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Cataclasite

Cataclasite is a cohesive granular fault rock^[1]. Comminution, also known as cataclasis, is an important process in forming cataclasites^[2]. They fall into the category of cataclastic rocks which are formed through faulting or fracturing in the upper crust (higgins). Cataclasites are destinguished from fault gouge, which is incohesive, and fault breccia, which contains coarser fragments (Sibson).

Types of cataclasite

Cataclasites are composed of fragments of the pre-existing wall rock as well as a matrix consisting of crushed microfragments, which cohesively holds the rock together^[2]. There are different types of classification schemes for cataclasites in the fault rock literature. The original classification scheme by Sibson classifies them by their proportion of fine-grained matrix to angular fragments^[1]. Many may also use the term fault breccia for describing a cataclasite with coarser grains. A fault breccia is a cataclastic rock with clasts that are larger than 2mm and make up at least 30% of the rock (woodcock).

. These are the varieties based on the original classifications scheme of cataclasites proposed by Sibson in 1997^[1]:

Protocataclasite: protocataclasite is a type of cataclasite in which the matrix takes up less than 50% of the total volume.
Mesocataclasite: mesocataclasite is a type of cataclasite in which the matrix occupies between 50 and 90 percent of the total volume.
Ultracataclasite: ultracataclasite is a type of cataclasite characterized by a matrix occupying greater than 90% of the total volume.

This classification scheme separates distinct features of cataclasites but any fault rock that has been formed through brittle deformation mechanisms containing pieces of the fractured pre-exisitng rock type are normally referred to as cataclasites. Cataclasites are different from mylonites, another type of fault rock, that is classified by the presence of a schistosity and was formed through ductile deformation methods.

Although cataclasites usually lack an oriented fabric, some researchers have classified a new type of cataclasite termed a “foliated cataclasite”^[3]. Normally these would be classified as mylonites although, through experiments, it was proven that some cataclastic mechanisms can form cataclasites with an oriented foliation solely due to brittle deformation^[3]. In this case, the fault rock would be still considered a cataclasite as it was created by cataclastic mechanisms.

Formation

Cataclasis

Cataclasites are formed through the process of cataclasis or comminution (higgins). Cataclasis is defined as the process in which rocks are broken up due to movement or faulting, which would normally refer to brittle deformation as opposed to plastic deformation (higgins). This deformation is normally formed through different brittle deformation mechanisms such as frictional sliding, rotation of mineral grains and granulation^[2].

Cataclastic Flow

The combination of all the brittle deformation acting on a rock to produce a cataclasite can be referred to as cataclastic flow ^[2]. These deformation mechanisms, such as comminution, along with frictional sliding and grain rotation allows a rock to macroscopically flow^[2].

Setting

The areas of the earth where cataclastic flow is dominant over ductile deformation are in the upper crust (sibson). Many faults near the earth’s surface are mainly brittle and do not show signs of ductile deformation^[2]. In these systems, cataclasites would be more likely to form as opposed to mylonites, which would require crystal plastic deformation. Cataclasites normally form in shallow crustal conditions where temperature and pressure are lower^[3]. At low temperatures, there is not enough energy for the crystal grains to deform plastically, thus each grain fractures as opposed to elongate or recrystallize. Due to quartz being the main mineral in many rocks in the brittle regime of the crust, the brittle-ductile transition for quartz in the crust can be a good indication of where cataclasites would form before ductile deformation plays a role^[2]. This normally refers to the first 10-12 km of the continental crust^[2].

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^ ^a ^b ^c Sibson, R. H. (1977). "Fault rocks and fault mechanisms". Journal of the Geological Society. 133: 191–213.
^ ^a ^b ^c ^d ^e ^f ^g ^h Fossen, Haakon (2010). Structural Geology. United Kingdom: Cambridge University Press. ISBN 978-0-521-51664-8.
^ ^a ^b ^c Chester, F.M.; et al. (1985). "Foliated Cataclasites". Tectonophysics. 111: 139–146. {{cite journal}}: Explicit use of et al. in: |last= (help)

[:1-1] Sibson, R. H. (1977). "Fault rocks and fault mechanisms". Journal of the Geological Society. 133: 191–213.

[:0-2] ^ ^a ^b ^c ^d ^e ^f ^g ^h Fossen, Haakon (2010). Structural Geology. United Kingdom: Cambridge University Press. ISBN 978-0-521-51664-8.

[:2-3] Chester, F.M.; et al. (1985). "Foliated Cataclasites". Tectonophysics. 111: 139–146. {{cite journal}}: Explicit use of et al. in: |last= (help)

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