UPLIFT AND EROSION OF THE COLORADO PLATEAU AND GRAND CANYON—IMPLICATIONS OF NEW CALCULATIONS OF LARGE-SCALE ROCK UPLIFT, EXHUMATION, AND RIVER INCISION

Construction of GIS databases of stratigraphic information and volume calculations for the Colorado Plateau and Grand Canyon provide key data on long-term uplift and erosion. Total mean Cenozoic rock uplift in the Colorado Plateau is 2133 m. Several hundred meters of uplift are the result of erosional unloading and isostatic rebound. A maximum of 1800 m remains to be explained by non-geomorphic sources of rock uplift. Scant paleobotanical and fission-track data suggest the Laramide Orogeny could easily account for all remaining rock uplift and that there has been no surface uplift since Eocene time. In fact, results indicate only half the Laramide-tectonic rock uplift estimated for the Rockies is evident on the Plateau. Add to this possible rock uplift due to bouyant mantle, and it is evident that there is more proposed uplift than there is actual uplift on the Plateau. Laramide uplift of the Plateau was probably significantly less than thought, and non-isostatic means of post-Laramide epeirogenic uplift are unnecessary. New Pleistocene bedrock incision rates of ~150 m/m.y. along the mainstem Colorado River in eastern Grand Canyon are similar to other estimates of long-term (105 to 106 yr) incision rates upstream along the Colorado River drainage in Utah and Colorado that are based on associations of volcanic deposits to landscape features. Only downstream of the active Hurricane/Toroweap fault is there evidence for significantly different (lower) incision rates at this timescale. Extrapolation of these rates relative to the depth of Grand Canyon suggest either incision rates have decreased through time since incision began or that cutting of Grand Canyon was augmented by occupation of already-cut paleocanyons as suggested by some workers. Calculating the volume of Grand Canyon and assuming this feature has been eroded in the past 6 m.y. results in a long-term sediment yield/denudation rate of 87.2 m/m.y. This is similar to historic sediment yield estimates for the Colorado River drainage based on gauge data. Similar calculations for the entire Plateau since Eocene time result in a mean denudation rate of only 24.1 m/m.y., confirming that the long-term erosion rate of Grand Canyon has been greater than the long-term mean for the region.