Mylonites

Mylonites are rocks that have undergone modification of original textures by predominantly plastic flow due to dynamic recystallization. The formation of mylonites occur at depth, below brittle faults, in continental and oceanic crust. Formation of mylonites occur in strike-slip, extensional or compressional regimes, and are characteristic of moderately high temperature deformation. Microstructures that form during mylonitization vary according to original mineralogy and modal compositions, temperatures, confining pressure, strain rate, applied stresses, and the presence or absence of fluids. Mylonites are foliated rock with strong lineation resulting from shear in a ductile fault shear zone. Mylonitization, the process of forming a mylonite generally consists of crystal plastic strain by dislocation climb and recovery. Mylonitization reduces the grain size of a protolith and commonly produces a very fine- grained, well foliated rock with a pronounced linear fabric defined by elongate minerals. Mylonites can be found in places call mylonite zones. These zones are located along major fault zones, and can range in lengths of hundreds of kilometers, and can also be several kilometers thick.

Vince Martin 2/21/97

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