Mylonite is a fine-grained, compact rock produced by dynamic crystallization of the constituent minerals resulting in a reduction of the grain size of the rock. It is classified as a metamorphic rock. Mylonites can have many different mineralogical compositions; it is a classification based on the textural appearance of the rock.
There are many different processes that control crystal-plastic deformation. In crustal rocks the most important processes are dislocation glide, dislocation creep and pressure solution. Volume and surface diffusion are important ductile deformation mechanisms at high metamorphic grades, particularly if the grain size is small. Dislocation glide and dislocation creep both act to increase the internal energy of crystals. This effect is compensated through recrystallization which reduces the internal energy by increasing the surface area and reducing the volume, storing energy at the mineral grain surface. Thus mylonites, which are characterized by small grain sizes relative to surrounding rocks, are interpreted to result from extensive ductile deformation.
Kinematic indicators are structures in mylonite that allow the sense of shear to be determined. Most kinematic indicators are based on deformation in simple shear zones and infer sense of rotation of the finite strain axis with respect to the incremental strain axis. Because of the constraints imposed by simple shear, displacement is assumed to occur in the foliation plane in a direction parallel to the mineral stretching lineation. Therefore a plane parallel to the lineation and perpendicular to the foliation is viewed to determine the shear sense.
The most common shear sense indicators are C/S fabrics, asymmetric porphyroclasts, vein and dike arrays, mantled porphyroclasts and mineral fibers. All of these indicators have a monoclinic symmetry which is directly related to the orientations of the finite strain axis. Although structures like asymmetric folds and boudins are also related to the orientations of the finite strain axis, these structures can form from distinct strain paths and are not reliable kinematic indicators.