Transient creep in the upper mantle

The rheological properties of the upper mantle are examined on the basis of recent advances in the study of the rheology of partially melted rocks. The presence of a few percent liquid phase in the lowvelocity zone, which is indicated but not required by the geophysical evidence, does not affect the long-term creep properties of the material. Thus, while partial melting in the upper mantle can easily account for a variety of geodynamic processes, it cannot alter the rheology which is in all likelihood non-Newtonian. This conclusion is only apparently at odds with the most commonly accepted interpretation of postglacial isostatic rebound, which is taken to be compatible with a Maxwellian earth with Newtonian viscosity. It is pointed out here that neither the total strain, nor the time since deglaciation are such that the widespread assumption of steady-state rheology is necessarily justified. Under the assumption that transient creep, and not steady-state creep, governs the behaviour of the upper mantle during postglacial rebound, the inferred transient-creep viscosities are shown to be of the expected order of magnitude and to grade into the usually accepted steady-state viscosities at larger strains