Numerical modelling of thermochemically driven fluid flow with non-Newtonian rheology : applied to the earth's lithosphere and mantle

Abstract

In the 25 years after the general acceptance of the concept of plate tectonics we have witnessed large progress in observational, laboratory, forward modelling and inversion techniques. These provide a clear view of the immense complexities that are facing us when studying the dynamics of the interior of the Earth. Plate tectonics can be seen as both an expression of, and the mechanism controlling, the dynamic cooling of the Earth. Traditionally, the soloistic and often opposing, simplifying views have been adopted of either the 'convectionist', who sees plate tectonics merely as the surface expression of mantle convection, or the 'tectonist', who views the plates as the only dynamic component in an otherwise passive mantle. It clearly emerges from the observational data, knowledge of deformation mechanisms, and the available modelling and inversion results, that a uniform approach, combining the two views would be more appropriate to describe the dynamics of the Earth. The generalized description is made difficult by the very distinct nature of the lithosphere, as expressed by the thermal, compositional and rheological differences from the underlying mantle. In this thesis I will present some model studies of the deformation of lithosphere and mantle, recognizing the strong influence of compositional and rheological differences. The approach is more 'convectionistic' than 'tectonistic', in that only ductile deformation is considered and that the brittle/elastic behaviour of the upper and colder lithospheric parts is ignored. Ductile creep has been used with varying success in 'teetonist' models of lithosphere dynamics, but the application of these methods to global deformation problems are, as yet, computationally too expensive.