2.5-D Earth Crust Density Structure Modeling of the Central Part of Cameroon Using Gravity Data

The knowledge of areas of high and low geophysical densities is of paramount importance to better understand the geodynamic, tectonic and geomorpho-logic evolution of the earth. Several geophysical methods have been devel-oped to achieve this, including a 2.5 D modeling from gravity data, which is the approach used in this work and whose aim is to highlight the causative geological structures of these contrasts in the central part of Cameroon. This zone extends between latitudes 3˚ and 7˚ North and longitudes 11˚ and 16˚ East. Several filters were applied to the gravimetric and topographic data using the Oasis Montaj software from geosoft in order to develop the different anomaly maps and Grav2dc in order to develop the different geological models of the subsoil. In the final analysis, it appears that this part of Cameroon suggests the presence of granitic rocks and sedimentary rocks whose density contrasts vary between −0.19 and −0.205 g/cm 3 observed in the localities of Banyo, Ngaoundere and Abong-Mbang. We also observe the presence of gneiss and volcanic rocks of contrast density varying between 0.0522 to 0.0534 g/cm 3 in the locality of Tibati, and granitic intrusions in addition to basic rocks of contrast density varying between 0.285 at 0.29 g/cm 3 in the localities of Yoko, Bertoua and Belabo.


Introduction
The Cameroon Pan-African chain contains rocks of different nature in its base-ment capable of creating gravity anomalies, which can be observed from the surface of the Earth. This chain encompasses central Cameroon which is our study area. Central Cameroon has a complex tectonics marked by the presence of large Precambrian faults, known as the Cameroon Shear Zone (Ngako et al.) [1], whose reactivation controlled the establishment of the sedimentary basins of the Mbere and Djerem, and the development of intraplate volcanism along the Cameroon Line. This region has already been the subject of several geophysical studies, the main results of which suggest the presence of a thin crust (Dorbath et al. [2]; Poudjom [3]), intruder of non-flush igneous rocks, of a basic nature, which would have been implemented thanks to the reactivation of the Cameroon Shear Center (Noutchogwe [4]; Kande [5]). The lithosphere would also be reduced following a rise in the asthenospheric fluid in a context of incipient rift (Poudjom [3]; Poudjom et al. [6]). The work of (Noutchogwe [4]) suspects the presence of basaltic rocks, gneiss, granite and sediments in this area precisely in Adamawa.
From this work, we ask ourselves what could be the nature of the rocks which structure this chain and more precisely our study site. Our work essentially proposes 2.5-D type basement models which present the different density contrasts of the bodies which contribute to the structuring of the whole region. To do this, we used the separation method to produce gravity maps and models. In the final analysis, we noticed that in the localities of Banyo, Ngaoundere and Abong-Mbang, the gravimetric anomalies observed suggest the presence of granitic and sedimentary rocks whose density contrast varies between −0.19 and −0.205 g/cm 3 .
Also, in the locality of Tibati, we observed the presence of gneiss and volcanic rocks with a density contrast varying between 0.0522 and 0.0534 g/cm 3 and in the localities of Yoko, Bertoua and Belabo, granitic and basic intrusions density contrast vary between 0.285 to 0.29 g/cm 3 .

Geology and Tectonics of the Area
The study area is bounded to the south by the 3˚N parallel, to the north by the 7˚N parallel, to the west by the 11˚E meridian border with Nigeria, to the east by the 17˚ meridian E (Figure 1). It covers four regions, including a part of the South, East, Center and Adamawa, the main cities of which are: Yaounde, Ngaoundere and Bertoua. This area has been the subject of several geological and geophysical studies, the main results of which are as follows: After recognition surveys carried out in central Cameroon in the 1950 (Guiraudie [8]; Lasserre [9]), the results of which are compiled in the geological map of the United Republic of Cameroon, more recent data have been obtained following work carried out on the central domain of the Pan-African chain of Cameroon (Soba [10]; Ngako et al. [1]; Ngako [11]; Nzenti et al. [12]; Ngnotué et al. [13]; Toteu et al. [14]; Kapajika [15]) and the Cameroon Line in Adamawa (Temdjim [16]; Dautria and Girod [17], Nono et al. [18], Menard et al. [19]). These detailed studies generally applied different  [20]). The CVL is formed at the base of Pan-African rocks made up mainly of schists and gneiss intersected by granites and diorites (Déruelle et al. [21]); • The Adamawa Plateau is an area that was raised after the Cretaceous activities on the northeast side of Cameroon (Nnange et al. [22]). It has a sedimentary section with a series of small synclines; • The CASZ is another characteristic tectonic zone extending from Darfur in Sudan to the Adamawa Plateau (Dorbath et al. [23]). It is characterized by high density crust rocks (Ayonghe [24]). Its extension to the Southwest is known as the Foumban Shear Zone (FSZ). The lithosphere is thin under the CASZ with a rise in the asthenosphere (Plomerova et al. [25]). The Sanaga Open Journal of Earthquake Research Fault (SF) is another dextral shear zone parallel to the CASZ (Dumort [26]).
The border between the Pan-African belt and the Congo Craton (CC) occupies part of southern Cameroon and progresses towards the north of the Central African Republic. The CC consists mainly of Archean rocks with certain resedimented materials formed in the Paleoproterozoic.
The Pan-African chain is the area that lies between the West African craton in the North West and the Congo craton in the South. Rocks such as: syenite, post and syn-tectonic granite with an extended part in the Center, East and Adamawa regions. The gneiss and migmatites spread through all the regions of the study area, mica schist and volcanic schist spread around the Center, South and the East regions, making this area to be amongst those that have been subjected to Pan-African tectonics, whose geochronological ages show a rejuvenation of 500 -600 Ma. The Djerem and Mbere basins belong to the central geodynamic domain of the north equatorial pan-African chain in Cameroon (Nzenti et al. [27]) or the Adamawa-Yaounde domain of the pan-African chain (Toteu et al. [28]).
These basins are located on the southern margin of the Adamawa plateau or the Cameroon Shear Center (CSC), which seems to mark the paleogeographic limit between a northern domain with characteristics of an active margin (Ngako [11]) and a predominantly continental southern domain (Nzenti et al. [27]).
The study site shows traces of the various tectonic events which marked the Pan-African geological period. Geochronological work allows the evolution of the northern edge of the Congo craton to be split into two major tectonic epi- ling the placement of many so-called "syn-tectonic" granites (Ngako et al. [29]; Nzenti et al. [12]). The Cretaceous region is marked by extensive tectonics in relation to the opening of the South Atlantic (Ngangom [30]; Popoff [31]).

Gravimetric and Topographic Data
The gravimetric data used in this study come from the Earth Gravitational Model, EGM08. These satellite data have been corrected for long wavelengths (greater than 300 km) of anomalies. The topographic data were obtained from the model of the structure of the earth's crust in the area modeled by Eyike and Ebbing [35].

Methodology
We used the separation method in this work. To avoid the subjectivity inherent in graphic methods, we opted for the least-square analytical method. The principle consists of constructing a polynomial equation of high order, which generates an analytical surface and adapts to an experimental surface by the method of least squares. This analytical surface represents the region, and the important variations of variable compared in this regional make it possible to separate the masses which can correspond more or less to buried sources. Let where N is the order of the polynomial and m B are the coefficients to be determined.
Equation (6) can also be written in the form: For a fixed value of N, we have We denote by the difference between the homologous points of the experimental and analytical surfaces respectively and by 0 N the number of stations i P where the Bouguer anomaly is known. Adjusting the surfaces consists of minimizing the quadratic deviation: This leads to: Taking into account (4-2), we finally have: We then obtain a system of equations with unknowns. The unknowns being the coefficients m B of the polynomial ( ) , i i F x y of order N . Once the coefficients have been determined, we calculate the regional analytical anomaly, and we deduce the residual: The calculations were carried out using the FORTRAN program 77 "POLYFIT" To constrain the interpretation, we proceeded to a two-layer modeling, taking into account our knowledge of the geology of the region. The density contrasts were chosen by referring to the ranges of average rock densities existing in the literature (Darly [38]; Sydney and Clark [39]; Telford et al. [40]).

Bouguer Anomaly Map
The simple Bouguer anomaly map shown in Figure 3 was produced using the

Isostatic Residual Map
The isostatic anomaly is obtained using a complex calculation process. It consists first of all in considering the Airy-Heisanen compensation model while taking into account parameters such as the density of the crust, the density contrast and the density of the mantle so the values are respectively 2.67, 0.66 and 3.27 g/cm 3 , then calculate the crustal root thickness of compensation followed by the topographic compensation effect at each of the measurement points. We deduced the isostatic anomaly by applying these corrections to the Bouguer anomaly; its residual and regional components are obtained using a separation method.

Choice of Profiles
In this work, we have chosen 11 profiles on the residual anomalies map in the N-S direction, perpendicular to the main direction of the network of isogal lines ( Figure 5). These profiles are separated from each other by a distance of 50 km, and each has a lenght of 500 km.

Modeling
• Geological model according to the profile P 1 .   • Geological models using the profiles P 5 and P 6 .
These models represented in Figure 8 reveal the presence of intrusions of basaltic and metamorphic rocks in sedimentary rocks.
• Geological models from the profiles P 7 , P 8 , P 9 , P 10 and P 11 . Figure 9 represents the geological models of subsoil calculated according to profiles P 7 , P 8 , P 9 , P 10 and P 11 . These models only reveal the presence of intrusions of volcanic rocks in the sediments, except for the P 11 profile where we also observe an intrusion of metamorphic rock which would be gneiss.

Discussion
The   . Geological models from the profiles P 7 , P 8 , P 9 , P 10 et P 11 . to the crust. This intrusion could probably be responsible for the vast anomaly observed in the center of the region and could be a consequence of the collapse of the basement of the region; • Analysis of the Bouguer residual anomaly map and its profiles along the N-S directions show areas of negative anomalies that could be a basement collapse and in the results of Koumétio et al. [41], he suspected a collapse of the basement and intrusions of rocks buried in depths ranging from 3 to 18 km in this area; • The following models profiles P 1 , P 2 , P 3 and P 4 suggest the presence of granite rocks and sedimentary rocks in the locality of Banyo and Abong-Mbang. Also, gneiss intrusions of volcanic rocks in the locality of Tibati, and granitic intrusions and basaltic rocks in the locality of Yoko. In the work of Noutchogwe [4], he suspected the presence of basaltic rocks, gneiss, granite and sediments in this area, especially in Adamawa; • Models of profiles P 5 , P 6 , P 7 , P 8 , P 9 , P 10 and P 11 suggest that the study area could contain granite rocks and sedimentary intrusions that extend to the Djerem basin; sedimentary intrusions at Abong-Mbang, Yokaduma, Nanga-Eboko; gneissic formations near Meiganga and granito-basaltic formations in the localities of Bertoua and Belabo. The results of Noutchogwe [4] suggest an evolution of magma in relatively superficial "chambers", between base rocks separated by faults and both the presence of rocks such as Gneiss, granite, sediments, basalt, observed from residual anomalies.
When we make the comparison between the negative anomalies of long wavelengths observed and the topography of the study area, characterized by an average altitude of 1100 m, the first idea that comes to mind about the origin of this anomaly is that of the isostatic compensation, which would have been affected by the sinking of the crust in the heavier upper mantle. However, seismological studies by Dorbath et al. [2] and Stuart et al. [42] have shown that the thickness of the crust is normal (33 km) in Adamawa and more reduced (23 km) when moving north. The same work also revealed the existence of an abnormally light structure in the upper mantle between 80 and 140 km. These results corroborate the gravimetric work carried out by Poudjom-Djomani [3] then Poudjom-Djomani et al. [6] who suggested a thin lithosphere under the Adamawa, following a lighter asthenospheric ascent of about 40 km. The great negative anomaly of Adamawa was therefore attributed to the resulting bulge while the zone of positive anomaly, narrower and centered on the plateau, is attributed on one hand to the thin crust and on the other hand, to the existence of non-flush magmatic pockets (Noutchogwe [4].

Conclusions
Analysis of Bouguer's anomaly map of his transformed maps and profiles made it possible to conclude that: The geophysical characterization of the gravimetric body in the study area integrates the results of gravimetric interpretations. The regional-residual separation of the Bouguer anomalies allowed us to establish a map of residual gravity anomalies essentially reflecting the gravitational effect of the sources found in the upper crust. The geological and tectonic context of the study area, located in the fault zone of the Cameroon Shear Center, allowed us to identify these intrusive bodies with basalts from the Cameroon Volcanic Line.