Mylonite

Mylonite is a fine-grained and compact metamorphic rock. It is created through a process called dynamic recrystallization. This process changes the minerals that make up the rock. As these minerals change, their grain size becomes much smaller.

Scientists classify mylonite based on its textural appearance. This means they group them by how the grains look and are arranged. Mylonite is important because it reveals how the Earth's crust moves and shifts deep underground.

These rocks form in ductile fault zones. In these zones, the rock undergoes large shear strain. This means the rock is stretched and pushed in a flowing way.

This flowing movement is called ductile deformation. Mylonites form at depths of no less than 4 km. They are the deep counterparts to brittle faults. Brittle faults create rocks called fault breccias near the surface. In contrast, mylonites show how rock behaves under intense heat and pressure.

Crystal-plastic deformation is the main way mylonites form. This involves two important processes: dislocation creep and diffusion creep. Dislocation creep increases the internal energy of the crystals. To fix this, the rock uses grain-boundary-migration recrystallization. This reduces internal energy by increasing the grain boundary area. It also reduces the grain volume by storing energy at the mineral grain surface.

This helps organize dislocations into subgrain boundaries. As more dislocations add up, these boundaries become high-angle boundaries. This turns a subgrain into a brand-new grain through subgrain rotation recrystallization. This specific process reduces the mean grain size. At high temperatures and small grain sizes, diffusion creep becomes important. This happens through volume and grain-boundary diffusion.

Geologists classify mylonites into several distinct types. Protomylonites have experienced limited grain-size reduction. In these rocks, the matrix grains make up less than 50% of the total. Because the process is incomplete, you can still see old grain textures.

Mesomylonites have undergone more change. Their matrix grains make up between 50% and 90% of the rock. Ultramylonites have undergone extreme grain-size reduction. These rocks consist of more than 90% matrix grains. Ultramylonite is often hard, dark, and looks like flint or cherty stone.

Some mylonites are called phyllonites. These are rich in phyllosilicate minerals like mica or chlorite. They usually have a well-developed secondary shear fabric.

Blastomylonites are another specific type of mylonite. They are coarse-grained and often look sugary. They do not have distinct tectonic banding. The history of the name mylonite is quite interesting. The name comes from the Greek word "mylos," which means "mill."

Originally, people thought the grains were formed by mechanical abrasion. They thought the grains were milled down like flour. We now know this is not true. Instead, the rock changes through the internal crystal processes described earlier.

Determining the movement of these rocks is a major task for geologists. This is called determining the shear sense. They look for structures called kinematic indicators to find the direction of movement.

Scientists often assume the deformation is plane strain simple shear deformation. This means the movement happens in a flat zone. In this model, the displacement is parallel to the shear zone boundary. The incremental strain axis stays at a 45-degree angle to the boundary. Over time, the finite strain axes rotate away from this axis.

Common kinematic indicators include C/S fabrics and asymmetric porphyroclasts. Other indicators are vein and dike arrays, mantled porphyroclasts, and mineral fibers. These indicators have a monoclinic symmetry. This symmetry is directly related to the orientation of the finite strain axes. While structures like asymmetric folds exist, they are not considered reliable kinematic indicators. They can form from different strain paths. By studying these specific markers, scientists can map the complex history of the Earth's crust.