An interpretation of the Variscan tectonics of SW Britain

Abstract

Summary The structure of SW Britain is interpreted in terms of thin-skinned tectonics, producing a NNW verging fold and thrust zone. In south Wales the thrusts involve high level imbricate stacks showing some 20–40% shortening with a shallow floor thrust in Namurian-Westphalian strata, cut to the south by deeper level folds and thrusts with a basal decoupling zone in or below the Lower Palaeozoic rocks. In Devon and Cornwall the Upper Palaeozoic rocks show a gently dipping, widespread and locally intense cleavage related to the progressive development of large thrust sheets and associated inclined to recumbent folds. In north Cornwall these northward verging structures have been redeformed and often completely overprinted by a large south verging backthrust. Some of this backthrust movement may be offset, by a lateral ramp, to a much lower decoupling zone, producing a major backfold in south Devon. The thrust transport direction is to the NNW as determined from mineral lineations, maximum extension directions, fold and thrust traces and tear faults. Fold hinges are generally normal to the transport direction but are locally rotated into the NNW trend in more intensely deformed zones. In north Devon and south Wales, the folds are oblique suggesting a component of differential movement. Considering SW Britain and southern Ireland the fold trends are arcuate and there is often extension parallel to the fold axes. These phenomena are interpreted as due to the obliquity of the thrusting and the Variscan front. Pre-tectonic facies changes along the front hinder lateral fault propagation giving sticking points at lateral tips which become poles of rotation. Deformation affected only the upper crust; there was no major crustal thickening during Variscan tectonics though the crust has been locally thinned probably by a factor of 2 before onset of compression in Devonian times. Deformation was diachronous with a slow displacement rate. Sedimentation occurred in an advancing series of fore-deeps ahead of the deformed and thickened zone. Estimates of shortening across the thin-skinned zone are about 50%, that is 150 km across SW England. The crust beneath the thin skin must extend back beneath the English Channel and northern France.