Deep beneath the Italian peninsula, Earth’s crust is pulling apart along the Apennine mountain range even as it is squeezed together along the range’s outer edge. Parts of the region are rising while others sink, and earthquakes on either side tell contradictory stories. Now, geologists led by Stefano Tavani of the University of Florence think they have found the answer: the crust is ‘unzipping’ under Italy, a process known as delamination.
What Is Delamination?
Delamination occurs when the dense lower crust and attached lithosphere peel away from the crust above and sink into the mantle. Beneath the Apennines, this peeling is not happening everywhere at once. Instead, like a zipper, it has a single front—a hinge—where the peeling is active, and that front is slowly migrating under Italy toward the Adriatic foreland.
The Apennine Puzzle
The Apennine Mountains stretch some 1,200 kilometers (745 miles) along the Italian peninsula and have long posed a geological puzzle. Like many mountain ranges, they were built by tectonic plates pushing together, crumpling and thickening Earth’s crust over millions of years. But the tectonic system beneath Italy did not simply keep pushing in the same way. As the slab of rock sinking into the mantle gradually retreated, the crust behind the growing mountain range was pulled apart, opening the Tyrrhenian Sea.
This left the Apennines in an unusual situation: even as the outer edge of the mountain range continued to be squeezed together, the crust farther back was being stretched apart. A 2006 Annals of Geophysics report described this contradiction, noting that “the paradox of how horizontal contraction and extension can occur simultaneously in convergent mountain belts remains a fundamental and largely unresolved problem in continental dynamics.”
Between around 10 and 2 million years ago, roughly 100 kilometers of shortening in the central Apennines was matched by a similar amount of extension in the Tyrrhenian region behind them. This simultaneous contraction and extension was previously attributed to slab rollback—the retreat of the sinking slab, which stretched the crust behind the mountain front even as contraction continued farther east.
But starting around 2 million years ago, something changed. The major phase of extension that opened the Tyrrhenian Sea came to an end, and shortening along the Apennine front subsequently slowed dramatically. Yet the paradoxical deformation of the mountain range continued. Something else seemed to be going on.
A Migrating Hinge Beneath the Mountains
To investigate, Tavani and his colleagues brought together several different views of the mountain range, from decades of earthquake and GPS measurements to satellite radar observations and maps of the boundary between Earth’s crust and mantle—a region known as the Moho, short for the Mohorovičić discontinuity.
A compelling pattern emerged. The different kinds of deformation appeared to be centered around the same structure deep beneath the Apennines. For more than 500 kilometers along the mountain range, the researchers found a zone where the Moho beneath the Tyrrhenian side overlaps the Moho beneath the Adriatic side. They interpret this doubled crust as the region where the lower crust is peeling away—the moving front of the unzipping process—like the point at which a piece of tape lifts away from a surface as you peel it.
Earthquakes cluster around this front, too. Behind and above it, earthquake mechanisms mostly indicate that the crust is being pulled apart; ahead of it, they mostly indicate compression. GPS measurements tell a similar story. Across the mountain belt, the researchers measured around 4 millimeters per year of extension, while toward its outer edge, some of that movement is balanced by roughly 2 millimeters per year of contraction. The researchers describe this as “accordion-like” deformation: the mountain belt stretches internally while simultaneously shortening at its front.
Why Slab Rollback Isn't Enough
Slab rollback could explain such a pattern during the earlier evolution of the Apennines, but that explanation isn’t enough for what we see now. Tavani and his colleagues argue that ongoing delamination beneath the mountains provides the missing internal engine.
Ahead of the migrating hinge, the lower crust and lithospheric mantle remain attached to the sinking slab, which pulls the crust downward. As the hinge passes and the lower layers peel away, however, that downward load is released. The remaining crust can then unbend and rebound upwards, as denser material beneath it is replaced by more buoyant mantle. This process produces extension behind the hinge even as the still-attached crust ahead of it experiences compression and subsidence.
An Unresolved
It’s not a complete . The model is deliberately simplified, and questions remain about the precise structure of the slab beneath the Apennines. More sophisticated models will be needed to understand the full complexity of the mantle and crust as they deform over time.
But the result suggests that the Apennines could be offering geologists a rare gift—”an empirical, geodetically constrained documentation of a laterally migrating delamination hinge that is tracking mantle-lithospheric peel-back in real-time,” the researchers write.
The findings have been published in Communications Earth & Environment.
