The conventional story of how the Andes Mountains formed is one of tectonic plate-powered mountain building, with the range rising slowly, steadily and in-sync with plate tectonic movements along South America's western edge.
But a new study published in Nature Communications tells a much speedier story for a key segment of the mighty mountain range, which is the longest in the world.
After analyzing a diverse array of geological evidence, researchers found that the Andes of west-central Argentina, which includes the highest peak in South America, formed in four distinct and rapid pulses of activity, each lasting about 5 to 8 million years — a growth rate that is faster and more punctuated than previously thought.
"The remarkable part was how short-lived and how frequent the pulses were," said the study's lead author Tomas Capaldi, who started the research as a doctoral student at The University of Texas at Austin Jackson School of Geosciences and is now an assistant professor at the University of California, San Diego. "You can almost think of it as a high-tempo beat."
This rapid growth outpaces the steady 50-100-million-year timeline expected if growth was driven purely by the subduction rate of the Nazca tectonic plate. The researchers think that magma and fluids introduced into the Earth's crust by the subduction process — but not controlled directly by the rate of subduction itself — could explain the geologically rapid rise of the Andes.
Whereas magma is generated above the subducting plate, additional geologic fluids that help influence mountain building start off deep below the surface, as seawater trapped in cracks and pores in the subducting plate. As the plate dives into the deeper mantle and pressure and temperature rise, the water escapes upward into the overriding crust. This sets in motion a process that, along with magmatic heating, weakens the overriding crust.
The researchers found that each phase of mountain building was preceded by volcanic activity, with the newly formed wave of volcanoes and mountains moving inland toward the center of the continent with each successive phase.
"This study shows that there's a form of internal self-organization that happens, maybe cyclically, and this process somehow regulates mountain building," said study co-author Brian Horton, a professor at the Jackson School.
Magma and geologic fluids are known to be influential players in Argentina's unique geology — including its abundance of gold, copper, critical minerals, volcanoes, and earthquakes. Capaldi said that the research suggests that mountain building is yet another process these fluids help control.
"Fluids help regulate large megathrust earthquakes along the Chilean margin, and fluids help create one of the most robust, enriched, copper and gold deposits in the world in this single segment of the Andes," he said. "What we're proposing is that these fluids have played a strong role in these deformation cycles that also drive these very rapid cycles of mountain building."
The case for the rapid rise is built on a range of geological data related to four pulses of mountain building activity over the past 30 million years. This includes geochemical and geochronological analysis of rock and sediment samples in the lab, along with larger scale basin analysis and structural geology conducted in the field.
"These datasets allow us to link the processes operating in the deep Earth, to the faults that are involved, to the evolution of the surface recorded in sedimentary basins," said co-author Chelsea Mackaman-Lofland, who also began the research as a doctoral student at the Jackson School and is now an assistant professor at the University of Tennessee, Knoxville. "It's multidisciplinary science that integrates a range of different proxies, to transform our understanding of how deep Earth dynamics affect the growth of mountain ranges."
The research is also a testament to the value of sustained field study and collaboration. The data were collected and analyzed over close to 15 years of collaboration between Capaldi, Horton and Mackaman-Lofland, along with their Argentine colleagues and co-authors Facundo Fuentes and Gustavo Ortiz.
The research was funded by the National Science Foundation.
