Mantle anomalies beneath southern Africa: evidence from seismicSandPreceiver functions

Abstract

To understand deep structure and processes beneath southern Africa, we apply the double-stacking version of the S receiver function (SRF) technique to the recordings of the South African Seismic Experiment. In this technique the wavefields of S and SKS are separated by space—time filtration, and the receiver functions are constructed separately for the S and SKS seismic phases. The results are consistent with those obtained from the SRFs of the permanent stations BOSA and LBTB. Evidence for a reduced S velocity atop the 410-km discontinuity is present in both SRFs and P receiver functions: the S velocity contrast at the 410-km discontinuity is ∼40 per cent larger than the norm, and there are observations of S350p and P350s seismic phases from the negative 350-km discontinuity beneath the Kaapvaal craton. The S350p and P350s phases display a dependence on the azimuth, which can be caused by anisotropy in the layer atop the 350-km discontinuity. This dependence is consistent with observations of shear wave splitting in SKS. There are observations of the S450p phase from the 450-km negative discontinuity in the transition zone. The most anomalous transition zone is found close to the region where the Kalahari craton was located in the Mesozoic. Lateral variations of the S velocity are found beneath the 660-km discontinuity. Teleseismic S and P traveltime residuals with respect to IASP91 model are evaluated from the traveltimes of the P410s and Pp410s seismic phases. In the uppermost mantle these residuals as well as S410p traveltimes require a reduced VP/VS ratio (around 1.75 ± 0.01 versus 1.8 in IASP91), an effect of the depletion of the mantle lithosphere in basaltic material. In the models obtained by a joint inversion of the receiver functions and the teleseismic traveltime residuals the low velocity zone (LVZ) with the onset of low velocity at a depth of ∼140 ± 20 km is present at most locations. The minimum S velocity in the LVZ is ∼4.5 km s−1. The LVZ in our models is consistent with the S velocity, VP/VS velocity ratio and depth range of high-temperature lerzholites in the mantle xenoliths from southern Africa. Our most intriguing finding is a very low quality factor QS (on the order of a few tens) in the upper mantle. We interpret high attenuation, the LVZ and the low S velocity atop the 410-km discontinuity as the effects of plume-like phenomena in the upper mantle.