DEEP STRUCTURE OF THE ANGOLA MARGIN FROM WIDE-ANGLE REFLECTION / REFRACTION OBS

Continental lithospheric thinning and break-up are major plate tectonics processes, which are still poorly understood. Tectonic and magmatic processes that occur during continental rifting can be better analyse if quantitative images of the deep crustal structure are provided. During the ZaïAngo deep seismic cruise, a collaborative programme between Ifremer and Total-Fina-Elf, the South Atlantic margin of Angola was surveyed. A 4.5-km long streamer was used with low frequency air gun array and 68 OBS were deployed. The OBS wide-angle reflection / refraction data were interpreted by using a ray tracing program. The shallow part of the model was built using the geometric constraints of MCS lines. The deep part of the model was constrained by OBS data and gravity modelling. On the eastern part of the margin, a deep subsalt basin (about 12 km depth) is filled by sediments with velocities ranging from 4.7 to 5.6 km/s. This basin is underlain by a crust with 6 to 6.8 km/s velocities. Over a distance of about 30 km from the shore, the continental crust thins from 30 km to 4 km beneath the basin. At the base of the crust, where the crust is highly thinned, a high velocity (7.2 to 7.4 km/s) body was evidenced. In the oceanic domain, the sediment velocity varies from 3.3 to 3.4 km/s. The crust is 5 to 6 km thick with velocities varying from 6 to 6.8 km/s. Despite the disturbance and complexity brought by salt tectonics, the combined use of MCS profiles, OBS and gravity data helps to constrain the crustal velocity model of the Angola margin. This velocity model is used to perform a pre-stack depth migration of the MCS line. These new data allows us to discuss some key points about the deep structure of the margin: the geometry and the nature of the thinned continental crust, the geometry of the sub-salt basin, the geometry and the nature of ridge at the oceanic limit, the nature of the transitional domain and to bring some constraints on the continental lithospheric thinning.