Displacement of the Moho by the Outer Isles thrust shown by seismic modelling

Abstract

Deep seismic data (Fig. 1) from the Caledonides, off the north-west coast of Scotland, collected (1981–82) by the British Institutions Reflection Profiling Syndicate (BIRPS), show evidence that Caledonian thrusts have been reactivated as Mesozoic normal faults^1,2. The best example is the Outer Isles (OI) thrust, which is mapped on land as a compressional fault, and is revealed by these data to be overlain offshore by a thick, wedge-shaped Mesozoic rift basin¹, known as the North Lewis (NL) basin. Here we have used synthetic seismic sections to identify distortions of the OI thrust reflection due to the NL basin. Models, which compensate for the effects of this distortion, show that the OI thrust has a steeper angle of dip than has been predicted previously assuming no distortion, causing the OI thrust to project into a 2–3 km offset of the Moho and hence to penetrate the entire crust. Our data, suggesting that shear strains affecting the lower crust may be localized in character, have adverse implications for rheological models which predict, to the contrary, that the lower crust is a region where strain is distributed.