On the effect of temperature and strain-rate dependent viscosity on global mantle flow, net rotation, and plate-driving forces

Global circulation models are analysed using a temperature and strain-rate dependent rheology in order to refine previous estimates of the nature of mantle flow and plate driving forces. Based on temperature inferred from a tectonic model and seismic tomography, the suboceanic viscosity is lower than underneath continents by similar to one order of magnitude. If net-rotations of the lithosphere with respect to a stable lower mantle reference frame are accounted for, the patterns of flow in the upper mantle are similar between models with layered and those with laterally varying viscosity. The excited net rotations scale with the viscosity contrast of continental roots to the ambient mantle; this contrast is dynamically limited by the power-law rheology. Surface net rotations match the orientation of hotspot reference-frame Euler poles well, and amplitudes are of the right order of magnitude. I compare prescribed surface velocity models with free-slip computations with imposed weak zones at the plate boundaries; velocity fields are generally consistent. Models based on laboratory creep laws for dry olivine are shown to be compatible with average radial viscosity profiles, plate velocities in terms of orientation and amplitudes, plateness of surface velocities, toroidal:poloidal partitioning, and fabric anisotropy formation under dislocation creep in the upper mantle. Including temperature-dependent variations increases the relative speeds of oceanic versus continental lithosphere, makes surface velocities more plate-like, and improves the general fit to observed plate motions. These findings imply that plate-driving force studies which are based on simpler mantle rheologies may need to be revisited.