Pre-peer review manuscript A reevaluation of the proposed ‘Lairg Impact Structure’ and its potential implications for the deep structure of northern Scotland Michael J. Simms1 and Kord Ernstson2 1. Department of Natural Sciences, National Museums Northern Ireland, Cultra, Holywood, Co. Down BT18 0EU, Northern Ireland.
[email protected] 2. Faculty of Philosophy, University of Würzburg, Germany.
[email protected] Abstract It has been suggested that the Lairg Gravity Low represents a buried impact crater ~40 km across from which the 1.2 Ga Stac Fada Member ejecta deposit originated. The structure is too large to represent a simple crater, and there is no evidence of a central peak. Reanalysis of the point Bouguer gravity anomaly data reveals a ring of positive anomalies around the central negative anomaly and we interpret it as the eroded central part of a peak ring crater. Peak ring craters show a consistent 2:1 relationship between peak ring diameter and total crater diameter, implying that the putative Lairg crater may be ~100 km across. This would place the rim of the crater within a few km of the Stac Fada Member outcrop, a proximity that is inconsistent with the thickness and clast size of the ejecta deposit. We propose that the impact crater was originally formed further east, at a substantially greater distance from the Stac Fada Member than today. Subsequently it was translocated westwards, to its present location beneath Lairg, during the Caledonian Orogeny. This suggests that a deep-seated thrust fault, analogous to the Flannan and Outer Isles thrusts, exists beneath the Moine Thrust in north-central Scotland.