The importance of inherited structures in slope evolution: the Ridnaun Valley, Italy

The south facing slope of the Ridnaun Valley (South Tyrol, Italy) is set on crystalline units belonging to the Austoalpine Nappe of the Alpine orogenic wedge and shows evidence of quaternary gravitational evolution which highly depends on the interaction between the slope trend and the brittle/ductile structural setting. The slope is carved within the paragneiss rocks of the Oetztal - Stubei Unit and the micaschists of the Schneeberg Unit. These two units are separated by a NNW gentle dipping tectonic contact, which obliquely intersects the E–W slope, and is underlined by multiple ultracataclasitic layers that follow the regional low angle north-dipping schistosity. Folds with sub-horizontal E–trending axes induce the change in the dip direction of the regional schistosity from N dipping (unfavorable to the slip) to SE dipping (favorable to the slip). NNE–SSW and N–S trending faults, associated to 1 m thick horizons of incoherent fault breccias, affect the entire slope. These, as well as the folds and the ultracataclastic layers, has significant consequences on rock mass mechanical properties and on mechanisms and timing of the recognized gravitational phenomena. Field works and ALS-HRDEM analysis revealed different gravitational movements along the slope. A fully evolved gravitational collapse, having the features of a Rock Avalanche, characterizes the central part covering an area of about 2.4 km2; whereas to the east and west of the RA, Deep Seated Gravitational Slope deformations (DSGSDs) still affect the slope. An ongoing gravitational deformation involves the uphill sections of the slope, next to the crown area of the RA. In addition, to the west and east of the RA, morphostructural features as double ridge, scarps – counterscarps, trenches are evident. PS and DS - SAR interferometry data (provided by the Geological Survey of the Autonomous Province of Bolzano, Italy), testify an ongoing movement on both the DSGSDs bordering the RA, highlighting a most unstable area at the western sector. The heterogeneous behavior of the slope is most likely controlled by the interaction between ductile and brittle structures: the small – scale folds ease the DSGSD formation and evolution and act as releasing factor for the RA crown area, whereas the recognized fault network acts as lateral release of the unstable areas. Finite element modeling approach and hybrid FEM/DEM modeling techniques were chosen to investigate, from the known structural setting, triggers and mechanisms of progressive rock mass degradation, as well as fracture propagation processes which led to the initiation and evolution of the catastrophic collapse generating the RA.