The Role of Slab Remnants in Modulating Free Subduction Dynamics: A 3-D Spherical Numerical Study

Seismic tomography of Earth's mantle images abundant slab remnants, often located in close proximity to active subduction systems. The impact of such remnants on the dynamics of subduction remains underexplored. Here, we use simulations of multi-material free subduction in a 3-D spherical shell geometry to examine the interaction between visco-plastic slabs and remnants that are positioned above, within and below the mantle transition zone. Depending on their size, negatively buoyant remnants can set up mantle flow of similar strength and length scales as that due to active subduction. As such, we find that remnants located within a few hundred km from a slab tip can locally enhance sinking by up to a factor 2. Remnant location influences trench motion: the trench advances toward a remnant positioned in the mantle wedge region, whereas remnants in the sub-slab region enhance trench retreat. These motions aid in rotating the subducting slab and remnant toward each other, reducing the distance between them, and further enhancing the positive interaction of their mantle flow fields. In this process, the trench develops along-strike variations in shape that are dependent on the remnant's location. Slab-remnant interactions may explain the poor correlation between subducting plate velocities and subducting plate age found in recent plate tectonic reconstructions. Our results imply that slab-remnant interactions affect the evolution of subducting slabs and trench geometry. Remnant-induced downwelling may also anchor and sustain subduction systems, facilitate subduction initiation, and contribute to plate reorganization events. Subduction, the process where cold oceanic lithosphere descends into the mantle, is a time-dependent process: old subduction zones cease while new subduction zones initiate, in cycles of tectonic plate motions. The cessation of subduction is accompanied by break-off of the subducting slab from the surface plate, forming a slab remnant. The remnant continues sinking into the mantle and, in doing so, generates a flow field that may influence adjacent subduction systems. In this study, we present numerical simulations of subduction in a 3-D spherical shell domain, and examine how subduction systems interact with a range of slab remnants. Our models show that sinking remnants can significantly enhance the sinking velocity of slabs within a few 100-1,000 km of the remnants, and can influence the spatial and temporal evolution of trench shape. Our results suggest that the existence of slab remnants may help to anchor and sustain subduction systems, and lead to an environment more favorable for the initiation of new subduction zones. Since such events are closely linked to reorganizations in global plate motions, we suggest that the location of pre-existing remnants influences tectonic plate movements and, potentially, super continent cycles. Subducted slab remnants can enhance the sinking velocities of actively subducting plates by up to a factor 2 Slab remnants strongly influence trench motions and the evolution of trench shape at subduction zones located within a few 100-1,000 s of km The flow fields interact such that the slab tip and remnant approach, thus strengthening mantle flow that can anchor subduction location