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

Abstract

Using 2D numerical subduction models, we compare deep slab behaviour with oceanic and continental overriding plates and a mantle viscosity structure where the lower mantle viscosity jump occurs either at 660 km or at 1000 km depth as suggested by the latest geoid inversions. We demonstrate that a strong, thick, and buoyant continental plate, combined with a 1000 km depth viscosity increase, promotes slab penetration into the lower mantle. Conversely, the same slab will deflect at 660 km depth if this subducts under an oceanic plate into a mantle where the viscosity increases at the canonical 660 km depth. To quantify these dynamics, we introduce a slab bending ratio, by dividing the deep slab tip angle by the shallow slab angle, reflecting the 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 1000 km depth viscosity increase separate from the endothermic phase change results in slabs with high ratio values, and stepped morphologies similar to that observed for the Nazca plate beneath the Southern Peruvian arc. 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.