Steady-state creep in the mantle

Abstract

SUMMARY - The creep equations for steady-state flow of olivine at high pressure and temperature are compared in an attempt to elucidate the rheological behaviour of the mantle. Results are presented in terms of applied deformation maps and curves of effective viscosity v depth. In the upper mantle, the transition stress between dislocation and diffusion creep is between 10 to 102 bar (as orders of magnitude) for grain sizes from 0.01 to 1 cm. The asthenosphere under continents is deeper, and has higher viscosity, than under oceans. Predominance of one creep mechanism above the others depends on grain size, strain rate, and volume fraction of melt; the rheological response can be different for different geodynamic processes. In the lower mantle, on the other hand, dislocation creep is predominant at all realistic grain sizes and strain rates. If the effective viscosity has to be only slightly higher than in the upper mantle, as some interpretations of glacioisostatic rebound suggest, then the activation volume cannot be larger than 11 cm3 mole^1.