New insights into deformation mechanisms in the gravitationally driven Niger Delta deep-water fold and thrust belt

We use two- and three-dimensional seismic reflection data from the deep-water Niger Delta fold and thrust belt to document evidence for two, discrete, postfaulting deformation mechanisms. An early phase of thrust-propagation folding is followed by folding caused by thickness changes within the basal shale detachment unit. The later phase of folding is caused by a lateral redistribution of the strata within the basal detachment unit. This example of late deformation occurred over a 4-5-m.y. period as a result of the displacement of approximately 590 km(3) (similar to 370 mi(3)) of the underlying strata within the detachment unit. In another deformation event in the basal detachment unit, about 160 km(3) (similar to 100 mi(3)) moved, laterally creating a synform in the overburden and parallel on-lap fill, indicative of the relatively rapid creation of accommodation space. On the basis of seismic reflection data from the delta and a consideration of the volumes and rates of movement of sedimentary rock, we conclude that the poorly imaged succession commonly referred to as mobile shale cannot deform solely by ductile mechanisms as interpretations of shale tectonic provinces have commonly suggested but instead by brittle processes that involve thickening by thrust faulting and subseismic accommodation structures. Processes such as liquefaction, where a complete loss of shear strength is observed, had a minimal function. Therefore, the term mobile shale in this setting is widely exaggerated. An awareness of postfaulting deformation mechanisms will be important for the successful exploration of gravitationally driven fold and thrust belts.