Plate motions and the viscosity structure of the mantle — Insights from numerical modelling

The viscosity structure of the Earthʹs mantle is likely to play an important role with respect to the motion of the lithospheric plates. A zone of low viscosity in the upper mantle, as postulated by seismological studies and also by investigations of postglacial uplift, has been proposed to facilitate plate motion. Another inferred feature of the viscosity profile in the mantle is a significant increase at greater depth. We have employed a three-dimensional mantle convection model to explore the relation between the appearance of plate motion and the viscosity structure of the mantle beneath. The model allows for a complex fluid rheology (strong temperature, pressure and stress dependence of the viscosity) and can so account for the self-consistent formation of plates at the surface of the convecting mantle. Plate-like motion appears only in a limited parameter range, and we demonstrate that this appearance is closely linked to a specific viscosity structure. The viscosity is characterized by a local minimum at shallow depth (low viscosity zone) and by a local maximum at greater depth. Self-consistent plate motion is only observed if such a viscosity structure exists. In order to determine the functional relationship between the viscosity structure and plate motion, we also employ an approach, in which a certain viscosity profile is prescribed. These studies again reveal the importance of the local extrema in the viscosity profile. Taking into account an increased interior viscosity (also obtained by the pressure dependence), we find stable plate motion. As a general result we observe that the balance of the temperature and stress dependence of the viscosity is necessary to obtain plate motion. At the same time we find a viscosity profile showing a minimum at shallow depth and a maximum at greater depth.