A Pliocene ignimbrite flare-up along the Tepic-Zacoalco rift: evidence for the initial stages of rifting between the Jalisco block (Mexico) and North America

The Tepic-Zacoaico rift, a NW-trending corridor ~50 x ~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 tufts and lavas, which were previously assumed to be Oligocene-Miocene in age, related to the Sierra Madre Occidental volcanic province, or older. New [sup.40]Ar/[sup.39]Ar 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 tufts 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 [sup.40]Ar/[sup.39]Ar 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. Keywords: ignimbrite flare-up, lithospheric extension, plate transfer, [sup.40]Ar/[sup.39]Ar geochronology, Rivera plate, Sierra Madre Occidental.