West African Rift System Research Articles

DRAINAGE BASIN MORPHOLOGY AND TERRAIN ANALYSIS OF THE LOWER BENUE RIVER BASIN, NIGERIA

The Lower Benue Trough is part of the Benue Trough of Nigeria and it consists of a series of rift basins which form a part of the Central West African Rift System of the Niger, Chad, Cameroon and Sudan Basement fragmentations. This research attempts to evaluate the geomorphological changes on the drainage basin interpreted using several techniques including the use of Remote Sensing and GIS. Several digital image enhancement techniques such as general contrast stretching and edge enhancement were applied to the Landsat image in ARCGIS, after which structures were mapped out on-screen using ArcGIS. The Digital Elevation Model (DEM) of the Trough was also generated from Shuttle RADAR Topographic Mission and used to enhance geomorphic features. The analysis carried out on the images revealed that the slope tool calculates the maximum rate of changes between each cell and its neighbors. The drainage density calculated shows that the drainage area = 86282.1km 2 , while the drainage density is 0.39872 km/km -2 . The result from the modeling concludes by revealing four sub-basins in the Lower Benue Trough with maximum thickness of 7-10km. Keywords: Benue, Remote Sensing, GIS, Geomorphology, River Basin

Topography of the Moho Undulation in Cameroon from Gravity Data: Preliminary Insights into the Origin, the Age and the Structure of the Crust and the Upper Mantle across Cameroon and Adjacent Areas

In this paper, the complete Bouguer anomaly data from Cameroon and part of the neighboring countries has been examined to compute the topography of the Moho undulations. This work is based on an improved filtering technique and an appropriate density contrast between the crust and the upper mantle. Comparison with seismic data shows that our Moho map defines better the continuity and morphology of the crust-mantle interface than the scattered seismic data in Cameroon. The present relief map, although may not give real depths at some areas provides a better surface correlation with the surface geology better than seismic techniques. Comparison between the Moho undulations and the topography maps reveals that the crust in Cameroon seems to not be in isostatic equilibrium. The Moho in the Central African Shear Zone (CASZ) must be linked with different dextral movements during the opening of the south Atlantic in the Cretaceous time. In the Chad basin, the Moho is associated to the opening of the central and south Atlantic ~130 Ma. In the case of the Congo basin, the Moho undulations are related to the post rift subsidence. The correlation between the Moho undulations map with the surface geology indicates that the actual morphology of the crust mantle interface in Cameroon can be related to the build-up of the West and Central African rift system dating back to the Early Cretaceous to Palaeogene, where the presence of intraplate tensional stresses reactivated previous shear zones of lithospheric weakness during the break up of Gondwana.

BURIAL HISTORY AND THERMAL MATURITY EVOLUTION OF THE TERMIT BASIN, NIGER

Reconstruction of the burial history and thermal evolution of the Cretaceous – Tertiary Termit Basin, a sub‐basin within the larger Eastern Niger Basin of Niger, indicates spatially and temporally variable conditions for organic matter maturation during the basin's multi‐phased evolution. Three episodes of tectonic subsidence which correspond to the observed fault mechanical stratigraphy within the Termit Basin are identified: Late Cretaceous, Maastrichtian to early Paleocene, and Oligocene. These episodes fall within the regional tectonic phases of the West African Rift System delineated by previous studies. The basin exhibits substantial heterogeneity in the magnitude of the tectonic episodes and in consequent thermal maturities. For this paper, 1D burial and thermal histories of eight widely dispersed wells in the Agadem permit area in the SW of the Termit Graben were modelled to investigate the maturation of organic matter in source rocks ranging from Santonian to Oligocene in age. The kinetic modelled maturities match with maturities based on Rock‐Eval Tmax values for four wells if present‐day heat flows are elevated. Future exploration strategies in the Termit Basin should take into consideration these heterogeneities in thermal histories and tectonic pulses, which may lead to the development of hydrocarbon accumulations with different oil‐gas compositions in different reservoir compartments.

Controlling factors of hydrocarbon accumulation in Termit rift superimposed basin, Niger

Abstract Based on the tectonic evolution and depositional characteristics of Central–West African Rift System and regional geologic background, the characteristics of oil and gas accumulation in the Termit rift superimposed basin were examined, and the factors controlling oil and gas accumulation there were discussed. The Termit basin is a rift superimposed basin formed by two stages of rifting in the early Cretaceous and Paleogene, where the Yogou Formation marine source rock is widespread. The two sets of reservoirs, the first member of the Paleogene Sokor Formation and the Cretaceous Yogou Formation, dominated by quartz sandstone, have relatively good porosity-permeability characteristics. They can form multiple sets of favorable caprock-reservoir assemblage below the regional caprock of the second member of Paleogene Sokor Formation. It is found that fault pattern and oil and gas migration pattern control the oil and gas accumulation patterns in the basin, including “Y-shaped” hydrocarbon migration and accumulation in the Araga graben, “multi-step” vertical hydrocarbon migration and accumulation in the Dinga faulted terrace, and the “composite” hydrocarbon migration and accumulation in the Fana transfer zone. Hydrocarbon enrichment in this basin was controlled by tectonic evolution of rift basins, that is, fault belts formed during the two stages of rifting controlled the horizontal distribution of oil and gas, while favorable reservoir-caprock assemblages in the sequences formed in the late rifting stage controlled the vertical hydrocarbon enrichment.

Igneous activity in the Bornu Basin, onshore NE Nigeria; implications for opening of the South Atlantic

The structure of igneous plumbing systems in circum-South Atlantic, intra-continental rift basins (e.g. the West and Central African Rift Systems) remains enigmatic owing to poor subsurface data coverage and quality. How magmatism in these basins related to the opening of the South Atlantic is thus poorly understood. We integrate 2D and 3D seismic reflection data ( c. 27 600 km 2 ), data from 23 boreholes, and field observations from the Bornu Basin and Upper Benue Trough, onshore NE Nigeria, to examine the timing and development of igneous bodies possibly related to opening of the South Atlantic. We identify numerous sills, which typically have saucer-shaped and en echelon morphologies, and extrusive volcanic cones. The igneous rocks are alkali basalts and dolerites. Seismic-stratigraphic relationships indicate that emplacement occurred in the Early Cretaceous (Albian to Cenomanian; c. 120 Ma), Late Cretaceous (Santonian to early Campanian; c. 83 Ma) and Cenozoic (Miocene; c. 22 Ma). Magmatism was broadly coeval with major plate boundary interactions, characterized by major azimuthal changes in fracture zones in the developing South Atlantic Ocean. The broad temporal correlation between intra-continental rift basin magmatism and plate boundary interactions suggests that periods of magma emplacement may have, in some way, been instigated by stress dissipation into intra-continental rift basins.

New insights into the crust and lithospheric mantle structure of Africa from elevation, geoid, and thermal analysis

AbstractWe present new crust and lithosphere thickness maps of the African mainland based on integrated modeling of elevation and geoid data and thermal analysis. The approach assumes local isostasy, thermal steady state, and linear density increase with depth in the crust and temperature‐dependent density in the lithospheric mantle. Results are constrained by a new comprehensive compilation of seismic Moho depth data consisting of 551 data points and by published tomography models relative to LAB depth. The crustal thickness map shows a N‐S bimodal distribution with higher thickness values in the cratonic domains of southern Africa (38–44 km) relative to those beneath northern Africa (33–39 km). The most striking result is the crustal thinning (28–30 km thickness) imaged along the Mesozoic West and Central African Rift Systems. Our crustal model shows noticeable differences compared to previous models. After excluding the Afar plume region, where the modeling assumptions are not fulfilled, our model better fits the available seismic data (76.3% fitting; root mean square error = 4.3 km). The LAB depth map shows large spatial variability (90 to 230 km), with deeper LAB related to cratonic domains and shallower LAB related to Mesozoic and Cenozoic rifting domains, in agreement with tomography models. Though crustal and lithosphere thickness maps show similar regional patterns, major differences are found in the Atlas Mountains, the West African Rift System, and the intracratonic basins. The effects of lateral variations in crustal density as well as the nonisostatic contribution to elevation in the Afar plume region, which we estimate to be ~1.8 km, are also discussed.

Aptian palaeoclimates and identification of an OAE1a equivalent in shallow marine environments of the southern Tethyan margin: Evidence from Southern Tunisia (Bir Oum Ali section, Northern Chott Chain)

Alternations between siliciclastic, carbonate and evaporitic sedimentary systems, as recorded in the Aptian mixed succession of southern Tunisia, reflect profound palaeoceanographic and palaeoclimatic changes in this area of the southern Tethyan margin. The evolution from Urgonian-type carbonates (Berrani Formation, lower Aptian) at the base of the series, to intervals dominated by gypsum or detrital deposits in the remainder of the Aptian is thought to result from the interplay between climate change and tectonic activity that affected North Africa.Based on the evolution of clay mineral assemblages, the early Aptian is interpreted as having been dominated by slightly humid conditions, since smectitic minerals are observed. Near the early to late Aptian boundary, the onset of a gypsiferous sedimentation is associated with the appearance of palygorskite and sepiolite, which supports the installation of arid conditions in this area of the southern Tethyan margin. The evaporitic sedimentation may have also been promoted by the peculiar tectonic setting of the Bir Oum Ali area during the Aptian, where local subsidence may have been tectonically enhanced linked to the opening of northern and central Atlantic. Stress associated with the west and central African rift systems may have triggered the development of NW–SE, hemi-graben structures. Uplifted areas may have constituted potential new sources for clastic material that has been subsequently deposited during the late Aptian.Chemostratigraphic (δ13C) correlation of the Bir Oum Ali succession with other peri-Tethyan regions complements biostratigraphic findings, and indicates that a potential expression of the Oceanic Anoxic Event (OAE) 1a may be preserved in this area of Tunisia. Although the characteristic negative spike at the base of this event is not recognized in the present study, a subsequent, large positive excursion with δ13C values is of similar amplitude and absolute values to that reported from other peri-Tethyan regions, thus supporting the identification of isotopic segments C4–C7 of the OAE1a. The absence of the negative spike may be linked to either non preservation or non deposition: the OAE1a occurred in a global transgressive context, and since the Bir Oum Ali region was located in the innermost part of the southern Tethyan margin during most of the Aptian, stratigraphic hiatuses may have been longer than in other regions of the Tethys. This emphasizes the importance of integrating several stratigraphic disciplines (bio-, chemo- and sequence stratigraphy) when performing long-distance correlation.

The role that plate tectonics, inferred stress changes and stratigraphic unconformities have on the evolution of the West and Central African Rift System and the Atlantic continental margins

The Muglad rift basin of Sudan, is a good example of polyphase rifting, with at least three major phases of basin development. Each phase has resulted in the generation of source rock, reservoir and seal geology with structural traps often closely linked to basement highs. In this paper we investigate on a regional scale the tectonic processes that have contributed to rift basin development.On a regional scale, the evolution of the Africa-wide Mesozoic rift system is intimately linked to relative movements of African sub-plates and to global plate tectonic processes and plate interactions. Changes in plate interactions are observed in the oceanic crust as azimuth changes of fracture zone geometries and by inference have caused significant modifications to both the orientation and magnitude of the motions of the African sub-plates. Such plate motion processes have controlled the polyphase development of the West and Central African Rift System. On the basinal scale, changes of sub-plate motions have resulted in changes in the stress field which have had a clear impact on the deformation and fault geometries of rift basins and on the resulting stratigraphy. The construction of the first unified stratigraphic chart for the West and Central African Rift System shows a close correlation in the timing of the major unconformities with the timing of changes in relative plate motion as observed in the changes of the azimuthal geometry of the oceanic fracture zones in the Central Atlantic. Since similarly timed unconformities exist along the continental margins of Africa and South America, we propose that the causative mechanism is change in relative plate motion which leads to an increase or decrease in the tension on the plate and thus controls the strength or effective elastic thickness, Te, of the crust/plate beneath the margins. This results in a focused change in isostatic response of the margin during short-period changes in relative plate motion; i.e. more tension will mean that loads are not compensated locally resulting in local uplift of the margin.

Mapping crustal thickness and oceanic lithosphere distribution in the Eastern Mediterranean using gravity inversion

Oceanic and continental lithosphere distribution within the eastern Mediterranean is not well understood. A gravity inversion, incorporating a lithosphere thermal gravity anomaly correction, has been used to map Moho depth, crustal thickness and lithosphere thinning for the eastern Mediterranean, from which the distribution of oceanic and continental lithosphere, the structure of the ocean–continent transition (OCT) and the location of the continent–ocean boundary (COB) can be determined. The gravity inversion results show thin crust and high continental lithosphere thinning under the Ionian Sea and the Herodotus Basin, consistent with these basins being underlain by oceanic crust. Moho depths from gravity inversion are in agreement with seismic refraction estimates in these basins. Highly thinned continental crust is predicted under the offshore Sirte and Levant basins. The sharp decrease in crustal thickness predicted by gravity inversion off the Libyan and Egyptian coast gives an indication of COB location. Crustal thickness and continental lithosphere thinning determined from gravity inversion have also been used to explore the relationship between the Cretaceous West and Central African Rift System (WCARS: Benue Trough, Chad, Central African Shear Zone (CASZ) and Sudan basins) and the eastern Mediterranean basins; continuity between the Cretaceous WCARS and the eastern Mediterranean basins is not apparent in the gravity inversion results.

Geophysical evidence of Cretaceous volcanics in Logone Birni Basin (Northern Cameroon), Central Africa, and consequences for the West and Central African Rift System

Detailed analyses and interpretation realized by combining existing 2D reflection seismic and Gravity/Magnetic data of the Logone Birni Basin (LBB) in the West and Central African Rift System (WCAS) have revealed the distribution of the main buried volcanic bodies as well as their relationships with the structural and tectonic evolution of this basin. The volcanic activity in the LBB is restricted to the Cretaceous period. Three main volcanic episodes are identified and are associated to the Neocomian, Late Albian and Cenomanian–Turonian rifting phases respectively. The volcanic bodies within the Lower Cretaceous are either lying directly on basement or are mainly interbedded with the contemporaneous sediments whereas the Upper Cretaceous bodies are morphologically expressed in the forms of dykes and sills. The volcanic activity was more intense in the western region of the central LBB (Zina sub-basin) along the Cameroon–Nigeria border whereas it was scanty and scattered in the other parts of the basin. The main volcanic dykes are found on the flanks of the major faults bounding basement horsts or in crestal positions in association with syndepositional faults.Although WCAS is associated with large amount of crustal extension and minor volcanism, the intense volcanic activity observed in LBB during the Cretaceous suggests that the intrusive zone during this period was confined to the basement beneath the study area flanked respectively to the north, south and southwest by the Lake Chad, Poli and Chum triple junctions. Tensional stresses generated by this localized domal uplift accounts for most of the observed tectonic structures where major faults transected the entire lithosphere, thus providing conduits for magma migration.

The enigmatic Chad lineament revisited with global gravity and gravity-gradient fields

Abstract The crustal structure of northern Africa is puzzling, large areas being of difficult access and concealed by the Sahara. The new global gravity models are of unprecedented precision and spatial resolution and offer a new possibility to reveal the structure of the lithosphere beneath the Sahara. The gravity gradients correlate better than gravity with geological features such as rifts, fold belts and magmatic deposits and intrusions. They are an ideal tool to follow geological units (e.g. basement units) below a stratigraphic layer of varying density (e.g. sediments). We focus on the Chad lineament, a 1300 km arcuate feature located between the west and central African rift system. The gravity fields show differences between the lineament and the west and central African rift system. Along the centre of the lineament high-density rocks must be present, which relate to either magmatic or metamorphic rocks. This is very different to the lineaments of the western and central-west African rift system which are filled with sediments. Considering present models of rifting and the absence of topography, the lineament cannot be coeval to the west and central African rift system and is most likely older. We suggest that the lineament is a structural element of the Saharan Metacraton.

On the use of global potential field models for regional interpretation of the West and Central African Rift System

The use in regional interpretations of the Earth Gravity Model (EGM08) and Earth magnetic model (EMAG2) is evaluated by comparison to ground gravity and aeromagnetic data in the central sector of the West and Central African Rift System (WCARS). The comparison includes upward continuation, spectral analysis and pseudogravity calculation and statistical evaluation. A correlation between EMAG2 (which contains roughly 25 km resolution aeromagnetic data in the region) and near-surface aeromagnetic data over WCARS is only true for the very low wavelength part but a strong similarity between EGM08 and ground gravity data can be confirmed. Interpretation of the EGM08 data allows identifying and confirming the position of major structural trends, and provides new information on the crustal architecture. The lineaments limiting different grabens forming the Logone Birni basin and in the southern Chad basin are identified. The results presented here show the use of EGM08 data for regional interpretations over Cameroon and adjacent countries, which can overcome the absence and sparseness of data in developing countries and remote areas.

A top to bottom lithospheric study of Africa and Arabia

We study the lithospheric structure of Africa, Arabia and adjacent oceanic regions with fundamental-mode surface waves over a broad period range. Including group velocities with periods shorter than 35 s allows us to examine shallower features than previous studies of the whole continent. In the process, we have developed a crustal thickness map of Africa. Main features include crustal thickness increases under the West African, Congo, and Kalahari cratons. We find crustal thinning under Mesozoic and Cenozoic rifts, including the Benue Trough, Red Sea, and East, Central, and West African rift systems, along with less abrupt crustal thickness changes at passive continental margins. We also find crustal thickness differences in North Africa between the West African Craton and East Saharan Shield. Crustal shear wave velocities are generally faster in oceanic regions and cratons, and slower in more recent crust and in active and remnant orogenic regions. Deeper structure, related to the thickness of cratons and modern rifting, is generally consistent with previous work. Under cratons we find thick lithosphere and fast upper mantle velocities, while under rifts we find thinned lithosphere and slower upper mantle velocities. However, we also find the lack of a thick cratonic keel beneath the central portion of the Congo Craton. There are no consistent effects in areas classified as hotspots, indicating that there seem to be numerous origins for these features. Finally, it appears that the African Superswell, which is responsible for high elevation and uplift over large portions of Africa, has had a significantly different impact (as indicated by features such as temperature, time of influence, etc.) in the north and the south. This is consistent with episodic activity at shallow depths, which is well-expressed in northeastern Africa and Arabia today.

A model for the three-dimensional evolution of continental rift basins, north-east Africa

Large areas of north-east Africa were dominated by regional extension in the Late Phanerozoic. Widespread rifting occurred in the Late Jurassic, with regional extension culminating in the Cretaceous and resulting in the greatest areal extent and degree of interconnection of the west, central and north African rift systems. Basin reactivation continued in the Paleocene and Eocene and new rifts probably formed in the Red Sea and western Kenya. In the Oligocene and Early Miocene, rifts in Kenya, Ethiopia and the Red Sea linked and expanded to form the new east African rift system.

Interior cratonic basins of Africa: relation to continental break‐up and role of mantle convection

AbstractThe history of the African continent since the beginning of fragmentation of Pangaea is one of widespread rifting in the Late Jurassic to Early Cretaceous, extensive inundation under elevated Cretaceous sea‐levels, widespread epeirogenic uplift beginning in the Early Tertiary and off‐rift domal uplift associated with alkaline igneous activity in the Tertiary and especially in the Neogene. The cratonic basins have developed against this post break‐up background,The cratonic basins of Africa are located between a relatively dense array of highspots and hotspots. Although there is a clear underlying structural fabric in the form of the Central African and West African rift systems, the present location of these basins cannot always be unequivocally linked to the presence of underlying rifts. The Congo Basin stands out as overlying a rigid rather than stretched lithosphere, and no reasonable density contrasts in the crust can explain the isostatic anomalies over the basin. Either there is a cold, dense region situated at depths typical of the upper mantle beneath the basin, or a downward‐acting dynamic force on the base of the lithosphere is necessary to explain the gravity field. Together, this points to the likelihood of a convective downwelling beneath the African plate under the Congo region.The African cold‐spot model may be applicable to the Early Palaeozoic cratonic basins of North America. Their location and timing of development with respect to the adjacent plate margin, size and shape may be analogous to the African examples.

Basement control in the development of the early cretaceous West and Central African rift system

Abstract The structural framework of the Precambrian basement of the West and Central African Rift System (WCARS) is described in order to examine the role of ancient structures in the development of this Early Cretaceous rift system. Basement structures are represented in the region by large Pan-African mobile belts (built at ca. 600 Ma) surrounding the > 2 Ga West African, Congo and Sao Francisco cratons. Except for the small Gao trough (eastern Mali) located near the contact nappe of the Pan-African Iforas suture zone along the edge of the West African craton, the entire WCARS is located within the internal domains of the Pan-African mobile belts. Within these domains, two main structural features occur as the main basement control of the WCARS: (1) an extensive network of near vertical shear zones which trend north-south through the Congo, Brazil, Nigeria, Niger and Algeria, and roughly east-west through northeastern Brazil and Central Africa. The shear zones correspond to intra-continental strike-slip faults which accompanied the oblique collision between the West African, Congo, and Sao Francisco cratons during the Late Proterozoic; (2) a steep metamorphic NW-SE-trending belt which corresponds to a pre-Pan-African (ca. 730 Ma) ophiolitic suture zone along the eastern edge of the Trans-Saharian mobile belt. The post-Pan-African magmatic and tectonic evolution of the basement is also described in order to examine the state of the lithosphere prior to the break-up which occurred in the earliest Cretaceous. After the Pan-African thermo-tectonic event, the basement of the WCARS experienced a long period of intra-plate magmatic activity. This widespread magmatism in part relates to the activity of intra-plate hotspots which have controlled relative uplift, subsidence and occasionally block faulting. During the Paleozoic and the early Mesozoic, this tectonic activity was restricted to west of the Hoggar, west of Air and northern Cameroon. During the Late Jurassic-Early Cretaceous the St. Helena mantle plume may have had an important effect in weakening the lithosphere across a broad zone from Brazil to Sudan. However, causes of extension and stretching are likely to be found in a plate tectonic framework. At that time, the opening of both the Indian and Atlantic oceans were the dominant forces which favoured the development of the WCARS and its propagation by strike-slip movements and extensional displacements along pre-existing Pan-African lithospheric zones of weakness.

Intracontinental movements in Western Gondwanaland: a palaeomagnetic test

Early Cretaceous intracontinental movements within Africa and/or South America, to account for misfit problem in the pre-drift reconstruction of Western Gondwana, have been tested using palaeomagnetic poles from both continents. Each continent has been considered as comprising separate subplates according to the boundaries proposed by Pindell and Dewey (1982) and Curie (1983): i.e., northern Africa ( naf), southern Africa ( saf), northern South America (nsa) and southern South America ( ssa). Visual and statistical distribution of Late Permian to Jurassic poles from the African subplates indicate two distinct groups before rotating saf relative to naf, while after the rotation they become indistinguishable. The distribution and sparsity of data from South America limit the study in this continent as the data are statistically indistinguishable before and after rotation of ssa with respect to nsa. When the test is extended to the reconstruction of Western Gondwana, the tightest grouping of the poles occurs when an intracontinental movement is assumed within Africa. This suggests that the misfit between the two continents is probably due to intracontinental deformation within Africa and not in South America. If confirmed, this may have profound implications in the kinematics of the processes related to the opening of the South and Central Atlantic Oceans and also the origin of the West African Rift System.

Rifting of Africa and pattern of mantle convection beneath the African plate

Abstract The Mesozoic and Cenozoic tectonic setting of the African continent is characterized by an extensional stress regime, as is evident from the evolution of the West, Central and East African rift systems, the opening of the Indian and Atlantic oceans and the development of Tethys domain, as well as from the results of geophysical investigations. The most prominent feature of the Bouguer gravity anomaly map of Africa is a great belt of negative Bouguer anomalies which extends across the entire African continent from Ethiopia in the East to Benguela in the West. In conjunction with the Red Sea spreading zone this belt appears to encircle a “centre” in Cameroon. It probably corresponds to a distinct zone of lithospheric thinning and extension. The entire African continent is characterized by higher-than-normal elevations. It is located inside the African residual geoid high. Earthquake fault-plane solutions confirm the extensional character of the present tectonic regime of eastern and southern Africa where normal faulting predominates. The average orientation of T axes is N-S in South Africa, NW-SE in Zambia, Mozambique and Madagascar, E-W in Kenya and Ethiopia, and NE-SW in the Red Sea region. From north to south, the orientation of T axes rotates systematically in a clockwise sense by 130° and describes a radial pattern centred on equatorial West Africa. This indicates that the African lithosphere has been subject to continent-wide radial extension around a hypothetical West African “spreading centre”, referred to as the African Spreading Centre A (ASC-A), which coincides with the geographical centre of the African plate. Shear traction exerted by the convecting mantle on the base of the lithosphere is considered to be a major driving force of plate movements. In the case of Africa, the continent-wide regularities in the pattern of lithospheric extension, the elevated position of the continent, as well as the evolution of the mid-oceanic ridge system surrounding Africa, provide distinct boundary conditions for the pattern of large-scale mantle flow beneath the African plate. It is proposed that relatively warm and less dense mantle material rises below Africa, forming a single mega-plume or a number of plumes. At the base of the African lithosphere, the ascending material diverges and flows radially away from the centre of ascent, corresponding to the ASC-A. This ascending flow forms part of a very large mantle convection cell which occupies the Eurafrican hemisphere. The descending branch of this convection cell is located at a distance of 80–90° from the ASC-A. The mid-oceanic ridges surrounding the African continent are located between the upwelling and the downwelling branches of this convection cell, at a distance of 50–60° from the ASC-A. Upper mantle flow below the sea-floor spreading axes is uni-directional and horizontal and is directed away from the ASC-A.

The West and Central African rift systems: Foreword

This chapter discusses about the West and Central African rift systems. The recognition of these intracontinental Cretaceous rift systems were based on the extensive gravity data base which exists for this region and on data made available by the Petroleum industry for many of the rift basins. This permitted delineating the extensional and strike-slip zones through West, Central, and North Africa. This was helped by understanding that the negative gravity anomalies associated with the sedimentary basins, forming part of this rift system, were superimposed on broad positive anomalies, which are up to 450 km wide and have amplitudes of up to 80 mGal. The spatial and temporal link between the West and Central African rift systems and plate tectonic processes within the Atlantic Ocean was recognized to indicate that their is a strong interaction between oceanic and intra-continental tectonics.

Chronology and geodynamic setting of Cretaceous-Cenozoic rifting in West and Central Africa

The development of the Early Cretaceous to Palaeogene West and Central African rift system, which extends from Nigeria (Benue trough) to Kenya (Anza trough), can be related to the build-up of intraplate tensional stresses during the break up of Gondwana, which caused reactivation of pre-existing zones of lithospheric weakness. Repeated changes in the intraplate stress regime of Africa are reflected by phases of crustal extension alternating with episodes of compression. Many of these events can be correlated with changes in rates of seafloor spreading in the Central and South Atlantic oceans, as reflected in flowline patterns. The West and Central African rifts can be considered as typical ‘passive’ rifts which evolved in response to the build-up of intraplate stresses. However, the St. Helena hot spot appears to have been located beneath the Equatorial plate boundary at approximately 120 Ma and may have played an important role in weakening the lithosphere during extension.