Abstract

The SWAT seismic reflection profiles recently acquired across the Celtic Sea demonstrate that the crust, which underlies the Munster and Pembroke Variscan foldbelts, contains extensive south-dipping reflections interpreted as Hercynian foreland-directed ramp thrusts largely involving the pre-Devonian basement. These large-scale crustal ramps developed without any intervening flat-lying decollement levels, and are thought to have originated in a deep infra-crustal decoupling zone not imaged on the SWAT profiles because it lies within the present-day layered lower crust at approximately 25 km depth. The pre-existing extensional basinal pattern is assumed to have highly influenced the geometry of these deeply rooted thrusts, and in particular that of the frontal ramp which emerges along a discontinuous N100°-trending thrust zone located above the faulted extensional margin of the Munster, Pembroke and Mendips Devonian basins. Shortening of the Irish and Welsh external zones is hence assigned to dominantly crustal stacking achieved along steep ramps dying out upwards, and giving way to mainly undisrupted folds in the thick overlying Devonian infill. In addition, a shallow thin-skinned fold-thrust system developed above an intra-Namurian master decollement still preserved in the footwall of the frontal ramp where it either stays blind (Ireland) or emerges (Pembroke) above a dominantly wrench-faulted Caledonian basement. Compiling SWAT seismic data and structural geology leads to a crustal evolutionary model for the Irish and South Welsh external Variscides from a Devonian basin template to a final deformation pattern.

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