S-wave velocity structure, mantle xenoliths and the upper mantle beneath the Kaapvaal craton

Abstract

SUMMARY Inversion of two-station Rayleigh-wave fundamental-mode phase velocities across the undisturbed region of the southern Kaapvaal craton south of the Bushveld Province produces velocity-depth models quantitatively similar to those estimated from low-T mantle xenoliths brought to the surface in Cretaceous-age kimberlite pipes that erupted in the same region. The cratonic xenolith suite was previously analysed thermobarometrically and chemically to obtain the equilibrium P-T conditions from which the seismic velocities and density of the cratonic mantle to about 180 km depth were calculated. As the xenoliths represent a snapshot of the mantle at the time of their eruption, comparison with recently recorded seismic data provides an opportunity to compare and contrast the independently gained results. We form a composite reference velocity model using xenolith values for the depth range 50–180 km with an interpolated join to PREM for the depth range 220–500 km, and a regionally determined crustal model for the upper 35 km. This composite served as the starting model for a linearized least-squares inversion (LLSI) using fundamental-mode Rayleigh-wave phase velocities in the period range 18–171 s measured for five events along 16 two-station paths within the southern Kaapvaal craton. Based on xenolith data, we constrain the vP/vS ratio in the inversion to vary from about 1.72 in the uppermost mantle to 1.78 at 180 km depth. The velocity structures determined by surface-wave inversion are consistent with those derived from the xenolith data, suggesting that the velocity structure (i.e. thermal structure) of the mantle to a depth of 180 km beneath the Kaapvaal craton today is similar to that at ∼70–90 Ma, the time of kimberlite eruption. Results from both surface-wave inversion and xenolith calculations indicate that S-wave velocities decrease slightly with depth beneath the craton, from a value around 4.7 km s−1 in the uppermost mantle to about 4.60–4.65 km s−1 at a depth of 180–200 km. We performed tests based on a wide range of starting models, and found no models with a minimum vS in the mantle less than about 4.55 km s−1 down to a depth of 250 km within the resolution possible from an inversion based on fundamental-mode Rayleigh waves. Additional analysis of synthetic models, using a combination of LLSI and the neighbourhood algorithm, shows that if there was a low-velocity zone such as that reported by Priestley in 1999, our analysis procedure would have found it.