CRUSTAL THINNING AND EXTENSION BENEATH THE BENUE TROUGH BASED ON GRAVITY STUDIES

Gravity profiles taken across the Benue Trough and over the flanking basement areas far removed from the sedimentary cover reveal that the Trough is associated with a broad residual positive Bouguer anomaly 90 mGal in amplitude and up to 400 km wide. Two dimensional model interpretation of the positive anomaly using constraints provided by seismological studies of the similarly evolved and contemporaneous Yola rift basin indicates that the crust beneath the Trough has thinned by about 14 km and is replaced by uplifted mantle over a lateral distance of approximately 300 km. The width of the thinned zoned and the known geology of the Trough are considered to reflect phases of extension and shear deformation that probably occurred during the initial stages of opening of the South Atlantic Ocean. Simple estimates of crustal movement give extension perpendicular to the Trough of 95 km and sinistral shear parallel to the Trough of 60 km.