On the Driving Forces of Plate Tectonics

Abstract

We suppose that the plates are pulled along on top of an effectively viscous asthenosphere by their cold dense sinking leading edges, and that they also tend to slide down the flanks of ocean ridge systems. Using reasonable literature values of density, viscosity and thickness, we find that a typical strong subduction zone pulls about seven times as hard as a typical mid-ocean ridge pushes. With the simplifying assumptions that other driving forces are much smaller, and the return current in the asthenosphere is everywhere (anti)parallel to the plate velocity, we perform a torque balance for each major plate in order to find its angular velocity, thus finding a set of relative angular velocities to compare with observations. The directions fit quite well but not the magnitudes. On the additional hypothesis that oceanic lithosphere thickens with age, owing to heat diffusion, the driving forces will be greater for large old plates. The fit is thereby greatly improved: predicted speeds are from 0.81 to 1.38 times the observed and the Cocos plate is not anomalous. The model predicts tension in plates near their subduction zones. The Red Sea and African rift appear to have begun spreading as a crack moved into the former African + Arabian plate from a re-entrant corner off the Gulf of Aden which would have concentrated the stress. Gondwanaland may possibly have been split in a similar way.