Deep structure of the southeastern margin of the West African craton from seismic reflection data, offshore Ghana

Abstract

Abstract Seismic reflection data, acquired as part of an oil exploration survey offshore Ghana, have been reprocessed to reveal the deep structure of the Proterozoic continental crust on the continental shelf, off West Africa. The seismic profiles presented are located across the Romanche transform margin, which transects the West African craton (WAC) margin where it is bounded by the Pan-African Dahomeyide orogen. The seismic data reveal highly reflective middle–lower crust of the WAC but a nearly transparent upper crust of c . 10 km thickness. Bundles of discontinuous reflections are located at the base of the unreflective upper crust and locally define the trough of a synformal structure interpreted as the keel of a Palaeoproterozoic (Birimian) greenstone belt exposed onshore. The deepest crust imaged is characterized by more continuous, subhorizontal reflections that are very similar to those observed in the lower crust of cratonic areas such as the Superior Province. Near the WAC margin the subhorizontal reflections are truncated by east-dipping reflections in the Pan-African domain, which are correlated with cratonward directed fold-and-thrust structures formed during the c . 600 Ma Pan-African orogeny. These seismic observations represent, to date, the first deep seismic reflection images of the West African craton margin and the continental lithosphere in the region.