The interplay between overriding plate kinematics, slab dip and tectonics

In subductions where the slab stagnates at the 660-km-mantle discontinuity, overriding plate kinematics largely controls slab dip and overriding plate tectonics. Although plates kinematics models suggest frequent velocity changes for most plates, the impact of temporal evolution of overriding plate velocity on subduction dynamics has been relatively little addressed. In this study, we use 2-D numerical models to assess the effects of changes in overriding plate far-field velocity on subduction geometry and on the horizontal stresses transmitted to the overriding plate. When a change in overriding plate velocity arises during slab stagnation, slab dip evolves during a transient period, called adjustment-time, to reach a state in equilibrium with the new boundary conditions. The models predict a dependency of the adjustment-time on the value of velocity change and on several internal parameters (subducting plate density, thickness, viscosity and mantle viscosity). We estimate that the adjustment-times may be similar to 10-35 Myr in nature, which suggests that most of present-day subduction zones with stagnating slabs might not be at a steady-state. Furthermore, the models predict that changes in overriding plate velocity generate high-temporary variations in the state of stresses of the plate.