GRAVITY AND AEROMAGNETIC INVESTIGATION OF THE GONGOLA ARM OF THE UPPER BENUE TROUGH, NIGERIA
GRAVITY AND AEROMAGNETIC INVESTIGATION OF THE GONGOLA ARM OF THE UPPER BENUE TROUGH, NIGERIA
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Date
1997-05
Authors
ELISHA, MUTUM SHEMANG
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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,