A brecciated metamorphic rock frequently found in a fault zone. The fractured texture is thought to form by the crushing actions of fault movement. strongly deformed rock from a ductile shear zone, commonly with a planar foliation and usually with a stretching lineation. Evidence for high strain such as quartz ribbons and porphyroclasts in a more fine grained matrix are common. Mylonite was originally (Lapworth 1885) defined as a brittle fault rock, but mylonites are now thought to have formed predominantly by crystal-plastic flow of the matrix, although some minerals suspended in the matrix may show brittle fracturing.
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