Styles of Extension Along the Liberia Segment of the West Africa Transform Margin

Joan F. Flinch, José Luís Huedo, Hernán Verzi, Héctor González, Ricardo Gerster, Promod Painluy, and Susana Jiménez
West Africa Exploration, Repsol YPF, Madrid, Spain

Regional 2D seismic data along the offshore Sierra Leona and Liberia Basins suggest a complex superposition of Jurassic to Lower Cretaceous extensional events with quite distinctive geometry and role within the evolution of the West African Transform Margin. Extensional activity is coeval with Transform fault tectonics like in other areas of this margin. The lateral superposition of differentially extended continent crust and oceanic crust segments results in strong lateral differential subsidence. Overlying the Pre-Cambrian and Palaeozoic Basement, extensional half-grabens bounded by high angle normal faults developed during Jurassic to Aptian time. A major Low-angle extensional detachment characterized by domino faulting in the hanging wall area, seem to be superimposed to pre-existing and underlying half-grabens of the rifting stage. Finally an overlying Aptian to Lower Albian low-angle extensional detachment, superimposed to the previously described detachment, results in distal toe-thrusting. This extensional event culminates the gravitational collapse of this segment of the West African transform margin, characterized by very steep slope. The Upper Cretaceous to Tertiary sedimentary evolution of the margin will be strongly influenced by this early tectonic development.

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