Mylonite is a fine-grained, compactmetamorphic rock produced bydynamic recrystallization of the constituentminerals resulting in a reduction of the grain size of the rock. Mylonites can have many differentmineralogical compositions; it is a classification based on thetextural appearance of the rock.
Mylonites areductilely deformed rocks formed by the accumulation of largeshear strain, in ductilefault zones. There are many different views on the formation of mylonites, but it is generally agreed that crystal-plastic deformation must have occurred, and that fracturing and cataclastic flow are secondary processes in the formation of mylonites. Mechanical abrasion of grains by milling does not occur, although this was originally thought to be the process that formed mylonites, which were named from theGreek μύλοςmylos, meaning mill.[1] Mylonites form at depths of no less than 4 km.[2]
There are many differentmechanisms that accommodate crystal-plastic deformation. In crustal rocks the most important processes aredislocation creep anddiffusion creep. Dislocation generation acts to increase the internal energy of crystals. This effect is compensated through grain-boundary-migration recrystallization which reduces the internal energy by increasing the grain boundary area and reducing the grain volume, storing energy at the mineral grain surface. This process tends to organize dislocations intosubgrain boundaries. As more dislocations are added to subgrain boundaries, the misorientation across that subgrain boundary will increase until the boundary becomes ahigh-angle boundary and the subgrain effectively becomes a new grain. This process, sometimes referred to assubgrain rotation recrystallization,[3] acts to reduce the mean grain size. Volume and grain-boundary diffusion, the critical mechanisms in diffusion creep, become important at high temperatures and small grain sizes. Thus some researchers have argued that as mylonites are formed by dislocation creep and dynamic recrystallization, a transition to diffusion creep can occur once the grain size is reduced sufficiently.
Mylonites generally develop in ductile shear zones where high rates ofstrain are focused. They are the deep crustal counterparts tocataclastic brittlefaults that create faultbreccias.[4]
Determining the displacements that occur in mylonite zones depends on correctly determining the orientations of the finitestrain axis and inferring how those orientations change with respect to the incremental strain axis. This is referred to as determining the shear sense. It is common practice to assume that the deformation isplane strainsimple shear deformation. This type of strain field assumes that deformation occurs in a tabular zone where displacement is parallel to the shear zone boundary. Furthermore, during deformation the incremental strain axis maintains a 45-degree angle to the shear zone boundary. The finite strain axes are initially parallel to the incremental axis, but rotate away during progressive deformation.
Kinematic indicators are structures in mylonites that allow the sense of shear to be determined. Most kinematic indicators are based on deformation in simple shear and infer sense of rotation of the finite strain axes with respect to the incremental strain axes. Because of the constraints imposed by simple shear, displacement is assumed to occur in thefoliation 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 axes. Although structures like asymmetric folds andboudinages are also related to the orientations of the finite strain axes, these structures can form from distinct strain paths and are not reliable kinematic indicators.
