Mesoplates

Abstract: THE concept of crustal plate motion over mantle hotspots has been advanced1 to explain the origin of the Hawaiian and other island chains and the origin of the Walvis, Iceland-Farroe and other aseismic ridges. More recently the pattern of the aseismic ridges has been used in formulating continental reconstructions2. I have shown3 that the Hawaiian-Emperor, Tuamotu-Line and Austral-Gilbert-Marshall island chains can be generated by the motion of a rigid Pacific plate rotating over three fixed hotspots. The motion deduced for the Pacific plate agrees with the palaeomagnetic studies of seamounts4. It has also been found that the relative plate motions deduced from fault strikes and spreading rates agree with the concept of rigid plates moving over fixed hotspots. Fig. 1 shows the absolute motion of the plates over the mantle, a synthesis which satisfies the relative motion data and quite accurately predicts the trends of the island chains and aseismic ridges away from hotspots.

Abstract: Summary 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.