GEOPHYSICAL INVESTIGATIONS OF THE CENTRAL PART OF NIGER STATE, NIGERIA.

摘要

The Nupe basin and the adjacent basement complex have been covered by very little geophysical work in the past. The purpose of the present work is to study the structural and tectonic nature of that area; to that end, gravity and magnetic surveys have been made in the central part of Niger State, Nigeria, between latitudes 9(DEGREES)00' and 9(DEGREES)48'N and longitudes 6(DEGREES)00' and 6(DEGREES)36'E.;The gravity measurements suggest the existence of up-warping of the Moho in the Nupe basin. Assuming that the depth to Moho is reflected in the 3rd order polynomial fit to the observed gravity, and taking densities of 2.70 gm cm('-3) for the crust and 3.27 gm cm('-3) for the mantle, we calculate a mantle rise of the order of 5 km.;Models were fit to the residual Bouguer anomalies assuming densities of 2.83 gm cm('-3) for the lower crust, 2.70 gm cm('-3) for the upper crust, and 2.45 gm cm('-3) for the sedimentary layer. The greatest sandstone thickness obtained from the models is about 3.5 km. The modeled lower crust rises to the surface between Minna and Paiko, and also near Kataeregi. The structural trend of this rise coincides with the trend of the quartz-mica-schist belt in the northeastern section of the surveyed area. The models also indicate major faulting at the edge of the basin around Kataeregi.;Magnetic models lead to an estimated thickness of overburden (laterite, gravels, etc.) in the basement complex area of about 0.4 km. An aeromagnetic map of a portion of the region indicates that magnetic minerals of economic importance are unlikely to be presented in this area.;The interpretation given here is of a rift origin for the Nupe basin. The initiation of rifting started in the adjacent Benue trough in the early Cretaceous; this gave rise to other rifted troughs (Yola, Gongola, etc.) in the Cretaceous period.