Depth to the bottom of the magnetic layer, crustal thickness, and heat flow in Africa: Inferences from gravity and magnetic data
Introduction
The Precambrian basement rocks underlie most of the African continent. The continent gathered and stabilized in the Proterozoic by amalgamation of ancient Precambrian continental kernels that included the Congo, Kalahari, Tanzania and West African cratons (Choubert and Faure-Muret, 1990; de Wit et al., 1992; Bleeker, 2003; van Hinsbergen et al., 2011). In general, the continental landmass of Africa preserves a long geological record that covers much of the of Earth's history, including the formation of some of the oldest continental crust known today in the Kalahari craton to billions of years of collisions, arc-accretions, amalgamation and fragmentation of continental land masses (de Wit et al., 1992; Stern, 1994; Zhao et al., 2002). The oldest rocks of Africa are represented by old continental cratons (Fig. 1). These cratons are surrounded by 650–450 Ma Pan-African fold belts (Kennedy, 1964, 1996; Shackleton, 1976; Cahen et al., 1984), where most of the rifts and faults are located.
Crustal thickness estimates under Africa were first provided by global models, using a compilation of seismic datasets (Soller et al., 1982; Cadek and Martinec, 1991). Nataf and Ricard (1996) prepared a tomographic upper mantle 3SAM model, which is a combined crustal model with chemical and geophysical information. CRUST2.0 model (Bassin et al., 2000) was commonly used in gravity and geodynamic modeling. It is most widely applied for crustal corrections in seismological investigations (Zhou et al., 2006). Tedla et al. (2011) have presented a continental-scale crustal thickness map beneath Africa based on gravity data using Euler deconvolution technique, however, their results were a subject of debate. Globig et al. (2016) have mapped the lateral variations in the Moho and the geometry of the lithosphere-asthenosphere boundary (LAB), and created a crustal thickness map of Africa.
The depth to the bottom of the magnetic layer (DBML) is an important parameter to constrain the temperatures in the crust (Idarraga-García and Vargas, 2018). The DBML may equal to the Curie point depth (CPD), where the Curie point of magnetite is about 580 °C (Eppelbaum et al., 2014). At the CPD, the ferromagnetic minerals change to paramagnetic minerals (Nagata, 1961). In other words, the DBML may equal to the depth where the magnetic minerals are replaced by the non-magnetic minerals (Ravat et al., 2007).
The geothermal gradient and heat flow in Africa is poorly studied and constrained at present. However, several local studies have been carried out to estimate the CPD, heat flow in Africa and the variations in the crustal thickness and structure. Leseane et al. (2015) have studied the thermal structure in the northwestern part of Botswana beneath the Okavango Rift Zone (ORZ) surrounding the basement using inversion techniques of high-resolution aeromagnetic data. Their results indicate that the CPD varies from 8 to 15 km, while the heat flow was estimated at 60–90 mW/m2 under a ~60 km wide NE-trending zone of the ORZ that is characterized by rift-related faults and thinning in the crust to less than 30 km in the southwestern and northeastern parts of the rift. Lawal and Nwankwo (2017) have used high-resolution aeromagnetic data to understand the thermal structure of the Chad basin. Their results indicate that the DBML varies between 18.18 and 43.64 km, while the geothermal gradient varies from 13.29 to 31.90 °C/km, and the heat flow values fluctuate between 33.23 and 79.76 mW/m2. Mono et al. (2018) have suggested anomalous geothermal conditions at the Loum-Minta area, based on aeromagnetic data. They have estimated the CPD at 5.22–14.35 km, with an average depth of 9.09 km, and the heat flow at ~101.10–~277.80 mW/m2 with an average value of ~180.6 mW/m2.
Begg et al. (2009) presented a new study of the lithospheric structure of the African continent, and the stages of its evolution from ~3.6 Ga to the present. Their study provides many insights into the processes of assembly and breakup of the continent. The lithospheric domains were delineated by integrating regional tectonics, geochronological and geophysical data. The lower lithospheric domains are interpreted from a global shear wave tomographic model, based on thermal/compositional modeling and xenolith data from volcanic rocks. Several Archean cratons and smaller cratonic fragments stitched together and were flanked by rifted margins of Proterozoic age. The larger cratons are underlain by the rigid subcontinental lithospheric mantle. The subcontinental lithospheric mantle is the uppermost solid part of the mantle associated with the continental lithosphere (Pearson and Nowell, 2002).
The objective of the current study is to estimate the DBML in Africa using the fractal distribution of the power spectrum of the magnetic field for data derived from the EMAG2 grid. The DBML was used for calculating the geothermal gradient and regional heat flow in Africa. As mentioned earlier, the DBML may be equivalent to the depth at which ferromagnetic minerals change to a paramagnetic state when they reach the Curie point temperature (Nagata, 1961), or the depth where the magnetic rocks are replaced by non-magnetic rocks (Ravat et al., 2007). According to the latter definition, the extent in the depth of the magnetic basement has been correlated with the Moho boundary, given that the mantle is generally considered to be non-magnetic (Wasilewski et al., 1979; Wasilewski and Mayhew, 1992; Idaárraga-García and Vargas, 2018). The DBML was correlated with the brittle/ductile boundary crust regime in subduction tectonic systems (Idaárraga-García and Vargas, 2018). The DBML map for South America obtained from the spectral analysis of magnetic data varies between 10 and 60 km. Within the Precambrian basement areas under the ORZ in NW Botswana, the CPD values were estimated at 16–30 km using aeromagnetic data (Leseane et al., 2015). The CPDs were also estimated at 8.6–35.7 km in Egypt using aeromagnetic data (Elbarbary et al., 2018). The other main aim of the current study is to calculate the crustal thickness/Moho depth in Africa from gravity data extracted from the Earth Gravitational Model (EGM2008).
Among other new data sources, the GRACE satellite mission is providing higher resolution datasets of global gravity field anomalies that are widely used for estimating the mass transport and distribution in the Earth’s fluid, caused by climate changes and/or anthropogenic activities (Mohamed et al., 2014b, Mohamed et al., 2015, MohamedSultan et al., 2017; Mohamed, 2019, Mohamed, 2020a, Mohamed, 2020b, Mohamed, 2020c, Mohamed, 2020d; Taha et al., 2021; Mohamed et al., 2021; Mohamed and Gonçalvès, 2021).
Section snippets
Geological setting
The African continent is made up of highly deformed metamorphic and granitic rocks (Wright et al., 1985). These rocks approximately constitute the subsurface over about half of the area of the African continent (Fig. 2). The second half of the continental landmass is underlain by thin sedimentary layers that occupy the sedimentary basins. The shallow sedimentary basins unconformably overlie the metamorphic and igneous rocks of the basement rocks. The sedimentary rocks in the region have ages
Global gravitational model
The Earth Gravitational Models (EGMs) of the Earth consist of spherical harmonic coefficients, published by the office of geomatics at the National Geospatial-Intelligence Agency (http://earth-info.nga.mil/GandG/wgs84/gravitymod/). The new global gravity model has optimally combined the gravitational information that is extracted from dedicated geopotential mapping satellite missions such as CHAMP and GRACE (Pavlis et al., 2008). The EGM2008 model is a complete to spherical harmonic degree and
Gravity data
The crustal thickness model is conducted using the method of Simpson et al. (1985). The main idea is that in most of the Earth's surface, the longer wavelengths of the Bouguer gravity field and the topography show an inverse correlation. The principle of Isostasy offers an explanation of the idea that loads of topographic features on the earth's surface are supported at depth by mass deficiencies as if the lighter crust were floating on the denser underlying mantle (Woollard, 1966; Heiskanen
Crustal thickness map of Africa
The estimated crustal thickness map beneath the African continent is shown in Fig. 6a. Examination of Fig. 6a shows that the average crustal thickness of the continent varies from lower values of ~29.9 km at its northern and western regions, close to the Mediterranean Sea and the North Atlantic ocean to higher values of ~48.0 km in its southern regions. Intermediate thickness values of ~30.0–35.0 km are shown in the northern and central parts of the continent. The highest crustal thickness
Discussion
Interpretation of the global gravity field data gives a better understanding of the crustal structure of Africa, given that the deep seismic data are unavailable for large regions of the continent (Reid et al., 2012). Tedla et al. (2011) have presented a new crustal thickness estimate of Africa, based on modeling of the global free-air gravity field data using Euler deconvolution. The minimum and maximum values of our crustal thickness model (Fig. 6a) are similar with the results (Fig. 6b) of
Conclusion
Our results indicate that the depth to the bottom of the magnetic layer in Africa varies between ~23.0 and ~37.2 km, with the shallowest values located in the northern, eastern, and western regions, whereas the deepest values are located in the central and southern regions. The depth to the Moho boundary shows lower values of ~29.9 km in the northern and western regions, and higher values of ~48.0 km in the southern regions. The results show that the heat flow values vary from lower heat flow
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to thank the editor and the anonymous reviewers of the Journal of African Earth Sciences for their instructive comments and suggestions.
References (101)
- et al.
Australia's Moho: a test of the usefulness of gravity modelling for the determination of Moho depth
Tectonophysics
(2013) A heat production model for stable continental crust
Tectonophysics
(1979)The late Archean record: a puzzle in ca. 35 pieces
- et al.
Quaternary oblique extensional tectonics in the Ethiopian rift (horn of Africa)
Tectonophysics
(1998) - et al.
Meso-archaean and Palaeo-Proterozoic sedimentary sequence stratigraphy of the Kaapvaal craton
Mar. Petrol. Geol.
(2012) - et al.
Curie point depth, heat flow and geothermal gradient maps of Egypt deduced from aeromagnetic data
Renew. Sustain. Energy Rev.
(2018) - et al.
Depth to the bottom of magnetic layer in South America and its relationship to Curie isotherm, Moho depth and seismicity behavior
Geodesy and Geodynamics
(2018) - et al.
Seismic structure of the uppermost mantle beneath the Kenya rift
Tectonophysics
(1994) - et al.
The oldest part of the Barberton granitoid-greenstone terrain, South Africa: evidence for crust formation between 3.5 and 3.7 Ga
Precambrian Res.
(1996) - et al.
Pn arrivals and lateral variations of Moho geometry beneath the Kaapvaal craton
Lithos
(2003)
Neoarchean tectonic history of the Witwatersrand Basin and Ventersdorp Supergroup: new constraints from high-resolution 3D seismic reflection data
Tectonophysics
Gravity based estimates of modern recharge of the Sudanese area
J. Afr. Earth Sci
Gravity applications to groundwater storage variations of the Nile Delta Aquifer
J. Appl. Geophys.
Hydro-geophysical monitoring of the North Western Sahara Aquifer System’s groundwater resources using gravity data
J. Afr. Earth Sci.
Estimation of Curie-point depths, geothermal gradients and near-surface heat flow from spectral analysis of aeromagnetic data in the Loum – Minta area (Centre-East Cameroon)
Egypt. J. Petrol.
3SMAC: an a priori tomographic model of the upper mantle based on geophysical modelingPhysics of the
Earth and Planetary Interiors
Fine structure of the lowermost crust beneath the Kaapvaal craton and its implications for crustal formation and evolution
Earth Planet Sci. Lett.
Active faults in Africa: a review
Tectonophysics
Interpretation of heat-flow density in the central Andes
Tectonophysics
Crustal thickness, discontinuity depth, and upper mantle structure beneath southern Africa: constraints from body wave conversion
Phys. Earth Planet. In.
Curie point depth based on spectrum analysis of magnetic anomaly data in East and Southeast Asia
Tectonophysics
The tectonic evolution of southern Africa: an overview
J. Afr. Earth Sci.
South African seismicity, April 1997 to April 1999, and regional variations in the crust and uppermost mantle of the Kaapval craton
Lithos
Moho depth and crustal composition in Southern Africa
Tectonophysics
Scaling spectral analysis: a new tool for interpretation of gravity and magnetic data
Earth Science India e-Journal
Estimation of depth to the bottom of magnetic sources by a modified centroid method for fractal distribution of sources: an application to aeromagnetic data in Germany
Geophysics
The current limits of resolution for surfacewave tomography in North America, EOS, Trans
Am. geophys. Un.
The lithospheric architecture of Africa: seismic tomography, mantle petrology, and tectonic evolution
Geosphere
Analysis of magnetic anomalies over Yellowstone national park: mapping and curie point isothermal surface for geothermal reconnaissance
J. Geophys. Res.
Spectral analysis of gravity and magnetic anomalies due to rectangular prismatic bodies
Geophysics
Potential Theory in Gravity and Magnetic Applications
Modelling the 2-D seismic velocity structure across the Kenya rift
Tectonophysics
Spherical harmonic expansion of the Earth's crustal thickness up to degree and order 30
Studia Geophys. Geod.
The Geochronology and Evolution of Africa
Carte Geologique Internationale de l'Afrique. Commission for the geological map of the world
Greenstone Belts
formation of an Archean continent
Nature
Cenozoic paleostress and kinematic evolution of the Rukwa–north Malawi rift valley (East African rift system)
Bull. Centr. Rech. Explor. Prod. Elf Aquitane.
Scaling behavior of real earth source distribution: Indian case studies
Crustal structure in Ethiopia and Kenya from receiver function analysis: implications for rift development in eastern Africa
J. Geophys. Res.
Applied Geothermics
Improvements to the Spector and Grant method of source depth estimation using the power-law decay of magnetic field power spectra
Geophysics
The GeoForschungsZentrum Potsdam/Groupe de Recherche de Géodésie Spatiale satellite‐only and combined gravity field models: EIGEN‐GL04S1 and EIGEN‐GL04C
J. Geodes.
New insights into the crust and lithospheric mantle structure of Africa from elevation, geoid, and thermal analysis
J. Geophys. Res. Solid Earth
A seismic refraction investigation of Namaqualand metamorphic complex, South Africa
J. Geophys. Res.
New global maps of crustal basement age
EOS, Trans. Am. geophys. Un. (90 Fall Meet. Suppl, Abstract T53B-1583)
Physical Geodesy, W. H. Freeman, San Francisco (Reprint Available from Section of Physical Geodesy
Evidence for mafic lower crust in Tanzania, East Africa, from joint inversion of receiver functions and Rayleigh wave dispersion velocities
Geophys. J. Int.
Seismicity
Cited by (22)
Structural mapping of the west central Arabian Shield (Saudi Arabia) using downward continued magnetic data
2024, Journal of King Saud University - ScienceLithospheric and asthenospheric properties of the saharan platform inferred from potential field, geoid and heat flow data
2024, Journal of African Earth SciencesGeophysical investigations for the identification of subsurface features influencing mineralization zones
2023, Journal of King Saud University - ScienceDeep learning of GPS geodetic velocity
2022, Journal of Asian Earth Sciences: XCitation Excerpt :The prediction process using RFA is much more time consuming compared to other algorithms. Proposed method can be investigated in the field of magnetic layer, crustal thickness and satellite based ground water storage (Mohamed and Al Deep. 2021, Mohamed, 2020; Taha et al., 2021). New geodetic velocity of NW Iran is estimated by using four methods (BPANN, LSC, BA and RFA) and 42 GPS velocity vectors.
Geophysics and remote sensing applications for groundwater exploration in fractured basement: A case study from Abha area, Saudi Arabia
2021, Journal of African Earth SciencesCitation Excerpt :Geoelectric methods have a wide range of applications beside groundwater exploration e.g., mineral resources (Araffa et al., 2020; Mousa et al., 2020). On regional scale the Earth’s gravity field anomalies, and TRMM data from Gravity Recovery and Climate Experiment satellite mission is widely used for estimating the groundwater storage variations across large and transboundary aquifers (e.g. Mohamed et al., 2014, 2021, Also for geothermal gradient and heat flow studies (Mohamed and Al Deep, 2021). The area under investigation is located 50 km northeast of Abha city, Saudi Arabia; the area is bounded by latitudes 18° 29' 46.32" and 18° 30' 22.31" N and longitude 42° 33' 26.71" and 42° 33' 57.29" E. Figure (1).