Localization of flow in natural quartzo-feldspathic shear zones at granulite-facies temperatures does not occur purely by dislocation creep and dynamic recrystallization. From a review of some natural shear zones, initial faulting focused along a pre-existing shear zone, and/or the boundary of a volume of relatively strong crust or lithosphere, guides emplacement of hot mantle-derived melts into the base of the crust, leading to extensive melting. When crystallized, the sub-solidus plutons represent a focused zone of thermally softened material, capable of deforming readily and developing the full range of crystal-plastic microstructures and fabrics associated with mylonites, contrasting with the intrusion-free wall rocks.

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