EVIDENCE OF PALAEOSEISMICITY IN THE KARSTIC CAVITIES OF THE GRANDS CAUSSES AS SHOWN IN THE AVEN DE LA PORTALERIE (LARZAC, AVEYRON, FRANCE)
CAGEP - URA 903 CNRS, Institut de géographie de l'Université de Provence 29, av. Robert Schuman, 13621 Aix-en-Provence cedex 01, FRANCE et GRD 1058.
CAGEP - URA 903 CNRS, Institut de géographie de l'Université de Provence 29, av. Robert Schuman, 13621 Aix-en-Provence cedex 01, FRANCE et GRD 1058.
CERAK, Faculté polytechnique de Mons 9, rue Houdain, 7000 Mons, BELGIQUE.
Abstract
ABSTACT : Instrumental and historical seismicity data reveal that the Grands Causses are a seismically and tectonically stable area. Conventional neotectonic analysis methods give similar results. However, the Aven de la Portalerie on the Causse du Larzac does show some evidence of a degree of seismotectonic activity expressed by the presence of speleothem breaking. A large number of stalagmites have been broken off and the break often repaired by a subsequent growth of calcite. Stalagmitic floors have been fractured and displaced by fault movements occurring after their formation. This destruction would seem to have been caused by several earthquakes, since stalagmite fragments lying on the ground show that they had already been broken and built up again by several centimetres.
In this context of apparent stability, only the endokarst is capable of retaining and revealing evidence of relatively recent earthquakes whose chronological order can be established using various dating techniques.