DEFINITION OF CATACLASIS

A process of rock deformation involving the

breaking and grinding of preexisting rocks. It amounts to a

macroscopic ductile flow produced by grain-scale fracturing,

rotation, and sliding of grains to produce smaller fragments.

It is typically confined to narrow, tabular-shaped zones

known as shear zones or faults, in which the original wall

rocks are crushed and ground up to produce fault rocks. Cataclasis

or cataclastic flow occurs under relatively low temperature

and low to moderate confining pressure conditions in

the Earth’s crust. The range of cataclastic deformation may

be extended to high pressure and temperature conditions by

increasing the strain rate, or rate of deformation. High fluid

pressures in a deforming rock may aid the cataclastic deformation

processes. Cataclasites are distinctive types of fault

rocks. Included are coarse-grained fault breccias with visible

fragments, fine-grained fault gouge, microbreccias, fractures

and microfractures, and even frictional melts known as pseudotachylites.

Cataclastic flow may increase the volume of a

deforming rock mass since the fracturing creates new open

void space. Fluids that flow in the faults commonly deposit

vein minerals that often become fractures themselves during

continued deformation.

Most cataclastic deformation occurs within a few kilometers

of the Earth’s surface. However, some minerals behave

brittlely under higher temperature conditions and may fracture

and deform cataclastically when other minerals are

deforming ductily. For instance, feldspar, garnet, and a few

other minerals may break and fracture at relatively high temperatures,

typically 930°F–1,830°F (500°C–1,000°C),

depending on the strain rate. Most other common minerals,

including quartz and calcite, stop behaving brittlely and

deform ductily at 570°F–1,470°F (300°C–800°C). It is often

possible to estimate the pressure and temperature conditions

of deformation by determining which minerals deformed cataclastically

and which minerals deformed ductily.

See also DEFORMATION OF ROCKS; STRUCTURAL GEOLOGY.

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