Deep structure of the foreland to the Caledonian Orogen, NW Scotland: Results of the Birps Winch Profile

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Abstract
The WINCH marine deep seismic reflection profile crosses the Hebridean shelf, the Proterozoic foreland to the Caledonian orogen, west of Scotland. The data quality is very good. The upper crust is largely devoid of coherent seismic reflections, although this may in part be due to acquisition techniques being inappropriate for this problem. In contrast, the middle and lower crust (10–25 km depth) exhibits good reflections; the mid crust contains reflectors which may be relics of early Palaeozoic, Caledonian (or earlier Grenvillian) eastward‐dipping thrust zones, which pass into an acoustically strongly layered lower crust. The Outer Isles Thrust is mapped from the surface to the mid crust, and tied into its land outcrop on north Lewis. Reactivation of this thrust offshore, in a normal sense, in Mesozoic times caused the formation of the Minch and North Lewis Basins. The Mono is defined by a strong band of reflections at about 27 km depth, which correlates well with results from the HMSE explosion seismic survey. Moho depth is apparently rather uniform in the area of the foreland crossed by WINCH. WINCH and HMSE results together suggest a mean crustal velocity for the Hebridean shelf of 6.4+0.1 km s−1. The eastward‐dipping Flannan Thrust can be mapped into the upper mantle on three lines from about 15 to 45 km depth, well into the upper mantle. Neither the Flannan Thrust nor the Outer Isles Thrust appear to pass straight through the reflective lower crust, suggesting that the lower crust is a region of high strain. The Outer Hebrides is a positive block probably formed as an isostatic response to Mesozoic normal faulting which reactivated the Outer Isles Thrust.

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