On the rheology of the upper mantle

Mechanisms permitting the steady state deformation of crystalline solids are critically reviewed, and an approximate constitutive relationship is derived for fluid phase transport in a partial melt. (Fluid phase transport has a linear stress dependence and an inverse squared grain size dependence.) A set of rheologic material constants for olivine (Fo 85 ‐Fo 95 ) are derived from a combination of experimental data and empirical generalizations. Our preferred power law exponent n is 4.2, and the associated Dorn parameter is 1.2 × 10 4 . Dislocation creep below the high‐stress breakdown is the dominant deformation mechanism in the upper mantle. The possibility exists that the bottom of the upper mantle is not deforming at a significant rate (≥10 −14 /sec) if the activation volume for diffusion is greater than 40 cm³/mole.