Abstract
The Larzac Plateau is delimited by vertical cliffs whose geometry is controlled by vertical joints. Cliff’s erosion involves landslides initiated by incremental enlargement of joints that progressively detach rock columns at very low velocities (1.2 mm/yr). We find that enlargement of joints is linked to intraseasonal thermal cycles ranging between 2-15 days in relation with dilation/contraction of rock blocks trapped inside the joints. The mechanism involves two successive stages in which blocks create a wedging and a ratcheting effect on the rock column. Wedging is associated with compressional forces acting on the rock column, resulting from temperature increase and dilation of the shallow rocks. Ratcheting is associated with downward displacement of blocks by gravity to a new equilibrium position, resulting from temperature decrease and contraction of shallow rocks. The displacement vector in a thermal cycle is split into a plastic and a thermal component; plastic displacements range between 10 — 200 μm according to the seasons, and are absorbed along a shear plane dipping ~40° beneath the rock column: they are largest during autumn and winter, minor during spring and negligible in summer. This deformation mechanism is termed thermomechanical creep as permanent deformations are associated to mechanical forces induced by short-term thermal cycles.
Highlights
The role of temperature fluctuations on the initiation of landslides is a controversial subject
We propose a simplified theoretical model for thermomechanical creep, based on displacement and temperature measurements on a rock column, and on many field observations in the Larzac Plateau (Southern France)
We present a simplified model explaining the correlations between short-term intraseasonal displacement and thermal cycles, based on: a) the analysis of data from the instrumented column presented in the previous section ; and b) geological and morphological observations of many rock columns in the Larzac Plateau
Summary
The role of temperature fluctuations on the initiation of landslides is a controversial subject. Thermal effects are normally considered to be secondary, recent studies suggest that temperature variations may induce permanent plastic deformations within jointed rock slopes, which may trigger landslide processes [1,2,3]. Some authors suggest that temperature variations during annual cycles may induce shearing beneath rock columns through a mechanism combining wedging and ratcheting [4,5,6]. We propose a simplified theoretical model for thermomechanical creep, based on displacement and temperature measurements on a rock column, and on many field observations in the Larzac Plateau (Southern France). Short-term (and not annual) thermal cycles linked to varying climatic conditions are at the origin of incremental plastic deformations beneath the rock column. The paper is organized as follows: section 2 presents the geological setting of our field observations, section 3 presents data form the instrumented rock column, section 4 explains the theoretical model based on measurements and observations, and section 5 discusses the results and perspectives of this research topic
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