A seismic reflection profile study of lithospheric flexure in the vicinity of the Cape Verde Islands

Seismic reflection profile data are used to determine the stratigraphic "architecture" of the flexural moat that flanks the Cape Verde Islands. The moat region is characterized by upward of 1-2 km of poorly to well-stratified material. The two lowermost units thicken from west to east and are attributed to sediment loading and flexure at the nearby West Africa continental margin during Early Cretaceous to early Miocene. The two uppermost units, in contrast, thicken concentrically around the islands and are attributed to the infilling of a flexural moat that formed by volcanic loading since the early Miocene. Flexure modeling shows that the thickness of the moat infill cannot be explained only by surface loading and requires that the downward flexure due to surface loads is opposed by an upward acting subsurface load. The best fit combined surface and buried loading model is for an elastic thickness, Te, of 29 km, a load and infill density of 2700 kg m-3, and a ratio of surface to subsurface loads, f, of 0.2. These results are in accord with spectral studies of free-air gravity anomaly and topography data based on the admittance technique. The subsurface loads are spatially limited to the islands and their submarine flanks. Nevertheless, they are associated with a broad regional uplift of up to ∼400 m. The uplift is large enough to explain why the moat infill is generally tilted away from, rather than toward, the islands. The uplift is too small, however, to account for the height of the swell, upon which the Cape Verde Islands are superimposed. The origin of the Cape Verde swell is not known. However, a normal Te and a modest heat flow anomaly suggest that the swell cannot be fully explained by uplift due to thermal reheating of the lithosphere by an underlying "hot spot" and that other, deep-seated, mantle processes must be involved