The deepwater Niger Delta is an ideal natural laboratory for studying the evolution of fold and thrust belts. There is an extensive grid of 2D seismic data, increasingly available 3D data, and recent deepwater well penetrations. This paper details some recent analysis of the structural styles and evolutionary history of deepwater folds and faults in this area of active exploration.

The Niger Delta is a large Tertiary delta that has prograded onto the oceanic crust of the Gulf of Guinea. The internal tectonics of the delta have partitioned into updip extension near the delta front, balanced by downdip contraction near the base of slope. These systems are linked by a basal detachment in ductile overpressured shales of the older and deeper section.

The contractional portion of the delta can be divided into an Inner Thrust Belt and an Outer Toe Thrust Belt, which are commonly separated by large Translational Basins. The deformation front was confined to the Inner Thrust Belt from Oligocene to Early Miocene time. The Outer Toe Thrust Belt began forming when the deformation front jumped outboard in Middle Miocene time, and it has progressively advanced to its current location.

The basal detachment of the Outer Toe Thrust Belt differs dramatically from that farther updip. The basal shear inboard of the Inner Thrust Belt is characterized by diffuse slip across a thick Cretaceous mobile shale sequence. The advance of the deformation front was initiated when some of the distributed shear within the Cretaceous mobile shale sequence began to transfer into more discrete detachments in Upper Cretaceous and Paleogene shales beneath the abyssal fan, rather than simply breaching to the surface. Broad regional anticlines developed where this slip transfer involved a climb in stratigraphic level, resulting in complex fault-bend folds above the ramp. The downslope transition from thick mobile shale to more discrete detachments has led to complicated imbrication of the deep section even where no obvious ramp is observed.

Within the Translational Basins the deformation is dominated by localized detachment folds and/or buckles above duplexes within the thin detachment layers. The boundary between the Translational Basins and the Outer Toe Thrust Belt is loosely defined by where the detached deformation breaks to the surface. The Outer Toe Thrust Belt typically consists of an in-sequence set of 10 to 20 fault-fold structures. While the majority of the structures are forward verging, there are local domains dominated by backthrusting and a frontal wedge. The structures are predominantly fault-propagation folds, often initiated by thrusting out of early low-relief buckles. Detailed reconstructions of the western belt suggest that the timing of progressive initiation of motion on the basal detachment is strongly linked to the onset of elevated fluid pressures within the detachment shales due to disequilibrium undercompaction in response to rapid burial by the advancing delta.

The sediments involved in the deformation of the Outer Toe Thrust Belt are unconsolidated to weakly consolidated muds, silts and sands that behave in a very ductile manner. Folds tend to develop very rounded forms, and the faults show only minor bending after ramping up from the detachment. Only minimal structural topography can develop because the surface sediment is unconsolidated, and downslope failure quickly removes the crest of the growing structures. As a result, breaching thrusts are quickly truncated and rarely roll over onto the seafloor.

Detailed analysis of the growth history of several fault-propagation folds suggests that once initiated they propagate rapidly to near their ultimate length. Subsequent deformation increases the fault slip and structural relief without significant increase in length. Later deformation along an individual structure frequently retreats to localized deformation near the structural crest, while the extremities are successively abandoned. It is proposed that this localization of deformation is being driven by the effective weakening of the shales at the crest of the fold due to elevated fluid pressures being transported from the back syncline along interbedded sands.