EVOLUTION OF THE NORTHERN RIO GRANDE RIFT, GORE RANGE, COLORADO FROM (U-TH)/HE THERMOCHRONOLOGY AND IMPLICATIONS FOR RIFT PROPAGATION MODELS

The Rio Grande rift system tapers northward into the center of the southern Rocky Mountains. There is no consensus on the geodynamic mechanisms responsible for the origin of the rift. Models include the extensional collapse of the southern Rockies, clockwise rotation of the Colorado Plateau, and mantle upwelling associated with the sinking Farallon slab. How the rift is related to the evolution of the Rockies is similarly controversial, with inconsistencies in the published literature as to whether the rift propagated spatially through time and how this bears on broader uplift in the southern Rockies region.

The Gore Range and adjacent Blue River Valley of central Colorado are the northernmost significant fault-related manifestations of the rift. Apatite (U-Th)/He data from the range record only mid- and late Cenozoic exhumation phases, unlike neighboring Rockies uplifts that record a single phase of Late Cretaceous-early Tertiary unroofing. Mid-Tertiary dates require that Gore Range basement remained covered by sedimentary rocks until at least the mid-Eocene. Normal faulting initiated during the Oligocene, inducing exhumation of basement to the surface and deposition of synrift fill in the Blue River Valley. Major cooling and unroofing in the Miocene was restricted to the southern Gores and continued until at least 7 Ma, with ~2.1 km of displacement along the Blue River normal fault. This mid- to late Tertiary unroofing history is strikingly similar to that inferred for other rift flank uplifts to the south, and suggests broad synchroneity in the mid-Tertiary onset and subsequent evolution of >700 km of the Rio Grande rift. The results indicate that the notion of a northward propagating rift as suggested by its northward tapering geometry is incorrect, and preclude models invoking rift propagation to explain late Cenozoic elevation gain of the Rockies.