Linear or non-linear rheology in the Earth's mantle: the prevalence of power-law creep in the postglacial isostatic readjustment of Laurentia

The great majority of postglacial rebound computations carried out during the ast three decades assumed a purely linear rheological relation for the mantle. Experimental data on high-temperature creep deformation and modelling of other tectonic processes, however, might also support the existence of non-linear creep mechanisms. We addressed postglacial rebound in North America through an axially symmetric finite-element model with a compos te (linear or plus non-linear) mantle rheology. In such a formulation, the transition stress sigma(T) governs the balance between linear and non-linear creep components, while the term sigma(B), added to the effective shear stress, accounts for the background (ambient) stress induced by convection and other tectonic processes. By varying sigma(T) and sigma(B) in the ranges 0-10 MPa and 0-5 MPa respectively, we found that composite models fit Relative Sea Level (RSL) variations at 29 North American sites better than the purely linear model. On the basis of the effective shear stress induced in the mantle by glacial forcing (1-3 MPa), our results indicate that power-law creep accounts for the majority of the strain rate.