Mylonite: The deep-crustal rock forged by intense shearing, not mechanical milling

Nature · Published 3 October 2026 · Updated 25 October 2026 Swipe more like this →

In short: Contrary to its name, which implies a milling process, mylonite is not ground down by friction. Instead, these fine-grained rocks are the result of ductile deformation deep within the Earth. They act as a record of intense shear, revealing the hidden movements of the crust through their unique internal structures.

Mylonite is a fine-grained, compact metamorphic rock that forms at depths of at least 4 km. While the name originates from the Greek word for mill, suggesting mechanical abrasion, geologists now understand that mylonites are created through crystal-plastic deformation. In these ductile fault zones, high rates of strain are focused, causing the rock to recrystallize rather than simply shatter.

The process relies on mechanisms like dislocation creep, where internal energy within crystals increases, prompting the formation of subgrain boundaries. Through subgrain rotation recrystallization, these boundaries eventually become high-angle, effectively creating new, smaller grains. Some researchers suggest that once grain size is sufficiently reduced, the rock may transition to diffusion creep, where volume and grain-boundary diffusion become the dominant drivers of change.

Geologists classify mylonites based on the percentage of matrix grains present. Protomylonites contain less than 50% matrix, mesomylonites range between 50% and 90%, and ultramylonites exceed 90%. Ultramylonites are particularly distinct, often appearing dark and flinty, sometimes resembling obsidian or pseudotachylite. Phyllonites, by contrast, are defined by their high content of phyllosilicates like mica or chlorite.

To understand the history of a shear zone, scientists analyze kinematic indicators—structures like asymmetric porphyroclasts or C/S fabrics—that reveal the direction of movement. By assuming plane strain simple shear, researchers can infer how the finite strain axes rotated during deformation. These indicators are essential for interpreting the complex, deep-seated tectonic processes that shape the crust, serving as the ductile counterparts to the brittle fault breccias found closer to the surface.

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