16 Aug 2022
16 Aug 2022
Status: this preprint is open for discussion.

The story of a summit nucleus: Hillslope boulders and their effect on erosional patterns and landscape morphology in the Chilean Coastal Cordillera

Emma Lodes1, Dirk Scherler1,2, Renee van Dongen3, and Hella Wittmann1 Emma Lodes et al.
  • 1GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
  • 2Freie Universität Berlin, Institute of Geological Sciences, 12249 Berlin, Germany
  • 3International Centre for Water Resources and Global Change, 56068 Koblenz, Germany

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.

Emma Lodes et al.

Status: open (until 30 Sep 2022)

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  • RC1: 'Comment on egusphere-2022-619', Anonymous Referee #1, 31 Aug 2022 reply

Emma Lodes et al.

Emma Lodes et al.


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Short summary
We investigate the effect of fractures on the location of hills and valleys in bedrock landscapes, by comparing erosion rates of unfractured bedrock versus soil. Unfractured bedrock erodes slower, and soil, which likely overlies fractured bedrock, erodes faster. We also find that streams generally follow the orientations of faults. Together, our data show that fractures influence landscapes by weakening bedrock, causing it to erode faster and to eventually form a valley where a stream may flow.