Chapter 30 Structural and Stratigraphic Evolution of the Faeroe-Shetland Channel and Northern Rockall Trough M. M. Earle BP Petroleum Development Limited Cairo, Egypt E.J. Jankowski I.R. Vann The British Petroleum Company Limited London, United Kingdom ABSTRACT Beneath the continental shelf north and west of Scotland, the upper crust consists of half-graben basins formed by extension and subsidence during the Devonian to Carboniferous and Permian to Jurassic periods. They developed on listric faults which dip southeastward and sole into a detach- ment surface previously used to emplace thrust sheets during the Caledonian orogeny, and subse- quently used to invert the basins in the Tertiary. West of the shelf is the deep Rockall Trough, which overlies a rift basin floored by intruded and very thin continental crust; this rift is a branch of INTRODUCTION West of the British Isles the North Atlantic margin is a com- plex mosaic of fault-controlled sedimentary basins formed after the Caledonian orogeny during three major cycles of extension and subsidence. In this chapter we deal with three aspects of the geology of the area, namely: the crustal struc- ture across the continental shelf and Rockall Trough; the main tectonic elements and structural features; and the rela- tionship between tectonic activity and sedimentation from the Devonian to the Tertiary. The area covered in detail includes the Hebrides Shelf, the West Shetland Shelf, the northern part of the Rockall Trough, and the southern part of the Faeroe-Shetland Channel (Figure 1). COMPOSITION AND STRUCTURE OF THE CRUST Since 1972 several different and incompatible explanations have been proposed for the origin and development of the the North Atlantic system, and extension began in the late Middle Jurassic. The rift continues north- ward and underlies the Faeroe-Shetland Channel, but is offset at the Orkney-Faeroe alignment, a cross-cutting tectonic element of early Paleozoic age or older. Sediment distribution, thickness, and fades were strongly influenced by faulting, uplift, and subsi- dence. Regional paleorelief became lower with time, and prevailing conditions evolved from continental during the Devonian to marine during the Late Jurassic to Tertiary. rift basins bordering the North Atlantic west of the British Isles (reviewed by Megson, 1987). In particular, there has been a protracted debate about the age and type of crust beneath the Rockall Trough and Faeroe-Shetland Channel— a debate which now appears to be resolved in favor of intruded and considerably thinned continental crust (Megson, 1987; Mudge and Rashid, 1987; Roberts et al., 1988). In 1986 British Petroleum (BP) and Britoil shot two seismic- refraction lines across the northern part of the Rockall Trough (lines 002 and 005 on Figure 1). Both lines reveal a two-layer continental crust which is thinner on the east side of the trough (Roberts et al., 1988). Because of this asymme- try our interpretation of the crustal structure is based on a simple shear model, not a pure shear model (Figure 2). Sim- ple shear models have been applied to other extensional margins, including the adjacent North Sea (Beach, 1986). In our interpretation the upper and lower plates decouple on an eastward-dipping fault which passes beneath the Heb- ridean Platform. We suggest that it links with the Flannan 461