KEYNOTE. The Oligocene to middle Miocene tectonic and magmatic evolution of western Mexico records a prolonged phase of waning subduction that culminated in the establishment of a transtensional plate boundary after ~12.5 Ma. Rifting in the Gulf of California has traditionally been linked exclusively to post–~12.5 Ma tectonics, whereas earlier extension and volcanism were attributed to Basin and Range deformation and subduction-related arc magmatism, respectively, implying no genetic connection to rifting. This interpretation relies on two assumptions: that rifting began only after subduction ceased, and that volcanic geochemistry uniquely reflects plate-boundary type. However, studies over the past 15 years demonstrate that extensional deformation along both Gulf margins predates the end of subduction, and that Sierra Madre Occidental (SMO) and Comondú magmatism reflects variable contributions from lithospheric mantle, asthenosphere, and crust, challenging simple geochemical classifications of tectonic setting.

Recent work on both margins further corroborates the timing of Gulf rifting proposed in our 2018 geologic, petrologic, and geodynamic model. Extensional fault systems and basins along the eastern Gulf margin (western SMO) developed during the latest Oligocene–early Miocene, also supported by apatite fission-track ages of ~25–19 Ma from exhumed Late Cretaceous–Paleocene plutons in Sinaloa. Mafic lavas with asthenosphere affinity are found since the Oligocene associated to some of these extensional faults. On the southern Sinaloa–northern Nayarit continental shelf, the buried Mazatlán (San Blas) Basin contains >3 km of sedimentary fill with a minimum age of ~19 Ma. On the western conjugate margin, the ubiquitous presence of Late Cretaceous detrital zircon populations in the Comondú Group implies deposition within an extensional basin flanked by the Peninsular Ranges Batholith since ~19 Ma. Comondú volcanism, consisting of intermediate domes and lava flows, began to be emplaced at this time, mirroring magmatic activity along the conjugate eastern margin in Sinaloa. Within the Gulf, dredged and ROV-recovered intrusive rocks record rapid cooling at ~18 Ma and thermochronological data indicate an early Miocene initiation of the San José del Cabo fault. Together, these observations support our model, in which post–12.5 Ma transtension localized along a lithosphere previously weakened by protracted orthogonal extension.