Mantle seismic structure beneath the Kaapvaal and Zimbabwe Cratons

Fouch, M. J.; James, David E.; VanDecar, J. C.; Lee, Suzan van der

doi:10.2113/107.1-2.33

Cited by 163 publications

(126 citation statements)

References 52 publications

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“…It is also consistent with the anisotropic surface wave models and with the results of P receiver functions and SKS waveforms inversion, which propose change in the orientation of the fast direction at depths > 160 km (Adam and Lebedev, 2012;Vinnik et al, 2012). However, such a vertical variation in seismic anisotropy should produce a backazimuthal dependence of the shear wave splitting, which was not described in the regional studies (Silver et al, 2001;Fouch et al, 2004a;Silver et al, 2004;Vinnik et al, 1995).…”

Section: Solidsupporting

confidence: 86%

“…Small-scale variations in seismic properties within the cratonic root also may not be imaged or interpreted unambiguously. Beneath the Bushveld complex, some, but not all body wave tomography models imaged velocities lower than the average ones in the craton, which have been attributed to either more fertile compositions associated with the 2 Gaold Bushveld magmatism (James et al, 2001;Fouch et al, 2004a) or to a hotter present-day geotherm (Fouch et al, 2004a).…”

Section: Baptiste and A Tommasi: Seismic Properties Of The Kaapvamentioning

confidence: 99%

“…The deformation of the Kaapvaal craton has also been extensively investigated by seismic anisotropy studies (Silver et al, 2001;Fouch et al, 2004a;Fouch et al, 2004b;Silver et al, 2004;Adam and Lebedev, 2012;Vinnik et al, 1995Vinnik et al, , 2012. Silver et al (2001) measured fast polarization directions that consistently followed the trend of geological structures and small SKS delay times (0.62 s on average), with almost null delays in the central Kaapvaal craton.…”

Section: Baptiste and A Tommasi: Seismic Properties Of The Kaapvamentioning

confidence: 99%

“…P and S wave tomography studies reported lateral velocity perturbations of 0.5 % (James et al, 2001;Fouch et al, 2004a), while surface wave studies imaged velocity perturbations as high as 5-7 % within the Kaapvaal cratonic root (Ritsema and van Heijst, 2000;Priestley et al, 2006;Chevrot and Zhao, 2007;Fishwick, 2010). Priestley and Tilmann (2009) a very narrow geotherm range and suggested that lateral S wave velocity variations within the cratonic root are mainly due to compositional changes.…”

Section: Comparison With Seismological Velocity Profiles and Tomograpmentioning

confidence: 99%

“…Priestley and Tilmann (2009) a very narrow geotherm range and suggested that lateral S wave velocity variations within the cratonic root are mainly due to compositional changes. A characteristic example of this intracratonic seismic velocity heterogeneity are the lower velocities (∼ 0.5 % for V p and ∼ 0.8 % for V s ) imaged below the Bushveld complex by body wave tomographies (James et al, 2001;Fouch et al, 2004a) (Fig. 1), but not by surface wave tomographies (e.g., Priestley and Tilmann, 2009), probably due to their poorer lateral resolution.…”

Section: Comparison With Seismological Velocity Profiles and Tomograpmentioning

confidence: 99%

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Petrophysical constraints on the seismic properties of the Kaapvaal craton mantle root

Baptiste

Tommasi

2014

Solid Earth

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Abstract.We calculated the seismic properties of 47 mantle xenoliths from 9 kimberlitic pipes in the Kaapvaal craton based on their modal composition, the crystal-preferred orientations (CPO) of olivine, ortho-and clinopyroxene, and garnet, the Fe content of olivine, and the pressures and temperatures at which the rocks were equilibrated. These data allow constraining the variation of seismic anisotropy and velocities within the cratonic mantle. The fastest P and S 2 wave propagation directions and the polarization of fast split shear waves (S 1 ) are always subparallel to olivine [100] axes of maximum concentration, which marks the lineation (fossil flow direction). Seismic anisotropy is higher for high olivine contents and stronger CPO. Maximum P wave azimuthal anisotropy (AV p ) ranges between 2.5 and 10.2 % and the maximum S wave polarization anisotropy (AV s ), between 2.7 and 8 %. Changes in olivine CPO symmetry result in minor variations in the seismic anisotropy patterns, mainly in the apparent isotropy directions for shear wave splitting. Seismic properties averaged over 20 km-thick depth sections are, therefore, very homogeneous. Based on these data, we predict the anisotropy that would be measured by SKS, Rayleigh (S V ) and Love (S H ) waves for five endmember orientations of the foliation and lineation. Comparison to seismic anisotropy data from the Kaapvaal shows that the coherent fast directions, but low delay times imaged by SKS studies, and the low azimuthal anisotropy with with the horizontally polarized S waves (S H ) faster than the vertically polarized S wave (S V ) measured using surface waves are best explained by homogeneously dipping (45 • ) foliations and lineations in the cratonic mantle lithosphere. Laterally or vertically varying foliation and lineation orientations with a dominantly NW-SE trend might also explain the low measured anisotropies, but this model should also result in backazimuthal variability of the SKS splitting data, not reported in the seismological data. The strong compositional heterogeneity of the Kaapvaal peridotite xenoliths results in up to 3 % variation in density and in up to 2.3 % variation of V p , V s , and V p /V s ratio. Fe depletion by melt extraction increases V p and V s , but decreases the V p /V s ratio and density. Orthopyroxene enrichment due to metasomatism decreases the density and V p , strongly reducing the V p /V s ratio. Garnet enrichment, which was also attributed to metasomatism, increases the density, and in a lesser extent V p and the V p /V s ratio. Comparison of density and seismic velocity profiles calculated using the xenoliths' compositions and equilibration conditions to seismological data in the Kaapvaal highlights that (i) the thickness of the craton is underestimated in some seismic studies and reaches at least 180 km, (ii) the deep sheared peridotites represent very local modifications caused and oversampled by kimberlites, and (iii) seismological models probably underestimate the compositional heterogeneity in the Kaapvaal man...

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Section: Solidsupporting

confidence: 86%

Section: Baptiste and A Tommasi: Seismic Properties Of The Kaapvamentioning

confidence: 99%

Section: Baptiste and A Tommasi: Seismic Properties Of The Kaapvamentioning

confidence: 99%

Section: Comparison With Seismological Velocity Profiles and Tomograpmentioning

confidence: 99%

Section: Comparison With Seismological Velocity Profiles and Tomograpmentioning

confidence: 99%

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Petrophysical constraints on the seismic properties of the Kaapvaal craton mantle root

Baptiste

Tommasi

2014

Solid Earth

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Seismic evidence for stratification in composition and anisotropic fabric within the thick lithosphere of Kalahari Craton

Sodoudi

Yuan

Kind

et al. 2013

Geochem Geophys Geosyst

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[1] Based on joint consideration of S receiver functions and surface-wave anisotropy we present evidence for the existence of a thick and layered lithosphere beneath the Kalahari Craton. Our results show that frozen-in anisotropy and compositional changes can generate sharp Mid-Lithospheric Discontinuities (MLD) at depths of 85 and 150-200 km, respectively. We found that a 50 km thick anisotropic layer, containing 3% S wave anisotropy and with a fast-velocity axis different from that in the layer beneath, can account for the first MLD at about 85 km depth. Significant correlation between the depths of an apparent boundary separating the depleted and metasomatised lithosphere, as inferred from chemical tomography, and those of our second MLD led us to characterize it as a compositional boundary, most likely due to the modification of the cratonic mantle lithosphere by magma infiltration. The deepening of this boundary from 150 to 200 km is spatially correlated with the surficial expression of the Thabazimbi-Murchison Lineament (TML), implying that the TML isolates the lithosphere of the Limpopo terrane from that of the ancient Kaapvaal terrane. The

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Erosion patterns and mantle sources of topographic change across the southern African Plateau derived from the shallow and deep records of kimberlites

Stanley

Flowers

Bell

2015

Geochem Geophys Geosyst

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Flow in the sublithospheric mantle is increasingly invoked as a mechanism to explain both modern and past surface topography, but the importance of this phenomenon and its influence at different localities are debated. Southern Africa is an elevated continental shield proposed to represent dynamically supported topography. However, this region is also characterized by a complex lithospheric architecture variably affected by Cretaceous heating, thinning, and metasomatic alteration. We used apatite (U-Th)/He thermochronometry on 15 Cretaceous kimberlites from an 600 km long transect across the Kaapvaal Craton, combined with information from xenoliths in these pipes, to determine the plateau interior erosion history. The goal was to determine the relationships with lithospheric modification patterns and thereby better isolate the sublithospheric contribution to elevation. The results document a wave of erosion from west to east across the craton from 120 to <60 Ma, initially focused along paleorivers and then retreating as a scarp across the landscape. This spatially variable erosion event was associated with limited modification of the Archean cratonic lithospheric mantle as recorded by mantle xenoliths and xenocrysts, implying that dynamic buoyancy sources may be required to explain the elevations. In contrast, off-craton to the southwest, a more pronounced regional erosion phase at 110-90 Ma was coincident with significant modification of the Proterozoic lithospheric mantle. This relationship suggests that lithospheric processes were more important contributors to erosion and topographic change off-craton than on-craton. Together, these results suggest that lithospheric architecture can have an important control on the surface expression of mantle dynamics.

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Mantle seismic structure beneath the Kaapvaal and Zimbabwe Cratons

Cited by 163 publications

References 52 publications

Petrophysical constraints on the seismic properties of the Kaapvaal craton mantle root

Petrophysical constraints on the seismic properties of the Kaapvaal craton mantle root

Seismic evidence for stratification in composition and anisotropic fabric within the thick lithosphere of Kalahari Craton

Erosion patterns and mantle sources of topographic change across the southern African Plateau derived from the shallow and deep records of kimberlites

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