Geologists studying the Apennine mountain range in Italy have identified a process that may explain one of the region's longest-standing puzzles: why one stretch of the earth's crust there is stretching apart while a nearby stretch is being squeezed together, and why some areas are rising as others sink.
A team led by Stefano Tavani of the University of Florence proposes that the lower part of the crust and the upper mantle beneath it, together known as the lithosphere, is gradually detaching and sinking into the mantle below. The researchers compared earthquake records, GPS measurements, satellite radar observations and maps of the Moho, the boundary between the crust and the mantle, to reach their conclusion. Their findings were published in the journal Communications Earth & Environment and point to a kind of "zipper" advancing beneath Italy.
The Apennines form a long mountain chain running roughly 1,200 to 1,400 kilometers from north to south along the Italian peninsula, the backbone of the country. For years, the mismatched pattern of stretching, compression, uplift and subsidence recorded across the range resisted explanation by a single tectonic mechanism.
What is happening under the Apennines?
The researchers call the process delamination. In simple terms, the denser rock layers at the base of the crust separate from the upper crust and descend into the mantle. According to the study, this detachment does not happen all at once along the entire mountain chain. Instead, the scientists identified a moving front, which they call a hinge, marking the point where the separation is taking place.
Evidence from earthquakes and satellite data
The data gathered by the team point to a structure extending more than 500 kilometers beneath the Apennines. In that zone, the Moho beneath the Tyrrhenian side of the range appears to overlap with the Moho on the Adriatic side. Tavani and his colleagues interpret this configuration as the region where the lower layers are losing their connection to the upper crust.
Earthquake data offer another clue. Behind the hinge, tremors are dominated by signals associated with crustal extension, while in front of it the mechanisms are linked to compression. The researchers say the hinge's shift toward the Adriatic foreland, the stable landmass bordering the range to the east, is consistent with their proposed model.
Why one part of Italy stretches while another compresses
The apparent contradiction has its roots in the tectonic history of the Apennines. Like other major mountain ranges, the chain formed as colliding plates compressed and thickened the crust over millions of years. That pattern changed when the plate descending into the mantle began to retreat.
That retreat produced extension behind the mountain front and contributed to the opening of the Tyrrhenian Sea, even as the outer edge of the Apennines remained under compression. Between roughly 10 million and 2 million years ago, about 100 kilometers of shortening in the central Apennines coincided with a similar amount of extension in the Tyrrhenian area, according to the study.
The main phase of extension linked to the opening of the Tyrrhenian Sea ended around 2 million years ago, and the shortening at the Apennine front also slowed. But the deformation did not stop. Current measurements cited in the study show about 4 millimeters of extension per year across the mountain range and roughly 2 millimeters per year of contraction toward its outer edge.
How the crustal zipper works
The model holds that before the hinge passes through a given section, the lower layers remain attached to the descending plate and exert a downward pull on the crust above. When the front advances and those layers break away, that downward load eases.
The crust left behind can then recover some of its original shape and rise, while the denser material beneath it is replaced by more buoyant mantle rock. That shift favors extension behind the hinge. At the same time, the section that still retains its connection to the descending plate continues to experience compression and sinking.
Tavani and his co-authors acknowledge that their proposal is a simplified model and that questions remain about the exact structure of the plate beneath the Apennines.
