Rates of erosion and topographic evolution of the Sierra Nevada, California, inferred from cosmogenic 26Al and 10Be concentrations

Concentrations of cosmogenic Al and Be in cave sediments and bedrock surfaces, combined with studies of landscape morphology, elucidate the topographic history of the southern Sierra Nevada over the past 5 Ma. Caves dated by Al/Be in buried sediments reveal that river incision rates were moderate to slow between c. 5 and 3 Ma (≤0·07 mm a), accelerated between 3 and 1·5 Ma (c. 0·3 mm a−), and then have subsequently become much slower (c. 0·02 mm a). Although the onset of accelerated incision coincides in time with both postulated Pliocene tectonism and pronounced global climate change, we argue that it primarily represents the response to a discrete tectonic event between 3 and 5 Ma. Dated cave positions reveal that, prior to 3 Ma, river canyons displayed up to 1·6 km of local relief, suggesting that Pliocene rock uplift elevated pre-existing topography. Renewed incision beginning c. 3 Ma deepened canyons by up to 400 m, creating narrow inner gorges. Tributary streams exhibit strong convexities, indicating that the transient erosional response to Pliocene uplift has not yet propagated into upland surfaces. Concentrations of Al and Be in bare bedrock show that upland surfaces are eroding at slow rates of c. 0·01 mm a. Over the past c. 3 Ma, upland surfaces eroded slowly while adjacent rivers incised rapidly, increasing local relief. Although relief production probably drove at least modest crestal uplift, considerable prePliocene relief and low spatially averaged erosion rates suggest that climatically driven rock uplift is not sufficient to explain all uplift implied by tilted markers at the western edge of the range. Despite the recent pulse of erosion, spatially averaged erosion rates are low, and have probably acted to preserve the broad topographic form of the Sierra Nevada throughout much of the late Cenozoic. Copyright © 2005 John Wiley & Sons, Ltd.