J.D. Fairhead

Mesozoic plate tectonic reconstructions of the central South Atlantic Ocean: The role of the West and Central African rift system

Abstract The Mesozoic opening history of the Central, Equatorial and South Atlantic oceans is closely linked in time and space to the development of the Cretaceous rift system in West and Central Africa. Geological and geophysical studies of the West and Central African rift system indicate it to be dominated by a set of sinistral and dextral strike-slip zones diverging from the Gulf of Guinea and terminating as extensional rift basins in Niger and Sudan. Plate tectonic studies of the opening history of the Atlantic basins indicate four stages of development: stage 1, the Jurassic opening of the Central Atlantic, stage 2, the Early Cretaceous (≈ 130-119 Ma) opening of the South Atlantic with rifting propagating deep into Africa via the Benue Trough, stage 3. the opening of the Equatorial Atlantic (119-105 Ma), and stage 4, the linkage of these oceanic basins and development of a single opening mid-oceanic rift system as seen today. The West and Central African rift system continued to develop until the end of the Cretaceous (i.e., into stage 4) when rifting was terminated by a compressional-shear deformation event causing the folding of sediments in the Benue Trough and reactivation of the Central African shear zone and extensional tectonics in the basins of southern Sudan. This event correlates with an important change in the relative opening of the Atlantic basins. During Tertiary-Recent times the West African rift system has not been active. However, during this period domal uplift and volcanism have occurred in central Cameroon and western Sudan and a very active phase of volcanism has occurred within the last 10 Ma along the present-day Cameroon volcanic line. Although these uplift and volcanic events indicate thermal processes in the upper mantle, there is, as yet, no clear link between their development and plate tectonic processes.

The structure of the lithosphere beneath the Eastern rift, East Africa, deduced from gravity studies

Abstract A compilation of all published and unpublished gravity data for the Eastern rift between latitudes 1°N and 5°S is presented. The Bouguer anomaly map reveals that the shape of the negative regional anomaly associated with the rift is approximately two-dimensional, striking east of north, of width 350 ± 50 km and amplitude500 ± 100 g.u. relative to the background value of−1300 ± 100 g.u. to the west. The regional anomaly is interpreted in terms of an upward thinning of the lithosphere and replacement by low-density asthenosphere. This model is different from previous interpretations in that major lithospheric thinning is restricted to the region of the Eastern rift affected by the domal uplift and does not extend beneath the Lake Victoria region to the west. The gravity and seismic models are compatible if the anomalous upper mantle (asthenospheric part), beneath the rift, is in a state of partial melt. A consequence of the revised regional anomaly is that it reduces previous amplitude estimates of the axial positive residual anomaly within the rift by at least 50% and generates negative anomalies over the rift shoulders in areas covered by Cenozoic volcanics. These negative anomalies are considered to be caused by the low density of the surface volcanics. Within the rift, elongated negative anomalies of amplitude 100–350 g.u. are associated with sedimentary basins and are attributed to low-density sediments up to 3 km thick. The positive residual anomaly along the axis of the rift can be interpreted in terms of either a dyke injection zone less than 15 km wide or by a dense infill body about 2.5 km thick. The positive anomaly is shown to be confined to the volcanic province of the Eastern rift and has its southern termination in the Magadi—Natron area, just north of where the Kenya rift valley changes to block faulting in N. Tanzania. This termination coincides with a change in the spatial distribution of the seismic and geothermal activity.

Gravity study of the Central African Rift system: A model of continental disruption 1. The Ngaoundere and Abu Gabra Rifts

Abstract A regional compilation of published and unpublished gravity data for Central Africa is presented and reveals the presence of a major rift system, called here, the Central African Rift System. It is proposed that the junction area between the Ngaoundere and Abu Gabra rift arms in Western Sudan forms an incipient intraplate, triple-junction with the as yet unfractured, but domally uplifted and volcanically active, Darfur swell. It is only the Darfur swell that shows any similarities to the uplift and rift history of East Africa. The other two rifts arms are considered to be structurally similar to the early stages of passive margin development and thus reflect more closely the initial processes of continental fragmentation than the structures associated with rifting in East Africa.

Differential opening of the Central and South Atlantic Oceans and the opening of the West African rift system

Abstract Plate tectonic studies of the development of the Central and South Atlantic Oceans using Seasat and Geosat altimeter and magnetic anomaly isochron data now provide quantitative models of seafloor spreading through time. Such models enable an initial assessment of the differential opening between these two oceanic basins to be determined. The Equatorial Atlantic is an integral part of this oceanic rifting process, allowing stresses arising from the differential opening to be dissipated into both the Caribbean and Africa along its northern and southern boundaries respectively. The tectonic model for the West African rift system, based on geological and geophysical studies, shows a series of strike-slip fault zones diverging into Africa from the Gulf of Guinea and dissipating their shear movement into the development of extensional basins orientated perpendicular to these faults zones. The development of the West African rift system was contemporaneous with the early opening of the South Atlantic, continued to develop well after the final breakup of South America from Africa and did not cease until the late Cretaceous when there was a major phase of basin inversion and deformation. Santonian ( ~ 80 Ma) deformation across the Benue Trough (Nigeria) is broadly contemporaneous with dextral shear reactivation of the central African fracture system which, in turn resulted in renewed extension in the Sudan basins during the late Cretaceous and early Tertiary. This paper illustrates the close linkage in both time and space between the history of the African rift basins and the opening of the Atlantic. Both exhibit distinct phases of evolution with the rift basins developing in direct response to the differential opening between the Central and South Atlantic in order to dissipate stresses generated by this opening. The Mesozoic tectonic model proposed is therefore one of an intimate interaction between oceanic and continental tectonics.

National Geophysical Data Center candidate for the World Digital Magnetic Anomaly Map

Marine and airborne magnetic anomaly data have been collected for more than half a century, providing global coverage of the Earth. Furthermore, the German CHAMP satellite is providing increasingly accurate information on large-scale magnetic anomalies. The World Digital Magnetic Anomaly Map project is an international effort to integrate all available near-surface and satellite magnetic anomaly data into a global map database. Teams of researchers were invited to produce candidate maps using a common pool of data sets. Here we present the National Geophysical Data Center (NGDC) candidate. To produce a homogeneous map, the near-surface data were first line-leveled and then merged by Least Squares Collocation. Long wavelengths were found to agree surprisingly well with independent satellite information. This validates our final processing step of merging the short-wavelength part of the near-surface data with long-wavelength satellite magnetic anomalies.

Magnetic source parameters of two‐dimensional structures using extended Euler deconvolution

The Euler homogeneity relation expresses how a homogeneous function transforms under scaling. When implemented, it helps to determine source location for particular potential field anomalies. In this paper, we introduce an additional relation that expresses the transformation of homogeneous functions under rotation. The combined implementation of the two equations, called here extended Euler deconvolution for 2-D structures, gives a more complete source parameter estimation that allows the determination of susceptibility contrast and dip in the cases of contact and thin-sheet sources. This allows for the structural index to be correctly chosen on the basis of a priori knowledge about susceptibility and dip. The pattern of spray solutions emanating from a single source anomaly can be attributed to interfering sources, which have their greatest effect on the flanks of the anomaly. These sprays follow different paths when using either conventional Euler deconvolution or extended Euler deconvolution. The paths of these spray solutions cross and cluster close to the true source location. This intersection of spray paths is used as a discriminant between poor and well-constrained solutions, allowing poor solutions to be eliminated. Extended Euler deconvolution has been tested successfully on 2-D model and real magnetic profile data over contacts and thin dikes.

A plate tectonic setting for Mesozoic rifts of West and Central Africa

Abstract Africa, by virtue of its central position within Gondwana, has recorded much of the complex history of plate interactions which have progressively fragmented this supercontinent since the Early Mesozoic. Continental reconstructions reveal both a temporal and spatial relationship between the development of the continental margins of Africa and the formation of rifted sedimentary basins deep within the African continent. The multi-stage opening of the Atlantic Ocean and associated rifting in West and Central Africa provide one of the most impressive examples of ocean-continent tectonic interactions. During the Early Cretaceous the onset of rifting along the future margins of the South Atlantic is contemporaneous with intra-continental rifting generating both strike-slip and extensional basins within West and Central Africa. The syn-rift phase of intra-continental basin development continued until the Santonian, by which time Africa and South America had been physically separated for approximately 10 Ma. Except for minor rifting during the Senonian and Tertiary, the short-lived phase of deformation at about 80 Ma marks the transition into the post-rift or “sag” phase of basin development. This deformational event can be correlated with a period of plate motion changes, recognised from fracture zone geometries, seen in both the Central and South Atlantic oceans. Using present day stress analogies, Cretaceous rifting in Africa can provide a means of indicating the regional palaeostress directions within Africa at this time.

Effective elastic thickness and crustal thickness variations in west central Africa inferred from gravity data

The west central African region is characterized by various geological features: Cretaceous rifts (Benue), Tertiary domal uplift (Adamawa volcanic uplift), Tertiary-Recent volcanoes (Cameroon Volcanic Line or CVL), Tertiary sedimentary basins (Chad basins), and cratonic region (Congolese craton). In this study, we investigate the relationship between these tectonic features and the flexural rigidity of the lithosphere in Cameroon, in terms of effective elastic thickness (Te), by the use of the coherence function analysis. For that purpose, we use a new dataset of ∼32,000 gravity and topography points from Cameroon and the adjacent countries. The Te contour map deduced from this study shows a good relationship between the tectonic provinces and the rigidity of the lithosphere, the minima (14–20 km) are beneath the active rifted and volcanic areas (Benue, CVL, and Adamawa), and the maxima (∼40 km) correspond to the Archean reworked unit in Congo. A spectral analysis of the gravity data is performed to determine the crust-mantle boundary in these tectonic provinces. The crustal thickness (Tc) contour map shows a variation from 14 km to about 45 km, consistent with other geophysical data. The lower values (14–20 km) are obtained in central Cameroon on the Adamawa uplift, and the highest values are found in southern Cameroon (Archean reworked Congolese craton). Comparing Te and Tc values shows that there is generally a positive correlation between the two parameters, with an exception in Chad where this correlation is rather negative.

A regional gravity study of the West African rift system in Nigeria and Cameroon and its tectonic interpretation

Abstract This study brings together existing and new gravity data to investigate the nature and cause of the Bouguer gravity field associated with the Cretaceous West African rift system in Nigeria and Cameroon. The new gravity measurements include data collected over the basement area between the Benue Trough and the Cameroon border and fill an important gap in the gravity coverage. After removal of the long wavelength negative anomaly from the observed gravity field the remaining positive gravity anomaly, associated with the rift system, is interpreted in terms of two and three dimensional crustal models. These models are constrained by crustal thicknesses derived from a seismic refraction study carried out across and to the south of the Yola rift. The results of simple three dimensional gravity modelling indicate the crust beneath the lower and middle Benue is approximately 20 km thick while beneath the Gongola rift the crust is approximately 25 km thick relative to a normal crustal thickness of 34 km away from the rift. Assuming the thinned crust associated with the rift system is the result of simple lithospheric stretching, then the maximum possible crustal extensions of 95, 65 and 55 km have taken place perpendicular to the Benue, Gongola and Yola rifts respectively. These crustal extensions do not necessarily reflect the total crustal movement affecting these rifts since the Benue and Yola rifts have been subjected to varying amounts of shear displacement during the Cretaceous and early Tertiary times. Crustal extension estimates across the West African rift system could be as much as four times greater than the published extension estimates across the East African rift system. These rift systems have the same plate tectonic setting being the failed third arms of successful triple junctions and are the result of extensional and shear tectonics. The differences in crustal extension and the resulting isostatic response of the lithosphere beneath these rifts can explain why the West African rift system has been associated with subsidence processes throughout its development whereas the East African rift system in Northeast Africa has been strongly affected by uplift.

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

Abstract 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.