The aim of this study is to determine the depth of deep tectonic structures observed in the Adamawa-Yadé zone (central part of Cameroon) and propose a new structural map of this area. The horizontal gradient associated with upward continuation and the 3D Euler deconvolution methods have been applied to the Earth Magnetic Anomaly Grid 2 (EMAG2) data from the study area. The determination of the maximum magnitude of the horizontal gradient of the total magnetic intensity field reduced to the equator, in addition to the main contacts deducted by Euler solution, allowed the production of a structural map to show the fault systems for the survey area. This result reveals the existence of two structural domains which is thus confirmed by the contrast of magnetic susceptibility in the Central Cameroon Zone. The suggested depths are in the range of 3.34 km to 4.63 km. The structural map shows two types of faults (minors and majors) with W-E, N-S, NW-SE, NE-SW, ENE-WSW, WNW-ESE, NNE-SSW, and NNW-SSE trending. The major faults which are deepest (3.81 km to 4.63 km) with NE-SW, W-E, and N-S direction are very represented in the second domain which includes the Pangar-Djerem zone. This domain which recovers many localities (Ngaoundéré, Tibati, Ngaoundal, Yoko Bétaré-Oya, and Yaoundé) is associated with the Pan-African orogeny and the Cameroon Volcanic Line.
The study area is located in the central part of Cameroon (Central Africa) between 11°30′00′′E and 15°30′00′′E and between 3°00′00′′N and 7°30′00′′N (Figure
Localization of the study area.
The study area is located to the SE part of the Tibati-Banyo Fault (TBF). It is partly combined of the Central Zone and Southern Zone of Cameroon which extends from the southern of TBF to the northern limit of Congo craton. It contains volcanic formations (Cenozoic), Post-Pan-African formations, synt- to post-tectonic granitoid (600–500 Ma), Meso- to Neoproterozoic formations (1000-700 Ma),and paleoproterozoic gneiss (2100 Ma).
The geologic context of the study area (Figure
Geological map of the study area ([
In the western part of CAR (between latitude 3°00′00′′N and 8°00′00′′N), the Yadé massive is marked by Precambrian rocks of date Proterozoic and a not well-known granito-gneissic complex of supposed Archean age [
The Lom group (West of the Sanaga basin) is a narrow belt of discontinuous and discordant rocks on the previous (gneiss and migmatites) of the basement complex [
The basement complex constitutes the cristallophyllites rocks (Ectinites and Migmatites) corresponding to the ancient sediments probably marine, associated with ancient eruptive and intrusive rocks represented by syn to post-tectonic granites.
The Ectinites which correspond to inferior gneiss are very present at the boundary of Adamawa and the South of Batouri. The geological features encountered are as follows: gneiss to garnet, gneiss to amphiboles, leptynites, leptynites garnetified, and garnets.
Migmatites are observed in north of Yaoundé, Bafia, Foumban, Banyo, Tibati, and south of Batouri. It represents accessory minerals identical to those of Ectinites. The migmatites are distinguished in embrechites and in anatexites of two micas, or at biotite and amphibole which are relative to the formations found at SE of Batouri [
The ancient eruptive and intrusive rocks are made of ancient syn-tectonic granites (630–620 Ma) intrusive in the paleoproterozoic. The late syn-tectonic granites (600–580 Ma) present with the ones before, similarities specially in their average composition which tends to be alkaline. Those granites are sometimes concordant and reveal in some areas xenoliths of ancient metamorphic rocks; they are massive and the most important ones are observed in the SE part of Sesse basin, between Yoko and Mankin [
Structural studies reveal that the area was affected by three deformation phases [
(i) The first (deformation D1) is implementing a subhorizontal foliation generally transposed by the second in a straight fold to the horizontal axis. It is responsible for the development of S1 cleavage or schistosity, L1 lineation, C1 shear, and P1 folding.
(ii) The second phase D2 is characterized by a regional foliation of NE-SW direction and a steep slope towards the SE or NW. This phase is associated with the development of granitoid intrusions syn-D2. A subhorizontal lineation L2 is equally noted (of NE-SW direction and dipping SW), L2 lineation, S2 schistosity, P2 folds, and the C2 shear markers. These markers indicate a sinister movement in the northern part of the domain, in relation to the Tcholliré-Banyo fault (FTB) [
(iii) The deformation D3 is marked by set of C3 shear zones and P3 folds.
Those different orogenesis phases have affected the study area, such as a multitude of tectonic structures (faults) which are linked to the shear zone at the Center of Cameroon and the Sanaga fault [
The global Earth Magnetic Anomaly Grid 2 (EMAG2) used in this study has been compiled from satellite, ship, and airborne magnetic measurements. EMAG2 is a significant update of the grid for the World Digital Magnetic Anomaly Map. The data were provided by the organizations listed at
For this study, three methods were applied with the final goal of enhancing the signature of hidden lineaments. The location and estimation of magnetic contacts, associated with tectonic structures and other structural discontinuities, were achieved by the application of horizontal gradient associated with the upward continuation method. In this study, the residual map was obtained after an upward continuation of the TMI-RTE grids at 4 km. The residual of upward map at 4 km is used to realize the horizontal derivative map. In order to estimate and characterize source depths from gridded magnetic data, we applied the 3D Euler deconvolution method.
The horizontal gradient magnitude (HGM) method is the simplest approach to identify areas of geological contacts sources of high susceptibility and determine tectonic structures. Generally, the horizontal gradient of magnetic anomaly which corresponds to a tabular body tends to overlie the edges of the body if the edges are vertical or horizontal and isolated from each other [
The upward continuation processing of the residual map was obtained at various altitudes. The horizontal gradient maxima computation for each level was carried out. The progressive migration while increasing upward continuation height indicates the direction of features outlined [
Reference [
In the current study, the 3D Euler process is to produce a map showing the location and the corresponding depth estimations of geologic structures associated with magnetic anomalies in two-dimensional grid. The standard 3D Euler is based on solving Euler’s homogeneity equation [
The total magnetic map (TMI) shows the variation of the magnetization field of the body buried under the ground (Figure
Total magnetic intensity map of the study area (TMI).
According to the intensity and orientation of these anomalies, the study area can be subdivided in two main domains.
(i) The first domain at the north between parallel 4°30′00′′N and 7°30′00′′N is constituted with positive anomalies located in Mbong and those which extend from Batouri to Bouar; Banyo to Ngaoundal and Ngaoundéré to the Pangar-Djerem reserve. The general direction of anomalies is NE-SW. These anomalies are concentrated on the Pan-African domain and can be correlated to the anatexite granites, ancient to late syn-tectonic granites, micaschists, granulites, schists, gneiss, migmatites, syenites, orthogneiss, basalts, amphibolites (para- and ortho-derivatives greenstones rocks), and cretaceous sedimentary formations.
(ii) The second domain at the south of the map underlines a large negative anomaly which extends over this area following the meridian 11°30′0′′E to 15°0′0′′E and the parallel 3°00′00′′N to 4°30′00′′N. It plunges to the North of Bertoua with intensity between −523.80
The TMI map is characterized by high magnetic anomalies of NE-SW and W-E trending directions. This configuration may be attributed to relatively deep-seated low relief basement structures.
The RTE map (Figure
Total magnetic intensity map reduced to the equator (TMI-RTE).
(i) The effect of the actual magnetic field
(ii) The fact that the parameters (declination and inclination) from the beginning are far from those which define the magnetization of the sources (in this present case will be remanent).
The positive anomalies are most amplified than those observed on TMI map. Their main direction NE-SW is preserved.
The negative anomalies are reduced compared to those of TMI map. This is remarkable at the level of Bertoua where the negative anomaly, well represented on the TMI, is not yet observed on the RTE.
The TMI map is the result of the superposition of high and low effects of geological structures which correspond to regional and residual anomalies. To have a good correlation between anomalies and geological sources, it is necessary to separate these two components. The upward continuation is an analytical method used to separate a regional anomaly resulting from deep sources from residual anomaly due to shallow sources. The upward continuation with increasing heights highlights the magnetic effect of deep body sources because the transformation attenuates high frequency signal components associated with shallow magnetic sources and tends to underline deep regional-scale magnetic anomalies.
Figure
Residual map of TMI-RTE upward continued at 4 km.
The horizontal gradient method is used to detect and interpret structural contacts regardless of their orientation. The method provides continuous contacts locations that are thin and straight. The HGM map of the study area has been realized by using the residual map of the TMI-RTE upward continued at 4 km (Figure
Horizontal gradient magnitude of TMI-RTE map upward at 4 km.
However, to highlight the contact direction represented on the HGM map, it is necessary to show the maxima of the HGM which is represented on Figure
Maxima of the horizontal gradient magnitude map upward at 4 km.
The maxima of the HGM reveal structural complexity such as faults inside the study zone.
According to the orientation of these maxima, two structural domains were recognized within the survey area.
(i) The southern domain (3°00′00′′N to 4°30′00′′N) where major lineaments are characterized by W-E trend. This domain which is geologically distinct to Congo craton formations was affected by Eburnean orogeny.
(ii) The northern domain located between 4°30′00′′N and 7°30′00′N reveals four major directions (W-E, N-S, NE-SW, and NW-SE) of supposed faults. This multitude direction proofs that the northern part of our area of study which corresponds to the Pan-African mobile zone in Cameroon has been affected by significant tectonic activities during the Pan-African orogeny.
The maxima of horizontal gradient of magnetic anomalies help to locate contacts associated with abrupt changes in susceptibility and the multiscale analysis of these maxima and involve the upward continuation. The maxima map of our study area (Figure
Multiscale peaks analysis is process of edge detection in potential field data which corresponds to the maxima gradient position. During the process, each location of gradient maxima is obtained across multiple heights of upward continuation. The total horizontal gradient map (Figure
Maxima of the horizontal gradient of magnetic anomalies map upward continued to 4.1, 4.3, 4.5, and 4.7 km.
To estimate the depth of the deep structures, the Euler method is applied the RTE-TMI map. In the study area, the Euler deconvolution was carried out by using the Standard Euler 3D method of Geosoft package. The Standard Euler 3D method leans on Euler’s homogeneity equation that relates the magnetic field and its gradient components; it also locates the source with the structural index. The location of contact boundaries and lineaments is realized with a window size of 10 km grid cells, a maximum distance acceptable of 20 km, a depth tolerance error of 15%, and a structural index N = 0.
The Euler solution map (Figure
Euler depths map of the study area with geological boundaries obtained with structural index
The Euler map given with coloured point shows the same main trends as the previous method (HGM). The output from the Euler method shows that there are many faults segments trending in W-E, N-S, NW-SE, NE-SW, and WNW-ESE directions.
The obtained results have permitted determining some deep tectonics structures. These different elements (lineaments) prove the presence of a probable deep contacts or geological boundaries.
The observation of the structural map of HGM can be used to analyze direction trends of lineaments, which are even more observed in the Euler solution map. The magnetic data by horizontal gradient and Euler deconvolution methods were combined to produce the final interpretation of contact locations. The structural map of the study area (Figure
Magnetic lineaments map obtained for the study area showing the TBF, CCSZ, SF, YF, and MF or KF.
Table
Characteristic orientation of different fault segments.
Fault segment | Direction | Depth (km) | Fault segment | Direction | Depth (km) | Fault segment | Direction | Depth (km) |
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F1 | NE-SW | 4.0 | F28 | W-E | 3.81 | F55 | N-S | 3.81 |
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F2 | NE-SW | 4.09 | F29 | W-E | 3.81 | F56 | N-S | 4.63 |
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F3 | W-E | 4.63 | F30 | W-E | 4.63 | F57 | NNE-SSW | |
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F4 | W-E | 4.09 | F31 | W-E | 3.97 | F58 | N-S | 3.97 |
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F5 | NW-SE | 3.85 | F32 | NE-SW | F59 | NNE-SSW | 3.97 | |
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F6 | ENE-WSW | 4.63 | F33 | WNW-ESE | 3.76 | F60 | WNW-ESE | 4.14 |
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F7 | NE-SW | 3.85 | F34 | W-E | 4.05 | F61 | ENE-WSW | 3.96 |
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F8 | W-E | F35 | NE-SW | 4.63 | F62 | ENE-WSW | 3.97 | |
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F9 | ENE-WSW | 3.87 | F36 | W-E | 3.74 | F63 | W-E | |
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F10 | NE-SW | F37 | NE-SW | 4.63 | F64 | NW-SE | 3.66 | |
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F11 | ENE-WSW | 3.87 | F38 | NW-SE | F65 | NW-SE | 3.92 | |
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F12 | N-S | 3.86 | F39 | ENE-WSW | 3.92 | F66 | W-E | |
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F13 | W-E | 3.94 | F40 | WNW-ESE | 3.62 | F67 | ENE-WSW | 3.66 |
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F14 | WNW-ESE | 3.93 | F41 | W-E | 3.87 | F68 | W-E | 3.74 |
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F15 | W-E | 4.63 | F42 | WNW-ESE | F69 | ENE-WSW | 4.05 | |
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F16 | W-E | 3.88 | F43 | NW-SE | F70 | W-E | ||
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F17 | W-E | F44 | W-E | 4.63 | F71 | W-E | 3.95 | |
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F18 | NW-SE | 3.92 | F45 | NNE-SSW | 4.63 | F72 | W-E | 3.93 |
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F19 | W-E | 3.95 | F46 | NNE-SSW | 4.63 | F73 | W-E | 4.63 |
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F20 | WNW-ESE | 3.9 | F47 | NW-SE | F74 | WNW-ESE | ||
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F21 | NW-SE | 3.71 | F48 | NNE-SSW | 3.34 | F75 | W-E | 3.81 |
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F22 | W-E | 3.71 | F49 | N-S | 3.71 | F76 | N-S | 3.92 |
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F23 | ENE-WSW | 4.05 | F50 | W-E | 3.94 | F77 | NW-SE | |
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F24 | NW-SE | 4.05 | F51 | W-E | 3.74 | F78 | N-S | 3.87 |
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F25 | W-E | 3.66 | F52 | W-E | 3.88 | F79 | N-S | 3.62 |
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F26 | NE-SW | 3.79 | F53 | W-E | 3.97 | F80 | N-S | 3.97 |
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F27 | NE-SW | 3.87 | F54 | N-S | 3.71 |
The orientations of the lineaments extracted from the structural map (Figure
(a) Rose diagram of the general lineament orientations of the area studied; (b) rose diagram of the major lineament orientations; (c) rose diagram of the minor lineament orientations.
Figure
Faults with N-S and W-E direction characterize the definitive stability of the Congo craton which would be associated with the Eburnean orogeny. The NE-SW and W-E trends represent the direction of tectonic structures associated with the Pan-African orogeny. The NE-SW and ENE-WSW directions are correlated with the direction of subduction of the Ntem Complex (cratonic plate) under the Pan-African mobile zone [
The interpretation of EMAG2 data using horizontal gradient associated with upward continuation and the 3D Euler deconvolution analysis methods has allowed identifying two structural domains. The two domains are located on the base complex which is associated with the Pan-African belt zone and the Ntem Complex or Congo craton. The results showed in Figure
The magnetic lineaments observed in our study area suggest that the area has been affected by an important regional field stress. The predominant W-E, N-S, NE-SW, ENE-WSW, and WNW-ESE fault directions prove that the regional stress field which affected the Base Complex in the Central Cameroon region is responsible for the reorientation of the former structures observed on the Ntem Complex or Congo craton [
The tectonic activity associated with the NW-SE and N-S predominant trending of magnetic lineament characterizes the stability of the Congo craton. The NE-SW and WSW-ENE directions are correlated with the direction of subduction of the Congo craton under the Pan-African [
The depth in the range of 3.34 km to 4.63 km is in accordance with the result of [
The interpretation of magnetic anomalies of the Adamawa-Yadé zone between the Tibati-Banyo fault and the northern limit of the Congo craton has been realized by using EMAG2 data. Horizontal gradient associated with the upward continuation in addition with the 3D Euler methods has been used to filter magnetic data and enhance the data, the features that would be difficult to detect without outcrop and determine the depth of faults. Application of selected filtering methods to the magnetic data reveals the presence of deepest tectonic structures. The structural map obtained for the area is materialized by many faults with different directions which indicate a complex tectonic history and different phase of deformation. The major faults with direction trends W-E, NE-SW, N-S, NW-SE, and WNW-ESE are associated with the Cameroon Volcanic Line located in the Pan-African belt. The depths of these geological contacts or tectonic structures are estimated between 3.66 km and 4.63 km. The structural and tectonic elements obtained in our study area are also in accordance with those which were recently discovered by many authors using interpretation of aeromagnetic and EMAG2 data based on horizontal gradient, vertical gradient, upward continuation, analytic signal, and the tilt angle methods [
The authors declare that they have no conflicts of interest.
The authors are grateful to the organizations listed at