Paraglacial gravitational deformations in the SW Alps: a review of field investigations, ¹⁰ Be cosmogenic dating and physical modelling

Abstract

Catastrophic deep-seated landslides (DSL) are generally considered to be the result of large slope deformations also known as deep-seated gravitational slope deformation (DSGSD). This paper aims to build a synthesis of multiple studies made in the Tinée Valley (southern French Alps) to assess the geometrical, kinematical, mechanical and chronological relationships between these two gravitational processes.At the scale of the valley, data issued from geological, geomorphological and 10 Be dating indicate a clear geometrical link between DSGSD and DSL occurring at the base of the slope and suggest that gravitational slope evolution began after the glacial retreat (13 ka BP). This is supported by the example of the well-documented La Clapière slope. A continuous evolution process is characterized geometrically and temporally from geomorphic observations and analogue modelling. Coupling structural, geomorphological, physical and chronological studies allowed us to propose a four-dimensional (4D) deformation model mechanically correlated with progressive failure concept. The validity and variability of this reference site are discussed at the valley scale (taking Isola and Le Pra slope deformation as examples).It allows a rough estimation of the state of slope deformation at the valley scale to be constructed and the slope evolution with time to be considered. This 4D model could then be considered as a reference for other deep-seated gravitational slope deformations in comparable Alpine valleys.

Large ice‐contact slope movements: glacial buttressing, deformation and erosion

“…Deep-seated gravitational slope deformation (DSGSD), also known as deep-seated creep, is the very slow (<1.6 m/yr, IUGSWGL, 1995) deformation of rock slope masses larger than about 200,000 m 3 (Dramis and Sorriso-Valvo, 1994;Pere, 2009). This type of slope movement is common in high-relief and glaciated terrains (Beck, 1968;Bovis, 1982Bovis, , 1990Bovis and Evans, 1996;Holm et al, 2004;Ambrosi and Crosta, 2006;Agliardi et al, 2009b;Hippolyte et al, 2009;Pere, 2009;El Bedoui et al, 2011;Pánek et al, 2011). DSGSDs are typically recognized by features such as scarps and uphillfacing scarps, sackungen, tension cracks and toe bulging, and can involve large (>100 m downslope) displacements of rock at rates of a few millimetres to several metres per year (Figure 1).…”

“…Most studies of DSGSDs in glaciated terrain have been of slopes long-since disconnected from the glacier. Several of these DSGSDs have been considered to have become active long after ice downwastage or retreat, which is sometimes supported based on either dating of landslide surface features (Cossart et al, 2008;Agliardi et al, 2009a;El Bedoui et al, 2011) or numerical modelling studies of failure conditions (Bovis and Stewart, 1998;Ustaszewski et al, 2008;Ambrosi and Crosta, 2011;Ghirotti et al, 2011). In some of these situations this may be the case.…”

Denudation systematics inferred from in situ cosmogenic 10Be concentrations in fine (50–100 µm) and medium (100–250 µm) sediments of the Var River basin, southern French Alps

“…In the northern and highest part of the basin, the Mercantour-Argentera Massif comprises migmatites, gneiss, and granite ( Fig. 1) inherited from the Paleozoic Hercynian orogen (El Bedoui et al, 2011). The late Paleozoic to Cenozoic sedimentary cover of the southern and western parts of the basin mainly comprises Mesozoic marls and limestones (Kergkhove and Montjuvent, 1977;Roure et al, 1976).…”

“…Topographic shielding factors were calculated using the ArcGIS toolbox of Codilean (2006), which computes both self-shielding and shading. Quartz-free areas were excluded from the basin-averaged production rate calculations based on 1 : 50 000 and 1 : 250 000 scale French Geological Survey geological maps (Bigot et al, 1967;Campredon et al, 1980;Faure-Muret et al, 1954, 1957Gèze et al, 1969;Ginsburg et al, 1980;Kergkhove and Montjuvent, 1977;Kergkhove and Roux, 1976;Kergkhove and Thouvenot, 2010;Roure et al, 1976). Quartz-bearing zones comprise 730 km 2 , corresponding to 26 % of the total surface area of the Var basin.…”

Influence of inherited topography on gravitational slope failure: three‐dimensional numerical modelling of the La Clapière slope, Alpes‐Maritimes, France

“…One of these movements – the La Clapiere landslide, historically active since the beginning of the 20th century (Casson et al ., ) – has shown dated movements back to ~10 ka (Bigot‐Cormier et al ., ; Sanchez et al ., ). The landslide is embedded in a larger deformation known as Deep Seated Gravitational Slope Deformation or Sagging (Agliardi et al ., ; Bois et al ., ; El'Bedoui et al ., ) with an upslope propagation (Merrien‐Soukatchoff and Gunzburger, ) and a failure surface depth ranging from 100 m to 200 m. The gneissic material in the landslide area is intensively weathered (Guglielmi et al ., ; Jomard et al ., ; Lebourg et al ., ), leading to linear evolutions of both effective cohesion and effective angle of internal friction (respectively decreasing and increasing) from the unweathered area to the weathered area since 3.6 ka (Lebourg et al ., ).…”