Tectonic and sedimentary evolution of the frontal part of an ancient subduction complex at the transition from accretion to erosion: the case of the Ligurian wedge of the Northern Apennines, Italy

Subduction can be either associated with accretion or removal of material from the overriding plate. These two processes can coexist or alternate in time along the same margin. Theirinception has the potential to change the dynamic equilibrium of a margin wedge resulting in the development of out-of-sequence thrusts, normal and strike-slip faults or large submarine landslides in the frontal part of the subduction zone.In this work we investigate the effects of the transition from frontal accretion to frontal erosion on the stability of a subduction complex through the study of a fossil example from the Northern Apennines.New structural data suggest that in the Aquitanian the removal and underthrusting of the toe of the wedge, formed by both the accreted sediments of oceanic affinity and the overlying wedge-top basin fill (i.e., the Subligurian Units), implied a process of frontal tectonic erosion. The presence, on top of the subduction complex, of a complete succession of mid-late Eocene to late Miocene slope apron sediments - i.e., the Epiligurian succession - allowed to reconstruct the sedimentary response to thisevent.In the Aquitanian large areas of the wedge were denudated of the lower-slope sedimentary cover through extensive gravitational mass movements. The subsequent deposition of a thick body ofsubmarine debris flow has been documented. The mass-wasting deposits are interpreted as the sedimentary response to the underthrusting of the frontal part of the Ligurian subduction complex formed by the Subligurian Units.