On the rheology of the upper mantle

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Abstract
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 (Fo85‐Fo95) 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 × 104. 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.

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