Effects of mantle and subduction-interface rheologies on slab stagnation and trench rollback

Trench rollback has been a widely discussed phenomenon in recent years, and multiple studies have concentrated on various parameters that may influence trench migration and related aspects of slab deformation in the (upper) mantle. Here we concentrate on the effects of rheological description (yield stress, lower-mantle viscosity, viscosity of crust) in controlling the rollback and associated stagnation of slabs in the transition zone (410-660 km depth). We perform numerical simulations of slab evolution in a 2D Cartesian model with strongly nonlinear rheology combining diffusion creep, dislocation creep and a power-law stress limiter. We demonstrate that trench retreat develops in most models considered, regardless of the subducting plate age or prescribed strength. Rollback then mostly produces slabs that are horizontally deflected at the 660-km phase boundary and remain subhorizontal at the bottom of the transition zone. Slab morphologies are in agreement with stagnant, horizontally deflected structures reported in the transition zone by seismic tomography. Furthermore, if the strength of the slab is limited to less than 0.5 GPa, the slab experiences a significant amount of horizontal buckling. The amplitude of the rollback velocity is sensitive to several model parameters. As one might expect, it increases with the age of the subducting plate, thus reflecting its increasingly negative buoyancy. On the other hand, rollback velocity decreases if we increase the viscosity of the crust and thereby strengthen the coupling between the subducting and overriding plates. High friction on the contact between the subducting and overriding plates may even result in slabs penetrating into the lower mantle after a period of temporary stagnation. Also, reducing the additional negative buoyancy imparted by the 410-km exothermic phase transition suppresses trench rollback. Interpretation of the controls on slab rollback and stagnation may be rather complex in strongly nonlinear rheological models, where, for example, buoyancy effects may be counteracted by associated yield-stress weakening.