LONG-TERM TECTONOTHERMAL HISTORY OF LARAMIDE BASEMENT FROM ZIRCON (U-TH)/HE AGE-EU CORRELATIONS

The broad temperature sensitivity of the zircon (U-Th)/He thermochronologic system, as a function of accumulated radiation damage, provides a unique tool to constrain long-term (> 1 Ga) thermal histories of cratons. We use zircon (U-Th)/He thermochronology to understand the Proterozoic-Phanerozoic thermal history of Laurentian basement in the northern Rocky Mountain region of the western United States. Zircon (U-Th)/He ages of Wyoming craton crystalline rocks from the hanging wall of major Laramide thrust faults in the Wind River Range (54 ages) show distinct negative correlations with effective uranium (eU) concentrations. Zircons show a range of ages from 540 Ma at low eU to a pediment of early Cenozoic ages across a wide range of higher eU from about 1000-7000 ppm. Using eU differences as a proxy for relative amounts of accumulated radiation damage in these zircon grains, we use a radiation damage and annealing model that accounts for the effects of radiation damage on He diffusion to document a multistage thermal history of the Precambrian basement in this mountain range. The history that best agrees with the observed ages consists of cooling below 300 °C at about 1300 Ma, followed by reheating to 160 °C and rapid cooling to the surface at 60 Ma. The most recent parts of this thermal history is consistent with published apatite He and fission-track constraints on the timing of Laramide exhumation in the Wind Rivers (~60 Ma), but our model also constrains the pre-Laramide history of these rocks, both through reheating events caused by Phanerozoic sedimentary burial, and the timing of much earlier Proterozoic cooling, consistent with early phases of plausible mid-continent tectonic activity associated with anorogenic magmatism (ca. 1500-1300 Ma).