THE SEARCH FOR THE SOURCE OF THE ANOMALOUSLY HIGH UPPER MANTLE SEISMIC VELOCITIES OF THE SIBERIAN CRATON: EVIDENCE FROM XENOLITHS

Peacetime nuclear explosions were used as sources for four seismic lines across the Siberian craton in the late 1970’s and early 1980’s. These seismic lines found anomalously high Pn-wave velocities (up to 8.8 km/s) under the kimberlite fields of western Yakutia. Later studies (Pavlenkova et al., 1996) confirmed that these velocities were real and not an artifact of sampling. It was concluded that these velocities must be due to upper mantle anisotropy. Olivine lattice orientations on three peridotite xenoliths from the Udachnaya kimberlite were measured and seismic anisotropy was calculated from the data. The maximum calculated seismic velocities of the xenoliths may be capable of explaining the observed velocities, but this requires that olivine [100] axes must preferentially be horizontal and have an azimuth of approximately 100°, parallel to two of the four seismic lines. The characteristic orientation must also be restricted to a relatively small area under the kimberlites, such as a shear zone, and abruptly change away from them. Eclogite xenoliths in kimberlites from the same region have average compressional velocities of 8.2 km/s, and so cannot explain the high velocities. Recently, broadband seismometers (Oreshin et al., 2002) have recorded shear wave splitting in SKS and SKKS phases around the shield but curiously, the fast split shear wave does not have the same azimuth as the high Pn velocities, as required for horizontal [100] axes. This supports Oreshin’s et al. (2002) conclusion that the anisotropy responsible for high Pn velocities must be limited only to the upper most mantle and the anisotropy responsible for the observed shear wave splitting must occur at greater depths.