The rheological properties of the upper mantle for geotherms representative of shield and Cordilleran regions are studied on the basis of olivine microrheology. The equation for power-law creep, where the strain rate is dependent upon the diffusion-controlled climb of edge dislocations, is found to yield realistic values of creep strength and viscosity, when the experimentally determined parameters for dry olivine are used. For grain sizes larger than 0.01 cm, power-law creep is predominant over linear creep in the upper mantle, but the increasing importance of grain-boundary diffusion with decreasing depth makes linear creep a very likely deformation mechanism in the upper lithosphere.The differences between shield and Cordilleran geotherms are reflected in lateral variations in the rheological properties of the lithosphere and upper mantle. The lithosphere is thinner, and the upper mantle softer, in the Cordilleran region. The lateral variations in effective viscosity in the 100–200 km depth range are between one and two orders of magnitude.