Gravity Study over Jos – Bukuru Younger Granite Complex, North Central Nigeria
By S. C. Alkali and S. N. Yusuf
Horizontal gradient filtering on the Bouguer anomaly data indicates contour values between -1.5 and 1.3. Zero contour zones are interpreted as faults and other abrupt geologic edges. Bouguer anomaly field is characterized by negative values between -90 and -10 mGal. Values of – 90 mGals are centred on the Jos - Bukuru Younger Granite complex. Lower values at the eastern side are separated from higher ones at the western side by a NW – SE diagonal line. The anomaly contour closures trend in the E – W, NE – SW, N – S and NNW – SSE directions. Third order polynomial trend surface fitting shows the contour levels of the regional anomaly field in a NNE – SSW direction, consisting of increasing N – S values. Map of the residual anomaly is characterized by both negative and positive values between -25 to +20 mGals. The negative values are probably located on the intrusive younger granite rocks while the positive values may explain areas underlain by volcanic rocks. The contour closures of the residual anomaly map are oriented along the same directions as the Bouguer anomaly map but without any line separating higher values from lower ones. Two and one half models of the residual anomaly along profiles indicate depths up to 18.75 km for the plutonic rocks and 13.96 km for the volcanic rocks. These large depths are attributed to the presence of a large-scale fault associated with the Romanche fracture zone in the basement around the Jos - Bukuru Younger Granite complex.
Key words: Positive, Negative, Anomaly, Residual and Gravity modeling.
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