Feedbacks between subduction dynamics and slab deformation: Combined effects of nonlinear rheology of a weak decoupling layer and phase transitions

Abstract

Seismic tomography reveals a wide variety of slab deformation in the mantle transition zone and shallow lower mantle. Numerical modeling of subduction has identified several factors that control slab deformation, among them the properties of the mechanical decoupling between subducting and overriding plates was shown to have a major influence on slab velocity, rollback and deformation in the transition zone. Models with weak crust generally yield fast rollback and slab stagnation while stronger crust results in slab penetration into the lower mantle. Here we perform a detailed analysis of the effects of this weak crustal layer. First, in models with constant crustal viscosity, we quantify combined effects of crustal viscosity and thickness. Further we apply nonlinear crustal rheology that combines dislocation creep and pseudoplastic deformation. We test the effects of varying parameterisations of the nonlinear rheological mechanisms and evaluate their effects on slab deformation. We conclude that the variations of subducting slab velocity (controlled by the buoyancy effects of the major phase transitions) induce strong time variations of the crustal viscosity that in turn enforce further acceleration or deceleration of the slab. This feedback between slab velocity and crustal viscosity strenghtens transient behavior of the subducting slabs and enforces slab penetration after a transient period of stagnation.