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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

We present a new analysis of the lithospheric architecture of Africa, and its evolution from ca. 3.6 Ga to the present. Upper-lithosphere domains, generated or reworked in different time periods, have been delineated by integrating regional tectonics and geochronology with geophysical data (magnetic, gravity, and seismic). The origins and evolution of lower-lithosphere domains are interpreted from a high-resolution global shear-wave tomographic model, using thermal/compositional modeling and xenolith/xenocryst data from volcanic rocks. These data are integrated to map the distribution of ancient highly depleted subcontinental lithospheric mantle (SCLM), zones of younger or strongly modified SCLM and zones of active mantle upwelling, and to relate these to the evolution of the upper lithosphere domains.

The lithospheric architecture of Africa consists of several Archean cratons and smaller cratonic fragments, stitched together and flanked by younger fold belts; the continental assembly as we see it has only existed since lower Paleozoic time. The larger cratons are underlain by geochemically depleted, rigid, and mechanically robust SCLM; these cratonic roots have steep sides, extending in some cases to 300-km depth. Beneath smaller cratons (e.g., Kaapvaal) extensive refertilization has reduced the lateral and vertical extent of strongly depleted SCLM. Some cratonic roots extend 300 km into the Atlantic Ocean, suggesting that the upper lithosphere may detach during continental breakup, leaving fragments of SCLM scattered in the ocean basin.

The cratonic margins, and some intracratonic domain boundaries, have played a major role in the tectonics of Africa. They have repeatedly focused ascending magmas, leading to refertilization and weakening of the SCLM. These boundaries have localized successive cycles of extension, rifting, and renewed accretion; the ongoing development of the East Africa Rift and its branches is only the latest stage in this process. The less depleted SCLM that underlies some accretionary belts may have been generated in Archean time, and repeatedly refertilized by the passage of magmas during younger tectonic events. Our analysis indicates that originally Archean SCLM is far more extensive beneath Africa than previously recognized, and implies that post-Archean SCLM rarely survives the collision/accretion process. Where continental crust and SCLM have remained connected, there is a strong linkage between the tectonic evolution of the crust and the composition and modification of its underlying SCLM.