THE existence of rapidly spinning microplates along the southern East Pacific Rise has been documented by geophysical swath-mapping surveys1–6, and their evolution has been successfully described by an edge-driven kinematic model7. But the mechanism by which such microplates originate remains unknown. Proposed mechanisms1–10 have generally involved rift propagation11, possibly driven by hotspots or changes in direction of sea-floor spreading. Here we present geophysical data collected over the Earth's fastest spreading centre, the Pacific–Nazca ridge between the Easter and Juan Fernandez microplates (Fig. 1), which reveal a large-offset propagating rift presently reorganizing the plate boundary geometry. A recent episode of rapid 'duelling' propagation of the historically failing spreading centre in this system has created a 120 120 km overlap zone between dual active spreading centres, which may be the initial stage of formation of a new microplate.