Offshore seismic reflection profiles crossing the Caledonian orogenic front (locally, ?Moine thrust) between Shetland and the Scottish Highlands show a singular coherent east-dipping reflection underlain by a highly reflective dipping zone in the middle crust extending down to the Moho discontinuity. This reflector pattern varies spatially with respect to the eastern edge of the Precambrian Lewisian foreland as well as to previously mapped locations of the Moine thrust. Southwest of Shetland, the reflector pattern coincides with the offshore projection of the Moine thrust, but further south, toward the Highlands, the reflector diverges to the west of the thrust and actually underlies autochthonous Lewisian foreland basement. Where this reflector pattern diverges from the Moine thrust, two interpretations are possible: the prominent reflector is part of a basement imbricate thrust duplex within the footwall (or ‘lower plate’) of the Moine thrust that merges with the thrust zone further north and/or it was originally a Proterozoic normal fault which, further north around Shetland, was reactivated or over-printed by the Moine thrust. The latter interpretation supports the importance of pre-existing crustal structure in controlling Caledonian compressional deformation.