Upper Triassic sandstone- and siltstone-infilled basins in the “Massif Ancien” of the High Atlas (Morocco) are distributed along a complex east-northeast trough at the boundary between the Hercynian orogen and the West African craton. The formation of these basins involved a dense pattern of pre-existing faults. Hydroplastic slickensides, which affect nonlithified sediments, help identify not only dip-slip but also strike-slip faults, together with other tectonic and sedimentary structures. The directional distribution of these faults leads to a model in which extension varies from north-northwest (during lower unit sandstone infilling) to northwest (during upper unit siltstone infilling). The latter direction is compatible with the formation of the lower Liassic intrusional doleritic dikes of the Anti-Atlas.