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 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 ...

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 ...

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