Implications of absolute plate motions and intraplate stress for mantle rheology

Recent tests of driving force models for plate tectonics include comparison of predicted absolute plate motions with those allowed by paleomagnetism and comparison of the directions of principal deviatoric stresses predicted within the lithosphere with those inferred from midplate earthquake mechanisms and in-situ measurements. These tests are extended in this paper to possible non-linear relationships between plate velocity and shear stress at the base of the plate, including the relationships expected if dislocation creep is the rate-limiting flow mechanism in the mantle and if viscous heating and a temperature-dependent viscosity control the shear stress. Neither absolute plate velocities nor intraplate stress can be used to distinguish linear viscosity in the asthenosphere from a rheology appropriate to dislocation creep. Absolute plate motions for effective rheologies controlled by shear heating at the base of the lithosphere are in conflict with paleomagnetic limits on net polar wander, thereby restricting viscous heating as a regulator of shear stress to at most the oldest oceanic and continental lithosphere.