Some simple physical models for absolute plate motions

Abstract

Although the relative angular velocities of the earth's plates are well known, the velocities relative to the underlying mantle and the nature of the driving forces are not. We calculate several solutions to the ‘absolute’ velocity field from the hypothesis that no net torque is exerted on the lithosphere as a whole and a series of assumptions about the forces driving plate motions. The force models include viscous drag at the base of all plates (plus arbitrary forces at ridges and trenches that exert torques of equal magnitude but opposite sign on adjacent plates), drag concentrated beneath continents, drag concentrated to oppose the horizontal translation of sinking slabs, gravitational pull by sinking slabs, and linear combinations of these. These models generally give absolute plate velocities that are very similar to those calculated on the premise that a set of ‘hot spots’ provides a fixed reference frame. This removes some of the rationale for attributing to these hot spots any significant contribution to the driving forces. The similarity of plate motions inferred from markedly different driving mechanisms precludes using the velocity fields to discriminate among proposed models. Many of the hot spots may be simply due to intraplate tensional stress associated with the forces acting on present plate boundaries. This suggests that calculation of intraplate stress can ultimately serve to choose among force models.