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 (<50 km depth) flux melting in the inner part of the mantle wedge at 3–5 Myr after the start of the model run, consistent with the timing of boninitic magmatism after the onset of subduction initiation in the Izu–Bonin–Mariana system. Our models that incorporate water release from the slab require average fracture zone (or transform fault) shear strengths of ≤7 MPa to attain a self-sustaining subduction zone. The fault strengths might be achieved in nature where seawater infiltrates deep into the fault zone.