BASEMENT CONTROL IN THE DEVELOPMENT OF THE EARLY CRETACEOUS WEST AND CENTRAL AFRICAN RIFT SYSTEM

The structural framework of the Precambrian basement of the West and C entral African Rift System (WCARS) is described in order to examine th e role of ancient structures in the development of this Early Cretaceo us rift system. Basement structures are represented in the region by l arge Pan-African mobile belts (built at ca. 600 Ma) surrounding the > 2 Ga West African, Congo and Sao Francisco cratons. Except for the sma ll Gao trough (eastern Mall) located near the contact nappe of the Pan -African Iforas suture zone along the edge of the West African craton, the entire WCARS is located within the internal domains of the Pan-Af rican mobile belts. Within these domains, two main structural features occur as the main basement control of the WCARS: (1) an extensive net work of near vertical shear zones which trend north-south through the Congo, Brazil, Nigeria, Niger and Algeria, and roughly east-west throu gh northeastern Brazil and Central Africa. The shear zones correspond to intra-continental strike-slip faults which accompanied the oblique collision between the West African, Congo, and Sao Francisco cratons d uring the Late Proterozoic; (2) a steep metamorphic NW-SE-trending bel t which corresponds to a pre-Pan-African (ca. 730 Ma) ophiolitic sutur e zone along the eastern edge of the Trans-Saharian mobile belt. The p ost-Pan-African magmatic and tectonic evolution of the basement is als o described in order to examine the state of the lithosphere prior to the break-up which occurred in the earliest Cretaceous. After the Pan- African thermo-tectonic event, the basement of the WCARS experienced a long period of intra-plate magmatic activity. This widespread magmati sm in part relates to the activity of intra-plate hotspots which have controlled relative uplift, subsidence and occasionally block faulting . During the Paleozoic and the early Mesozoic, this tectonic activity was restricted to west of the Hoggar, west of Air and northern Cameroo n. During the Late Jurassic-Early Cretaceous the St. Helena mantle plu me may have had an important effect in weakening the lithosphere acros s a broad zone from Brazil to Sudan. However, causes of extension and stretching are likely to be found in a plate tectonic framework. At th at time, the opening of both the Indian and Atlantic oceans were the d ominant forces which favoured the development of the WCARS and its pro pagation by strike-slip movements and extensional displacements along pre-existing Pan-African lithospheric zones of weakness.