Ceophys. J. Int. zyxwvutsrqponm (1991) 106, 149-160 zyxwvutsrq Interpretations of geoid anomalies around the Iceland hotspot G. Marquart University of Uppsala, Institute of Geophysics, Department of Geodesy, S-75592 Uppsala, Sweden Accepted 1991 February 11. Received 1991 February 11; in original form 1990 September 29 SUMMARY Geoid height anomalies around the Iceland hotspot in an area extending between z 50"W to 14"E and 45"N to 72"N have been investigated to search for evidence of an upper mantle thermal anomaly. After removing wavelengths longer than 3800 km, the effect of the cooling lithosphere was eliminated by low-pass filtering of age data and applying the boundary model. The effects of the various platforms and swells in the North Atlantic were removed by an Airy model, since the geoid-to-topography ratio is lower than 1.5 m km-' for all areas of elevated topography independent of their origin. The residual geoid shows a large positive anomaly with a steep constant slope towards a location (north)west of Iceland beneath Greenland. The amplitude of the anomaly can be estimated to at least 8m. The geoid-to-topography ratio reaches a value of 7mkm-' which is in good agreement with a plume-related anomaly. Key words: admittance, geoid anomaly, Iceland hotspot, mantle convection, North Atlantic, residual geoid. 1 INTRODUCTION The northernmost Atlantic or Norwegian-Greenland Sea is one of the most exciting areas on earth from a plate tectonic point of view. This area is characterized by a number of complex geophysical and structural phenomena. Here we have the only place where a mid-ocean ridge reaches the surface; there is clear evidence of an extinct ridge, having been active until 25Myr ago (Talwani zyxwvutsr & Eldholm 1977; Nunns 1983), there are two major (Jan-Mayen and Charlie Gibbs) and some minor fracture zones and a number of elevated platforms of oceanic or continental origin. Talwani & Eldholm (1977) reconstructed the tectonic history of this area and found that seafloor spreading started in the Norwegian-Greenland Sea about 55Myr ago at the Mohns and the Agir ridge. The Agir ridge was active until anomaly 7 (about 25 Myr), then the spreading centre jumped westward into the Greenland continental margin which caused the split off of the Jan Mayen continental fragment. Directly west of the Jan Mayen Block the anomalies zyxwvutsrq 6A to 5D (about 22 to 18Myr) have been identified. Then, at the time of anomaly 5 the ridge migrated again to the west forming the recent Kolbeinsey ridge. The Greenland-Faeroe and the Voering plateaus are two other anomalous structures in the North Atlantic whose origin remains controversial. Even though the Greenland- Faeroe plateau is generally believed to reflect a hotspot track, its location, orientation and age progressing cannot be simply explained by a fixed mantle plume beneath Iceland. If a hotspot is understood as the surface expression of a mantle temperature anomaly it must not necessarily coincide with the centre of dynamic processes in the mantle directly beneath. The strong anomalous volcanism around Iceland however, suggests that it might be underlain by a very large mantle plume of a diameter of several hundreds of kilometres. If such a plume is close to a mid-ocean ridge it might result in channelling asthenospheric material to the section of the ridge closest to the hotspot. Vink (1984) proposed a model in which the plume was originally (about 50 Myr ago) situated far to the west of the early spreading ridge beneath Greenland. By lateral asthenospheric flow it formed the Voering plateau and the earliest parts of the Greenland-Faeroe plateau in the narrow North Atlantic. In contrast to Vink's model the Voering plateau might also be interpreted to be of continental origin (Hinz & Schluter 1978). In this case rocks are formed in an epicontinental, strong tensional environment prior to seafloor spreading and have been overflown by large amounts of flood basalts after the onset of spreading. Numerical modelling of ocean spreading driven by gravitative instability of the plate and arbitrary convective flow beneath also shows that asthenospheric material can be channelled over thousands of kilometres towards the spreading centre (Schmeling, unpirblished results). Generally accepted is the continental origin of the Rockall 149 by guest on October 13, 2016 http://gji.oxfordjournals.org/ Downloaded from