Cataclasites

Cataclasite is a type of fault rock formed by cataclasis, the brittle fragmentation and grinding of rocks within a fault zone. It consists of angular to subangular clasts embedded in a fine-grained groundmass created by mechanical disintegration during deformation. The rock is typically interpreted as having undergone rapid brittle deformation at shallow crustal depths, with the texture ranging from matrix-supported to clast-supported depending on the relative abundance of groundmass and fragments.

Formation and setting: Cataclasites form under brittle conditions where high strain rates drive repeated fracturing and sliding along faults or shear zones. The process produces a mixture of crushed grains and rock fragments, commonly in a clay- to silt-sized groundmass. Fluids may contribute to cementation or mineral precipitation, strengthening the fault rock after fragmentation. Cataclasites are commonly associated with fault zones and record aspects of slip history and deformation temperature typical of brittle regimes.

Textures and mineralogy: The characteristic texture includes a mosaic of angular to subangular clasts set in a finer matrix of crushed grains. The mineral compatibility generally reflects the protolith, with quartz, feldspar, micas, and accessory minerals present in varying proportions. Some samples preserve porphyroclasts—larger original grains—that remain within the crushed groundmass. The degree of fragmentation and matrix development helps distinguish cataclasites from other breccia-type rocks.

Relation to other fault rocks: Cataclasites are distinct from breccias by their finer groundmass and pronounced fragmentation produced by cataclasis rather than simple cementation of large clasts. They differ from mylonites, which form under ductile, crystal-plastic deformation and exhibit foliated microstructures; cataclasites record brittle, shock-like processes in contrast to the ductile regime indicated by mylonites.

Significance: In structural geology, cataclasites indicate brittle deformation and provide constraints on fault mechanics, slip history, and stress conditions in fault zones. They are widely described in studies of orogenic belts, rift zones, and other tectonically active regions.