EROSION RATES FROM VARIABLE GRAIN SIZES IN THE TECTONICALLY ACTIVE CALIFORNIA TRANSVERSE RANGES USING 10Be COSMOGENIC NUCLIDES

The concentration of 10Be in alluvial sediments can be used to interpret catchment-scale erosion rates in a variety of landscapes. We have applied this technique to two tectonically active catchments: the Ventura River drainage and the San Gorgonio Pass within the TopaTopa and San Bernardino Mts. in S. California, respectively. We use 10Be TCN concentrations in multiple grain-sizes to determine the erosion rates and understand the role of geomorphic process, sediment transport and source terrains. Within the Ventura River drainage, spatially distributed samples of both sand (0.25 - 0.50 mm) and pebbles (70-100 mm) were collected from the active channel and below Late Pleistocene and Holocene terrace surfaces. Results indicate paleo-erosion rates from the sand size fraction are 0.1mm/yr but rates determined from pebbles are 0.35 to 0.8mm/yr. Pebbles thus imply at least a three times greater erosion rate than sand. The pebble erosion rates, however, may be more appropriate than rates determined from the sand fraction for this high-energy, landslide dominated geomorphic setting. Our data indicate that 10Be concentration, and thus erosion rates and exposure ages, vary with grain size and suggest sediment transport pathways should be considered in any TCN study. To test this hypothesis, four grain sizes (0.25- 0.500 mm, 1-2 mm, 11-22 mm, 100-250 mm) were collected from three transects within the active channel on the tectonically active Millard Canyon Fan. These data show how 10Be concentration varies as a function of grain-size, source-lithology, and downstream transport distance, and provide a calibration dataset to evaluate cosmogenically determined erosion rates in other tectonically active landscapes.