Temperature, velocity and viscosity profiles for coupled thermal and mechanical models of the upper mantle beneath continental shields and old ocean basins show that under the continents, both the tectonic plate and the asthenosphere, are thicker than they are beneath the oceans. The minimum value of viscosity in the continental asthenosphere is about an order of magnitude larger than in the shear zone beneath oceans. The shear stress or drag underneath continental plates is also approximately an order of magnitude larger than the drag on oceanic plates. Effects of shear heating may account for flattening of ocean-floor topography and heat flux in old ocean basins.
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This work was partly supported by the Earth Sciences Section, National Science Foundation, NSF Grant GA 40749 and by the National Aeronautics and Space Administration, NSG 7002.