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The lithospheric architecture of Africa: Seismic tomography, mantle petrology, and tectonic evolution

¹ Minerals Targeting International, 26/17 Prowse Street, West Perth, Western Australia 6005, Australia, and GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia
² GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia
³ Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA
⁴ GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, New South Wales 109, Australia
⁵ Western Mining Services (Australia), 26/17 Prowse Street, West Perth, Western Australia 6005, Australia
⁶ GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia, and Geological Survey of New South Wales, Maitland, New South Wales 2320, Australia
⁷ Western Australian Centre for Geodesy, Curtin University of Technology, Perth, Western Australia 6845, Australia
⁸ GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia, and British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK
⁹ Department of Geology-Petrology-Geochemistry, Université Jean-Monnet, 42023 Saint-Étienne, France

View larger version (63K): [in this window] [in a new window]   Figure 1. Map of the bedrock geology of Africa, outlining the major subdivisions of the crust discussed in the text. Cratons and Micro-continents: West African Craton (Ia—Reguibat Shield; Ib—Man-Lèo Shield); Congo Craton (IIa—Gabon-Kamerun Shield; IIb—Bomu-Kibalian Shield; IIc—Kasai Shield; IId—Angolan Shield); Ugandan Craton—III; Tanzanian Craton (IVa—Northern Terrane; IVb—Southern Terrane; IVc—Dodoma Zone); Kaapvaal Craton (Va—Southern Terrane; Vb—Central Terrane; Vc—Pietersburg Terrane; Vd—Western Terrane); Zimbabwe Craton—VI; Limpopo Block—VII; Bangweleu Block—VIII. West African Mobile Zone: TB—Tuareg Block; BNB—Benin-Nigerian Block. East African Orogenic Zone: ANS—Arabian-Nubian Shield; MB—Mozambique Orogenic Belt. Fold Belts: Paleoproterozoic Belts: ub—Ubendian; us—Usagaran; rb—Ruwenzory; kb—Kheis; oi—Okwa inlier; mb—Magondi; wb—West Central African; nekb—North-Eastern Kibaran. Paleo-Mesoproterozoic Province: rp—Rehoboth. Mesoproterozoic Belts: krb—Kibaran; ib—Irumide; sib—Southern Irumide; chk—Chomo-Kolomo; nnb—Namaqua-Natal. Neoproterozoic Belts: zb—Zambezi; la—Lufilian arc; db—Damara; kob—Kaoko; gb—Gariep; ob—Oubanguides; aab—Anti-Atlas; phb—Pharusian; dab—Dahomeyean; rob—Rockellides; mrb—Mauritanides; lb—Lurio; sb—Saldania. Neoproterozoic Basins: bsC—Congo; bsTa—Taoudeni; bsTi—Tindouf; bsV—Volta.

View larger version (65K): [in this window] [in a new window]   Figure 2. Map of the African upper lithosphere showing the tectonothermal age of different domains.

View larger version (81K): [in this window] [in a new window]   Figure 3. Checkerboard test of shear-wave tomographic model. Each column shows input anomalies introduced into a layer, together with output anomalies at layers 1–5 (0- to 400-km depth). A—anomalies introduced in Layer 1 (0–100 km); B—anomalies introduced in Layer 2 (100–175 km); C—anomalies introduced in Layer 3 (175–250 km).

View larger version (70K): [in this window] [in a new window]   Figure 4. Test of depth smearing for a high-velocity lid of craton dimensions. Tomographic model in left column; recovered anomalies from synthetic model in right-hand column. Synthetic anomalies were introduced in Layers 1 and 2, as shown by box patterns in right-hand column. The results demonstrate that the observed anomalies at 250- to 325-km depths are unlikely to be produced by smearing from real anomalies in the upper two layers.

View larger version (89K): [in this window] [in a new window]   Figure 5. Upper panel shows tomographic image (S-wave velocity [Vs]) of Africa, 0- to 100-km depth slice; reference velocity is 4.6 km/sec. Red to white colors denote velocities much higher than the starting model; blue-green colors show Vs much lower than the starting model. Lower panel shows the density of ray paths through individual cells, expressed as log (path length/100 km). Areas with blue colors are poorly constrained, whereas yellow to white areas have relatively good ray-path coverage. White line outside the continental margins is the 2000 m bathymetric contour.

View larger version (85K): [in this window] [in a new window]   Figure 6. Tomographic image (S-wave velocity [Vs]) of Africa, 100- to 175-km depth slice, with map of ray-path coverage as per Figure 5. Reference velocity is 4.5 km/sec.

View larger version (78K): [in this window] [in a new window]   Figure 7. Tomographic image (S-wave velocity [Vs]) of Africa, 175- to 250-km depth slice, with map of ray-path coverage as per Figure 5. Reference velocity is 4.48 km/sec.

View larger version (85K): [in this window] [in a new window]   Figure 8. Tomographic image (S-wave velocity [Vs]) of Africa, 250- to 325-km depth slice, with map of ray-path coverage as per Figure 5. Reference velocity is 4.56 km/sec.

View larger version (66K): [in this window] [in a new window]   Figure 9. Cross sections through the upper 1000 km (10 layers) of the tomographic model, along lines shown in the map (0- to 100-km tomography from Fig. 4); no vertical exaggeration. See text for explanation of velocity scale. Vertical arrows on each section indicate the coastline. WAC—West Africa Craton; ESC—Eastern Saharan Craton; EAR—East Africa Rift; ANS—Arabian-Nubian Shield; CC—Congo Craton; DO—Damaran Orogen; KC—Kalahari Craton; H—Hoggar Swell; Vs—S-wave velocity.

View larger version (74K): [in this window] [in a new window]   Figure 10. Three-dimensional representation of the tomographic model, looking up and to the NE from below the south Atlantic Ocean. Velocity anomalies have been normalized at each depth layer (equal standard deviations) for comparison. Red volumes have S-wave velocity (Vs) 1.9% above the normalized model; blue volumes have Vs 1.9% below the model. The higher Vs zones beneath the cratons are distinct from lower Vs (upwelling hot) material (East Africa Rift and offshoots) to depths >300 km. LAB—"Lithosphere-Asthenosphere Boundary."

View larger version (61K): [in this window] [in a new window]   Animation 1. 3D representation of isocontours of positive-velocity anomalies beneath Africa (shaded red). African coastline and 0- to 100-km horizontal tomography slice plotted for reference. The anomalies are isocontoured between 1.2% and 2.4%, at intervals of 0.1%, and the results are presented as a time series. If you are viewing the PDF of this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00179.S1 or the full-text article on http://geosphere.gsapubs.org/ to view Animation 1.

View larger version (52K): [in this window] [in a new window]   Animation 2. A perspective view of the depth-normalized, fast-velocity anomalies beneath Africa, isocontoured at +1.9%. The time series represents varying azimuth, viewed from the southwest. If you are viewing the PDF of this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00179.S2 or the full-text article on http://geosphere.gsapubs.org/ to view Animation 2.

View larger version (49K): [in this window] [in a new window]   Figure 11. Chemical tomography section (after O'Reilly and Griffin, 2006) showing the vertical distribution of rock types and meta-somatic signatures, the mean XMg of olivine, and the mean S-wave velocity (Vs) in a generalized subcontinental lithospheric mantle (SCLM) section sampled by the Group I (90 Ma) kimberlites of the SW Kaapvaal Craton. XMg is estimated from garnet-xenocryst compositions (Gaul et al., 2000); Vs is estimated by applying the algorithms of Hacker and Abers (2004) to whole-rock compositions and modes calculated from garnet xenocrysts as described by O'Reilly and Griffin (2006). Values of mean Vs calculated from xenoliths (James et al., 2004) are shown for comparison.

View larger version (121K): [in this window] [in a new window]   Figure 12. Map of southern Africa showing the distribution of temperature at 150-km depth, estimated from paleogeotherms defined by xenolith and xenocryst data on kimberlite pipes of different ages. Approximate ages of some kimberlites are shown in brackets; unlabeled ones were intruded at 85–100 Ma.

View larger version (80K): [in this window] [in a new window]   Figure 13. Chemical tomography sections for the subcontinental lithospheric mantle (SCLM) beneath selected kimberlite fields (after Griffin et al., 2003a), showing the relationships between SCLM composition and thickness and the seismic tomography model in the 100- to 175-km depth slice.

View larger version (52K): [in this window] [in a new window]   Figure 14. Cartoon showing the sequential tectonic assembly of Africa, based on the upper lithosphere domains and reflecting the crustal terrane history. Plate boundaries and magmatic arcs omitted for simplicity.

View larger version (122K): [in this window] [in a new window]   Figure 15. Distribution of low-volume melts (alkaline rocks, carbonatites, and kimberlites) and Mesozoic to Cenozoic rifts relative to the velocity structure and cratonic blocks (see Fig. 1) of Africa. Data on alkaline rocks and carbonatites from Woolley (1987); locations of kimberlites from Faure (2006). Blue polygons—rifts; white asterisks—volcanoes; green stars—carbonatites; pink circles—nepheline syenites; white squares—kimberlites; CVL—Cameroon Volcanic Line. S-wave velocity (Vs) image is 100- to 175-km depth slice.

View larger version (81K): [in this window] [in a new window]   Figure 16. Detailed S-wave velocity (Vs) tomography model at 200-km depth across the Kalahari Craton of southern Africa (Fouch et al., 2004), with locations of kimberlites (Faure, 2006). Yellow dashed line outlines the Kaapvaal Craton; solid yellow line—Bushveld Complex. Circles and ovals mark locations of chemical tomography sections shown in Figure 13. At this level of detail, as well as on the regional scale (Fig. 15), the kimberlites cluster around the margins of high-Vs volumes and in low-velocity zones.

View larger version (71K): [in this window] [in a new window]   Figure 17. Interpretation of the seismic tomography model (100- to 175-km depth) in terms of the tectonothermal age of the sub-continental lithospheric mantle (SCLM). A—Archon; P—Proton; T—Tecton.