A Surface-Wave Study of Structure and Anisotropy of Tuscany

Large-scale Mediterranean tectonics has been dominated by the slow, N-S convergence of Africa and Eurasia. The opening of the Tyrrhenian back-arc basin in the western Mediterranean has been associated with a rapid east-ward retreat of a subduction zone at which Adriatic lithosphere is thought to have subducted westward. The Apennines orogen now forms the Adriatic-Tyrrhenian boundary, but it is debated whether any subducted lithosphere is still present beneath it at the moment. Recently, a large array of broad-band seismic stations (RETREAT) was deployed across Tuscany and Northern Apennines in order to constrain seismic structure and anisotropy beneath the area. A major result of the shear-wave splitting analysis was a consistent, laterally varying pattern of fast-propagation directions that indicated, in particular, NNW oriented fabric in the mantle beneath central Tuscany, just west of the Apennines. Questions remain, however, regarding the depth distribution of anisotropy, as well as regarding structure of Tuscany’s lithosphere and mantle below. In order to obtain new constraints on the current structure of the lithosphere and asthenosphere, we measure the dispersion of surface waves, a data type that is particularly sensitive to vertical distribution of shear speeds and anisotropy. Using RETREAT data, we measure inter-station surface-wave dispersion with a combination of cross-correlation and waveform-inversion approaches. Because most inter-station distances are short and because the distances to regions of abundant seismicity are long, broad-band dispersion curves can be measured accurately for only a minority of station pairs. Combinations of measurements for the entire central Tuscany, however, do provide robust, region-average dispersion curves in the period range of around 5-100 s, with sensitivity from the upper crust to the deep lithosphere and asthenosphere. Preliminary results indicate azimuthal anisotropy both in the lithosphere and asthenosphere, with fast-propagation axes oriented along N-NNW azimuths, similar to those inferred from shear-wave splitting in the area. The new data also constrains an isotropic shear speed profile, characterising the structure of the lithosphere and asthenosphere beneath Tuscany.