11. GEODYNAMIC EVOLUTION OF THE CÔTE D’IVOIRE-GHANA TRANSFORM MARGIN: AN OVERVIEW OF LEG 159 RESULTS1Christophe Basile,2 Jean Mascle,3 Jean Benkhelil,4 and Jean-Pierre Bouillin2 |
A synthesis of shipboard and postcruise studies performed on Ocean Drilling Program (ODP) Leg 159 data leads to several important modifications on previously proposed models for the Côte d’Ivoire-Ghana Transform Continental Margin evolution. Siliciclastic sedimentation occurred from Aptian through late Albian times in a wrench tectonic (possibly pull-apart basin) setting and was submitted to high geothermal gradients. This tectonically active transform zone jumped twice from the southern edge of the Ivorian Basin (the adjacent divergent margin) to the southernmost oceanic border of the Gulf of Guinea. Each jump correlates with a localized uplift, coeval with the end of tectonic deformation and heating. As a consequence, the erosional unconformities capping the Early Cretaceous deformed sequences are not thought to be coeval at all ODP sites. A progressive southward migration of the active transform may have been initiated in early Albian times, when continental breakup occurred within the adjacent rifted Ivorian Basin. The early northward tilting of the marginal ridge appears to be prior to the late Albian, and has led to the development of a fringing reef along its northern slope. A second migration of the transform fault zone occurred in the late Albian–early Cenomanian at the end of the active shear between the African and Brazilian continental crusts. During Late Cretaceous times, the transform margin experienced continuous subsidence leading to the progressive up-slope migration of the reef. An increasing subsidence rate characterized the early Coniacian, close to the time when the first continent/ocean transform transition occurred. No specific thermal event or uplift can be associated with this continent/ocean active transform boundary, which is characterized only by an increase of the marginal ridge’s northward tilting. ODP Leg 159 results clearly exclude thermal conduction from the oceanic lithosphere to the continental one as the main process resulting in transform margin tilting and uplift. Clues to these processes have to be searched in the functioning of the active transform fault during both intracontinental and continental/oceanic stages. |
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