Recent deformations of the deep continental root beneath southern Africa

Vinnik, Lev; Green, R. W. E.; Nicolaysen, L. O.

doi:10.1038/375050a0

Cited by 104 publications

(74 citation statements)

References 19 publications

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“…Because olivine is anisotropic and develops strain-induced lattice-preferred orientation, splitting results may be used to infer mantle strain fields [see review by Savage, 1998]. However, there is an ongoing dispute over whether •p at continental cratons is parallel to past geologic features and reflects the fossil lattice-preferred orientation (LPO) of olivine minerals within the Precambrian continental lithosphere [Silver and Chan, 1991;Silver, 1996], or if •p is directed parallel to the present-day absolute plate motion and reflects LPO associated with asthenospheric shear [Vinnik et al, 1992[Vinnik et al, , 1995. Silver [1996] presents an extensive compilation of the continental splitting results, showing that several continental shields (the Canadian Shield, Brazilian Craton, and Kaapval Craton) display •p parallel to ancient geologic structures, consistent with anisotropy caused by vertically coherent deformation of the crust and mantle during Precambrian orogenesis.…”

Section: Introductionmentioning

confidence: 99%

Shear‐wave splitting across western Saudi Arabia: The pattern of upper mantle anisotropy at a Proterozoic Shield

Wolfe

Vernon

Alamri

1999

Geophysical Research Letters

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Abstract. We constrain upper mantle anisotropy across the Arabian Shield from shear-wave splitting analyses of $K$ phases at eight temporary broadband stations that operated in Saudi Arabia. The direction of fast polarization is consistently aligned north-south and the delay time between fast and slow shear waves is generally 1.0 to 1.5 s, indicating that the mantle anisotropy is relatively homogeneous and coherent. We cannot distinguish between two possible models for the origin of this signal. The observed splitting may reflect fossil upper mantle anisotropy associated with the dominantly east-west accretion of oceanic terranes and formation of the Proterozoic Arabian lithosphere. Our results may also be compatible with present-day asthenospheric anisotropy caused by the northward absolute plate motion of the Arabian plate or northward asthenospheric flow from an Ethiopian mantle plume.

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Section: Introductionmentioning

confidence: 99%

Shear‐wave splitting across western Saudi Arabia: The pattern of upper mantle anisotropy at a Proterozoic Shield

Wolfe

Vernon

Alamri

1999

Geophysical Research Letters

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“…They attributed this anisotropy to fossil deformation in the mantle lithosphere. Vinnik et al (1995) attributed the same NNE-SSW fast polarization directions to asthenospheric deformation in response to African plate motion. Significant variations of SKS delay times were measured near Kimberley, which was interpreted as the boundary between a strongly and a weakly anisotropic domain (Fouch et al, 2004b).…”

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%

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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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“…9a, b). The APM directions are consistent with the present ongoing asthenospheric flow (Vinnik et al, 1992(Vinnik et al, , 1995Vinnik and Montagner, 1996). By using different plots of the plate motion (APM of Eurasian and Indian plates referenced to the no-net-rotation (NNR) frame or the relative plate motion of the Indian Plate referenced to the Eurasian Plate), we want to examine the contribution of APM to explaining the observed anisotropic variation and to check which plate motion best explains the observed φ s of the splitting measurements.…”

Section: Deformation Pattern Revealed From the Comparison Of The Gpsmentioning

confidence: 99%

Seismic anisotropy inferred from direct <i>S</i>-wave-derived splitting measurements and its geodynamic implications beneath southeastern Tibetan Plateau

et al. 2017

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Abstract. The present study deals with detecting seismic anisotropy parameters beneath southeastern Tibet near Namcha Barwa Mountain using the splitting of direct S waves. We employ the reference station technique to remove the effects of source-side anisotropy. Seismic anisotropy parameters, splitting time delays, and fast polarization directions are estimated through analyses of a total of 501 splitting measurements obtained from direct S waves from 25 earthquakes ( ≥ 5.5 magnitude) that were recorded at 42 stations of the Namcha Barwa seismic network. We observe a large variation in time delays ranging from 0.64 to 1.68 s, but in most cases, it is more than 1 s, which suggests a highly anisotropic lithospheric mantle in the region. A comparison between direct S-and SKS-derived splitting parameters shows a close similarity, although some discrepancies exist where null or negligible anisotropy has been reported earlier using SKS. The seismic stations with hitherto null or negligible anisotropy are now supplemented with new measurements with clear anisotropic signatures. Our analyses indicate a sharp change in lateral variations of fast polarization directions (FPDs) from consistent SSW-ENE or W-E to NW-SE direction at the southeastern edge of Tibet. Comparison of the FPDs with Global Positioning System (GPS) measurements, absolute plate motion (APM) directions, and surface geological features indicates that the observed anisotropy and hence inferred deformation patterns are not only due to asthenospheric dynamics but are a combination of lithospheric deformation and sub-lithospheric (asthenospheric) mantle dynamics. Direct S-wave-based station-averaged splitting measurements with increased back-azimuths tend to fill the coverage gaps left in SKS measurements.

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Recent deformations of the deep continental root beneath southern Africa

Cited by 104 publications

References 19 publications

Shear‐wave splitting across western Saudi Arabia: The pattern of upper mantle anisotropy at a Proterozoic Shield

Shear‐wave splitting across western Saudi Arabia: The pattern of upper mantle anisotropy at a Proterozoic Shield

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

Seismic anisotropy inferred from direct <i>S</i>-wave-derived splitting measurements and its geodynamic implications beneath southeastern Tibetan Plateau

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Recent deformations of the deep continental root beneath southern Africa

Cited by 104 publications

References 19 publications

Shear‐wave splitting across western Saudi Arabia: The pattern of upper mantle anisotropy at a Proterozoic Shield

Shear‐wave splitting across western Saudi Arabia: The pattern of upper mantle anisotropy at a Proterozoic Shield

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

Seismic anisotropy inferred from direct &lt;i&gt;S&lt;/i&gt;-wave-derived splitting measurements and its geodynamic implications beneath southeastern Tibetan Plateau

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Seismic anisotropy inferred from direct <i>S</i>-wave-derived splitting measurements and its geodynamic implications beneath southeastern Tibetan Plateau