the northwestern part of the African-Arabian Craton, covered by a sedimentary mantle. The foundation of the Saharan Platform crops out in the massifs that extend from west to east along the central part of the platform, constituting the central Sahara uplift zone. Early Precam-brian rocks are exposed at the surface in the Rigaibat massif in the west; to the east, separated by the Tanezrouft Trough, is the Ahaggar (Tuareg) massif, which consists of alternating horsts and grabens that developed in the late Precambrian. The horsts are composed of highly metamorphosed early Precambrian rocks (suggarian), whereas the grabens are less metamorphosed late Precambrian sedimentary and volcanic rocks (pharusian). Ancient Precambrian rocks also make up the Tibesti and al-Uwaynat massifs to the east and the western tip of the Nubian-Arabian Shield.
North and south of the central Sahara uplift zone of the foundation are regions that experienced significant subsidence during the Phanerozoic. In the Mesozoic (until the mid-Cretaceous), a continental stratum accumulated that was covered by late Cretaceous and early Paleogenic marine sediments. Since the Oligocene, the area of the Saharan Platform has developed under continental conditions, and the ancient massifs—especially the Ahaggar and Tibesti—have undergone significant uplift.
The south Sahara zone of subsidence has less variation and less development of marine sediment. It includes the level Taoudenni Syneclise, which formed on the West African Shield in the Riphean; the Mesocenozoic Gao Graben; the Mali-Niger Syneclise; and the Chad Syneclise.
V. E. KHAIN