THERMAL HISTORY OF THE WHEELER RIDGE PALEOCANYON AND IMPLICATIONS FOR THE EROSION KINEMATICS OF WESTERN GRAND CANYON

We acquired 27 new (U-Th)/He ages from seven samples of crystalline basement rocks at Wheeler Ridge, a Basin and Range fault block immediately west of the Colorado Plateau. Wheeler Ridge hosts a pre-tilt (pre-15 Ma) erosion surface, the Wheeler Ridge paleocanyon, with the same dimensions and erosion level as western Grand Canyon (WGC). A key question in regard to the Wheeler Ridge paleocanyon is whether it is a fragment of a pre-15 Ma Grand Canyon, “calved off” of the western margin of the Colorado Plateau by normal faulting (Wernicke, 2011, GSAB). One test is whether the thermal history of the basement beneath the Wheeler Ridge paleocanyon is similar to that of western Grand Canyon basement rocks. If similar, it would strengthen its correlation with WGC and suggest that Grand Canyon had already formed by 15 Ma, inconsistent with the hypothesis that it was carved by the post-6 Ma Colorado River. The new data yield distributions of mean sample age, overall mean age, and eU that are indistinguishable from a population of 42 published ages from western Grand Canyon, indicating their thermal histories, however interpreted, are essentially the same. A new stratigraphic compilation of the thickness of pre-Laramide Mesozoic strata in the region indicate temperatures in basement rocks just below the Great Unconformity were both regionally uniform and within the range of 106-130 °C for tens of millions of years prior to unroofing, assuming a range of 20-30 °C/km in geothermal gradient. This result is confirmed independently by an AFT transect on the western edge of the Hualapai Plateau that demonstrates that the top of the pre-Laramide PRZ lay 300 m below the Great Unconformity in the WGC area (Fitzgerald et al., 2009, Tectonics), indicating a temperature of c. 110 °C prior to Laramide unroofing. These data, combined with the lack of age-eU dependence of the sample population, preclude recent speculation (e.g., Winn et al., 2017, EPSL) that WGC basement apatites (1) resided at a broad range of relatively low pre-Laramide temperatures (70 to 105 °C) during the Mesozoic, (2) were host to significant radiation damage prior to unroofing, or (3) resided at temperatures >35°C after 60 Ma. The data instead strongly favor incision of WGC to within 200-400 m of its current depth in the Late Cretaceous (Wernicke, 2011; Flowers and Farley, 2012, Science).