Understanding the geology of the Andes

"Understanding how the Andes Mountains have evolved over time is of paramount importance for understanding non-collisional mountain building, global climate change and the impact of lateral orogenic plateau accretion on evolutionary and biotic changes"

Dr Aude Gébelin, Lecturer in Tectonics


With a mean elevation of ~4000 m, the Andes represent the second highest mountain range on Earth, after the Himalaya! In contrast to the Himalaya-Tibet orogen, the Andes represent a non-collisional subduction orogen, situated at the convergent margin between the subducting oceanic Nazca plate and the overriding South American continent. One of the main assets of this mountain belt is the Puna Plateau, a high elevation orogenic plateau of ~3700 m with thick crustal roots characterised by low internal relief and arid conditions and where the volcanic activity is intense!

However, despite decades of research, the mechanisms responsible for the plateau development and associated features – but also the impact of this topographic high on the long-term climatic evolution and atmospheric circulation patterns of the Earth – are the focus of considerable debate. As a consequence, all of these aspects make this active growing subduction orogen an exciting place to work on and the perfect place to study the interactions among tectonics, topography and climate!

I started as a PhD student to focus on the processes of large-scale deformation of continental lithosphere in the Variscan belt of Western Europe. This old mountain chain was located 300 million yeas ago near the Equator but is now totally eroded

The challenges of Andes research

Working in such old areas is very challenging because the outcrop conditions are very poor, due in part, to the flat landscape that characterized them. As a consequence, you don’t (or hardly ever) see the geological structures in 3D which is very frustrating! It means you need to be tenacious to find good outcrops and have a good sense of the 3D visualization if you want to have a chance to reconstruct the geological history of this belt.

What I really like now, working in Ecuador, is the geographical and environmental situation which is comparable to that of the Variscan belt of western Europe during the upper Carboniferous!

Although totally different in age and derived from a different tectonic context, the two mountain chains evolved in a warm equatorial environment! Part of my work aims at quantifying the elevation history of orogen using stable isotopes and recent results obtained, as part of the project of my PhD student Camille Dusséaux, indicate that the hinterland regions of the Variscan belt in western Europe were very likely at similar elevation than the Ecuadorian Andes. This allowed me to project myself 300 million years earlier, observing the landscape and vegetation from near sea level till 4000 m elevation. I couldn’t resist to send an SMS to Camille while observing banana trees at 3000 m elevation!

I have spent quite a lot of time working in the North American Cordillera that extends to the south through the mountain ranges of the Andes! My work in the Western US consisted of trying to understand the mechanisms of fluid-rock-deformation interactions in crustal-scale shear zones and to use these deformation zones as paleoaltimetry proxy.

Therefore, having demonstrated that 20 million years ago the Basin and Range was twice higher than it is today, it appears logical to go further south in the Northern Andes and try to obtain paleoaltimetry estimates for a similar period of time.

This would allow us to define the difference between modern and Miocene mean elevation along the north-trending axis of the Cordillera!

Dr Aude Gébelin on Mount Rainier in the United States

I am also currently working on the Himalaya trying to reconstruct the paleoelevation / paleoclimate history of the highest mountains on Earth that formed by the collision between two main continents, India and Eurasia. In contrast, the Andes developed as a result of ocean-continent convergence and although lower in elevation, the Andes represent the second highest mountain range on Earth where the continental crust can reach in some areas 60-70 km! It is therefore extremely exciting to focus on the geological processes of these two orogens, that although being associated to different geodynamic contexts, led to high elevation and similar crustal thickening!

A complex climate

Topography exerts an important control on atmospheric moisture fluxes over the Northern Andes. However, the geometry of the Ecuadorian Andes is characterized by two north-south trending calderas separated by a valley.

Their locations near the Equator and in northwestern South America, but also the ocean oscillations present in Ecuador, complicate the trajectories of air masses and so the source of moisture. Currently, the weather observed in this part of the Andes is dominated by moisture-bearing winds from the Atlantic and the Amazon Basin and not by the Pacific westerlies.

Global circulation models have shown that this might be due to the South American low-level jet (SALLJ) that developed as a result of the Central Andes surface uplift at the expense of the westerlies that have been blocked from the South Pacific to the southern Andean Plateau. However, the role of the Northern Andes in deflecting air masses is not taken into account by these models. Therefore, further studies on this topic are needed to understand the complexity and the diversity of the weather and subsequent environment observed in the Ecuadorian Andes!

<p>Volcanic pyramids - South of Cuenca</p>
<p>Chimborazo</p>
<p>Lake at 4000m</p>

The aims of my current research

First, it provides geologists with fundamental information to elucidate the relationship between the Andean topography evolution and geodynamic processes through time. 

Second, as the topography of mountain ranges influences the trajectories of air masses, studying how the topography and associated tectonics processes evolved through time informs scientists who are interested in past climate reconstructions by providing new boundary conditions for a better understanding of the role of the entire Andes in Southern hemisphere climatic evolution and atmospheric circulation patterns over geologic time. Understand what happened in the past, is essential to understand today’s changes, and can help to anticipate future scenarios.

Third, Earth’s surface processes including the topography evolution are important to study because any vertical variations of the Earth’s surface trigger changes of the landscape that in turn impact on the appearance and disappearance of plant / animal species.

Nowadays, one of the main challenges for the scientific community is to try to better understand the respective roles played by natural factors and anthropogenic factors in the biological and climate modifications that have been identified these last years. The Andes represent an amazing natural laboratory by the geological, biological and environmental diversity they offer. As a consequence, understanding the link between Earth’s surface processes and those that govern the internal dynamics of this orogen is of direct relevance to all of us, as we are all facing this dramatic climate and environment change…and this, on a human scale!!!

Our final goal is to understand the relationships between tectonic processes, topography and climate. To address this key question we will reconstruct the paleoaltimetry and paleoclimate history of the Northern Ecuadorian Andes based on a technique that recovers the isotopic composition of ancient meteoric water that scales with elevation. In order to obtain paleoaltimetry estimates, our plan is to retrieve this isotopic signal from volcanic glasses which incorporate large amounts of meteoric water (3-5 wt%) into their structures when they fall on the Earth’s surface. This involves to collect fresh volcanic rocks from different areas and to measure the hydrogen isotope ratios (D/H) of volcanic glass shards that equilibrated with surface-derived waters during hydration.

One of the major advantages of investigating volcanic glasses is that they can be temporally linked due to their potential for high precision geochronology. We will date the volcanic glasses in order to make the link between their age and the isotopic signal of Earth’s surface-derived waters preserved during hydration. Based on the record of ancient meteoric waters retrieved from our samples, we will be able to quantify the spatial and temporal evolution of the topography of this part of the Andes.

However, if the altitude effect on the isotopic composition of precipitation is well constrained from the mid to low latitude, it is not the case near the Equator. The complex atmospheric and ocean oscillations (El Niño-Southern Oscillation System and La Niña) present in Ecuador, but also the multiple sources of moisture, render the relationships between the altitude and the precipitation a bit more complicated.

As a consequence, a fundamental requirement when using stable isotope data for paleoaltimetry reconstructions is to determine the modern lapse rate that will be needed to determine paleoaltimetry estimates from our samples. In this way, this work involves collecting water samples from rivers, tributaries, springs and or weather stations from different altitudes in order to define a precipitation-altitude relationship for the specific region we focus on.

Working with students and partners

Maksymilian Radkiewicz came into the field last September with me and my colleague Dr Cesar Witt from the Université of Lille (France) and has participated in the collection of volcanic rock samples as well as water samples. As part of his MSc project, Maksymilian will help to interpret the water samples in order to check if a lapse rate can been defined in our studied area, separate volcanic glass shards of the samples we have collected together, and measure the hydrogen isotopic composition of volcanic glasses.

Ecuador is just an amazing country where in a few kilometres the climate, landscape/environment, as well as the vegetation can change! Guess the diversity you can get by doing East-West trending transects from the Pacific coast to 4000 m elevation while moving towards the continental interior! Together with Maksymilian and my colleague Dr Witt, who originates from the country, we covered a quite large area observing the geology and geomorphology as well as collecting water samples and fresh volcanic rocks from varied geological units.

Research

Our world-class research keeps staff at the cutting edge of recent scientific developments within the field.

Explore the interdisciplinary research carried out within Earth sciences.

Find out more about our research