HOW DATA CONCERNING THE DEEP STRUCTURE OF SELECTED NORTH ATLANTIC CONJUGATE MARGINS CAN HELP IN THE DATA INTERPRETATION OF THE ANGOLA MARGIN

The Zaïango deep seismic data have been acquired during spring 2000 on the Angola margin within the framework of an Ifremer/TotalFinaElf collaborative program. A 4.5-km long streamer was used and 68 OBSs were deployed. With lessons gained from previous experiments and due to the presence of a deep and thick sub-salt basin, we operated a source array with a maximum of energy available in the low frequency domain by using a 4800 ci air gun array shot in single bubble mode. This array was able two provide 3 times more energy in the 10-25 Hz bandwidth than the most powerful array (in the same bandwidth) used by service companies. The preliminary MCS and refraction results are interpreted by comparison with data acquired on the starved continental margins of the North Atlantic. There, two conjugate transects have been constructed, across the Galicia Bank-SE Flemish Cap and Iberia Abyssal Plain-Newfoundland margins. These two sets of conjugate profiles present different features. The northern one is asymmetric, concerning the crustal structure and the shape of the underlying serpentinized peridotite body located at the base of the thinned continental and transitional crusts. The southern one is symmetric with the presence of a 300 km-wide transitional domain, which raises questions about its origin. Deep tow magnetics and drilling data, as well as the geometry and velocity structure of these two conjugate margins, bring fresh insight to the origin of such a transitional domain in the ongoing exhumation versus very slow spreading debate concerning the origin of oceanic crust. Results of these experiments in the North Atlantic combined with results obtained in the close environment of the Zaïango project (Southeast Atlantic margins) will be used to discuss problems related to the Angola margin: i) the geometry and nature of the ridge located at the foot of the thinned continental crust and its relationship with the transitional crust of the Angola basin and ii) thinning mechanisms in liaison with the emplacement of early transitional and oceanic crusts.