Modelling lithospheric ageing during subduction: Implications for the Izu-Bonin-Mariana trench migration

Lithosphere bends and subducts into the mantle at trenches. Subducting plate cools, thickens, and contracts in time, inducing changes in the strength of the litosphere and its buoyancy as it grows older. Recent worldwide databases reveal that most of the trenches roll back, but a significant amount of them migrates in advancing toward the upper plate. On Earth this dichotomy is exhibited in the Pacific area: most of the advancing slabs are located in the Western Parific, whereas the Eastern Pacific trenches retreat. It seems that the age of the slab, and consequently the slab strength, to a certain extent controls the subduction style. In this study we select a region where geological studies illustate a variability in trench migration in time. The Izu-Bonin-Mariana (IBM) system, the oldest subducting lithosphere on Earth, represents a significant esample. The reconstruction of the trench migration of the IBM subducting system shows a long episode of asymmetric trench rollback, followed by a recent phase of trench advance. We suggest that this evolution of trench migration from retreating to advancing mode would be the consequence of the subduction of progressively older and stiffer lithospheric material. We tested this speculation by performing 2-D numerical, fully dynamic models, implemented in order to mimic the influence of the progressive lithospheric ageing on trench kinematics. The arrival of older and stiffer lithosphere at the trench has, as consequence, the gradual slow down of the trench migration, and the continuous increase of the resisting force for bending. Hence, the plate is forced to move in advancing, and the change in trench migration occurs. Despite the simplified assumptions of the model, we are confident in the robustness of our results. This model can be applied to the IBM subduction system reconciling down-dip slab shape with the recent transition of trench kinematics from retreating to advancing style.