Mantle plume heads and the initiation of plate tectonic reorganizations

We present results of numerical simulations which examine the plausibility of a reorganization of plate motions brought about by the interaction of a hot, low-viscosity plume head with the underside of a tectonic plate. A numerical model of highly viscous fluid flow driven by thermal buoyancy is employed to examine the interaction of mantle plumes with surface plates in a two-dimensional cartesian geometry. Plate-like boundary conditions applied to the upper surface consist of a large active plate with fixed speed plus a smaller passive plate whose speed and direction of motion are permitted to react to tractions induced by the underlying mantle flow (driven by the active plate). The passive plate is also subject to applied end-loads intended to simulate incipient subduction. The mantle plume is initiated with a hot patch on the lower boundary, and viscosity varies with temperature following an Arrhenius rheology. The calculations, performed with fixed aspect ratio (4 × 1) and fixed Rayleigh number (Ra = 106, where Ra refers to the strength of the starting plume), were found to depend on three parameters: the strength of the viscosity variation, the size of the applied end-load, and the speed of the active plate. These calculations indicate that plate motions can be sharply influenced by the lubricating effects of mantle plume heads even to the point of causing a reversal in the direction of plate motion.