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

Abstract

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...

Seismic Anisotropy and Its Geodynamic Implications in Iran, the Easternmost Part of the Tethyan Belt

“…This kinematic change leads to a transition from transpressional to transtensional (TT) deformation regimes from the west to the east of the belt (Shabanian et al, ). The horizontal lineation and vertical foliation produced in SSD transpressional deformation (Baptiste & Tommasi, ; Fossen et al, ; Sullivan & Law, ; Tommasi et al, ) cause an aggregation of belt‐parallel lithospheric and NNR‐parallel asthenospheric splittings in central Alborz. Toward the eastern part of Alborz where the relative motion of SCB and central Iran are almost parallel with mountain trends, the deformation pattern changes to the SSD and PSD TT regimes.…”

“…This kinematic change leads to a transition from transpressional to transtensional (TT) deformation regimes from the west to the east of the belt (Shabanian et al, 2012). The horizontal lineation and vertical foliation produced in SSD transpressional deformation (Baptiste & Tommasi, 2014;Fossen et al, 1994;Sullivan & Law, 2007;Tommasi et al, 1999) cause an aggregation of (Lü et al, 2017). (b) The slope of the fitted second-order polynomial to the Alborz range trend is plotted by red line.…”

“…As discussed previously in the 2-D model, the large suture-parallel component of convergence in western Zagros results in the domination of simple shear deformation and horizontal lineation fabrics (Figures 8 and 10). To the southeast and in central and eastern Zagros the suture-parallel component of the convergence decreases; hence, the PSD transpression and axial shortening could lead to a more vertically oriented lineations and a decrease in the strength of the mountain belt-parallel splitting caused by lithospheric anisotropy (Baptiste & Tommasi, 2014;Fossen et al, 1994;Sullivan & Law, 2007;Tommasi et al, 1999; Figure 10). Such a transition between different deformation regimes may give rise to deviation of the lithospheric splittings from the observable belt-parallel trend and even cause the domination of plate motion-parallel fast axis orientations originating from the asthenospheric flow filed in eastern Zagros.…”

Mantle Anisotropy and Asthenospheric Flow Around Cratons in SE South America

“…Figure 3.2 we see the main basement faults In these areas, the SWS anisotropy directions may be due only to frozen anisotropy imprinted at the lithosphere, from past tectonic events, or they can be a compound effect of the asthenospheric anisotropy, generated by mantle flow, plus the lithospheric frozen anisotropy. Baptiste and Tommasi (2014) argues that small delays and null SWS measurements may result from vertical variations of seismic anisotropy, which could be the reason of the observed small delays and many null results in northern Brazil.…”

“…A null measurement is one where splitting cannot be detected for one of four reasons: there is no splitting or the polarization of the XKS wave is parallel or perpendicular to the orientation of the olivine foliation plane. A different and more complex type of anisotropy, such as orientations varying with depth, or tilted fast axes of the olivine crystals (Baptiste and Tommasi, 2014). Figure 1.3: Representation of a shear wave passing through an anisotropic medium.…”