Reflectivity and seismic properties of the deep continental crust

Abstract

The recovery of drill core from a section of upper amphibolite facies terrane located in the Inner Piedmont of South Carolina provides a rare opportunity to study in detail the seismic character of rocks which were once part of the deep continental crust Seismic reflection profiles over the Inner Piedmont in the vicinity of the drill hole reveal subhorizontal high‐amplitude reflections. The origin of these reflections has been investigated by measuring compressional wave velocities as a function of confining pressure and densities in 150 samples obtained from 305 m of drill core. Synthetic seismograms generated from the velocity and density measurements demonstrate that reflections originate from subhorizontal metamorphic layering, probably originating from lower crustal ductile flow. Fine tuning of the metamorphic layering, which is temperature and pressure related, can produce bright spots in the deep crust. Anisotropy, resulting from preferred mineral orientation, is another important property of this section. Velocities are low for vertical wave propagation, whereas horizontal velocities are relatively high. Horizontal velocities show only a slight variation with azimuth, which is unlikely to be detected using standard refraction techniques. Reflectivity decreases with increasing anisotropy. Critical to this study has been the continuous coring of a major section of high‐grade metamorphic terrane, which has provided the necessary stratigraphic and lithologic control for the generation of accurate synthetic seismograms.