Thrusting in oceanic crust during continental drift offshore Niger Delta, equatorial Africa

Abstract

Two‐ and three‐dimensional (3‐D) seismic reflection data acquired over oceanic crust in the deepwater west Niger Delta reveal convincing evidence for compressional tectonics during oceanic crustal spreading. Using the 3‐D seismic data set we describe numerous inclined seismic reflections that dissect the entire oceanic crust from the top of the crust to the level of the Moho that are interpreted as thrusts. Thrust propagation results in the development of associated hanging‐wall anticlines and footwall synclines. These structures are orthogonal to and clearly postdate normal faults that formed during the accretion of oceanic crust during continental drift and strike at right angles to them. The Charcot Ridge is located 140 km south of these thrusts and is a significantly larger structure. It is a triangular‐shaped uplifted region of oceanic crust measuring 80 by 150 km and is located along the NE–SW‐oriented Charcot Fracture Zone. Two interpretations are possible for the role of the fracture zone in the development of the Charcot Ridge: (1) A thin‐skinned model whereby the oceanic crust west of the fracture zone has been thrust southeastward, with detachment occurring close to the level of the Moho. The ridge forms as a result of translation and folding above a crustal‐scale ramp‐flat thrust geometry. (2) A thick‐skinned model where there is no detachment close to the Moho, with the thrust fault being much steeper, penetrating the crust and probably the mantle lithosphere. In this interpretation the structure formed owing to the compressional reactivation of the fracture zone. Approximate dating of onlapping reflections on either side of the ridge constrains the timing of its formation as between 25 and 120 Ma ago. The Charcot Ridge represents one of the largest thrust structures to be identified in a passive margin setting. Many other compressional folds with the same orientation formed to the northeast in the Benue Trough, probably during the Santonian, as a result of a change in the spreading direction during South Atlantic rifting. We speculate that the same causal mechanism applies for the formation of the Charcot Ridge.