The tectonic setting of north-western Cameroon includes Pan African collision tectonics [14], Cretaceous rifting forming the WCARS [22] [23] and the CVL [24] - [31]. The dominant outcropping lithologies of north-western Cameroon are Proterozoic metamorphic, igneous and sedimentary units related to the Pan African orogeny of central Africa (Figure 1(a)). This orogeny consists of a mobile belt formed by the collision of several Precambrian terranes [26]. The Pan African belt in Cameroon is bordered on the south by the Congo Craton and by the Benue Trough on the north-northwest. The belt was formed by tectonic interactions of portions of the West African and Congo Cratons [26] (Figure 1(b)). These interactions are first represented by eastward dipping subduction in eastern Cameroon followed by a collision between the Nigerian Shield and the West African Craton [14]. However, the tectonic evolution along the northern border of the Congo Craton still rises questions due to the lack of oceanic rocks, but structural and geochemical evidence points to a collisional boundary [27]. This lack of agreement has led to a number of different tectonic models that all involve collision between various blocks [28] [29]. Additionally, at least two major strike-slip tectonic episodes have been reported that are neither coeval nor post-date the major collisional events [26] but are most likely related to the collision between the Congo Craton and some blocks to the northwest of the craton.

The Pan African belt in Cameroon consists of three main lithological units: 1) the Poli-Lere Group, 2) the Adamawa Domain and the 3) Yaounde Group [26] (Figure 1(a)). Of these, the Poli-Lere Group is the most prevalent within the study area (Figure 1(a)) with the Adamawa Group occurring in the southern portion of the study area. The Neoproterozoic Poli-Lere Group consists of volcanic and sedimentary origin medium- to high-grade schists and gneisses, formed in a magmatic arc context probably within a back-arc basin [29] and that pre-, syn- and post-Pan African tectonic calc-alkaline granitoids [30]. Post-Pan African tectonic alkaline granitoids contain mafic and felsic dykes that are cross-cut by intrusive granites and syenites. These intrusions intrude the schists and gneisses, and form a NNW-trending line of batholiths [14]. Overlying the above units are unmetamorphosed sediments and volcanic rocks contained in several small basins [17] [31].

Three tectonic events affected the structural development of northern Cameroon, particularly the Poli area. These include a crustal shortening and thickening event caused by collision between Neoproterozoic blocks followed by syn to post-orogenic shear tectonic movements [26]. The crustal shortening event is seen as gentle folding outcropping in isoclinal folds with flat-lying foliations which were formed by thrust faulting (Figure 1(a)) [26]. Later compressional deformation formed vertical and NNE-trending foliations, and tight upright to recline folds [32].

The tectonic phase of compression and late stage shear zone development was followed by NE-SW to N-S sinistral shear zones. The Tchollire-Banyo (TBSZ), Gode-Gormaya (GGSZ), and Balche (BSZ) shear zones are the major sinistral shear zones that are mainly represented by mylonites (Figure 1(a)). The BSZ is classified by a secondary shear zone that cuts across the GGSZ but these are considered coeval with the NE-trending shear zones despite trending in a different direction [14].

The WCARS is a series of extensional and transtensional basins that extend from Nigeria and Cameroon eastwards into Chad through Kenya and northwards through Lake Chad into southern Algeria [23]. These basins were active [22] and, in Nigeria and Cameroon are represented by the thick clastic sediments of the Benue Trough (Figure 1(a), Figure 1(b)). Within the study area, the Cretaceous clastic sediments [33] and alkaline plutonic and volcanic rocks [34] are scattered throughout the study area (Figure 1(a)). The origin of the alkali igneous rocks is controversial, but most models are related to one or two hot spots in the Mesozoic [34].

The CVL (Figure 1(b)) overlies the Proterozoic basement in Cameroon [21] and covers over a 1600 km long Y-shaped trend of Cenozoic volcanoes that extend to the southern extent of the study area (Figure 1(b)). These volcanoes and sub-volcanic complexes [24] trend from the ocean-continent boundary in the Gulf of Guinea into the northern Cameroon. The volcanoes consist of mainly alkali basalts and some more evolved phonolites and trachytes with an inferred asthenospheric source [21]. Seismic studies indicate that low velocity nearly vertical P- and S-wave zones extend to at least 300 km in depth beneath the CVL, suggesting that edge flow convection against the Congo Craton may be responsible for these anomalies [20] [25]. These low velocity zones are more prevalent in western and northwestern Cameroon implying that these regions might be more susceptible to future volcanic eruptions.

A series of aeromagnetic surveys were carried out in 1970 by SURVAIR with a flight height above the topography of 235 m and 750 m flight line spacings oriented at N˚135. Each individual survey was corrected for diurnal variations and was gridded into one coherent dataset. Then the regional magnetic field of the Earth or the IGRF (International Geomagnetic Reference Field) was removed from the dataset to produce a residual magnetic dataset. Then the residual magnetic dataset was gridded at a 0.75 km spacing interval and displayed as a color image (Figure 2). Next, the gridded TMI data were reduced to the equator (RTE) using inclination and declination angles of the IGRF of January 1, 1970 of −1.01˚ and −3.96˚, respectively. The RTE anomaly map (Figure 3) contains both small wavelength anomalies, high amplitude anomalies formed by magnetic lithologies at the surface or at depths near the surface, and longer wavelength anomalies related to deeper magnetic sources. To amplify the effect of the deeper source or longer wavelength anomalies, the RTE data were upward continued to 1 km above the Earth’s surface. Figure 4 shows the upward continued RTE map.

The RTE magnetic map (Figure 3) has anomaly values ranging from approximately −77 to 300 nT. These values are in general reflected by variations in the lateral variations in the amount of magnetite within the crustal rocks. Within northwestern Cameroon, these anomalies can be usually correlated with the Proterozoic granitic and metamorphic lithologies including metavolcanics, metaigneous rocks, gneisses and schists (Figure 1(a)). The RTE map contains mainly northeast trending minima and maxima anomalies that are roughly parallel to the strike-slip tectonic episodes that related to the Proterozoic collisional events [26]. The most prominent anomalies are a series of large wavelength, low amplitude magnetic maxima (anomalies 1 and 2) and slightly higher amplitude magnetic minima (anomalies 3 and 4) in the northern portion of the map (Figure 3). The magnetic maxima are caused by lithologies with lower magnetic susceptibility content. The area of anomalies 1 and 2 occur over the medium to high grade gneisses of the NW Cameroon terrane and post Pan African sediments (Figure 1). The extent of this large amplitude magnetic maxima implies that the magnetic susceptibility of the gneisses is relatively low with some regions (small wavelength maxima within the region of anomaly 1) containing higher susceptibilities. This could be due to higher grade gneisses or isolated bodies of mafic metaigneous rocks.

The upward continued RTE map shown in Figure 4(a) exhibits a similar pattern of high and low magnetic anomalies that are seen in Figure 3. The upward continued RTE map is characterized by anomalies values ranging from about −164.47 to 288.31 nT. The upward continued RTE map is basically a smoothed version of the RTE map and contains prominent negative (long wavelength) anomalies distributed at the northeastern, central and southern parts of our study area. The persistence of the prominent anomalies with large wavelength, low amplitude magnetic maxima (anomalies 1 and 2) and slightly higher amplitude magnetic minima (anomalies 3 and 4) observed in the northern portion of the map (Figure 4(a)) which indicates that they are associated to deeper structures. In addition, the upward continued RTE map shows attenuated anomalies (anomalies 5, 8, 9 and 10) that are still related to upper crustal lithologies. However, all these anomalies are also present on the RTE map (Figure 3).

The CPD’s and depths to the bottom magnetic sources were determined using the methods of [4] where a 2D radial power spectra analysis of the upward continued RTE data was used (Figure 4(a)) utilizing a 50% overlapping of 30 × 30 km square blocks, as is illustrated in Figure 4(b). The study area was subdivided into twenty-one blocks. It is well known that the using of a small window width may produce an error in using the spectral methods in aeromagnetic interpretation [44]. Furthermore, it was showed that [12] window size is dependent on the depth to the bottom of a magnetic susceptibility source and that the window size should be two to three times the depths to be determined. Accordingly, the radial power spectra for different window widths were calculated, it is ranging from 10 to 55 km with a 5 km increase in window width and found that a 30 × 30 km window best delineated depths between 6 and 13 km. The 2D radial power spectrum (Figure 4(c)) of the upward continued RTE data for each window was then computed. Next, the long wavelengths of regional magnetic anomalies were removed using a first-order polynomial trend surface for each block, and the grids were expanded by 10% to lessen the edge effects of using the Fourier Transform in determining the power spectrum [45].

Following [4] [36] [37] and references therein, for each block, the depth to bottom of magnetic source in the present study is estimated in four main steps using [4] method:

1) compute the radially averaged power spectrum of the magnetic data;

2) firstly, pick two points to determine the slope on first spectrum (black line) in order to estimate the depth to the top of the magnetic source (Z_t);

3) afterwards pick two points to determine the slope on the frequency-scaled spectrum (red line) to estimate the depth to the centroid of the magnetic source (Z_c);

4) thus, deduce the depth to the bottom of the magnetic source (Z_b) using relation (1).

The results of the power spectrum analysis are shown in Table 1, on which the heat flow and geothermal gradient were deduced using relation (2) and (4). The estimated depths to the centroid range from 4.06 km for block 13 to 9.74 km for block 9. However, the depths to the top of magnetic susceptibility sources ranged from 1.74 to 5.73 km. Hence, the depths to the bottom of the magnetic susceptibility sources range (which was interpreted to be the CPD) from a minimum depth of 6.36 km, to a maximum depth of 13.72 km. The spectrally determined depths reflect the average depths beneath each block [46]. The ranges of the estimated depths to the top boundary of the magnetic sources are well correlated with the other regional work in and surrounding the study area

[18] [46] [47]. From seismic refraction studies, the maximum depth to the basement in the central sector of the WCARS is around 7 km [18].

Previous CPD studies [12] have shown that the CPD is related to the geological makeup of an area. The shallowest depth to the bottom of the magnetic susceptibility sources (CPD), ranging between 6.37 and 10.01 km, is obtained within the blocks 2, 3, 4, 5, 6, 7, 8, 13, 14, 19 and 21. These blocks are located in different regions of the study area including the northern portion (Hina Marbak, Gawel and north of Moutouroua), the southeastern portion (Kaele), southern portion (Guider) and southwestern portion (Mayo Oulo). The majority of these areas (e.g., Hina Marbak, Gawel and north of Moutouroua) (Figure 1(a)) are located where Lower Cretaceous sediments outcrop. Numerous lineaments and shear zones are located in this region [15]. These shallow depths may be due to magmatic intrusions which could be related to the CVL, and the other shallow depths are likely due to the rifting related to the Benue Trough [48]. [47] showed that this region is mainly related to the CVL, although this region has been affected by a failed arm of the Benue Trough. The thermal effect of the igneous activity on the Cretaceous sedimentary basins and the basement complex, led to local thermal anomalies observed in the region [49]. [4] using Japan as an example showed that CPDs less than 10 km occur in volcanic and geothermal areas, between 15 - 25 km in island arcs, and deeper than 20 km in oceanic plateaus, while [12] showed that in Precambrian terrains these depths are greater than 15 km.

From our analysis, four shallows (depths less than 10.38 km) CPD regions: SCPD1, SCPD2, SCPD3 and SCPD4 can be identified. These regions might be related to elevated heat flow in the study area. SCPD1 is in the northwestern part of the study area and is located near the Hina-Gawel area where it trends in a northwest direction. SCPD2 is located in the eastern part of the study area and is located in the Kaele area, approximately trending north-south. SCPD3 is located in the southern portion of the study area near the Guider neighbourhoods where it trends approximately north-south. SCPD4 is located in the Mayo Oulo area and also approximately trends north-south. However, the shallower regions of SCPD1 and SCPD2 could also be considered as a separate shallow CPD regions as this region is split into two parts by a broad northeast trending of deeper CPD region centered at longitude 14˚10'. This slightly deeper trend occurs over the mafic to intermediate metaigneous lithologies of the Mayo-Kebbi domain and the metavolcano-sedimentary units of the Poli-Lere Group (Figure 1(a)). This region also is the site of numerous thrust faults related to the collision tectonics of the Pan African belt and thus may be a region of thicker crust with lower heat flow values and may have been harder to rift during the formation of the Central African Rift system. The other shallow CPD regions are found in the SCPD3 and SCPD4 regions (Figure 5). These regions are adjacent to the inferred trend of the CVL and the elevated CPD may be related to magmatic activity within the CVL.

The deeper depths to the bottom of magnetic sources, ranging between 10.38 and 13.72 km, is found in the northeast portion of the study area (DCPD4, Mindif region) and in the southwest (DCPD3, Moutouroua, Guider, Bossoum). DCPD4 is within the Cretaceous sediments related to the Benue Trough but lies along trend to the deeper values mentioned above related to the thrust zone with the Mayo-Kebbi domain. DCPD3 is in a region of syntectonic granites and medium to high grades gneisses of the NW-Cameroon domain (Figure 1). Additionally, two other deeper regions (DCPD1 and DCPD2) are seen on Figure 5. This northwest trending zone occurs mainly over the Neoproterozoic gneisses of the NW-Cameroon zone (Figure 1) suggesting that the shallow regions to the northeast and southwest are related to small isolated geothermal anomalies. Recently, the studies done by [50] in Mindif-Kaele area revealed potential geothermal reservoirs at Djangal and Moundjou, located in northeastern portion of the study area (SCPD1, Mindif neighbourhood), which is corroborated by the CPD regions.

The shallow CPDs in regions SCPD4 and SCPD3 (Figure 5) can be associated with regions that have been extended in the Mesozoic during the formation of the WCARS which formed the Benue Trough. Regions SCPD1 and SCPD2 are not so readily interpretable as these regions occur over Quaternary sediments and Proterozoic units (Figure 1). Seismic S-wave tomographic images locate these regions over low velocity zones indicating that these shallow regions may be related to magma in the subsurface. However, the S-wave tomographic images were constructed by 25 broadband stations covering the entire country of Cameroon [25] and do not have the resolution to image small-scale differences in the S-wave velocity structure. The S-wave tomographic results [25] indicate that the main zone of the CVL is underlain by deep low velocity zones, but in northern Cameroon the low velocity zones are of a smaller magnitude and cover isolated areas. The most prominent low velocity region, which occurs over the Cretaceous sediments, is correlated to shallow CPD regions SCPD3 and SCPD4. Additionally, the P-wave velocity images, which are less sensitive to partially melted material, indicate similar but more subtle trends than the S-wave results. The P-wave images also indicate small amplitude low velocity regions under our shallow CDP regions SCPD1 and SCPD2. This lack of correlation may be due to other factors other than thermal variations such as the wide spacing of the broadband stations, or that the northern low P-wave velocity zones are more indicative lithological variations at depth than the S-wave results. In fact, the shallow CDP regions seen in Figure 5 may represent regions of potentially higher magmatic and/or geothermal activity that cannot be imaged by widely spaced broadband seismic experiments. To determine that cause of our shallow depths, more detailed crustal scale seismic studies in the form of seismic refraction or closely spaced broadband stations, and/or magnetotelluric studies are needed to confirm if these regions are indeed related to geothermal activity. In contrast, the deeper CPD regions (DCPD1-4) may be related to thicker crust [51] or are regions with less Quaternary magmatic activity. The CPD depths occur in general trends which are consistent with the prevailing tectonic environment, where a series of NNW to NW-trending extensional basins occur, caused by the strain of NE to ENE trending strike-slip faults coming from the Gulf of Guinea [22].

The determination of the surface heat flow requires a CPD and a thermal conductivity of the most common lithologies in the region. For the thermal conductivity, 2.5 W∙m⁻¹∙˚C⁻¹ was used based on the average thermal conductivity values for Proterozoic lithologies [52]. Using the CPDs from Table 1 and a Curie isothermal point of 580˚C, a geothermal gradient was calculated. The heat flow values were then determined by multiplying the geothermal gradient by the thermal conductivity using equation 8. The lowest heat flow value (105.68 mW/m²) was obtained for block 9 and the highest heat flow value (227.63 mW/m²) was obtained for block 4 (Table 1). The heat flow values are all greater than 100 mW/m² and worldwide heat flow values average between 80 - 100 mW/m². Thus, our heat flow values indicate that northern Cameroon has geothermal anomalous conditions [53]. In particular, the regions with heat flow values greater than 180 mW/m² are indicative of regions with geothermal potential. The region with the greatest geothermal potential is found in the northern portions of the study area, particularly in the neighbourhood of Hina Marbak where the heat flow value was the highest. The study area exhibits two main potential geothermal areas which are characterized by low and high heat flow. The high heat flow values are concentrated around the Hina-Gawel, Kaele, Guider and Mayo Oulo areas (Figure 5). The results also show that shallow depths and the attendant high heat flow values may be related to the effects of large-scale tectonic events. In northern Cameroon, the most likely magma rising event is the formation of the volcanic material in the CVL that has risen along the northeast-trending strike-slip faults. The known volcanoes occur just to the south of the study area [24] but a recent broadband seismic study indicates that potentially partially melted material may exist in isolated areas in northern Cameroon [31]. Heat flow studies in other regions have shown that the heat flow in sedimentary basins the amplitude of the temperature effect is larger for thermal events within the basin (intrusions or diapirism) [54] and in Proterozoic metamorphic and igneous terranes where the lithologies have an average higher thermal conductivities [55]. This same reasoning applies that high heat flow found in northern Cameroon may be indicative of wide scale thermal events. Potential regions for geothermal exploration are regions characterized by high heat flow, high temperature gradient, and therefore shallow depths to the bottom of magnetic susceptibility sources [56].

Figure 6(a) and Figure 6(b) show the variations of the geothermal gradient and heat flow with the calculated CPDs, respectively. The geothermal gradient and heat flow decrease significantly with the deeper depths to the CPD. The results obtained also indicate that the geothermal gradients in this study area vary between 42.27 and 91.05˚C/km; with an atypical maximum of 91.05˚C/km stands out in the in the neighbourhood of Hina Marbak.