PATTERNS OF INCISION, KNICKZONES, AND DEFORMATION IN THE CENTRAL COLORADO PLATEAU

Recent research in the Colorado Plateau provides multiple hypotheses for late Cenozoic epeirogenic uplift, reveals intriguing patterns of differential incision and river knickzones, and indicates variable degrees of normal-fault activity. Yet, few quantitative constraints exist for incision and deformation in the heart of the plateau. New OSL chronostratigraphic records of river terraces in southeastern Utah add to other well-constrained records and provide a snapshot of incision and deformation patterns over the past 10^5 yr timescales across the transient landscape of the Colorado Plateau.

The Green River at Crystal Geyser, below Desolation Canyon, has incised at a relatively rapid pace of 450 m/my over middle-late Pleistocene time, but there has been no deformation of terraces or surface rupture here where the river crosses the Little Grand Wash normal fault. Along the Colorado River upstream of Moab, Utah the fastest incision rates in the region are documented in an area confirmed to have active salt-related uplift and subsidence. Upstream of these two study areas lie Desolation and Westwater Canyons, respectively, and above those steeper knickzones, incision rates calculated by other workers are much slower. These apparently rapid decreases in incision rates upstream are more abrupt than predicted by just the “bull’s-eye” pattern of isostatic rebound from region-wide erosion. These knickzones have distinctly different patterns than does the incision around Cataract and Grand Canyons downstream, with these upstream canyon-knickzones in the central plateau holding up the upstream-migration of incision from regional baselevel fall.

Regarding deformation, the inactive Little Grand Wash fault contrasts with related structures of the ancestral Paradox Basin, where it has long been suggested that active salt diapirism and graben subsidence is due to the generation of high local relief by recent erosion combined with groundwater and river dissolution. I hypothesize there may also be an older Plio-Pleistocene component of fault slip in the broader region starting at the onset of significant erosional unloading and domal rebound, enhancing extension. Thus, all the faults of the central plateau may share erosional unloading, at different scales, as a common driver for late Cenozoic deformation.