Dynamical and thermal structure of a subduction zone: influence of slab geometry on the convective state of the Earth's upper mantle; preliminary results

Abstract

The aim of this study is to investigate the influences of plate kinematics and of the downgoing slab's geometry on internal dynamics and thermal structure in the vicinity of a subduction zone. A systematic study depending on these parameters has been carried out based on a two-dimensional finite differences code. In a convective domain representing the upper mantle, plate displacements are prescribed by kinematic boundary conditions. In particular, the subducting plate moves and dips at a constant velocity under the slower overriding one. Typical subduction dips (30°, 45° and 70°) have been studied for a realistic Rayleigh number of about 10 5 and for plate velocities around a ‘coupling’ velocity of 2–3 cm year −1. This limit value was inferred from the balance between surface forces and buoyancy forces. For any subduction dip and plate velocities lower than this coupling velocity, plate kinematics variations induce significant effects on thermal and mineralogical patterns. At greater plate velocities, these patterns stabilize. For plate velocities of the order of the coupling velocity, a subduction dip decrease of 40° involves a heating of the downgoing plate of 100 degrees and a depth increase of 20 km of the olivine-spinel phase transition in this plate. In the particular case of a non-moving overriding plate, the balance between this plate and underlying internal dynamics requires a moving plate velocity lower than 2–3 cm year −1 for a lower dip. Moreover, a shallow subduction enhances a lateral large-scale convection under the upper plate even if this plate is motionless, in contrast to the case of a steep subduction. This is consistent with observed back-arc extension at the surface, which is often associated with steep subductions. Our model suggests a spreading zone of between 500 and 1000 km in the upper plate for a subduction velocity lower than 2–3 cm year −1 and a Rayleigh number of about 10 5.