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. In Geology, rock is a naturally occurring aggregate of Minerals and/or Mineraloids The Earth's outer solid layer the ‘ Lithosphere It is classified as a metamorphic rock. Metamorphic rock is the result of the transformation of an existing rock type the protolith, in a process called Metamorphism, which means "change Mylonites can have many different mineralogical compositions; it is a classification based on the textural appearance of the rock. Mineralogy is an Earth Science focused around the Chemistry, Crystal structure, and physical (including optical) properties of Minerals
Mylonite is a ductilely deformed rock formed by the accumulation of large shear strain, in ductile fault zones. Ductility is a mechanical property used to describe the extent to which materials can be deformed plastically or "stretched" into "wires" without Shear strain is a strain that acts parallel to the face of a material that it is acting on In Geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement There are many different views on the formation of mylonite, 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 mylonite. Mechanical abrasion of grains by milling does not occur, although this was originally thought to be the process that formed mylonite.
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. Creep is the tendency of a solid material to slowly move or deform permanently under the influence of stresses It occurs as a result of long term exposure to levels of Pressure solution or pressure dissolution in Structural geology and Diagenesis is a Deformation mechanism that involves the dissolution 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.
Mylonites generally develop in ductile shear zones where high rates of strain are focused. Ductility is a mechanical property used to describe the extent to which materials can be deformed plastically or "stretched" into "wires" without They are the deep counterparts to cataclastic brittle faults that create fault breccias. Cataclasite is a metamorphic rock that is formed by mechanical Shear stress during faulting. In Geology a fault, or fault line, is a planar rock fracture which shows evidence of relative movement Breccia (ˈbrɛtʃiə ˈbrɛʃiə breach is a rock composed of angular fragments of several Minerals or rocks in a matrix, that is a cementing material
Determining the displacements that occur in mylonite zones is dependent on correctly determining the orientations of the finite strain axis and inferring how they change their orientation 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 is a plane strain simple shear deformation. Simple Shear is a special case of Deformation of a fluid where only one component of Velocity vectors has a non-zero value \ V_x=f(xy 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 axis are initially parallel to the incremental axis, but rotate away during progressive 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. In Mathematics, a foliation is a geometric device used to study manifolds Informally speaking a foliation is a kind of "clothing" worn on a manifold 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. This article is about the Sausages For the Geological feature see Boudinage.