Dynamic (cataclastic) metamorphism is metamorphism of rock masses caused primarily by stresses that yield relatively high strain (deformation) rates. More simply, it is metamorphism resulting from deformation. The deformation may be dominantly brittle, in which case rock and mineral grains are broken and crushed, or it may be dominantly ductile, in which case plastic behavior and flow occur via structural changes within and between grains.' Temperatures during dynamic metamorphism are typically elevated and may be caused by the deformation process. Fluids commonly contribute to the metamorphic process, both by altering chemistry and by aiding recrystallization.

Both local and regional dynamic metamorphism are recognized. At the local scale, in narrow zones from less than 1 cm to several meters wide, brittle or ductile deformation along faults and fold limbs causes rock to break, recrystallize, and even to melt. Similarly, both brittle and ductile deformation, as well as melting, occur during impacts of extraterrestrial bodies. Brittle and ductile deformation processes also operate at the regional scale.

The rocks produced at all scales by dynamic metamorphism are rocks composed of fragments of preexisting material (porphyroclasts), surrounded by a deformed matrix, the texture or mineral composition of which was produced by metamorphic processes. Such rocks, which fit into the broad category of clastic rocks, referred to as dynamoblastic rocks.

Occurrences of Dynoblastic Rocks

Faults are common within the crust of the Earth. Since faults are deformation zones, dynamoblastic rocks associated with faults are a common feature. In addition, folds and related deformation zones are relatively common in the roots of mountain belts. Even in zones in which newly formed rocks are only partially lithified, for example, in soft sediments on the seafloor, deformation may yield dynamically metamorphosed rocks. Particularly noteworthy among the local- to regional-scale zones of dynamoblastic rock are the mylonite zones associated with metamorphic core complexes and the melanges of outer metamorphic belts. Melanges are, in fact, mappable masses of dynamoblastic rock of local to regional dimensions. Impact structures with dynamoblasric rocks include Meteor Crater in Arizona and the Ries Basin of Germany.

Regional zones of dynamoblastic rocks occur at plate boundaries. Along spreading ridges, regional stress may be widespread enough to yield dynamically metamorphosed zones of rock. Perhaps more commonly, ductile deformation is concentrated in narrow zones within a regional terrane of schistose ultramafic rocks. Most local and regional zones of this type are probably subducted and are not preserved. Nevertheless, evidence of their existence is preserved locally in mantle slabs of accreted ophiolites. More commonly, oceanic crustal rocks are deformed along transform faults. Examples of rocks deformed in this way are exposed in the Sierra Nevada of California, in northern Italy, and on the island of Cyprus. Exposures of transform faults that transect the continents also reveal brittly and ductily deformed rocks, such as those along the San Andreas Fault System in California.