GRAVITY AND AEROMAGNETIC INVESTIGATION OF THE GONGOLA ARM OF THE UPPER BENUE TROUGH, NIGERIA

No Thumbnail Available
Date
1997-05
Authors
ELISHA, MUTUM SHEMANG
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
A regional gravity survey of the Gongola Arm of the Benue Trough was carried out with the aim of determining structures of local and regional interest. A total number of 1005 detailed gravity stations and 95 base stations were occupied during the survey which took place in June/July 1992 and May/June 1994. Aeromagnetic data of the area was also digitised and the interpretation carried out alongside the gravity interpretation. The results of the study indicate that the area is characterised by negative Bouguer anomalies that basically trend in the NE-SW direction and range in values between -75 to -10 mGal, with an average value of-42 mGal. The Free-Air anomalies in the area range between -10 to 45 mGal and generally suggest that the area is isostatically compensated. A first order polynomial was used to approximate the regional anomalies in the area. The results showed that the regional in the area is a plane dipping NNE-SSW at a rate of 0.04 mGal/ km, and range in value from -46 mGal in the eastern part of the area to -35 mGal in the western part. The residual gravity map of the Gongola arm of the upper Benue trough showed anomalies that range in value from -25 mGal to 20 mGal. A detailed interpretation of these gravity anomalies of the area is hereby presented along with the magnetic anomalies with the sole aim of determining the source, type and nature of the anomalies. The total magnetic intensity composite map showed intensity values ranging from 7400 nT to 8100 nT, with a dynamic range of 700 nT. A major prominent lineament that trends NE-SW passing through the axis of the trough in the form of a continuous narrow belt that has an average width of about 25 km, and stretches for a distance of over 250 km was observed in the area. This lineament is believed to be associated with the history of the trough and most probably a geosuture zone. Two and Three-Dimensional modelling of the residual gravity and magnetic anomalies suggest that the structure of the area is that of a horst and graben, with the grabens being as large as 80 km in width and are filled with sediments that are up to 4.5 km thick. The interpretation of the residual gravity anomalies also showed that most of the positive anomalies are caused by basement uplift (horst): a reflection of the structure of the trough. Basic to ultrabasic rocks are also considered possible sources of the prominent positive residual anomalies. Models of the magnetic anomalies also suggested that these anomalies are caused by basic to ultrabasic bodies at depth in the area. Some of these bodies were seen to occur as deep as 8 km, while others occur at shallow depth at instances 1 km. The magnetic modelling results also suggest the existence of a geosuture zone in the area, which passes through the axis of the trough forming a continuous narrow belt and trending NE-SW direction. This geosuture zone stretches for a distance of about 250 km from around Wikki to Bajoga and beyond and has an average width of 25 km. The crustal structure of the area of study suggests that the area has undergone crustal thinning (Moho uplift) of about 7 km, with the smallest crustal thickness not less than 27 km. The general indications from the results of these interpretations is that the area is characterised by horst and graben structures, with associated magmatic intrusion at depth, thus suggesting that the area is a rifted structure
Description
A DISSERTATION SUBMITTED TO THE POSTGRADUATE SCHOOL OF AHMADU BELLO UNIVERSITY, ZARIA NIGERIA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF DOCTOR OF PHILOSOPHY (Ph.D.) DEGREE IN APPLIED GEOPHYSICS. May, 1997
Keywords
GRAVITY,, AEROMAGNETIC,, GONGOLA ARM,
Citation
Collections