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As part of the RRISP 77 combined land-sea refraction seismic experiment, observations were carried out on Iceland itself with special emphasis on resolving the deep structure beneath Iceland and its transition towards the eastern flank of Reykjanes Ridge. The data, interpretational procedures, and results for the land part are described in this paper. A structural model of Iceland is presented which is characterized by a generalized two-layered crust of variable thickness underlain by anomalous mantle with P-wave velocities of 7.0 km/s at the base of the crust increasing to 7.4 km/s at 30 km depth. Two regions of relatively low velocity have been identified in the lower crust, possibly indicating zones of high melt concentration. A normal P- to S-wave velocity ratio of 1.76 is found within the crust, whereas this ratio reaches unusually high values of up to 2.2 in the anomalous mantle. From this and the P-wave velocity distribution the amount of partial melt is calculated. The melt content is highest (17%-23%) at the top of the mantle and decreases with increasing depth indicating differentiation processes in the upper mantle. The anomalous mantle is confined to Iceland and a sharp transition exists in the area of the shelf edge where normal oceanic lithosphere replaces the updoming asthenosphere.
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