Relationship between Deep-Seated Gravitational Slope Deformations (DSGSD) and mountain springs in NW Alps (Italy)

DSGSDs and “sackung” deformations are complex processes of gravitational movement that involve large volumes of rock, often several tens of meters thick and several kilometers long. DSGSDs are extremely slow moving, (0.4-5.0 mm/y) and produce distinctive morphological elements on the slope. These morphological features, including doubled ridges, ridgetop depressions, scarps and counterscarps, linear trenches, open tension cracks and ridge-crest grabens, indicate the presence of a DSGSD, particularly when observed in combination. The presence of a DSGSD and the correlated open cracks affecting also the rock crystalline bedrock can strongly influence the instability phenomena in the overlaying detritus cover, the hydrographic network development, the infiltration dynamics and the groundwater circulation. In this context the spring location and their hydrodynamic regimes could provide additional information in order to understand the DSGSDs characteristics and evolution. The authors examined the Rodoretto Valley, the west tributary of the Germanasca Valley, located in the Italian Western Alps. The site is located in the Penninic Domain along the North-South tectonic thrust between the Dora Maira Massif (DM) which outcrops on the valley's east side, and the Greenstone and Schist Complex (GS), visible on its west side. The study area is interested by two DSGSD, developed on eastern and western valley side respectively. The gravitational features often dislocate some glacial esaration forms (roches moutonnées). Several transversal trenches are also present, forming locally small closed depressions or counterscarps. The numerous minor scarps are delineated by the broken plane-view profiles of the watercourses. Some spring occur along the longitudinal trenches and give rise to small streams, demonstrating that the pattern of the hydrographic network is affected by the gravitational features. The two DSGSDs present different deforming characteristics which influence the springs location. In particular the western DSGSD highlights several springs, mostly located on the contacts between the crystalline bedrock and the overlaying quaternary glacial deposits which probably represent their supplying aquifers. Instead on the eastern side of the valley there is a lack of perennial spring.