Mantle Structure in the Central Mediterranean Region From P and S Receiver Functions

We investigate the upper mantle discontinuities in the central Mediterranean region by applying the P and S receiver function techniques on waveforms recorded at broadband stations located around the Tyrrhenian basin. P and S wave velocity profiles (down to 300-km depth) are calculated with joint inversion of P and S receiver functions. We could identify the Moho, lithosphere-asthenosphere boundary, and an underlying low-velocity layer between similar to 60- and similar to 200-km depth. The low-velocity layer is interpreted as asthenospheric material, and its lower boundary is identified below the western Ionian and Tyrrhenian basins as a sharp Lehmann discontinuity. Although the stations are located on different lithospheric domains we find a strong correlation between Moho and the lithosphere-asthenosphere boundary depths, which suggests ubiquitous coupling of the crust and lithospheric mantle, consistently with the southward opening of the Tyrrhenian basin. The Tyrrhenian and western Ionian basins present thinning of the transition zone of similar to 14 km, as inferred from a reduced P660s-P410s differential time. Below the southern Apennines we observe a standard differential time that implies an average mantle transition zone thickness. We explain these mantle transition zone thickness variations as due to temperature heterogeneity linked to the area's subduction history. Finally, under central Europe (the location of the deep S-to-P conversion points) two strong signals from nonstandard discontinuities within the mantle transition zone are observed. These signals can be explained as being generated at the boundaries of high seismic velocity layers that are spatially correlated with stagnant slabs in the transition zone detected by seismic tomography.