Upper mantle S-wave speed heterogeneity and anisotropy beneath the North Atlantic from regional surface wave tomography:: the Iceland and Azores plumes

We present a high-resolution S-v-wave velocity and azimuthal anisotropy model for the upper mantle beneath the North Atlantic and surrounding region derived from the analysis of over 3000 fundamental and higher mode Rayleigh waveforms. Much of the data set comes from global and national digital seismic networks, but to improve the path coverage we have also deployed a number of instruments at coastal sites in northwest Europe, Iceland and eastern Greenland. The dense path coverage, wide azimuthal distribution, substantial higher mode content and the relatively short path-lengths in the data set have enabled us to build an upper-mantle model for the region with a horizontal resolution of a few hundred kilometres extending to 400 km depth. Three major hotspots, Iceland, Azores and Eifel, exist within the region of the model and slow upper-mantle velocities are associated with each of these areas. The best depth resolution in our model occurs in NW Europe and in this area low S-v velocities in the vicinity of the Eifel hotspot extend to approximately 400 km depth. Major negative velocity anomalies exist in the North Atlantic upper mantle beneath both Iceland and the Azores hotspots. Both anomalies are, above 200 km depth, 5-7 per cent slow with respect to PREM and are elongated along the Mid-Atlantic ridge. Low velocities extend to the south of Iceland beneath the Reykjanes ridge where other geophysical and geochemical observations have indicated the presence of hot plume material. A similar but somewhat weaker feature exists beneath the Kolbeinsey ridge north of Iceland, where there is also supporting evidence for the presence of hot plume material. This observation might also be associated with a plume beneath Jan Mayen. The low-velocity structure beneath the Azores hotspot is also elongated along the Mid-Atlantic ridge. The fast propagation direction of horizontally propagating S-v waves in the Atlantic south of Iceland correlates well with the east-west ridge-spreading direction at all depths and changes to a direction close to NS in the vicinity of Iceland.