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.

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Marine geophysical surveys show that the Escarpment is the buried feather-edge of a thick pile of flood basalts of early Eocene age, overlying a thinner, more widespread layer of basalts of late Palaeocene age. The Escarpment does not, therefore, define the continent-ocean boundary in the southern Norwegian Sea, but instead marks the contemporary shoreline separating terrestrially erupted basalt flows in the north from a restricted shallow-water shelf to the south. The basalts overlie 5-6 km of Mesozoic sediments, which have completely buried a large conical flat-topped seamount of similar dimensions to Anton Dohrn and Rosemary Bank. We call this newly postulated body the Brendan seamount. The Mesozoic sediments are at least as old as early Cretaceous in age, therefore precluding the possibility that the mid-Cretaceous seafloor spreading episode (supposed by some to have created the Rockall Trough) could have also created the thin crust inferred to underlie the Faeroe-Shetland Trough and Mbre Basin.

Detailed analysis of marine magnetic profiles from the western part of the East Scotia Sea confirms continuous, organized back-arc spreading since at least 15 Ma ago. In the eastern part of the East Scotia Sea, the South Sandwich arc lies on crust that formed at the back-arc spreading centre since 10 Ma ago, so older back-arc crust forms the basement of the present inner forearc. Interpretations of two multichannel seismic reflection profiles reveal the main structural components of the arc at shallow depth, including evidence of trench-normal extension in the mid-forearc, and other features consistent with ongoing subduction erosion. The seismic profile interpretations have been used to constrain simple two-dimensional gravity models. The models were designed to provide constraints on the maximum possible thickness of the arc crust, and it is concluded that this is 20 and 19.2 km on the northern and southern lines, respectively. On the northern line the models indicate that the forearc crust cannot be much thicker than normal oceanic crust. Even with such thin crust, however, the magmatic growth rate implied by the cross-section of the arc crust is within the range recently estimated for two other arcs that have been built over a much longer interval.