Since Middle Miocene time sea-floor spreading activity in the southeast Pacific has shifted westward from the Galapagos Rise, then the last active segment of the Farallon Ridge system in the South Pacific, to the East Pacific Rise. This shift was accomplished by three large jumps of fracture-zone-bound axial sections which progressed from south to north. The jumps spanned an average distance of 750 km and occurred 8.2, 6.4 and 5.7 m.y. ago respectively. Positioning of the newly-formed axes aslong the EPR trend apparently was controlled by asthenospheric rather than lithospheric phenomena.