The deep structure of the Moine Thrust, south-west of Shetland

The Caledonian orogenic front (the Moine thrust zone) has been described in detail from outcrops in the NW Highlands and Shetland (see Flinn et al. 1979; McClay and Coward 1981). Recently however, the use of deep seismic reflection profiling on the adjacent continental shelf has provided new information on the deep structure of the orogen (Smythe et al. 1982; Brewer et al. 1983). This note presents further evidence for the offshore position and orientation of the thrust plane as interpreted from seismic profiles in the vicinity of Foula. In the extreme NW of Shetland, Lewisian orthogneisses which crop out on North Roe and Ve Skerries belong to the Caledonian foreland (Fig. 1). These are separated from the younger metasediments of Shetland and Foula by a steeply (60°) east-dipping Caledonian thrust (the Wester Keolka shear), which is generally considered to be the northward extension of the Moine Thrust (Pringle 1970; Flinn et al. 1979). Further west, an area of strongly developed magnetic and gravity anomalies has been attributed by Bott and Watts (1970) to basement gneiss at or near the seabed (Fig. 1). A further prominent seabed feature is the Foula Ridge, which connects Foula and Ve Skerries (Fig. 1). This is composed of metamorphic rocks and has yielded samples of banded hornblendic gneisses and granulite (May et al. 1984). The youngest tectonic feature in the area is the Permo-Triassic West Fair Isle Basin which is preserved on the downthrown side of the Walls Boundary Fault (Fig. 1; Bott and Browitt . . .