds). Cambridge: Cambridge University Press, pp. 24-31.
Caine, J.S., Evans, J.P., Forster, C.B., 1996. Fault zone architecture and permeability structure. Geology 24, 1025-1028.
Chester, F. M., Friedman, M., Logan, J.M., 1985. Foliated cataclasites. Tectonophysics 111, 139-146.
Cladouhos, T. T., 1999. Shape preferred orientations of survivor grains in fault gouge. Journal of Structural Geology 21, 419-436.
Cowan, D.S., Cladouhos, T.T., Morgan, J.K., 2003. Structural geology and kinematic history of rocks formed along low-angle normal faults, Death Valley, California. Bulletin of the Geological Society of America 115, 1230-1248.
Engelder, J.T. , 1974. Cataclasis and the Generation of Fault Gouge. Bulletin of the Geological Society of America 85, 1515-1522.
Hatcher, R.D. Jr., 1978. Comment and reply on ‘Eastern Piedmont fault system: Speculations on its extent’. Geology 6, 580-582.
Killick, A.M., 2003. Fault rock classification: an aid to structural interpretation in mine and exploration geology. South African Journal of Geology 106, 395–402.
Kirkpatrick, J.D., Rowe, C.D., 2013. Disappearing ink: How pseudotachylytes are lost from the rock record. Journal of Structural Geology 52, 183-198.
Lapworth, C., 1885. The Highland controversy in British geology; its causes, course and consequences. Nature 32, 558-559.
Lin, A., 2008. Fossil Earthquakes: The formation and preservation of pseudotachylytes. Berlin ; London : Springer, pp.348.
Lin, A., 1999. Roundness of clasts in pseudotachylytes and cataclastic rocks as an indicator of frictional melting. Journal of Structural Geology 21, 473-478.
Mort, K., Woodcock, N.H., 2008. Quantifying fault breccia geometry: Dent Fault, NW England. Journal of Structural Geology 30, 701-709.
Higgins, M.W., 1971. Cataclastic Rocks. Professional Paper, United States Geological Survey, 687-784.
Sibson, R.H., 1977. Fault rocks and fault mechanisms. Journal of the Geological Society, London 133, 191– 213.
Shand, S.J., 1916. The pseudotachylyte of Parijs (Orange Free State), and its relation to `trap-shoten gneiss’ and `Flinty crush-rock’. Quarter Journal of Geological Society of London 72, 198-221.
Snoke, A.W., Tullis, J., Todd, V.R., 1998. Fault-related rocks : a photographic atlas. Princeton, N.J. : Princeton University Press, pp. 617.
Winter, J.D., 2001. An introduction to igneous and metamorphic petrology. New Jersey: Prentice Hall 697, pp. 460.
Woodcock, N.H., Mort, K., 2008. Classification of fault breccias and related fault rocks. Geological Magazine 145, 435–440.