Fig. 2: Model evolution.

Model setup (a) and evolution (b–e) of the second invariant of the strain rate (\({\dot{{{{{{\rm{\varepsilon }}}}}}}}_{{{{{{\rm{II}}}}}}}=\sqrt{0.5{\dot{{{{{{\rm{\varepsilon }}}}}}}}_{{{{{{\rm{ij}}}}}}}^{2}}\)) and temperature for an 800 km wide slab which is used as reference model. Arrows represent absolute motion with reference to the fixed model edge. Model evolution plots show an inset area of panel (a) of surface strain rate in the centre of the model domain and the vertical-temperature profile up to 200 km depth along the red line. a A narrow slab subducts between two buoyant continents and below an overriding continental plate. The slab is initially placed up to 150 km deep in the mantle and between two prescribed transform faults ending after 660 km in x-direction (dashed red lines) to ensure subduction initiation. b After opening of an initial back-arc basin, transform faults form, connecting the trench and spreading centre. c At a critical distance, increased coupling between back-arc and neighbouring plates forces the transform faults to become inactive and localization of stresses closer to the trench. d A rapid ridge jump occurs. e The slab width is decreased and new transform faults are formed.