Morphology and geomorphological significance of relict landslides in the Tertiary basin of Puy-en-Velay (Massif Central, France)

The study of a set of relict landslides in the Tertiary basin of Puy-en-Velay (Massif Central, France) provides information about the types of landslide that affect a part of the hillslopes and the predisposing factors controlling their pattern. Two clusters of landslides were defined, located in similar morphostructural contexts, but characterised by basaltic covers of different ages. The first factor controlling pattern was lithological, in particular the occurrence of two sandy-clay geological formations. One, 'Sables de la Laussonne', is an Oligocene formation, and the other corresponds to the sandy-clay fraction of a Quaternary formation. The second factor was the age of the basaltic cover, which controls the degree of catchment dissection. The 'old' catchments are most strongly widening, with a more differentiate topography than younger ones, and the density of relict landslides was greater in these catchments. The main type in the set of relict landslides seems to be complex slides with a rotational component at the top and a slow-moving earthslide making up the body. So far we cannot advance any firm hypothesis about the kinetics of these landslides. These relict landslides constitute major processes underlying basaltic scarp back-tilting in the basin and are a key factor in catchment geomorphology development.