Groundbreaking study shakes up established views on mountain formation

Contrary to traditional belief of crustal wrinkling and densification, a new study suggests that mountain formation, particularly in subduction zones such as southern Italy, may be significantly affected by subsidence of a tectonic plate through the Earth’s mantle and altered mantle flow, providing a more accurate understanding of the mountain-building process.

New research led by Colorado State University points to the answers to how and why mountains are buried deeper than previously thought.

“Mountain building is a fundamental process of how the Earth behaves, and this study suggests we may not understand it as well as we thought,” said Sean Gallen, lead author and assistant professor of earth sciences at California State University.

Galen and his team have produced new datasets and methodologies for using landscapes to reconstruct extensive histories of mountain buildings in southern Italy. Their unique approach produced what Gallen called “disconcerting” results.

In subduction zones, such as the one in Calabria in southern Italy, one tectonic plate sinks beneath another. The mountains in these places are believed to have been formed by the wrinkling and condensation of the earth’s crust.

The team collected measurements that recorded geologically short and long time scales, from thousands of years to tens of millions of years. Like the “geological recorder” of tectonic history, the landscape filled in for the rest.

View of Calabria overlooking the Tyrrhenian Sea with a sea terrace in the foreground (flat area). Credit: Sean Gallen

“In southern Italy, the landscape is actually the bridge between these different styles that we usually use,” Gallen said.

The flat, elevated patches of landscape along the “toe” of the Italian peninsula represent a time when mountain formation was slow, and the sharp shift down below indicates rapid acceleration. These clues in the landscape allowed the researchers to produce a long-term, continuous record of rock elevation, the longest and most complete record of its kind.

“We expect to see a correlation between the rate at which one plate sinks below the other plate through time and our history of rock uplift, and we don’t see that,” Galen said.

Crust wrinkling and condensation appears to be secondary to another process in the formation of the Calabrian mountains. The data point to lower plate descent through the Earth’s mantle and change of mantle flow field as the primary factor controlling rock uplift.

CSU graduate students take samples from the bedrock of a thermohistochemistry in Calabria, Italy. From left, Nikki Seymour, second author on the study, Joanna Edman and Eyal Marder. Credit: Sean Gallen

“The results suggest that the typical way we look at mountain building does not apply to southern Italy,” Gallen said. “It appears to be controlled by things much deeper within the Earth system. This behavior has been seen in models but never in nature. This is the first time we think we have observed it.”

Galen cautioned that more data is needed to confirm whether or not their interpretation is correct, but that it is supported by existing numerical models. Scientists previously linked mountain height to tectonic plate interactions within Earth’s plastically flowing mantle, but this research indicates for the first time that this mechanism is the dominant force in mountain building at subduction zones.

“The records we presented indicate that deep Earth signals appear to dominate what is happening at the surface,” Galen said. “I’ve been working in the Mediterranean for 15 years, and this finding has profoundly changed the way I think about these subduction zones.”

View of Calabria overlooking the Tyrrhenian Sea with a sea terrace in the foreground (flat area). Credit: Sean Gallen

Transformational and transparent research

The new techniques developed for this study offer a breakthrough in the construction of long-term histories of rock lifting.

The team created a unified framework based on a set of standard geomorphology measurements — thermochronology, cosmic nuclides, river-rock profiles, and a record of past sea levels found in sea terraces. The new approach goes back further than others and uses different data sets to constrain modeling in a unique way.

The method is best applied to active systems, where recent landscapes provide clues to their histories. The more active a system has been in geological time, the more difficult it is to reconstruct its history with confidence.

A program for the study was developed and published in DOI: 10.1038/s41561-023-01185-4