A cataclastic rock is a type of fault rock that has been wholly or partly formed by the progressive fracturing and

comminution Comminution is the reduction of solid materials from one average particle size to a smaller average particle size, by crushing, grinding, cutting, vibrating, or other processes. In geology, it occurs naturally during faulting in the upper part of ...

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 An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or me ...

breccias.

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. Sibson's 1977 classification of fault rocks was the first to include an understanding of the

deformation mechanism A deformation mechanism, in geology, 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 th ...

s involved and all subsequent schemes have been based on this. Fault breccias have been further classified in terms of their origins; attrition, distributed crush and implosion brecciation, and, borrowing from the cave-collapse literature, crack, mosaic and chaotic from their clast concentration.

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 cl ...

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

Types

Cataclasite

Cataclasite is a fault rock that consists of angular

clast Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...

s in a finer-grained

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. It is normally non-foliated but some varieties have been described with a well-developed planar fabric that are known as ''foliated cataclasites''. 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 In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...

veins Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated b ...

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 Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay minerals ...

content are known as ''clay gouges''.

Pseudotachylite

Pseudotachylite is a fault rock that has the appearance of the

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) of ...

tachylyte Tachylite ( ; also spelled tachylyte) is a form of basaltic volcanic glass. This glass is formed naturally by the rapid cooling of molten basalt. It is a type of mafic igneous rock that is decomposable by acids and readily fusible. The color ...

. 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 A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...

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.

Cataclastic flow

Cataclastic flow is the main deformation mechanism accommodating large strains above the

brittle–ductile transition zone __NOTOC__ The brittle-ductile transition zone (hereafter the "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 c ...

. It can be regarded as a ductile mechanism, although one that takes place within the elastico-frictional regime of deformation. 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.

References

{{Structural geology Petrology Structural geology