163-G 1.2

The role of mass wasting in glacial forelands of the Andes

Large segments of the Andean foreland have been repeatedly shaped by Quaternary glaciations. The many diagnostic landforms include large glacial lakes, staircases of moraine ridges, and extensive outwash plains, and have inspired generations of Quaternary geologists to reconstruct the processes, magnitude, and timing of ice build-up and decay along the mountain front, adding to a reference chronology of Southern hemisphere glaciations. What only a few of these studies have noticed are several hundreds of very large (>>106m3) mass-wasting deposits that fringe the Andean foreland. Many of these debris mounds intersect with many well-dated moraine ridges or former meltwater-lake shorelines and offer exciting opportunities of exploring the hitherto largely unknown role of mass wasting in the glacial forelands of the Andes.

Studying the timing of these large landslides provides a stringent test for models of paraglacial landscape evolution. Preliminary work on large landslides glacial moraines indicates that moraines can fail catastrophically several thousand years after they formed. Several landslide bodies entered former glacial lakes shown by distinct horizontal breaks in landslide deposit morphology, thus raising the possibility of past and future landslide tsunamis.  

The project aims to understand what we can learn from and what can we generalize about the mass-wasting activity of low-gradient glacial forelands. The Andean foreland may well host the largest cluster of (relatively) dated large, low-gradient landslides on Earth, which so far has been elusive in studies of the Andean sediment cascade. How does this estimate compare to sediment transport data, and what do we learn about sediment transfer from glaciated mountain belts to their proximal forelands?

10|2015 – 09|2018

Rock slides vs. rock glaciers: feeding the central Andean sediment cascade

This project explores the role of both large (>106 m3) catastrophic rock slides and rock glaciers as prime movers of the central Andean sediment cascade. Recent hypotheses concerning the triggers of large non-volcanic bedrock landslides in the central Andes favor earthquakes, judging from the distribution of tell-tale landslide deposits with respect to active faults and shallow seismicity. Rock glaciers share a very similar topographic niche, but are traditionally viewed as diagnostic of sporadic alpine permafrost, though they may have also originated from earthquake-triggered supraglacial rock slides. Rock slides and rock glaciers are not only voluminous point sources of coarse debris, but also decisive barriers to incoming sediment flux. The aim of this project is to quantify to first order the regional net balance between such sediment release and sequestration by large rock slides and rock glaciers in central Andean headwaters using a multi-scale methodology: First, we will expand an existing regional inventory of large landslides and rock glaciers in the region to quantify the spatial pattern, topographic characteristics, and volumetric distribution of large Andean debris deposits from digital topography and remote sensing data. We will compare classic operator-based mapping with state-of-the-art automated object-oriented mapping techniques. Second, fieldwork will involve local ground truthing of landslide and rock-glacier geometries and provide vital input data for gauging regional volumetric budgets of denudation rates and intermittent sediment storage. We will estimate the fraction of valley fills causally linked to catastrophic slope failure and rock-glacier dynamics to gauge the overall relevance of catastrophic hillslope input to the central Andean sediment cascade. Samples collected in the field will further provide age constraints of strategically selected rockslide and rock-glacier surfaces or correlate backwater sediments with 14C, 10Be, lichenometry or dendrochronology, depending on available samples. Third, we will quantify metrics of geomorphic impact of these deposits on the fluvial network (changes in fluvial transport capacity, formation of knickpoints, epigenetic bedrock meanders, etc.), and the sediment cascade (barrier lakes, floodplain aggradation, etc.). We will expand existing numerical models of channel adjustment to landslide and rock glacier impacts to estimate fluvial response and recovery times. Similar work that we conducted in other regions revealed decisive controls of large landslide deposits on bedrock channel geometry, and the size and age distribution of valley fills that are potential sinks of alluvial georesources. Moreover, large river-blocking rock slides and rock glaciers may form important temporary buffers to incoming, and potentially adverse, sediment pulses from local disturbances.