A modern analog of past climatic impacts on denudation rates and sediment transport: The Del Medio fan, NW Argentina

Abstract:

The combined effects of tectonic and climatic forcing govern the evolution of landscapes, modifying erosional processes, changing river profiles, and ultimately impacting depositional systems. The complexity of the response, however, often means that we cannot predict a priori how a given landscape will react to climatic changes.

The Del Medio catchment is located in the Humahuaca Basin, an intermontane valley of the Eastern Cordillera. This area coincides with a climatic divide between a sub-humid and a semi-arid environment. An extensive debris-flow fan sourced in the Del Medio catchment covers ca. 18.6 km² of the lower basin. To investigate rates and timing of its evolution, we analyzed cosmogenic ¹⁰Be concentrations on large boulders from the fan surface, river sands and pebbles in 3 active channels, a depth profile, and bedrock exposed atop the drainage basin margins. Our CRN results illustrate the rapid rate at which the fan surface is subject to change, with each of the 16 analyzed boulder samples providing ages of < 400 years. The 15-m deep depth profile exposes three cycles of activity in the recent past. In addition, river sands record very high denudation rates (from several mm/yr to tens of mm/yr), despite bedrock denudation rates of < 0.04 mm/yr from most bedrock ridges. Interestingly, modern pebbles from tributaries with recent landslide activity consistently yield denudation rates that are faster than those from sands, while tributaries without landslide activity show the opposite pattern, documenting the importance of grain-size dependency for inferring the main mechanism of sediment transport within a catchment.

Farther upstream in the more arid Humahuaca Basin, abandoned fluvial terrace sequences have been linked to wetter episodes during the late Quaternary. Within those fills, preliminary paleo-denudation rates reach values similar to those currently observed in the Del Medio catchment, while modern denudation rates from river sands are similar to the low rates observed from bedrock ridges. Taken together, these observations indicate that the Del Medio fan could be a modern analog of those older fill sequences, providing a unique opportunity to link present-day processes to those that were likely active in the past during wetter climate episodes.