A DETAILED GRAVITY INVESTIGATION OF THE KWELLO ANOMALY IN THE NORTH CENTRAL AREA OF NIGERIA

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Date
1997-05
Authors
AKAOLTSA, Casmir Zanders
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Abstract
A detailed gravity survey of the Kwello area of the Basement Complex of Northwestern Nigeria has been carried out to determine the structure and probable mode of emplacement of the causative body of the Kwello Bouguer gravity anomaly. Two hundred and sixty seven gravity stations were occupied at 2 km intervals using a LaCoste and Romberg Model G gravimeter for relative gravity measurements and two Wallace and Tiernan aneroid altimeters for the elevation determination. Density measurements of two hundred and twenty nine rock samples were carried out. Results of gravity measurements in the Kwello area show that the area is characterized by negative Bouguer anomaly values ranging in amplitudes from -30 mGal to -58 mGal. Free-air anomalies distributed about a mean of +28 mGal seem to suggest that the area is isostatically compensated. Interpretation of the Bouguer anomaly field shows that the area is characterized by prominent residual anomalies of -8 mGal to +12 mGal amplitude. The residual anomalies were enhanced by the second vertical derivatives and the lithologies with contrasting densities in the area were delineated by the zero contours of the second derivative map. A residual gravity high of +12 mGal amplitude correlate well with mapped schist body in the area, while residual gravity lows are correlated to the granite bodies in the area. A N - Strending regional anomaly with an E - W gradient of the order of 0.18 mGal/km is obtained from a first order polynomial fitting to an upward continued Bouguer anomaly data. A 21/2 - Dimensional modelling of the residual anomaly reveals that the thickness of the schist vary between 1 to 5 km with inward dipping walls of dips 60° and the granite bodies have a depth extent of about 6 km with an outward dipping wall of dip 48°. The sharp contacts of the bodies indicate that the contacts are possibly faulted. The general configuration of the modelled bodies and the pattern of the gravity anomalies suggest that the Older Granites make sharp contact with the country rocks and were magmatically emplaced into the Basement Complex. The Bouguer gravity high over the schists in the Kwello area have a maximum amplitude of only -30 mGal. These values are thought to be too low to indicate the existence of more mafic rocks at depth. The schists in the Kwello area are therefore believed to have evolved in an ensialic environment.
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A THESIS SUBMITTED TO THE POSTGRADUATE SCHOOL, AHMADU BELLO UNIVERSITY, ZARIA IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE AWARD OF THE DEGREE OF MASTER OF SCIENCE, APPLIED GEOPHYSICS. DEPARTMENT OF PHYSICS FACULTY OF SCIENCE AHMADU BELLO UNIVERSITY, ZARIA NIGERIA.
Keywords
DETAILED,, GRAVITY,, INVESTIGATION,, KWELLO,, ANOMALY,, NORTH,, CENTRAL,, NIGERIA
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