The effects of rheological decoupling on slab deformation in the Earth's upper mantle

Processes within subduction zones have a major influence on the plate dynamics and mantle convection. Subduction is controlled by a combination of many parameters and there is no simple global relationship between the resulting slab geometry and deformation and any specific subduction parameter. In the present work we perform a parametric study of slab dynamics in a two-dimensional model with composite rheology including diffusion creep, dislocation creep and stress limiter or Peierls creep. The mechanical decoupling of the subducting and overriding plates is facilitated by a low viscosity crust. We are particularly interested in the effect of the contact of subducting and overriding plates on the plate geometry in the upper mantle. We also study the influence of the surface boundary condition and of the rheological description (yield stress of stress-limiting rheology, additional viscosity contrast at 660-km discontinuity). Our results demonstrate that the slab morphology and deformation in the upper mantle and the transition zone is sensitive not only to the slab strength, but also to the decoupling mechanism at the contact of the subducting and overriding plates. Weak crust with a viscosity of 10(20) Pa s effectively decouples the subducting and overriding plates and produces reasonable slab morphologies. The geometry of the slab in the upper mantle is strongly influenced by the initial geometry of the contact between the subducting and overriding plates. Further, a step-wise viscosity increase by about an order of magnitude at 660 km depth is necessary to limit the plate velocities to a reasonable value around 5 cm/yr.