The Tepic-Zacoalco rift, a NW-trending corridor ∼50 × ∼250 km, is one arm of a triple-rift system in western Mexico. Together with the Colima rift and the Middle America Trench, it bounds the Jalisco block, a portion of western Mexico that may be moving independently of North America. The predominant basement rock types in the Tepic-Zacoalco rift are rhyolitic ash-flow tuffs and lavas, which were previously assumed to be Oligocene-Miocene in age, related to the Sierra Madre Occidental volcanic province, or older. New 40Ar/39Ar dates on 41 volcanic samples reveal a previously unrecognized, voluminous flare-up of rhyolitic ignimbrites between 5 and 3 Ma throughout the entire corridor of the Tepic-Zacoalco rift; they are often associated with Pliocene high-Ti basalts. The eruption rate during this Pliocene time period was an order of magnitude higher (hundreds of m/m.y.) than that documented in the Tepic-Zacoalco rift over the last 1 m.y. The Pliocene ash-flow tuffs have been faulted along NW-trending lineaments, producing vertical offsets up to at least 500 m. The voluminous ignimbrite flare-up in the Tepic-Zacoalco rift at 5–3 Ma may reflect the initial stages of rifting of the Jalisco block away from North America, analogous to what occurred in the proto-gulf region at 12–6 Ma, prior to the transfer of Baja California from North America to the Pacific plate.
Additionally, new 40Ar/39Ar dates show that the Sierra Madre Occidental volcanic province extends across the entire width of the Tepic-Zacoalco rift and terminates abruptly at the northern boundary of the Jalisco block near the Rio Ameca. In contrast, Paleocene-Eocene basement from the Jalisco block extends northward into the Tepic-Zacoalco rift, where it is locally overlain by Sierra Madre Occidental rhyolites.