EXHUMATION HISTORY OF THE COLORADO PLATEAU

Questions about the interaction of erosion, rock uplift, and surface uplift in shaping the scenic landscape of the Colorado Plateau have been contemplated since the days of John Wesley Powell. We attempt to address these questions using apatite fission-track (AFT) and (U-Th)/He (AHe) thermochronometry on sandstones and basement rocks from drill holes and canyons to provide key data about the low temperature (<110°C) cooling and exhumation history of the Colorado Plateau. Stratigraphic and geomorphic landscape reconstructions, coupled with geodynamic modeling, place constraints on rock uplift. Preliminary analysis of these integrated data sets reveals a pattern of variable erosional and rock uplift patterns across the Plateau. Laramide unroofing and a distinct pulse of middle Cenozoic exhumation are recorded in thermochronometric data from the southwestern portion of the Plateau in the Grand Canyon region. In contrast, late Cenozoic exhumation dominates the erosional history of the interior and the eastern edge of the Colorado Plateau. AHe data from shallow (<800 m) drillholes in the Canyonlands to Lake Powell region of southern Utah record <10-6 Ma cooling from T>70°C. To the north, the Uinta Basin has been slowly exhuming since Oligocene time, based on AFT data from three different drill holes. The San Juan Basin cooled from T > 110°C due to erosional exhumation starting at <15 Ma. AFT ages in the San Juan Basin decrease toward the north. The Piceance Basin began cooling <7 Ma, with cooling ages decreasing toward the east. The Aspen anomaly, a region of slow seismic velocities in the upper mantle at depths of about 100 km beneath southwestern Colorado, appears to influence the exhumation history of the southeastern Colorado Plateau. Sources of Cenozoic rock uplift across the Plateau include Laramide deformation, isostatic flexure due to Cenozoic erosion, and Cenozoic buoyancy modification of Colorado Plateau lithosphere. Even though modeling indicates mild plateau-averaged net uplift due to unloading and flexure, this rebound is focused in the central Plateau and is focused in late Cenozoic time. Thus this mechanism has a notable influence on more recent patterns of erosion and uplift of the Colorado Plateau.