Chapter 14 Review of the tectonic controls and sedimentary patterns in late neogene piggyback basins on the barbados ridge complex

Abstract A review of the tectonic control and sedimentary patterns of the late Neogene piggyback basins of the Barbados Ridge Complex is proposed, mainly based on the analysis of seismic reflection profiles and sidescan sonar images. The geometrical relationships between the thrust faults and the syntectonic deposits show that the frontal piggyback basins are controlled by active thrusting. Deformations, with distinct scales, control their evolution: (1) a rapid tilting, strictly localized at the back limb of anticlines, and attributed to migration of active axial surfaces during fault-bend fold propagation; (2) a complex activation of the major thrust system, at a scale of a few kilometers: blind thrusts corresponding to frontal propagation develop seaward whereas motion along thrusts occurs backward. At about 50 km back of the deformation front, abundant muddy material raises up through pre-existing faults and disturbs arcward piggyback basins. The superimposition of the diapiric structures upon deformations linked to tectonic accretion (development of backthrusts and reactivating of forward verging thrusts) and inherited oceanic basement ridges leads to the individualized development of sub-basins bounded by steep topographic features. Most of the sediments of the South Barbados piggyback basins originate from the South American continent and are massively transported to the abyssal plain through canyons. Their course is driven by the main regional structures and their morphology reflects the tectonic activity of the features where they run. Oceanic ridges, by damming and collecting turbidite material control the thickness of sediments added to the complex and then the depth of the decollement and size and filling of the piggyback basins. Tectonics, by generating routes along the faults and excess fluid pressures control the main location and importance of mud diapirs and authigenic deposits. These stiff carbonate crusts preferentially develop on diapiric domes or on the back limb of anticlines up to the edge of supra-prism basins. Clayey diapiric material may be found within the basins where they form important mud flows as well as sliding masses. Tectonic and diapiric structures control gravity reworking processes, whereas structural relieves locally disturb bottom currents and hence control some erosion processes. The development of the piggyback basins of the BRC is closely linked to the evolution of a thrust wedge. The formation of frontal basins is mainly controlled by a forward-verging thrust system that forms a brittle wedge, whereas the development of the arcward basins is mainly controlled by subcretion of deep muds that induces mud diapirism, ductile deformation in the lowermost part of the wedge, a regional gentle slope, and the occurrence of both backward and forward verging thrusts. The evolution of the piggyback basins of the BRC also reflects the north to south changes in width and thickness of the wedge which are mainly related to variations of the increase of the sediment supply. Piggyback basins then evolve from minor depressions filled with very poor sediments in the north to about 10 km long overfilled basins in the south of the complex where abundant sedimentation occurs.