(U-Th)/He thermochronologic constraints on the evolution of the northern Rio Grande Rift, Gore Range, Colorado, and implications for rift propagation models

Abstract

The Rio Grande Rift system tapers northward into the center of the southern Rocky Mountains. How rift initiation and evolution are related to development of the Rocky Mountains is not well understood. The Gore Range and adjacent Blue River Valley of central Colorado are the northernmost significant fault-related manifestations of the rift. New apatite (U-Th)/He data from the range record only middle and late Cenozoic exhumation phases, unlike neighboring Rocky Mountain uplifts that record a single phase of Late Cretaceous–early Tertiary unroofing. Middle Tertiary dates require that Gore Range basement remained covered by sedimentary rocks until at least the Middle 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 were restricted to the southern Gore Range and continued until at least 7 Ma, with ∼2.3 km of total displacement along the Blue River normal fault. This middle 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 middle Tertiary onset and subsequent evolution of >700 km of the Rio Grande Rift. The results highlight 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 Rocky Mountains.