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Seismic Tomography, Mantle Petrology, and Tectonic Evolution

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Seismic Tomography, Mantle Petrology, and Tectonic Evolution The lithospheric architecture of Africa: Seismic tomography, mantle petrology, and tectonic evolution G.C. Begg 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 W.L. Griffi n L.M. Natapov Suzanne Y. O’Reilly GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia S.P. Grand Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA C.J. O’Neill GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, New South Wales 109, Australia J.M.A. Hronsky Western Mining Services (Australia), 26/17 Prowse Street, West Perth, Western Australia 6005, Australia Y. Poudjom Djomani 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 C.J. Swain Western Australian Centre for Geodesy, Curtin University of Technology, Perth, Western Australia 6845, Australia T. Deen 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 P. Bowden Department of Geology-Petrology-Geochemistry, Université Jean-Monnet, 42023 Saint-Étienne, France ABSTRACT ancient highly depleted subcontinental litho- refertilization has reduced the lateral and spheric mantle (SCLM), zones of younger or vertical extent of strongly depleted SCLM. We present a new analysis of the litho- strongly modifi ed SCLM and zones of active Some cratonic roots extend ≥300 km into the spheric architecture of Africa, and its evolu- mantle upwelling, and to relate these to the Atlantic Ocean, suggesting that the upper tion from ca. 3.6 Ga to the present. Upper- evolution of the upper lithosphere domains. lithosphere may detach during continental lithosphere domains, generated or reworked The lithospheric architecture of Africa breakup, leaving fragments of SCLM scat- in different time periods, have been delin- consists of several Archean cratons and tered in the ocean basin. eated by integrating regional tectonics and smaller cratonic fragments, stitched together The cratonic margins, and some intra- geochronology with geophysical data (mag- and fl anked by younger fold belts; the conti- cratonic domain boundaries, have played a netic, gravity, and seismic). The origins and nental assembly as we see it has only existed major role in the tectonics of Africa. They evolution of lower-lithosphere domains are since lower Paleozoic time. The larger cratons have repeatedly focused ascending magmas, interpreted from a high-resolution global are underlain by geochemically depleted, leading to refertilization and weakening of shear-wave tomographic model, using ther- rigid, and mechanically robust SCLM; these the SCLM. These boundaries have localized mal/compositional modeling and xenolith/ cratonic roots have steep sides, extending successive cycles of extension, rifting, and xenocryst data from volcanic rocks. These in some cases to ≥300-km depth. Beneath renewed accretion; the ongoing development data are integrated to map the distribution of smaller cratons (e.g., Kaapvaal) extensive of the East Africa Rift and its branches is Geosphere; February 2009; v. 5; no. 1; p. 23–50; doi: 10.1130/GES00179.1; 17 fi gures; 2 animations. For permission to copy, contact [email protected] 23 © 2009 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/5/1/23/3337106/i1553-040X-5-1-23.pdf by guest on 23 September 2021 Begg et al. only the latest stage in this process. The less relation between crustal history and SCLM REGIONAL GEOLOGY AND depleted SCLM that underlies some accre- composition implies a strong linkage between TECTONICS OF AFRICA—AN tionary belts may have been generated in crust and mantle, and the processes affecting OVERVIEW Archean time, and repeatedly refertilized by both, over time spans measured in eons. the passage of magmas during younger tec- The secular evolution of SCLM composi- This brief review of the complex basement tonic events. Our analysis indicates that orig- tion has major consequences for the nature of geology of Africa (Fig. 1) focuses primarily on inally Archean SCLM is far more extensive crustal tectonics through time. Archon SCLM the timing and nature of major tectonothermal beneath Africa than previously recognized, is buoyant relative to the underlying astheno- events that have affected specifi c areas. It is and implies that post-Archean SCLM rarely sphere, and this buoyancy may have played an intended to provide a background for examin- survives the collision/accretion process. important role in the stabilization of cratons ing correlations between mantle composition, Where continental crust and SCLM have (Poudjom Djomani et al., 2001). Tecton SCLM, the geophysical signature of the SCLM, and remained connected, there is a strong linkage being less depleted, is buoyant relative to the the evolution of the overlying crust. Details between the tectonic evolution of the crust asthenosphere as long as its geotherm remains of age relationships and alternative structural and the composition and modifi cation of its elevated, but on cooling it is likely to become interpretations have necessarily been subordi- underlying SCLM. unstable, and may delaminate and sink. At some nated to this larger aim. To simplify reading, convergent margins, crustal thickening and the key references are gathered at the beginning of INTRODUCTION transformation of mafi c lower crust to eclogite each section. may result in a more continuous drip-style of Since late Proterozoic time, Africa has been The lithosphere is the outermost, relatively lithosphere removal (e.g., Sobolev et al., 2005). a center of continental accretion. Today it is rigid shell of the Earth, made up of the crust In either case, the ensuing upwelling of astheno- surrounded on three sides by divergent plate and the underlying lithospheric mantle. Beneath spheric material can lead to widespread crustal boundaries; despite minor compression on its the continents, the subcontinental lithospheric melting, as in the Central Asian Orogenic Belt northern edge, it is essentially stationary. Global mantle (SCLM) varies widely in thickness, (Zheng et al., 2006; references therein). Refer- seismic tomography and geodynamic modeling from a few tens of kilometers beneath active tilization of older SCLM by asthenosphere- suggests that it sits atop a major mantle upwell- rift zones to >250 km beneath some ancient cra- derived melts leads to an increase in SCLM den- ing, the African Superswell, and is beginning tonic blocks. The SCLM, as sampled by xeno- sity, enhancing the probability of delamination. to break apart along the East Africa Rift Zone liths in volcanic rocks and some exposed mas- The nature and history of the SCLM therefore (Grand et al., 1997; Nyblade and Langston, sifs, consists almost entirely of peridotitic rocks will affect the response of the overlying crust to 1998; Lithgow-Bertelloni and Silver, 1998; (olivine + orthopyroxene ± garnet ± clinopyrox- tectonic stresses. Lithospheric architecture, and Ritsema et al., 1998, 1999; Gurnis et al., 2000; ene ± spinel). Re-Os isotopic studies show that particularly the presence of boundaries between Kieffer et al., 2004; Simmons et al., 2007). beneath some Archean cratons, the SCLM is at domains with different types of SCLM, will be least as old as the oldest known crustal rocks important in controlling crustal tectonics, and Archean Cratons (Archon, Proton/Archon, (Carlson et al., 1999; Griffi n et al., 2004; refer- especially the transport of fl uids and magmas and Tecton/Proton/Archon) ences therein). from depth. This control may have important From studies of mantle-derived xenoliths and implications for the distribution of major ore The basic fabric of Africa (Fig. 1) consists of the distribution of diamonds, Janse (1994) rec- deposits; an understanding of crust-mantle link- several major Archean cratons and smaller cra- ognized that the nature of the SCLM is related ages and lithospheric architecture therefore has tonic fragments, stitched together and fl anked by to the tectonothermal age of the overlying crust, direct economic relevance. younger fold belts; the framework of the mod- as defi ned by the timing of the last major tec- Most of our detailed knowledge of SCLM ern continent was established in the Late Neo- tonothermal event. We will use a modifi ca- composition comes from the study of xenoliths proterozoic to earliest Paleozoic Pan- African tion of Janse’s terminology, in which the last and xenocrysts brought up in volcanic rocks orogenic event. The cratons themselves are major crustal tectonothermal event in Archons (Griffi n et al., 1999; O’Reilly and Griffi n, 2006; aggregates of continental fragments 3.6–2.5 Ga occurred at ≥2.5 Ga, in Protons between 2.5 and references therein). However, these samples are in age, and differ markedly from one another in 1 Ga, and in Tectons at <1 Ga. scattered in time and space, and are not avail- their structure and evolution. Data from xenoliths and exposed massifs able for large areas of the continents. To map show a marked secular evolution in SCLM com- the
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