Initial results from the ICEMELT experiment: Body-wave delay times and shear-wave splitting across Iceland

We present results from the first stage of the ICEMELT broadband seismometer experiment designed to determine upper mantle structure beneath Iceland, a hotspot located on the Mid-Atlantic Ridge. Relative delays of teleseismic body waves across Iceland are in excess of 1 s for P waves and as large as 3 s for S waves. The patterns of P and S wave delays suggest a low-velocity anomaly in the upper few hundred kilometers beneath central Iceland, consistent with the signature of mantle upwelling beneath a hotspot. Shearwave splitting measurements of the fast polarization direction phi and the delay time delta t between the fast and slow shear waves have been obtained at several network stations. Splitting times range from 0.7 to 1.7 s, and fast directions are generally between N20 degrees W and N45 degrees W. While splitting times of this magnitude must be primarily signatures of the anisotropy of the Icelandic upper mantle, the directions of fast polarization are inconsistent with simple models of horizontally diverging flow either in the plate spreading direction or radially from the center of the hotspot. A hypothesis consistent with splitting data obtained to date is that the dominant contribution to upper mantle anisotropy is from the large-scale mantle flow field of the North Atlantic.