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MASCLE, JEAN, Centre National de la Recherche Scientifique, Villefranche-Sur-Mer, France

ABSTRACT: Atlantic Equatorial Margins Off Africa: A Case Study for Transform Margin Development

Continental transform (sheared or translational) margins result from plate motion more or less parallel to active boundaries between parting continents. Such a specific initial geodynamic setting has direct bearing on their morphology, sedimentary evolution, tectonic characteristics, and crustal structures. Off Africa, within the Equatorial Atlantic domain, most of the continental margin segments result from a dominantly initial transform motion between the two parting subcontinents, western Africa and Brazil, in Early Cretaceous times.

In the past 15 years several continental margin segments of the surrounding area, particularly off Cote d’Ivoire and Ghana, have been surveyed using various marine geophysical and geological tools including swath bathymetry, single channel and multichannel seismic reflection profiling, seismic refraction data acquisition, deep-sea dives, and, finally, deep-sea drilling (ODP leg 159). This rather complete set of data has helped to better characterize the different sedimentary and tectonic regimes that successively prevailed during the construction of one of the major results of transform margin; i.e., the development of important bordering marginal ridges that lie at the boundary between continental and oceanic crusts.

Off Cote d’Ivoire the thick sedimentary pile exposed along the marginal ridge slope, over more than 3000 m, is made of a repetitive clastic sequence typical from prodeltaic to deltaic environments and has likely been delivered by the erosion of the passing Brazilian craton in early Cretaceous times. This exposed sedimentary pile, which can be viewed as one of the oldest and still outcropping sedimented continental slopes, has recorded several stages of transform margin sedimentary and structural evolution. Syn- to post-lithification deformations first recorded extensional deformations generated by early rifting; then wrench tectonics has produced intense fracturing and local folding and has potentially participated to regional tectonic uplift. Integration of geophysical data and ground truth, as directly derived from in situ and from deep drilling results, support qualitative models for transform margins that are based on three main periods: intracontinental, ocean-continent active transform, and finally passive margin stages. An approximately 15 minute video, from deep dives up to 5000 m of water depth, might be shown at the end of this presentation.

AAPG Search and Discovery Article #90910©2000-2001 AAPG Distinguished Lectures