Continuous deformation can result from elementary mechanisms taking place at the scale of a single crystal. Under directed (deviatoric) external stress, a single crystal can deform by gliding upon reticular planes, this is plastic deformation in a strict sense, by directional diffusion of atoms through the crystal lattice or along its faces, or by direct crystallization of a new phase at the expense of an older one, this is syntectonic crystallization. Deformation by gliding and by diffusion can take place entirely within the solid state and without a change in volume: this is plastic deformation in the most general sense. Diffusion and new crystallization are however favoured by the presence of fluids which can thus enhance the ductility of the medium. Although they are sometimes associated, plastic deformation in the solid state and fluid-assisted deformation differ by their mechanism and are generally found operating in distinct mineralogical assemblages and geological situations. Thus deformation aided by fluids is above all found during prograde metamorphism, that is to say metamorphism of increasing grade in relation to the release of fluids accompanying the dehydrating reactions.
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