The Guinea continental margin: an example of a structurally complex transform margin

Abstract

The Guinean continental margin corresponds to a wide hinge zone which developed between the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic. The Guinea Marginal Plateau that characterizes the southern border of this margin is a wide triangular morphological feature bounded to the west by a typical rifted margin slope segment, while its southern edge corresponds to a narrow and morphologically complex continental slope probably resulting from intracontinental transcurrent motion during the Early Cretaceous. On the basis of seismic profiles recorded in two selected areas located along the southern Guinea margin, a seismic stratigraphy is established. At depth, the plateau is underlain by a thick Mesozoic and Cenozoic sedimentary wedge made of two main ensembles. A lower ensemble (or sequence 1) consists of Early Cretaceous clastics. An upper ensemble includes a series of four distinct sequences attributed to Albian to Tertiary deposits. Eastward, the sedimentary cover of the southern plateau edge is locally pierced by volcanic bodies of Paleocene age. Beneath the southern margin of the Guinea Plateau, the sedimentary sequences appear deformed by faults and folds resulting from, at least, two main tectonic episodes. A first event only affected sequence 1, which is cut by a set of extensional faults. A second tectonic phase including folding, reverse faulting and transcurrent faulting is responsible for the deformation of sequences 1 and 2. During this tectonic episode, former normal faults have been reactivated as reverse faults resulting in a structural inversion. A third tectonic phase was responsible for structures transverse to the general E-W trend of the southern Guinea margin and consisting of N-S-trending normal faults and associated volcanoes. The spatial distribution of the tectonic trends clearly substantiates a polyphase tectonic activity that can be related to plate motion changes during early stages of the continental separation. A first stage is characterized by a divergent rifting leading to N-S-trending synsedimentary normal faults. During the Cretaceous, the tectonic activity increased as a general transform motion occurred between the Guinea Plateau and its American conjugate margin (the Demerara Plateau). Local tensional stresses were responsible for a splay fault system active near the tip of the Guinea Plateau. In late Albian times, a slight adjustment of the Africa plate, with respect to the South American one, resulted in local squeezing of the whole sedimentary wedge. After this short compressive event, final continental separation occurred in an extensional regime characterized by a general collapse along two major scarps and volcanic activity.