A mountain front model for the Variscan deformation of the South Wales coalfield

Abstract: Access to the considerable geological database of British Coal, together with seismic evidence of subsurface relationships, has enabled the development of a structural model for the South Wales coalfield. Detailed site investigations are used as key examples. The simple broad synclinal nature of the coalfield masks a more complex structural system. Thrusting of Variscan age is largely confined to the Lower and Middle Coal Measures and throughout the central and northern coalfield is dominantly a forethrust system. However, along the south-crop a major backthrust system is developed which in places penetrates the Upper Coal Measures. The south-crop shows features analogous with a mountain front and is interpreted as a triangle zone, in which pre-existing basement faults have facilitated uplift. The lack of thrusting within the Upper Coal Measures, in contrast to the underlying Lower and Middle Coal Measures, suggests they have acted as a passive roof to the thrust system. Geographical variation in the intensity of the deformation can be related to the proximity of, and buttressing against, the Caledonian massif. Although broadly analogous with current models of foreland basin development and deformation, the South Wales coalfield demonstrates the importance of pre-existing massifs and lineaments in determining the precise evolution of an area