Discussion on ‘A reassessment of the proposed ‘Lairg impact structure’ and its potential implications for the deep structure of northern Scotland’ Journal of the Geological Society, London, 176, 817-829

The interpretation of the Stac Fada Member of the Stoer Group of NW Scotland as a proximal ejecta blanket of late Mesoproterozoic age (Amor et al. 2008) has led to the search for the impact crater from which it originated. One prominent suggestion has been that this crater lies buried beneath east-central Sutherland, broadly centred on the village of Lairg (Fig. 1; Simms 2015). Simms & Ernstson (2019) develop this suggestion by interpreting the well-known negative anomaly that dominates the Bouguer gravity map of northern Scotland to represent the roots of this crater. However, they qualify their interpretation, noting that the geology at the Stac Fada outcrops implies a more distant crater location, and go on to propose hitherto unreported ‘crustal shortening structures' to explain this discrepancy. In doing so, they challenge current understanding of structural evolution in the Moine Thrust Belt and for crustal structure in NW Scotland. The purpose of this discussion is to demonstrate not only that the Simms & Ernstson (2019) proposals are incompatible with the known structural geology of northern Scotland, but that the geology of the Moine Thrust Belt renders the Lairg site to be one of the very few places that can be regarded as a highly unlikely source for the Stac Fada impact rocks. Internal Earth processes, rather than extra-terrestrial interventions, should be sought as explanations for the ‘Lairg gravity low'. A more general aim is to show, through this discussion, that making small changes to tectonic models commonly carries unintended consequences for geological interpretations that can go unrecognized if only illustrated on unscaled sketches.