Variscan compressional structures within the main productive coal-bearing strata of South Wales

The structure of the main productive Coal Measures within the anthracite zone in the western part of the South Wales coalfield is re-examined using data from recently operational opencast coal sites. Variscan thrusts and/or folds and bed-parallel shear structures are present within all of the sites studied. At East Pit, unaffected by any large-scale zone of disturbance, bed-parallel shear structures are developed along many of the gently dipping coal seams, which form a series of simultaneously active detachments, representing a process of easy-slip thrusting. A NE-SW-striking, fold dominated disturbance at Gilfach Iago is interpreted to overlie a reactivated basement fault. A more complex disturbance zone at Ffos Las combines both E-W-trending folds and more variably oriented thrusts which together shorten the coal bearing strata by up to 2 km and this is interpreted to be the major Variscan frontal thrust ramp in the extreme west of the coalfield. The Variscan compressional structures within South Wales are not consistent with a simple thin-skinned thrust system. The strain induced by deep-seated Variscan thrusts was distributed throughout much of the coal bearing sequence by easy-slip deformation rather than being localized along a single regional detachment, and additional thrusts present around the northern margin of the coalbasin appear to have formed as isolated autochthonous structures. The inherent weakness of coal seams across South Wales is attributed to the presence of greatly increased fluid pressures, which would have been generated both by the maturing coal and by the influx of fluids along deep-seated disturbance zones.