The effect of rheological parameters on plate behaviour in a self-consistent model of mantle convection

In a three-dimensional numerical model of mantle convection we have explored the self-consistent formation and evolution of plates. This was done by investigating the influence of temperature, stress and pressure dependence of the viscosity on plate-like behaviour. Further, the role of a depth-dependent thermal expansivity and of internal heating was examined. First-order self-consistent plate tectonics can be obtained for temperature- and stress-dependent viscosity: we observe virtually undeformed moving plates and the phenomenon of trench migration. However, plate motion only appears during short intervals, which are interrupted by long phases characterised by an immobile lid. An improvement is achieved by adding pressure dependence of the viscosity and of the thermal expansivity to the model. This leads to extended plates with continuous plate motion and the appearance of a low-viscosity zone. Internal heating accelerates subduction and gives an increase in plate velocity-the plates are otherwise left intact. We find that the surface velocity is largely determined by subduction rather than by upwellings. Plate-like behaviour is restricted to a narrow parameter window. The window is bordered by an episodic or a stagnant lid type of flow on the one side and by a domain in which convection is akin to constant viscosity flow on the other side. With these different convective regimes the behaviour of various terrestrial planets can be described self-consistently. Occasionally even a change from the plate regime to the stagnant lid regime occurs. Such a type of transition has possibly changed the style of tectonics on some terrestrial planets, Such as Mars. (C) 2004 Elsevier B.V. All rights reserved.