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

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 v P / v S 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 v S 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.