Manuele Faccenda | Central Mediterranean Structure and Dynamics

The Tertiary tectonic evolution of the Central Mediterranean and its first-order present-day structure have been relatively well constrained by abundant geological and geophysical data. Yet, several uncertainties persist about the mechanisms that led to the present-day surface morphology and deep slab geometry. With this respect, over the past few years new geodynamic and seismological modeling techniques have been combined to reproduce the recent large-scale evolution of the Central Mediterranean and provide mechanical constraints through the mapping of seismic anisotropy. The geodynamic simulations were designed and calibrated according to paleogeographic-tectonic reconstructions and seismological observations available in the literature. It is found that, although the opening of back-arc extensional basins in response to the retreat of the Ionian slab is a common feature in all models, structural heterogeneities within the Adria plate and/or the geometry of its Tyrrhenian passive margin profoundly impact on the segmentation of the subducting slab and the amount of Ionian trench retreat. This scenario is supported by anisotropic P-wave travel-time and S-wave splitting-intensity tomography models of the upper mantle covering the entire Mediterranean basin. The isotropic component of our preferred tomography model is dominated by numerous fast anomalies associated with retreating, stagnant, and detached slab segments. In contrast, relatively slower mantle structure is related to slab windows and the opening of back-arc basins. The anisotropy patterns are interpreted as the result of asthenospheric material flowing primarily horizontally around the main slabs in response to pressure exerted by their mid-to-late Cenozoic horizontal motion, while sub-vertical anisotropy possibly reflects asthenospheric entrainment by descending lithosphere. The last part of the seminar is then dedicated to the discussion of a recent, stochastically-based, anisotropic tomographic model of the Etna volcanic field (Sicily, Italy), where a cylindrical pattern of P-wave slow axes is imaged in the 6-16 km depth range. According to the predictions of geodynamic modeling, this peculiar and unprecedently imaged structure should be primarily related to a radially distributed vertical dikes departing from a pressurized magma chamber.