The seismic structure of the mantle beneath Iceland

Seismic images of seismic wave velocity variations in the mantle have suggested that a cylindrically shaped low-velocity structure beneath Iceland extends deep into the upper mantle. Whereas low-velocity anomalies are confined to the upper 100 km beneath the Atlantic Ridge, the low-velocity anomaly extends deeper than 300-400 km beneath Iceland. In this presentation, we discuss two new and independently derived images of shear velocity variation in the North Atlantic upper mantle. S40RTS is based on the inversion of Rayleigh wave dispersion and body-wave traveltime measurements. ECOS.12 is derived by full waveform inversion of three-component surface waves and long-period body waves. Despite the different modeling approaches, S40RTS and ECOS.12 show consistent heterogeneity if the heterogeneous resolution is considered. We will make a quantitative comparison of the new models and discuss the origin of the extremely strong shear-velocity reductions in the Icelandic upper mantle (3-4% when averaged over a 500-km wide region). Topics to be addressed include: (1) the resolution of strong heterogeneity on the small to intermediate (~500 km) scales, (2) the effects of wavefront healing on the imaging of plumes in the lower mantle. (3) thermodynamic interpretations of seismic velocity heterogeneity.