Seismic tomography shows that upwelling beneath Iceland is confined to theupper mantle

We report the results of the highest-resolution teleseismic tomography stud y yet performed of the upper mantle beneath Iceland. The experiment used da ta gathered by the Iceland Hotspot Project, which operated a 35-station net work of continuously recording, digital, broad-band seismometers over all o f Iceland 1996-1998. The structure of the upper mantle was determined using the ACH damped least-squares method and involved 42 stations, 3159 P-wave, and 1338 S-wave arrival times, including the phases P, pP, sP, PP, SP, PcP , PKIKP, pPKIKP, S, sS, SS, SKS and Sdiff. Artefacts, both perceptual and p arametric, were minimized by well-tested smoothing techniques involving lay er thinning and offset-and-averaging. Resolution is good beneath most of Ic eland from similar to 60 km depth to a maximum of similar to 450 km depth a nd beneath the Tjornes Fracture Zone and near-shore parts of the Reykjanes ridge. The results reveal a coherent, negative wave-speed anomaly with a di ameter of 200-250 km. and anomalies in P-wave speed, V-P, as strong as -2.7 per cent and in S-wave speed, V-S, as strong as -4.9 per cent. The anomaly extends from the surface to the limit of good resolution at similar to 450 km depth. In the upper similar to 250 km it is centred beneath the eastern part of the Middle Volcanic Zone, coincident with the centre of the simila r to 100 mGal Bouguer gravity low over Iceland, and a lower crustal low-vel ocity zone identified by receiver functions. This is probably the true cent re of the Iceland hotspot. In the upper similar to 200 km, the low-wave-spe ed body extends along the Reykjanes ridge but is sharply truncated beneath the Tjornes Fracture Zone. This suggests that material may flow unimpeded a long the Reykjanes ridge from beneath Iceland but is blocked beneath the Tj ornes Fracture Zone. The magnitudes of the V-P, V-S and V-P/V-S anomalies c annot be explained by elevated temperature alone, but favour a model of max imum temperature anomalies <200 K, along with up to <similar to>2 per cent of partial melt in the depth range similar to 100-300 km beneath east-centr al Iceland. The anomalous body is approximately cylindrical in the top 250 km but tabular in shape at greater depth, elongated north-south and general ly underlying the spreading plate boundary. Such a morphological change and its relationship to surface rift zones are predicted to occur in convectiv e upwellings driven by basal heating, passive upwelling in response to plat e separation and lateral temperature gradients. Although we cannot resolve structure deeper than similar to 450 km, and do not detect a bottom to the anomaly, these models suggest that it extends no deeper than the mantle tra nsition zone. Such models thus suggest a shallow origin for the Iceland hot spot rather than a deep mantle plume, and imply that the hotspot has been l ocated on the spreading ridge in the centre of the north Atlantic for its e ntire history, and is not fixed relative to other Atlantic hotspots. The re sults are consistent with recent, regional full-thickness mantle tomography and whole-mantle tomography images that show a strong, low-wave-speed anom aly beneath the Iceland region that is confined to the tipper mantle and th us do not require a plume in the lower mantle. Seismic and geochemical obse rvations that are interpreted as indicating a lower mantle, or core-mantle boundary origin for the North Atlantic Igneous Province and the Iceland hot spot should be re-examined to consider whether they are consistent with upp er mantle processes.