Cataclastic rock

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A cataclastic rock is a type of fault rock that has been wholly or partly formed by the progressive fracturing and comminution of existing rocks, a process known as cataclasis. Cataclasis involves the granulation, crushing, or milling of the original rock, then rigid-body rotation and translation of mineral grains or aggregates before lithification. Cataclastic rocks are associated with fault zones and impact event breccias.

Contents

Classification

Various classification schemes have been proposed for the cataclastic rocks, but changes in understanding of the processes involved in their formation and better knowledge of the variety of such rocks has made a simple classification difficult, particularly where distinctions cannot be made in hand specimens. [1] Sibson's 1977 classification of fault rocks was the first to include an understanding of the deformation mechanisms involved and all subsequent schemes have been based on this. [2] [3] Fault breccias have been further classified in terms of their origins; attrition, distributed crush and implosion brecciation, [4] and, borrowing from the cave-collapse literature, crack, mosaic and chaotic from their clast concentration. [5]

Mylonite was originally defined as a cataclastic rock but is now understood to have formed mainly by crystal-plastic processes. [1]

Types

Cataclasite

Cataclasite is a fault rock that consists of angular clasts in a finer-grained matrix. [1] It is normally non-foliated but some varieties have been described with a well-developed planar fabric that are known as foliated cataclasites. [3] Cataclasite grades into fault breccia as the percentage of visible clasts increases to more than 30%.

Fault breccia

Fault breccia is a fault rock that consists of large fragments of rock in a fine-grained matrix. It may be either cohesive or incohesive. The matrix may also include mineral veins formed in voids between the clasts, which may themselves become fractured by later movements on the fault.

Fault gouge

Fault gouge is an unconsolidated and incohesive type of fault rock consisting almost entirely of finely crushed material. Varieties that have a large clay mineral content are known as clay gouges.

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. It is generally found either along a fault surface or as veins injected into the fault walls. Most pseudotachylites have clearly formed by frictional melting, associated with either seismic faulting, some large landslides or meteorite impacts.

Formation

Cataclastic rocks form by brittle processes in the upper part of the crust in areas of moderate to high strain, particularly in fault zones. The two main mechanisms involved are microfracturing (breaking the original rock into fragments) and frictional sliding/rolling of the fragments, combined with further fracturing. [6]

Cataclastic flow

Cataclastic flow is the main deformation mechanism accommodating large strains above the brittle–ductile transition zone. It can be regarded as a ductile mechanism, [7] although one that takes place within the elastico-frictional regime of deformation. [2] Deformation is accommodated by the sliding and rolling of fragments within the cataclastic rock. Cycles of cementation and refracturing are generally recognised in such rocks.

See also

Related Research Articles

<span class="mw-page-title-main">Breccia</span> Rock composed of angular fragments

Breccia is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.

<span class="mw-page-title-main">Fault (geology)</span> Fracture or discontinuity in displaced rock

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 forces, with the largest forming the boundaries between the plates, such as the megathrust faults of subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes. Faults may also displace slowly, by aseismic creep.

<span class="mw-page-title-main">Metamorphism</span> Change of minerals in pre-existing rocks without melting into liquid magma

Metamorphism is the transformation of existing rock to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of 150 °C (300 °F), and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism is distinct from weathering or diagenesis, which are changes that take place at or just beneath Earth's surface.

<span class="mw-page-title-main">Shear zone</span> Structural discontinuity surface in the Earths crust and upper mantle

In geology, a shear zone is a thin zone within the Earth's crust or upper mantle that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shear zone takes the form of a fracture called a fault. In the lower crust and mantle, the extreme conditions of pressure and temperature make the rock ductile. That is, the rock is capable of slowly deforming without fracture, like hot metal being worked by a blacksmith. Here the shear zone is a wider zone, in which the ductile rock has slowly flowed to accommodate the relative motion of the rock walls on either side.

<span class="mw-page-title-main">Mylonite</span> Metamorphic rock

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 classification based on the textural appearance of the rock.

<span class="mw-page-title-main">Vein (geology)</span> Sheetlike body of crystallized minerals within a rock

In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation. The hydraulic flow involved is usually due to hydrothermal circulation.

<span class="mw-page-title-main">Shear (geology)</span> Response of rock to deformation

In geology, shear is the response of a rock to deformation usually by compressive stress 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 or rock texture and fault mechanics.

<span class="mw-page-title-main">Clastic rock</span> Sedimentary rocks made of mineral or rock fragments

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks, and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic to refer to sedimentary rocks and particles in sediment transport, whether in suspension or as bed load, and in sediment deposits.

<span class="mw-page-title-main">Texture (geology)</span>

In geology, texture or rock microstructure refers to the relationship between the materials of which a rock is composed. The broadest textural classes are crystalline, fragmental, aphanitic, and glassy. The geometric aspects and relations amongst the component particles or crystals are referred to as the crystallographic texture or preferred orientation. Textures can be quantified in many ways. The most common parameter is the crystal size distribution. This creates the physical appearance or character of a rock, such as grain size, shape, arrangement, and other properties, at both the visible and microscopic scale.

<span class="mw-page-title-main">Brittle–ductile transition zone</span> Strongest part of the Earths crust

The brittle-ductile transition zone is the zone of the Earth's crust that marks the transition from the upper, more brittle crust to the lower, more ductile crust. For quartz and feldspar-rich rocks in continental crust, the transition zone occurs at an approximate depth of 20 km, at temperatures of 250–400 °C. At this depth, rock becomes less likely to fracture, and more likely to deform ductilely by creep because the brittle strength of a material increases with confining pressure, while its ductile strength decreases with increasing temperature.

<span class="mw-page-title-main">Fracture (geology)</span> Geologic discontinuity feature, often a joint or fault

A fracture is any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. A fracture will sometimes form a deep fissure or crevice in the rock. Fractures are commonly caused by stress exceeding the rock strength, causing the rock to lose cohesion along its weakest plane. Fractures can provide permeability for fluid movement, such as water or hydrocarbons. Highly fractured rocks can make good aquifers or hydrocarbon reservoirs, since they may possess both significant permeability and fracture porosity.

<span class="mw-page-title-main">Pseudotachylyte</span> Glassy, or very fine-grained, rock type

Pseudotachylyte is an extremely fine-grained to glassy, dark, cohesive rock occurring as veins that form through frictional melting and subsequent quenching during earthquakes, large-scale landslides, and impacts events. Chemical composition of pseudotachylyte generally reflects the local bulk chemistry, though may skew to slightly more mafic compositions due to the preferential incorporation of hydrous and ferro-magnesian minerals into the melt phase.

<span class="mw-page-title-main">Cataclasite</span> Rock found at geological faults

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

<span class="mw-page-title-main">Fault breccia</span> Breccia formed by tectonic forces

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

In geology, a deformation mechanism is a process occurring at a microscopic scale that is responsible for changes in a material's internal structure, shape and volume. The process involves planar discontinuity and/or displacement of atoms from their original position within a crystal lattice structure. These small changes are preserved in various microstructures of materials such as rocks, metals and plastics, and can be studied in depth using optical or digital microscopy.

<span class="mw-page-title-main">Fault gouge</span> Crushed rock found near faults

Fault gouge is a type of fault rock best defined by its grain size. It is found as incohesive fault rock, with less than 30% clasts >2mm in diameter. Fault gouge forms in near-surface fault zones with brittle deformation mechanisms. There are several properties of fault gouge that influence its strength including composition, water content, thickness, temperature, and the strain rate conditions of the fault.

<span class="mw-page-title-main">Deformation bands</span> Geological mechanism producing small fault lines.

Deformation bands are small faults with very small displacements. In the past, these bands have been called Luder's bands or braided shear fractures. They often precede large faults. They develop in porous rocks, such as sandstone. Material in a deformation band has a much smaller grain size, poorer sorting, and a lower porosity than the original sandstone. They can restrict and/or change the flow of fluids like water and oil. They are common in the Colorado Plateau, where examples occur in the Entrada Sandstone in the San Rafael Swell in Utah.

The Thiviers-Payzac Unit is a metasedimentary succession of late Neoproterozoic and Cambrian age outcropping in the southern Limousin in France. The unit geologically forms part of the Variscan basement of the northwestern Massif Central.

<span class="mw-page-title-main">Ductility (Earth science)</span>

In Earth science, ductility refers to the capacity of a rock to deform to large strains without macroscopic fracturing. Such behavior may occur in unlithified or poorly lithified sediments, in weak materials such as halite or at greater depths in all rock types where higher temperatures promote crystal plasticity and higher confining pressures suppress brittle fracture. In addition, when a material is behaving ductilely, it exhibits a linear stress vs strain relationship past the elastic limit.

<span class="mw-page-title-main">Fault zone hydrogeology</span>

Fault zone hydrogeology is the study of how brittlely deformed rocks alter fluid flows in different lithological settings, such as clastic, igneous and carbonate rocks. Fluid movements, that can be quantified as permeability, can be facilitated or impeded due to the existence of a fault zone. This is because different mechanisms that deform rocks can alter porosity and permeability within a fault zone. Fluids involved in a fault system generally are groundwater and hydrocarbons.

References

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  2. 1 2 Sibson, R.H. (1977). "Fault rocks and fault mechanisms". Journal of the Geological Society. 133 (3): 191–213. Bibcode:1977JGSoc.133..191S. doi:10.1144/gsjgs.133.3.0191. S2CID   131446805 . Retrieved 2009-11-03.
  3. 1 2 Lin, A. (2007). Fossil earthquakes: the formation and preservation of Pseudotachylytes. Springer. p. 348. ISBN   978-3-540-74235-7 . Retrieved 2009-11-01.
  4. Sibson, R.H. (1986). "Brecciation processes in fault zones: Inferences from earthquake rupturing". Pure and Applied Geophysics. 1241 (1–2): 159–175. Bibcode:1986PApGe.124..159S. doi:10.1007/BF00875724. S2CID   129350442.
  5. Mort, K.; Woodcock N.H. (2008). "Quantifying fault breccia geometry: Dent Fault, NW England". Journal of Structural Geology. 30 (6): 701–709. Bibcode:2008JSG....30..701M. doi:10.1016/j.jsg.2008.02.005.
  6. Blenkinsop, T. (2000). Deformation Microstructures and Mechanisms. Springer. p. 150. Archived from the original on 2013-02-03. Retrieved 2009-11-03.
  7. Ismat, Z (2006). "Cataclastic flow: a means for ensuring ductility within the elastico-frictional regime" . Retrieved 2009-11-03.