The island of Foula located 25 km SW of Shetland preserves a gently folded, 1.6 km thick sequence of Middle Devonian sandstones spectacularly exposed in kilometre-long cliff sections over 350 m high. These rocks unconformably overlie likely Precambrian-age amphibolite facies basement rocks, intruded by sheeted granites. The onshore succession is similar in age to the nearby Lower Clair Group offshore to the west. New mapping which incorporates use of drone imagery in inaccessible cliff sections uses down-plunge projections to show that growth folding and faulting on Foula were contemporaneous with sedimentation during basin filling. The large-scale structural geometry is consistent with regional constrictional strain due to sinistral transtension associated with movements along the Walls Boundary - Great Glen Fault Zone system during the Middle Devonian. Detrital zircon provenance studies indicate that the Devonian sequences of Foula - and nearby Melby in western Shetland - show similarities with the Clair Group and Orkney successions. We suggest that NE-SW transtensional fold development contemporaneous with regional subsidence may be more widespread than previously realised in the Devonian basins of Scotland. Large, kilometre-scale folds previously interpreted to be related to Permo-Carboniferous inversion may therefore have initiated earlier in the basin evolution than previously realised.

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