Dr G. D. Williams writes: W. M. Dunne has produced an interesting model for the tectonic evolution during the Upper Palaeozoic of SW Wales with important emphasis on the syn-depositional faulting during the Devonian. In his discussion of Variscan deformation, Dunne states that evidence for a northward-propagating deformation front is found in the larger folds in the S of the area and a decrease in finite strain to the N. Clearly, a piggyback thrusting model (Butler 1982) with deformation migrating northwards is the most logical interpretation of the Variscan deformation. However, the evidence quoted by Dunne is erroneous; internal deformation of thrust sheets, in this case seen as major folds in the ORS rocks, may be simply a function of a relatively slowly propagating thrust fault with very few imbricate splays (Williams & Chapman, in press). The lack of major folding in the Upper Carboniferous rocks to the N of the Johnston thrust may be attributed to their thin-bedded nature relative to the ORS and the existence of numerous small thrust in a trailing imbricate fan (Boyer & Elliott 1982) beneath the Johnston thrust. Finite strain in the northern region may be as great as that in the folded ORS, but has been attained through multiple brittle thrusting rather than internal deformation of thrust sheets.
Displacement on both the Ritec fault and Johnston thrust apparently dies out laterally, to the W on the former and to the E on the latter. Has Dunne considered that the Musselwick fault and Ritec
- © Geological Society of London 1984