THE CENTRAL COLORADO PLATEAU BULLSEYE—LINKING PATTERNS OF QUANTIFIED INCISION, EXHUMATION, AND FLEXURE

Robust calculations of Quaternary incision based on terrace chronostratagraphic records across the Colorado Plateau reveal patterns that can be quantitatively related to lithospheric processes. Fluvial incision integrated over the past several hundred thousand years of glacial cycles is 2-3x faster in the central canyonlands district than at the peripheries of the plateau, including the Grand Canyon area.

This pattern cannot be explained by faulting or the passage of knickzones alone. Instead, we hypothesize that faster incision in the central plateau is causally linked to broader and deeper total recent exhumation of the measurably weaker bedrock there. Keeping in mind the history of the Colorado River drainage, we know that the vast majority of this exhumation has been accomplished in the past 6 my. Geomorphic reconstructions reveal a concentration of 1.5 to 4 km of erosion centered over the central canyonlands region, tapering to zero at the edges of the plateau.

A 3-D flexural model calculation of rock uplift in response to this unloading results in a bullseye of more than 1 km of recent epeirogenic uplift in the central plateau. Analyzing this in the context of the Colorado River long profile, this flexural rebound can account for much of the higher Pleistocene incision rates towards the center of the plateau.