Teleseismic receiver functions from a seismic experiment in a small area of the northern Apennines, Italy, reveal strong crustal structure variations across the mountain chain. Receiver functions imaging and full waveform inversion technique are used to constrain the S-wave velocity profile in the crust and to reconstruct the geometry of the main seismic discontinuities at depth. We highlight the presence of the main mode-converting discontinuities in the study area. Most importantly, we identify the crust-mantle transition which is represented, almost everywhere in the study area, by a sharp S-wave velocity increase (over 4 km/s) at depth between 35 and 40 km. However, farther west, the S-wave velocity reaches values typical for the sub-crustal mantle at about 54 km depth, possibly marking the locus where the subducting Adriatic plate starts to dip into the mantle. Here the presence of a shallower discontinuity at about 36 km depth, with S-velocity values around 4 km/s, can be interpreted as the Moho signature of the overriding Tyrrhenian plate.