The structure of the Moho in the Northern Apennines: Evidence for an incipient slab tear fault?

• A discontinuity in the Moho is found along the strike of the orogen.
• A trench-normal deformation zone in the subducted lithosphere is suggested.
• A regional scale “Moho doubling” is found beneath the Northern Apennines.
• The subduction point (S-point) lies beneath the westernmost extensional basins.

This study investigates the geometry of the crust–mantle boundary along the Northern Apennines (Italy) and its relationship to the tectonic evolution of a retreating subduction system. Using receiver function analysis from teleseismic records, the Moho depth was computed beneath 26 seismic stations located in key positions along the orogen. The results show a relatively shallow (~ 20 km) Moho in the Tyrrhenian side, as expected from an extended overriding plate, and a deeper Moho (~ 40 km) in the subducted Adriatic plate, SW-dipping 15–45° beneath the chain. Stations deployed on the Northeastern edge of the Plio-Quaternary extensional basins provide maximum Moho depth estimates, thus supporting the hypothesis that these basins define the zone where the two plates separate at depth. In addition, an abrupt change in the Moho depth is recognized along strike of the orogen, attributed to a 45 km-wide, trench-normal deformation zone that crosses the Northern Apennines in the proximity of the Livorno–Sillaro lineament. This zone is interpreted as an incipient lithospheric tear fault, which accommodates varying rates of trench retreat along the length of the subduction zone.