Spectral modelling of mantle convection in a non orthogonal geometry: application to subduction zones

Abstract

We present a two-dimensional numerical convection model, in a parallelogram-shaped domain adapted to the geometry of subduction zones. The Navier–Stokes problem is solved by a pseudo-spectral solver (named projection-diffusion) coupled to the Richardson iterative scheme. In a convective domain, representative of the upper mantle, plate tectonic parameters are fixed by imposing the velocities on the domain boundaries. In particular, the subducting plate, located on one side of the domain box, moves and dips at constant velocity. The model is validated for the zero obliquity configuration for three boundary conditions: first, for free boundaries, second, for a rigid upper boundary; and third, for a rigid upper and lateral boundary. We also report preliminary results of experiments for a non-zero obliquity at a Rayleigh number value of 10 4, a motionless overriding plate, subduction velocities smaller than 4 cm/yr, and subduction dip angles steeper than 60°. Concerning the evolution of the flow and thermal structures with the subduction movement and geometry, the model predicts the occurrence of a recirculation process, characterised by the formation of an extra convection cell above the subducted plate. Recirculation starts at a critical subduction intensity that is dependent on the subduction geometry: the lower the subduction dip angle, the lower the critical subduction velocity. A critical subduction geometry is predicted by the model because the extra convection cell exists even at very low subduction velocities and subduction dip angles less than 65°.