Structural constraints on deep-seated slope deformation kinematics

A significant sackung-type deep-seated slope gravitational deformation (DSGSD) was recognised for the first time by the authors in the middle part of Valfurva, east of Bormio (Rhaetian Alps, Italy). The reconstruction of its kinematics, age and state of activity is presented, through a detailed description of its morphological, geomechanical and structural features. An integrated multi-disciplinary approach was performed to achieve a clear comprehension of the phenomenon. Field surveys and aero-photo interpretation were carried out in order to clarify the structural, geological and geomorphological setting in which the DSGSD developed. A kinematic conceptual model of the slope deformation was developed through the analysis of morpho-structures, of their significance and relationships with lithological markers and Quaternary deposits. After a geomechanical characterisation of the rock mass involved in the slope deformation, numerical modelling was performed to verify the hypotheses made on kinematics and driving factors of the phenomenon. The sackung affects pre-Permian metapelites, metabasites and marbles belonging to the Upper Austroalpine basement of the Campo Nappe, as well as Late Pleistocene and Holocene glacial and rock glacier deposits. The deformation started after the Late-Wurmian age (15,000-11,000 years B.P.), and continued until few centuries ago, not excluding a present-day low-rate activity. Deformation consists in a large oblique "sagging" along a deep confined sliding surface, associated with gravitational reactivation of pre-existing (late-Alpine and recent) tectonic brittle structures, leading to the formation of N-S and WNW-ESE trending gravitational morpho-structures. The evolution of the WNW-ESE trending system, resulting in asymmetric trenches, led to progressive failure of the lower part of the slope during the last 10,000 years, as testified by large paleo landslide accumulations, and it is still in progress. Numerical modelling indicates post-glacial unloading as the main triggering factor of the slope deformation. The importance of this deep-seated slope deformation is enhanced by the occurrence of the 30 Mm3 active "Ruinon" landslide in the lower part of the slope. Such landslide is subjected to rapid evolution and threatens the valley floor, establishing an important risk factor connected to human lives and socio-economic activities