Recent geological and geophysical data from a marine survey conducted off Guinea and the Ivory Coast-Ghana Region (Equatorial Atlantic) are discussed. We interpret the southern Guinean margin firstly as an area linked to the Central (Jurassic) Atlantic, and secondly as an area affected by the progressive transform opening of the Equatorial Atlantic in the Early Cretaceous.

Seismic profiles across the transform continental margin off the Ivory Coast and Ghana (Western Africa) illustrate the structural style resulting from the early Cretaceous phase of shear stress which leads to the final separation between the African and Brazilian continental margins in this area. Most of the characteristic tectonic features observed along this portion of margin (asymmetric grabens on the Ghanean platform, folds of the deep Ivory Coast basin, the Ivory Coast—Ghana marginal ridge) are believed to result from progressive transform contacts between the African and Brazilian continents as their margins were created during early Cretaceous time. A major tectonic unconformity inferred to be of upper Albian-lower Cenomanian age, may be a direct consequence of the final separation of the continental margins. The later evolution of the transform margin is chiefly explained by thermal subsidence.

Other| September 01, 1987 Geologie de la pente continentale ivoiro-ghaneenne; resultats de dragages de la campagne Equamarge E. Blarez; E. Blarez Author Lab. geodynam. sous-mar., Villefranche-sur-Mer, France Search for other works by this author on: GSW Google Scholar J. Mascle; J. Mascle Author Search for other works by this author on: GSW Google Scholar P. Affaton; P. Affaton Author Search for other works by this author on: GSW Google Scholar C. Robert; C. Robert Author Search for other works by this author on: GSW Google Scholar J. P. Herbin; J. P. Herbin Author Search for other works by this author on: GSW Google Scholar G. Mascle G. Mascle Author Search for other works by this author on: GSW Google Scholar Author and Article Information E. Blarez Author Lab. geodynam. sous-mar., Villefranche-sur-Mer, France J. Mascle Author P. Affaton Author C. Robert Author J. P. Herbin Author G. Mascle Author Publisher: Société Géologique de France First Online: 03 Mar 2017 Online Issn: 1777-5817 Print Issn: 0037-9409 GeoRef, Copyright 2012, American Geosciences Institute. Reference includes data from PASCAL, Institute de l'Information Scientifique et Technique, Vandoeuvre-les-Nancy, France Bulletin de la Société Géologique de France (1987) III (5): 877–885. https://doi.org/10.2113/gssgfbull.III.5.877 Article history First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation E. Blarez, J. Mascle, P. Affaton, C. Robert, J. P. Herbin, G. Mascle; Geologie de la pente continentale ivoiro-ghaneenne; resultats de dragages de la campagne Equamarge. Bulletin de la Société Géologique de France 1987;; III (5): 877–885. doi: https://doi.org/10.2113/gssgfbull.III.5.877 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin de la Société Géologique de France Search Advanced Search This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The relative motions between lithospheric plates can be reduced to three major types: convergence, divergence and transform motion. Convergence leads to an active margin (and eventual collision); divergence results in the construction of a rifted (passive) margin; transform motion generates a transform (or sheared) margin. Although passive margins have been extensively studied1,2, and many models have been proposed for their origin and subsequent evolution3–5, little is known about transform margins, with the exception of a few studies of their morphologies6,7, shallow structures or crustal sections8,9. Here we present the main conclusions derived from a recent study of the northern Gulf of Guinea margins, particularly off the eastern Ivory Coast and Ghana, where the continental margin is one of the best-preserved examples of an extinct transform margin10–12. Our observations support a four-stage model for transform margin evolution. Technically active transform contacts, first between normal continental crusts and then between thinned margins, induce characteristic structures such as pull-apart grabens and shear folds. The next stage, in which thermal exchange between oceanic and continental lithos-pheres controls a complex subsidence, is followed by the transition to a true intra-oceanic fracture zone.

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.

Several seismic profiles recorded south of Abidjan (off Ivory Coast) show the presence of a relatively thick Neogene/Quaternary sedimentary wedge along the continental margin. Features considered as typical of a deep-sea fan are observed; among them, we note characteristic acoustic facies (chaotic, transparent, bedded) associated with a system of meandering channels and lateral levees. This deep-sea fan has developed since the late Oligocene (in response to the Oligocene regression) with the Trou Sans Fond Canyon acting as its sediments feeder, although there is no direct relationship with a modern river system.