Two-Phase Development of the Nevadaplano, Western Nevada and Southern California, from Low-Temperature Thermochronology

Abstract:

We use new apatite and zircon (U-Th)/He ages, combined with published thermochronology data from the Sierra Nevada, western Great Basin, and Mojave Desert, to evaluate the timing of development, regional extent, and dynamic controls on the formation of the Nevadaplano. Basement samples from the Argus Range, Avawatz Mountains, Clark Mountains, Lucy Gray Range, and Spring Mountains record cooling at two distinct periods, ~65 Ma and/or ~50-55 Ma. Detrital He dates from the Miocene Bedrock Spring Formation in the Lava Mountains, a proxy for the cooling age of basement sources in the northern Mojave Desert, also show two distinct peaks. Zircon He dates (n=85) have a peak frequency at ~65 Ma, with ~60% of the grains analyzed ranging from 70-60 Ma; whereas, apatite He dates (n=101) have a strong peak at ~52 Ma, with > 30% of the grains analyzed between 55-50 Ma and the majority between 60-45 Ma. These new ages and dates are similar to published (U-Th)/He, fission-track, ⁴⁰Ar/³⁹Ar, and K/Ar ages from across the region, suggesting that Late Cretaceous and Eocene cooling events were areally extensive. Paleodepth reconstructions indicate that exhumation during these events was rapid and on the order of kilometers. Late Cretaceous cooling is well documented in the region and may be linked to tectonic exhumation and crustal refrigeration associated with Laramide flat-slab subduction. In contrast, widespread Eocene cooling is more difficult to explain because structures of that age are not known, implying tectonically quiescence. We suggest that this exhumation event may have been caused by rapid erosion following major shifts in global climate associated with the Late Paleocene Thermal Maximum and Early Eocene Climatic Optimum. To strengthen this hypothesis, we link the thermal signature of this denudation event to the climate record and to the terrestrial and marine sedimentary record. Regional cooling age data also show that Eocene exhumation is followed by ~35 m.y. of inactivity in cooling histories, indicating that subsequent erosional exhumation was very slow. Collectively, these data suggest that rapid tectonic exhumation of the Nevadaplano occurred in the latest Cretaceous, whereas erosional beveling and stabilization of the plateau was dominantly an early to middle Eocene phenomenon related to a changing climate.