SPHERICAL SHELL MODELS OF MANTLE CONVECTION WITH TECTONIC PLATES

Abstract

A simple three-dimensional spherical model of mantle convection, where plates are taken into account in the top boundary condition, allows to investigate the plate tectonics-mantle convection coupling in a self-consistent way. Avoiding the strong difficulties inherent in the numerical treatment of rheology, the plate condition appears efficient in reproducing the Earth-like features as subduction, mid-oceanic ridges and hotspots. Whereas the free-slip condition leads to a classical polygonal cell pattern with cylindrical hot plumes surrounded by downwellings, the plate condition favors the development of strong linear downwellings associated to passive diverging zones along plate boundaries. These cold currents, very similar to subductions, act the main role in mantle convection: they drive the whole circulation. In that context, hot plumes remain almost independent, except if on the long term, cold material spreading at the core surface induces a slight migration, below a few mm/yr, of their surface impingement. The main result is that plate tectonics appear to be more than a simple mode of organization of the surface movements, it is the essence of the Earth mantle dynamics.