Mylonite

Mylonite is a fine-grained metamorphic rock produced by intense ductile deformation and grain-size reduction in a shear zone. Its foliated fabric records the movement and strain that formed it.

Ductile shear zone: A region of rock that accommodates displacement through distributed ductile deformation. Mylonite forms as intense deformation concentrates within these zones.
Foliation (geology): A planar arrangement of minerals or compositional layers in a metamorphic rock. Mylonite commonly develops a strong planar fabric during shearing.
Ultramylonite: A mylonite in which the fine-grained matrix forms more than 90 percent of the rock. It represents an especially strong degree of grain-size reduction and matrix development.
Geologic strain: The change in shape or size of a rock body caused by deformation. Mylonite fabrics preserve evidence of the strain accumulated in a shear zone.
Fault rock: Rock transformed by deformation and displacement along a fault or shear zone. Mylonite is one member of the broader family of fault rocks.
Dynamic recrystallization: The formation of new mineral grains during deformation, driven by changes in grain boundaries and crystal defects. It commonly reduces grain size as mylonitic fabrics develop.
Lineation (geology): A linear fabric in rock produced by aligned minerals, elongated grains, or other structures. Stretching and mineral alignment can form lineations within mylonite.
Protomylonite: A weakly developed mylonite containing abundant relict, relatively undeformed grains. It captures an early or lower-intensity stage of mylonitic deformation.
Geothermobarometry: Methods for estimating the temperature and pressure at which a mineral assemblage formed or equilibrated. Mineral assemblages in mylonites can constrain conditions during deep deformation.
Mylonitization: The process of forming mylonite through intense ductile deformation and grain-size reduction. It names the deformation process that produces the rock.

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