(U-Th)/He thermochronometry constraints on unroofi ng of the eastern Kaapvaal 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 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- sic portion of the major escarpment that encircles the plateau. The o study area encompasses ~1500 m of relief within basement of 25oS Kaapvaal the Barberton region of the eastern . 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 , 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, 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 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 , is embedded in the Precambrian 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 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 . 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 (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 KPV99-91 claystones 94, 106 Ma 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 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. Sample results are reported as the sample mean and 1σ sample Gondwana volcanic margin, nonconformably overlie the eastern study standard deviation for the nine samples with three or more analyses. The area (Fig. 1) (Wilson, 1982; Schlüter, 2006). Most downwarping of 11 samples yielded dates from 78 ± 9 Ma to 136 ± 24 Ma, with six of Karoo units and development of relatively minor steeply dipping north- these characterized by mean sample dates from 96 to 101 Ma. All coastal striking faults in the study area occurred during continental breakup plain samples (<600 m) yielded mean dates within error or younger than (Wilson, 1982; Dingle et al., 1983; Klausen, 2009). Shallowly dipping ca. 100 Ma (Fig. 2; Fig. DR1A in the GSA Data Repository1). These sam- Cretaceous sedimentary rocks unconformably overlie the Karoo volca- nics to the east of the study area (e.g., McMillan, 2003). Nearly horizon- 1GSA Data Repository item 2010230, analytical methods, Table DR1 (ana- lytical results), Figure DR1 (additional plots of data), and Figure DR2 (expanded tal Tertiary and younger sedimentary units unconformably overlie the thermal history simulation results), is available online at www.geosociety.org/ Cretaceous section (Fig. 2C) (Frankel, 1972). The Archean basement pubs/ft2010.htm, or on request from [email protected] or Documents Sec- exposures of the eastern Kaapvaal craton range from 1800 to 300 m in retary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

828 GEOLOGY, September 2010

Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/38/9/827/3539730/827.pdf by Princeton University user on 19 October 2020 ples lack positive correlations between date and eU (effective U concen- are characterized by Cenozoic temperatures ≤21 °C, suggesting the data tration) as observed in other cratonic data (Fig. DR1B) (Flowers, 2009). are best explained by temperatures within a few degrees of the surface in Two of four plateau samples (≥900 m) yielded dates older than 120 Ma. post-Cretaceous time. (U-Th)/He data were also obtained for four individual titanite fragments from two tonalite gneisses. The He closure temperature for titanites of UNROOFING OF THE EASTERN KAAPVAAL CRATON AND typical grain size and cooling rate is 220–190 °C (Reiners and Farley, IMPLICATIONS FOR ELEVATION GAIN OF THE SOUTHERN 1999). The titanite fragments yielded dates from 636 ± 26 to 871 ± 36 Ma. AFRICAN PLATEAU Refer to Table DR1 and GSA Data Repository text for analytical results The apatite (U-Th)/He data constrain the relative magnitudes of and methods. Mesozoic and Cenozoic unroofi ng seaward of the escarpment, relevant for opposing models of scarp development and plateau uplift in Meso- MESOZOIC–CENOZOIC THERMAL HISTORY OF THE zoic or Cenozoic times. Estimates of unroofi ng below employ the mod- EASTERN KAAPVAAL CRATON ern mean annual surface temperature of 18 °C on the coastal plain. Peak The implications of the (U-Th)/He data for the thermal history of the Mesozoic temperatures of 88–175 °C indicate minimum coastal plain eastern Kaapvaal craton were explored using time-temperature (t-T) simu- burial by ~3000 m of overburden, assuming a typical cratonic geotherm of lations. We specifi ed the following six major restrictions on the thermal 20 °C/km implied by heat fl ow values of 33–51 mW/m2 in the Kaapvaal history imposed by geological information. First, during and immediately craton (Jones, 1988). A higher geothermal gradient of 30 °C/km, possibly following accretion of the Natal metamorphic province to the south, we caused by the low thermal conductivity of subsequently denuded Karoo permit temperatures of 300–220 °C at ca. 1.1 Ga, and an expanded tem- basalts, would reduce the minimum burial estimate to 2300 m. Greater perature range of 300–150 °C from 1.1 to 0.9 Ga. The youngest biotite burial depths than this are probable, however, based on the preservation 40Ar/39Ar dates in the eastern Kaapvaal craton are Paleoproterozoic (Layer of ~900 m of Karoo sedimentary rocks in eastern Swaziland and up to et al., 1992), limiting temperatures to ≤300 °C since Paleoproterozoic 5000 m of Karoo volcanics in the Lebombo monocline to the east. time. Second, we permit a phase of reheating at 580–480 Ma during Pan- The mid- to Late Cretaceous cooling phase records the removal of African tectonism. Third, the basement was at the surface at ca. 300 Ma, most of these overlying rocks by Cenozoic time. Coastal plain tempera- prior to Karoo Supergroup deposition. Fourth, burial continued from tures of ≤35 °C during the Cenozoic would indicate ≤850 m of Cenozoic Permian through Middle Jurassic time. Fifth, we assume that peak tem- unroofi ng, assuming a cratonic geotherm. Application of higher geother- peratures were attained between the 183 Ma extrusion of the Karoo volca- mal gradients decreases the inferred denudation magnitude. As noted nics and the end of the Cretaceous. Finally, the basement was exhumed to above, the thermal history simulations suggest the (U-Th)/He data are the surface by the present. most consistent with Cenozoic temperatures within a few degrees of the Apatite data were simulated using the radiation damage accumula- current surface temperature. If correct, this would imply at most several tion and annealing model (RDAAM), which accounts for the evolution hundred meters of denudation in the Cenozoic. The depositional record of apatite He retentivity owing to the buildup and elimination of radia- in the Kwazulu Basin east of our study area is complementary to the ero- tion damage in the apatite crystal (Flowers et al., 2009). A pre-Mesozoic sional history inferred from the apatite (U-Th)/He data. This basin con- portion of the thermal history was included in the simulations because tains up to 2000 m of Cretaceous sedimentary rocks, but only ~100 m of any accumulated radiation damage would infl uence the apatite He reten- Tertiary sediments (McMillan, 2003). tivity in Mesozoic–Cenozoic time. We used the mean and 1σ standard These results help fi ll a critical gap in our knowledge of the unroofi ng deviation of the titanite results as a constraint on this earlier history, and history of southern Africa. Apatite fi ssion-track (AFT) thermochronology applied the titanite diffusion parameters of Reiners and Farley (1999) in work has yielded important insight into the 120–60 °C cooling history the simulations. Simulations were accomplished using the inverse model- along transects across the southwestern, southern, and southeastern pas- ing capabilities of HeFTy to pose 10,000 candidate t-T paths satisfying the sive margins of South Africa (Brown et al., 2002; Tinker et al., 2008; Kou- thermal history constraints (see Ketcham, 2005, for details). References to nov et al., 2009). These studies identifi ed accelerated unroofi ng episodes temperatures below are based on the “good-fi t” simulation results that are in both Early and mid- to Late Cretaceous time, but only strictly limited consistent with the dates, eU values, and equivalent spherical radii of the Cenozoic denudation to ≤2100 m (assuming a cratonic geotherm) owing specifi ed samples. to the bottoming out in temperature sensitivity of the AFT method at Only a limited set of thermal histories can reproduce the ca. 100 Ma ~60 °C. Cosmogenic studies resolve short-term (a few hundred thousand apatite (U-Th)/He dates. The additional age variability in the dataset sug- years) denudation rates that are an order of magnitude lower than those gests some intriguing spatial heterogeneity in the region’s cooling history, estimated for the Cretaceous (Kounov et al., 2007), but do not constrain but does not impact our key conclusions below. For the simulation of Tertiary rates. Our data across the eastern escarpment are compatible with, coastal plain samples, we included the four samples with more than two and better refi ne, the broader unroofi ng brackets imposed by AFT and cos- analyses apiece that cluster at ca. 100 Ma in order to utilize the moderate mogenic data elsewhere in southern Africa. eU range (12–25 ppm) permitted by these data. Satisfactory t-T paths (1) Together this information indicates that unroofi ng rates substantially limit peak Mesozoic temperatures to ≤175 °C to preserve the Neopro- decelerated between Mesozoic and Cenozoic times. The greater magni- terozoic titanite dates, (2) require peak temperatures ≥88 °C in Middle tude of Mesozoic than Cenozoic unroofi ng is consistent with erosion in Jurassic to Cretaceous time to induce complete He loss from the apatite response to scarp development and elevation gain of the eastern Kaapvaal crystals, (3) demand relatively rapid cooling from peak temperature, with craton in Mesozoic time. Some degree of uplift and seaward tilting is a the majority of t-T paths indicating accelerated cooling between 130 and predicted isostatic response to scarp development (e.g., Tucker and Sling- 100 Ma, and (4) imply the maintenance of temperatures no greater than erland, 1994; Brown et al., 2002). Perhaps the angular unconformity ~35 °C in Cenozoic time (Fig. 2D; Fig. DR2). The uniformity of the apa- between shallowly dipping (1°–6°) Cretaceous units (Frankel, 1972; Ken- tite (U-Th)/He dates, despite a moderate eU span, precludes histories that nedy and Klinger, 1975) and the overlying subhorizontal Tertiary sedi- entail protracted Cretaceous cooling through the apatite He partial reten- mentary rocks (Fig. 2C) (Frankel, 1972) on the coastal plain is attributable tion zone (e.g., Flowers et al., 2009). We carried out additional simulations to warping during Cretaceous scarp development, with negligible tilting that included three or more of the ca. 100 Ma samples to further explore since deposition of the earliest coastal plain Tertiary sediments in Paleo- the constraint on Cenozoic temperatures. In all cases, the “best-fi t” results cene time (Dingle et al., 1983; McMillan, 2003).

GEOLOGY, September 2010 829

Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/38/9/827/3539730/827.pdf by Princeton University user on 19 October 2020 Mesozoic unroofi ng temporally overlaps with continental rifting and Jourdan, F., Feraud, G., Bertrand, H., Kampunzu, A.B., Tshoso, G., Watkeys, M.K., and Le Gall, B., 2005, Karoo large igneous province: Brevity, origin, associated phases of magmatism across southern Africa. If unroofi ng were and relation to mass extinction questioned by new 40Ar/ 39Ar data: Geology, a response to Mesozoic elevation gain of the plateau, this would imply v. 33, p. 745–748, doi: 10.1130/G21632.1. that mechanisms related to the breakup of Gondwana contributed to pla- Kennedy, W.J., and Klinger, H.C., 1975, Cretaceous faunas from Zululand and teau uplift (e.g., de Wit, 2007). In more detail, however, it is noteworthy Natal, South Africa: Introduction, Stratigraphy: Bulletin of the British Mu- seum (Natural History), Part A (Geology), v. 25, p. 263–315. that Cretaceous unroofi ng of the study area postdates the more local Early Ketcham, R.A., 2005, Forward and inverse modeling of low-temperature thermo- Jurassic breakup of Gondwana and signifi cant 183 Ma Karoo volcanism chronometry data, in Reiners, P.W., and Ehlers, T.A., eds., Low-temperature along the Lebombo monocline. Rather, study area unroofi ng overlaps in thermochronology: Techniques, interpretations, and applications: Mineral- time with the 132 Ma Etendeka-Parana LIP associated with fi nal continen- ogical Society of America Reviews in Mineralogy and Geochemistry, Vol- tal breakup in the South Atlantic (e.g., Renne et al., 1996), the 100–90 Ma ume 58, p. 275–314. King, L.C., 1951, South African scenery: Edinburgh, UK, Oliver and Boyd, Agulhas LIP along the south coast of southern Africa (e.g., Parsiegla et 379 p. al., 2008), and signifi cant phases of inland kimberlite magmatism (e.g., Klausen, M.B., 2009, The Lebombo monocline and associated feeder dyke Jelsma et al., 2004). The local temporal decoupling of Mesozoic magma- swarm: Diagnostic of a successful and highly volcanic rifted margin?: Tec- tism and plateau unroofi ng in the eastern Kaapvaal craton may be signifi - tonophysics, v. 468, p. 42–62, doi: 10.1016/j.tecto.2008.10.012. Kounov, A., Niedermann, S., de Wit, M.J., Viola, G., Andreoli, M., and Erzinger, cant for deciphering the geodynamics of mantle processes responsible for J., 2007, Present denudation rates at selected sections of the South Afri- plateau uplift. Our results do not entirely preclude a phase of uplift dur- can escarpment and the elevated continental interior based on cosmogenic ing Cenozoic time as proposed on the basis of geomorphic studies (e.g., 3He and 21Ne: South African Journal of Geology, v. 110, p. 235–248, doi: Burke, 1996). If a signifi cant Cenozoic uplift episode occurred, however, 10.2113/gssajg.110.2-3.235. it coincided with a broad interval of decelerated erosion rates and post- Kounov, A., Viola, G., de Wit, M.J., and Andreoli, M., 2009, Denudation along the Atlantic passive margin: New insights from apatite fi ssion-track analysis dated the most signifi cant phase of post-Paleozoic denudation along the on the western coast of South Africa, in Lisker, F., et al., eds., Thermochro- eastern passive margin of the plateau. The overlap of the predominant nological methods: From paleotemperature constraints to landscape evolu- unroofi ng episode with continental rifting, voluminous LIP magmatism, tion models: The Geological Society of London Special Publication 324, and kimberlite emplacement points toward deep mantle processes as the p. 287–306. Layer, P.W., Kroner, A., and York, D., 1992, Pre-3000 Ma thermal history of the most likely buoyancy sources for initial rise of the southern African Pla- Archean Kaap Valley pluton, South Africa: Geology, v. 20, p. 717–720, doi: teau in the Mesozoic. 10.1130/0091-7613(1992)020<0717:PMTHOT>2.3.CO;2. McMillan, I.K., 2003, Foraminiferally defi ned biostratigraphic episodes and ACKNOWLEDGMENTS sedimentation pattern of the Cretaceous drift succession (Early Barremian Initial sample collection was supported and facilitated by S. Bowring and to Late Maastrichtian) in seven basins on the South African and southern M. de Wit. We thank Rod Brown, Kevin Burke, and an anonymous reviewer for Namibian continental margin: South African Journal of Science, v. 99, insightful comments that improved the manuscript. National Science Foundation p. 537–576. grant EAR-0951518 to Flowers partly supported this work. Ni, S., Tan, E., Gurnis, M., and Helmberger, D., 2002, Sharp sides to the African Superplume: Science, v. 296, p. 1850–1852, doi: 10.1126/science.1070698. REFERENCES CITED Parsiegla, N., Gohl, K., and Uenzelmann-Neben, G., 2008, The Agulhas Plateau: Brown, R.W., Summerfi eld, M.A., and Gleadow, A.J.W., 2002, Denudational his- Structure and evolution of a Large Igneous Province: Geophysical Journal tory along a transect across the Escarpment of southern Africa International, v. 174, p. 336–350, doi: 10.1111/j.1365-246X.2008.03808.x. derived from apatite fi ssion track thermochronology: Journal of Geophysi- Partridge, T.C., and Maud, R.R., 1987, Geomorphic evolution of southern Africa cal Research, v. 107, no. B12, 2350, doi: 10.1029/2001JB000745. since the Mesozoic: South African Journal of Geology, v. 90, p. 179–208. Burke, K., 1996, The African Plateau: South African Journal of Geology, v. 99, Reiners, P.W., and Farley, K.A., 1999, Helium diffusion and (U-Th)/He thermo- p. 341–409. chronometry of titanite: Geochimica et Cosmochimica Acta, v. 63, p. 3845– Burke, K., and Gunnell, Y., 2008, The African Erosion Surface: A continental-scale 3859, doi: 10.1016/S0016-7037(99)00170-2. synthesis of geomorphology, tectonics, and environmental change over the Renne, P.R., Glen, J.M., Milner, S.C., and Duncan, A.R., 1996, Age of Etendeka past 180 million years: Geological Society of America Memoir 201, p. 66. fl ood volcanism and associated intrusions in southwestern Africa: Geology, de Wit, M.J., 2007, The Kalahari epeirogeny and climate change: Differentiat- v. 24, p. 659–663, doi: 10.1130/0091-7613(1996)024<0659:AOEFVA>2. ing cause and effect from core to space: South African Journal of Geology, 3.CO;2. v. 110, p. 367–392, doi: 10.2113/gssajg.110.2-3.367. Ritsema, J., van Heijst, H.J., and Woodhouse, J.H., 1999, Complex shear wave Dingle, R.V., Siesser, W.G., and Newton, A.R., 1983, Mesozoic and Tertiary ge- velocity structure imaged beneath Africa and Iceland: Science, v. 286, ology of southern Africa: Rotterdam, Netherlands, A.A. Balkema, 375 p. p. 1925–1928, doi: 10.1126/science.286.5446.1925. Flowers, R.M., 2009, Exploiting radiation damage control on apatite (U-Th)/ Schlüter, T., 2006, Geological atlas of Africa: Berlin, Heidelberg, Springer-Ver- He dates in cratonic regions: Earth and Planetary Science Letters, v. 277, lag 272 p. p. 148–155, doi: 10.1016/j.epsl.2008.10.005. Schoene, B., de Wit, M.J., and Bowring, S.A., 2008, Mesoarchean assembly and Flowers, R.M., Ketcham, R.A., Shuster, D.L., and Farley, K.A., 2009, Apatite stabilization of the eastern Kaapvaal craton: A structural-thermochronolog- (U-Th)/He thermochronometry using a radiation damage accumulation and ical perspective: Tectonics, v. 27, p. 1–27, doi: 10.1029/2008TC002267. annealing model: Geochimica et Cosmochimica Acta, v. 73, p. 2347–2365, Tinker, J., de Wit, M., and Brown, R.W., 2008, Mesozoic exhumation of the south- doi: 10.1016/j.gca.2009.01.015. ern Cape, South Africa, quantifi ed using apatite fi ssion track thermochro- Frankel, J.J., 1972, Distribution of Tertiary sediments in Zululand and southern nology: Tectonophysics, v. 455, p. 77–93, doi: 10.1016/j.tecto.2007.10.009. Mozambique, southeast Africa: American Association of Petroleum Geolo- Tucker, G.E., and Slingerland, R.L., 1994, Erosional dynamics, fl exural isostasy, gists Bulletin, v. 56, p. 2415–2425. and long-lived escarpments: A numerical modeling study: Journal of Geo- Jelsma, H.A., de Wit, M.J., Thiart, C., Dirks, P.H.G.M., Viola, G., Basson, I.J., physical Research, v. 99, p. 12,229–12,243. and Anckar, E., 2004, Preferential distribution along transcontinental cor- Wilson, A.C., 1982, Geological map of Swaziland (including Geological sum- ridors of kimberlites and related rocks of Southern Africa: South African mary): Mbabane, Geological Survey and Mines Department, scale 1:250,000, Journal of Geology, v. 107, p. 301–324, doi: 10.2113/107.1-2.301. 1 sheet. Johnson, M.R., Van Vuuren, C.J., Hegenberger, W.F., Key, R., and Shoko, U., 1996, Stratigraphy of the Karoo Supergroup in southern Africa: An over- view: Journal of African Earth Sciences, v. 23, p. 3–15, doi: 10.1016/ Manuscript received 18 December 2009 S0899-5362(96)00048-6. Revised manuscript received 26 April 2010 Jones, M.Q.W., 1988, Heat fl ow in the Basin and environs and its Manuscript accepted 28 April 2010 signifi cance for the South African shield geotherm and lithosphere thick- ness: Journal of Geophysical Research, v. 93, p. 3243–3260. Printed in USA

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