Gravity studies have been carried out in the Douala sub-basin which is a sedimentary basin located both onshore and offshore on the South coast of Cameroon between latitudes 3˚03'N and 4˚06'N and longitudes 9˚00' and 10˚00'E, covering a total surface area of 12,805 km 2 . On its onshore portion, the Douala sub-basin has a trapezoic shape and covers a total surface area of about 6955 km 2 while the offshore part covers an area of about 5850 km 2 . Gravity data used in this study are constituted of 912 gravity data points located between longitudes 8˚10.2' to 10˚59.4'E and latitudes 2˚30.6' to 4˚59.4'N and the study area is located to the NW section of the onshore portion of the Douala sub-basin. This study area is characterised by considerably high positive anomalies attaining peak values of about 104.1 mGals at longitude 9˚9.9' and latitude 4˚1.1' with contour lines which are mostly oriented in the NNE direction. Residual anomalies were extracted by upward continuation of the Bouguer anomaly field at an optimum height of 30 km. This residual field and those obtained by the separation of polynomial of order 4 had a very high correlation coefficient factor of 0.979. The multi-scale horizontal derivative of the vertical derivative (MSHDVD) method was applied on the extracted residual anomalies for the delimitation of possible contacts in the source while the amplitude spectrum was used to estimate the depth to the top of the po-* Corresponding author. E. N. Ndikum et al.525 tential field source. The MSHDVD method did not delimite any clear cut contacts in the source but the amplitude spectrum estimated the potential field source at a depth of about 4.8 km. The ideal body theory was used to determine the density contrast along a 65 km NW-SE profile yielding a value of 0.266 g/cm 3 . 2.5D modelling aimed at bringing out the underlying structural layout of this study area presents a source body which is very probably an intrusive igneous block surrounded by sedimentary formations and having a density of 2.77 g/cm 3 at a depth of about 5.88 km below the surface and an average thickness of about 26.95 km.
The Bouguer anomaly map of the region between latitudes 3 0 N and 3 0 45 ' N and longitudes 9 0 30 ' E and 10 0 10'E and which forms the southern part of the Douala Basin, shows ring-like positive contour lines. The Bouguer gravity profiles obtained across the gravity anomaly contour lines in the region have been interpreted using 2D 1/2 gravity modelling. The results reveal that in the southern part of the Douala Sedimentary Basin, two major structures exist: a half-dome of mantle material explained by isostatic compensation and a pillar of high density rocks probably representing an upper mantle intrusion down to a depth of about 14 km. The similarities of these results with those obtained in the coastal sedimentary basin of Mauritania-Senegal suggest a more extensive movement that would have affected the whole of West African coast.
Gravity data have been processed in the Douala sedimentary sub-basin in a section consisting of a set of 116 gravity data points located between latitudes 3˚03'N and 4˚06'N and longitudes 9˚00'E and 10˚00'E. The established Bouguer anomaly fields and the Residual anomaly fields, extracted by upward continuation at an optimum height of 30 km, were both characterized by considerably high positive anomalies. These anomalies showed many ring-like positive gravity anomaly contour lines in the study region. Gravity gradients were analysed using the multi-scale horizontal derivative of the vertical derivative (MSHDVD) method, and this excluded the existence of fault lines across this region. Amplitude spectrum was used to estimate the potential field source at a depth of about 4.8 km. The ideal body theory capable of handling sparse data contaminated with noise was applied along a 50.2 km WWS-EEN profile to determine a density contrast of 0.266 g/cm 3 . Using these results as constraints, 2.5 D modelling carried out along this profile presented two major blocks with density contrast of 0.266 g/cm
162sub-basin and the coastal sedimentary basin of Mauritania-Senegal and thus foster the suggestions of a more extensive movement that would have affected the whole of the West African coast.
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