Formation and Stability of Same‐Dip Double Subduction Systems

A specific configuration of the global subduction system is the “same‐dip” double subduction, which occurs when two parallel subduction zones dip in the same direction. Double subduction convergence rates can be significantly faster than across single subduction systems because of pull by two slabs. However, questions regarding double subduction remain largely unexplored in terms of factors controlling its initiation, duration, and dynamics. In this study, we perform numerical simulations to investigate (i) subduction initiation of a secondary system in a mature single subduction system and (ii) the dynamics and stability of the newly formed system. We start from a single subduction setup, where subduction is already initiated and we stress the system by controlling the convergence rate. Two sets of models (Model A—upper mantle and Model B—deep mantle) are performed, and we compare our results to single and double subduction systems. We find that the forcing needs to overcome the right (front) subduction rate in order to obtain a self‐sustaining double subduction system. However, once initiation of double subduction is successful, the left (rear) subduction dominates the dynamics of the system until simulations reach a steady state with convergence speed‐ups of 2.5 relative to single subduction. Our model results are consistent with previous findings in that convergence rates across double subduction are higher than across single subduction, elevated dynamic pressures are accumulated in between the two slabs, or that the left trench advances while the right trench retreats. Finally, we find that upper mantle models create artificially high pressures in between slabs. Key Points Influx/outflux boundary conditions are used to investigate initiation of a secondary subduction in a mature single subduction system Analysis of the stability of double subduction systems shows that the left (rear) subduction largely controls the force balance Convergence rates are higher across double subduction systems, but they reach maximum speed‐ups, despite higher forcing magnitudes