DEVELOPMENT OF THE CRETACEOUS-PALEOGENE UNCONFORMITY FROM THERMOCHRONOLOGY DATA IN THE HINTERLAND OF THE NORTH AMERICAN CORDILLERA

The Cretaceous-Paleogene unconformity, also known as the sub-Tertiary or Paleogene subvolcanic unconformity, is a regionally extensive and widely recognized erosion surface in the hinterland of the North American Cordillera. The unconformity separates late Neoproterozoic to Early Cretaceous rocks from overlying Eocene to Miocene volcanics and localized lacustrine limestone and conglomerate. Although routinely incorporated into a range of geologic analyses (e.g., cross-section restorations and paleodepth reconstructions), the timing and pace of development of this vital datum remains unresolved. Here we present a compilation of published and unpublished low- to moderate-temperature thermochronology data from Nevada, southeastern California, western Utah, and northwestern Arizona that inform the Late Cretaceous to Pliocene landscape evolution of this region. The database includes radiometric ages from more than fifty K/Ar, ⁴⁰Ar/³⁹Ar, fission track, and (U-Th)/He studies, using a variety of minerals (e.g., apatite, zircon, titanite, feldspar, micas, and hornblende) with a range of closure temperatures (75-500 °C). In all, over 1700 ages have been compiled, with ages ranging from Cretaceous through Quaternary. Although the data are limited in certain regions (e.g., southeastern and central Nevada) where public access is restricted or studies have not been completed, three main phases of cooling are recognized. Cooling in the Cretaceous is linked to magmatic cooling, Sevier contraction, and syn-convergent extension, while Miocene to Pliocene cooling is linked to Basin and Range extension and dextral transtension. An Eocene cooling event is also recognized in the southern Sierra Nevada, the northern and western Great Basin, and Mojave, that may be linked to the unconformity’s development, but its cause is not entirely clear. In addition, the lack of middle Eocene to early Miocene cooling ages across the region suggests that tectonic and erosional exhumation was negligible during this ~30 Myr period. The complete database will be used in combination with sub-crop maps, cross-sections, and palinspastic reconstructions to resolve crustal thickness and paleotopographic patterns, which will contribute to a better understanding of the processes involved in the unconformity’s development.