The Ksour mountains (northwest Algeria) belong to the western part of the Saharan Atlas. They are located between the High Plateau to the north, the South Atlas Front to the south and the Moroccan High Atlas to the west (fig. 1a). During the last decades, the Ksour mountains were the focus of several studies, mainly based on sedimentary analyses [Cornet, 1952; Bassoulet, 1973; Baiche, 1975; Ameur, 1978; Ait Ouali, 1991; Mekahli, 1995; Elmi et al., 1998]. These studies indicate that the Ksour mountains correspond to an old asymmetric rift, trending NE-SW, which underwent extension during the Triassic and Liassic [Ait Ouali, 1991; Frizon de Lamotte et al., 2000]. In order to precise the timing of the rifting processes we have analysed subsurface data (seismic lines and borehole data) from the Sonatrach Company.