Linking the deep structure to surface deformation: Body wave tomography of the Ligurian Sea and South-Western Alps

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The Alpine orogenic belt, which represents the scientific focus of the priority programme 4D-MB, is characterized by its highly complex tectonic nature resulting from the interaction of oceanic and continental lithosphere between the converging European and African plates. The Ligurian Basin is a back-arc basin generated by the southeastward trench retreat of the Apennines-Calabria-Maghrebides subduction zone, which also triggered the opening of the adjacent western Mediterranean basins. The Ligurian Sea is also a key location due to its complex geodynamic setting characterized by pronounced variations in crustal nature and thickness. In particular, in the center of the basin, the nature of crust and mantle is still debated. The proposed project will specifically harvest the rich data set of the marine component of AlpArray (LOBSTER) to extract a maximum of information from the OBS data. The 23 LOBSTER OBS comprises the offshore component of the AlpArray seismic network to define subsurface structures at the transition from the Western Alps to the Apennines and to improve our understanding of the 3D-geometry of the system and its kinematics. We will use body waves from primary and secondary arrivals (P, S, Pn, Sn) from OBS and AlpArray permanent and temporary stations to invert for velocity models of the Ligurian Sea and surrounding. The arrival times used onset will be from the following sources a.) airgun shots registered on the LOBSTER and landstations. b.) from local seismicity during the deployment of the LOBSTER stations and c.) from arrivals listed in the ISC catalog since the year 2000. Using velocities from the three-dimensional velocity structure will allow extracting information of the distribution and nature of the structural units, in particular for the units continental crust, hyper-extended continental crust, serpentinized mantle, and oceanic crust. Shear wave arrivals and models will contribute to the degree of serpentinization (i.e. ‘hydration’) of mantle rocks. Waves travelling just below the crust-mantle boundary (MOHO) allow determining the velocity of the uppermost mantle and yield information on the thermal and tectonic history of the upper mantle. These waves (refracted waves) allow inverting for transverse anisotropy for the uppermost mantle. We hypothesize that stretching of the crust should have modified the mantle flow and hence the fast polarization directions provide information on the width of the extensional zone at depths below the MOHO. The obtained results from the velocity models will be interpreted in relation to gravity data, sediment information, and geomorphology. In the last year of the project balancing techniques, we will quantify the amount of continental extension along those profiles using modeling techniques. Timing of extension can be constrained with the age of syn- and post-rift sediments.