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Department of Geology, Imperial College, London SW72BP, UK
The West Orkney Basin developed in Devonian times, as the western part of the Orcadian intermontane basin. It has been studied using commercial speculative seismic reflection data and the MOIST deep seismic data. The NW edge of the West Orkney Basin is formed by listric faults which are also strongly arcuate in plan, while the SE part is composed of straight domino-type faults which formed parallel to earlier (Caledonian) layering in the basement. Fault restoration and balancing suggest that initial extension in the basin occurred on low-angle reactivated Caledonian thrust faults. Steeper breaching faults cut the low-angle set, forming planar (domino-type) faults in the centre of the basin but listric faults at the NW margin. The maximum extension is about 45% in the basin centre, most of this being taken up on the later breaching fault system. This extension decreases to the SW, where fault tips occur on-shore, but some may transfer to fault systems in the Minches. The faults apparently decouple at a depth of 18-20 km and the extension suggests an initial post-Caledonian crustal thickness of up to 40 km. However, the sedimentary thickness is an average of only 3 km in the basin centre, much less than would be expected had the lithosphere thinned homogeneously, and there is no evidence of a thermal subsidence phase to the basin. This suggests that the extension shown by the West Orkney Basin was transferred to lower lithospheric levels to the east along the deep decoupling zone.
The Devonian sediments on-land show facies changes and periods of uplift and erosion which may be related to extension during basin development. They also show a phase of pre-Late-Permian tectonic inversion where the beds are locally folded and thrusted, probably related to the Hercynian events further south. The West Orkney Basin is capped by Mesozoic sediments and was probably reactivated during Mesozoic basin development in the Minches and Moray Firth.
The shape of the faults, their orientation and decoupling levels are strongly controlled by the earlier Caledonian structure, in particular by the layering and crustal anisotropy developed along and above the Moine thrust. The West Orkney Basin with its 20 km deep decoupling level formed by extension of Caledonian thickened crust. It is notable that the major basin-bounding faults to the NW, the Outer Isles and Flannan faults, which developed where the crust was thinner and hence less ductile at depth, decoupled at much deeper structural levels, at the Moho or below.
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