Neotectonics and current hydrologically-induced karst deformation. Case study of the Plateau de Calern (Alpes-Maritimes,France)

The astronomical and geodetic observatory OCA, located on the karst plateau of Calern (Caussols, France), has been monitoring earth deformation for several years. Two medium baseline tiltmeters have been installed in a shaft in 2007, along with classical hydrogeological monitoring tools in order to investigate the relationship between current karst deformation and hydrology. Dye tests have shown that the plateau is drained towards the East, to the spring of Bramafam, except for its Western third. Karst tilting, as recorded by the tiltmeters, is linked to rainfall events. These instruments bring additional information to characterize different reservoirs: the deep karst aquifer of Bramafan with high amplitude oscillations of its water table, up to 100 m, the perched aquifer of Moustiques shaft whose response is attenuated, and several slope aquifers with reduced oscillations (Fontaniers, Castel Bon Pré).

Tilt deformation reaches 8 μad with a definite orientation between N90°E and N100°E. The best correlation between hydrology and tilts is observed for the deep aquifers. If the first autumn rainfall is ineffective on tilt, it recharges the epikarst and refills the reserves. The winter rains cause the water to flush towards the eastern deep aquifer and provokes a quick tilting of the plateau. Finally, the long term variations in tilt and water table show a very good correlation.

The relationship between current hydrological deformations and tectonics is also analyzed; broken and shifted speleothems in the shafts indicate a general shift of the plateau towards the south.