Slab Dehydration: mechanical consequences on subduction zones dynamics

Subduction zones occur at the boundaries where tectonic plates converge and their dynamics is strongly coupled to the mantle wedge one. In this area, three main mechanical actors interact: the diving lithosphere, the overriding one, and the asthenosphere. To better understand the coupling phenomenon in the mantle wedge, we study the case of two converging oceanic plates, and we focus on the slab dehydration effect on the mantle wedge dynamics. Numerical experiments are performed using a thermomechanical code of convection. Water transfers are controlled by dehydration reactions within the slab and by hydration in the overlaying rocks. Dehydration and hydration reactions are both estimated according to accurate phase diagrams (Schmidt and Poli 1998, Bousquet et al. 1997). Rocks are assumed to be H₂O-saturated. Mantle rocks can be strongly weakened by the presence of water, that we model by adecreasing rock viscosity as a function of water content. Simulations show firstly that the amount of water released into the mantle wedge can hydrate the upper plate on about a 80 km thickness. Secondly, the hydrated rock softening in the mantle wedge enhances the corner flow. Furthermore, if the viscosity reduction coefficient, visco_dry/visco_wet}=f{ν , due to the presence of water is sufficiently large (greater than 50), secondary convection cells appear. As a consequence, the overriding lithosphere is delaminated by small blobs detachment and thins progressively. In these cases, the upper plate base is delaminated until the hydrated sublithospheric layer disappears. The delamination characteristic time seems to be proportional to f_ν ^-2/3. What brings about this upper plate erosion? Is it the enhanced corner flow, or the mechanical structure of the hydrated lithosphere? Simulations without subduction and localized hydration reactions in selected areas of the lithosphere and of the underlying asthenosphere are performed. For f_ν ≥ 50, a convective destabilization appears as in subduction experiment with similar characteristics. Therefore, for high hydrous strength reduction, the upper plate thinning during subduction is not controlled by the corner flow dynamics, but by the hydrated lithosphere strength. Thus we test the influence of the lithosphere bulk composition, using Gibbs free energy minimization calculations (de Capitani and Brown, 1987) to recalculate amounts of hydration. The water-contents we obtain in the sublithospheric layer are low because of amphiboles disappearance. This decrease of water contents strongly limits the hydrous softening in the upper plate. As a consequence, one can conclude from our preliminary results that the presence of amphiboles within the lithosphere favors local convection by reducing the viscosity.