IMBRICATE SHEAR FAULT-BEND FOLDING IN THE DEEP-WATER NIGER DELTA

Non-conventional imbricate thrust structures are common in deep-water environments of passive margins worldwide, including the deep-water Niger Delta. These imbricate systems are the product of contraction due to gravity-driven extension on the shelf. The Niger Delta offers a unique opportunity to study this style of imbricate thrust systems, as these structures are extremely well imaged at deep levels in seismic reflection profiles and because they preserve growth strata that record fold kinematics. Using fold shape, fault plane reflections, and patterns of growth sedimentation, we model the geometry and kinematics of these imbricate systems using shear fault-bend folding theory. These imbricate systems are formed as a weak décollement layer of finite thickness at the base of fault ramps undergoes pure shear or externally imposed bedding-parallel simple shear. Individual shear fault-related folds within the imbricate system are characterized by long planar backlimbs with increasingly shallower dips to growth strata, reflecting a component of progressive limb rotation. We present forward models of both break-forward and break-backward imbricate shear fault-bend fold systems with the correspondent patterns of growth sedimentation. Unlike conventional fault-bend fold systems where imbrication causes a steeping of overlying thrust sheets, imbrication by shear fault-bend folding may locally increase and decrease the ramp and bed dip angles in overlying thrust sheets. The geometry of shear-imbricated thrust sheets is a function of both underlying fault geometry and shear magnitude, and can be distinctive of other folding mechanism. For example, a local decrease in ramp and dip angles by imbrication effectively produce an increase in accommodation space for growth sedimentation, resulting in structural down lap and other distinctive stratigraphic patterns in growth. We use these models and the resulting patterns of growth sedimentation to describe patterns of shear imbrication in the deep-water Niger Delta.