VARISCAN COMPRESSIONAL STRUCTURES WITHIN THE MAIN PRODUCTIVE COAL-BEARING STRATA OF SOUTH WALES

The structure of the main productive Coal Measures within the anthraci te zone in the western part of the South Wales coalfield is re-examine d using data from recently operational opencast coal sites. Variscan t hrusts and/or folds and bed-parallel shear structures are present with in all of the sites studied. At East Pit, unaffected by any large-scal e zone of disturbance, bed-parallel shear structures are developed alo ng many of the gently dipping coal seams, which form a series of simul taneously active detachments, representing a process of easy-slip thru sting. A NE-SW-striking, fold dominated disturbance at Gilfach Iago is interpreted to overlie a reactivated basement fault. A more complex d isturbance zone al Ffos Las combines both E-W-trending folds and more variably oriented thrusts which together shorten the coal bearing stra ta by up to 2 km and this is interpreted to be the major Variscan fron tal thrust ramp in the extreme west of the coalfield. The Variscan com pressional structures within South Wales are not consistent with a sim ple thin-skinned thrust system. The strain induced by deep-seated Vari scan thrusts was distributed throughout much of the coal bearing seque nce by easy-slip deformation rather than being localized along a singl e regional detachment, and additional thrusts present around the north ern margin of the coalbasin appear to have formed as isolated autochth onous structures. The inherent weakness of coal seams across South Wal es 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-sealed disturbance zones.