WEAK BEDROCK, FAST INCISION, DEEP EXHUMATION, AND FLEXURAL REBOUND IN THE CENTRAL COLORADO PLATEAU: A CONFLUENCE OF NEW DATASETS

A series of recent and ongoing studies in the Colorado Plateau are revealing similar patterns that distinguish the center of this province from its edges, in terms of both late Cenozoic erosion and geodynamics. The surficial and geophysical processes represented by these data may be intertwined in an elegant and messy way. First is a rich dataset comparing field and laboratory measurements of bedrock strength to hydraulic and geomorphic characteristics across Grand and Glen canyons. Reaches of Grand Canyon have older bedrock with significantly greater tensile and compressive strengths and statistically smaller fracture spacing than Glen Canyon in the central plateau. These measurements correspond to Grand Canyon having reaches that are, on average, four times steeper, one half as wide, and expending nine times more unit stream power than Glen Canyon reaches. This implies that the remarkably irregular profile of the Colorado River reflects some level of dynamic equilibrium with bedrock resistance, rather than hosting a series of transient knickzones. Second are increasingly robust calculations of Quaternary incision rates from terrace chronostratigraphic studies in Grand Canyon and from upstream at Lees Ferry and other spots in the central plateau. These indicate that fluvial incision integrated over glacial cycles is 2-3x faster in the central-plateau canyonlands than along surrounding reaches of trunk rivers. Thirdly, calculations of total late Cenozoic exhumation based upon improved geomorphic reconstructions indicate a concentration of up to 4 km of erosion centered over the Glen Canyon-Canyonlands region, tapering to zero at the edges of the plateau. Key to an explanation for these converging patterns are model calculations of flexural rock uplift in response to this unloading, which suggest a welt of up to 1 km of recent epeirogenic uplift in the central plateau.

Linking these results together, it is clear that the post-6 Ma pulse of incision that has diffused up the Colorado drainage has recently expressed itself with rapid erosion of the central canyonlands. Transient erosion may be focused in the region partly due to the measurably weak bedrock. Finally, the positive feedback of flexural response to this exhumation could account for part of the anomalously high Quaternary incision rates in the same region.