Seismic velocities in the Lewisian metamorphic complex, northwest Britain—‘in situ’ measurements

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Abstract
Synopsis: Sixteen reversed seismic refraction profiles, each 1 km long, in Sutherland between Kylesku and Durness have demonstrated a difference in P-wave velocity between the northern and central belts of the mainland outcrop of the early Precambrian Lewisian rocks. The variably-retrogressed pyroxene granulites of the central belt (of andesitic-dioritic composition) yielded a velocity of 6.34 ± 0.17 km/s (6 observations); the quartzo-feldspathic gneisses of the northern belt yielded a velocity of 5.64 ± 0.18 km/s (6 observations); the rocks of the boundary region, near the Ben Stack line, yielded a velocity of 5.28 ± 0.23 km/s (4 observations). Correlations are made with the velocity layering of crustal models derived for the area, confirming the wide occurrence in the upper crust of granulite-facies rocks. Such high values of velocity in surface rocks place strict limits on velocity gradients in such rock types in the deeper continental crust.