Validation of gravity data from the geopotential field model for subsurface investigation of the Cameroon Volcanic Line (Western Africa)
© The Author(s) 2018
Received: 14 December 2017
Accepted: 4 March 2018
Published: 13 March 2018
Volcanic flows generally cover areas where eruptions occur, which bury structural features like faults making it difficult for geological surveys to be carried out. However, geophysical studies enable to highlight structural features and understand the subsurface structure in such areas. For example, Noutchogwe (2010) presented a close correlation between the lineaments inferred from magnetic anomalies, the sites of thermo-mineral springs and the hydrographical network in Adamawa Cameroon. Using gravity data, Jaffal et al. (2010), Fan et al. (2014) and Abate Essi et al. (2017) showed the importance of geophysical lineaments in studying ore bodies and mineralized areas. Therefore, geophysical investigation is helpful to delineate outcropped or buried faults. The aim of this paper is to use gravity data derived from the Earth Gravitational Model EGM2008 to investigate the subsurface of the CVL with an emphasis on structural features.
Geological and structural setting
The study area comprises the main domain of the continental part of the Cameroon Volcanic Line (CVL). This volcanic line, which crosscuts the Pan-African Fold Belt, is also surrounded at its southern-eastern edge by the Congo craton (Fig. 1). The CVL is a 1600-km-long alignment of Cenozoic to recent volcanic massifs and plutons striking N30°E (Le Maréchal 1976; Déruelle et al. 1991, 2007). The oceanic section of the CVL lies within the Gulf of Guinea consisting of the islands of Pagalu, São Tomé, Principe, and Bioko, while the continental part is made of two main plateaus (Biu and Ngaoundéré) and several volcanic mountains. Main mountains are mount (Mt) Cameroon (4095 m) which is the highest and the most active volcanoes of the CVL, mainly formed by alkaline basalts (Hedberg 1968; Déruelle et al. 2007); Mt Manengouba (2420 m) characterized by basaltic, trachytic and rhyolitic formations; Mt Bambouto (2679 m) with alkaline basalts and trachytes, and Mt Oku (3011 m) consisting of transitional basalt, quartz trachyte and rhyolite flows (Tchoua 1974; Fitton and Dunlop 1985).
The continental structure of the CVL is marked by an alternation of horsts and grabens (Nkouathio et al. 2008). The horsts are made of large polygenetic volcanoes or volcanic plateaus, characterized by complete magmas series, whereas the grabens are monogenetic volcanic fields displaying basic magmas suites (basanites, basalts, and accessory hawaiites) (Tamen et al. 2007). Van Houten (1983) and Ngounouno et al. (2000) interpreted the CVL as a volcanic and subvolcanic alignment resulting from hotspot activity. Fitton (1980, 1983) presented it as an active rift system produced by a thermal anomaly in the asthenosphere. Some authors like Déruelle et al. (1991), Moreau et al. (1987) and Nkono et al. (2014) described it as the consequence of the rejuvenation of a Pan-African N70°E fracture zone which took place at the opening of the Atlantic Ocean.
Another major shear zone is identified near Ngambe and Edea localities called the Rocher du Loup shear zone (RLSZ) by Ngako et al. (2008) or SW Cameroon Shear zone (SWCSZ) by Nsifa et al. (2013). It is described as a sinistral transcurrent deformation along the western border of the Congo craton.
Data and methodology
In this work, we used gravity data from the geopotential field model EGM2008 released by National Geospatial Intelligence Agency (NGA), which is an improved version of the Earth Gravitational Model EGM96. EGM2008 combines marine, airborne, satellite-altimetry-derived and terrestrial gravity data (Collignon 1968; Poudjom-Djomani 1993; Poudjom-Djomani et al. 1996) to model the global gravity field with a spatial resolution of 5 by 5 arc minutes. It is complete to spherical harmonic degree and order 2159 and contains additional coefficients extending to degree 2190 and order 2159 (Pavlis et al. 2012). The spherical harmonic coefficients of the EGM2008 are used to derive a geoid referenced to WGS 1984 and to calculate free air anomalies (Eyike et al. 2010). Assuming a density of 2.67 g cm−3 for Bouguer slab, we applied topographic correction to free air anomalies using the digital elevation model Etopo 1 (Amante and Eakins 2008) to obtain Bouguer anomalies.
The EGM2008 Bouguer anomaly map of the study area is presented in Fig. 4. The gridding method used to realize the Bouguer anomaly map is minimum curvature with a grid size of 0.01° (about 1.1 km). Ngatchou et al. (2014) have successfully experimented this grid size while studying the structure of crust beneath Cameroon from EGM2008. Gridding method generates interpolated surface analogous to a thin, linearly elastic plate passing through each of the data values with a minimum amount of bending.
Bouguer anomaly values range from − 255 to 198 mGal. From Manyemen to Hossere Mandam localities, negative anomalies (− 255 to − 80 mGal) appear along a corridor particularly trending NE–SW. The corridor of negative values coincides with the Cameroon Volcanic Line and a part of the sedimentary basin of Mamfe (which belongs to the southern part of Benue Trough). This corridor crosscuts an area marked by a relatively high Bouguer anomaly representing the granite–gneiss basement of Pan-African Fold Belt. The contact zone between Congo craton and Pan-African Fold Belt in Ngambe area is underlined by a positive anomaly with high amplitude (more than 100 mGal). The highest positive anomaly (greater than 150 mGal) is located in the Atlantic Ocean including Limbe and Mount Cameroon areas. From the analysis, it is possible to perform a correlation between the geological (Fig. 2) and Bouguer anomaly maps (Fig. 4).
Main gravity anomalies of residual map
Douala Sedimentary basin
Swamp and sedimentary alluvia (Sanaga River)
Mamfe basin (southern part of Benue Trough)
North of Takamenda (Nigeria)
Volcanic mountain of the CVL
East of Makak locality
Iron mineralization located at the northern edge of Congo craton
Successive circular trending NE–SW
Volcanic mountains of the CVL
Negative anomalies are nominally distinguished as N1, N2, N3 and N4. These zones referred as sedimentary basins (Fig. 5) or swamp zones, suggesting low-density materials. N1 anomaly brings out Douala sedimentary basin described as a Lower Cretaceous basin (Regnoult 1986; Nguene et al. 1992). Along Eseka and Bafia localities, negative anomalies named N2 expose low-density geological formations. This specific zone is intensely drained by Sanaga River and its tributaries, indicating that it is made of alluvias; thus, less geological outcrops are found. Both anomalies N3 and N4 represent the southern part of Benue Trough filled by cretaceous sedimentary deposits (Benkhelil 1986) of low density.
EGM2008 residual anomaly map reveals a positive anomaly (P5) with values higher than 100 mGal. In this area, the geological map (Fig. 2) exposes some granitoid intrusions. Mount Cameroon is located on a positive anomaly (P1). Similarly, successive located positive anomalies (P4) are appreciable in this volcanic area with the same orientation NE–SW of the CVL. Ngambe zone presents a circular positive anomaly around the contact between Congo craton and Pan-African Fold Belt corresponding to a particular garnet gneiss in the previous geological map. Positive anomaly (P3) may display iron mineralization described by Ngoumou et al. (2014) around the locality of Eseka at the northern edge of Congo craton.
Bouguer anomaly map obtained from the geopotential field data is filtered. The methodology used in this work involves a combination of techniques comprising upward continuation, horizontal gradient, maxima of horizontal gradient coupled to upward continuation technique and Euler deconvolution. This combination of techniques has the particularity in studying gravity signatures of subsurface geological features. The grid size of Bouguer anomaly map (0.01°) is maintained during the filtering.
The Bouguer anomaly map is smoothed with upward continuation technique. This operation consists of the application of a low passed filter that attenuates short wavelengths while amplifying long wavelengths (Jacobsen 1987). The Jacobsen’s theory suggests that the field resulting from upward continuation to a level of Z focuses on sources situated at a minimum depth of Z0 = 1/2Z. Thus, this method is suitable to study deeper and major crustal structure of the regions of interest.
Maxima of horizontal gradient coupled to upward continuation method
This method combines the two techniques mentioned above. This combination is used not only to bring out lineaments but also to evaluate different dips (vertical and oblique). It entails applying the upward continuation filter to the Bouguer anomalies at progressive heights and to determine the horizontal gradient of each upward continued distance (Blakely and Simpson 1986; Everaerts and Mansy 2001; Jaffal et al. 2010; Hadhemi et al. 2016). For each upward continued map, we represent essentially the maxima of the horizontal gradient in the map (Blakely and Simpson 1986). A displacement of maxima will correspond to the dip orientation. Thus, a vertical dip will display superimposed maxima of different altitudes.
Upward continuation for regional structure
Figure 7 presents the horizontal gradient of the study area obtained from the Bouguer anomalies of the study area (Fig. 4). Mount Cameroon and Ngambe localities expose heavy-density materials. Referring to geological map (Fig. 3), Mount Cameroon is made of extrusive volcanic deposits, while Ngambe area carries garnetiferous metamorphic formations. The eastern part of Sanaga River presents water flow related to a NE–SW fault.
Several horizontal gradient maxima (some are curvilinear and others are linear) are located along the Cameroon Volcanic Line. This result demonstrates that the CVL is a fractured zone and consists of mountains represented by circular signature. Moreover, Benue Trough coming from Nigeria is evidently revealed around Mamfe and Takamanda zones. Boundary of Manyemen gneiss is revealed in Fig. 2 although partially covered at the surface by volcanic flows of the CVL. Along the western part of Douala and Edea localities, we identify the limits of the Douala sedimentary basin.
Maxima of horizontal gradient coupled with upward continuation
A general overview of this map helps to assert that the littoral zone of the study area (SW of Edea, SW of Douala, SW of Mount Cameroon) shows lineaments dipping W to SW. The sedimentary areas highlight very few lineaments especially in Douala basin and Benue Trough (North of Efolofo and Mamfe areas). Concerning the CVL area, we note abundant fractures with both inclined and vertical dips. Deadly lakes (Nyos and Monoun) are located on this fractured area. The zone carrying Lake Nyos is characterized by the deepest maxima down to 10 km.
Validation of results
This work was carried out in an area where volcanic activities are still active (materialized by gas emissions in lakes Nyos and Monoun, and eruptions in Mount Cameroon). Based on the identification of known geological features, the main findings demonstrate the reliability of the gravity data derived from the Earth Gravitational Model EGM2008. Ground gravity data are usual tools to study the subsurface of terrestrial crust (Poudjom-Djomani 1993; Marcel et al. 2010; Jaffal et al. 2010; Hadhemi et al. 2016). The study area comprises volcanic mountains which are hardly surveyed by ground gravity campaigns. It presents therefore an actual challenge for a continuous spatial investigation. The Earth gravitational Model EGM2008 which integrates terrestrial and satellite gravity data enables to overcome the sparseness in gravity maps due to lack of data.
Upward continued maps highlighted a NE–SW trending regional structure which corresponds to the CVL direction (Le Maréchal 1976). Besides, subsurface formations of previous geological studies are well expressed through residual anomaly map. Gravity data derived from EGM2008 expose efficiently the geophysical response of the geology in the area of study. Located dense materials of volcanic formations like basalts (Telford et al. 1990) are aligned in the same direction with the CVL (N30°). This residual anomaly map presents obviously sedimentary basin characterized by low amplitude anomaly such as Douala basin filled by cretaceous deposits (Regnoult 1986; Nguene et al. 1992) and the cretaceous Benue Trough. Positive residual anomaly of Ngambe exposes the garnetiferous gneiss. Abate Essi (2010) studied the density of diverse geological formations of Pan-African Fold Belt and showed that garnetiferous gneiss of Yaoundé Group and especially where there is intense accumulation of garnet called garnetite or garnet rock has high density above 3 g/cm3. EGM2008 is therefore useful for geological investigation based on rock density variation. Furthermore, horizontal gradient maxima derived from upward continuation at different heights and deconvolution of Euler enable to characterize some structural linear features. These two last techniques present approximately similar limit depth of features at 10 km. The lineaments highlighted in this work are discussed in the next section.
Summary of main lineaments identified in the study area
Boundary fault continent—ocean
Limit Douala sedimentary basin—Pan-African belt
L15, L17, L19
Contact Pan-African Belt—Congo craton
Sanaga River fault
SW Cameroon SZ
Lineament of Bioko island
Granit anticlinal of SE Takamanda
Geological contact granite—high-grade gneiss
ENE–WSW to NE–SW
Foumban SZ (CASZ)
Lineaments L2 and L3 follow an ENE–WSW and NE–SW directions, respectively. They tally the Foumban shear zone representing the ending of Central Africa shear zone CASZ (Ngako et al. 1991, Njonfang et al. 2008). Sanaga River is delineated by lineaments L11- and L16-oriented NE–SW. Lineament L18 trending N–S superimposes the South-Western Cameroon shear zone (SWCSZ) and corresponds to a deep fault (Euler deconvolution displayed approximatively 8 km of depth). Besides, another deep fracture trending NW–SE is detected at the contact ocean-continent (lineament L20); therein, the maxima of horizontal gradient coupled with upward continuation method reveal its westward dip. At the east side of L20, the contact between sedimentary basin of Douala and Pan-African Fold Belt is put in evidence under L24. L22 is oriented NE–SW and represents a lineament crossing the Bioko volcanic island. The northern edge of Congo craton in contact with Pan-African Fold Belt describes a thrust front (Ngako et al. 2008; Toteu et al. 2001) whither L15, L17, L19 are highlighted with a NE–SW orientation. In the Pan-African Fold Belt around Hosere Mandam, the geological contact between granite and high-grade gneiss, put in evidence by Koch (1953), is confirmed in this study with lineament L1 trending NE–SW. In addition, NE–SW Benekuma fault, N–S Mundemba anticlinal, Bikoki anticlinal as well as anticlinal in Takamanda granite are identified in this work by L39, L34, L33 and L37, respectively. Sedimentary basin of Mamfe, the southern part of Benue Trough, underlines lineament L40.
The new lineaments revealed in this study added to the previous ones show that the study area is very fractured. The findings of this study provide an opportunity for more research on the CVL. Some of the previous lineaments (Fig. 3) have not been highlighted in this study. This may be due to the resolution of EGM2008 (5 arcminutes) which may not be suitable to locate and detect more detailed information of the subsurface. Other techniques such as seismic, aeromagnetic investigations, satellite imagery, ground geological verification, etc. can be integrated for more efficient results. However, this statistical result of lineaments (Fig. 11) confirms the reliability of EGM2008.
Geohazard and land-use: development planning implications
Numerous lineaments are highlighted in this continental part of the CVL. Many of them confirm earlier studies while other ones are revealed. They generally refer to faults or geological contacts as described above. These lineaments express locally weakness zones of the subsurface in the terrestrial crust. Tamen et al. (2007) suggest that lineaments in the basement rocks work as pathways for magma ascent. In the study area, two different types of volcanic activities still occur: volcanic eruptions in Mount Cameroon (latest eruptions in 2000, 1999, 1982) and the famous deadly gas emissions from maar lakes of Monoun (in 1984) and Nyos (in 1986). It is also important to point here that several other maars are numbered in this volcanic sector. Maar defines shallow volcanic craters with steep sides. Some of the CVL maars have been studied: Nyos (Lockwood and Rubin 1989), Barombi Koto (Tamen et al. 2007), Debunschar (Ngwa et al. 2017). The multitude of lineaments puts in evidence in this work attest of the vulnerability of this sector. Thus, special attention should be paid on this zone for geohazard prevention.
The filtering of gravity data derived from EGM2008 is efficient to explore the Cameroon Volcanic Line. Its NE–SW direction is the main structural feature revealed as confirmed by the rose diagram. In addition, important faults like Foumban (Central Africa) and SW Cameroon shear zones or the contact between Pan-African Fold Belt and Congo craton provide the reliability of this methodological approach. A correspondence is found between gravity anomalies and geological formations. This work corroborates once more the vulnerability of the CVL zone. The distribution of faults and maar lakes shows that special attention should be paid in this sector to prevent natural disasters such as gas emissions of “asleep” maars. The earth gravity model EGM2008 resulting of the combination of terrestrial and altimetry-derived gravity data is therefore advantageous for subsurface investigations in volcanic mountainous areas as the CVL where terrestrial gravity surveys cannot easily reach.
JM conceived of the methodology of data analyses and interpreted geophysical maps. JMAE contributed to the geological aspect of the paper and generated maps. JM and JMAE wrote the first draft of the paper. PN performed critics and improved the interpretation of results. OS participated in designing this study. EM supervised the work and revised the paper. All authors read and approved the final manuscript.
The authors are grateful to the BGI (Bureau Gravimétrique international) for their kind collaboration by providing EGM2008 data used in this paper as well as the anonymous reviewers for their constructive reviews which improved the quality of our manuscript.
The authors declare that they have no competing interests.
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