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(U-Th)/He Thermochronometry Constraints on Unroofing of the Eastern Kaapvaal Craton and Significance for Uplift of the Southern

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(U-Th)/He Thermochronometry Constraints on Unroofing of the Eastern Kaapvaal Craton and Significance for Uplift of the Southern (U-Th)/He thermochronometry constraints on unroofi ng of the eastern Kaapvaal craton and signifi cance for uplift of the southern African Plateau Rebecca M. Flowers1* and Blair Schoene 1Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA 2Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA ABSTRACT 15oE20oE25oE30oE35oE The timing and causes of the >1.0 km elevation gain of the south- ern African Plateau since Paleozoic time are widely debated. We Africa o Zimbabwe craton report the fi rst apatite and titanite (U-Th)/He thermochronometry 20 S Limpopo belt Lebomb data for southern Africa to resolve the unroofi ng history across a clas- monocline sic portion of the major escarpment that encircles the plateau. The o study area encompasses ~1500 m of relief within Archean basement of 25oS Kaapvaal the Barberton Greenstone Belt region of the eastern Kaapvaal craton. craton Titanite dates are Neoproterozoic. Apatite dates are Cretaceous, with Figure 2 most results clustering at ca. 100 Ma. Thermal history simulations confi rm Mesozoic heating followed by accelerated cooling in mid- to o Late Cretaceous time. The lower temperature sensitivity of the apa- 30 S Kimberlites, tite (U-Th)/He method relative to previous thermochronometry in mostly 143-45 Ma southern Africa allows tighter constraints on the Cenozoic thermal 183 Ma Karoo sills/lavas 0 km 500 history than past work. The data limit Cenozoic temperatures east of o > 2.8 Ga, mostly the escarpment to ≤35 °C, and appear best explained by temperatures 35 S granitoid gneisses Great Escarpment within a few degrees of the modern surface temperature. These results Figure 1. Topography of southern Africa, with Archean base- restrict Cenozoic unroofi ng to less than ~850 m, and permit negligible ment depicted in orange. erosion since the Cretaceous. If substantial uplift of the southern Afri- can Plateau occurred in the Cenozoic as advocated by some workers, then it was not responsible for the majority of post-Paleozoic unroof- the Mesozoic (e.g., Brown et al., 2002; de Wit, 2007; Tinker et al., 2008). ing across the eastern escarpment. Signifi cant Mesozoic unroofi ng is These two hypotheses for the timing of surface uplift suggest fundamen- coincident with large igneous province activity, kimberlite magma- tally different causes of elevation gain. Cenozoic uplift could be due to tism, and continental rifting within and along the margins of south- shallow convection and thermal modifi cation of the upper mantle when ern Africa, compatible with a phase of plateau elevation gain due to Africa attained a stationary position in mid-Tertiary time (e.g., Burke and mantle buoyancy sources associated with these events. Gunnell, 2008). In contrast, Mesozoic elevation gain would point toward deep mantle processes associated with the breakup of Gondwana as the INTRODUCTION source of buoyancy for plateau uplift (e.g., de Wit, 2007). How cratonic plateaus undergo elevation gain with little upper crustal Low-temperature thermochronology is widely used to resolve cool- deformation is a long-standing problem. Cratonic southern Africa, charac- ing during unroofi ng, which in turn can help constrain the uplift history of terized by anomalous elevations and a distinctive topography, is a unique a region. Here “unroofi ng” refers to the thickness of rock removed through example of plateau uplift (Fig. 1). The southern African Plateau, encom- erosion or tectonism, and “uplift” refers to the increase in surface eleva- passing the southeastern half of the African continent, attained >1.0 km tion. We report the fi rst (U-Th)/He data for southern Africa, acquired in of elevation while distal from convergent plate boundaries, unlike most an area characterized by ~1500 m of relief across the Great Escarpment other major continental plateau settings in which horizontal contraction in ancient basement of the eastern Kaapvaal craton (Fig. 2). We exploit was substantial during plateau elevation gain (e.g., Burke, 1996; de Wit, the ability of the apatite (U-Th)/He method to resolve lower temperatures 2007). The “Great Escarpment” encircles much of southern Africa, sepa- (down to 30 °C) than prior thermochronometry in southern Africa to better rating the more highly denuded passive margins from the less denuded limit the magnitude of Cenozoic unroofi ng across the escarpment, evalu- and low-relief plateau interior. The Kaapvaal craton, one of the best-pre- ate the erosional history from the Mesozoic to the present, and consider served Early Archean cratons, is embedded in the Precambrian shield that the implications for uplift of the southern African Plateau. makes up the core of the plateau (Fig. 1). Positioned beneath the cratonic lithosphere is one of the most signifi cant low-seismic-velocity structures TECTONIC SETTING in the deep mantle on Earth, the “African superplume” (e.g., Ritsema et The core of southern Africa is underlain by Archean basement of the al., 1999; Ni et al., 2002). The dramatic escarpment, the atypical eleva- Kaapvaal and Zimbabwe cratons sutured by the Limpopo orogenic belt. tions of the cratonic nucleus, and the distinctive mantle seismic structure Younger terranes were accreted subsequently during the Mesoprotero- all contribute to the unusual character of southern Africa. zoic Namaqua-Natal, Neoproterozoic Pan-African, and Paleozoic Cape Many workers advocate either (1) a mid- to late Tertiary origin of orogenies. The region is last known to have been at sea level in Paleozoic most of the modern topography (e.g., King, 1951; Partridge and Maud, time during deposition of the Karoo Supergroup (Johnson et al., 1996). 1987; Burke, 1996; Burke and Gunnell, 2008), or (2) elevation gain in Deposition began ca. 300 Ma and was terminated by widespread 183 Ma Karoo volcanism during the breakup of Gondwana (e.g., Jourdan et al., *E-mail: rebecca.fl [email protected]. 2005). Other Mesozoic magmatism included the Etendeka fl ood basalts © 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, September September 2010; 2010 v. 38; no. 9; p. 827–830; doi: 10.1130/G30980.1; 2 fi gures; Data Repository item 2010230. 827 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/38/9/827/3539730/827.pdf by Princeton University user on 19 October 2020 A Cenozoic sediments B 183 Ma Karoo lavas Lower Permian Ecca Group KPV99-91 claystones 94, 106 Ma Paleoproterozoic Transvaal Supergroup sedimentary rocks KPV99-94 AGC01-5 81 ±10 Ma Archean Pongola Supergroup 97 ±9 Ma sedimentary rocks and volcanics AGC01-4 Archean granitic crust 26°S 101 ±8 Ma Archean Barberton KPV99-96 t: 636, 725 Ma Greenstone Belt 131, 122 Ma EKC02-40 100 ±7 Ma A t: 689, 871 Ma A/ BS04-7 AGC01-2 111 ±22 Ma 136 ±24 Ma EKC02-64 Mozambique EKC02-65 101 ±11 Ma 96 ±9 Ma EKC03-35 78 ±9 Ma South Africa 27°S Swaziland 31°E32°E C / Time (Ma) A A 300 200 100 0 2000 D 0 W 100 ±7 Ma 94, 106 Ma E 136 ±24 Ma 96 ±9 Ma 101 ±8 Ma 50 81 ±10 Ma 97 ±9 Ma 101 ±11 Ma 1000 111 ±22 Ma 131, 122 Ma 78 ±9 Ma Permian Tertiary overlain by 100 Ecca Group younger sediments Cretaceous sedimentary rocks Elevation (m) 0 Precambrian basement 150 183 Ma Karoo lavas (°C) Temperature 0 40 80 120 160 200 Onset of Karoo Karoo Etendeka- Agulhas ~25x vertical exaggeration Distance (km) Supergroup deposition LIP Parana LIP LIP Episodes of significant kimberlite magmatism Figure 2. A: Topography of the study area. Darker and lighter grays represent lower and higher elevations, respectively. Sample locations and mean apatite (U-Th)/He dates are shown. For the two samples with only two analyses, the individual dates are listed. The individual ti- tanite dates are denoted by “t”. A–A′ marks the location of the cross section in C. B: Simplifi ed geological map of the study area, from Wilson (1982). C: Cross section through the eastern Kaapvaal craton, with samples projected at the appropriate elevations. Relationships of Cre- taceous and Tertiary units are from Frankel (1972). D: Post–300 Ma inverse modeling simulation results depicted as individual t-T paths for the ca. 100 Ma coastal plain samples. The black square and gray rectangles show the constraints imposed on the thermal history. The dark and light gray lines represent good and acceptable fi ts, respectively. The bold black line depicts the “best-fi t” history. LIPs and episodes of signifi cant kimberlite magmatism are noted. (e.g., Renne et al., 1996), the Agulhas large igneous province (LIP; e.g., elevation, and include the escarpment that separates the elevated plateau Parsiegla et al., 2008), and the emplacement of numerous kimberlite pipes from the coastal region (Fig. 2). (e.g., Jelsma et al., 2004). The Barberton Greenstone Belt and Ancient Gneiss Complex APATITE AND TITANITE (U-Th)/He in Swaziland and eastern South Africa contain the oldest recognized THERMOCHRONOMETRY (3.66 Ga) and best-exposed rocks of the Archean Kaapvaal craton We acquired (U-Th)/He data for 43 individual apatite crystals (Fig. 2B) (e.g., Schoene et al., 2008). The two major post-Archean tec- from 11 samples ranging in elevation from 315 m to 1515 m across the tonic events in this area were accretion of the ca. 1.2–1.1 Ga Natal meta- southeastern African escarpment (Fig. 2). Samples were 3.6–2.75 Ga morphic province to the south and ca. 580–480 Ma Pan-African over- orthogneisses collected from basement terranes fl anking the Barberton printing to the northeast. Permian to Jurassic Karoo sedimentary rocks Greenstone Belt. Data were acquired at the California Institute of Tech- and volcanics of the southern Lebombo monocline, marking the rifted nology.
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