A SYNTHESIS OF RECENT RESULTS FROM GEODYNAMIC MODELS OF SUBDUCTION INITIATION AND TERMINATION (Invited Presentation)

While subduction initiation and termination appear as symmetric bookends on the long-term subduction process, from a geodynamic modeling perspective they provide very different challenges. First, it easy to create conditions to stop subduction in a model, but quite difficult to start subduction in a way that is consistent with what we know about the available forces and pre-existing structures. Second, modeling the initiation process requires methods focused mainly on sinking of the lithosphere and mantle flow, while the termination process requires methods that can also model the complex long-term evolution of the crustal deformation during collision. However, one common factor for both these processes is a strong dependence on the rheological model of the lithosphere and crust, which controls how the forces available in the system localize on pre-existing or newly-formed shear zones and faults. For subduction initiation recent work has explored the role of oceanic crustal strength, hydration-related weakening, shear heating, upper-plate buoyancy, the conditions driving convergence both in intra-oceanic and passive-margin settings, and the initiation of the first subduction zones prior to plate tectonics. Termination of subduction ultimately occurs when the slab driving subduction breaks off. Recent models have shown how this process occurs in 3D, both in settings with continental collision and approach of a spreading ridge. In both cases the condition for slab breakoff is controlled by the strength of the slab, and in the case of collision also depends on the ability of the evolving buoyancy structure of the colliding feature to provide sufficient resistance. Geodynamic studies provide insight into the evolving balance of these competing forces, which lead to the protracted nature of continental collision. From a geodynamic perspective, it is clear from these recent models that 1) a weak crustal layer is a key factor in facilitating subduction initiation and the protracted nature of continental collision, and 2) strong oceanic lithosphere is required to limit the conditions that lead to slab breakoff and sustain continental collision. In the future, the effects of 3D structures, surface deformation and mantle flow need to be more fully explored, in particular for the subduction initiation process.