Influence of mantle flow on slab geometry and subduction cessation. Insights from numerical models

Slab dip variations both along a single and among different subduction zones are sometimes at odds with predictions of slab pull theory. It has been proposed that the relative motion of the subducting plate with respect to the mantle influences slab dip and state of stress. This influence differs depending on the different dip direction of the slab with respect to a horizontal background mantle flow. We investigate the influence of mantle flow relative to the lithosphere on subduction dynamics. We develop 2D time-dependent thermo-mechanical models assuming incompressible non-Newtonian fluid rheology. We systematically study the effect of different mantle flow velocities and directions (either sustaining the slab or opposing its dip) consistent with absolute plate motion models, and different slab strengths. We find that a significant amount of slab dip variability found in nature can be explained by the interaction of slabs with mantle flow. Moreover, when the imposed mantle flow has the same direction as the slab, an initial stage of slab steepening is followed by a stage of continuous slab dip decrease. For long evolution times, this slab flattening eventually leads to mantle wedge closure, subduction cessation and slab break-off, possibly driving to subduction flips.