Magmatic Surge Requires Two-Stage Model for the Laramide Orogeny

Abstract The beginning of the Laramide orogeny is a pivotal time in the geological development of the western United States, but the driving mechanism responsible for mountain building, basin formation and ore mineralization is controversial. Most prominent models suggest this event was caused by the collision of an oceanic plateau with western North America at ca. 88 Ma which caused the angle of subduction beneath the continent to shallow. This subhorizontal (flat) subduction is thought to have led to shut-down of the arc, crustal cooling, and the formation of deep, basement-involved thrust faults that penetrated far into the continental interior. In contrast to these predictions, we show that the arc experienced a magmatic surge, the lower crust was hot (835-750°C) and partially molten from 90 to 70 Ma, and cooling occurred after 75 Ma. These data contradict plateau underthrusting as the driving mechanism at 90-80 Ma; therefore, the Laramide orogeny cannot have been initiated by flat-slab subduction. We propose that the Laramide orogeny is best explained as a two-stage orogeny consisting of a syn-magmatic phase at 90-75 Ma, and a widespread mountain building phase at 75-50 Ma. Only that latter phase is linked to flat-slab subduction.