MESOPROTEROZOIC TO CENOZOIC THERMAL HISTORIES OF GRAND CANYON BASEMENT FROM ZIRCON (U-TH)/HE AND K-SPAR 40AR/39AR THERMOCHRONOLOGY

Precambrian basement exposures of the Grand Canyon provide unique insight into the Meso- to Neoproterozoic tectonic evolution of the Laurentian margin. We combine (U-Th)/He zircon (ZHe) and ⁴⁰Ar/³⁹Ar K-spar thermochronology to model the < 250 °C thermal history of these basement rocks. Forward models of date-effective uranium (eU) correlations indicate that the ZHe system is sensitive to a main phase of Precambrian cooling to <200 °C between 1600 and 1300 Ma, after the Yavapai orogeny. This agrees with K-spar MDD models showing rapid post-1400 Ma cooling and both are consistent with the 1255 Ma depositional age for the Unkar Group. Forward models are also sensitive to the late heating due to burial beneath ~ 3-4 km of Phanerozoic strata prior to the Laramide Orogeny and show a best match to the date-eU data with maximum temperatures of 120-140 °C, in agreement with apatite thermochronologic data. Forward models also test “old” (70 Ma) versus “young (~ 6 Ma) models for carving of Grand Canyon and require “young” (post- 7 Ma) rapid cooling to explain 3 to 7 Ma ZHe bulk dates over a wide range of eU. Inverse modeling using Ar data as constraint boxes supports the forward model results and conforms with geologic constraints. Thus, the greatest magnitude of basement exhumation below the Great Unconformity was Mesoproterozoic (1300-1250 Ma), which predates proposed Snowball Earth related erosion, and ZHe data do not support “old” (70-20 Ma) carving of Grand Canyon.