Changes in denudation rates and erosion processes in the transition from a low-relief, arid orogen interior to a high-relief, humid mountain-front setting, Toro Basin, southern Central Andes

In tectonically active mountain belts positive correlations between denudation rates and hillslope angles are commonly observed, supporting the notion that landscape morphology may reflect tectonic forcing. However, this relationship generally breaks down at 30°, when hillslopes reach threshold angles. Beyond this threshold, faster denudation may occur by an increased contribution from mass-wasting processes. We test this idea in the 4000 km2 Toro Basin, a fault-bounded basin in the Eastern Cordillera of the southern Central Andes. This N-S oriented basin is located between low-relief, arid conditions in the orogen interior (N) and a high-relief, humid setting at its fluvial outlet (S). We measured in-situ produced 10Be concentration in fluvial sediments, which can be converted into basin-mean denudation rates, assuming a spatially uniform contribution of sediment from the catchment. However, in landslide-influenced areas, this assumption is often violated. Previous studies have suggested that clast-size material is mainly contributed by mass-wasting processes, whereas sand is derived from a broad range of erosional processes. Hence, a combination of clast and sand samples can reveal information about the basin-mean denudation rate as well as the contribution of mass-wasting processes. We sampled 13 pebble (1-3 cm) and sand (250-500 µm) pairs across the basin. The sand-derived denudation rates increase from N to S, ranging from 0.010 mm/yr to 1.337 mm/yr, and reveal a non-linear positive correlation with median basin slope. The clast/sand ratios also increase from N to S, indicating amplified mass-wasting processes with increasing slopes. To test if our ratios represent a real shift in erosional processes, we mapped different erosional processes in the study area (e.g. deep-seated landslides, scree erosion,.., diffusion). We assume that today's distribution of processes has not changed over the integration time of 10Be derived denudation rates. This detailed erosion inventory indicates a shift in the dominant erosional processes with increasing clast/sand ratios and thus with increasing slopes. We provide empirical data supporting the hypothesis that higher denudation rates can be achieved by an increased contribution of mass-wasting processes after threshold slopes have been reached.