Depth to the bottom of the magnetic layer, crustal thickness, and heat flow in Africa: Inferences from gravity and magnetic data

https://doi.org/10.1016/j.jafrearsci.2021.104204Get rights and content

Highlights

  • Mapping crustal thickness variations beneath Africa using gravity data.

  • Determination of the depth to the bottom of the magnetic layer in Africa using magnetic data.

  • Calculation of the geothermal gradient and heat flow in Africa.

Abstract

Data from the Earth Gravitational Model (EGM2008) and the Earth Magnetic Anomaly Grid (EMAG2) were used to develop a continental scale crustal thickness model for Africa, and to estimate the depth to the bottom of the magnetic layer (DBML) and the geothermal gradient and heat flow. The results are: (1) the estimated DBML from the magnetic data varies from ~23.0 to ~37.2 km. The shallowest DBML values are located in the northern, eastern, and western parts of the continent, whereas the deepest values are observed in the central and southern regions. (2) The estimated crustal thickness based on gravity data varies from ~29.9 km in the northern and western parts of Africa to ~48.0 km in its southern regions, with an average thickness of 35.1 km for the whole continent. (3) The estimated heat flow varies between high values of 46–59 mW/m2, observed in the northern, eastern, and western regions to low values of ~< 41 mW/m2, observed in the central and southern parts of the continent. (4) The geothermal gradient values vary between 14.5 and 23.6 °C/km (5) The East African rift zone is underlain by shallow DBML characterized by high heat flow values that vary between 42 and 59 mW/m2 (6) The heat flow anomalies in Egypt and Libya may be associated with the zone of the Pelusium megashear system, and it shows heat flow values that vary between 36.3 and 59.0 mW/m2. The current study has taken advantage of the availability of the EGM2008 and EMAG2 datasets to map crustal thickness variations and DBML beneath the continental landmass of Africa.

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.

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