How multiple foliations may control large gravitational phenomena: A case study from the Cismon Valley, Eastern Alps, Italy

摘要

The right slope of the High Cismon Valley (Trento Province, Italy), carved into the poly-deformed phyllites of the South Alpine Basement, shows evidence of differential Quaternary slope evolution which highly depends on how the slope intersects the inherited structures. In the study area, the regional schistosity outlines a kilometer-scale NNW-SSE trending fold, with close flanks and an axial plane dipping to the NE. The structure obliquely intersects the NNE-SSW trending slope so that the northern part of the slope follows the upper limb of the fold and the southern sector coincides with the lower limb and the hinge. The secondary axial-plane foliation is typically incipient at the fold flanks, and much more pervasive and fan-shaped near the hinge zone. This foliation, as well as the asymmetric polyharmonic secondary folds, has significant consequences on rock mass mechanical properties and on mechanisms and timing of the gravitational phenomena developed along the slope. In particular the Joint Compressive Strength (JCS) and the Geological Strength Index (GSI), obtained on stable outcrops outside the deforming area, display a decrease from north to south. This points to a progressive deterioration of the rock mass strength which directly reflects the influence of the pre-existing fabric. The results obtained by the analysis of LiDAR-derived digital elevation model show evidence of two different gravitational movements, located in the northern and southern sectors of the slope respectively. The northern side is characterized by an ongoing deep-seated gravitational slope deformation (DSGSD) likely triggered by post-glacial unloading, derived from the retreat of the ice tongue that filled the Cismon Valley during the Last Glacial Maximum (LGM). Conversely, the southern part of the slope is the expression of a fully evolved pre-LGM gravitational collapse. This heterogeneous behavior of the slope is most likely controlled by the secondary foliation and asymmetric polyharmonic folds, the former being of paramount importance for the release of the mobilized mass at the crown area, and the latter easing or impeding the downslope movement. Although it is widely accepted that the regional foliation is the dominant controlling factor of DSGSDs on poly-deformed and highly foliated metamorphic basement, our work suggests that the secondary foliation, as well as parasitic folds, may also have remarkable effects on slope evolution if favorably oriented and sufficiently pervasive.