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.