Mylonites

Mylonites are a type of rock formed by intense ductile shearing at relatively shallow crustal depths. They are characterized by a fine-grained, foliated texture that reflects the severe deformation experienced during their formation. The original rock, often a granite, gneiss, or basalt, is mechanically ground down and recrystallized under high pressure and temperature. This process leads to the development of distinct mineral alignments and a layered appearance, where larger relict grains may be embedded within a matrix of finer, dynamically recrystallized minerals.

The foliation in mylonites is a key feature, typically oriented parallel to the shear zone. Within this foliation, specific mineral structures like mica fish or asymmetric porphyroclasts can indicate the direction of shear movement. Mylonites are commonly found in fault zones, particularly in the brittle-ductile transition region of the Earth's crust. Their presence signifies significant tectonic activity and deformation. They are distinct from cataclasts, which are formed by brittle fracturing at shallower depths. The study of mylonites provides crucial insights into the kinematics and mechanics of fault systems and the deformation history of the Earth's crust.