Beneath Italy, a deep part of the Earth's crust appears to be gradually detaching, with effects visible all the way to the surface.
The Apennines cross the Italian peninsula for about 1,200 km. This mountain range has long displayed surprising behavior. Its center is stretching, while its eastern edge continues to undergo compression. Some areas are rising, others are sinking, and earthquakes are also recording these opposing movements.
Orographic and hydrographic map showing the subdivisions of the Apennine chain.
Credit: Gaëlix / Wikimedia Commons, CC BY-SA 4.0.
To understand what is happening, researchers combined several types of observations. In particular, they used GPS measurements of displacement, deformations detected by radar from satellites, and data provided by earthquakes. Each method provides information about a different part of the structure and its movements.
Together, these observations outline a scenario in which the lower part of the crust separates from the section above it. This deeper, denser rock then sinks into the mantle, the Earth's layer beneath the crust. Geologists call this phenomenon delamination.
However, the detachment does not occur everywhere at the same time. It advances along a front that slowly moves eastward toward the Adriatic. This progression resembles the opening of a zipper, which is why the term “detachment” is used to describe the mechanism.
This deep movement helps connect several observations that had appeared contradictory. When dense materials sink, they alter the forces exerted on the crust above them. The crust can then stretch in one area while remaining compressed elsewhere, without these two behaviors being incompatible.
The history of the Apennines also helps explain this situation. The mountain range formed when tectonic movements brought together and thickened portions of the crust over millions of years. Later, part of the rocks descending into the depths retreated, encouraging the opening of the Tyrrhenian Sea and the stretching of the region behind the mountain range.
The significance of this study extends beyond Italy. Researchers now have a current example in which a deep mechanism can be compared with surface movements measured directly. Future observations will make it possible, in particular, to track the movement of the detachment front and better connect this evolution with the deformations and earthquakes of the Apennines.