Crustal structure of the Mamfe basin, West Africa, based on gravity data

Abstract

The Mamfe basin is the smallest and most southerly of three side rifts associated with the Benue Trough. It extends 130 km east from the Benue Trough into Cameroon as far as the Cameroon Volcanic Line and until recently it was the only rift devoid of geophysical studies to indicate its crustal structure and mode of formation. Between 1984 and 1986 three gravity surveys were made over the basin and adjacent basement areas of Nigeria and Cameroon to reveal that the western half of the basin is associated with a broad positive Bouguer anomaly at least 100 km wide and amplitude in excess of 20 mGal (200 gu). Superimposed on this gravity “high” is a negative Bouguer anomaly which closely correlates with the area of sedimentary cover. Using the crustal thickness and density contrasts determined from seismological and gravity studies over the Yola side rift, located farther north, two-dimensional modelling of the regional and residual gravity fields have been attempted. The regional “high” has been modelled as a reduction in crustal thickness of about 8 km beneath the basin and the residual “low” as a sedimentary basin reaching at least 3 km in depth at the Nigeria/Cameroon border. The eastern half of the Mamfe basin has a gravity expression to indicate no large sedimentary thickness. Thus the gravity expression of the western end of Mamfe basin is similar in character to that found elsewhere within the West African rift system. Crustal models indicate that extension perpendicular to the Mamfe basin could be at most 19 km with geological data indicating only small amounts of compression/shear deformation within the basin during the late Cretaceous times. This lack of deformation probably results from the Mamfe basin orientation being more or less perpendicular to the Benue Trough and Foumban (or Central African) shear zone along which most of the shear deformation has occurred during and since Cretaceous times.