Mylonite

A fine grained, compact metamorphic rock produced by the dynamic recrystallization of its constituent minerals, a process that reduces the grain size of the original rock; because mylonite is defined by texture rather than by mineral content, it can form from rock of widely varying composition. It develops through ductile deformation within fault zones that have accumulated intense shear strain, at depths of at least 4 kilometers, through crystal-plastic processes, particularly dislocation creep and diffusion creep, in which dislocations organize into subgrain boundaries that progressively rotate into new, smaller grain boundaries, rather than through simple mechanical grinding despite the rock's name coming from the Greek word for mill. Mylonites generally develop in ductile shear zones where strain is strongly concentrated, and they represent the deep crustal equivalent of the brittle fault breccias that form at shallower depths, marking zones of significant tectonic deformation. Depending on how much the grain size has been reduced, mylonites can display foliation, rotated mineral fragments called porphyroclasts, and various banding patterns that record the history of shearing the rock has undergone.