The lowermost mantle beneath northern Asia-II. Evidence for lower-mantle anisotropy

Abstract

We have analysed prediffracted S-waves with turning points beneath northern Siberia in a study of anisotropy in the lowermost mantle. Deep-focus earthquakes beneath the Marianas, Izu Bonin and the Sea of Japan recorded at stations in western Europe are used. A correction for upper-mantle anisotropy is applied to the data. Comparisons of the data with synthetic data for models with and without a high velocity D″ layer suggest that there is a velocity discontinuity at the top of the D″ region and that the style of anisotropy is transversely isotropic in this region. Time separations between S-waves on the radial and transverse component show a weak trend where the separation increases with epicentral distance. A normalization of this separation with the travel distance within D″ (300 km thick in this region) suggests that the anisotropy is uniformly distributed within this layer and has an average value of 0.5 per cent. A combination of different studies which investigate the structure of the lowermost mantle beneath Europe and northern Siberia reveals a complicated picture. Tomographic models from this area and evidence of D″ anisotropy, lower mantle scatterers, reflections from a D″ discontinuity and ultra-low-velocity zones suggest two distinct regions. One exhibits high velocities, D″ anisotropy, a D″ discontinuity and no evidence of scatterers or ultra-low-velocity zones. These features are likely associated with the palaeosubduction of the Izanagi plate well into the lowermost mantle. The other region has a lower overall velocity and shows evidence of scatterers and ultra-low-velocity zones, perhaps suggesting the presence of partial melt. These results suggest dramatic lateral variations in the nature of the lowermost mantle beneath northern Asia over a length scale of roughly 30 degrees.