Structural evolution of the Laxford front

Abstract

SIRS,Beach, Coward and Graham (1974) in their recent paper on the structure of the 'Laxford Front' have criticised our interpretation (Park & Cresswell 1972, 1973 of the structural relationships between the Scourie dykes and the host gneisses. We feel that the authors have misrepresented our views and have also presented an oversimplified picture of the geological relationships in question.We argued that the lateral variation in geometry of the dykes (eg. from discordance to concordance, thick to thin, regular to irregular, etc.) was only partly due to the effects of subsequent deformation as had formerly been thought and that there was a significant original variation in geometry for which we tried to find an explanation.The criticism of Beach et al. (op. cit) is in two parts: (a) They claim that our statement "concordance and discordance (of dykes) are to a large extent primary and not the result of deformation" is contradicted by the observation that concordant bodies are more deformed than discordant.We would refute this criticism on semantic grounds: what we are saying is that dykes intruded concordantly subsequently become more deformed, in general, than those inintruded discordantly. There is no contradiction here: one could disbelieve our evidence but hardly quarrel with our logic! (b) Their second criticism is more significant: that we "fail, apparently, to see the significance of deformation in the modification and/or obliteration of discordant structures". In our paper, where we present several field sketches and diagrams in addition to a detailed discussion of the structural relationships, it is clear that we do in fact recognise the role of deformation in modifying the dykes although we were mainly concerned with trying to establish the nature and extentof the original variation before deformation. We would admit that there is room for disagreement as to the regional extent or proportion of original as compared with deformational variation, because of the ambiguous nature of much of the evidence. However, in the area in question the evidence in many places is quite adequate to demonstrate pre-Laxfordian variation in trend as well as thickness. Moreover our observations over a large area of the Lewisian have revealed numerous examples of variation in geometry of dykes without any trace of corresponding deformation in the host gneisses or, often, in the dykes themselves. There can be no question therefore that there is considerable primary variation in geometry regionally.We state in our paper: "these figures (9a-c) demonstrate that the dykes in this area (ie. Scourie-Laxford) are post-ISi (Fi of Beach et al. op. cit) and generally discordant although with local concordance accompanied sometimes by abrupt changes in direction Scott.