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We have obtained an accurate bathymetric map of the lower slope and deformation front in the area of Leg 78A sites by means of a multibeam survey. We interpret N-S trends revealed by the survey to be asymmetric structural features linked to the westward subduction. The influence of the Tiburon Rise is not obvious in the drilling area, but clearly induces an E-W trend of structures immediately to the south of the rise. In the southern part of the Barbados Ridge the asymmetric folds are well expressed in the bathymetry as N-S elongate anticlines. INTRODUCTION The deformation front of the Barbados Ridge complex runs from south to north for several hundred kilometers (Fig. 1). As pointed out by Westbrook et al. (this volume), conventional geophysical techniques are not always adequate to get a good idea of the internal deformation in the accretionary prism. Moreover, the grid of seismic profiling generally does not allow a three-dimensional picture of the structure to be obtained in any case. A very detailed geomorphological and structural study of the Barbados accretionary prism has therefore been conducted by the Comité d'Etudes Pétrolières Marines (CEPM) using multibeam and high-resolution seismic tools (CARVEN—Caraibe-Venezuela—project). Recently Biju-Duval et al. (1982) have presented some preliminary results from the southern part of the Barbados Ridge complex, and have shown how the multinarrowbeam sonar system (Sea Beam) has provided a third dimension to the cross-sectional structure visible on the seismic reflection profiles. As will be summarized here, the very detailed bathymetric survey has allowed us to obtain a good correlation between the present seafloor topography and the geological structures at the frontal part of the accretionary prism. In the Leg 78A drilling Biju-Duval, B., Moore, J. C , et al., Init. Repts. DSDP, Vol. 78A: Washington (U.S. Govt. Printing Office). 2 Addresses: (Fontas) Institut de Géologie du Bassin D'Aquitaine, Laboratoire de Géologie et Oceanographic, Avenue des Facultés, 33405 Talence Cedex, France; (Valéry) Société Nationale Elf Aquitaine (P), Tour Generate, La Defense 9, 5 place de la Pyramide, 92800 Puteaux, France; (Le Quellec) Compagnie Française de Pétroles, Tour Mirabeau, 39-43 Quai Andre Citroen, 75739 Paris, Cedex 15, France; (Mascle) Institut Français du Pétrole, B.P. 311, 92506 Rueil Malmaison, Cedex, France; (Renard) Centre Océanologique de Bretagne, 29272 Brest, Cedex, France; (Tardy) Université de Savoie, B.P. 1104, 73011 Chambery, France; (Biju-Duval, present address) Centre National pour PExploitation des Oceans, 66 Avenue d'Iéna, 75116 Paris, France. area, where seismic reflection profiling does not provide precise imagery of the internal deformation of the accretionary wedge (see Ngokwey et al., this volume), could this geomorphological approach give information about the tectonic style? That was the question when, before Leg 78A, the Centre National pour 1'Exploitation des Oceans (CNEXO) conducted a restricted bathymetric survey of the site area using the multibeam tool of the N/O Jean Charcot. The resultant limited coverage available at the time of the drilling has since been enhanced by a new survey made in July 1981 by CEPM-CNEXO. This chapter will present briefly the new data obtained around the drilling area, our interpretation of these data, and comparisons with conditions at the southern part of the ridge. Multibeam Tool The Sea Beam is a multibeam echo-sounder which represents a considerable technological advance in marine cartography (Renard and Allenou, 1979). The Sea Beam produces 16 narrow beams (each 2A° wide) which, after roll stabilization, are processed in real time by a minicomputer. Data are acquired from a swath of the seafloor over which the ship is sailing (Fig. 2); coverage is roughly two thirds of the water depth. Contour charts are traced in real time on a digital plotter (Fig. 3). Sound velocity used is 1500 m/s. The scale is chosen by the operator, and the along-track distance is obtained by integration of the ship's speed. All data from the 16 beams are stored on a magnetic tape for later processing. Playback of bathymetric data and navigation data allows production of a bathymetric chart in geographic coordinates. The preliminary survey was carried out with a Transit satellite navigation system. During the CARVEN project, the new survey used a radio positioning system (ARGO), but, owing to the distance of the Leg 78A drilling area from the land stations, the final map of the site was drawn in geographic coordinates using satellite navigation and, because of the reproducibility of the acquired data, by track-to-track correlation of the bathymetric contours.

Multibeam Bathymetric Survey of the Leg 78A Drilling Area and Comparison with the Southern Part of the Barbados Ridge Deformation Front