On the relation between trench migration, seafloor age, and the strength of the subducting lithosphere

Oceanic lithosphere thickens and strengthens as it grows older. Worldwide databases reveal that the age of the slab to a certain extent controls the subduction style. Old and thick (and consequently strong) slabs show a trench advance, while younger, thin (weak) slabs are migrating in retreating style (trench rollback). We performed numerical models to show that this configuration could be the result of the dynamic equilibrium between gravity and resisting forces. In particular we show that energy dissipation caused by bending and unbending of the slab, although less important than other resisting forces, could be a primary control on trench migration. Our results fit well with global compilations of kinematic data from modern subduction zones in two reference frames with different amounts of net rotations. Based on the age at which the transition from retreating to advancing style occurs, we propose an effective lithosphere/mantle viscosity of similar to 200 during bending of the lithosphere into the subduction zone.