Mantle-wedge alteration facilitates intra-oceanic subduction initiation along a pre-existing fault zone

Intra-oceanic subduction initiation may occur where a transform fault or a fracture zone is located between the younger and older lithospheres. However, the mechanisms and conditions controlling the initiation of subduction remain debated. Here, we present two-dimensional visco–elasto–plastic numerical models of vertically-forced (or spontaneous) subduction initiation along such a pre-existing fault zone where slab-derived fluids enter the overlying mantle wedge. The models show that just after sinking of the older lithosphere, hydrothermal alteration to form serpentine (antigorite) or talc occurs within a juvenile, thin mantle wedge, resulting in the development of a low-viscosity shear zone (1019–1020 Pa s) at its base. The low-viscosity shear zone acts to decouple the mantle wedge from the slab, thereby facilitating slab subduction. Continued slab subduction generates shallow (