RATES OF KNICKPOINT MIGRATION AND BEDROCK EROSION FROM COSMOGENIC BE-10 IN A LANDSCAPE OF ACTIVE NORMAL FAULTING

Superb exposures of active normal faults are present in The Grabens, Canyonlands National Park, Utah where a ~500m thick brittle surface layer is extending in response to subsurface ductile flow of an evaporite sequence. We examined geomorphic response to normal faulting with longitudinal and cross-sectional stream profiles and cosmogenic Be-10 exposure dating of bedrock with quartz from the Permian Cedar Mesa Sandstone. Streams crossing normal faults typically exhibit knickpoints up to 5m high in our study site at 109 degrees 53 min W. 38 degrees 5 min N. Knickpoints migrate upstream by processes of groundwater sapping that cause undercutting and collapse of sandstone and limestone beds with thickness of >1m. This process effectively resets the cosmogenic isotope system permitting rates of knickpoint migration (1.2+/-0.5m/ka) and timing of faulting (7.7+/-3.8ka) to be estimated. In contrast, normal streamflow is ineffective at zeroing the cosmogenic clock in this semi-arid environment. This is shown by modern stream channels with apparent Be-10 exposure ages of ~22ka to 55ka. These results suggest that bedrock incision in the study area is accomplished primarily by knickpoint retreat rather than by normal stream erosion.