We have seen that significant forces are associated with the changes in potential energy, accompanying the generation and ageing ocean lithosphere and its subduction; forces which we refer to as ridge push (about 2-3 x 1012 N m-1 as seen by old ocean lithosphere) and slab pull (up to the order of 1013 N m-1). Similarly, potential energy variations associated with continental topography are capable of generating large buoyancy forces. In addition, the passage of the lithosphere over the deeper, hotter convective mantle, or, viceversa, the traction exerted by the convective flow on the base of the lithosphere, may act to either resist or drive plate motion (Fig 11.1). An important constraint on the way these tectonic forces interact is provided by the observation that individual plates are not accelerating. This requires that a basic force balance or, more strictly, a torque balance operates on individual plates. In the following discusion we consider the implications of this reuirement for torque balance for the driving mechanisms a number of the Earth's plates.
The notion that the plates are not appreciably accelerating implies a torque balance, which must reflect the interaction between forces which drive plate motion and those that resist plate motion. It seems reasonable (to me at least) to assume that the torque pole of the force and/or combintaion of forces driving plate motion is correlated with the velocity pole, and thus we should be able to identify the driving forces from the resistive forces, by comparison of torque poles and velocity poles.
There is substantial variation in the absolute velocity of the Earth's major plates which correlates to some degree with the gravitational torques acting on the plate ( see reprint Coblentz et al. On the Potential energy of the Earth's lithosphere)
Traditionally, the rifting that has occurred in the continental regions of both plates has been seen as the consequence of a process actively involving the sub-lithospheric mantle, such as the impingement of a plume on the base of the lithsophere (such as illustrated in Fig. 11.1). It is estimated that the imingement of a plume at the base of the lithdophere can jack the lithsophere upo by as much as 1-2 kms, with a corresponding increase in potential energy of up to 3 x 1012 N m-1. While this is undoubtedly an important process in generating extensional stress regimes in continents, another possibility is simply related to the way in which potential energy is distributed across the plate and how this potential energy distribution evolves in time as the oceans spread outward around the continent. To understand this it is useful idealise the African Plate as a completely circular plate which has grown uniformily with time (Fig.11.3), and imagine how the potential energy of the plate evolves with time. What is growing with time is largely the old ocean lithosphere which represents a potential energy low within the plate. Consequently, the mean plate potential energy must decline with time, and the difference between the continental potential energy and the mean platte potential energy becomes progressively greater producing significant tension in the most elevated parts of the continent.
For those interested more details can be found in the reprints Sandiford & Coblentz :Plate scale potential energy differences and the fragmentationof ageing plates), and Coblentz & Sandiford, Tectonic Stresses in the African Plate: Constraint on the Ambient Stress State.