Shale 3D Flow and Interaction With Basement Faults in the Niger Delta Deep-Water Fold and Thrust Belt

Based on a large 3D seismic data set in the deep-water domain of the Niger Delta, this study challenges previous interpretations involving the occurrence of multiple detachments and extensive thrust flats, illustrating timing and mode of shales flow at the toe of the gravity system. Five units of syn-kinematic sediments, reaching a maximum thickness of similar to 800 m, accumulated in the tectonically subsiding synclines during fold amplification between similar to 9.5 and similar to 1.4 Ma. The volumes of syn-kinematic units roughly balance those of the shales accumulated in the thickened cores of WNW trending anticlines. This feature is consistent with folding resulting from buckling controlled by the competence contrast between isopach Cenozoic units and underlying overpressured shales of the Akata Formation. A dense network of NE-SW striking oblique extensional faults offsets a prominent anticline characterized by a NE-SW trend (which is almost perpendicular to the regional fold trend). These faults form a narrow, continuous deformation zone extending for tens of kilometers along and beyond the length of the anticline. The faults, rooting within the shales of the Akata Formation, formed since similar to 5 Ma and deform the seabed. Displacement distribution suggests mechanical interaction between isolated fault segments within the deformation zone. The latter is interpreted as the shallow expression of a deep-seated fault zone inherited from the segmented passive margin and marked by gravity and magnetic data. Our results, providing a comprehensive picture of active deformation features and their relationships with deep-seated faults, shed new light into the modes of interaction between gravity systems and underlying basement structures.The occurrence of thick successions of weak shales underlying sandy deltaic deposits control gravity-driven deformation in deep-water basins worldwide. In these conditions, large sedimentary bodies such as the Niger Delta of this study are characterized by (a) a spreading inner shallow-water sector, which undergoes collapse due to the fast accumulation of deltaic sediments, and (b) a deep-water domain characterized by shortening. Volume balancing in the latter domain points out that a 3D flow pattern must accommodate thickening of mobile shales into anticlinal cores as a result of flow of material from adjacent synclines. Folding occurred by buckling, a process characterized by mechanically active layering. This process was controlled by the strong viscosity contrast between the thick shales and the overlying sandy units, which tend to maintain a constant thickness in seismic lines. Fold nucleation was locally controlled by the occurrence of basement structures, which is revealed by magnetic and gravimetric data. These folds show "anomalous" trends that do not conform to the regional fold orientation. The interaction between the delta deposits and the underlying basement structures also led to the development of faults that display a clear morphology at the sea bottom and are therefore considered active.Volume balancing indicates that mobile shales accumulated into anticlinal cores as a result of 3D flow from surrounding synclinesThe incompetent Akata shales thickened in the core of the anticlines during buckle folding accompanied by late-stage thrustingThe detachment at the base of the shales interacted with basement structures, localizing deformation in overlying delta sediments