Elucidating the development of the basal Tertiary unconformity from a synthesis of regional thermochronologic data

The basal Tertiary unconformity is a striking, semi-continuous erosional surface that is widely recognized across the hinterland of the North American Cordillera in the Sierra Nevada, Great Basin, and Mojave Desert. The unconformity forms the boundary between Paleozoic sedimentary rocks or Mesozoic granitoids and overlying Cenozoic volcanic and sedimentary rocks. Recognition of the unconformity dates to 1911, where its presence was documented beneath `auriferous gravels' in the Sierra Nevada. The unconformity is an important datum for cross section restorations and for paleodepth reconstructions, but the history of exhumation and erosion associated with its development are poorly understood. Here we present a synthesis of thermochronologic data from this broad geographic region. The synthesis is based on a compilation of more than 1000 published and unpublished (U-Th)/He, fission track, 40Ar/39Ar, and K/Ar dates. The data are derived from a breadth of thermochronometers (e.g. apatite, zircon, biotite, muscovite, hornblende, titanite) that provide constraints over a wide temperature window of 500-75°C. To evaluate spatial and temporal trends in the data, we group the data into 10 m.y. time step maps to show the Cretaceous to Pliocene thermal evolution of the study region. Preliminary analysis of the data suggests development of the unconformity is linked with the exhumation history of the Late Cretaceous to Eocene Nevadaplano, a high-standing, low-relief orogenic plateau that developed in the hinterland of the Sevier orogeny. The thermochronology data also demonstrate that exhumation in the Eocene was succeeded by protracted quiescence across the region. For most of the region, exhumation does not begin again until the early to middle Miocene, suggesting that erosion rates over this interval were very low. The full suite of data will be combined with sub-crop maps and balanced cross sections to elucidate paleotopographic and crustal thickness trends, erosion rates, and sediment dispersal patterns. The results will also be used to evaluate dynamic drivers responsible for the Late Cretaceous to Pliocene exhumation of the region.