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The Relationship Between Detachment Folding and Duplexing: Constraints From the Niger Delta


Detachment folds are common to many contractional belts, but poor seismic resolution into the core of these structures has hindered understanding of the mechanisms responsible for these fault-related folds. New long-offset, deep-tow acquisition, combined with depth-imaging methods now provide seismic criteria to facilitate interpretations of these structures. A variety of folding processes have been proposed to explain detachment folds. A certain class of models involve incompetent thickening of a weak core, with overlying strata deforming primarily by flexural slip. The internal nature and geometry of the weak core is rarely specified, but is proposed to be due to either chaotic thickening, imbrication of thrust sheets at a scale below the resolution of the data, or true flow, such as in case of salt-cored detachment folds, or unconsolidated mud. The fold is bound by a basal detachment and implicit is the idea that the incompetent core is disharmonically related to the overlying section, i. e. is also fault-bounded. Duplexes by definition have a floor thrust with an imbricated interior, and a folded section above a roof thrust. Thus the distinction between duplexes and some detachment folds is ultimately a question of scale and resolution. Alternatively, some detachment folds have been proposed to be due to a form of pure-shear thickening across many stratigraphic horizons, bound by a basal detachment. A mix model is one with pure-shear thickening in the core that also has a bounding roof thrust with flexural slip dominating above. Recently acquired long-offset, deep-tow, prestack depth-migrated seismic from across the offshore Niger Delta basin shows all of these phenomena are present. These high-resolution depth images reveal internal duplexing in many of the detachment folds of the Outer Fold Belt, whereas other detachment folds show pure-shear thickening. The distinctive mobile-shale structures of the Inner Fold Belt in these images appear to also be large detachment folds, sometimes with clear thrust imbrication at depth, sometimes with chaotically thickened cores and associated fluid-expulsion features. The upper and basal detachments of the detachment folds are regionally present throughout much of the offshore delta, residing within the upper part of Eocene Akata Fm. a weak, commonly overpressured, prodelta section. In places, the upper detachment also acts as an additional basal detachment for thrusting above these detachment folds.