New aspects of the deep structure of the Scottish Caledonides are revealed by the Moine and Outer Isles deep seismic reflection traverse (MOIST). The Caledonian foreland is underlain by an easterly-dipping, strongly reflecting surface cutting through the Moho and traceable to more than 45 km depth. Thrusts within the foreland basement and Caledonian orogen have been reactivated as normal faults bounding half-grabens filled with sediments of late Palaeozoic to Mesozoic age. The Moine Thrust, which carries rocks of the orogen over Lewisian foreland, dips at 20–25° to the E on MOIST, and is either: (a) the westernmost of a series of easterly-dipping reflections (thrusts) which flatten or terminate at 17–20 km depth, or (b) a thrust lying further E which structurally overlies these easterly-dipping reflectors. By comparison with easterly-dipping reflections on COCORP lines in the northern and southern Appalachians the latter interpretation is preferred, implying that many of the easterly-dipping reflections on MOIST correspond to off-shelf metasedimentary rocks, with slivers of basement, stacked and imbricated against the Lewisian basement edge. Regional seismic refraction and conductivity data from northern Scotland suggest that regions of the Caledonian orogen have lower mid-crustal velocity gradients and higher conductivities than the foreland, which might be explained by this interpretation of the easterly-dipping reflections.
Despite similarities in crustal reflector geometries, there are important differences between the MOIST and COCORP lines, including (1) a remarkably continuous, relatively horizontal Moho seen on MOIST data at about 25 km depth, and (2) the sedimentary basins offshore from northern Scotland which have formed by reactivation under crustal extension of the easterly-dipping thrusts. Furthermore, it is probable that the easterly-dipping reflectors in these areas of the MOIST and COCORP lines were formed at different times in the early Palaeozoic.