LOW-TEMPERATURE THERMOCHRONOMETRIC CONSTRAINTS ON RETREAT OF THE VERMILLION CLIFFS AND CARVING OF MARBLE CANYON

Previously published and unpublished apatite (U-Th)/He (AHe) and apatite fission track data in the Marble Canyon region of the Grand Canyon, Arizona, provide constraints on the denudation of this part of the Kaibab Plateau by retreat of the ~ 1 km high Echo and Vermillion cliffs, and the timing of carving of Marble Canyon. AFT ages east of the Kaibab uplift indicate that river- level rocks remained at temperatures of > 100 C until ~70 Ma near the Butte fault and 35-40 Ma near Lee’s Ferry. AHe is the preferred thermochronologic system for constraining timing of cliff retreat and canyon incision. Mesozoic strata in the Vermillion and Echo cliffs form a ~1 km cliff above the Kaibab plateau. Assuming parallel cliff retreat, a surface temperature of 10-25°C, and a geothermal gradient of 20-25°C/km,rocks on the plateau should have cooled through 30-50°C as the cliffs retreated.. Samples PGC-011, -006, -002 (Flowers et al., 2007; 2008) near the top of the Kaibab uplift have minimum AHe ages of 20-25 Ma suggesting the uplift was covered by much of the Mesozoic section until 20-25 Ma. Unpublished data from Stockli along Highway 89A at the base of the Vermillion cliffs also give minimum AHe ages of 20-40 Ma and suggest this area resided in the AHe partial retention zone over this timeframe. River-level samples from Lee et al. (2013) at River Mile 12 shows rapid cooling 20-15 Ma. Collectively, these data suggest rapid retreat of the Mesozoic escarpment during carving of the East Kaibab paleocanyon 25-15 Ma such that the Mesozoic escarpment was near its present position ~ 20-15 Ma. Samples at RM 4 and near Lees Ferry cooled rapidly after ~ 10 Ma. We interpret these to reflect timing of initiation (~ 6 Ma) of carving of Marble Canyon below the Kaibab surface by the Colorado River. Sampling and AHe analysis along both horizontal and vertical transects, on the plateau and at river level throughout Marble Canyon, is underway to better resolve the timing of integration of the Colorado River, the role and rates of cliff retreat of the surrounding cliffs, and the rate of migration of the Lees Ferry knickpoint.