Kaapvaal Research Articles

Constraints from eclogite and MARID xenoliths on origins of mantle Zr/Hf–Nb/Ta variability

New trace-element data of rutile in kimberlite-borne ~1.85 Ga eclogite and pyroxenite xenoliths from the central Slave craton, as well as ~110 Ma MARID xenoliths from the Kaapvaal craton, provide constraints on the origins of lithospheric and sublithospheric mantle variability in high field strength element ratios. Rutiles in eclogites and pyroxenites have Zr/Hf ranging from 20 to 62 and Nb/Ta ranging from 10 to 40. Rutiles in MARID xenoliths have Zr/Hf from 24 to 33 and Nb/Ta from 10 to 41. Calculated whole-rock Zr/Hf is suprachondritic for eclogites with suggested gabbroic protoliths and subchondritic for boninite-like eclogites; the latter is consistent with cpx-controlled depletion in the protolith source. Within each eclogite type, positive correlations of Zr/Hf with La/Lu and negative correlations with Lu/Hf likely reflect fractionation of cpx and/or plagioclase during crystallisation of the protoliths. Zr/Hf–Nb/Ta relationships of some MARID-type rocks, which are products of lithospheric mantle metasomatism, and eclogite xenoliths plot on a silicate differentiation trend, whereas other samples have higher Nb/Ta at a given Zr/Hf. Fractionation of a few percent rutile from an HFSE-rich mafic melt can generate a trend towards strongly increased Nb/Ta at minimally changed Zr/Hf in the residual melt. Superposition of rutile fractionation on the effects of silicate differentiation, which fractionates Zr/Hf more strongly than Nb/Ta, can explain the Zr/Hf–Nb/Ta relationships of most eclogites from the central Slave craton as well as those of MARID rocks, metasomatised peridotites and group II kimberlites. By contrast, Zr/Hf–Nb/Ta relationships suggest that Group I kimberlites are mixtures between depleted peridotite and carbonatite. Thus, high Nb/Ta is a signature of lithospheric processes and may not be important in deeply subducted eclogites that bypass extended residence in the lithosphere. Conversely, considerable primary Zr/Hf variability was inherited by the eclogites, which is indicative of the compositional diversity of ancient subducted oceanic crust, which is expected to have generated substantial heterogeneity in sublithospheric basalt sources.

Paleomagnetic and geochronological evidence for large-scale post–1.88 Ga displacement between the Zimbabwe and Kaapvaal cratons along the Limpopo belt

Proterozoic reconstructions of the Kaapvaal and Zimbabwe cratons have been limited by the scarcity of precisely dated paleomagnetic poles for the Zimbabwe craton. We present new U-Pb baddeleyite and apatite dates from two diabase sheets that have previously yielded paleomagnetic data from the Mashonaland igneous province in the Zimbabwe craton. Discordant baddeleyite analyses yield upper intercept dates of 1871.9 ± 2.2 and 1882.7 +1.6/–1.5 Ma. Apatite data from the same samples give less precise but statistically indistinguishable dates, providing direct constraints on the post-magmatic thermal history of the diabases. The new U-Pb dates and other recently published baddeleyite dates from the Mashonaland province are coeval with mafic magmatism in the adjacent Kaapvaal craton (1879–1872 Ma), but paleomagnetic poles from the two intraplate suites differ by 39°, suggesting that the two cratons underwent substantial relative motion after ca. 1.88 Ga. Paleomagnetic reconstructions are consistent with >2000 km of lateral displacement being accommodated in the Limpopo orogenic belt that separates the two cratons.

Electrical lithosphere beneath the Kaapvaal craton, southern Africa

[1] A regional-scale magnetotelluric (MT) experiment across the southern African Kaapvaal craton and surrounding terranes, called the Southern African Magnetotelluric Experiment (SAMTEX), has revealed complex structure in the lithospheric mantle. Large variations in maximum resistivity at depths to 200–250 km relate directly to age and tectonic provenance of surface structures. Within the central portions of the Kaapvaal craton are regions of resistive lithosphere about 230 km thick, in agreement with estimates from xenolith thermobarometry and seismic surface wave tomography, but thinner than inferred from seismic body wave tomography. The MT data are unable to discriminate between a completely dry or slightly “damp” (a few hundred parts per million of water) structure within the transitional region at the base of the lithosphere. However, the structure of the uppermost ∼150 km of lithosphere is consistent with enhanced, but still low, conductivities reported for hydrous olivine and orthopyroxene at levels of water reported for Kaapvaal xenoliths. The electrical lithosphere around the Kimberley and Premier diamond mines is thinner than the maximum craton thickness found between Kimberley and Johannesburg/Pretoria. The mantle beneath the Bushveld Complex is highly conducting at depths around 60 km. Possible explanations for these high conductivities include graphite or sulphide and/or iron metals associated with the Bushveld magmatic event. We suggest that one of these conductive phases (most likely melt-related sulphides) could electrically connect iron-rich garnets in a garnet-rich eclogitic composition associated with a relict subduction slab.

Shear wave model of southern Africa from regional Rayleigh wave tomography with 2-D sensitivity kernels

SUMMARY Rayleigh-wave phase velocities are computed at the period range of 20–167 s in southern Africa from a two-plane-wave (TPW) tomography method using 2-D, Born approximation sensitivity kernels. These phase velocities are compared with the results of the TPW method using Gaussian sensitivity functions and the correlation between the two methods is good up to 100 s. The correlation coefficient becomes less than 0.5 at periods greater than 111 s, indicating significant finite-frequency effects at long periods. The correlation of phase velocity between adjacent frequencies increases with period for inversions using the 2-D sensitivity kernels, while such a systematic trend is not observed for inversions using Gaussian sensitivity functions. In addition, low phase velocity anomalies at long periods (111–167 s) are better and more consistently imaged from inversions using 2-D sensitivity kernels. To account for finite-frequency effects and improve resolution, the 2-D sensitivity kernels are therefore recommended in regional TPW tomography. A 3-D shear wave model is developed from the phase velocities obtained in southern Africa. The new model confirms the existence of a fast mantle lid underlain by a low-velocity zone revealed in a previous model constrained from the TPW method using Gaussian sensitivity functions. One new feature in this model is the vertical alignment of a shallow low-velocity anomaly with a deep high-velocity anomaly at the western Bushveld province. The alignment provides new evidence to support the interpretation of the slow anomaly as the result of high iron content from the Bushveld intrusion and the fast as a more depleted residual mantle. A low-velocity channel at the depths of 220–310 km from the Namaqua–Natal Belt to the northwest of the Kaapvaal Craton is also imaged. It suggests that the hot asthenosphere beneath the mobile belt can migrate into the craton area through a weak channel and produce thermal disturbance at the base of the craton. Further more, low-velocity anomalies from 100 to 140 km in the 3-D model agree well with the localities of kimberlites erupted at 65–104 Ma in the Kaapvaal Craton. This observation supports that kimberlites are generated in the lithosphere by thermal disturbance and provides additional evidence for the depth extent of mantle xenoliths.

Fractal behavior in continental crustal heat production

Abstract. The distribution of crustal heat production, which is the most important component in the elucidation of continental thermal structure, still remains a theoretical assumption. In general the heat production values must decrease with depth, but the form of decrease of heat production in the crust is not well understood. The commonly used heat production models are: "block model", in which heat production is constant from the surface to a given depth and the "exponential model", in which heat production diminishes as an exponential function of depth. The exponential model is more widely used wherein sources of the errors are heterogeneity of rock and long wavelength changes due to changes in lithology and tectonic elements, and as such exponential distribution does not work satisfactorily for the entire crust. In the present study, we analyze for the first time, deep crustal heat production data of six global areas namely Dharwar craton (India), Kaapvaal craton (South Africa), Baltic shield (Kola, Russia), Hidaka metamorphic belt (Japan), Nissho pluton (Japan) and Continental Deep Drilling site (KTB, Germany). The power spectrum of all the studied data sets exhibits power law behaviour. This would mean slower decay of heat production with depth, which conforms to the known geologic composition of the crust. Minimum value of the scaling exponent has been found for the KTB borehole, which is apparently related to higher heat production of gneisses, however for other study areas, scaling exponent is almost similar. We also found that the lower values of scaling exponents are related to higher heat production in the crust as is the case in KTB. Present finding has a direct relevance in computation of temperature-depth profiles in continental regions.

The Kaapvaal Craton, South Africa: no evidence for a supercontinental affinity prior to 2.0 Ga?

We briefly examine the possible antiquity of the supercontinental cycle while noting the likely unreliability of palaeomagnetic data >ca.1.8 Ga, assuming a gradual change from a magmatically dominated Hadean Earth to a plate tectonically dominated Neoarchaean system. A brief review of one of Earth's oldest cratons, Kaapvaal, where accent is placed on the lithostratigraphic and geodynamic-chronological history of its cover rocks from ca. 3.1 to 2.05 Ga, forms the factual basis for this article. The ca. 3.1–2.8 Ga Witwatersrand–Pongola (Supergroups) complex retroarc flexural foreland basin developed while growth and stabilization of the craton were still underway. Accretion of relatively small composite granite-gneiss-greenstone terranes (island arc complexes) from both north and west does not support the formation of a Neoarchaean supercontinent, but may well have been related to a mantle plume which enhanced primary gold sources in the accreted terranes and possibly controlled the timing and rate of craton growth through plate convergent processes. Subsequent deformation of the Witwatersrand Basin fill with concomitant loss of ≤1.5 km of stratigraphy must have been due to far-field tectonic effects, but no known mobile belt or even greenstone belts can be related to this contractional event. At ca. 2714–2709 Ma, a large mantle plume impinged beneath the thinned crust underlying the Witwatersrand Basin forming thick, locally komatiitic flood basalts at the base of the Ventersdorp Supergroup, with subsequent thermal doming leading to graben basins within which medial bimodal volcanics and immature sediments accumulated. Finally (possibly at ca. 2.66–2.68 Ga), thermal subsidence enabled the deposition of uppermost Ventersdorp sheet-like lavas and sediments, with minor komatiites still present. Ongoing plume-related influences are thus inferred, and an analogous cause is ascribed to a ca. 2.66–2.68 Ga dike swarm to the north of the Ventersdorp, where associated rifting allowed formation of discrete ‘protobasinal’ depositories of the Transvaal (ca. 2.6–2.05 Ga Supergroup, preserved in three basins). Thin fluvial sheet sandstones (Black Reef Formation, undated) above these lowermost rift fills show an association with localized compressive deformation along the palaeo-Rand anticline, north of Johannesburg, but again with no evidence of any major terrane amalgamations with the Kaapvaal. From ca. 2642 to 2432 Ma, the craton was drowned with a long-lived epeiric marine carbonate-banded iron formation platform covering much of it and preserved in all three Transvaal Basins (TB). During this general period, at ca. 2691–2610 Ma, the Kaapvaal Craton collided with a small exotic terrane [the Central Zone (CZ), Limpopo Belt] in the north. Although farfield tectonic effects are likely implicit in TB geodynamics, again there is no case to be made for supercontinent formation. Following an 80–200 million years (?) hiatus, with localized deformation and removal of large thicknesses of chemically precipitated sediments along the palaeo-Rand anticline, the uppermost Pretoria Group of the Transvaal Supergroup was deposited. This reflects two episodes of rifting associated with volcanism, and subsequent thermal subsidence within a sag basin setting; an association of the second such event with flood basalts supports a plume affinity. At ca. 2050 Ma the Bushveld Complex intruded the northern Kaapvaal Craton and reflects a major plume, following which Kaapvaal–CZ collided with the Zimbabwe Craton, when for the first time, strong evidence exists for a small supercontinent assembly, at ca. 2.0 Ga. We postulate that the long-lived evidence in favour of active mantle (cf. plume) influences with subordinate and localized tectonic shortening, implicit within the review of ca. 3.1–2.05 Ga geological history of the Kaapvaal Craton, might reflect the influence of earlier Precambrian mantle-dominated thermal systems, at least for this craton.

2.05-Ga Isotopic Ages for Transvaal Mississippi Valley–Type Deposits: Evidence for Large-Scale Hydrothermal Circulation around the Bushveld Igneous Complex, South Africa

Abstract The known dimensions of hydrothermal fluid circulation associated with emplacement of the 2.055-Ga Bushveld Igneous Complex (BIC) are extended with new isotopic ages for Paleoproterozoic mineral deposits in wallrocks (Transvaal Supergroup) surrounding the BIC. A fluorite-rich Mississippi Valley–type (MVT) deposit in the Zeerust district, dated at 2.05 Ga (Sm-Nd isochron age), has Nd isotopic compositions that are identical to coeval F-rich Lebowa Granite late in the BIC intrusive sequence, suggesting transport of magmatic fluids ∼100 km beyond the present margins of the BIC. Hydrothermal illite from the Zn-Pb MVT Bushy Park deposit, dated at 2.05 Ga by K-Ar geochronology, links widespread MVT deposits of the Kaapvaal craton to large-scale basinal brine circulation associated with emplacement of the BIC. These data, in combination with gold vein, phosphate mineral, and paleomagnetic resetting ages, indicate a regional-scale Bushveld-age hydrothermal system circulating 700 km outward from the prese...

New Regional Moment Tensors in South Africa

Regional moment tensors (RMTs) provide important information for seismotectonic and hazard studies in regions with low to moderate seismicity, where infrequent earthquakes of Mw ≥ ∼4.0–4.5 occur that are too small for global momenttensor techniques. Moment-tensor analysis involves fitting theoretical waveforms with observed broadband waveforms and inverting for the moment-tensor elements ( e.g. , Aki and Richards 1980; Jost and Herrmann 1989). One powerful tool to calculate RMTs is the time domain surface wave waveform inversion code TDMT_INVC (Dreger and Helmberger 1993; Pasyanos et al. 1996; Dreger 2003). In recent years RMTs have been routinely calculated with this software in many parts of the world such as western Canada (Ristau et al. 2003, 2007), California (Dreger and Helmberger 1993; Romanowicz et al. 1993; Pasyanos et al. 1996), Alaska (Ratchkovski and Hansen 2002), Japan (Kubo et al. 2002), Taiwan (Kao et al. 1998), the European–Mediterranean region (Bernardi et al. 2004), and New Zealand (Ristau 2008). Only a few moment tensors/focal mechanisms are available for South Africa. This is due to moderate tectonic and deep mine-related seismicity, as well as, until recently, a sparse distribution of broadband seismometers in the South African National Seismograph Network (SANSN) (Saunders et al. 2008). A unique opportunity presented itself when the dense, very broadband Incorporated Research Institutions in Seismology (IRIS) PASSCAL Kaapvaal craton array was deployed in South Africa ( e.g. , Nguuri et al. 2001). From this array we identified three near regional Mw ∼4.0 earthquakes with suitable waveform data to calculate RMTs with the TDMT_INVC software. Our goal is to determine the moment magnitude, earthquake mechanism, and focal depth in order to 1) make progress in resolving the difference between local and moment magnitudes routinely determined with the SANSN; and 2) expand our understanding of the regional …

Zircon ages and metamorphic evolution of the Archean Assegaai-De Kraalen granitoid-greenstone terrain, southeastern Kaapvaal Craton

The Assegaai, De Kraalen and Witrivier greenstone belts in the southeast part of the Kaapvaal craton contain strongly deformed volcano-sedimentary successions affected by amphibolite- to granulite-facies metamorphism at ∼3.22 to 3.20 Ga. These greenstone belts consist predominantly of mafic-ultramafic volcanic successions intercalated with chert and BIF. Single zircon SHRIMP ages of 3222±8 and 3193±5 Ma for two granitoids intrusive into the Assegaai greenstone belt provide a minimum age for the succession. Calc-silicates and amphibolites of the De Kraalen and Witrivier greenstone belts record a high-pressure metamorphic event (M₁) at ∼12 to 15 kbar and ∼600 °C and 800 °C, respectively. This high-pressure event was overprinted by a medium-pressure (7-8 kbar) amphibolite-facies event (M_2A) that was recorded in all three greenstone belts, suggesting that the De Kraalen and Witrivier supracrustal rocks were buried to a deeper crustal level than the Assegaai rocks during crustal thickening. Decompression of the rocks along clockwise P-T paths possibly occurred during subsequent crustal extension at ∼3.22 to 3.20 Ga. The low geothermal gradient of this area (15-20 °C/km) contrasts with higher geothermal gradients reported for other Archean terrains worldwide and may support the view that crustal thickening processes related to convergence in a setting similar to subduction zones were operating in Paleoarchean times.

From BIF to red beds: Sedimentology and sequence stratigraphy of the Paleoproterozoic Koegas Subgroup (South Africa)

The ~ 2.4 Ga old Koegas Subgroup of the Transvaal Supergroup contains siliciclastics and iron formations. It occupies a transitional stratigraphic position between the major iron formations on the Kaapvaal craton, and strata with evidence for oxygenation of the environment. Koegas siliciclastics reflect a new phase of uplift and erosion on the craton after a period of chemical deposition. Paleocurrent data indicate transport to the west and northwest, away from the craton; however, no precise source area can as yet be defined. Rocks were deposited in foreshore to offshore environments of a prograding delta or submarine fan system. Immature arkosic wackes and subarkoses occur in proximal parts and pass to terrigenous mudstones and chemical iron formations in offshore environments with low detrital input. Lateral facies transitions and the hierarchical stratigraphic stacking pattern of the Koegas Subgroup allow a sequence stratigraphic interpretation in which regressive coarse siliciclastics prograded repeatedly over transgressive units of chemical iron formations. Metals in the transgressive units have an intrabasinal (hydrothermal) source, similar to underlying iron formations. Localized iron-stained detrital units resemble red beds. They reflect additional diagenetic Fe remobilization from deeper-water iron formations, or possibly a continental Fe source. The second possibility would imply the presence of some free oxygen in the environment.

Composition of the Lithospheric Mantle in the Siberian Craton: New Constraints from Fresh Peridotites in the Udachnaya-East Kimberlite

We present petrographic, major element and trace element data for bulk-rocks and minerals for a suite of 34 remarkably fresh peridotite xenoliths in the Udachnaya kimberlite recovered from deep horizons of the open pit mine. The xenoliths are spinel peridotites, granular and deformed garnet peridotites (as well as a megacrystalline dunite) in proportions similar to those reported by previous work on altered xenoliths from less deep levels in the mine. Equilibration temperatures (T) range from 760–965°C for the spinel peridotites to 1200–1320°C for sheared garnet peridotites; the latter yield pressures (P) of 5·4–6·6 GPa. The majority of the granular garnet peridotites equilibrated at 860–1000°C and 2·6–5·5 GPa, but two samples yield much higher T and P values (1176°C, 6·1 GPa; 1340°C, 6·8 GPa) indicating the presence of undeformed rocks near the base of the lithosphere. Strong enrichments in silica and opx relative to dry melting residues are not common in our granular peridotites (e.g. 11 samples out of 18 analyzed plot on Boyd’s ‘oceanic trend’); they have distinctly lower modal opx than low-T peridotites from the Kaapvaal craton and could be formed by ∼30–45% of melt extraction at 1–5 GPa. Minor to moderate enrichments in silica relative to experimental melting trends seen in some xenoliths may indicate that subduction-related settings were involved in their origin. Garnets are usually unzoned but some are enriched in Ti at the rims. Their contents of CaO (4–7 wt %) and Cr2O3 (2–12 wt %) show inverse power-law correlations with whole-rock CaO, Al2O3 and Al/Cr ratios; the latter can be estimated from garnet analyses. Garnets from the dominant cpx-free and cpx-bearing harzburgites and less common low-cpx (5–6%) lherzolites fall into the G9 ‘lherzolite’ field; garnets from three cpx-rich (9–15%) lherzolites are in the G5 ‘pyroxenite’ field. Rare earth element (REE) patterns in cpx and garnet range from sinusoidal to bell-shaped and light REE (LREE)-depleted; the latter are common in sheared rocks and may be close to equilibrium with metasomatic liquids. Garnets from some granular rocks show low heavy REE (HREE) and continuous depletion in LREE, as in melting residues. Trace element patterns and ratios in many bulk peridotites are distinct from those in the host kimberlites and may not be much affected by contamination. Bulk-rock abundances of moderately incompatible elements are lowest in spinel harzburgites (20–50 times lower than in primitive mantle for HREE) and highest in sheared rocks. Some bulk-rocks show depletion from Lu to Er attributed to 30–40% of melt extraction first in the presence of garnet, then in the spinel field. Overall, the observations on fresh Udachnaya peridotites show less evidence for silica and opx enrichments and smaller compositional effects of syn- and post-eruption alteration than earlier work. The abundances of cpx and garnet and bulk-rock Al2O3 do not vary systematically with depth except that rare sheared xenoliths anomalously enriched in cpx and garnet occur near the base of the lithosphere. The remarkable preservation of olivine and opx shows in detail the transformation of granular microstructures into different types of porphyroclastic fabrics and their relations to chemical compositions.

PETROLOGY AND GEOCHEMISTRY OF THE GRANITOID ROCKS OF THE JOHANNESBURG DOME, CENTRAL KAAPVAAL CRATON, SOUTH AFRICA

The Johannesburg Dome (JD) in the central Kaapvaal Craton (KC) is dominated by granitoid rocks of the tonalite-trondhjemite-granodiorite (TTG) series. Based on modal analysis as well as a major and trace element investigation the JD granitoids could be subdivided into three main suites, i.e. a Tonalitic gneiss suite (TG) around the southern boundary, a Granodiorite to Adamellite Gneiss suite (GAG) across the northern part, and a Granodiorite/adamellite to Granodiorite suite (GG) occurring between the TG and GAG suites. These rocks are dominantly I-type and peraluminous with the tonalites (TG and partly the GAG suites) falling in the metaluminous field. TTGs of the JD are high-K calc-alkaline to calc-alkaline and are dominantly high silica rocks (~70 weight %), aluminous (Al2O3 >15wt%) with low Yb ( 30), high Na2O/K2O (>1), and have Na2O contents of between 3wt% and 5wt%, comparable to that of the average TTG. The JD tonalities (TG suite) have higher Al2O3, Sr, Na2O/K2O, Mg#, Ni, Cr and LILE contents compared to the more calc-alkaline granitoids (GG suite and trondhjemites of the GAG suite), which are typically richer in HREE (lower REE fractionation), Y and show a negative Sr and Eu anomaly. Other characteristic features of the JD TTG’s include HFSE depletion and distinct enrichment of fluid sensitive elements such as Pb. The strongly fractionated REE pattern, high (La/Yb)N ratio and depletion in HREE (Yb) of the JD TTGs are characteristics shared with modern adakites. The TG suite most probably formed through melting of a subducted oceanic slab with the melt interacting with mantle peridotite during its accent through a thin mantle wedge. The remaining JD granitoids (GAG and GG) most probably formed through the remelting of a TTG protolith, which has a subducted slab and mantle wedge signature (similar to the TG suite).

(U-Th)/He thermochronometry constraints on unroofing of the eastern Kaapvaal craton and significance for uplift of the southern African Plateau

The timing and causes of the >1.0 km elevation gain of the southern African Plateau since Paleozoic time are widely debated. We report the first apatite and titanite (U-Th)/He thermochronometry data for southern Africa to resolve the unroofing history across a classic portion of the major escarpment that encircles the plateau. The study area encompasses ∼1500 m of relief within Archean basement of the Barberton Greenstone Belt region of the eastern Kaapvaal craton. Titanite dates are Neoproterozoic. Apatite dates are Cretaceous, with most results clustering at ca. 100 Ma. Thermal history simulations confirm Mesozoic heating followed by accelerated cooling in mid- to Late Cretaceous time. The lower temperature sensitivity of the apatite (U-Th)/He method relative to previous thermochronometry in southern Africa allows tighter constraints on the Cenozoic thermal history than past work. The data limit Cenozoic temperatures east of the escarpment to ≤35 °C, and appear best explained by temperatures within a few degrees of the modern surface temperature. These results restrict Cenozoic unroofing to less than ∼850 m, and permit negligible erosion since the Cretaceous. If substantial uplift of the southern African Plateau occurred in the Cenozoic as advocated by some workers, then it was not responsible for the majority of post-Paleozoic unroofing across the eastern escarpment. Significant Mesozoic unroofing is coincident with large igneous province activity, kimberlite magmatism, and continental rifting within and along the margins of southern Africa, compatible with a phase of plateau elevation gain due to mantle buoyancy sources associated with these events.

Olivine water contents in the continental lithosphere and the longevity of cratons

Cratons, the ancient cores of continents, contain the oldest crust and mantle on the Earth (>2 Gyr old). They extend laterally for hundreds of kilometres, and are underlain to depths of 180-250 km by mantle roots that are chemically and physically distinct from the surrounding mantle. Forming the thickest lithosphere on our planet, they act as rigid keels isolated from the flowing asthenosphere; however, it has remained an open question how these large portions of the mantle can stay isolated for so long from mantle convection. Key physical properties thought to contribute to this longevity include chemical buoyancy due to high degrees of melt-depletion and the stiffness imparted by the low temperatures of a conductive thermal gradient. Geodynamic calculations, however, suggest that these characteristics are not sufficient to prevent the lithospheric mantle from being entrained during mantle convection over billions of years. Differences in water content are a potential source of additional viscosity contrast between cratonic roots and ambient mantle owing to the well-established hydrolytic weakening effect in olivine, the most abundant mineral of the upper mantle. However, the water contents of cratonic mantle roots have to date been poorly constrained. Here we show that olivine in peridotite xenoliths from the lithosphere-asthenosphere boundary region of the Kaapvaal craton mantle root are water-poor and provide sufficient viscosity contrast with underlying asthenosphere to satisfy the stability criteria required by geodynamic calculations. Our results provide a solution to a puzzling mystery of plate tectonics, namely why the oldest continents, in contrast to short-lived oceanic plates, have resisted recycling into the interior of our tectonically dynamic planet.

A review of the stratigraphy and geological setting of the Palaeoproterozoic Magondi Supergroup, Zimbabwe – Type locality for the Lomagundi carbon isotope excursion

The Palaeoproterozoic Magondi Supergroup lies unconformably on the Archaean granitoid-greenstone terrain of the Zimbabwe Craton and experienced deformation and metamorphism at 2.06–1.96 Ga to form the Magondi Mobile Belt. The Magondi Supergroup comprises three lithostratigraphic units. Volcano-sedimentary rift deposits (Deweras Group) are unconformably overlain by passive margin, back-arc, and foreland basin sedimentary successions, including shallow-marine sedimentary rocks (Lomagundi Group) in the east, and deeper-water shelf to continental slope deposits in the west (Piriwiri Group). Based on the upward-coarsening trend and presence of volcanic rocks at the top of the Piriwiri and Lomagundi groups, the Piriwiri Group is considered to be a distal, deeper-water time-equivalent of the Lomagundi Group. The Magondi Supergroup experienced low-grade metamorphism in the southeastern zone, but the grade increases to upper greenschist and amphibolite facies grade to the north along strike and, more dramatically, across strike to the west, reaching upper amphibolite to granulite facies in the Piriwiri Group. Carbonates form prominent horizons in the lower Lomagundi Group, occur in the Deweras Group as thick packages in the northern part of the basin, but form only thin discontinuous beds elsewhere, and are rare in the Piriwiri Group. Beds of anhydrite and sulphate pseudomorphs are relatively common in the Deweras Group, and also occur in the Lomagundi Group. Schidlowski et al. (1975, 1976) found extreme enrichment in 13C in carbonates of the Lomagundi Group, with an average δ 13C value of +8.2‰ VPDB. Subsequent work in the Magondi Basin has shown that high δ 13C carbonates are also present in the continental rocks of the underlying Deweras Group. The initiation of the Deweras rift is not well constrained geochronologically, but it may have started as early as 2.26 Ga, and was followed by deposition of the lower part of the Lomagundi Group on a passive continental margin. Assuming that the deepening trend in the upper Lomagundi Group and upward-coarsening trend in the Piriwiri Group reflect subsidence in a back-arc and subsequent foreland basin setting, with sediment derivation from an approaching volcanic arc, the age of the onset of Magondi deformation at ca. 2.0 Ga provides an upper age limit for sedimentation. Combining all available data for the Magondi Basin, the Limpopo Belt between the Zimbabwe and Kaapvaal cratons, and the Kheis Belt on the southwestern margin of the Kaapvaal craton, a new tectonic model is presented for the assembly of the Kaapvaal and Zimbabwe cratons at ca. 2.1–2.0 Ga. These cratons along with the Archaean cratons in West Africa and South America record an assembly of a large continent at the time when Archaean cratons in North America and Fennoscandia experienced extension and breakup. Carbonates of the Magondi Basin, the Kheis Belt, and the northern margin of the Kaapvaal craton therefore reflect the carbon isotope composition of the open ocean at 2.2–2.1 Ga and provide further evidence that the δ 13C values reaching to +10‰ VPDB and higher record a seawater signal rather than local diagenetic or closed basin conditions. The emerging picture is that Earth experienced dramatic tectonic reorganization between 2.1–2.0 Ga, which likely influenced ocean circulation and redox state and potentially ended the conditions that promoted high burial of organic matter with sediments during the Lomagundi excursion.

Age, Composition and Thermal Characteristics of South African Off-Craton Mantle Lithosphere: Evidence for a Multi-Stage History

A mineral chemistry and whole-rock major element, platinum group element (PGE), and Re–Os isotope dataset is presented for a large suite of mantle xenoliths from 13 kimberlites that erupted through the Proterozoic mobile belts surrounding the Archean Kaapvaal craton of South Africa. The peridotites have compositions that are unusually infertile compared with post-Archean lithosphere outside southern Africa (e.g. mean olivine Mg-number = 91·4; mean whole-rock Al2O3 = 1·3 wt %) but are similar to xenoliths from the Gibeon kimberlites of southern Namibia, which also erupted through Proterozoic lithosphere. Rhenium depletion model ages (TRD) determined from 58 Os isotope compositions of peridotites span a range from 2·6 to 0·6 Ga, with an average of 1·67 Ga. Latest Archean TRD model ages were determined for three samples from different localities, but whether these indicate the off-craton presence of reworked Archean lithosphere or are simply artifacts of heterogeneity in the convecting mantle is unclear. Low and relatively restricted Al2O3 contents combined with variable 187Os/188Osi in the peridotites are most consistent with a two-stage melt extraction history for southern African off-craton lithosphere, with initial formation in the earliest Proterozoic by widely varying (but, on average, moderate) degrees of melting, followed by a second melt extraction episode associated with the Namaquan orogeny at 1·3–1·0 Ga. Extensive metasomatism in the lithosphere beneath East Griqualand, SE of the craton, resulted in extreme clinopyroxene enrichment and addition of ilmenite along with disturbance of PGE abundances in many samples. This is probably due to percolation of melts similar to those parental to the Cr-poor megacryst suite, and related pyroxenites, which are abundant in East Griqualand kimberlites. In other regions, there is evidence for less extensive clinopyroxene addition and cryptic metasomatism. Southern African off-craton mantle xenoliths record evidence of a Mesozoic heating episode probably brought about by mantle upwelling linked to continental break-up and/or Karoo flood basalt magmatism. Prior to this, the thermal state, and hence the thickness, of southern African off-craton and cratonic lithosphere were probably roughly similar. The mantle upwelling responsible for lithospheric heating also appears to have caused moderate (≈30 km) thermal erosion of the off-craton portions of the southern African lithosphere.

Petrogenesis of the end-Cretaceous diamondiferous Behradih orangeite pipe: implication for mantle plume–lithosphere interaction in the Bastar craton, Central India

We present mineral chemistry, geochemistry and Sr and Nd isotope data of drillcore samples from the Late Cretaceous (65 Ma), diamondiferous Behradih ultramafic pipe, Bastar craton, Central India, which is emplaced synchronous with the Deccan flood basalt eruption. The rock is affected by pervasive serpentine–talc–carbonate alteration and consists of pelletal lapilli and variously sized olivine and phlogopite macrocrysts, set in a groundmass of abundant clinopyroxene, chrome spinel, apatite, Fe-rich perovskite (<50 μm), zircon, titanite, rutile and calcite. Mineralogical studies identify the Behradih pipe as orangeite (formerly termed as Group II kimberlite) and establish the occurrence of such rocks outside the Kaapvaal craton, southern Africa. As the age of the Behradih orangeite overlaps with that of the main phase of the Deccan flood basalt magmatism, we infer a common tectonomagmatic control vis-a-vis the Deccan-related mantle plume. Trace element ratios and the Nd isotope signatures of the Behradih pipe imply that the Deccan plume has only contributed heat, but not substantial melt, to the Behradih magma with a cause-and-consequence relationship between them. Our study highlights (a) a striking similarity in the genesis of Late Cretaceous orangeites associated with the continental flood basalts in the Kaapvaal and Bastar cratons but related to different mantle plumes and (b) the role of plume–lithosphere interaction in the generation of orangeites.

Empirically Based Ground Truth Criteria for Seismic Events Recorded at Local Distances on Regional Networks with Application to Southern Africa

Wepresentanewapproachtoobtainingempiricallybased(EB)criteriafor estimating the epicentral location accuracy (i.e., ground truth, GT) of seismic events recorded at local distances on a regional network. The approach has been developed usinga jackknife resampling method appliedto carefully pickedPgphasearrivaltimes for GT reference events from several South African gold mines. The events were well recorded locally by Southern African Seismic Experiment (SASE) stations within the Archean Kaapvaal craton, an area of relatively simple crustal structure. The region- specific criteria obtained specify an EBGT395% level of epicentral accuracy if events are recorded on eight or more stations at distances less than the Pg=Pn crossover (215 km) when the stations have a primary azimuthal gap <202 degrees. In addition, when nine or more stations are used for event location and one of them is within 79 km of the event, we find that a focal depth accuracy of 4 km at the 95% confidence level can be obtained and that an accuracy of 6 km can be obtained if eight stations are used. This result illustrates that GT criteria commonly applied to global event catalogs can be relaxed if an accurate velocity model and carefully picked phase-arrival times are used for event locations. Consequently, it is likely that additional events can be added to GT compilations by developing EBGT criteria for other regional networks and using them to identify candidate GT events. For example, the EBGT criteria developed in this study, when applied to the SASE seismicity catalog, yields 10 new GT events.

U–Pb baddeleyite ages linking major Archean dyke swarms to volcanic-rift forming events in the Kaapvaal craton (South Africa), and a precise age for the Bushveld Complex

The Archean basement in the northeastern part of the Kaapvaal craton is intruded by a large number of mafic dykes, defining three major dyke swarms, which collectively appear to fan out from the Bushveld Complex. Herein we present U–Pb baddeleyite ages for two of these dyke swarms, the northwest trending Badplaas Dyke Swarm and the east-west trending Rykoppies Dyke Swarm, and infer their correlation with tectonic events in the Kaapvaal craton. We also present a U–Pb baddeleyite age for a noritic phase of the Marginal Zone of the Rustenburg Layered Suite (Bushveld Complex). The age of the Badplaas swarm is determined from two dolerites dated at 2965.9 ± 0.7 Ma and 2967.0 ± 1.1 Ma. These ages coincide with units of the Nsuze Group lavas (2967–2985 Ma), which constitute the world's oldest preserved rift basin, and suggest that dykes of this swarm are feeders to basaltic units of this group. Similarly, the E–W trending Rykoppies swarm has earlier been interpreted as a potential feeder system to the Bushveld Complex. However, the emplacement ages of six dolerites fall in the range 2.66–2.68 Ga, thus ∼600 Myr earlier than the intrusion of the Bushveld Complex (herein dated at 2057.7 ± 1.6 Ma). Rather, these ages coincide with the Allanridge Formation at the uppermost part of the Ventersdorp Supergroup as well as volcanic rocks of the “protobasinal” sequences preserved at the base of the overlying Transvaal Basin. The Rykoppies Dyke Swarm probably marks the initial stages of rifting of the Transvaal Basin and reflects a major shift from a NW–SE to an E–W trending tectonic setting. The origin of the Rykoppies Dyke Swarm can be linked either to prolonged mantle plume activity or to the onset of back-arc extension associated with south-directed subduction of oceanic lithosphere in a compressional setting along the northern margin of the Kaapvaal craton.

1-D Velocity Model for Use by the SANSN in Earthquake Location

Reliable local earthquake locations depend on many factors, a major one of which is the velocity model. Locating earthquakes using unreliable models contributes in part to the uncertainties of active fault mapping and unexplained scatter of s eismic locations. Given that we strive to continually improve our locating abilities, it is necessary to always improve on the model, when possible. The South African National Seismograph Network (SANSN) has been using a model (Saunders et al. 2006) from work published by Wright et al. (2002) and refined by Brandt (2005). Even though there have been many crust and mantle structure studies in South Africa ( e.g. , Nguuri et al. 2001; Simon et al. 2002; Wright et al. 2002; James et al. 2004), the models obtained are too complicated for use in seismic event locations, especially when using the earthquake analysis software SEISAN (Havskov and Ottemoller 2008). The HYPOCENTER program (Lienert 1994), which is the location program in SEISAN, cannot use models with low-velocity layers, because these usually introduce instabilities. Modification/refinement of the results of one of these studies (Wright et al. 2002) by Brandt (2005) produced a model that the SANSN is currently using in earthquake locations. This model can therefore be used as a starting point in conducting local earthquake tomography to obtain a more realiable model. Our study aims to compute a 1-D velocity model for South Africa by inverting P -wave travel times recorded by SANSN. Ideally, SANSN should use different models for the different major tectonic units in South Africa, i.e. , Kaapvaal craton, Namaqualand, and Cape Fold belt. However, available data dictates what can be done, and for now we can only produce a 1-D model whose velocities are approximately equal to the average velocity of the 3-D structure within the same …