Constraints on crustal structure in eastern Iceland based on extremal inversions of refraction data

Abstract

In the summer of 1978 the Iceland Research Drilling Project undertook the drilling of a deep crustal hole near Reydarfjörder in eastern Iceland. As a part of this project, the Scripps Institution of Oceanography and the University of Washington undertook a small‐scale seismic refraction experiment near the drill site in an attempt to compare surface geophysical measurements with observations of samples from and logging in the hole. Using recent advances in the methods of extremal inversion of seismic data, we have determined an approximate one‐dimensional velocity structure for the drill site. This structure indicates that the 1.9‐km hole failed to penetrate the layer 2 layer ‐3 transition which was at some 3.0–4.5 km beneath the drill site. The transition appears to be rather abrupt, unlike that beneath the ocean, with velocity increasing from 5.2–5.5 km/s in the upper crust to about 6.7 km/s in layer 3. We observe a steep eastward dip and a shallow westward dip in the lower crust away from the nearby Thingmúli and Reydarfjördur volcanic centers, respectively, in agreement with previous work associating shallow depths to layer 3 with Tertiary volcanic centers as a result of increasing metamorphic grade and increased dyke swarm intensity.