Contribution of advanced edge-detection methods of potential field data in the tectono-structural study of the southwestern part of Cameroon

The southwest part of Cameroon is composed of a range of geological structures and sedimentary basins, whose geological history dates from the Cretaceous, and their establishment would have originated from the opening of the South Atlantic. Among these structures, the most important is the Cameroon Volcanic Line, generally denoted CVL, followed by the southern part of the Central African Shear Zone (CASZ) called the Foumban Shear Zone (FSZ), the Ntem Complex, the Benue Trough, the northern margin of the Congo Craton, and many others. The major structures identified in this part are formed as a result of geodynamic activity between the Oligocene and the recent period, to which we can add the volcanism that exists along with the continental limit. The XGM201e_2159 model is used to highlight the geological structures of Southwestern Cameroon. First, we separated the regional and residual anomalies from Bouguer gravity data. Then, we used a combination of filters to enhance the signature of the residual anomalies in Southwest Cameroon. These filters identified various geological structures in the area. Finally, we applied the enhanced horizontal gradient amplitude (EHGA) and multi-scale horizontal derivative of the vertical derivative (MSHDVD) methods to reveal the geological contacts, allowing us to establish the corresponding structural map of the region. In addition, the results obtained in this study are the first to precisely define the circumferential demarcation of the continental and oceanic expansions of Mount Cameroon, while clearly illustrating the Bao, Bomana, Tiko, and Ekona faults that extend to the Bakassi Peninsula and the Douala outlet. Furthermore, they highlight the strike–slip faults in the summit vicinity. The structural map shows that most of the geological boundaries identified in the area are trending in the NE–SW, NNE–SSW, ENE–WSW, N–S, and NW–SE directions.

1 Introduction

Located in the south-west of Cameroon, the study area spanned between longitudes 8˚00′ E and 11˚25′ E and latitudes 3˚00′ N and 7˚25′ N (Figure 1). It encompasses various geological structures and sedimentary basins whose origins remain subjects of reflection within the scientific community. Among these structures, the most important would be the succession of mountain ranges that form the Cameroon Volcanic Line (CVL). The CVL owes its origin to a large subvolcanic and volcanic sequence to which it belongs, which in turn would originate from its presence within several hot spots (Ngounouno et al., 2000; Ngounouno et al., 2003). This great lineament, which extends till the West African coast, is composed of both an oceanic and a continental part. It separates the sedimentary Rio Del Rey Basin from that of Douala–Kribi Campo. The mountain range that forms it is generally alkaline and formed between the Tertiary and the recent period (Fitton, 1987). This mountain range is subject to volcanism, which extends from the Atlantic islands of Pagulu to the interior of the African continent (Lee et al., 1994). The islands of Bioko and Sao Tomé et Principe constitute the marine component, while the Manengouba, Bamboutos, Oku, and Mandara mountains constitute the continental portion (Figure 1). This continental part has an average crustal density of 400 kg/m³, with an average Moho depth of about 32.5 km (Ghomsi et al., 2020; Ghomsi et al., 2022b). In addition to the CVL, we can see other structures in this area, including the Ntem Complex and the Mamfe Sedimentary Basin, which is a part of the Central Cameroonian Shear Zone.

FIGURE 1

FIGURE 1. Map location of the study area with the representation of the Cameroon Volcanic Line.

According to Ubangoh et al. (2005), the positive anomalies seen along the CVL point are due to the presence of multiple active volcanic craters and cones as well as a strong geothermal gradient. Also, an analysis of the samples collected along the CVL highlights the existence of a predominant magnetization spectrum within it. The main lineaments of the region are highlighted in the past research by Nzeuga et al. (2019) and Ghomsi et al. (2022a) based on the interpretation of aeromagnetic data. Noutchogwé et al. (2011) and Ndikum et al., 2014; Ndikum et al., 2017 highlighted the geological contacts of this part of Cameroon from the interpretation of gravity data using the MSHDVD method. Koumetio et al. (2012) applied multi-scale elevation of the maxima of the horizontal derivative on the vertical gradient of the Bouguer anomaly, followed by 3D modeling to highlight geological structures in the region, visible on the signal as negative gravity anomalies. The basement of this region is intruded by bodies of variable shapes and sizes located at a depth varying between 1 and 10 km and a density range from 2.57 to 2.87 g/cm³. These intrusions observed in the crust of this region could be potential reservoirs of rare minerals (Fosso-Téguia et al., 2020), and the crust thickness is between 35 and 39 km (Tokam et al., 2010; Ghomsi et al., 2020; Ghomsi et al., 2022b).

Considering the results of previous work in the study area (Ghomsi et al., 2020; Ghomsi et al., 2021; Tchoukeu et al., 2021; Ghomsi et al., 2022b), the use of the recent XGM 2019e2159 Gravity Field Model presented by Zingerle et al. (2020) and the regional/residual separation of the Bouguer anomaly map by using the method of Zeng et al. (2007) could increase the signature gravity anomalies. Additionally, determining gravity source parameters is crucial for finding new mineral deposits and helps maximize exploratory drilling operations (Pham et al., 2019; Pham et al., 2020a; Duong et al., 2021; Long et al., 2021).

Gravity and magnetic data can provide vital information on the subsurface structures (Dung and Minh, 2017; Hang et al., 2019; Eldosouky et al., 2021a; Eldosouky et al., 2021b; Oksum 2021). The determination of geological boundaries from the interpretation of gravity data has been the subject of several studies, and many different techniques have been introduced (Melouah et al., 2021; Eldosouky et al., 2022a; Pham et al., 2022a; Eldosouky et al., 2022b; Pham et al., 2022b; Eldosouky et al., 2022c; Ghomsi et al., 2022c; Eldosouky et al., 2022d; Ghomsi et al., 2022d), such as, for example, the peak detection methods (Blakely and Simpson, 1986; Hang et al., 2017; Pham et al., 2018; Pham et al., 2021), vertical derivatives, the filters of horizontal gradient (Cordell, 1979; Cordell and Grauch, 1985), the analytical signal (Nabighian, 1972; Nabighian, 1984; Roest et al., 1992), tilt angle (Miller and Singh, 1994), the horizontal gradient amplitude of tilt angle (Verduzco et al., 2004), enhanced horizontal gradient amplitude (Pham et al., 2020a), improved logistic function (Pham et al., 2020b), and balanced horizontal gradient amplitude filters (Prasad et al., 2022a; Prasad et al., 2022b).

From the Bouguer anomaly data, we shall extract anomalies of importance in this study. Then, in order to emphasize the various contacts, we will use the first-order derivation, the horizontal gradient (HG), the analytical signal (AS), the tilt angle (TDR), the horizontal gradient of the tilt angle (HG_TDR), and the enhanced horizontal gradient amplitude (EHGA) filters. Finally, we will compare the geological contacts obtained on the TDR, HG_TDR, and EHGA maps with those obtained by using the MSHDVD method to reveal a new tectono-structural map of the region.

2 Geological setting

The geology of the southwestern part of Cameroon illustrated in Figure 2 is very large and complex. The CVL consists of a succession of mountains ranging in age from the Tertiary to the recent period (Fitton and Dunlop, 1985). Among the elements that constitute it, we can cite the Adamaoua–Yade Plateau and the Western Cameroon Highlands, generally called the Cameroon Highlands. The highlands of Cameroon form the androgenic plutonic complexes of the CVL and encompass a succession of volcanoes oriented at 30˚E with peaks ranging from 900 to 1,800 m for the smallest to 3,010 m for Mount Oku and 4,020–4,100 m for Mount Cameroon, located near the coastal plain (Deruelle et al., 2007; Beckline et al., 2018; Wembenyui et al., 2020). The CVL consists of eight monogenetic volcanic fields with an estimated total area of 17,000 km², with individual fields ranging in size from 700–4,000 km² (Schmidt et al., 2022). Because the CVL is located in a unique tectonic environment at the transition from the oceanic to continental crust, it serves as a natural laboratory for the study of the geochemical evolution of alkaline magmas with varying degrees of crustal contamination and mixing. Mount Cameroon is the only active mountain in the mountain range that constitutes the CVL, and its last seven eruptions date from 1909, 1922, 1954, 1959, 1982, 1999, and 2000 (Geze, 1943; Fitton et al., 1983; Suh et al., 2003). The basanitic and alkaline basaltic nature of its flows, adding small quantities of pyroclastic materials and volcanic cinder cones, make it a composite volcano (Suh et al., 2003; Yokoyama et al., 2007; Suh et al., 2008). The CVL has a crystalline basement embedded in the mobile orogenic Ubangid belt, bounded by the Pan-African range and located between the Saharan Metacraton, the Congo Craton, and the West African cratons (Meyers et al., 1998; Ghomsi et al., 2020; Ghomsi et al., 2021; Ghomsi et al., 2022b). Ghomsi et al. (2022a) applied the intensity of the enhanced horizontal gradient amplitude (EHGA) on the aeromagnetic data to highlight the major geological structures of the CVL and their effects on the topography and the cross-correlation analysis to highlight the principal directions of magnetization of certain anomalies. The Central African Shear Zone (CASZ), a component of the Pan-African Range, is a major fault belt still active. Its nature is identical to that of its counterpart, known as the Pernambuco lineament, located in Northeast Brazil, which would be an extension of the CVL in South America (Torsvik et al., 2009; Aslanian and Moulin, 2010; Moulin et al., 2010).

FIGURE 2

FIGURE 2. Geology map of Southern Cameroon (after Ghomsi et al., 2022a).

The Ntem Complex, part of which is in our study area, represents the northern part of the Congo Craton. It is formed mainly of Archean rocks, intrusive rocks, leptynites, and gneisses (Tchameni et al., 2001). The intrusions encountered in the Ntem Complex are mainly granites, tonalites, and syenites. The Nyong unit is the part of the Ntem Complex which appears in our study area. This unit consists mainly of gneisses and greenstones in the form of a belt and represents the NW section of the Ntem Complex (Maurizot et al., 1986; Minyem and Nedelec, 1990; Tchameni, 1997). Basic and ultrabasic rocks and ferriferous and barren quartzites are the constituent elements of greenstone belts. The blastomylonitic shear zones observed irregularly on the Nyong and Ntem units owe their origin to faulting deformation. Neoproterozoic intrusions of nepheline syenite in a sinistral shear zone in the Nyong unit also owe their origin to the last magmatic episode in this one (Maurizot et al., 1986; Nsifa, 2005).

Another part of our study area is the Benue Trough. It is a large sedimentary basin oriented in the NE–SW direction, distributed over a length of about 1,000 km, which extends from the Niger Delta in Southern Nigeria to the vicinity of Lake Chad. Its origin dates from the Cretaceous with the opening of the South Atlantic (Guiraud and Maurin, 1992; Nouayou, 2005). The Benue Trough includes many faults of various kinds in its upstream part, and the main ones are oriented along the N115˚E structural direction (Regnoult, 1986). It is represented in Cameroon in the North by the Garoua Rift, also called Yola Trough, and in the south-east region by the Mamfe Sedimentary Basin. The volcanic massifs of Cameroon are at the origin of the interruption of the extension of the Cameroonian coastal basins of Douala/Kribi–Campo toward the Benue Trough, and these basins (Mamfé, Rio Del Rey, and Douala/Kribi-Campo) would have the same geological history as this one (Nzeuga et al., 2019).

3 Data

The satellite-derived gravity data for the entire Southwest Cameroon are derived from the XGM 2019e_2159 (eXperimental Gravity field Model 2019) (Zingerle et al., 2020), which is represented through spheroidal harmonics, corresponding to a spatial resolution of 2’ (∼4 km), and has lesser artifacts. It is developed from altimetry, satellite-only model GOCO06s (Kvas et al., 2021), topography (Amante and Eakins, 2009), and ground data. It is a static global combined gravity field model with degree and order of 5,399. It has the long wave from GRACE+GOCE v5 and the short wave from land gravity averaged at 1/15° and marine and altimetric gravity (DTU15) provided by the National Geospatial-Intelligence Agency (NGA). The topographic heights are calculated from the spherical harmonic model of the topography (ETOPO1) used up to the same maximum degree as the gravity field model.

4 Methodology

To determine the lineaments and propose a geological model of the subsoil, highlighting the intrusions of igneous bodies in the southwestern part of Cameroon, many filtering operations are needed and should be applied to the gravity anomaly data. First, we have performed the regional–residual separation from the Bouguer anomalies using the method of Zeng et al. (2007). Then, the filters such as the horizontal gradient, analytical signal, tilt angle, and enhanced horizontal gradient amplitude have been applied to the residual anomaly data to enhance the geological edges. Finally, we have applied the improved maximum detection method (Pham et al., 2020c) to the multi-scale horizontal derivative of the vertical derivative to detect the geological contacts.

4.1 Regional/residual separation

The observed gravity anomaly is the sum of gravity effects of density differences at various depths (Oksum et al., 2019; Pham et al., 2022c; Ghomsi et al., 2022d). Here, we used the method of Zeng et al. (2007) for separating the regional anomaly resulting from deep sources from the observed gravity. Then, the residual anomaly generated by the shallow sources was calculated by removing the regional anomaly from the observed anomaly. This method calculates a series of cross-correlations between the upward continuations at two successive heights (Zeng et al., 2007). The height associated with the maximum deflection of these cross-correlation values yields the optimum height for regional/residual separation (Zeng et al., 2007).

4.2 Use of derivative filters

We consider ΔT as the value of the total gravimetric field caused by a random distribution of the density sources in the soil. When the gravity fields of several sources interfere, the application of the derived filters is important to dissociate them and to determine the location of the contacts at the origin of each source (Fedi and Florio, 2001). It also has the advantage of operating calculations at different heights via the upward extension stable operator (Jacobsen, 1987).

The first-order horizontal and vertical derivatives can be written as follows:

$\frac{\partial ∆T}{\partial x}=\frac{{∆T}_{i+1,j}-{∆T}_{i-1,j}}{2∆x}(1)$

$\frac{\partial ∆T}{\partial y}=\frac{{∆T}_{i,j+1}-{∆T}_{i,j-1}}{2∆y}(2)$

$\frac{\partial ∆T}{\partial z}=F^{-1}\left\{\left|k\right|F\left(∆T\right)\right\}(3)$

where Δx is the sampling distance along the longitudes; Δy is the sampling distance along the latitudes; i and j represent the data collection points of ΔT in x and y, respectively; F and F⁻¹ represent the Fourier transform and inverse Fourier transform, respectively; and |k| is the radial wave number.

We can combine these elements to calculate the horizontal gradient (HG) (Cordell, 1979; Cordell and Grauch, 1985) and the analytical signal (AS) (Nabighian, 1972; Nabighian, 1984; Roest et al., 1992) as follows:

$HG=\sqrt{{\left(\frac{\partial ∆T}{\partial x}\right)}^2+{\left(\frac{\partial ∆T}{\partial y}\right)}^2}(4)$

$AS=\sqrt{{\left(\frac{\partial ∆T}{\partial x}\right)}^2+{\left(\frac{\partial ∆T}{\partial y}\right)}^2+{\left(\frac{\partial ∆T}{\partial z}\right)}^2}(5)$

The maximum value of the HG and AS is used to detect the source edges (Grauch and Cordell, 1987; Phillips, 2000).

4.3 Tilt angle

The tilt angle (TDR) is also known as a phase filter. The tilt angle and its total horizontal gradient are recommended for the mapping of geological structures buried in the ground at medium depths and for the determination of ore deposits (Miller and Singh, 1994). This filter is applied on potential data, and it is defined as the ratio of the vertical derivative in the z-direction to the amplitude of the horizontal gradient as follows:

$TDR={\mathit{tan}}^{-1}\left[\frac{\frac{\partial ∆\mathit{T}}{\partial \mathit{z}}}{\sqrt{{\left(\frac{\partial ∆\mathit{T}}{\partial \mathit{x}}\right)}^{\mathbf{2}}+{\left(\frac{\partial ∆\mathit{T}}{\partial \mathit{y}}\right)}^{\mathbf{2}}}}\right](6)$

The horizontal gradient of the tilt angle (HG_TDR) was developed by Verduzco et al. (2004) to optimize the geological information given by the tilt angle. It is given by

$HG\_TDR=\sqrt{{\left(\frac{\partial TDR}{\partial x}\right)}^2+{\left(\frac{\partial TDR}{\partial y}\right)}^2}(7)$

4.4 Enhanced horizontal gradient amplitude

Pham et al. (2020a) proposed to use the enhanced horizontal gradient amplitude (EHGA) filter to highlight both the shallow and deep geological contacts responsible for the gravity anomalies observed at the surface. It can be given as follows:

$EHGA=\mathcal{R}\left(asin\left(p\left(\frac{{HG}_z}{\sqrt{{HG}_x^2+{HG}_y^2+{HG}_z^2}}-1\right)+1\right)\right)(8)$

where $\mathcal{R}$ is the real part of the function and p is a positive number. The EHGA ranges from—π⁄2 to + π⁄2 when the peaks are directly located over the edges, and we obtained optimal results when using p ≥ 2 (Pham et al., 2020a).

4.5 Multi-scale horizontal derivative of the vertical derivative

As explained previously, the upward continuation on the data field is indicated to enhance the geological information in the ground at greater depths. For determination of geological contacts, the greater the extension height, the more information is enhanced on the deep contacts. Suppose the contact is located vertically from the source, then the maxima of the horizontal gradient of the extended map will make it possible to determine it, and the direction of shift of the maxima in question defines the dip direction of said contact (Fedi and Florio, 2001).

The following is a summary of the steps used for the MSHDVD method:

• Computing the first-order vertical gradient of the gravity data extended upward at different experimental altitudes, called multi-scale vertical derivative (MSVD).

• Determining the maxima of the horizontal gradient of the maps resulting from the MSVD method.

• Superposing the results determined for different continuation heights.

5 Results

5.1 Bouguer anomaly map

In this section, we outlined the Bouguer anomalies resulting from the combination of the XGM 2019e_2159 and the ETOPO1 models to plot the Bouguer anomaly map. A 5 min × 5 min gridded spatial distribution of the Bouguer gravity anomaly was obtained. The total gravity intensity (TGI) clearly showed the differences in the locations of high and low gravimetric intensities (Figure 3). Due to the effects of region on the distribution of gravity anomalies observed on this map, the interpretation may contain errors, and we must first eliminate these effects to interpret it. This will be the subject of the next section.

FIGURE 3

FIGURE 3. Bouguer anomaly map of the study area with its perspective representation, estimated on a 0.1° × 0.1° geographical grid (black lines represent national boundaries).

5.2 Regional/residual separation

The Bouguer anomalies map (Figure 3) showed positive long-wavelength anomalies reaching ∼174 mGal at the coast and negative anomalies in the eastern part of the study area, reaching peaks of −130 mGal. These positive anomalies would be the extension of the oceanic crust, which has a high density put in place during the opening of the South Atlantic (Torsvik et al., 2009; Lawrence et al., 2017). The negative anomalies observed to the northeast would be the deposits of ancient Precambrian rocks that would form the region’s bedrock. Furthermore, the 3D Bouguer anomalies map showed a high correlation between the regional distribution of anomalies and the topography, thus indicating the presence of isostatic compensation. In addition, we observed a great correlation between the Bouguer anomalies map and the geological map of the study area.

Figure 4A shows the cross-correlations between continuations to two successive heights versus the height. Figure 4B shows the deflection C versus the height for the Bouguer anomaly of the southwestern part of Cameroon. The curve of the deflection shows a maximum at a height of 20 km (Figure 4B). Thus, this height is regarded as the optimal height (Figure 4B). The upward continuation of the Bouguer anomaly data at a height of 20 km yields the regional anomaly (Figure 5A). The regional anomaly map obtained by extension shows a large positive anomaly at the coast and a large negative anomaly in the northwest region, separated by a medium-density structure. These anomalies would be caused by the effects of surface structures present in the study area. The residual anomaly caused by localized gravity effects was calculated by removing the regional anomaly data in Figure 5A from the observed anomaly. The calculated residual field anomaly map is shown in Figure 5B. The residual anomaly map provides precision on the distribution of gravity anomalies in the region. The positive anomalies observed along the coast and at the east of the region would be the representation of the CVL. The negative anomalies represent ancient Precambrian deposits, and mid-density anomalies represent sedimentary deposits.

FIGURE 4

FIGURE 4. (A) Cross-correlations between the upward continuations at two successive heights. (B) Deflection C versus the height.

FIGURE 5

FIGURE 5. (A) Bouguer regional map; and (B) Bouguer residual map (black lines represent national boundaries).

5.3 Derivatives

To highlight the gravimetric signatures of the study area, the first-order derivative filters were applied, as explained in Section 4. Figure 6 shows the X, Y, and Z derivatives of gravimetric data. The X-derivative map shows that the positive anomalies observed are oriented NE–SW and the existence of sedimentary deposits along the coast and in the bordering part between the north-west and the south-west of Cameroon. This information is practically contradictory to that observed on the Y-derivative map. On the other hand, the Z-derivative map is similar to the residual anomaly map with positive anomalies oriented NE–SW, well dissociated compared to those observed in this one. Furthermore, Z-derivative presented anomalies of higher amplitudes, ranging between −4 and 8 mGal/km, than those observed on the X- and Y-derivatives and corroborated the existing information on the geological map.

FIGURE 6

FIGURE 6. First-order derivative of the Bouguer residual anomaly map: (A) in the X-direction; (B) in Y-direction; and (C) in Z-direction (black lines represent national boundaries).

5.4 Gradient filters

The HG and AS were applied to the Bouguer residual map to characterize the isolated sources since, in most cases, the filtered anomaly is generally positive, and its maxima are located over the source edges. The gradient maps represented in Figure 7 show that the anomalies grouped in the Bouguer regional map are isolated, and the gradient filters aim to center the gravity anomalies below their geological sources. We can observe that the HG is more effective than the AS in mapping the source edges from gravity data. Here, Figure 7A also shows more edges than Figure 7B.

FIGURE 7

FIGURE 7. (A) Horizontal gradient and (B) analytical signal amplitude of the Bouguer residual map (black lines represent national boundaries).

5.5 Filters based on the ratio of gradients

The TDR and HG_TDR maps of the residual data are shown in Figures 8A, B, respectively. On the TDR map, the observed positive anomalies well-corroborate the information of the geological map distinguished by the Eocene, Miopleocene, and Paleo-Neogene age sand covers that cover the Douala and Rio Del Rey sedimentary basins. Pham (2020) showed that the TDR filter is useful for enhancing information on discordant signals. However, it is not an ideal filter for edge detection because it does not provide sharpened responses at the source edges. The HG_TDR map represents the same anomalies observed on the TDR map but in the form of geological contacts existing below them. The EHGA map (Figure 8C) isolates edges better than TDR and HG_TDR since it can extract the edges of both shallow and deep structures.

FIGURE 8

FIGURE 8. (A) Tilt derivative (TDR); (B) the horizontal gradient of the tilt derivative (HG_TDR); and (C) the enhanced horizontal gradient amplitude (EHGA) of the Bouguer residual map (black lines represent national boundaries). In green: the Fako and Tiko faults meet in a ring formation constituting the continental junction, spreading out to the Bakassi Peninsula and the Douala outlet. The yellow arrow denotes the average of the green arc.

5.6 Determination of geological contacts using the multi-scale horizontal derivative of the vertical derivative method

The detailed methodology used to apply this method has been elaborated in Section 4. In order to reveal the geological contacts between the depths of 1–5 km, first, the first-order Z-vertical derivative was calculated on the residual map and extended successively to depths of 1, 2, 3, 4, and 5 km. Second, we calculated the horizontal gradient amplitudes of these vertical derivatives and used the improved maximum detection method (Pham et al., 2020c) to determine the source edges (Figure 9). It is noticed that there is a great similarity between the maps in Figure 9 and those in Figure 8. The rose diagram of EHGA and MSHDVD lineaments represented in Figure 10 shows the distribution and orientations of the main faults identified in Southwestern Cameroon. As can be seen, most of the geological boundaries identified in the area are trending in the NE–SW, NNE–SSW, ENE–WSW, N–S, and NW–SE directions. Based on the information from the geological map, these maps will be analyzed to propose a tectono-structural map of the southwest part of Cameroon as shown in Figure 11. This map (Figure 11) highlights the main structures highlighted by the results of previous studies and corroborates the information provided by the geological map.

FIGURE 9

FIGURE 9. Source edges of the study area obtained by using the MSHDVD method.

FIGURE 10

FIGURE 10. Rose diagram that shows the fault orientations within Southern Cameroon.

FIGURE 11

FIGURE 11. Representation of new faults compared to the existing ones. Legend and abbreviations as shown in Figure 2.

6 Discussion

The Bouguer anomaly map (Figure 3) highlights a large positive anomaly at the coast and a large negative anomaly at the northeast region of the study area. This positive anomaly would have originated from the extension of the oceanic crust at the level of the coast set up during the opening of the Atlantic Ocean in the Cretaceous and corroborates the results of Guiraud and Maurin (1992), Torsvik et al. (2009), and Lawrence et al. (2017). The negative anomaly observed indicates the presence of isostatic compensation as presented by Balogun (2019) and would be an accumulation of syn-tectonic granite. Moreover, we can observe an accumulation of low-intensity positive gravity anomalies covering the upper cretaceous sandstone of the Mamfe Basin. Thus, these observations are consistent with the existing geological and tectonic information in the study area.

The regional map (Figure 5A) highlights all the regional effects that interfere with the gravity signal. It is similar to the Bouguer anomaly map but is smoother than this one. Its resemblance rate to the Bouguer anomaly map justifies the chosen separation method and validates the height of 20 km for upward continuation. The residual map (Figure 5B) represents the anomaly map after removing the regional anomaly from the Bouguer anomaly. It presents the distribution of the different anomalies of the region more precisely. The positive anomalies observed are all oriented along the mean structural direction of N75° E, which is the major structural direction of the CVL (Tokam et al., 2010; Nzeuga et al., 2017), and this anomaly would be due to a melting point in the form of a “Y” below the crust (Fitton, 1980). The negative gravimetric anomalies observed in the northeast and southeast of the region would represent old syn-tectonic granitic rocks to which are added felsic gneisses and meta-sediments which constitute the crystalline basement (Toteu et al., 2004; Nzenti et al., 2006; Reusch et al., 2010; Anderson et al., 2014). According to Jacobsen (1987) and Kebede et al. (2020), when the gravity field is upward continued to a height z, it maps the sources located at and below the depth z/2. Since the Bouguer anomalies continuing upward to 20 km represent the regional field and were subtracted from the Bouguer anomaly map to obtain the corresponding residual, our results are related to the sources located at and above 10 km.

The first-order derivative maps (Figure 6) enhance the geological information contained in the residual map. The Z-derivative shows some positive anomalies that were not visible on the residual map east of the region. The HG and AS maps (Figure 7) isolated the different sources that were arranged as a block and placed the gravity anomalies directly above their sources. Since the THG and AS use the derivative amplitude to outline the edges, they cannot balance anomalies from shallow and deep structures. In other words, these methods cannot determine geological boundaries located at different depths at the same time. Both methods are dominated by large-amplitude responses from the shallow structures. In addition, for thin or deep sources, the peaks of the AS are shifted inward from the edges, making the structural bodies seem smaller than they are, as reported by Pham et al. (2020a), Pham et al. (2020b).

The TDR map (Figure 8A) highlights the different gravimetric domains, while the HG_TDR map (Figure 8B) highlights the different contacts that delimit these different domains. It should be noted that the contacts obtained from the TDR and HGTDR are connected, complicating the geological interpretation (Eldosouky et al., 2022e). The EHGA filter is effective in highlighting a wide range of density structures in southwestern Cameroon. Since the EHGA is based on the ratio of derivatives of the horizontal gradient, it can balance anomalies from different structures. Thus, the use of this method brings clearer images for all the edges of both the shallow and deep sources. In other words, the EHGA can outline the geological boundaries located at different depths at the same time. In addition, the EHGA map can avoid bringing false information in the edge maps, as reported by Pham et al. (2020a) and Ghomsi et al. (2020). Thus, we believe that the edges in the EHGA map are suitable for highlighting geological contacts as faults.

The source edges determined by the MSHDVD method represented in Figure 9 highlight most of the contacts shown on the EHGA map. The presence of a wide range of structures in the MSHDVD map is also verified by the results of the EHGA. So, we can observe in Figure 9 that these contacts bound the anomalies. Thus, contacts that form closed contours delineate intrusions, while those that are linear determine the presence of structures like faults. The combined analysis of the maps in Figures 8, 9 highlights the geological structures in this region. We also compared the density structures determined in Figures 8, 9 to the geological structures in Figure 2 (Figure 11). The present study is the first of its kind to accurately determine the circular margin of the continental and oceanic extensions of Mount Cameroon, while perfectly illustrating the Bao, Bomana, Tiko, and Ekona faults, as well as highlighting the strike–slip faults at the top as simulated by Mathieu et al. (2011) and Kervyn et al. (2014). We can see that our results not only highlighted some structures that already existed according to the observations of the geological map but also highlighted new faults.

7 Conclusion

In this study, we separated the regional/residual anomalies from the Bouguer anomaly data to obtain the best map of residual anomalies for the southwestern part of Cameroon. Then, we applied filters such as the HG, AS, TDR, HG_TDR gradient, and EHGA to the residual anomaly data to highlight the geological source edges. Finally, we used the MSHDVD method to highlight the same contacts. The main results of this work show with precision the region’s major lineaments, which in turn made it possible to highlight the CVL, intrusions of rocks of low densities known as syn to late tectonic granitoid, and faults of various directions. These results are comparable to those of the existing one and corroborate the information that the basement of the studied region would be made up of Precambrian volcanic rocks with a large concentration of basalt and gneiss. The observed geological structures show that the tectonic activity in this region is still ongoing. The current study is the first of its type to precisely define the circular boundary of Mount Cameroon’s continental and oceanic expansions, while also perfectly displaying the Bao, Bomana, Tiko, and Ekona faults and highlighting the strike–slip faults near the peak. However, it would be important to delineate the depths of the structures that form the CVL by quantitative analysis by inversion and/or by 2D or 3D modeling. Also, the use of remote sensing data will make it possible to highlight a structural map of the said region precisely.

Data availability statement

The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.

Author contributions

ARN contributed to writing, scientific development and early computations of this study. FEKG contributed to writing, result interpretation, scientific development, review, data curation and figure preparation. LTP contributed to writing, result interpretation, scientific development, review and performed the computations. AME contributed to writing, result interpretation, review and scientific development. ZA, JDK, ABKF, and RN contributed to writing and scientific development. KA, MSF, and PA reviewed, provided critical feedback and helped shape the research.

Funding

This research was supported by Researchers Supporting Project number (RSP-2021/249), King Saud University, Riyadh, Saudi Arabia.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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