Inversion of the Lower Palaeozoic Welsh Basin during the Early to Mid-Devonian is generally thought to have been achieved by a combination of approximately co-axial shortening and transcurrent movement along major faults to produce a strongly partitioned transpressional strain. However, new field observations from Rhydwilym in southwest Wales reveal superimposed deformations which indicate that thrust tectonics operated within the Welsh Borderland Fault System (WBFS) along this segment of the basin margin. An increasing regional magnetic response towards the south suggests that contrasting depth to magnetic basement across the WBFS may have buttressed basin shortening and provided the focus for thrusting and late-Caledonian or proto-Variscan reactivation. British Geological Survey © Nerc 2009. All rights reserved.