The Role of the Overriding Plate and Mantle Viscosity Structure on Deep Slab Morphology

Using 2D numerical subduction models, we compare the morphology of deep slabs in the presence of an oceanic or continental overriding plate and viscosity jumps at either 660 km or 1,000 km depth as suggested by the latest geoid inversions. We demonstrate that a continental plate, combined with a 1,000 km depth viscosity increase, promotes slab penetration into the lower mantle. The same slab will deflect at 660 km depth if it subducts under an oceanic plate into a mantle where the viscosity increases at 660 km depth. To quantify these dynamics, we introduce a slab-bending ratio, dividing the angle of the deepest tip of the slab (slab tip angle) by its dip angle below the plate interface (shallow slab angle), reflecting the overall steepness, and sinking history of the slab. Ocean-ocean convergence models with a viscosity increase coincident with the phase transition at 660 km depth have low ratios and flattened slabs comparable to ocean-ocean cases in nature (e.g., Izu-Bonin). Coupling a continental overriding plate with a 1,000 km depth viscosity increase separate from the endothermic phase change results in slabs with high ratio values, and stepped morphologies similar to those observed for the Nazca plate beneath Southern Peru. Our results highlight that slab morphologies ultimately express the interaction between the type of overriding plate, slab-induced flow, and phase transitions, modulated by the viscosity structure of the top of the lower mantle and transition zone, complementing studies of slab folding, buckling, and other deformation in the upper mantle.