The Andes Natural Laboratory
I study landscape evolutions in mountainous regions using principles of tectonic and process geomorphology (i.e. river incision into bedrock).
Mountain ranges are a function of the competition between mass added by tectonic processes and removed by erosive processes on the surface. As surface processes depend on climate, herein lies a competition between tectonics and climate in driving the evolution of Earth's mountain ranges. In my group, we study the partitioning between climate and tectonics in the evolution of mountains.
mountain range evolution
climate vs tectonicS
Figure 1: Topographic map of western South America highlighting the Andes orogen. Source: Val and Willenbring (2022).
One of the fundamental questions in geoscience, which has cross disciplinary implications is the question: what processes govern the creation, maintenance, and removal of high mountain topography above subduction zones?
At the core of this question is a central hypothesis: if wetter and/or colder climatic regimes can drive surface processes to erode the surface more efficiently over the timescale of mountain building, then spatial and temporal changes in climate may control how wide an orogen gets as well as how either side of an orogen may be wider or narrower due to rain shadow and orographic rainfall effects, respectively.
One way to assess the hypothesis and question above is by quantifying spatial patterns in mountain topography, orogen width, and erosion rates in a mountain range which experiences different climatic regimes or gradients. The Andes orogen (Figure 1 - on the left) is a great natural laboratory with these kinds of conditions. Notice in Figure 1 how the orogen narrows from the center (the Altiplano-Puna Plateau) to the north and south. These changes vary systematically with changes in the amount of rainfall that either side of the orogen experiences, but also the age of the subjecting Nazca plate (see Figure 2).
I have investigated the current distribution of topography and cosmogenic-nuclide erosion rates in the active Cordilleran mountains in the southern Central Andes. Here, I observed similar erosion rates across climatic gradients (including rainfall, runoff, and storm frequency):
• Tectonic control of erosion rates in the southern Central Andes
Val, P; Venerdini AL; Ouimet W; Alvarado P; Hoke GD. 2018 Earth Planet. Sci. Lett. 482, 160-170, doi: 10.1016/j.epsl.2017.11.0004
I sought to evaluate the same question along the entire Andes. The results corroborate the results from the southern Central Andes: rainfall rates cannot explain erosion rates. Instead, the age and geometry of the subducting Nazca plate explain how steep and wide the orogen is on the west versus the east! Interestingly, a counter-intuitive finding when looking at all of the erosion rate data available in the Andes is that how dry it gets (i.e. 0-0.5 m/yr) matters more in terms of climate’s influence on landscape evolution, not how wet it gets (i.e. orographic rainfall)! See:
• Across-strike asymmetry of the Andes orogen linked to the age and geometry of the Nazca plate
Val, P; Willenbring, JK, 2022 Geology, 50 (12), 1341–1345, doi: 10.1130/G50545.1
Figure 2: Rainfall map of western South America showing across-Andes lines showing the width of the Orogen and also highlighting the ages of the subducting Nazca plate. Source: Val and Willenbring (2022).
The erosional response to mountain building
Figure 3: Conceptual models of erosional response to mountain building. Source: Val et al (2016).
What happens to the landscape during mountain building? As mountain ranges grow, rivers are in charge of incising into their underlying bedrock. How fast does this happen and can they keep up with the tectonic forces literally lifting the ground underneath? The answer depends in side of the mountain range we're talking about (i.e. upstream or downstream of the growing range).
First a bit of background: the temporal geomorphic response to mountain building had only scarcely been demonstrated until 2010s, in part due to the scarcity of methods available that could capture sediment fluxes in deep time (i.e. millions of years ago) but still with thousand-year resolution (i.e. a real snapshot of erosion in a moment in the geologic past). However, we new what to expect based on analytical and numerical models (Figure 3).
To do this, one needs a sedimentary record (i.e. foreland basin deposits) that contains a continuous history of sediment infill coming from the adjacent growing mountains. Like in the most common use of cosmogenic nuclides, sediments currently carried by rivers contain concentrations of 10Be that are proportional to
erosion rates. In old buried sediments, cosmogenic nuclides can still be measured to quantify erosion rates (paleoerosion rate) provided that the age of those sediments is independently well known and not too old that the 10Be hasn't decayed completely. If those conditions are met, records of 10Be from foreland basins upstream and downstream of an active fold-and-thrust belt can reveal the temporal erosional signature of the adjacent, growing mountain ranges.
Back to the erosional response: In southern Central Argentina, more especifically in Valle de Iglesia (Figure 4), foreland basin stratigraphy is preserved with remarkable temporal continuity and age-constraints both upstream and downstream of the growing mountain range. Using sediments from these basin rocks which are dated between 8 and 3 million years (i.e. when the mountains were growing), I obtained paleoerosion rates history to constrain the dynamics of erosion during mountain building. The data demonstrated that the attainment of peak erosion lags mountain building by ~2 Ma downstream of the growing mountains (Figure 5). The data also showed that erosion rates decrease upstream of the growing mountains either related to the creation of internally drained conditions or due to base-level rise (likely the latter).
Figure 4: Valle de Iglesia, an intermontane valley near Rodeo, Argentina. The photo points NE. The shown mountain range is the backlimb of the westernmost E-verging thrust-sheet of the Precordillera fold-and-thrust belt. Photo credit: Self.
Figure 5: Paleo-erosion rate data from the Precordillera fold-and-thrust belt. Dashed line shows the shortening history (i.e. when mountains were growing); squares show the paleo-erosion rate data; lines show the sediment accumulation in basins. (A) Data from the intermontane valley (shown in Figure 4); (B) Data downstream of the fold-and-thrust belt. Source: Val et al (2016).
• Reconciling tectonic shortening, sedimentation and spatial patterns of erosion from 10Be paleo-erosion rates in the Argentine Precordillera.
Val, P.; Hoke, GD; Fosdick, JC; Wittmann H. 2016 Earth Planet. Sci. Lett. 450, 173–185 doi: 10.1016/j.epsl.2016.06.015
Interestingly, the temporal patterns (i.e. the ups and downs) of erosion rates that we obtained from the Argentine Precordillera were reproduced in numerical models of landscape evolution (see video bellow). In a work spearheaded by Greg Ruetenik, we simulated a landscape responding to the formation of a fold-and-thurst belt:
• Regional landscape response to thrust belt dynamics: The Iglesia basin, Argentina.
Ruetenik, G; Moucha, R; Hoke, G; Val, P. 2018, Basin Research. 30, 6, 1141-1154
Video simulating a scenario representative of an evolving fold-and-thrust belt (Ruetenik et al., 2018).
If you want to learn more about applying Cosmogenic Nuclides and paleoerosion rates, I wrote a paper and software to help others apply it:
• A practical tool for examining paleo-erosion rates from sedimentary deposits using cosmogenic radionuclides: examples from hypothetical scenarios and data.
Val, P.; Hoke, GD. 2016 Geochemistry, Geophysics, Geosystems 17, 1-11, doi: 10.1002/2016GC006608