Fig. 1: Mechanism sketch.

a–c Extinct and active back-arc spreading centre in the narrow Tyrrhenian Sea (adapted from Gueydan et al.³³), Scotia (adapted from Eagles & Jokat¹⁸, and Lesser Antilles at ~40 Ma (adapted from Allen et al.¹⁹. All subduction zones retreat within the bounds of STEP- or similar transform faults. d Sketch of a spreading centre jump in a retreating subduction zone, bound laterally by transform faults. The subducting slab tears along STEP (subduction-transform-edge-propagator) faults, whereas the back-arc moves relative to the neighbouring along transform faults (grey shaded regions). Within a short period is old spreading centre and the associate volcanic arc extinct, and a new spreading centre forms closer to the retreating trench.