Photo: Stemonitis/Wikimedia Commons
Scientists say they may have solved a long-standing geological mystery beneath Italy’s Apennine Mountains, where the Earth’s crust is stretching along the mountain range while simultaneously being compressed along its outer edge.
Geologists led by Stefano Tavani of the University of Florence say the process resembles unzipping a zipper, creating unusual geological structures beneath the Italian peninsula, ScienceAlert reports.
The mechanism is known as delamination. Dense lower layers of the Earth’s crust, together with part of the underlying lithosphere, separate from the layers above and sink into the mantle.
According to Tavani and his colleagues, this process is not occurring everywhere at once. Instead, it has a distinct front — similar to the slider of a zipper — where the layers are separating. This front is slowly moving beneath Italy toward the Adriatic foreland.
The Apennines stretch for roughly 1,200 kilometers along the Italian Peninsula and have long presented a geological puzzle. Like many mountain systems, they formed through the collision of tectonic plates, which compressed, folded and thickened the Earth’s crust over millions of years.
But tectonic activity beneath Italy involved more than compression. As the subducting plate retreated into the mantle, the crust behind the growing mountain range began to stretch, contributing to the formation of the Tyrrhenian Sea.
This left the Apennines in an unusual state: while the outer part of the mountain system continued to experience compression, the crust deeper within the range was being stretched.
For millions of years, these opposing processes formed part of the same tectonic system. Between roughly 10 and 2 million years ago, crustal shortening in the central Apennines — amounting to around 100 kilometers — was accompanied by a comparable amount of extension in the Tyrrhenian region behind the mountains.
Scientists had previously explained this behavior through slab rollback, in which the subducting plate retreats and stretches the crust behind the mountain front while compression continues to the east.
But around two million years ago, conditions changed. The main phase of extension that created the Tyrrhenian Sea ended, and the rate of crustal shortening along the Apennine front subsequently declined sharply. Yet the mountain range continued to deform in seemingly contradictory ways.
A hidden boundary beneath the mountains
For the study, researchers combined a wide range of data, including long-term seismic and GPS measurements, satellite radar observations and maps of the boundary between the Earth’s crust and mantle, known as the Mohorovičić discontinuity or “Moho.”
They found that different types of deformation appear to be concentrated around the same deep structure beneath the Apennines.
Over more than 500 kilometers along the mountain range, the researchers identified a zone where the Moho surface beneath the Tyrrhenian side overlaps with the Moho surface beneath the Adriatic side.
The scientists believe this is where the lower part of the crust is peeling away, creating what they describe as a “delamination front” — similar to the point where adhesive tape separates from a surface as it is peeled away.
Seismic activity is also concentrated around this zone. Behind and above the front, earthquake mechanisms mainly indicate crustal extension, while ahead of it they predominantly show compression.
The researchers describe the resulting deformation as resembling an accordion: the mountain belt stretches internally while simultaneously being compressed along its front.
The findings provide a new explanation for the unusual combination of stretching, compression, uplift, subsidence and seismic activity observed across the Apennines.
