Bangui Magnetic Anomaly Research Articles

Understanding the crustal architecture beneath the Bangui magnetic anomaly and its interactions with central African tectonic megastructures based gravity and magnetic analysis

The Bangui magnetic anomaly (BMA) in Central Africa is one of the largest continental magnetic anomalies on Earth in terms of amplitude and lateral size. Determining the sources of the BMA can lead to an increased understanding of the crustal dynamic in the Central African sub-region and the African continent as a whole. Magnetic and gravity analysis-based derivative, two-dimensional forward modelling and a Curie isothermal depth, showed that (a) the bottoms of the magnetic sources were between 15 and 35 km; (b) the BMA is a coalescence of several anomalies that trend E-W and roughly NE-SW. These directions coincide with regional Pan African-aged shear zones along the Central African orogenic belt and to thrust sheets at the northern edge of the Congo Craton. The depth of magnetization does not exceed 35 km with the amplitude of magnetization becoming smaller in the Central African Republic. The potential magnetic susceptibility sources have an average density of 2850 kg/m3 and magnetic susceptibilities between 0.06 and 0.25 SI. The BMA is interpreted to be a combination of middle and lower crustal bodies that are not continuous and consist of magnetic mineral rich granulites and banded iron formations. The gravity and magnetic modelling indicate that the entire crust was involved in the Pan African collisional event similar to what is seen in the Mozambique belt in East Africa. Combined with geological and geochemical studies, the models add evidence that one or two subduction zones were involved in accreting terranes on the northern edge of the Congo Craton. The tectonic accretions caused a crustal remobilization along major shear zones that has locally contributed to a probable circulation of fluids enriched in ferromagnesian minerals during late Neoproterozoic magmatism that created the BMA sources.

Coherence of Bangui Magnetic Anomaly with Topographic and Gravity Contrasts across Central African Republic

The interactions between the geophysical processes and geodynamics of the lithosphere play a crucial role in the geologic structure of the Earth’s crust. The Bangui magnetic anomaly is a notable feature in the lithospheric structure of the Central African Republic (CAR) resulting from a complex tectonic evolution. This study reports on the coherence in the geophysical data and magnetic anomaly field analysed from a series of maps. The data used here include raster grids on free-air altimetric gravity, magnetic EMAG2 maps, geoid EGM2008 model and topographic SRTM/ETOPO1 relief. The data were processed to analyse the correspondence between the geophysical and geologic setting in the CAR region. Histogram equalization of the topographic grids was implemented by partition of the raster grids into equal-area patches of data ranged by the segments with relative highs and lows of the relief. The original data were compared with the equalized, normalized and quadratic models. The scripts used for cartographic data processing are presented and commented. The consistency and equalization of topography, gravity and geoid data were based using GMT modules ‘grdfft’ and ‘grdhisteq’ modules. Using GMT scripts for mapping the geophysical and gravity data over CAR shows an advanced approach to multi-source data visualization to reveal the relationships in the geophysical and topographic processes in central Africa. The results highlighted the correlation between the distribution of rocks with high magnetism in the central part of the Bangui anomaly, and distribution of granites, greenstone belts, and metamorphosed basalts as rock exposure. The correspondence between the negative Bouguer anomaly (<−80 mGal), low geoid values (<−12 m) and the extent of the magnetic anomaly with extreme negative values ranging from −1000 to −200 nT is identified. The integration of the multi-source data provides new insights into the analysis of crustal thicknesses and the average density of the Earth in CAR, as well as the magnitude of the magnetic fields with notable deviations caused by the magnetic flux density in the Bangui area related to the distribution of mineral resources in CAR.

Coherence of Bangui Magnetic Anomaly with Topographic and Gravity Contrasts across Central African Republic

Coherence of Bangui Magnetic Anomaly with Topographic and Gravity Contrasts across Central African Republic

The origin of Bangui magnetic anomaly - one of the largest magnetic anomalies in the world

The origin of Bangui magnetic anomaly - one of the largest magnetic anomalies in the world

Inversion of Magnetic Measurements of the Swarm A Satellite of the Bangui Magnetic Anomaly

We wanted to make a satellite altitude magnetic anomaly map of the large magnetic anomaly in the Central African Republic, the Bangui magnetic anomaly, with data from the Swarm satellites. In the first part of our study, we summarize the earlier investigations and their interpretation. In the second we discuss our data processing applied to produce a magnetic anomaly map. We used the IGRF

The mechanical behaviour of the lithosphere beneath the Chad Basin and the Bangui Magnetic Anomaly. Insights from Moho depth and effective elastic thickness estimates

We used high-resolution gravity data to analyse crustal structure beneath the Bangui Magnetic Anomaly (BMA) and the Chad Basin within the North-Central African region. Moho depths and effective elastic thickness values, calculated respectively by a regularized non-linear inversion method and by analysis of the coherence function between gravity and topography, have been interpreted with reference to seismic data, previous gravity studies and regional geology. Our aim is to determine the crustal architecture and to analyse its mechanical behaviour. Estimated Moho depths range from 30 to 43 km in the North-Central African region. Beneath the uplift zones (Adamawa-Yade plateau, Ouaddaï, Tibesti, Hoggar, Tuareg shield) and the Congo Craton, the Moho depths range between 35 and 37 km. These values can be interpreted by tectonic and seismic events recorded along the regional shear zones; but also to the remobilization of the Precambrian basement. Analysis of the coherence function between the Bouguer gravity and the topography shows two different domains of rigidity: the highest values (100 km < Te < 180 km) are observed in Central African Republic, Chad, Niger, Algeria, and Egypt; the lowest values (20 < Te < 100 km) are observed in Cameroon, Nigeria, and Sudan. The lithosphere beneath the Chad Basin has a higher rigidity, similar to that observed in Central African Republic. In Chad and the Central African Republic, there is a positive correlation between the Te and the Moho depths; this indicates that cratonic remnant (Chad craton) beneath the Chad Basin is well preserved and covered by the sediments. The low Te values in Cameroon and the constant value in the Moho depth, are a proof of the reactivation of the Precambrian basement. This reactivation is related to the tectonic, seismic and thermal events occurred during the Eburnean and Pan-African orogenies. The contrast between the Te values in Cameroon compared to others regions (Chad, Central African Republic, Niger, Egypt, Algeria) show the magnitude of these events on the crustal architecture. This being defined by the presence of cratonic cores, the Cameroon Volcanic Line (CVL) and hot spots as well as large regional shear zones, explain the fact that thermo-tectonic and seismic events that occurred at the northern edge of the Congo Craton in Cameroon, are more intense than in the Central African Republic. So according to the geodynamic process in Cameroon and Central African Republic, related to the hot spots along the CVL, thermo-tectonic and seismic events recorded along the regional shear zones (Sanaga Shear Zone and the Central Cameroon Shear Zone) and the Congo Craton, the BMA is associated to two different geotectonic contexts. The sources of the BMA are closely linked to the Congo Craton; they bottom should not exceed −37 km depth. The present results agree with previous studies which allow us to understand the evolution of the Saharan metacraton and the preservation of cratonic remnants (Murzuq, Al-Kufrah, Chad); similarly the estimated Moho depths is an important step that would serve as an input to model the sources of the Bangui Magnetic Anomaly.

Initial scalar lithospheric magnetic anomaly map of China and surrounding regions derived from CSES satellite data

The China Seismo-Electromagnetic Satellite (CSES), China’s first satellite to measure geophysical fields with scientific goals in both space and solid earth physics, was launched successfully in February 2018. It carries high-precision magnetometers to measure the geomagnetic field. In this study, the CSES magnetic data were used to extract the signal of the lithospheric magnetic field caused by magnetized rocks in the crust and uppermost mantle. First, an along-track analysis of the CSES magnetic data was undertaken near the Bangui magnetic anomaly in central Africa and the Tarim magnetic anomaly in China, demonstrating that the CSES magnetic data are indeed sensitive to the lithospheric magnetic anomaly field. Then a lithospheric magnetic anomaly map over China and surrounding regions was derived. This map is consistent with the lithospheric part of the CHAOS-7 model. In particular, it clearly reveals four major magnetic anomalies containing long-wavelength signals at the altitude of low-earth-orbiting satellites. Three magnetic highs are located over the Tarim, Sichuan and Songliao basin, the origins of which could be related to large-scale tectonic-magmatic activities during geological history. A prominent magnetic low is otherwise found in the southern Himalayan-Tibetan plateau, possibly caused by the shallow Curie depth in this region. To further improve the precision of the lithospheric magnetic field model, more detailed data processing and multi-source data merging are needed.

Crustal thickness, depth to the bottom of magnetic sources and thermal structure of the crust from Cameroon to Central African Republic: Preliminary results for a better understanding of the origin of the Bangui Magnetic Anomaly

The Bangui Magnetic Anomaly (BMA) is one of the largest magnetic anomalies in the world whose origin is still not known. This research investigated the crustal thickness, Curie depths and thermal structures in the Central African sub-regions – Cameroon, Central African Republic and adjacent countries – which are largely characterized by the Bangui Magnetic Anomaly. To achieve a better understanding and clearer idea of the location of the possible sources of the BMA, analyses of geothermal structures were conducted. Two potential methods were used: gravity to evaluate the crustal thickness and magnetics for geothermal analysis. Spectral analysis of gravity data shows that crustal thickness range between 14 and 55 km. The highest depths were found in Central African Republic. The lower values of crustal thickness were obtained in South-Chad basin with a minimum of roughly 14 km. Geothermal analysis is carried out using the Curie point depth, thermal gradient and heat-flow evaluations. The results show that the BMA is related to a thick crust of roughly 40 km. Depth to the bottom of possible sources does not exceed the lower crust. The mean Curie point depth estimated is 38 km with an error of ±2 km. Geothermal results also show the difference in the thermal behaviour between the crust in the Pan African and Precambrian domain. The mobile zone which constitutes the Pan African domain is associated with a thin crust of high heat-flow values of 65 mW/m2. However, the Precambrian domain beneath the BMA is associated with a thick crust with lower heat-flow values (roughly 45 mW/m2). The difference between crustal thickness and Curie point depths shows that all the sources of the BMA are crustal. The present results are in favour of a geological origin for the Bangui Magnetic anomaly.

Global High-Resolution Magnetic Field Inversion Using Spherical Harmonic Representation of Tesseroids as Individual Sources

In this study, we present a novel approach combining the advantages of tesseroids in representing geophysical structures though their voxel-like discretization features with a spherical harmonic representation of the magnetic field. Modelling of the Earth lithospheric magnetic field is challenging since part of the spectra is hidden by the core field and the forward modeled field of a lithospheric magnetization is always biased by the spectral range used. In our approach, a spherical harmonic representation of the magnetic field of spherical prisms (tesseroids) is used for high-resolution magnetic inversion of lithospheric field models. The use of filtered spherical harmonic models of the magnetic field of each tesseroid ensures that the resulting field matches the spectral range of the input data. For the inversion, we use the projected gradient method. The projected gradient method easily allows us to assign an initial guess (i.e., a-priori assumption) for the inversion and avoids negative values of susceptibilities. The latter is providing more plausible models since induced magnetization is assumed to be dominant over the continents and, for the oceans, a remanence model can be subtracted. We show an application of the technique to a synthetic dataset and a satellite-derived lithospheric field model where the model geometry is based on seismic information. We also demonstrate a proof-of-concept for high-resolution tile-wise inversion for the Bangui anomaly in Africa.

Rock magnetic investigation of possible sources of the Bangui magnetic anomaly

The Bangui magnetic anomaly (BMA) is the largest lithospheric magnetic field anomaly on Earth at low latitudes. Previous studies investigated its geological source using constraints from satellite and ground magnetic field measurements, as well as from surface magnetic susceptibility measurements on rocks from the Panafrican Mobile Belt Zone (PMBZ). Here we combine magnetic field data modelling and rock magnetic property measurements (susceptibility and natural remanent magnetization, NRM) on many samples from this PMBZ and the surrounding formations. It reveals that NRM is a significant component of the total magnetization (Mt) of the BMA source, which reaches 4.3A/m with maximum thicknesses of 38 and 54km beneath the western and eastern parts of the BMA. Only the isolated and relatively thin banded iron formations and some migmatites show such Mt values. Thus we suggest that the thick BMA source may be composed either by overlapped slices of such metamorphic rocks, or by an iron-rich mafic source, or by a combination of these two geological structures.

Paleomagnetic and paleosecular variation study of the Mt. Cameroon volcanics (0.0–0.25 Ma), Cameroon, West Africa

Mt. Cameroon (4075 m) is a large volcanic horst and is presently the only active volcano on the Cameroon Volcanic Line (eight eruptions during the last century). This location belongs to the famous Bangui magnetic anomaly and is thus of particular interest for the investigation of the persistence and the evolution of the anomaly with time. We have conducted a paleomagnetic study of 94 samples drilled from 13 lavas flow on Mount Cameroon (0.0–0.25 Ma). Low-field susceptibility versus temperature ( k– T) and SIRM experiments indicated that magnetite (575 °C) was the dominant magnetic carrier and that several sites were characterized by low-temperature mineral phases (e.g. 100–200 °C and 300–400 °C), reflecting the presence of titanomagnetite with low Titanium content. At least eight samples from each site were demagnetized using both thermal treatment and alternating fields (a.f.). One contemporaneous site provided inconsistent and scattered directions and was rejected. Successful mean paleomagnetic directions were obtained for 12 sites but two sites exhibited inclinations which were significantly away from the inclination of the geocentric axial dipole (GAD) and VGP latitudes lower than 60°. Their age indicates that they were likely recorded during a period of reduced dipole intensity and they were thus discarded. The time-averaged direction derived from this final selection has a mean declination of 354.2° and a mean inclination of 5.1°, which is 3° lower than the GAD inclination. The corresponding VGP position is slightly far sided but contains the north geographic pole. The angular standard deviation (13.7°) of the VGPs is also not significantly different from the predictions of model G at this latitude. Thus we found no evidence for a long-term anomaly in this area but additional sites are needed to provide more conclusive indications.

Gradients in the interpretation of satellite-altitude magnetic data: an example from central Africa

The Euler and the analytic signal methods have been implemented in spherical coordinates to facilitate the interpretation of spherically-registered potential-field anomaly data collected by satellites. Model studies show that the methods are able to delineate the edges of idealized models even from 400 km altitude; the depth resolution is, however, not adequate from this altitude. A very large magnetic anomaly, the Bangui anomaly in central Africa, was studied using these methods. These results, from Magsat data, suggest that impact related remagnetization (Girdler, R.W.,Taylor, P.T., Frawley, J.J., 1992, A possible impact origin for the Bangui magnetic anomaly (Central Africa), Tectonophysics, 212, 45–58) and presence of a highly magnetic source (Ravat, D., Hinze, W.J., von Frese, R.R.B., 1992, Analysis of Magsat magnetic contrasts across Africa and South America. Tectonophysics 212, 59–76) near the center of the impact structure are likely contributors to the anomaly.

Improvement of equivalent source inversion technique with a more symmetric dipole distribution model

A technique which generates an equal-area distribution model for numerical integration on the surface of a sphere by equally subdividing the faces of a spherical icosahedron was applied to the analysis of polar orbiting geophysical observatory crustal magnetic anomaly data and compared with the longitude-latitude gridding model currently being used. The region in Bangui, Central African Republic, which includes the Bangui anomaly was chosen for comparison of the two models. Using the equivalent source inversion technique and the technique of principal component regression a magnetization function was calculated for each model. With an average dipole spacing of 2.3° for each model the results indicated good correlation of the inverse solution between the models up to 99.99% of the trace retention of the singular value decomposition. At 99.99% of the trace retention, equatorial instabilities begin to arise. Further increase of the percentage of trace retention led to greater equatorial instabilities; however, the severity of the instabilities were less for the proposed model. At 99.99% of trace retention, a 15% smaller standard deviation of magnetization function was found for the proposed model than for the current model, and at 99.9999% at 42% smaller standard deviation of magnetization function was found for the proposed model. The better results seen with this model are thought to be due to the high symmetry and shape consistency of the model's surface area sectors which are qualities lacking in the longitude-latitude distribution model currently in use. The proposed model is, however, limited in that only discrete dipole spacings are possible, as opposed to the currently used model which permits a continuous range of dipole spacings.

A possible impact origin for the Bangui magnetic anomaly (Central Africa)

Bangui is one of the most impressive magnetic anomalies on Earth and by far the largest over Africa. At Magsat altitudes, it has an amplitude of 28 nT and covers an area of 700,000 km 2. Evidence presented suggests that the anomaly might be related to a large impact structure early in the history of the Earth: north-south, west-east, and vertical derivatives of the 5-min topographic data base all reveal a double ring structure with outer ring diameter ~ 810 km and inner ring diameter ~ 490 km. The Precambrian geology shows several small ancient basins within a large basin with a basement high in the middle such as found in large impact structures. The basement complex is early Precambrian and hence the impact must be of this age. The magnetic anomaly is interpreted as being due to strong remanent magnetism in the ancient crater floor and surrounds. The anomaly can be satisfactorily modelled by a 4.5-km-thick disc, 800 km in diameter beneath the ancient basins. To model the amplitude and shape of the anomaly requires an intensity of 10 A m −1 with direction D = N 18° W, I = + 25°, i.e., significantly different from the present dipole field. The magnetisation is likely to include strong shock remanence (SRM) acquired at the time of impact, thermal remanence (TRM), partial thermal remanence (PTRM), thermochemical remanence (TCRM) and chemical remanence (CRM) acquired soon after the impact. All depend on the size of the impact and all contribute to the high intensity required to explain the anomaly.

Seismological investigation of the Bangui magnetic anomaly region and its relation to the margin of the Congo craton

Inversion of teleseismic travel times of P-waves recorded by a temporary linear array of seismological stations has been carried out in order to investigate the deep P-wave velocity structure in the Central African Republic in a region where a large magnetic anomaly—one of the largest occurring over a stable continent—has been revealed by satellite and surface surveys. Because of the good azimuthal coverage of seismic sources, it turned out that most of blocks are sampled by rays crossing in widely different directions. Lateral inhomogeneities strongly bias 2-D inversion and explain why 3-D inversion improves the fit of the data. Structural variations persist down to a depth of 160 km. Crustal thickness is about 40 km. Lateral inhomogeneities may be related to the northern margin of the Congo craton where up to 4% higher velocities are observed. The similar patterns of the Bangui magnetic anomaly and of the seismic velocity distribution suggest a close relation between the two. The abnormal magnetic field could be due to a deep structure with a higher susceptibility.

Reduction of satellite magnetic anomaly data : Langel, R.A., E.V. Slud and P.J. Smith, 1984. J. Geophys. (Z. Geophys.), 54(3):207–212. Goddard Space Flight Center, Greenbelt, Md. 20771, USA

Analysis of global magnetic anomaly maps derived from satellite data is facilitated by inversion to the equivalent magnetization in a constant thickness magnetic crust or, equivalently, by reduction to the pole. Previous inversions have proven unstable near the geomagnetic equator. The instability results from magnetic moment distributions which are admissible in the inversion solution but which make only small contribution to the computed values of anomaly field. Their admissibility in the solution could result from noisy or incomplete data or from small poorly resolved anomalies. The resulting magnetic moments are unrealistically large and oscillatory. Application of the method of principal components (e.g. eigenvalue decomposition and selective elimination of less significant eigenvectors) is proposed as a way of overcoming the instability and the method is demonstrated by applying it to the region around the Bangui anomaly in Central Africa.

Preliminary correlations of MAGSAT anomalies with tectonic features of Africa

An overview of the MAGSAT scalar anomaly map for Africa has suggested a correlation of MAGSAT anomalies with major crustal blocks of uplift or depression and different degrees of regional metamorphism. The strongest MAGSAT anomalies in Africa are closely correlated spatially with major tectonic features. Although a magnetic anomaly caused by a rectangular crustal block would be offset from the block's center by the effects of magnetic inclination, an anomaly caused by real crustal blocks of varying uplift, depression, and degree of regional metamorphism would be located nearer to the locus of greatest vertical movement and highest grade of metamorphism. Thus, the Bangui anomaly may be caused by a central old Precambrian shield, flanked to the north and south by two relatively young sedimentary basins.

The Bangui Magnetic Anomaly: Its geological origin

Satellite magnetometer data have revealed a long‐wave length magnetic anomaly in the vicinity of Bangui, Central African Republic. Substantiation of the anomaly has been made through analysis of air and ground magnetic surveys. A correlative gravity anomaly has also been observed in ground gravity data. The anomaly, in an area of highly metamorphosed Precambrian rocks, does not correlate with any surficial features but is definitely crustal in origin. An interpretation consistent with all available geophysical and geological data is that the anomaly could be the result of an early intrusion into the crustal rocks of this region, followed by subsidence and deformation.

The N. A. S. A. Earth and ocean dynamics programme

The central theme of N. A. S. A.’s applications programme in general is to provide a platform for a broad effort to develop practical tools, predictive models and observational spaceborne and ground systems to further social and economic benefits to society. This programme should serve as a tool for and as a positive impetus to ( a ) realizing the equitable and efficient use and conservation of natural resources, ( b ) expanding the educational opportunities and medical services for all people, ( c ) providing new opportunities for exchange of information and lessening of international tensions, and ( d ) providing increased business and social communications between nations. The Earth and ocean dynamics programme is part of N. A. S. A.’s overall applications programme and consists in essence of two major parts; one dealing with the solid Earth and the other with the world’s oceans. In this paper, the main emphasis will be placed upon the first, since this part fits best into the theme of this special session. Practical uses and applications of space systems, science and technology will be discussed. What are the phenomena to be observed, their state of the art and their possible future needs? For instance, the present state of tectonic motion determination is to estimate its average motion over millions of years. Our aim is to understand and determine these motions to say 0.5 cm/year over one or two years in order to help in the development of earthquake prediction models, to quote an example. Is there a correlation between the Earth’s gravity and magnetic fields and what is its implication for earth resources exploration? The Bangui anomaly in central Africa is a good example. Large mineral deposits are associated with this gravity and magnetic anomaly.