Deep-seated gravitational slope deformation in the central Apennines: a contribute to Slope Tectonics

Deep-Seated Gravitational Slope Deformations (DSGSDs) are gravity-driven mass movements involving entire slopes or large portions of mountain ridges over prolonged periods of time. Over slope-to-valley systems, DSGSDs are characterized by peculiar associations of geomorphological features such as double ridges, ridge-top depressions, grabens, trenches, scarps, counterscarps, etc., in the upper and middle sections, toe bulging, buckling folds and secondary landslides in the lower sectors. The rock masses involved can be many tens or even hundreds of meters thick and present areal dimensions up to several square kilometers. If compared to their size however, DSGSDs show small displacement rates, in the order of few mm/y, as demonstrated by remote sensing methodologies. A main scientific question concerns the classification of DSGSDs. Indeed, despite the increasing number of published papers in the last three decades, an exhaustive reference frame is still missing. In this thesis, after a review of historical papers, an original classification system is proposed as the first goal of the work. The further goal of the PhD was to give a contribute to the documentation of DSGSDs in mountain areas. DSGSDs are reported in different geological environments worldwide but for the Italian Apennines few cases were documented. The scientific question about it was whether a low-relief and low-energy belt can host large-scale gravitational phenomena in large numbers. Results of this thesis address the issue and fill the gap in knowledge between the Apennine and other mountain belts by completing an original geodatabase with almost 300 DSGSD records distributed over the Abruzzi and Molise regions. The relationship between the DSGSDs regional distribution within the mountain belt and the main tectonic features, both inherited from Mio-Pliocene thrusting or due to Quaternary normal faulting, is evident. In this context, the PhD thesis gives a contribute to the merging scientific branch of “Slope Tectonics,” which deals with the interaction between structural, engineering-geological, and geomorphic constraints and the evolution of large-scale slope deformation. Given the linear concentrations of DSGSD along main compressive and extensional tectonic features in central Apennines, the role of inherited structures is here considered as fundamental on the control of DSGSD onset and development. The last scientific topic discussed in this thesis regards the discovery of the first case - in the whole Apennine belt - of a Basal Shear Zone (BSZ) found at the lower boundary of an active DSGSD. Some cases of DSGSDs observed worldwide were proved to move over a continuous or discontinuous BSZ, which is generally composed of cataclastic breccias up to few tens of meters thick. However, well-exposed outcrops are rare and the description of the BSZ found at the base of the Luco dei Marsi DSGSD, on the western edge of the Fucino Basin, can be of general scientific interest. The origin of the Luco dei Marsi DSGSD was framed into the Quaternary morphotectonic evolution of the local slope-to-valley system through a multiple-step numerical modelling. Modelling results demonstrated, also for this specific case-history, the significant role played by Quaternary tectonics on the DSGSD formation and a gravity-driven deformation mechanism for the development for the BSZ breccias.