Abstract

Dr B. Lynas said after the presentation of the paper by Dr Woodcock: Whilst mapping the Shelve Ordovician inlier for the British Geological Survey, I began to suspect major strike-slip faulting due to the observed close juxtaposition of two unlike yet time-equivalent volcanic formations, Subsequently, I found unequivocal evidence of strike-slip faulting parallel to the Pontesford–Linley Fault that dextrally offsets an Ordovician sandstone. After considering the classical wrench tectonic models, I believe that there is strong evidence in the pattern of both folds, faults, and synorogenic dolerite intrusions to support dextral wrench faulting. The secondary set of structures induced by the major NE-trending strike-slip faults is clear, for they closely fit the classical models of Wilcox et al. 1973.

Would Dr Woodcock agree with the following? First, the Shelve results fit well with his own conclusions drawn from different evidence; secondly, the strain ellipsoid model makes a useful interpretive tool in the region; thirdly, the identification of the Shelve structural ‘microcosm’ suggests that such a stress field would have been operative throughout the Welsh Basin. The largest faults (Bala, Menai Strait, Pontesford–Linley, Church Stretton) must have been reactivated many times since initiation and though there is evidence of sinistral movement along some of these, this appears to be related to a post-Caledonian orogeny. That large-scale dextral strike-slip did occur during the Lower Palaeozoic was first implied by the plate tectonic model of Phillips et al. 1976. The Menai Strait line was later proposed as a large dextral strike-slip

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