Variscan compressional deformation in the western and central coalfield of South Wales is dominated by thrust faulting. Opencast extraction records and field data have shown the existence of distinct levels of thrust development as isolated structures and basal detachments within imbricate systems.
Two areas have been studied where the thrust faults exhibit similar features which have formed by different propagation mechanisms. At Nant Helen opencast site major thrusts are separated by smaller isolated structures, which apparently do not link to a basal décollement below the site. The isolated thrusts are adduced to have formed due to fluid over-pressuring and mechanical anisotropies in the rock sequence. In the Ffyndaff district deformation at the Ffyndaff opencast site is characterised by a series of foreland propagating thrusts linked by a detachment below the Nine Foot Seam, whereas to the west at Rhigos and Dunraven the detachment rises to a higher level and is interpreted as a gentle eastwards dipping lateral ramp.