mylonite

An amphibolitic mylonite showing a number of (rotated) porphyroclasts: a clear red garnet left in the picture while smaller white feldspar porphyroclasts can be found all over. Location: the tectonic contact between the (autochthonous) Western Gneiss Region and rocks of the (allochthonous) Blåhø nappe on Otrøy, Caledonides, Central Norway.

A mylonite (through a petrographic microscope) showing rotated so-called δ-clasts. The clasts show that the shear was dextral in this particular cut. Strona-Cenery zone, Southern Alps, Italy.

Mylonite, Owl Mountains, Poland

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.

Formation

Mylonite is a ductilely deformed rock formed by the accumulation of large shear strain, in ductile fault zones. 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, which was named from the Greek μύλος mylos, meaning mill.^[1]

There are many different mechanisms that accommodate crystal-plastic deformation. In crustal rocks the most important processes are dislocation creep and diffusive mass transfer. 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. They are the deep counterparts to cataclastic brittle faults that create fault breccias.^[2]

Classification

• Blastomylonites are coarse grained, often sugary in appearance without distinct tectonic banding.
• Ultramylonites have undergone extreme grainsize reduction. They are hard, dark, cherty to flinty in appearance and may be confused with pseudotachylite and obsidian.
• Phyllonites are phyllosilicate(e.g. chlorite or mica)-rich mylonites. They typically have a well-developed secondary shear (C') fabric.

Interpretation

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. 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. 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.

References