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

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.

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

“…9b). In steeper landscapes, such as LC, boulders will be more mobile and may roll down the hillslopes, eventually ending up in stream channels where they may shield the channel bed from denudation (DiBiase et al, 2017;Shobe et al, 2016;Fig. 9c).…”

“…Larger boulders require greater forces to be moved, which can be achieved by steepening slopes (Granger et al, 2001;DiBiase et al, 2018;Neely and DiBiase, 2020) or by lengthening residence time until subaerial weathering has decreased their size sufficiently to be transported downslope. During this prolonged residence time, boulders can shield hillslopes from erosion (Glade et al, 2017;Chilton and Spotlia, 2020) and stream channels from incision (Shobe et al, 2016;Thaler and Covington, 2016). In terrain where spatial gradients in bedrock fracture spacing result in spatial gradients of hillslope sediment size, it is thus reasonable to expect the resistance of surface boulders to weathering and transport to retard erosion locally, resulting in spatially differential erosion.…”

“…In terrain where spatial gradients in bedrock fracture spacing result in spatial gradients of hillslope sediment size, it is thus reasonable to expect the resistance of surface boulders to weathering and transport to retard erosion locally, resulting in spatially differential erosion. Moreover, fractured and therefore weaker bedrock facilitates erosion via both abrasion and plucking by streams (Lamb et al, 2015;Sklar and Dietrich, 2001), and smaller blocks are also more easily transported in fluvial systems (Shobe et al, 2016). Therefore, we would expect rivers to preferentially incise in zones of more intensely fractured rocks (Buss et al, 2013) that align with the orientation of faults (Molnar et al, 2007;Roy et al, 2016).…”