Great presentation, thanks

54:45 the answer to this is: there are two major active back-arc basins with retreating arcs in the Mediterranean Sea. The first is the combined Gulf of Lion/Balearic Sea/Ligurian Sea and Tyrrhenian Sea, with the Corsica-Sardinia microcontinent stuck in between, and the second is the Aegean Sea. Like all back-arc basins, these seas formed as subduction zones (and their volcanic arcs) retreated, creating sea floor spreading behind them. In the case of the strange geometry of Italy, the story begins with the peculiar way in which Africa collided with Europe during the Alpine Orogeny. The African continent included several pieces of lithosphere that stuck out from its northern edge-Arabia, now a separate plate, having rifted from Africa, was one of them, but another major promontory on the northern edge of the African plate has been given the name of “Adria” (after the Adriatic Sea), and it was a large piece of generally submerged extended continental crust, covered in carbonate platforms that jutted out into the Tethys Sea. During the Alpine Orogeny, this promontory called “Adria” was the first portion of Africa to impinge upon the European plate, and it became an indentor, with the Alpine Orogeny (technically the “Eoalpine Orogeny”) occurring where it struck the European plate, during the Paleocene. This collision took place because the southern edge of Europe, which was attached to oceanic lithosphere of the Tethys Sea, was subducted southwards under the African plate and Adria promontory, resulting in the consumption of the intervening ocean basin and eventual collision of the Adria promontory with Europe. Like with many collisional events in earth history, once oceanic lithosphere was completely consumed by subduction, the continental lithosphere attached to the downgoing plate became lodged in the subduction zone, too buoyant to subduct, and a continental collision resulted, forming the Alps. And like with many continental collisions, though southward subduction of the European plate under the African plate had ended, relative motion between the two plates was still convergent, driven by global tectonic plate kinematics, and so subduction then “jumped” outboard of the Alpine collision zone, and the African plate instead began to subduct under the European Plate, because a fringe of denser Tethyan oceanic lithosphere still existed on the northern edge of the African plate. Subduction was re established all along the southern margin of Europe, on the other side of the initial Alpine collisions, with a vergence opposite in sense to the original collision. But because the continents as a whole were unable to collide any closer, due to the fact that the Adria promontory was completely jammed against Europe, instead of moving Africa further to the north, subduction along what is now the Balearic coast of Spain and southern coast of France began to consume the fringing oceanic lithosphere north of the African plate, sweeping backwards to the south and east, forming a back-arc basin. At this time, the volcanic arc was the crust that now sits underneath Corsica and Sardinia, and this arc and block of crust rotated counter clockwise from the European coast, opening up the Balearic Sea/Gulf of Lion/Ligurian a sea behind it as it rotated, which grew by sea floor spreading. For some reason, about 20-15 million years ago, this back arc ocean basin stopped growing, but subduction kept happening, and trench retreat kept consuming oceanic lithosphere and extended continental lithosphere north of Africa, and so a new ocean basin began to open up to the east of Sardinia and Corsica, and the arc migrated eastward. This basin is the Tyrrhenian Basin, and the new arc that continued rotating counter-clockwise from Corsica and Sardinia is the Italian arc (the same one that contains Vesuvius and Campi Flegrei today). Subduction kept retreating and the arc kept rotating counter clockwise, but eventually all the intervening oceanic lithosphere attached to the northern edge of Africa would be consumed, and the subduction zone would collide against the continental crust of the Adria promontory in the east, and the northern edge of continental Africa in the south. This collision created the Apennine mountains and the Italian peninsula as we know it today in the east, with the promontory of Adria still stuck under the eastern half of the peninsula and the Adriatic Sea (which is floored by continental lithosphere and very, very shallow-it wasn’t even a sea at various times in recent geological history when sea level was lower), and the Hyblean mountains of Sicily and the Calabrian mountains of southern Italy in the south. Indeed, Sicily is a piece of the far northern edge of Africa that has had part of Europe, which was pushed backwards by retreating the Tyrrhenian back arc basin, thrust over it, and the eastern edge of the Italian peninsula and Adriatic Sea are also indigenous to Africa, with thrust faults along the spine of the Apennines carrying the migrating volcanic arc over it. This subduction zone only has a little more oceanic lithosphere belonging to Africa which it can consume, before back arc extension and collision is completely terminated in the Mediterranean. Then it will become tectonically quiescent, unless global plate kinematics allows for some of the new oceanic lithosphere of the back arc basins to be consumed in a new phase of subduction, sometime in the future. The second back arc basin in the Mediterranean is the Aegean Sea, and it formed similarly to the Tyrrhenian and Balearic/Ligurian/Gulf of Lion, in that the Aegean subduction zone has been retreating for tens of millions of years, opening up a back arc basin, the Aegean basin north of it, with the volcanic arc arcing along its southern edge. There is more oceanic lithosphere yet to be consumed here. And there was also a third back arc basin formed in the wake of the initial Alpine collision, the Alboran basin, all the way to the west of the Mediterranean, which formed the Betic and Rif mountains of Southern Spain and Morocco, respectively, and Gibraltar, and is implicated in a very interesting geologic story about how the Mediterranean was cut off from the Atlantic Ocean about 5-6 million years ago, leading to a drastic fall in sea level in the Mediterranean, and the deposition of huge amounts of evaporite minerals, the so-called “Messinian Salinity Crisis”. But this arc retreated all the way to the west against Gibraltar, Spain, and Morocco, and it is now extinct, providing a glimpse into the future of the still-active Calabrian and Aegean arcs. As to why these back arc basins begin in the first place, and why subduction retreats, it has to do with convection currents in the mantle. Essentially, the major convection currents in the mantle have what is called “poloidal motion”, which is convection motion that follows the direction of subduction, down with the downgoing plate, and then back up. But when a narrow piece of continental crust like the Adria promontory collides with another continent, and the subduction slab is narrow, it allows material from the rest of the mantle to rush sideways _around_ the downgoing slab, in what is called “toroidal motion”, creating smaller convection zones within the context of the larger global convection currents. It is this toroidal motion and the smaller convection cells it creates, which drives the opening of the back arc basins in the Mediterranean. I highly recommend checking out the work of the Italian geoscientist Claudio Faccenna and German geoscientist Thorsten Becker on this topic-they have created excellent models and visuals that will better explain what I have tried to explain above.