STRUCTURAL INTERPRETATION OF GRAVITY AND AEROMAGNETIC ANOMALIES OVER SOKOTO BASIN, NORTHWESTERN NIGERIA.
STRUCTURAL INTERPRETATION OF GRAVITY AND AEROMAGNETIC ANOMALIES OVER SOKOTO BASIN, NORTHWESTERN NIGERIA.
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Date
1990-03
Authors
MARIUS, NNANNA UMEGO
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Abstract
The first composite magnetic anomaly map of the
Sokoto Basin has been compiled at a scale of
1:3,000,000 from a total of thirty Geological Survey of
Nigeria aeromagnetlc maps. The composite map revealed
major structures in the Precambrian basement beneath
the sediments, and delineated areas for more detailed
investigations. A system of large scale NE-SW and NW-SE
trending magnetic lineations were also revealed, and
these have been interpreted to be part of the
continent-wide conjugate strike-slip fault system of
probable tectonic origin. The magnetic lineations and
intrusive activities are concentrated along three
prominent linear belts which probably represent preexisting
zones of crustal weakness in the continental
crust. The significance of these zones with regard to
mineralization and seismicity have been highlighted. A
magnetic depth to basement analysis has revealed that
the basement surface is generally shallow, and attain a
maximum depth of 1.8 km at the extreme NW corner of the
basin. The regional dip is gentle, and measured 0.4
in the northwestern direction.Also gravity data were collected along all
motorable roads in the basin and in parts of the
adjoining basement block of northern Nigeria. Threedimensional
interpretation of the gravity data led to
the following conclusions: (1) The isolated Older
Granite outcrops around Gusau coalesce into one big
(ix)
pluton just beneath the ground surface and extend to a
depth of 4.3 km with steeply dipping contacts which
suggest a magmatic origin for these granite bodies. (2)
The metasediments of the Anka Schist Belt are generally
outward dipping, and attain a general thickness of 2.5
km, terminating in a characteristic flat base. This
simple structural style suggests that the sequence was
probably affected by only one tectonic event which
left a N-S trending structural grain on these rock
units. (3) A number of depressions in the subsurface
basement rocks have been revealed where sedimentary
thicknesses range from 0.5 to 1.0 km.
Over the basin, both gravity and magnetic data
have revealed an irregular basement floor resulting
from gentle folding of the basement rocks along a
general N-S axis, at wavelength scales of roughly 100
km and amplitudes of only a few hundred meters. Several
sediment-filled troughs and depressions on the basement
surface have been revealed which form targets for
groundwater exploration. The absence of major faulting
even at the sediment-basement boundary tend to suggest
that this southeastern sector of the Iullemmeden Basin
probably originated from gentle epeirogenic warpings in
contrast to the western and northeastern parts
Description
A THESIS SUBMITTED TO THE POSTGRADUATE SCHOOL
AHMADU BELLO UNIVERSITY, ZARIA, IN PARTIAL
FULFILMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY IN GEOPHYSICS.
DEPARTMENT OF PHYSICS
FACULTY OF SCIENCE
AHMADU BELLO UNIVERSITY
ZARIA, NIGERIA.
MARCH, 1990.
Keywords
STRUCTURAL,, INTERPRETATION,, GRAVITY,, AEROMAGNETIC,, ANOMALIES,, SOKOTO BASIN,, NORTHWESTERN NIGERIA.