First results from the Reykjanes Ridge Iceland Seismic Project 1977

PALAEOGEOGRAPHIC reconstructions of the North Atlantic area, based on plate tectonic principles^1-3, suggest that Iceland is of pure oceanic origin as Greenland and Europe (including Rockall Plateau) seem to have been closely connected before 56 Myr BP (magnetic lineation 24). Oceanic origin is also suggested by the location of Iceland astride the axis of the Mid-Atlantic Ridge, by its surface geology and by the young age (16 Myr) of its oldest rocks. Further evidence comes from refraction seismic investigations⁴ yielding a layering of the Icelandic crust resembling the typical oceanic crust in velocity values, but with greater thickness of individual layers. A remarkable result was the discovery of a layer with P-wave velocities close to 7.2 km s+1 at a depth of 8-15 km, which was interpreted as the top of an anomalous upper mantle below Iceland. According to studies of teleseismic travel time residuals^5,6, as well as apparent velocity measurements of P-arrivals from Mid-Atlantic Ridge earthquakes⁷, this anomalous mantle may extend to 240 km depth. A very different model, however, has been derived from more recent long range refraction seismic measurements⁸. It is characterised by a more or less continental deeper crust and a normal upper mantle with a P-wave velocity of 8.0 km s⁺¹ at a depth of 50-60 km below Iceland. This model could also account for the low apparent velocities given by Francis⁷ and, if true, would require drastic modifications of current ideas of seafloor spreading. It has been used as an argument against the possibility of any significant amount of drift at the latitude of Iceland⁹. Geophysical evidence from Reykjanes and Kolbeinsey Ridges^10,11, on the other hand, strongly favours an oceanic origin of the surroundings of Iceland. Earlier refraction seismic measurements on the crest and western flank of the Reykjanes Ridge^12,13 indicate a systematic change of crustal structure with age and a crustal structure typical of oceanic basins has been found some 200 km southeast of the ridge ¹⁴. All the marine refraction work so far has shown anomalously low mantle velocities (7.2-7.8 km s⁺¹) but very little information is available on the structure of the oceanic upper mantle itself because of the limited range of the seismic profiles. In 1977 an 800-km long seismic refraction profile was shot across Iceland and along the southeastern flank of the Reykjanes Ridge (Fig. 1). The main purpose of the experiment was to resolve the structure of the crust and upper mantle to greater depth than previously possible and to study the transition from the oceanic to the Icelandic structure. This experiment was planned and carried out by an international working group (Reykjanes Ridge Iceland Seismic Project, RRISP).