Dislocations and nonelastic processes in the mantle

Dislocations in solids contribute to anelastic attenuation, relaxation of the shear modulus, transient creep, and steady state flow. These properties of the mantle may therefore be related. The glide and climb of dislocations appear to have the appropriate time constants to explain seismic wave attenuation and mantle viscosity, respectively. The dislocation density of the mantle depends on the ambient stress. The characteristic time scales of dislocation relaxation depend on dislocation length and temperature. These time scales for the mantle can be inferred from seismic wave attenuation and postglacial rebound, thereby potentially yielding information about dislocation density, stress, and temperature. The thickness of the 'rheological' lithosphere depends on stress and duration of load as well as age. Kilobar level stresses can be supported in the lithosphere for times greater than 10 • years. The relaxation time decreases rapidly with temperature. The asthenosphere can therefore only support small stresses on time scales of geological interest.