Competing influences of plate width and far-field boundary conditions on trench migration and morphology of subducted slabs in the upper mantle

Seismic tomography of the mantle provides a snapshot view of the present-day structure of subducted tectonic plates (slabs) in the mantle. However, a correct interpretation of how such features developed requires knowledge of both the history of plate motions, in particular the locations of subduction zones over time, as well as an understanding of how slabs deform and interact with the lower mantle. The understanding of seismic tomography requires a detailed knowledge of the motion of the trench with time, as well as the time-evolution of subduction zones and their associated slabs. We use 3-D numerical models to investigate two effects which can strongly influence the kinematics of subduction, in particular the migration of the trench: the width of the plate and the far-field boundary conditions of the subducting plate. These models study the full range of plate widths (300–6000 km) and are subjected to the full range of boundary conditions (“fixed,” “free,” and “pushed”) representative of the tectonic settings found on Earth. Additionally, our results indicate that trailing edge boundary conditions dominate over the effect of plate width. Each boundary condition is identified to produce a particular trench migration behavior with an associated upper mantle slab morphology: (1) fixed edges result in retreating trenches and retrograde motion of the slab which ends up lying flat atop the lower mantle, (2) free-trailing edges tend to exhibit quasi-stationary trenches with slabs developing into folded piles atop the lower mantle, and (3) pushed trailing edges generally result in advancing trenches that overrun previously subducted material to form forward-draping slabs lying prone atop the lower mantle. This range of behavior can be observed in seismic tomography and an example of each is provided. Within each suite of models with similar boundary conditions, plate width has a subsidiary role in the observed slab morphology but three distinct regimes of plate width can be distinguished in regards to trench migration. Thus plate width is an important effect on trench migration and does result in notable differences in slab morphologies between narrow and wide plate widths.