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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.

A combined GLORIA sidescan and Hydrosweep multi-narrow-beam sonar survey shows striking evidence for fan-shaped spreading systems, bounding pseudofaults and compression ridges associated with 90° of clockwise rotation of the Juan Fernandez microplate in the past 4 Myr. A quantitative analysis of this evolution suggests that the microplate has rotated as a rigid 'roller bearing' for most of its history, driven at its edges by shear between the surrounding major plates. The remarkable similarity between the Juan Fernandez and Easter microplates, despite their different tectonic settings, suggests that this is a general model for microplate tectonics.

There is a growing Easter microplate between the Pacific and Nazca plates. The eastern and western boundaries of the microplate are actively spreading and propagating north and south, respectively. The northern and southern boundaries are broad transform zones. Most of the interior of the microplate contains the sort of oblique sheared structures characteristic of zones of transferred lithosphere between propagating and failing spreading centres.