In order to analyze and determine the geological structures of the Adamawa plateau, the terrestrial gravity data were combined to data computed from GGM02C gravity model. The dense gravity net obtained were further introduced into qualitative and quantitative interpretations. The resulting Bouguer anomaly map obtained from combined data shows NE-SW direction which nearly coincides with the main direction of the fractures affecting the basement in the region and indicates strong gradients marking the presence of discontinuities between heavy and negative gravity anomaly. In order to conduct the quantitative interpretation of the combined gravity data, three profiles were drawn on the residual Bouguer anomaly map and therefore were interpreted using spectral analysis method and 3D density inversion. The knowledge of the depth and density of the geological structures show an uplift of dense rocks under the granite-gneiss substratum. This dense material found in the ENE-WSW direction of the Adamawa Plateau is interpreted as basaltic intrusion probably resulting from tectonic processes. According to this study, the depths of 3.83 km and 9.62 km are the new values of depths obtained for futures investigations in the Adamawa plateau.
The study area lies over the Adamawa Plateau (northern Cameroon) between latitudes 5˚ and 8˚N, and longitudes 13˚ to 15˚E. Previous geophysical and geological studies have been conducted in this area. These include the works of [
The main objective in this work is to interpret a new Bouguer anomaly map obtained from combined terrestrial and GRACE gravity model GGM02C, then determine the depth and density of the geological structure associated with the crustal formation from a dense gravity net in the study area. On residual Bouguer anomaly map, spectral analysis technic and 3D density inversion were obtained. This method has already been successfully used by [
Previous geological and geophysical studies conducted in the Adamawa plateau (
lavas and tuffs. The Foumban Shear Zone is a succession of major accident covered by a series of tertiary volcanoes that one follows from Sudan, through the Central African Republic and Chad up to Foumban in Cameroon [
In this study terrestrial gravity data and data from the global geopotential model GGM02C are available.
In this study, terrestrial gravity data were obtained by the “Office de la Recherche Scientifique et Technique d’Outre-Mer” (ORSTOM) [
organization has a very large database and excellent quality recognized internationally. These data were acquired with the Worden and Lacoste Romberg gravimeters. All gravity measurements are tied to the International Gravity Standardization Network 1971 (IGSN71) datum after correction of luni-solar effect and instrumental drift. To determine the free air anomaly, the linear vertical gradient of 0.3086 mGal/m was used to approximate free air correction. Coordinates stations were determined from topographic maps. Elevation values were obtained with Wallace and Tiernan altimeters. An average rock density of 2.67 g/m3 was introduced for simple Bouguer reduction. Due to the presence of relatively smooth topography, no terrain correction was added. The resulting Bouguer anomaly contour map in the study area
The first sector, located in the central part of the study area consists of a vast negative anomaly that goes from Garoua-Boulai to Djohong passing through
Meiganga, covers the entire city of Ngaoundere. An overview of the configuration of anomalies shows a dominant direction NE-SW, which corresponds to the large fault that extends from Foumban to the Mbere Ditch. The magnitude of these anomalies is approximately −100 mGal with minimums reaching −120 mGal. They can be interpreted as the effect of low density formations. The second sector, situated in the southern part of the study area consists of heavy gravity anomaly. This anomaly with amplitude of about −50 mGal can be link to the presence of dense rocks in this sector. The third sector, situated in the northern part of the region, between parallels 7.30˚ to 8˚N, which covers the Ngaoundere mountain zone is characterized by negative anomaly (−75 mGal) with dominant EW trend. This anomaly does not indicate any geological formation, because the zone is composed of basaltic rocks inside the basement. It is due probably either to the lack of gravity data in the region, or the effect of neighboring formations. The orientation of iso-lines of anomalies in this zone does not objectively identify the direction of the Ngaoundere mountain. The Bouguer anomaly map shows some limitations making a difficult to do the proper gravity investigations. The structure of Adamawa plateau remains poorly knows. To solve this problem, terrestrial gravity data must undergo using data from the global geopotential model GGM02C [
In this part of study data from the Global Geopotential Model were used [
The first sector, located in the middle part of the study area is constituted by negative anomaly with NE-SE direction that goes from Betare-Oya to Djohong passing through Graoua-Boulai and covers the entire city of Ngaoundere and Belel. This anomaly with amplitude of −120 mGal can be considered as the effect of the low densities formations. In this sector heavy anomalies (−20 to −30 mGal) appear in the Mbere basin area. The shape of these anomalies indicates that they would have been affected by folding which occurred during different tectonic phases that have affected the region [
map shows that weak anomalies appear in the central part of the massif. These anomalies are due to the collapsing of the crust or are due to the presence of granitic rocks constituting the socle. Heavy anomalies appear in the northern and southern slope of the mountain corresponds to the basaltic intrusion in this area. The direction of anomalies in this map is visibly more developed than that before densification. This direction coincides perfectly with ENE-WSW direction of the Adamawa Plateau and suggests that the intrusion could have been controlled by the tectonic processes.
In gravity method, the Bouguer anomaly map generally overshadow the effects of density contrasts, deep and shallow, wide and local. Regional-residual separation allows to isolate the anomaly due to deep sources and extended than those from density contrast reduced and shallower extension. This separation was made in our previous works and the residual of order 5 was the best amenable for the geological structures near surface in the Adamawa plateau [
The regional anomaly map (
anomaly is dominated by the effect of structures located beyond the Moho. At the level of Mbe the iso-lines of anomalies were NE-SW direction then passed EW to ESE-WNW in the center of study area. This multiple direction of iso- lines indicates that the basement has found at great depth [
Residual Bouguer anomaly map of the study area (
and Mbe (P1), north-east of Ngaoundere (P2) and north-east of Djohong (P3) localities indicates uplift basement and lateral high dense rocks. In geological considerations, at level of Djohong the positive residual Bouguer anomaly is assigned to the thinning down of the crust on the one hand and on the other hand to the existence of invisible basaltic rocks under the granite-gneisses basement. Between Ngaoundere and Mbe and north-east of Ngaoundere, the positive anomaly corresponds to the intrusion of dense rock of high density under the low density formation. The negative sector observed around Ngaoundere and north of Belel trending NE-SW. A comparison of this negative anomaly with the geological map shows that, this anomaly is due to the collapsing of the crust and the effect of low density formation. In south and east of Mbe, three peaks of negative anomalies appear in the central part of the massif, the average value of these Bouguer anomalies is −30 mGal. In general Bouguer anomalies are usually negative in the massif and mountains because of the isostasy. The rock density of their root is lower than that of the surrounding earth’s mantle. The presence of these peaks of anomalies indicates that, the massif has a variable thickness and is more rooted in these areas than elsewhere. The setting of this massif is linked to a general collapse of the socle in the region. The positive and negative sector is separated by a steep gradient, which corresponds to the effect of discontinuity between two different structures. In order to provide more information in the study area and to show the capability of densified data to detect unknown geological structures, spectral analysis and 3D density inversion based on the geological map were used along three profiles drawn on the residual Bouguer anomaly map.
Spectral analysis as described by [
where
For 3D density inversion of the structures in the Adamawa plateau we used GRABLOX2 (version 2.1) by [
To estimate the average depths of perturbing bodies responsible for gravity data, we used spectral analysis of three profiles (P1, P2, P3) drawn on the residual Bouguer anomaly map. These profiles are executed perpendicular to the main direction of the structure under study and crossed largely the zone where the structure is suspected [
The depths obtained by [
our results are integrating new gravity data in the existing. The second discontinuity associated with the shallowest depths for profiles P1, P2 and P3 is respectively 3.52 km, 5.86 and 2.10 km for a mean of 3.83 km. This value indicates that the sources of anomalies are not deep in the sedimentary basin. These depths are probably due to the base of Mbere sedimentary basin which could be associated with volcanic intrusions due to the positive gravity anomaly. These bodies may be due to the intrusions of dense materials in the basement and could be the boundary between the lower crust and the upper crust. The depths of 3.83 km and 9.62 km are the new values of depths obtained for futures investigations in the Adamawa plateau.
The 3D density inversion was obtained using three profiles P1, P2 and P3 trending NE-SE. This inversion informs us to the structure of the Adamawa Plateau.
The structural model associated with profile (P1) (
bodies with different compositions. The first body (1) with lower densities between 2.60 g/m3 and 2.73 g/m3 is situated at 5 km in the middle and 8 km in the north and south of the model. This formation corresponds to the granitic rock under the crust which the signature is due to the negative Bouguer anomalies observed in this zone. The second body (2) located at the end of the model has a lateral extension and roof that can reach 4 km. This formation with densities ranging from 2.75 g/m3 to 2.78 g/m3 can be associated to the gneisses formations. The third body (3) extended laterally beneath the granites in the study area has high density between 2.8 g/m3 and 2.8 g/m3. This body corresponds to the basaltic intrusion which the signature is due to positive gravity anomaly observed on the Bouguer anomaly map.
The structural model of the second profile (P2) (
ends of the model, is constituted with lower density ranging from 2.60 g/m3 to 2.73 g/m3. This body corresponds to the granitic intrusion at great deep with an extension of about 40 km. The second body (2) with density ranging from 2.75 g/m3 to 2.78 g/m3 designed gneisses formations in the basement of the study area. This body begins from surface area and reaches at 8 km in the middle of the model. A layer with density between 2.80 g/m3 and 2.90 g/m3 corresponds to the high basement structure body (3) located in the middle of the model. This body constitutes the main pluton of the complex, extends laterally at depth beyond the gneisses and corresponds to the basaltic formations. These formations have been put in place with the aid of the asthenospheric rise which leads to a lithospheric bulge. They are responsible for the positive Bouguer anomaly observed in this area.
Like the other profiles, the general features of profile (P3) (
2.45 g/m3 is located in the north and south extremity of the model. This body begins at 6 km and 4 km respectively at the start and at the end of model. The negative Bouguer anomaly observed in the northern and southern border of the profile can be attributed to the sedimentary deposit with low density in the study area. The second body (2) with density ranging from 2.68 to 2.78 g/m3 in the middle of the model can be associated with gneiss formation. This formation is at the origins of the positive Bouguer anomaly observed in this zone and related the crustal thickening of the region followed by the fracturing which must have lead to the collapse of large blocks of crust with dimensions of several kilometers. The third body (3) with density between 2.84 g/m3 and 3.00 g/m3, corresponds to the main pluton of the complex and extends laterally at depth beyond granites. This body with high density has a depth beyond 6 km and an extension of about 18 km under the basement. This body is interpreted as a rise of the basaltic rocks in the socle. It generates a positive Bouguer anomaly observed in this area and it installation is carried out in the following way: During a volcanic eruption, the magma crystallizes in deep into magmatic pockets without completely crossing the crust. In this study we observed a spectacular ascent of magmatic rocks, whose roof is about 2.5 km. The topographic expression of this process is the formation of the Adamawa plateau which is accompanied by brittle tectonics characterized by cracks and fractures.
In this work, the main objective is to analyze the new Bouguer anomaly map and to determine the depth and density of subsurface structure of the Adamawa plateau by using combined gravity data. The resulting residual Bouguer anomaly map shows positive and negative anomaly. Positive residual anomalies are due to the basaltic structures under granite-gneisses basement while negative gravity anomalies are due to the granite formations or the sedimentary basin. Spectral analysis and 3D density inversion show the depths and densities of geological structures associated with the positive residual Bouguer anomaly. The depths of 3.83 km and 9.62 km are the new values of depths obtained for futures investigations in the study area. This area is composed by granitic formation with density ranging from 2.60 to 2.73 g/m3, sedimentary formation with density ranging from 2.20 to 2.45 g/m3, gneisses and basaltic rocks with density between 2.75 to 2.78 g/m3 and 2.84 to 3.0 g/m3 respectively. Our results contain more additional information than those obtained from terrestrial gravity data. Some recent models such as GOCE are also available, but have not yet been fully tested in the study area. For future investigation we will compare several field models from GOCE to choose the one that represents the best gravity data in order to improve the gravity analysis in the Adamawa plateau.
The authors would like to thank Markku Pirttijärvi (Universty of Oulu Finland) for helpful the 3D density inversion and Late Henry Duquenne (LAREG, France) for putting at our disposal the GGM program. The authors also acknowledge all the reviewers for their constructive comments.
Apollinaire, B., Joseph, K., Tabod, T.C., Loudi, Y., Robert, N., Ludovic, K.H. and Valentin, O. (2017) Subsurface Structural Mapping Using Com- bined Terrestrial and Grace Gravity Data of the Adamawa Plateau (North-Cameroon). International Journal of Geosciences, 8, 869-887. https://doi.org/10.4236/ijg.2017.87050