a) Shaded high-resolution bathymetric data focused on the southern hydrothermal area. Black circles and squares are the locations of fluid-emission echoes; red triangles indicate the locations of oil discharges (see text for explanations). b) High resolution backscatter data on the same area. High backscatter is shown as black areas and low backscatter as grey areas. Red dotted lines represent the extent of hydrothermal vents, blue dotted lines are the limits of basin structures, green dotted lines surround areas of strong backscatter out of hydrothermal vents. Black and green arrows emphasize some structures at the edges of basins (see text for explanations).