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British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK
A number of marine deep seismic reflection profiles cross the Hebridean shelf, the Lewisian foreland to the Caledonian orogen. They include the BIRPS MOIST, WINCH and DRUM profiles, several recent commercial deep lines, and many old and new conventional commercial exploration profiles. Basement, comprising the upper crust, is largely devoid of coherent seismic reflections. In contrast, the mid-crust contains many reflectors which may be relics of early Palaeozoic, Caledonian (or earlier Grenvillian) eastward-dipping thrust zones, which pass into an acoustically strongly layered lower crust.
The Outer Isles Thrust is mapped from the surface to the mid-crust, and tied into its land outcrop on north Lewis. Reactivation of this thrust as a normal fault caused the formation of the Sea of the Hebrides, Minch and North Lewis Basins. The Moho is defined by a strong band of reflections at a rather uniform 27 km depth. The eastward-dipping Flannan Thrust can be mapped into the upper mantle from about 15 to 45 km depth.
The Moine Thrust, which carries rocks of the orogen over Lewisian foreland, dips at 20–25° to the east on MOIST, and is either the westernmost of a series of easterly-dipping reflections (thrusts) which flatten or terminate at 17–20 km depth, or a more easterly thrust which structurally overlies these easterly-dipping reflectors. In neither case are the easterly-dipping reflectors themselves likely to be simply ‘Caledonised’ Lewisian foreland.
