Cataclasite

Cataclasite under a petrographic microscope. The rock in part of the Malm-formation of the Helvetic nappes; sample found in a landslide near Engelberg in the Swiss Alps.

Cataclasite is a metamorphic rock that is formed by mechanical shear stress during faulting . It is either incohesive or cohesive with poor schistosity. It is usually non-foliated and consists of angular clasts in a finer-grained matrix.

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Formation

Cataclasite forms by the progressive fracturing of mineral grains and aggregates, a process known as cataclasis. The fracturing continues until a distribution of clast sizes is developed that allows the sliding of clasts past each other, without high enough frictional stresses to further fracture the rock significantly. From then on deformation is accommodated by continued sliding and rolling of fragments, a deformation mechanism known as cataclastic flow. In poorly consolidated or unconsolidated sediments, deformation in fault zones also occurs by rolling and sliding of grains but the porosity of the rock accommodates the resulting strains without significant grain fracturing.