Seismic structure of a Tethyan back-arc: transdimensional ambient noise tomography of the Black Sea lithosphere

The Black Sea is the largest European back-arc basin connected to the subduction and final closure of the Tethys ocean. Its origin and type of crust are widely debated, with contrasting views suggesting it is either a relic of Paleotethys or a rifted back-arc basin formed within the thick and cold Precambrian lithosphere. To investigate the structure of this atypical intra-continental basin, we constructed the highest resolution seismic tomography of the region using the latest techniques of probabilistic inversion of ambient noise data recorded at seismic stations around the sea. Our results indicate the presence of thinned continental crust beneath the basin, likely of Precambrian lithospheric origin, thus invalidating the existence of either a relic Paleotethys fragment or younger oceanic crust. Extension and rifting probably exploited pre-existing sutures, but the rheologically strong lithosphere resisted transition to seafloor spreading. Seismic anisotropy shows complex paleo-deformational imprints within the crust and upper mantle related to the closure of Tethys. Extension caused by subduction roll-back generated anisotropic lithospheric fabric parallel to the rifting axis within the thinnest sections of the crust in the western basin. The eastern part developed on a distinct lithospheric domain that preserves paleo-extension anisotropy signatures in the form of lower crustal viscous deformation. Further south, anisotropy orients along the Balkanide-Pontide collisional system that records the final stages of Neotethys closure. Our results place key constraints on the type of deformations that occurred throughout the Tethyan realm, with fundamental implications for the development and evolution of back-arc basins and continental break-up.