Evidence for upper mantle intrusion in the west African coastal sedimentary basins from gravity data: the case of the southern part of the Douala basin, Cameroon

The Bouguer anomaly map of the region between latitudes 30N and 3045’N and longitudes 9030’E and 10010’E and which forms the southern part of the Douala Basin, shows ring-like positive contour lines. The Bouguer gravity profiles obtained across the gravity anomaly contour lines in the region have been interpreted using 2D1/2 gravity modelling. The results reveal that in the southern part of the Douala Sedimentary Basin, two major structures exist: a half-dome of mantle material explained by isostatic compensation and a pillar of high density rocks probably representing an upper mantle intrusion down to a depth of about 14 km. The similarities of these results with those obtained in the coastal sedimentary basin of Mauritania-Senegal suggest a more extensive movement that would have affected the whole of West African coast. KEYWORDS: Bouguer anomaly, dome, graben.


INTRODUCTION
The region under study lies between latitudes 3 0 N and 3 0 45 ' N and longitudes 9 0 30 ' E and 10 0 10 ' E ( Fig.1). It includes part of the Congo Craton to the East and the southern part of the Douala Sedimentary Basin to the West. The basin has the form of a half-graben (Njike, 1984). However, the detailed structure around the crust-mantle limit under this basin is poorly defined.
The Bouguer anomaly map produced by Legeley-Padovani et al. (1996) shows a group of isogals oriented NNW-SSE to N-S within the study area (Fig.2). A zone of positive anomalies reaching a maximum value of 30 mgal is present in the region. The gravity anomaly contour lines are ring-like and present an E-W gradient of 2.2 mgal/km approximately. This could be due to an uplift of the dense material that forms a dome with a small diameter. However, this uplift of the mantle formation could have been expected to have the form of a half-dome since the basin has the form of a half-graben.
The aim of this study is to carry out gravity modelling along some profiles using the 2D1/2 IGAO modelling program (Chouteau et Bouchard, 1993) in order to better understand the cause of the Bouguer anomaly observed in this region.

GEOLOGY AND TECTONIC SETTING OF THE REGION
Outcrops over the part covered by the Congo Craton form a metamorphic Precambrian structure ( Fig.1). The basement is probably syenitic and shows an overall SSW-NNE orientation for the geological structures in the region (Koumetio et al., 2001;Koumetio, 2004).  , 1996) showing the profiles: P 1 , P 2 , P 3 .
According to Njike (1984) and Manguelle-Dicoum et al. (1993), the Douala Basin has a gneissic basement. The basin has the form of a half-graben presented as a distributive fault and there is a progressive uplift of the basin basement from the west to the east; going from depth of 8000m to 3000m approximately. The creation of the coastal Douala Basin could have been controlled by an E-W distension that could have provoked the complete rupture between the African and South American continents giving rise to the opening of the South Atlantic. This explains the halfgraben form of the Douala Sedimentary Basin according to Reyre (1964), Hoffman (1971) and Njike (1984).
The 2D½ IGAO gravity and magnetic program by Chouteau and Bouchard (1993) was used to carry out the modelling. This program calculates the theoretical Bouguer anomaly created by proposed structures and plots a graph, which is then matched with the observed anomaly.
Through the modelling, an optimal density value was retained for each formation (Table 1) with the constraint that this density value lies within the range of possible density values given in the literature (Dobrin, 1976;Telford et al., 1981). Density contrasts for each formation were calculated relative to the reference density value of 2.67 g/cm 3 for the crust. These density contrasts are given in Table 1. The models of the three profiles (Figs. 3i, 3ii and 3iii) show that the sediments extend from the Coast to the western margin of the Congo Craton which have syenitic basement. The depth of the sediments vary as one goes from west to east, from 6 to 3 km on profile P 1 ; from 8 to 3 km on P 2 and from 8 to 2 km on P 3 . These sediments lie on a gneissic basement. The top of this basement is presented as a distributive fault. The gneissic basement presents an uplift of the upper mantle formation. The top of this structure is at the depth of about 14 km on profile P 1 , 13 km on P 2 and 15 km on P 3 .

DISCUSSION OF THE RESULTS
The upper mantle formation seen on our geophysical model could be the result of the association of two different geological structures. One is a halfdome of mantle formation in accordance with the halfgraben structure of sediments in response to isostatic compensation. The other is a pillar of high density rocks (Fig.4). Considering the density value of this structure alone, it can represent a pillar of upper mantle formation, of gabbro or of eclogite, rocks whose density can be up to 3.5 g/cm 3 (Telford et al., 1981). In accordance with the fact that the thickness of the crust is generally greater in stable zones (Craton) than in mobile zones and considering the fact that the basement of the pillar of dense material merges with the limit between mantle and crust of the Congo Craton, this pillar of rocks may represent an upper mantle intrusion in the crust. The top of this pillar of upper mantle formation is at the average depth of 14 km.
The margin of the Congo Craton in our study region coincides with the eastern margin of the Douala Sedimentary Basin. This limit is associated with a seismic zone parallel to the volcanic line of Cameroon (Tabod et al., 1992). It could have been thought that the pillar of dense material is a consequence of volcanic activity but the absence of plumes observed at surface does not favour this explanation. There are similarities between our study region and the one studied by Roussel and Lecorche (1983) in Mauritania. The latter has to the east, part of the West African Craton and to the west, part of the coastal sedimentary basin of Mauritania-Senegal which extends into the Atlantic Ocean. The Bouguer anomaly map in this region shows many ring-like positive gravity anomaly contour lines. Roussel and Lecorche (1983) have interpreted this gravity anomaly in the Senegal-Mauritania region as due to an uplifted block of mantle material down to a depth of about 15 km, a depth comparable to that found in this work. These similarities suggest a more extensive movement that would have affected the whole of West African coast or suggest that the uplift of dense material is recurrent around the margins of Craton-sedimentary basins. A study of the correlation between the high regional anomaly and the boundaries of the Craton and the sedimentary basin will bring more understanding to the modelled features.

CONCLUSION
The regional gravity anomaly data has revealed in the southern part of the Douala Sedimentary Basin, the existence of two major structures: a half-dome of mantle material probably explained by isostatic compensation and a pillar of high density rocks probably representing an upper mantle intrusion up to the depth of about 14 km. The similarities of these results with those obtained in the coastal sedimentary basin of Mauritania-Senegal suggest a more extensive movement that would have affected the whole of West African coast.