Abstract

Detailed geological surveys carried out recently on the Layens have permitted to redefine the lithostratigraphy and to establish the structure of its south slope where several rockslides have been precisely located. This south slope is composed of a carbonaceous Mesozoic series ranging from the Triassic up to the Lower Cretaceous which is located on the reverse flank of a large overfolded north north-eastern syncline. The lithological and structural heritage, together with its special geodynamic setting, generate many brittle zones which favour the mechanical instability of the slope. On this basis the morphostructural position of the rockslides has been established, some of which are unstable while others are stable. Two stages in the evolution of the rockslides have been deduced from the geomorphology and mapping. These field observations have enabled us to distinguish three different rockslides (A, B, C) which are either deep or superficial. The analysis of the causes of their instability have been considered by taking into account the orientation of the major mechanical discontinuities, inherited from the structural context, as well as the sensitivity to the slipping of Triassic clay formations. The natural causes of the destabilization of the south slope of the Layens have been established for the most active rockslide : the geological surveys permit an evaluation of the volume and the total displacements.

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