Evolution stages of large deep-seated landslides at the front of a subalpine meridional chain (Maritime-Alps, France)

International audienceStudying long term-evolution of gravitational slope evolution is a key to understanding deep-seated landslide processes. This paper deals with three large Deep-Seated Landslides (DSLs) at a front of a subalpine meridional chain, on the "La Marbrière" slope near the town of Grasse (Alpes-Maritimes, France). The geological framework controlling the stability and morphology of the DSLs is associated with thick and tamped Triassic layers of mudstone with gypsum overlain by highly faulted Jurassic limestone. Gravitational deformation affects the entire slope, involving a movement of about 1.1 × 108 m3 of rock material. It creates large disturbances in landscape morphology, such as scarps, counter-slope scarps, trenches and other typical gravitational morpho-structures. Geomorphological mapping coupled with deep electrical resistivity tomography (ERT) reveals a strong correlation between these morpho-structures and inherited brittle tectonic features. This observation relies on spatial and geometrical relations (on the surface and at the depth of more than 150 m, checked by ERT) between the most persistent fault and the gravitational morpho-structures. The specific distribution of the morpho-structures on the basis of their morphological typologies and variations in the stage of evolution of three DSLs provides an interpretation of their kinematics during the last 400 ka. It appears that soft substratums combined with inherited persistent anisotropies are key factors in the development of the DSLs. Indeed, outflow of mudstone due to the lithostatic pressure imposed by individual limestone compartments has led to general slope subsidence. Then, a progressive toppling of a rock mass may have led to the catastrophic rock collapse along bedding planes. The evolution of the DSLs can be divided into three distinct stages represented by three zones: a young collapse stage (zone 1), a pre-collapse stage (zone 2) and an old mature stage (> 400 ka, zone 3). As the DSLs occur on the same slope and in the same geological context, this area offers interesting perspectives for understanding factors controlling the long-term gravitational evolution of slopes

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

International audienceGravitational slope failure involves rock slopes at various scales. Nowadays, it is accepted that different factors influence slope destabilization, including topography. In many cases, slope failure occurs between tributary valleys cutting the slope. In this study, we ask what influence tributary valleys have on slope failure. To tackle this question, we developed a 3-D numerical model of the La Clapière Slope and then examined a series of simplified 3-D models with different geometries of tributary valleys (spacing and depth). Our results show that: (1) whatever considered in situ stresses are, including the third dimension reduces the destabilization threshold compared with 2-D models; and (2) the spacing and the depth of tributary valleys influence slope failure. For shallow incisions, increasing the lateral spacing between tributary valleys does not affect failure localization but does increase slope damage (to a stable value from 2000 m). However, deeper incision does not affect slope damage but does contribute to failure localization. When the spacing is less than 1500 m, the part of the slope between tributary valleys is not involved in the failure process, but for spacings above 1500 m slope failure occurs between tributary valleys

Typologie et modèles de glissements de terrain : exemples de sites des Pyrénées occidentales et centrales

Plusieurs sites d’étude (Pyrénées) soumis à des glissements de terrain sont présentés avec leurs caractéristiques géologiques et géomorphologiques permettant de les classer suivant leur typologie. Pour l’ensemble des glissements de terrain, on analyse la géométrie des glissements ainsi que les facteurs géologiques et mécaniques favorisant les instabilités. Parmi les facteurs géologiques permanents on retrouve : la lithologie, la fracturation du massif, le plissement des couches, l’orientation des structures et la stratification. Sur les quatre sites étudiés, deux correspondent à des glissements de la moraine quaternaire et les deux autres à des glissements rocheux. La connaissance des différentes formes de surface de glissement, nous permet de les classer en : glissements translationnels, rotationnels ou complexes. Cette classification associée à huit modèles géométriques ainsi qu’aux différents facteurs géologiques et mécaniques permet d’expliquer les causes de la rupture et de proposer une aide à la cartographie de l’aléa « glissements de terrain » dans une perspective de gestion des risques

Morphological analysis of deep-seated gravitational slope deformation (DSGSD) in the western part of the Argentera massif. A morpho-tectonic control?

International audienceThe western part of the Argentera–Mercantour massif (French Alps) hosts very large currently active landslides responsible of many disorders and risks to the highly touristic valleys of the Mercantour National Park and skiing resorts. A regional scale mapping of gravitational deformations has been compared to the main geo-structures of the massif. A relative chronology of the events has been established and locally compared to absolute 10Be dating obtained from previous studies. Two types of large slope destabilisations were identified as follows: deep-seated landslides (DSL) that correspond to rock volumes bounded by a failure surface, and deep-seated gravitational slope deformations (DSGSD) defined as large sagging zones including gravitation landforms such as trenches and scarps or counterscarps. Gravitational landforms are mainly collinear to major N140°E and N020°E tectonic faults, and the most developed DSGSD are located in areas where the slope direction is comparable to the orientation of faults. DSL are mostly included within DSGSD zones and located at the slopes foot. Most of DSL followed a similar failure evolution process according to postglacial over steepened topographies and resulting from a progressive failure growing from the foot to the top of the DSGSD that lasts over a 10 ky time period. This massif-scale approach shows that large-scale DSGSD had a peak of activity from the end of the last deglaciation, to approximately 7000 years bp. Both morphologic and tectonic controls can be invoked to explain the gravitational behaviour of the massif slopes

Etude du matériel sécrété dans la chambre de régénération d'un appendice locomoteur de Porcellio dilatatus Br. (Crustacea : Isopoda)

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Long-term monitoring of a large deep-seated landslide (La Clapiere, South-East French Alps): initial study

International audienceThe large-scale deformation of high mountain slopes finds its origin in many phenomena (inherent parameters, external stresses) with very different time constants (instantaneous to geological scale). Gravitational effect, tectonic forces and water infiltration are generally the principal causes of slope instability. However, it can be very difficult to distinguish which cause is dominant and which are their respective effects. To gain a better understanding of the complex processes taking place during the evolution of an unstable slope and separate the causes responsible of the landslide dynamic, an observational study based on geodetic, meteorological, seismological and electrical data has been performed on the La Clapière rockslide (Southern French Alps). This deep-seated landslide (DSL) is known for many years as one of the largest and fastest rock slide in Europe (60 million m3 of highly weathered metamorphic material, moving at 1 to 3 m year−1). The set-up of the “Observatoire Multidisciplinaire des Instabilités de Versants” (OMIV, http://omiv.osug.fr) in 2011 has allowed the production and availability of an important and original data set over several years of accurate monitoring. Thus, for the first time, the long-term study of geodetic data permitted us to highlight acceleration phases in the general movement of the landslide that affect its dynamic. These modifications are associated with variations of the velocity by a factor 3 to 6. The characterization of the origin of these variations was possible due to the comparison with meteorological, electrical and seismological data. Based on these various signals, we were able to establish correlations and contributions of meteorological water infiltration in the dynamic evolution of the La Clapière slope. We determine several response times to the meteorological stress for seismic endogenous events (mainly rockfalls), the resistivity of the ground (quasi-instantaneous) and the kinematics of the slope (from 2 weeks to 2.5 months). Moreover, our results strongly suggest the existence of rainfall threshold of 3.5 ± 1 mm day−1 from which the number of seismic endogenous events is highly increased

Frequency and triggering of small-scale submarine landslides on decadal timescales: Analysis of 4D bathymetric data from the continental slope offshore Nice (France),

International audienceTime-series bathymetric data acquired from 1967 to 2011 are used to evaluate the morphological evolution of the continental shelf and upper continental slope off the city of Nice (SE France, Ligurian Sea). Mapping in water depths of 0–300 m was undertaken to identify the changing morphology of landslide scars and their erosive chutes. Quantitative Digital Terrain Model (DTM) comparisons reveal areas of erosion and deposition over intervals of 5–8 years. Sediment remobilization events on the upper slope (above depths of 200 m) are frequent and significant; landslide scars with volumes > 25,000 m3 can occur with frequencies of 200 m). Periods of quiescence (1980–1990 and 2006–2011) are seen to alternate with periods when rapid retrogressive failure increase sediment volumes eroded from the upper slope-shelf transition by an order of magnitude (1999–2006). Temporal variations in landslide activity were correlated to several potential triggering factors that individually would not induce failures, including earthquake activity, rapid deposition of fine-grained sediments on a steep slope, and rainfall leading to fresh groundwater circulation below the shelf. This 4D bathymetric study suggests that over the last 50 years the most important factor triggering landslides offshore Nice is overpressure due to freshwater outflows

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

International audienceCatastrophic 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 10Be 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