2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 3
Presentation Time: 8:40 AM


STOCK, Gregory M. and ANDERSON, Robert S., Department of Earth Sciences, Univ of California, Santa Cruz, Santa Cruz, CA 95064, gstock@es.ucsc.edu

Considerable debate surrounds the uplift and geomorphic history of the Sierra Nevada range of California. West-tilted Miocene volcanic flows and increasing dip of late Tertiary strata with age on the west flank of the range have been used to argue for late Cenozoic (2-10 Ma) surface uplift of the range crest. Recent geophysical and themochronologic work instead suggest that the mean elevation and local relief of the range were greatest during the late Mesozoic (60-80 Ma) and have been steadily decreasing since that time. One means of investigating the uplift history of mountain belts is by documenting changes in river incision rates, as river incision is particularly sensitive to changes in slope. We focus on the western slope of the Sierra Nevada, where deeply incised river canyons punctuate broad, low-relief interfluves. The most deeply incised portions of these canyons are typically cut in metamorphic pendants that flank the Sierra Nevada batholith; where the dominant lithology is marble, the canyons walls often contain numerous caves. Many caves and cave levels, now high in the canyon walls, can be shown to have formed along water table surfaces graded to former river positions. With age constraint, these caves provide an unique and important means of determining incision rates for many of the major river systems. We use 230Th/234U dating of speleothems (age range 0-0.5 Ma) to constrain ages for young caves close to modern river levels, and paleomagnetism and cosmogenic 26Al/ 10Be burial dating (age range 0.3-5 Ma) of sediment for older, higher caves. These cave ages, going back 2.5 Myr, present a detailed picture of the response of Sierra Nevada rivers after (and perhaps during) purported late Cenozoic uplift. Numerical models of landscape evolution provide insight into the expected response of rivers to the proposed forcing mechanisms of late Cenozoic uplift. Using physically-based models incorporating orographic precipitation, stream power-based river incision, and the flexural-isostatic response to both erosional unloading and delamination of an eclogitic root, we strive to find the best match between the forcing mechanisms and the cave-derived incision data. In this manner, caves offer valuable, and previously unrecognized, insight into the late Cenozoic evolution of the Sierra Nevada topography.