La deformazione gravitativa profonda di Sauze D’Oux (Alpi occidentali): analisi dei meccanismi deformativi in relazione all’evoluzione tardo quaternaria dell’Alta Valle di Susa

Abstract

Fioraso G., Tararbra M. & Negro N., The deep-seated gravitational slope deformation of Sauze d’Oulx (Western Alps):
analysis of the deformation mechanisms related to the late Quaternary evolution of the upper Susa Valley. (IT ISSN 0394-3356, 2010).
The upper Susa and Chisone Valleys represent a peculiarity in the landscape of the Western Alps because of the extraordinary concentration of deep-seated gravitational slope deformations (DSGSD) and large landslides, covering on the whole 285 km2
, equivalent to 38% of the mountain region. The Sauze d’Oulx DSGSD, extended over an area of 20 km2 , represent one of the biggest gravitational collapse of the western Alpine chain. Placed along the right slope of the Susa Valley near the confluence of Dora di Bardonecchia and Dora di Cesana catchment basins, the Sauze d'Oulx phenomenon extends between the Susa-Chisone drainage divide and the valley bottom, filled with a thick (up to 125 m) fluvio-lacustrine post-glacial succession.
Bedrock geology is characterized by a stack of tectono-stratigraphic units belonging to the Piemontese Zone and made up of monotonous calcschistes with ophiolitic intercalations (mainly serpentinites and ophicalcites with minor metabasites). Morphological, morphostructural and hydrogeological features characterizing the Sauze d'Oulx phenomenon are somewhat different respect to the other DSGSD of the Western Alps. Within the collapsed slope, two main morpho-evolutive domains are easily recognizable:
a) the Jouvenceaux sector, covering an area of over 5.3 km2 and characterized by a spectacular and well definited fan shaped morphology. Slope deformation involves a huge serpentinitic mass (at least 4 km2 ) in which jointed rock masses alternate with wide metric to pluridecametric cataclastic shear zones; these latter are crossed by a three-dimensional network of irregular closely spaced veinlets composed of fibrous tremolite and calcite.
b) the Sportinia-Richardette sector, involving an area of 14.9 km2 , that presents a clear amphitheatre-shaped morphology surrounding the Sauze d'Oulx village. In the middle and upper part of the deforming slope, displacement is accompanied by morpho-structural features such as scarps and counterscarps hectometric to kilometric in extent, and wide collapse dolines. Movement of rock masses takes place along multiple listric sliding surfaces reactivating at least three pre-existing fault systems: i) WSW-ENE striking subvertical transcurrent faults with pluri-kilometric extent influencing the trends of Susa and Chisone Valleys; ii) NW-SE oriented set of normal faults; iii) N-S trending subordinate system of normal faults and fractures.
To understand the kinematic evolution and the actual state and style of activity of the Sauze d'Oulx DSGSD, a multi-disciplinary
approach was performed collecting over time a variety of data from field and remote sensing investigation, both inside and outside the
collapse perimeter: detailed geological, geomorphological and hydrogeological field mapping; subsurface exploration with boreholes
up to 200 m deep; semi-quantitative informations about the rate of movement of the Jouvenceaux sector recorded in the first half of
the last century; monitoring ground-displacement with inclinometric and piezometric systems; measurement of superficial deformation by means of Permanent Scatterer (PSInSARTM) technique (1992-2001) and ground-based SAR interferometry (LISA©) (2002-2008), with the aid of GPS surveying system.
Integrated analysis of field data and instrumental monitoring suggest a dual response of the rock masses to the gravitational stress: the Jouvenceaux sector, subjected to a rapid evolution, can be recognized as a rock flow controlled by a deep-seated continuous creep
process, which involves as a whole the cataclastic serpentinites partially detached from the underling undeformed bedrock; instead,
the Sportinia-Richardette sectors creep processes develops along multiple rupture sufaces with a prevailing retrogressive sliding
mechanisms similar to those observed in the others rock flow phenomena of the Susa and Chisone Valleys.
On the whole the onset evolution of the Sauze d'Oulx DSGSD, controlled by the litostructural array of the bedrock, is due to the strong
downcutting of the Dora di Cesana and Dora di Bardoneccia glaciers and overdeepening of the valley bottoms during the last upper
Pleistocene glacial episode.