Seismic Stratigraphy of the Cape Verde Flexural Moat: Implications for Surface and Sub-surface Loading on Mid-plate Swells

Multichannel seismic reflection profiles acquired during RRS Charles Darwin cruise 8/85 and Shacklelton cruise 83/1 have been used to determine the stratigraphy of the Cape Verde flexural moat. The reflection data have been processed through to a final time migrated stack. The two way travel time to prominent reflectors has been determined and converted to depth using interval velocities from semblance and all available sonobouy data. The data show that the Cape Verde moat is underlain by a well stratified sedimentary sequence which overlies a rough and irregular oceanic basement. A total of 4 stratigraphic units have been recognised, on the basis of their reflective character and the nature of their bounding surfaces. The two lowermost units thicken towards east and are interpreted as sediments associated with the distal part of the West Africa continental margin. The two uppermost units thicken concentrically around the Cape Verde islands and are interpreted as volcanogenic sediments which have been derived from the islands and have partly infilled the flexural moat. The upper and lower units are separated by an angular unconformity which is interpreted as the result of flexural loading of the oceanic basement and the overlying Mesozoic margin related sediments by the Cape Verde Islands. The seismic data show that unlike the Canary Islands moat, the Cape Verde flexural moat is uparched such that the individual reflectors are tilted away from the islands. Stratigraphic data, together with 3-D elastic plate modeling, have been used to investigate the origin of the loading and the tilting of the moat sediments. Our studies show that the amplitude and wavelength of the moat infill cannot be explained by models that only consider surface (i.e. topographic) volcanic loading. Calculations based on an elastic thickness of 37 km, which is similar to what would be expected for the thermal age of the underlying lithosphere, predicts, for example, a flexural depression which is about 500 m shallower than observed. This suggests that other, sub-surface, loads may be present in the Cape Verde Islands. We have used models that combine surface and sub-surface loading to determine the elastic thickness, \ensuremath{T_e}, and ratio of surface and sub-surface loading that best fits the stratigraphic data. The best fit model is one in which \ensuremath{T_e} = 25 km and f = 0.3. These results, which are consistent with gravity modeling using the admittance and coherence technique, suggest that sub-surface loading is required in order to explain the thickness of sediments that is observed in the moat. Sub-surface loading of a relatively weak lithosphere may also explain the local uparching of the moat sediments and, possibly, at least part of the relief of the Cape Verde mid-plate swell.