Time dependent subduction dynamics driven by the instability of stagnant slabs in the transition zone

Seismic tomographic images indicate that subducted slabs are commonly deflected within the transition zone and that accumulated slab material may be subject to localized flux into the lower mantle. These observations are consistent with numerical models of mantle flow in which slab flux through phase transition at 660 km depth is impeded by the combination of rheological contrast and thermodynamic effects. We use a suite of 2D numerical experiments to explore the impact of 'slab avalanches' on plate boundary dynamics. The experiments are initiated at the onset of the avalanche and two distinct classes of model geometry are considered. In the first, the stagnant, deflected slab within the transition zone is connected to subduction at the surface of the model domain. In this case, we find that the location of the avalanche relative to the trench governs whether the slab dip steepens or shallows in response to the instability at 660 km depth. These experiments show a relatively modest impact on plate dynamics at the surface. In the second class of models, the stagnant slab material is not connected to the shallow subduction, and the former is interpreted as a remnant of either a relic subduction event or an accumulation of material from a distant zone of convergence. In this case, the impact on plate boundary dynamics is more profound. For example, when the avalanche is positioned under, or forward, of the island arc, it drives a significant retreat of the trench. In contrast, an avalanche that initiates under the back-arc leads to a thrusting of the island arc over the retro-plate and a reversal in the polarity of subduction. We test the influence of various boundary conditions on the evolution of these models and consider several different rheological parameterizations for the lithosphere and mantle. The models are intended as general templates for the impact of slab avalanches; however, we discuss the potential connection of these results to various zones of convergence along the western Pacific margin.