Quaternary, catastrophic rock avalanches in the Central Apennines (Italy): Relationships with inherited tectonic features, gravity-driven deformations and the geodynamic frame

摘要

Five cases of Quaternary rock avalanches detached from carbonate mountain ridges in the Central Apennines are presented. Due to the large amount of rock masses involved, the width of accumulation and detachment areas and the damming in the host environment, the analysed rock avalanches can be considered as catastrophic rock slope failures, sporadic events in a mountain region characterized by low elevation but where mountain ridges can have a relative elevation of up to 1 km above the lowermost valley floors. The geological setting of tectonic structures that originated during the Apennine orogenesis influenced rock avalanche characteristics, determining the location and shape of detachment areas, the kind of rock mass involved, and the failure mechanisms. Two main types have been identified: ⅰ) forelimb rock-slide avalanches (FRSA) such as the Lettopalena and Mt. Arezzo rock avalanche which involved Cenozoic, heterogeneous sequences of carbonate ramp deposits detached from box-shaped source areas according to a rock sliding mechanism; and ⅱ) backlimb slide-wedge rock avalanches (BSWRA) such as the Campo di Giove, Scanno and Celano rock avalanches that detached from sub-circular source areas carved on fault-bounded ridges and involving Meso-Cenozoic carbonate rocks with a combined sliding and rock wedge failure mechanism. The Campo di Giove, Lettopalena and Scanno rock avalanches originated from mountain ridges bounded by inactive fault zones and undergoing deep-seated gravitational slope deformations (DSGSDs) at the mountain scale. These three rock slope failures are considered as isolated events of long-lasting deformative processes featuring creep deformation. Gravity-driven deformations firstly generated as a response to stacking processes and synchronous normal faulting during the Neogene-Early Pleistocene Apennine tectonics. In particular, the Caramanico Fault System (CFS) and the Genzana Fault (GF), bordering the carbonate ridges from which the Campo di Giove and Scanno rock avalanches originated respectively, are here considered as backlimb collapse structures accommodating the passive uplift and deformation of positive tectonic structures. Gravity-driven deformations persisted during the post-Early Pleistocene dome-like uplift of the whole Apennine region. The regional uplift created the first-order (200 km) topographic wave-length of the belt, i.e. a periodic loading which has been balanced by the deflection of the Apennine crust and lithosphere. On the contrary, shorter topographic wave-lengths inherited from former thrusting and synchronous normal faulting determined local isostatic imbalances bearing a large potential for the mature development of DSGSDs on mountain ridges, favoured also by lateral unloading due to linear erosion and increase of topographic stress. Thus, a cause-effect relationship is hypothesized between the geodynamic evolution of the belt and mountain-sized gravity-driven deformations including large rock slope failures.