Spatial variations of magmatic crustal accretion during the opening of the Tyrrhenian back-arc from wide-angle seismic velocity models and seismic reflection images

The structural complexity of back-arc basins is related to the evolution of the associated subduction system. Here we present an integrated geophysical and geological study that constrains the 3D spatial variability of magmatic activity along the Tyrrhenian back-arc basin. We use wide-angle seismic and gravity data, acquired in 2010 within the MEDOC experiment along a ~300 km-long NW-SE transect that extends from SE Sardinia Island to the NW Sicily continental margin, across the Cornaglia Terrace. The geophysical transect is coincident with a seismic reflection line from the Italian CROP experiment that we have re-processed. The geophysical results, together with available basement dredges, support a basement along the profile fundamentally composed of continental-type rocks, locally affected by subduction-related magmatism. The continental nature of this region contrasts with the nature of the basement inferred along two geophysical cross-sections located to the north of the Cornaglia Terrace in which seismic velocity of the lower crust supports significant magmatic crustal accretion. The comparison of these three cross-sections supports that the highest magmatic activity occurred in the central and most extended region of the basin, whereas it was less important in the North and practically non-existent in the South. These observations indicate abrupt variations of magmatism during the basin formation. As in other back-arcs, the temperature, water content and composition of the mantle might have played an important role in such variation, but they fail to explain the abruptness of it. We propose that the interaction of the overriding continental lithospheres of Adria and Africa with the Apenninic-Calabrian subduction system caused changes in slab rollback and trench retreat dynamics, which in turn resulted in variations of back-arc stretching and magmatism. Based on our observations, we suggest that the Cornaglia Terrace formation process might share some similarities with the formation of oceanic crust in the Red Sea