Steady-state creep in the mantle

Abstract

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