BIRPS has acquired two deep seismic reflection profiles across the Walls Boundary Fault, a major late Caledonian strike-slip fault that is probably the northern continuation of the Great Glen Fault of Scotland. Where crossed by the SHET deep seismic survey north of the Shetlands, the Walls Boundary Fault appears to be a narrow vertical structure which penetrates the entire crustal thickness and juxtaposes crusts of different thicknesses. The seismic data show a Moho offset in travel-time of 1.0–1.5 seconds and two high-amplitude diffractions originating at Moho travel-times. These diffractions are, as yet, unique among seismic data across strike-slip faults and can be used to argue that the Moho is offset in depth by 2–3 km over a lateral distance of less than 6 km. Seismic modelling is used to constrain this interpretation. The preservation of this Moho topography suggests that the structure and rheology of the lower crust of the Shetland Platform was not significantly modified during post-Devonian extension of the North Sea and Atlantic margin.