Crustal structure and evolution of the Tyrrhenyian Sea: insights for the formation of Mediterranean back-arc basins

The Calabrian subduction system is a classic example of young Mediterranean back-arc basin that opened as a response to the rollback of the Ionian slab beneath the Calabrian arc. The large-scale structure of the subduction system has been imaged mainly by regional earthquake tomography studies. These studies, however, lack the resolution to define small-scale details on the location, nature and transition of different lithospheric domains, which are crucial to understand the geodynamic evolution of the system. Here we show a compilation of results from offshore and onshore active-source wide-angle and multichannel seismic profiles acquired in the region in the framework of projects MEDOC-2010 and CHIANTI-2015, both funded by Spanish agencies. These profiles have allowed defining the 2D Vp (and density) structure of the entire Calabrian subduction system (from the Ionian fore-arc to the back-arc) and of the different domains of the back-arc basin. The models and images provide a radically new view of the nature and limits of the different geological domains, which have profound implications regarding the underlying process of basin formation. First, our results show that the central part of the deepest basins (i.e. Vavilov and Marsili) is floored by unroofed mantle rocks instead of a “Penrose-type” oceanic crust as it has previously been assumed, and are intruded by large, deep-rooted volcanos in their axial part. Second, the transition from the deep ocean basins to the continent at the Sardinian and Campanian margins display conspicuous high-velocity lower crustal anomalies (e.g. at the Cornaglia terrace), which require the presence of a magmatic crust or of robust magmatic intrusions. The same spatial sequence from exhumed mantle intruded by localized volcanism to continental crust, crossing a section of “magmatic crust” (i.e. the Aeolian arc) is also observed in the transition towards the Calabrian arc. In summary, the new data reveal a juxtaposition of geological domains that contrasts with that of magma-poor Atlantic-type margins (e.g. W Iberia), where a transition from continental crust to oceanic crust through a section of unroofed continental mantle is observed. This means that Atlantic-type margins are probably not the best analog of the subduction systems that once evolved into Alpine-type orogenic belts: Mediterranean back-arc basin systems such as the Tyrrhenian one should be taken instead as a reference to better understand their process of formation.