Gravity anomalies and segmentation of the continental margin offshore West Africa

The free-air gravity 'edge effect' anomaly at passive continental margins provides constraints on the thermal and mechanical properties of the lithosphere in extensional regions. Like many margins, the edge effect 'high' offshore Gabon, West Africa is located at the shelf break in the region of maximum sediment thickness. Gabon differs, however, from other margins in that the edge effect 'low' is displaced from over the continental rise to the slope. The origin of the low has been investigated by backstripping the sediments at the margin. We have used the backstrip to calculate the amount of crustal thinning, assuming Airy isostasy. Such a model implies zero strength during rifting. Alternatively, this amount of crustal thinning can be explained by a strong margin and a depth of necking that exactly balances the upward and downward forces that act on the crust during rifting. We have developed a method, which takes into account variations in the strength during rifting and the depth of necking, to compute the thinning directly from the sediment backstrip. The backstrip and the crustal thinning have been used to calculate the rifting gravity anomaly. Differences between the backstrip and the present-day water depth give the geometry of the sediment load and hence, the contribution to the gravity anomaly of sedimentation. By comparing the combined rifting and sedimentation anomaly to the observed free-air gravity anomaly, we have been able to constrain the flexural rigidity land equivalent elastic thickness, T-e) of the lithosphere during and after rifting. The best fitting model is one in which the 'low' is caused by a 125 km wide zone of relatively thin (i.e. similar to 15 km) continental crust which was weak (i.e. 0 < T-e < 10 km) during rifting and has remained so since then. We speculate that the weak zone extends for 350-400 km along-strike of the Gabon margin and that passive margins may be highly segmented as regards their long-term strength. (C) 1998 Elsevier Science B.V. All rights reserved.