Pre-Peer Review Manuscript

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. Introduction The Stac Fada Member is a 4-12m thick unit exposed intermittently along the ~50km outcrop of the fluviatile and lacustrine Stoer Group (Mesoproterozoic, ~1.2 Ga) in north-west Scotland. It contains abundant devitrified angular melt clasts in a sandstone matrix and, for decades, was thought to be volcanic in origin (Stewart 2002). However, the discovery of shocked mineral grains (Amor et al. 2008, Osinski et al. 2011, Reddy et al. 2015) prove that it is the product of a km-scale meteorite impact. The near continuity of the Stac Fada Member outcrop was interpreted as evidence of its relative proximity to the impact crater (Amor et al. 2008) yet Stoer Group strata beneath the impact deposit are undisturbed, indicating that the present outcrop lies significantly beyond the crater rim. 1 Amor et al. (2008) suggested that the crater was located to the west, perhaps buried beneath a thick cover of younger rocks in what is now the Minch Basin, but Stewart (2002) had contended previously that the source of the Stac Fada Member, which he considered volcaniclastic in origin, actually lay to the east. Simms (2015) similarly argued for an eastern source leading to the inference that the crater, if it still exists, may lie beneath mainland Scotland. However, Proterozoic and Archean target rocks across much of the region now lie buried beneath a cover of predominantly Moinian (Neoproterozoic, ~1 Ga) metasediments that were thrust westwards across northern Scotland ~430 Ma ago (McClay and Coward 1981). As such it is unlikely that any physical manifestation of the crater will exist at the surface, even assuming that it survived many millions of years of post-impact erosion and the effects of subsequent tectonism associated with the Moine Thrust. However, geophysical methods offer the potential for locating a buried impact crater (Pilkington and Grieve 1992). Geophysical surveys (BGS map; Rollin et al. 2009; Leslie et al. 2010) have revealed a deep gravity anomaly centred on the town of Lairg, little more than ~50 km east of the Stac Fada Member outcrop at its closest point. There is a remarkable correspondence between the location of the Lairg Gravity Low, as it is known, and the location of the impact crater as predicted from inferred source directions of the Stac Fada Member impact ejecta sheet (Simms 2015). Comparing the Lairg Gravity Low with gravity signatures of other impact structures led to the suggestion that it might represent an impact structure in the Archean basement that now lies buried by overthrust Moinian metasediments (Simms 2015). The Moine Thrust which underlies these metasediments is considered to be near horizontal, and at relatively shallow depth (~1 km) across much of northern Scotland. Simms (2015) estimated the putative Lairg impact crater to have a diameter of at least 40 km based on the original geophysical analyses of Rollin et al. (2009), although recognising that erosion and/or tectonic effects might significantly have modified an originally larger structure (Simms 2016). This would place it among the fifteen largest of almost 200 impact craters currently known on Earth (Hergarten and Kenkmann 2015). Erosion of the impact crater A pronounced angular unconformity exists between the Stoer Group (Mesoproterozoic, ~1.2 Ga) and the Torridon Group (Neoproterozoic, ~1 Ga) on the west coast of northern Scotland. Further east the Stoer Group is absent and the Torridon Group rests directly on a deeply eroded surface of Lewisian Gneiss (Archean, ~3 Ga). These observations testify to the scale of erosion that the crater may have experienced in the almost 200 million year interval between the impact and deposition of the Torridon Group. This is comparable with the time elapsed since the Manicouagan impact structure in eastern 2 Canada was formed 214 million years ago. It is estimated that 2 km or more of post-impact denudation has occurred in this region since the late Triassic (Degeai and Peulvast 2006), reducing the crater from an original estimated diameter of 100 km to the 72 km diameter structure currently visible. As such the Manicouagan Crater might be considered broadly analogous with the putative Lairg crater at the onset of Torridon Group deposition. Post-impact erosion and/or the effects of Caledonian tectonics might have removed shallower parts of the Lairg impact crater, similarly reducing its apparent diameter to what we see today in the published geophysical data. Although nothing of the Lairg structure is visible at the surface, reanalysis of the gravity data may help to clarify aspects of the Archean crust beneath the Lairg Gravity Low and ascertain if what we see today is a true reflection of the original structure. Crater scale vs. structure If pre-Torridon erosion removed the outer parts of the Lairg Crater, how might we determine if what remains is a reasonable representation of the original crater or is merely part of a once larger structure? The answer lies in identifying some of the fundamental differences in crater structure that accompany an increase in crater size, from simple bowl-shaped craters to complex multi-ring structures (Morgan et al. 2016), and ascertaining if any of these critical features are evident in the gravity data. Bowl-shaped Simple Craters on Earth, such as Meteor Crater in Arizona, USA, are no more than a few kilometres across and approximate more closely to the shape and dimensions of the original transient crater than do larger impact structures (Melosh 1989). In larger and deeper structures the walls of the transient crater collapse and, coupled with uplift of the central part of the floor, generate Complex Craters that are substantially wider and shallower than the original transient cavity. Failure of the walls along concentric fractures may produce ring-shaped troughs and terraces in the upper part of the crater while accommodation factors associated with inward slumping commonly give rise to radial transpressional ridges (Kenckmann and van Dalwigk 2000). Impact structures become increasingly complex with size and at diameters greater than ~25 km the central peak is replaced by a basin surrounded by a raised ring (Osinski and Pierazzo 2013). The very largest terrestrial structures have multiple rings but on Earth only Sudbury, Chicxulub and Vredefort fall into this category (French 1998). It is these observed changes in crater structure with scale that have implications for reevaluating the putative impact crater beneath Lairg. The Lairg Gravity Low appears to represent a bowl-like structure, yet it is far too large to be a Simple Crater. The extent and thickness of the Stac Fada Member impact deposit at outcrop also implies a structure at least several tens of kilometres across, which is broadly consistent with the 40 km diameter estimated for the putative Lairg crater (Simms 2015). Such a crater might be anticipated to have either 3 a central peak or a peak ring, yet neither can be discerned in the original analyses of the Lairg Gravity Low by either Rollin (2009) or Leslie et al. (2010). Our reanalysis of the gravity data aims to shed light on the some of the finer details of the structure responsible for the Lairg gravity anomaly. This need not detract from the basic hypothesis that the Lairg Gravity Low represents an entire impact crater since the coarseness of previous analyses may have masked critical diagnostic features or, alternatively, that it actually represents the central basin of a substantially larger peak ring impact structure. Re-evaluation of the Lairg gravity field The original gravity dataset analysed by Rollin et al (2009) is freely available from the British Geological Survey. It is this data that has been reanalysed by one of us (KE). 1. Data and new processing From the Bouguer gravity anomaly database of the British Geological Survey (Rollin 2009) a rectangular field of gravity stations was selected (Fig. 2), located with respect to the negative anomaly centred on Lairg (Fig. 3). The size of the rectangle is somewhat arbitrary and is constrained by the presence of sea in the north-western and south-eastern corners of the region, and by the distribution of the surrounding regional gravity anomalies that may influence the Lairg local anomaly.

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