Roberta Carluccio, Louis N. Moresi , Fabio Capitanio, Rebecca Farrington, Lorenzo Colli, Ben Mather

The partitioning of the three-dimensional mantle flow into toroidal and poloidal components is a diagnostic element to characterise tectonics regimes, with both fields being substantially active and representative of the contemporary mode of mantle convection. The toroidal:poloidal ratio (T/P) affects several aspects of mantle circulation, including the sinking of the subducting lithosphere into the mantle and the lateral transport of melt and volatiles around the subduction zone. Geophysical observations suggest the presence of a very weak and thin sub-lithospheric layer (SLL) underneath several segments of the subducting Pacific lithosphere; however very little is known about its lateral extent. To address the impact of an SLL on mantle circulation in three dimensions, we perform buoyancy-driven subduction numerical models where we introduce an SLL and systematically vary its width and viscosity. Our results show that a non-uniform SLL produces a variety of subduction regimes and significant T/P (0.18-0.52). Most importantly, a considerable toroidal flow component is found in models where the slab has no free edge, where the deformation field would otherwise be completely poloidal, and the toroidal component is suppressed. As such, these outcomes provide a novel contribution to the long-standing debate on the link between the slab pull driving force in buoyant convection and the generation of toroidal flow. Furthermore, these findings aid the interpretation of subduction zones characterised by lateral variability in slab (c-to-s) morphology and trench (advance-to-retreat) migration where geophysical studies have reported an SLL, including Mariana, Izu-Bonin, and Nazca subduction regions, among others.