Seismic anisotropy of South African upper mantle xenoliths

Seismic properties of six upper mantle xenoliths from South Africa were determined both by laboratory measurements at elevated pressure and numerical calculations from petrofabric analyses, in order to provide constraints on the structural interpretation of the field seismic data obtained in recent years. The samples studied include harzburgite, garnet harzburgite, mica harzburgite, lherzolite and garnet lherzolite. Average anisotropies calculated from petrofabrics are 5.4% for Vp and 4.4% for Vs. Ultrasonic measured anisotropies from xenolith rock cores are approximately 1% lower than calculated values. This difference is likely due to simplifications in modal mineralogy used in the calculations and small amounts of alteration in the samples used in the velocity measurements. Comparison of laboratory data with the SKS and Pn data yields possible structural orientations in the upper mantle beneath the South Africa continent. Our results suggest that foliation in the upper mantle is likely horizontal, and the stretching lineation (olivine a-axis) is oriented N30°E, which coincides with the absolute plate motion (APM) of the South African continent. It is apparent from our data that the fast S-wave polarization depends on the propagation direction with respect to petrofabric orientations and does not always indicate the stretching lineation.