The ‘Lewisian Units Seismic Traverse’ (LUST) was executed in an attempt to demonstrate the presence of granulite-facies rocks below the amphibolite-facies quartzo-feldspathic gneisses of the northern belt of the Precambrian Lewisian complex in Sutherland, NW Scotland. Shots fired in Badcall Bay, Lochs Laxford and Inchard, and near Durness were detected along the 40 km line by seismometers spaced at about 400 m intervals. Interpretation of the first-arrivals by ray-tracing methods suggests that no laterally-extensive body of granulite-facies rocks, like those of the central belt, exists at depths less than 4 km below the northern belt, though some such rocks may be included in the cores of major antiforms in the area. Gravity modelling, based on the velocity distribution obtained, suggests various alternatives including less dense rocks below the granulites of the central belt and a ‘brown-schist’ succession between deep granulites and surface gneisses in the northern belt.

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