Fold architecture predisposing deep-seated gravitational slope deformations within a flysch sequence in the Northern Apennines (Italy)

This research illustrates the case of a deep-seated gravitational slope deformation (DSGSD) affecting a folded structure that developed during Miocene tectonics in the Northern Apennines of Italy, deforming a heterogeneous turbiditic sequence characterised by pelitic-marly and arenaceous intervals. The investigated area includes the headwater portion of the Bidente River basin that has experienced intensive valley incision during the late Quaternary and the sizeable Poggio Baldi landslide with failure episodes in 1914 and 2010. The aim of this work was to unravel the local morphostructural setting determined by the interplay between the active DSGSD, inherited fold geometry, and fluvial dynamics. Dip data collection and structural analysis were used to reconstruct the fold geometry. A complex type of fault-propagation fold was outlined, with later breakthrough thrusts causing the backlimb rotation. Gravity-driven geomorphological features were inventoried and displayed on two geological cross-sections together with Geological Strength Index measurements, while their distribution was analysed to conceive a conceptual model for the development of the DSGSD. Fold setting and lithological anisotropies were considered as the predisposing factors of the gravitational slope deformation, whereas the multi-step entrenchment of the Bidente River valley, testified by the presence of the remnants of at least three generations of fluvial erosional terraces, was considered as the main cause of its initiation. Finally, the reconstruction of the original slope topography in the Poggio Baldi area and the results of structural analysis performed in the landslide detachment zone through remote sensing techniques allowed for the estimation of the size of the wedge-shaped rock block which detached in 1914, an event that occurred in the wider frame of the DSGSD.