African transform continental margins: Examples from Guinea, the Ivory Coast and Mozambique

Abstract

Marine Studies of selected sheared-type margins have been undertaken along both West and East African coastal regions in order to determine the geological structure and the evolution of this specific type of passive margin. Geological and geophysical data have been obtained in the coastal waters off Guinea, Ivory Coast, and Ghana (Western Equatorial Africa) and off Mozambique (Eastern Africa). The continental margin off the Guinea coastline can be divided into three distinct sectors. To the Northwest, the observed seismic stratigraphy and structures are typical of a rifted Central Atlantic margin of Jurassic age. To the south, several features such as steep scarps, basement ridges, volcanic piles, magnetic and gravity lineaments, etc., are interpreted in relation to a transcurrent motion. A southwestern marginal area contains lower tectonized sequences of Early Cretaceous age covered by post-Albian detrital sediments. The overall structure is interpreted as a Jurassic rifted margin reactivated by shearing during the Lower Cretaceous. Off shore from the Ivory Coast and Ghana, two main marginal areas are also distinguished. To the east, south of Ghana, the upper margin is fractured by dissymetric and sedimented grabens, with the continental slope representing the unsedimented truncated African craton. South of the Ivory Coast, thick sedimentary basins, containing a Lower Cretaceous tectonized sequence developed between the continent and the Ivory Coast–Ghana Ridge, correspond to a composite feature consisting of deformed sedimentary wedges and basement structures. Off the Mozambique coast, the strike-slip motion of Madagascar with respect to Africa, during the Upper Jurassic, has generated a series of en echelon structures. The Davie Ridge includes a series of sedimentary shear folds and basement structures. Later, in Cretaceous times. throughout the Cenozoic, and since Miocene times, the continental margin has been submitted to different tectonic stresses generating diversely-trending extensional features. When shearing commences between two continental margins, it seems to generate either tensional features (e.g. Ivory Coast) or transpressive features (e.g. Mozambique) depending on the area. Later on, basement-involved structures and sedimentary wedges develop at the boundary between a thinned continental crust and a thick continental crust before connecting laterally with a typical oceanic fracture zone. The southern Guinea margin represents an unusual example where a rifted margin has been reactivated by transcurrent motion.