Crustal structure above a retreating trench: Receiver function study of the northern Apennines orogen

Teleseismic receiver functions recorded along a densely-spaced transect of 20 broad-band seismic stations highlight systematic spatial variation of crustal thickness across the Northern Apennines (Italy), revealing the two sides of an active continental orogenic zone. The crustal structure has been investigated using a 1D grid search technique, based on the coherent arrivals of forward and reverberated P-to-S converted phases from the Moho. At the SW end of the transect, the Tyrrhenian Sea and Tuscany are characterized by a shallow Moho (20–25 km) and a sharp impedance contrast. At the opposite edge, the definition of the crust–mantle boundary is problematic, suggesting a gradual and/or complicated transition from the crust to the mantle in the Adriatic domain. However, where we can resolve it, the crustal thickness is larger (∼ 35 km). The boundary between these two crustal regimes coincides with the high crest of the Apennines. Shear-wave splitting measurements (an upper-mantle texture indicator) in the area reveal a correspondence between crustal thickness and mantle fabric transitions. We suggest that these spatially coincident changes in the crustal structure and the upper-mantle texture beneath high Apennines mark the edge of the Eurasian plate in Italy.