How did the Andes Cordillera become so immense? A new geological research method may hold the answer

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Absolute plate motion change of the Pacific as obtained from a hotspot reference frame (red; from Wessel and Kroenke (2008)) compared to that obtained from RPM analysis (blue ; fixed Eurasia plate). The map shows the ellipse of the 30% confidence interval for the PA Euler pole shift for each set. The inset shows the distribution of amplitude changes for the aforementioned sets of Euler vectors. The map uses the Mollweide projection centered at 60E. Credit: Earth and Planetary Science Letters (2023). DOI: 10.1016/j.epsl.2023.118009

How did the Andes, the longest mountain range in the world, reach their enormous size? This is just one of the geological questions that a new method developed by researchers at the University of Copenhagen may be able to answer. With unprecedented precision, the method allows researchers to estimate how the Earth’s tectonic plates have changed speed over the past few million years.

The Andes are the longest mountain range above water on Earth. It stretches 8,900 kilometers along the western periphery of South America, is up to 700 kilometers wide, and in some places rises almost seven kilometers into the sky. But geologists aren’t sure exactly how this colossal mountain range emerged from inside the Earth.

Researchers from the University of Copenhagen propose a new hypothesis. Using a new method developed by one of the researchers, they closely studied the tectonic plate on which the range rests. Their discovery shed new light on the birth of the Andes.

Tectonic plates cover the Earth’s surface like huge puzzle pieces. They move a few centimeters each year, at about the same rate as our fingernails grow. Occasionally, these plaques may suddenly speed up or slow down. However, we know little about the fierce forces behind these events. The UCPH researchers have arrived at more accurate estimates than ever before, both of the magnitude and frequency of plate velocity changes in the past.

The researchers’ new calculations show that the South American plate has suddenly and dramatically shifted gears and slowed down twice significantly in the past 15 million years. And this may have contributed to the widening of the huge chain. The results of the study have been published in the journal Earth and Planetary Science Letters.

Remarkably, the two sudden downturns occurred between periods when the Andean Range was under compression and growing rapidly:

“In the periods preceding the two downturns, the plate immediately to the west, the Nazca Plate, pushed down the mountains and compressed them, causing them to grow. This result could indicate that part of the pre-existing range acted as a brake on both the Nazca Plate and the South American Plate. As the plates slowed in speed, the mountains widened instead,” says first author and Ph.D. student Valentina Espinoza from the Department of Geosciences and Natural Resource Management.

The mountains weigh down the plate

According to the new study, the South American plate slowed down by 13% during a period that occurred 10 to 14 million years ago, and by 20% during another period there. 5 to 9 million years old. In geologic time, these are very rapid and abrupt changes. According to the researchers, there are mainly two possible reasons for the sudden downturns in South America.

One could, as mentioned, be linked to the extension of the Andes, where the pressure has eased and the mountains have widened. The researchers hypothesize that the interaction between the expanding mountains and the lower velocity of the plate was due to a phenomenon called delamination. That is, much of the unstable material below the Andes broke off and sank into the mantle, causing major readjustments in plate configuration.

This process caused the Andes to change shape and expand laterally. It was during these periods that the mountain range extended into Chile in the west and Argentina in the east. As the plate accumulated more mountain material and became heavier, the movement of the plate slowed.

“If this explanation is correct, it tells us a lot about how this huge mountain range came to be. But there’s still a lot we don’t know. Why did it get so big? How fast did it form? How does the mountain range hold together? And will it eventually collapse?” says Valentina Espinoza.

According to the researchers, another possible explanation for the slowing of the plate is that there was a change in the heat flow from the interior of the Earth, known as convection, which moved in the layer the highest viscous layer of the mantle on which the tectonic plates float. top of. This change manifested itself as a change in plate motion.

Researchers now have the information and tools to begin testing their hypotheses through modeling and experimentation.

Can become a new standard model

The method for calculating changes in tectonic plate motion builds on previous work by associate professor and study co-author Giampiero Iaffaldano and Charles DeMets in 2016. What’s unique about the method is that it uses geological data at high resolution, generally used only to calculate the movement of the plates relative to each other. Here, the same data was used to calculate changes in plate motion relative to the planet itself. It provides estimates with unprecedented accuracy.

After testing the method with a combination of six other tectonic plates, the researchers believe it could become a new standard method:

“This method can be used for all plates, as long as high-resolution data are available. I hope that such a method will be used to refine historical models of tectonic plates and thus improve the chances of reconstructing geological phenomena that remain unclear to us”, says Giampiero Iaffaldano, who concludes:

“If we can better understand the changes that have taken place in plate motions over time, we may have a chance to answer some of the greatest mysteries of our planet and its evolution. We still know so little about , for example: the temperature of the Earth’s interior, or roughly when the plates started moving. Our method can most likely be used to find pieces for this large puzzle.”

More information:
Valentina Espinoza et al, Rapid changes in absolute plate motion inferred from high-resolution relative propagation reconstructions: a case study of the South American plate and its Atlantic/Pacific neighbors, Earth and Planetary Science Letters (2023). DOI: 10.1016/j.epsl.2023.118009

Journal information:
Earth and Planetary Science Letters

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