Insights into the evolution of the Italian lithospheric structure from S receiver function analysis

To advance our understanding of the complex and episodic tectonic history of the central Mediterranean, we image the characteristics and structure of the convergent African and Eurasian lithospheric plates using S receiver functions. Specifically we investigate the lithospheric structure in terms of seismic velocity discontinuities, including the crust–mantle boundary (Moho) and the lithosphere–asthenosphere boundary (LAB) beneath Italy. The geometry and continuity of these structures are explored using teleseismic S-to-P converted phases, applied extensively throughout this region for the first time. The continental lithospheric thickness varies between ∼60 and ∼170 km with an average of ∼90 km, and the pulse characteristics of the converted phases can be characterized by tectonic region and history. Beneath the Northern Apennines, where subduction is still active, we find evidence for a complex lithospheric structure, where two different negative S-velocity jumps are present at approximately 90 and 180 km depth. The shallowest negative phases are interpreted as the base of the lithosphere, but the second negative phases are much deeper than expected for the overriding plate lithosphere. When compared to hypocenter locations and P-wave tomography these deep converted phases correlate with the base of fast velocities perturbations in the upper mantle and regions with subcrustal earthquakes. In contrast beneath the Southern Apennines, the lithospheric structure appears less complex, with only one significant negative arrival in the receiver functions and where no subcrustal earthquakes are present. These pulses are generally deeper (∼110 km) than those beneath the Northern Apennines and are broader in shape indicating a less sharp velocity contrast. Farther south along the Italian peninsula, the structure becomes complex again, with two sharp negative amplitude phases at depth. These deep converted phases that originate in the uppermost mantle beneath the Calabrian arc and Sicily are also co-located with deep (>35 km) earthquakes. These observations suggest that these deepest negative phases correspond to the base of the actively subducting lithosphere beneath the Northern Apennines and Calabria, and where these phases are absent in the receiver functions there is no subducting slab. Seismic imaging of the lithosphere and uppermost mantle using S-to-P conversions provides another geophysical dataset to aid in interpretation of the complex, segmented slab structure beneath Italy.