PLEISTOCENE RELIEF PRODUCTION IN LARAMIDE MOUNTAIN-RANGES, WESTERN UNITED-STATES

Gently sloped summits and ridges (collectively referred to as summit f lats) are abundant in many Laramide ranges in the western United State s, The erosion rate of summit flats is similar to 10 m/m.y., on the ba sis of the concentrations of cosmogenic radionuclides. Because erosion rates in valleys between summit flats are an order of magnitude faste r, relief within these ranges is currently increasing by about 100 m/m .y. If summit-flat erosion is slower than rock uplift driven by the is ostatic response to valley erosion, then this relief production could result in increased summit elevations. The mean depth of material erod ed from a smooth surface fit to existing summit flats varies from 280 to 340 m in four Laramide ranges, based on geographic information syst em (GIS) analyses of digital elevation models, This erosion would resu lt in a maximum of 250-300 m of rock uplift, assuming Airy isostasy, H owever, because the Laramide ranges examined here are narrow relative to the flexural wavelength of the lithosphere, erosionally driven rock uplift is limited to similar to 50-100 m, Over the past several milli on years, summit erosion would approximately offset this rock uplift, Therefore, we conclude that summit elevations have remained essentiall y constant even though several hundred meters of relief has been produ ced, On the basis of valley and summit erosion rates and the average d epth of erosion, we estimate that relief production in Laramide ranges began at ca. 3 Ma. We hypothesize that this relief production was cli matically driven and was associated with the onset or enhancement of a lpine glaciation in these ranges.