Postspreading volcanism has built large seamounts and volcanic ridges along the short axes of a highly segmented part of the East Pacific Rise crest that ceased spreading at the end of the middle Miocene, offshore Baja California Sur, Mexico. Lava samples from Rosa Seamount, the largest volcano, are predominantly alkalic basalts, mugearites, and benmoreites. This lavas series was generated through fractional crystallization and is compositionally similar to the moderately alkalic lava series in many oceanic islands. Samples from volcanic ridges at three adjacent failed spreading axes include mildly alkalic, transitional, and tholeiitic basalts and differentiated trachyandesites and andesite. The subtle but distinct petrologic and isotopic differences among the four sites may be due to differences in the degree of partial melting of a common, heterogeneous source. Postspreading lavas from these four abandoned axes off Baja California Sur together with those from other fossil spreading axes and from seamount volcanoes that grew on the East Pacific Rise flanks define a compositional continuum ranging from normal mid-ocean ridge basalt (NMORB)-like to ocean island basalt (OIB)-like. We propose that the compositional spectrum of these intraplate volcanic lavas is due to different degrees of partial melting of the compositionally heterogeneous suboceanic mantle in the eastern Pacific. A large degree of partial melting of this heterogeneous mantle during vigorous mantle upwelling at an active spreading center produces NMORB melts, whereas a lesser degree of partial melting during weak mantle upwelling following cessation of spreading produces OIB-like melts. The latter melts have a low (<8 RA) 3He/4He signature indicating their formation is different from that of OIBs from major “hot spot” volcanoes in the Pacific with high 3He/4He ratios, such as Hawaii and Galapagos.