Alignment Of Anisotropic Minerals Research Articles

The origin of D″ reflections: a systematic study of seismic array data sets

Regional seismic studies of the Earth's deep mantle indicate the presence of P- and S-wave reflectors at the top of the D′′ layer. Such reflectors appear to be laterally intermittent, occur at a range of depths, and produce waveforms whose amplitude and polarity differ at different locations. A number of possible explanations for D′′ reflections have been proposed, including: a phase change from perovskite to post-perovskite; thermal or thermochemical discontinuities generated by subducted slabs; alignment of anisotropic minerals and global or small-scale chemical heterogeneity arising perhaps from core–mantle interactions or primitive material. However, some of the observed heterogeneity in D′′ structure may be related to lack of consistency between the style, quality and number of seismic observations in different regions, as well as complexity in wave propagation due to 3-D structure. In this study, we present a compilation of observations of D′′ reflections for regions in which (1) a reflector is seen in both P- and S-wave arrivals and (2) consistent array seismology techniques have been applied to infer the properties of the reflectors. Using a recent Monte Carlo thermodynamic modelling method to convert changes in chemistry and mineralogy into changes in seismic velocity, we compare the fit of different isotropic thermochemical structures to the observed seismic properties of D′′ reflections, namely the polarity and amplitude of the waveforms. Our results indicate that it may be possible to discriminate between different possible origins for D′′ reflections on the basis of polarity observations alone. Amplitude measurements may provide further constraints but must be treated with caution due to a large number of factors, which can influence waveform amplitudes. In regions such as the Caribbean, which are characterized by high-amplitude, negative polarity P waveforms and positive polarity S waveforms, a phase change from perovskite to post-perovskite provides the most likely explanation for the discontinuity. However, in other regions, notably Siberia and the central Pacific, P- and S waveforms both display a positive polarity, and multiple explanations for the discontinuity are possible, including subduction-related chemical heterogeneity and anisotropy.

S-wave anisotropy off Lofoten, Norway, indicative of fluids in the lower continental crust?

SUMMARY P- and S-wave velocity models along two 145 and 175 km long perpendicular profiles on the continental shelf off Lofoten, northern Norway, have been obtained from a study of nine three-component ocean-bottom seismographs. The S-wave model has been achieved from a study of the high-quality, horizontal component data. On one OBS the S-wave reflection from the Moho can be followed almost continuously from vertical to wide-angle incidence. This unique observation indicates the presence of about 10 per cent S-wave anisotropy in the lower crust along the NE-SW profile. No such anisotropy is observed along the NW-SE profile, and it is suggested that the inferred anisotropy might be caused by liquid-filled microcracks or pores aligned vertically along this profile. The vertical NW-SE alignment of the microcracks/pores might be a result of the present-day stress-field (the maximum compressive stress trends NW-SE), or it might be influenced by recent ductile strain fabrics or by ductile strain fabrics inherited from earlier deformation episodes. Another possible explanation for the inferred anisotropy might be alignment of anisotropic minerals. Many highly anisotropic minerals like olivine, pyroxene and hornblende can be excluded in this case, since the inferred S-wave anisotropy is at least three times higher than the P-wave anisotropy. Alignment of the mineral kyanite, however, might possibly explain the observations, since this mineral has much higher S-wave anisotropy than P-wave anisotropy. The hypothesis involving fluids is preferred since the very high seismic velocities of kyanite do not seem to be compatible with the estimated lower crustal velocity in this area (V, = 6.8 km s-').

Seismic anisotropy in the mantle beneath an oceanic spreading centre

BENEATH an active mid-ocean ridge, the mantle upwells in response to the divergence of the newly formed plates, leading to high temperatures and pressure-release melting below the ridge axis. The width of the upwelling region and the amount of melting depend on mantle rheology1–5, but all models predict a maximum decrease in seismic velocity at the ridge axis. It has also been suggested, however, that the alignment of anisotropic minerals by shear in the upwelling mantle will increase seismic velocity for rays travelling subvertically through the upwelling zone6,7. Here we report the observation of a consistent pattern of anomalously early P-wave arrival times at an array of ocean-bottom seismographs deployed across the axis of the southern Mid-Atlantic Ridge: P-waves from distant earthquakes arrive earlier at stations near the axis than at those further away. Our results are consistent with a model of anisotropy in which the degree of mineral alignment is greatest directly beneath the ridge axis, and significant anisotropy extends tens of kilometres from the axis.