ROCKY MOUNTAINS RIDE HIGH ON ANCIENT RIFTED MARGIN

The modern topographic expression of the Southern Rocky Mountains, largely defined by the Late Cretaceous–Eocene Laramide Orogeny, is hypothesized to result from the inversion of Proterozoic extensional faults that accommodated intracontinental rifting of western Laurentia and the break-up of supercontinent Rodinia. The idea that Phanerozoic orogenesis in continental interiors may reactivate ancient plate boundaries is geologically compelling; however, we commonly lack direct evidence to support such a linkage. Here, we document the Neoproterozoic-Recent tectonic history of Paleoproterozoic crystalline rocks from the Front Range near Boulder, Colorado using apatite and zircon (U-Th)/He (He) thermochronology, regional unconformities, and constraints from sedimentary rocks. Apatite He ages are 54 ± 7 Ma across ~2500 m of sampled relief. Zircon grains have He ages of 50–607 Ma that vary as a function of crystal U-Th composition (i.e., radiation damage). Zircon He ages older than 85 Ma (n=11) suggest that a pre-Laramide thermal history is retained, at least in part, by these grains. We used the thermal history modeling program HeFTy (v.1.9.3, with the apatite and zircon radiation damage accumulation and annealing models) to explore the range of histories both permitted by our He data and consistent with the Neoproterozoic Tavakaiv Quartzite and Phanerozoic nonconformities that place our crystalline basement samples near Earth’s surface (<2 km depth) at ca. 700-650, 500, and 300 Ma. Good-fit thermal histories require a long-lived (76±30 My) Neoproterozoic heating event with a mean peak temperature of 330±14°C. We argue that this heating is parsimoniously explained by sample burial to 4-12 km depth in an active extensional basin and therefore documents the origin of the Front Range as a rift that experienced km-scale burial and exhumation coeval with Neoproterozoic fragmentation of western Laurentia. Our thermal history models also imply that the rift-basin sediments that buried the Front Range basement were eroded away within ~40 My of Rodinia breakup. This post-rifting exhumation may reflect the topographic evolution of the Laurentian passive margin and provide a measure of the erosional impact of the late Neoproterozoic “Snowball Earth” glaciations prior to Cambrian deposition on the Great Unconformity.