GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 156-10
Presentation Time: 10:40 AM


MURRAY, Kendra E.1, CLARK, Marin K.2 and NIEMI, Nathan A.2, (1)Geosciences Department, Hamilton College, 198 College Hill Rd, Clinton, NY 13323; Earth and Environmental Sciences, University of Michigan, 1100 N University Ave, Ann Arbor, MI 48109, (2)Earth and Environmental Sciences, University of Michigan, 1100 N University Ave, Ann Arbor, MI 48109

It has been hypothesized that modern mountainous topography in continental interiors may delineate ancient plate boundaries. Fault-bounded ranges that comprise North America’s Southern Rocky Mountains are postulated to be the result of such a tectonic inversion—the reactivation of a Neoproterozoic rifted margin formed during Rodinia breakup. However, evidence directly supporting this origin is scarce because the geologic record is overprinted by multiple younger orogenic events, most recently the Cretaceous–Paleogene Laramide Orogeny. Here, we use (U-Th)/He (He) thermochronology to reveal the Neoproterozoic history of the largest basement-cored Laramide uplift, Colorado’s Front Range, and demonstrate the sensitivity of the helium chronometer to polygenetic tectonic histories. We sampled Paleoproterozoic crystalline bedrock in the Front Range near Boulder, CO. These samples yielded He ages of 50–607 Ma from zircon grains with a range of compositions (eU) ideal for using thermal history modeling (in HeFTy), leveraging the coevolution of radiation damage and He diffusivity in zircon to explore the pre-Laramide thermal history. Time-temperature (tT) constraints from published 40Ar/39Ar ages require temperatures colder than ~350 ˚C since ca. 1300 Ma. This bound is supplemented by several proximal geologic constraints that require our samples to be near surface temperatures (T <60 ˚C) at various points in the geologic past: (1) the late Neoproterozoic Tavakaiv (Tava) quartzite, which outcrops as sandy bodies and clastic dikes formed from unlithified parent sands deposited on a proto-Great Unconformity ca. 800-680 Ma and injected into the basement within ~2 km of the paleosurface, (2) the Great Unconformity at ca. 500 Ma, and (3) Ancestral Rocky Mountain unconformity ca. 300 Ma. Given these constraints, tT histories compatible with our zircon He ages require that our samples reached T = 250-350 ˚C after Tava formation and then cooled to near surface temperatures before Cambrian deposition on the Great Unconformity at ca. 500 Ma. We argue that this history documents, for the first time, the origin of the Front Range as a late Neoproterozoic rift basin that likely experienced km-scale burial and exhumation as western Laurentia transitioned from an intracontinental rift to a passive margin.