crenulations

Crenulations are a texture observed in metamorphic rocks characterized by fine-scale, scalloped corrugations on foliation surfaces or along a cleavage plane. In structural geology they are most often described as crenulated cleavage, a micro-scale pattern that forms when a preexisting planar fabric is overprinted by a younger, more finely spaced cleavage under shear. The resulting surface shows alternating ridges and troughs, giving a wrinkled, crenellated appearance. The crenulations typically dip at a shallow angle to the main fabric and indicate deformation by simple or pure shear during progressive tectonic work.

Formation and characteristics: Crenulations develop during progressive deformation when mineral grains rotate, recrystallize, or reorient to accommodate shear. The texture is most conspicuous in pelitic rocks, quartzites, and marbles subjected to moderate to high strain, though it can occur in a range of metamorphic rocks. Scale is usually millimetres to centimetres; the ridges tend to be curved rather than perfectly straight, producing the characteristic "crenulated" look.

Significance: The presence of crenulations provides information about the deformation history and kinematics of the rock, including the direction and sense of shear and the timing of overprinting fabrics. They are diagnostic of late-stage deformation features in many metamorphic terrains and help distinguish between different generations of cleavage and layering.

See also: Cleavage (geology), Foliation, Shear zone, Deformation fabric, Mylonite.