Seismic anisotropy of South African upper mantle xenoliths

Seismic properties of six upper mantle xenoliths from South Africa were det ermined 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 ha rzburgite, Iherzolite and garnet Iherzolite. Average anisotropies calculate d from petrofabrics are 5.4% for V-p and 4.4% for V-s. Ultrasonic measured anisotropies from xenolith rock cores are approximately 1% lower than calcu lated values. This difference is likely due to simplifications in modal min eralogy used in the calculations and small amounts of alteration in the sam ples used in the velocity measurements. Comparison of laboratory data with the SKS and P-n data yields possible structural orientations in the upper m antle beneath the South Africa continent. Our results suggest that foliatio n in the upper mantle is likely horizontal, and the stretching lineation (o livine a-axis) is oriented N30 degrees 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 directio n with respect to petrofabric orientations and does not always indicate the stretching lineation. (C) 2000 Elsevier Science B.V. All rights reserved.