EarthRef.org Reference Database (ERR) -- Smith & Lewis 1999

Uncertainties regarding the relative importance of basal drag and boundary forces in plate tectonic models are a consequence of flawed assumptions imposed by the use of the hotspot reference frame. Velocities of lithospheric plates are influenced not only by lateral boundary forces, but also by basal drag forces resulting from Earth rotation. Drag is exerted on the base of the asthenosphere and motion is transmitted upwards to the lithosphere. However, as the transmission of stress in the mantle is viscosity-dependent, the reduction in viscosity through the asthenosphere results in plates suffering a net westward lag. This 'differential rotation' effect causes continental plates to be more strongly coupled to the deep mantle as they are separated from the mesosphere by only relatively thin regions of asthenosphere. For such plates, the calculated drag forces are of the same order of magnitude as the boundary forces. Intraplate volcanism during continental rifting and in opening basins is related to transverse convection cells set up where topographic structures in the lithospheric root oppose mantle flow. The motion of oceanic plates is dominated by conventional plate boundary forces which may either reinforce or oppose drag from an eastward mantle flow. Reinforcement (e.g., Nazca plate) gives rise to Couette flow in the asthenosphere. Opposition (e.g., Pacific plate) results in counterflow. Shear stresses in both regimes are concentrated in the upper asthenosphere and lead to melting of concentrations of hydrous minerals ('wetspots') derived from eroded continental mantle introduced by lateral asthenospheric flow. Under a counterflow regime, melt collects in a stationary layer at shallow depth in the asthenosphere, at the crossover point between plate- and mesosphere-induced flow regimes. Release of melt to the surface is governed by lithospheric stress trajectories set up by convergence along plate boundaries. Intraplate volcanism thus has a common source although, due to different interactions between the boundary and drag forces, asthenosphere flow profiles will differ between basins, giving relative motions between melting anomalies of a few centimetres per year. The overall effect, however, is to give an illusion of a series of quasi-fixed melting anomalies, though in reality all these and the lithospheric plates are moving relative to the deep mantle. (C) 1999 Elsevier Science Ltd. All rights reserved.