Abstract

Results from magnetic field modelling of the Lower Benue Trough, West Africa revealed thick sedimentation with maximum values in the neigbourhood of 7,000m-10,000m. This is in contrast to the average value of 5000m suggested by earlier studies. The thick sedimentation represent thermal sag which is a regional post-rift subsidence associated with West and Central Africa rift systems. N-S, NNW-SSE and E-W structural fabrics interpreted from the transformed magnetic data bound the trough. The subsurface magnetic models also revealed extended graben structures that form the major depoceters that are segmented by intrarift horsts and average crustal thickness of 22km. The qualitative interpretation comprising, analytic signal, directional derivatives and wavelength filtering in frequency domain and subsurface magnetic modelling show that the rift architecture/geometry is controlled by high angle faults and some sinistral transtensional movements that are predominant in the Benue rift system. The low mean anisotropic susceptibility (kb = -6.7x10-4 SI) correlating with the directional horizontal derivative (dy) of the magnetic field conform with the sinistral movement in the trough indicating that magnetic anomalies align themselves along fractures/faults/shear zones owing to their variations in physical properties. The characteristics/patterns of the magnetic anomaly wavelength and inferred results from earlier studies on geophysical potential field methods, geological investigation and physical parameters (susceptibility and remanence) obtained from the subsurface magnetic modelling are pointers to non-magmatic origin of the Lower Benue Trough. The basement structuring, basin framework and predominance of anisotropic susceptibilities (ka, kb and kc) in three orthogonal directions and remanence suggest tectonic setting in the trough due to Early Cretaceous opening of the South Atlantic Ocean and interplate movement in Africa. The grabens, half grabens, faults and deep sedimentation (depocenters) interpreted from the magnetic data are hydrocarbon related structural features.

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