Patterns of magnetic anomalies associated with sea-floor spreading are not precisely parallel but undulate slightly with reference to each other. This produces deviations from ideal two-dimensionality. Such undulating patterns are well displayed in a region southwest of the Blanco fracture zone where coherent spreading-velocity anomalies show amplitudes of 5 to 15 mm/yr and wavelengths of 40 to 160 km. Possible mechanisms for the velocity anomalies are deformation of the zone within which rocks were magnetized, discrete volcanism at the ridge axis, or small-scale asymmetrical spreading produced by local movement of the spreading center. If the latter interpretation is correct, the motion of the fissure must have been nearly continuous.