TECTONIC CONTROLS ON CENOZOIC VOLCANISM, WESTERN U.S. AND MEXICO: AN OVERVIEW

Cenozoic volcanism in the western U.S. and Mexico occurred under extensional to transtensional strain regimes, resulting in excellent preservation of volcanic stratigraphy in deep, fault-controlled basins. This provides a detailed record of Cordilleran tectonic evolution, synthesized here.

(1) 50 – 16 Ma: Fallback of the subducting Farallon slab resulted in long-distance westward-migration of arc front volcanism across the southwest US and western Mexico, accompanied by E-W extension. Pre-22 Ma asthenospheric upwelling beneath pre-thickened crust produced supervolcano silicic caldera fields in the east; continued fallback produced stratovolcano/lava dome chains on thinner crust in the west (southern Ancestral Cascades; Comundú arc). Meanwhile, in the OR-WA segment of the margin, subduction stepped abruptly westward due to accretion of the Siletzia terane, so the northern Ancestral Cascades is mostly buried beneath the modern arc.

(2) 16-12 Ma: The Pacific/North America plate boundary lengthened, causing E-W extension over a broad arc/backarc region in the southwest U.S. and Mexico (northern and southern Basin and Range). Thermal softening weakened the continent in the arc front, just inboard of a strong lithospheric block created by the Cretaceous batholith in California and Baja California; this became exploited by:

(3) focused NNW-SSE transtension at ~12 Ma, in response to a change in Pacific plate motion, from more westerly to more northerly. Baja California was quickly rifted off of North America at 12-6 Ma, due to stalling of large Farallon microplates. California is calving off more slowly, following northward migration of the Mendocino triple junction (MTJ). The onset of transtension is marked by a burst of ~12-10 Ma high-K arc volcanism. In the wake of the MTJ, removal of the Farallon slab causes asthenospheric upwelling/slab window magmatism that extends far inboard of the trench, to the Basin and Range, leading to south-to-north degradation of the continental mantle lithosphere. The leading tip of the transtensional rift exploited a series of large arc volcanic centers localized at major transtensional stepovers (Sierra Crest-Little Walker, Ebbetts Pass), presently occurring at Mount Lassen; in its wake, the largest rift volcanic centers (Long Valley, Coso) lie in transtensional stepovers.