The Alarcon Basin in the southern Gulf of California originated by intracontinental rifting of the southwestern margin of North America and has evolved into an intercontinental ocean basin by 3.7 m.y. of seafloor spreading. Lava samples collected on the axial ridge along Alarcon Rise, two of the many near-axis seamounts on the northwest flank of the rise, and an abyssal hill on 0.6 Ma crust on the southeast flank were examined petrographically and analyzed for major and trace element and He-Sr-Nd-Pb isotope composition. Most samples are typical mid-ocean ridge basalt (MORB), but rocks at the axial ridge site closest to continent are more differentiated. Rocks from the axial ridge and abyssal hill are genetically related through separate episodes of fractional crystallization. Seamount samples are relatively more primitive than seafloor samples and may have been formed by smaller degree of melting of the same mantle source as the seafloor lavas. Tamayo transform fault has no apparent effect on the geochemistry of axial ridge lavas erupted close to it; this is contrary to the postulated transform fault effect. Alarcon Basin differs from the Salton Trough and Guaymas Basin in that its youngest seafloor shows no evidence for contamination with continental crust or a subduction component. We propose that Pacific MORB source mantle underlies the Gulf of California because the western North American margin has overridden the Pacific suboceanic mantle. Miocene rifting and Pliocene spreading in Alarcon Basin have created a window allowing the most recent MORB melts to rise uncontaminated to the surface.