On the Relative Importance of the Driving Forces of Plate Motion

TLDR

Recent theories propose that lithospheric plate motions are driven by forces such as ridge push, slab pull, trench suction, and mantle flow coupling, and that plates are in dynamic equilibrium with negligible inertia. The study introduces an observational method to test these driving-force theories by determining the relative magnitudes of forces that minimize net torque on each plate. The authors solve the inverse problem using observed plate motions and geometries to estimate the relative strengths of the plausible driving forces that minimize each plate’s net torque. The analysis shows that slab pull dominates plate motion, being an order of magnitude stronger than other forces, with oceanic plates reaching a terminal velocity where slab pull balances resistance, and that bottom drag is greater beneath continents than oceans.

Abstract

A number of possible mechanisms have recently been proposed for driving the motions of the lithospheric plates, such as pushing from mid-ocean ridges, pulling by downgoing slabs, suction toward trenches, and coupling of the plates to flow in the mantle. We advance a new observational method of testing these theories of the driving mechanism. Our basic approach is to solve the inverse problem of determining the relative strength of the plausible driving forces, given the observed motions and geometries of the lithospheric plates. Since the inertia of the plates is negligible, each plate must be in dynamic equilibrium, so that the sum of the torques acting on a plate must be zero. Thus, our problem is to determine the relative sizes of the forces that minimize the components of net torque on each plate. The results indicate that the forces acting on the downgoing slab control the velocity of the oceanic plates and are an order of magnitude stronger than any other force. Namely, all the oceanic plates attached to substantial amounts of downgoing slabs move with a 'terminal velocity' at which the gravitational body force pulling the slabs downward is nearly balanced with the resistance acting on the slab; regardless of the other features of the trailing horizontal part of the plates. The drag on the bottom of the plates which resist motion is stronger under the continents than under the oceans.