Earth’s Crust Is Pulling Itself Apart Like a Zipper

3 min readHere’s what you’ll learn when you read this story:The Apennine Mountains form the geologic “spine” of Italy, but their formation is somewhat confusing, as some sections are being squeezed while others are pulling apart, creating an “accordion-like” shape.A new study challenges the idea that slab rollback alone explains this geologic conundrum, instead arguing that the modern primary driver is delamination, where the lower crust and lithosphere separate from the upper crust. The authors conclude that a 500-kilometer overlap of crust forms the “hinge” of a zipper, with compression forming ahead of the zipper and extension forming behind it.Italy lies at the heart of a geologic traffic jam where the Eurasian and Adria tectonic plates crash together. Because of this perilous geography, Italy is consistently ranked among the highest in Europe for seismic risk and earthquake losses, and according to the European Facilities for Earthquake Hazard and Risk, the country lies in a “constant stress regime” at the juncture of the two major plates.One of the consequences of this tectonic collision over millions of years was the formation of Italy’s “spine,” also known as the Apennine Mountain range. However, the Apennines are a bit peculiar, because instead of the plate collision thickening the crust and pushing the mountains ever higher, certain sections of the range are being squeezed while others are being actively pulled apart. This has led some geologists to describe the range as “accordion-like,” and this peculiar shape is particularly hard to square with some leading theories of mountain formation.One of those theories is slab rollback, according to which an older oceanic plate retreating into the mantle can cause surface uplift and extension. However, a new study published in the journal Communications Earth & Environment led by scientists at the University of Florence argues that slab rollback doesn’t fully explain the Apennine spine’s strange geology.While slab rollback opened the Tyrrhenian Sea off of Italy’s western coast over millions of years, things changed roughly two million years ago. The stretching that formed the sea slowed down and compression along the mountain range’s front decreased. But deformation continued, a condition that can’t be explained by slab rollback alone. Instead, the authors suggest that the Apennines are experiencing an “unzipping” process, known as delamination, where the lower crust and lithosphere peel away from the upper crust.“Building on previous tomographic, geological, and geochemical evidence for lower-crustal delamination in the Apennines, here we integrate geodetic, seismic, and geological data within an elastic flexural model to show that the unzipping front is not just a passive response but instead the first-order control on the spatiotemporal evolution of the Apennines orogen,” the authors write.Much like a zipper, this delamination isn’t occurring across the entire range. Instead, it’s concentrated at a “hinge” where the front is subducting under Italy toward the Adriatic foreland (part of the Adria microplate), a basin that runs parallel along the eastern flank of the Apennines. The researchers came to this conclusion by analyzing decades of earthquake and GPS data and the boundaries of the Mohorovičić (Moho) discontinuity—the separation of Earth’s crust and mantle. A 500-kilometer area of the Moho on both the Tyrrhenian and Adriatic sides overlap, forming the “unzipping front,” according to the researchers.Data shows that crust is extending behind this migrating hinge at roughly four millimeters per year, while compression occurs at two millimeters per year ahead of the hinge.“Consequently, the present-day force balance and seismotectonic framework of the Apennines…are no longer driven by back-arc subduction dynamics nor by the Europe-Africa Convergence,” the authors write. “Instead, the present-day velocity budget depends entirely on ongoing lower-crustal delamination and the forelandward migration of the unzipping front.”The authors note that this unzipping process didn’t suddenly start two million years ago when slab rollback abated, but likely was already happening by at least the late Miocene epoch, around 10 million years ago. Once slab rollback ended, the region underwent a “geodynamic transition” that increased the importance of this lower-crust delamination.Italy is world renowned for history, food, and culture, but maybe there’s room for an increasing appreciation for its stunningly complex geology.Darren lives in Portland, has a cat, and writes/edits about sci-fi and how our world works. You can find his previous stuff at Gizmodo and Paste if you look hard enough.