Seismic properties, density, and composition of the Icelandic crust near Reydarfjördur

Abstract

Compressional and shear wave velocities, densities, and porosities have been measured to confining pressures of 6 kbar for 135 samples from a deep borehole in Eastern Iceland. The measurements are from a random sampling of dikes and sills through the 2 km hole. Velocities within the flow units are relatively low at the tops, reach maximum values within central portions, and decrease near the bases. This systematic variation correlates well with variations in porosity and bulk density. The dike units, on the other hand, have relatively uniform velocities, which are approximately equivalent to maximum velocities measured within flow units. Below 700 m the dike and flow velocities show systematic increases with depth which are related to changes in mineralogy and independent of porosity. The velocity gradient (dVp/dZ)T within this region is well defined by the laboratory velocities and approximates 0.6 s−1, in good agreement with field refraction measurements. High velocities near the base of the hole are related to increasing alteration with depth and, in particular, an increase in epidote content. The abundance of epidote in the rocks recovered from the lower portion of the drillhole supports a metamorphic origin for the layer 2‐layer 3 boundary in this region.