Fitting Calabria through a Rocky Doorway
One of the challenges of studying the Calabrian subduction zone is the enormous variation over relatively short distances. Etna is located just 120 kilometers from Stromboli, yet the volcanoes have completely different sources of magma. Fluvial conglomerates in the Crotone Basin have lots of chert, yet conglomerates of the same age just 15 kilometers to the south don’t have any.
On our last day of fieldwork, Nano took me just north of the Sibari Basin, at the southern tip of the Apennines, to investigate another dramatic shift. Here, we are looking at the transition from subduction to collision. An oceanic plate (like the Ionian Sea, east of Calabria) can be subducted easily: it’s made of oceanic crust, which is often colder, older, and more dense than the plate next to it. However, in space, oceanic crust transitions into continental crust, which is warm, young, and less dense. For example, the crust under the Atlantic Ocean is oceanic near the Mid-Atlantic Ridge, but continental off the coast of the United States.
The situation is similar in the Mediterranean. The Ionian Sea is made of oceanic crust but on its southern edge, the crust transitions into the African continental crust. In addition, just north of the Crotone Basin, the oceanic crust transitions into the Apulian Platform, a piece of continental crust that extends from the Gargano Peninsula to the Salento Peninsula. Since the Apulian Platform is too buoyant to subduct, the two plates are colliding, building mountains, and their convergence rate is slowing down. However, a few kilometers to the south, subduction continues and the convergence rate is steady.
To understand and work through this problem, I like to picture a comedic sketch in which someone carrying a two-by-four lengthwise tries to walk through a doorway. One side of the two-by-four hits the wall and generates the Apennines while the other side hits the wall and generates the Maghrebides in Sicily. Calabria is stuck in the door. Since tectonics continue to force Calabria through the open door, the parts that are stuck must somehow detach so that Calabria can push forward and continue subducting.
Most commonly, scientists think this process is accomplished through a vertical shear zone, or strike-slip fault. So the two by four behaves more like a piece of foam that will bend around the corners and eventually break completely. In the Sibari Basin, however, Nano and I have found little evidence of strike-slip faulting. Instead, what we’ve found are normal faults that are moving rocks near the surface through the doorway, while leaving deeper rocks behind. In this way, the crust acts more like a layered cake, in which the bottom layer remains in the doorway while the top slides through on a slippery layer of frosting. We need a lot more data before we know which mechanism is working. The fun part now is thinking of other ways that Calabria might slip through the doorway.