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British Institutions Reflection Profiling Syndicate, Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK
Although the Moine Thrust has been well studied on land in northwestern Scotland, the subject of its deep crustal structure and regional continuation offshore remains controversial. Here, we use post-stack migration and filtering of deep seismic reflection profiles crossing the projection of the Moine Thrust and coincident Caledonian orogenic front around the western margin of the Shetland Platform in order to characterize the crustal structure of the thrust in an area not substantially affected by basin formation and extension, and where onshore geological constraints can be more readily applied. This study provides for the first time seismic reflection constraints on middle and lower crustal structure down to the level of the Moho discontinuity underlying the thrust. The seismic data reveal a thick wedge of east-dipping reflections in the lower crust capped by a highly coherent reflection which can be correlated to the postulated Moine Thrust as previously mapped on Shetland. The continuation of this wedge from the middle crust down to the Moho, against which individual reflections are abruptly truncated, suggests a ‘thick-skinned’ interpretation for the Caledonian orogen beneath the, western flank of the Shetland Platform. The available seismic data from around Shetland imply that the Moine Thrust/Caledonian front forms a boundary between an upper plate which is relatively featureless structurally and a lower plate composed of interpreted northwest-verging thrusts, possibly a crustal duplex, structurally below the level of the Moine nappe. Our observations are thus in contrast with previous interpretations made from seismic data in the West Orkney Basin north of the Northwest Highlands that describe a highly reflective Moinian upper plate.
