The story of a summit nucleus: Hillslope boulders and their effect on erosional patterns and landscape morphology in the Chilean Coastal Cordillera
Emma Lodes, Dirk Scherler, Renee van Dongen, Hella Wittmann
Abstract
Abstract. While landscapes are broadly sculpted by tectonics and climate, on a catchment scale, the density of bedrock fractures can influence hillslope denudation rates and dictate the location of topographic highs and valleys. In this work, we used 10Be cosmogenic radionuclide analysis to measure the denudation rates of bedrock, boulders, and soil, in three granitic landscapes with different climates in Chile, with the hypothesis that high fracture density reduces grain size and increases denudation rates. Denudation rates range from 10 to 15 m Myr-1 for bedrock and boulders and from 15 to 20 m Myr-1 for soil in the humid and semi-arid climates, and are higher in the mediterranean climate (~40–140 m Myr-1), likely due to steeper slopes. We found that hillslope bedrock and boulders erode more slowly than the surrounding soil in the diffusively-eroding study sites. Furthermore, across a bedrock ridge in the humid site, bedrock denudation rates increase with fracture density. These findings are consistent with the observation that streams in our field sites follow the orientation of at least one major fault orientation. Our results suggest that tectonically-induced fractures and faults dictate landscape evolution through reducing grain size and thus enhancing differential denudation rates.