Basins and bedrock; spatial variation in (super 10) Be erosion rates and increasing relief in the Southern Rocky Mountains, USA

We used measurements of cosmogenic (super 10) Be in alluvium to estimate erosion rates on a 10 (super 3) - 10 (super 4) yr time scale for small (0.01- 47 km (super 2) ), unglaciated basins in northern Colorado, southern Wyoming, and adjacent western Nebraska (western United States). Basins formed in Proterozoic cores of Laramide ranges are eroding more slowly (23 + or - 7 mm k.y. (super -1) , n = 19) than adjacent basins draining weakly lithified Cenozoic sedimentary rocks (75 + or - 36 mm k.y. (super -1) , n = 20). Erosion rates show a relationship to rock resistance and, for granitic rocks, to basin slope, but not to mean annual precipitation. We estimated longer-term (>10 (super 5) yr time scale) erosion rates for the granitic core of the Front Range by measuring the concentration of (super 10) Be and (super 26) Al produced mainly by muon interactions at depths 1.7-10 m below the surface. Concentrations imply erosion rates of 9-31 mm k.y. (super -1) , similar to shorter-term erosion rates inferred from alluvial sediment. The spatial distribution of erosion rates and stratigraphic evidence imply that relief in the southern Rocky Mountains increased in the late Cenozoic; modern relief probably dates from post-middle Miocene time.