A continuous Sea Beam swath along 2500 km of the crest of the fast-spreading East Pacific Rise (EPR) defines the location and morphology of seven systems of spreading centers. They are bounded by six transform fault systems, where reconnaissance swaths mapped additional short axes of plate accretion. The spreading center systems are subdivided by nontransform offsets, most of which step right on the central part of the plate boundary (12¿S--4.5¿S) and left on the northern and southern parts. These staircases of small lateral offsets were probably created by changes in spreading direction, the right steps by a counterclockwise rotation as the 12¿S--4.5¿S EPR matured after 6 Ma from the propagating western boundary of the Bauer microplate. Because the evolution of this microplate also eliminated transform offsets inherited from the Pacific-Frallon boundary (the pre-24-Ma ancestor of the Pacific-Nazca EPR), led to segmentation of the (fossil) Galapagos Rise, and created the present right-offset transform fault systems on the EPR, it is identified as the principal event in the development of the mapped plan pattern. A recent small clockwise rotation in Pacific-Nazca relative motion is inferred to be responsible for most of the small left steps along the boundary and for segmentation of the transforms into systems of an echelon fault zone linked by short intratransform spreading centers.

The plan segmentation that results from the plate and microplate histories obstructs the shallow horizontal distribution of magma along the rise crest, allowing spatial variations in the rate of vertical magma delivery to be topographically expressed by the variable cross section of the axial ridge and by the along-strike relief of the neovolcanic zone. The pattern of variations is much more complex than that predicted by models of transform-bounded asthenosphere upwelling and regularly spaced melt diapirs. For example, the shallowest spreading segments, as well as the deepest, are short links between long fault zones in transform fault systems, and some long straight parts of the rise crest have humped long profiles consistent with a diapir-fed model, whereas others have a remarkably flat crest line with no topographic evidence for spreading cells. ¿ American Geophysical Union 1989