Extended Data Figure 6: Material point trajectories for points on the central line of model 80w.

The points are sampled along a direction initially parallel to the plate-motion (right to left in this figure), which bisects the domain and passes along the short edge of the accreting ribbon. Colours (see key) indicate the times at which individual trajectories have been recorded. Inset, the plan view of the trench at time intervals indicated by the same colouring. Lateral motion of material points is negligible (<25 km in 650 km of vertical sinking). As is typical of rollback subduction, material trajectories are much steeper than the slab orientation (grey lines). The slab at the left-hand side hangs from the back of the accreted ribbon and undergoes slow stretching. Material in this hanging part of the slab has been all subducted in the right-to-left direction in the orientation of this diagram and only moves left-to-right during stretching.