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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. michael.simms@nmni.com 2. Faculty of Philosophy, University of Würzburg, Germany. kernstson@ernstson.de

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.

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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

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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

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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. The window

comprises about 10,000 data, corresponding to approximately one gravity station per square kilometre.

For the Bouguer gravity field (Fig. 3) a regional trend field was computed applying strong low-pass

filtering of the data by a multifold moving average. We recognise that constructing a gravity regional

field is an ambiguous process that may preferentially accentuate long-wavelength or short-wavelength

anomalies but, after several attempts with various filters, the regional field shown in Fig. 4 is

considered a reasonable compromise. A Bouguer residual anomaly (Fig. 5) was then derived by

subtracting the regional field (Fig. 4) from the measured Bouguer field (Fig. 3). This generated a

complex picture of short-wavelength anomalies within which a ring of relatively positive anomalies

surrounds the conspicuous negative anomaly centred on Lairg.

2. Gravity profiles and model calculations

Three profiles of Bouguer data were constructed from the map of the Lairg residual anomalies, shown

in fig. 5, with each crossing the centre of the Lairg negative anomaly (Fig. 6). Their orientation was

chosen to evaluate various features of the Lairg anomaly, with the three gravity profiles compared in

Fig. 7. All three profiles have a similar shape with a somewhat structured slope towards the central

gravity minimum. The peripheral ring of positive anomalies is striking and is broken only by a gap in

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the north. The NW - SE and SW - NE gravity profiles were selected for a very simple 2.5D model

calculation (Fig. 8). In the absence of more specific density data, a straightforward modelling produces

a two-layer density distribution that assumes a mass deficit responsible for the Lairg gravity anomaly.

Because of this simple assumption the shape of the negative mass follows more or less the shape of the

gravity curve and reveals a step-like slope of the central depression with a depth extending to about 3

km below the thrust plane. This is surrounded by a rim wall with a diameter of approximately 50 km,

and a peripheral shallow mass deficit.

From potential theory it is known that the integration over a gravity anomaly provides a measure of the

total mass responsible for the anomaly, regardless of the density distribution within the mass. Once a

model adaptation to a measured gravity anomaly has been performed, its mass is the same for all other

density distributions providing they fit the measured gravity. This is the basis for a very coarse

evaluation of the central mass deficit producing the Lairg central negative anomaly. Because of the

simple mass calculation for a spherical segment the geological mass of the Lairg central structure has

been replaced by such a segment with a density of 0.15 g/cm3 (Fig. 9) and a total mass defecit of 7 x

1014 kg.

3. The Lairg Gravity Low as an impact structure

Gravity surveys across terrestrial impact craters have contributed to understanding their specific

structure and impact cratering processes in general (Pilkington and Grieve 1992, Sharpton et al. 1993,

Pohl 2015, and others).

The Lairg Gravity Low lacks any evidence of a central uplift but our reanalysis of the gravity data

does reveal a ring of relatively positive anomalies surrounding the central negative anomaly. We

interpret this circle of positive anomalies as an Inner Peak Ring, developed through elastic rebound

and slumping of the transient crater walls at the end of the excavation stage (Melosh 1989, Kenkmann

et al. 2013). Rock from greater depths may be uplifted in these ring structures which, because of the

general increase in density with depth, can account for the positive gravity anomalies. It is not

uncommon for the inner ring of a peak ring crater to be incomplete (Morgan 2016) and hence the

presence of gaps in the ring-like structure that we have identified does not detract from our

interpretation of it as an impact crater. On the contrary, the existence of a ring of negative gravity

anomalies beyond the ring of positive anomalies (Fig. 7) supports our model of a peak ring crater. To

interpret the ring of positive anomalies as representing the rim of a smaller 50 km-diameter impact

structure is inconsistent with density reduction that might be expected in the rim region through

deposition of low-density ejecta and slumping in the modification stage.

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We have compared the gravity profiles across the Lairg anomaly with gravity profiles for several

proven impact structures of diameters ranging between 25 km (Ries crater) and ~180 km (Chicxulub

impact structure) (Fig. 10). Craters comparable in size to that postulated for Lairg are Manicouagan

(~100 km), Popigai (100 km) and Chesapeake (85 km), but there is not a standard gravity profile for

these large complex impact structures. This is underlined by the statement of Wünnemann et al. (2005)

that pre-impact target properties may exert a considerable influence on the structure of complex craters

on Earth. This is reflected in considerable variations in gravity signature even among structures of

similar size, such as the Manicouagan and Popigai impact structures (Fig. 11). From this it is evident

that the Lairg gravity anomaly has most in common with the 100-km diameter Popigai impact

structure but also bears similarities to smaller complex craters, particularly the Rochechouart impact

structure in south-west France. The Rochechouart crater has experienced significant erosion such that

clear morphological features of a crater structure are no longer evident at the surface. Nonetheless

there is clear lithological evidence at the surface for its impact origin, in the form of impact melt rocks,

suevites, breccias and breccia dikes. Originally thought to have a diameter of between 20 and 35 km

(e.g., Pohl 2015), others have considered a diameter of 40 or 50 km to be more realistic (Sapers et al.

2014). This larger estimate corresponds more closely to the pronounced gravity signature shown in

Fig. 10, although not actually referred to by Sapers et al. (2014). In less eroded impact structures in

sedimentary or mixed lithology targets, peak ring uplift may be recognized through lithological

contrasts, but this has proven difficult for the deeply eroded Rochechouart structure where the target is

entirely crystalline. Hence, although the Rochechouart gravity signature suggest a peak-ring uplift

there is, as yet, no direct lithological evidence to confirm this (Fig. 10). At the Ries crater, in southern

Germany, low-density, post-impact, lake sediments up to 400 metres thick occur within the central part

of the crater and contribute significantly to the magnitude of the negative gravity anomaly. The

presence of an inner peak ring at Ries is proven by inliers of uplifted crystalline basement rock

projecting through the post-impact lake sediments, and also indicated by various geophysical

measurements (e.g. Pohl et al. 1977). After subtracting the lake sediment contribution from the overall

Ries gravity anomaly the resultant gravity curve closely resembles the Rochechouart and Lairg

anomalies. The results from our reanalysis of gravity data across the Lairg region compare favourably

with gravity signatures across proven impact structures. Although it does not prove conclusively that

an impact crater exists at this location, it does lend support to our hypothesis that the Lairg Gravity

Low may represent part of a once much larger impact structure.

4. Extrapolating crater size from peak-ring diameter

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A consistent relationship has been observed for the relative dimensions of peak ring craters, with total

crater diameter approximately twice that of peak ring diameter (Fig 11). Although there are relatively

few well-documented examples on Earth, many others are known from rocky bodies elsewhere in the

Solar System and this relationship appears to be independent of both crater size and location on other

Solar System bodies (Melosh 2015). This ‘Factor 2’ rule for complex impact craters with inner peak

rings has significant implications for interpreting the gravity signature of the Lairg structure. If our

estimate of 50 km diameter for the Lairg Gravity Low represents the full crater diameter, then we

might anticipate a peak ring 25 km across. By comparison with other impact crater gravity signatures,

previously discussed, then we would expect to recognize this in the Lairg gravity data as at least a

subtle inflexion or change of gravity gradientsuggesting a diameter of roughly 100 km for the

complete Lairg impact crater.

Applying this estimated figure to the approximate mass deficit calculated from the gravity anomaly

indicates that the Lairg structure is comparable with the mass deficit associated with many other

terrestrial impact structures (Fig. 12) and, as such, this would make Lairg the largest impact structure

yet discovered in Europe.

Constraints on the size of the Lairg impact structure

The Stac Fada Member impact deposit was described as a 'proximal ejecta blanket' on account of its

near continuous outcrop (notwithstanding recent erosion) (Amor et al. 2008). The absence of large

scale disruption of strata or significant soft-sediment deformation in the pre-impact Stoer Group

succession indicates that the present outcrop, which at its closest point is just 55 km from the centre of

the Lairg Gravity Low, lay beyond the area materially affected by seismic shocks associated with the

impact and ensuing crater formation. Claims have been made for impact-induced seismites associated

with, but hundreds of kilometres from, several large impact craters, among them Chicxulub (Renne et

al. 2018), Manicouagan (Clutson et al. 2018) and Sudbury Addison et al. 2005). For many other craters

no such claims have been made, although this may reflect an absence of appropriate strata in which

they might be preserved. However, the absence of any impact-induced seismite in the Stoer Group,

among sediments that demonstrably are prone to soft sediment deformation (Stewart 2002) remains

enigmatic and suggests that the impact was a considerable distance from the present outcrop of the

impact deposit.

But what other constraints might be placed on the original extent of the Lairg impact structure? In

particular, how might a potentially 100 km diameter impact crater be accommodated in the gap

between Lairg, at the approximate centre of the structure, and the present outcrop of the Stac Fada

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Member on the coast to the west? A crater of 50 km diameter centred on Lairg would not impinge on

the main Lewisian outcrop west of the Moine Thrust, which is consistent with the absence of any

impact related structures there. However, this apparent absence of evidence need not necessarily

discount the possibility that the crater was originally twice this size, at ~100 km across, extending

significantly beyond the present limit of the Moine Thrust.

The scale of post-impact erosion experienced by the crater can be inferred from the angular

unconformity between the Stoer Group, dipping at 23o to the west, and the near horizontal Torridon

Group that truncates it. This may be significant for delimiting the original extent of the crater.

Hundreds of metres, and perhaps even 2 km or more, of Stoer Group strata were removed from areas

to the east of its present outcrop where Torridon Group strata now rest directly on the Lewisian

basement. No direct evidence of impact, in the form of shatter cones, pseudotachylite, or large-scale

brecciation, has been identified in any of the Lewisian outcrops between the Moine Thrust and the

Stoer Group outcrop, while published geological maps do not reveal any pattern of large-scale

fractures that might reflect the existence of an impact crater. However, if we consider the processes of

complex crater formation it is possible that the outer edge of the crater, once extending into this

foreland region, might have been removed by erosion.

During the initial moments of the impact process a deep transient crater is formed, of the order of 8 km

for a final crater width of ~50 km. Gravity-driven collapse processes rapidly transform this transient

cavity into a wider, and shallower, complex crater in which shallow structures, such as listric faults

and subhorizontal basement detachments, develop towards the margins (Kenkmann and Dalwigk

2000). Inevitably these shallow marginal structures are more vulnerable to erosion than the deeper

more central parts of the crater. The potential scale of erosion already described may then have

removed the shallow outer edges of the crater across the current Lewisian outcrop before deposition of

the Torridon Group commenced. The westward dip of the Stoer Group beneath the basal Torridon

Group unconformity (Stewart 2002) has further implications for the potential destruction of any

westward extension of the crater. It implies that the Lewisian basement, on which the Stoer Group

rests, was similarly tilted for an unknown distance east of the present Stoer Group outcrop. By

inference, the sub-horizontal basal Torridonian unconformity, passing as it does onto successively

older Stoer Group strata and then onto the underlying Lewisian, reflects an eastward increase in the

scale of erosion. Assuming that the westward tilt evident in the Stoer Group did not extend across the

entire outcrop/subcrop of the Lewisian basement then some sort of 'hinge' structure, or perhaps a series

of rotational blocks tilted to the west, must exist to the east of the present Stoer Group outcrop. Were

this not the case then simple geometry would necessitate more than 30km depth of pre-Torridonian

erosion towards the eastern side of the subcrop. Cambrian strata rest uncomformably upon the

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Torridon Group and today dip eastwards (Peach et al. 1907). This implies a still steeper westward

tilting of the Stoer Group during the early Palaeozoic and, as such, the depth of erosion of a

hypothetical ‘unhinged’ basement would be greater still.

Resolving the paradox

It is impossible to accommodate a 50 km radius impact crater between the Stoer Group outcrop and its

centre at Lairg solely by invoking post-impact erosion of the outer parts of the crater. Our upper

estimate of crater size, approximately 100 km, reduces the distance between the original crater edge

and the current Stoer Group outcrop to a minimum of just a few kilometres. This is incompatible with

what is, or rather what is not, observed within the Stac Fada Member and contiguous strata. The

absence of any impact-generated seismite within the sandstones of the immediately pre-impact Stoer

Group, sediments that demonstrably are prone to soft-sediment deformation (Stewart 2002), has

already been alluded to. Furthermore, the Earth Impact Effects programme (Collins et al. 2005)

predicts that an ejecta deposit in such close proximity would be several hundred metres thick and

would incorporate very much coarser debris than is found in the Stac Fada Member ejecta deposit. The

inevitable conclusion is that if the putative Lairg impact crater is as large as our interpretation of the

gravity data suggests, then the spatial relationship between the crater and the Stac Fada Member

outcrop must have changed since they formed.

The original interpretion of the Lairg Gravity Low as an impact crater (Simms 2015) followed the

consensus view that the Moine Thrust did not descend significantly into the Archean basement. As

such the relationship between the crater and the present Stoer Group outcrop would not have changed

significantly. With an estimated crater diameter of just 40 km this did not present a major issue, since

the closest approach of the Stoer Group outcrop was more than 30 km beyond the supposed rim of the

crater. However, to reconcile a 100 km diameter crater centred on Lairg with an impact ejecta deposit

only ~10 metres thick and just 60-70 km to the west of Lairg, it would be necessary to invoke crustal

shortening of the Archean basement sufficient to translocate the crater westwards for at least several

tens of kilometres from its original position. Is this possible and, if so, is there any evidence that might

support such a model?

The Moine Thrust has been the focus of geological research for more than a century (Law et al. 2010).

Its outcrop has been mapped in extraordinary detail and is the subject of numerous papers interpreting

its structure. The current prevailing view is that the Moine Thrust is part of a thin-skinned thrust belt in

which movement is located largely above the Archean basement (in which the putative crater is

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located) and extends east at relatively shallow depth for tens of kilometres from its present surface

trace (Coward 1980, 1983, Elliott and Johnson 1980, Butler and Coward 1984, Butler 2010).

Interpretation of the deep crustal structure further east in northern Scotland remains far from resolved

(Butler 2007) and is based to a significant extent on extrapolation from observations at outcrop to the

west and from direct observations of basement inliers and shallow structures within the Moinian

further east. However, Brewer and Smythe (1984) specifically state that "surface mapping alone...is

incapable of revealing all the fundamental structures of an orogenic belt".

In contrast to the thin-skinned models, others have used essentially the same field observations, often

coupled with geophysical data, to argue for a much steeper thrust extending down to the Moho

(Watson and Dunning 1979, Stewart 1982, Soper and Barber 1982), with some of these models not far

removed from current thinking on the nature of thick-skinned tectonics (e.g. Soper and Barber 1982).

Significantly in this respect, geophysical evidence does suggest that a component of thick-skinned

Caledonian thrusting may exist beneath northern Scotland.

Several large-scale geophysical surveys of parts of northern Britain, including LISPB (Lithospheric

Seismic Profile in Britain; Barton 1992), MOIST (Moine and Outer Isles Seismic Traverse; Blundell et

al. 1985) and BIRPS (British Institutions Reflection Profiling Syndicate; Brewer et al. 1983, Brewer

and Smythe, 1984), have identified prominent reflectors broadly parallel to the Moine Thrust and

descending eastwards at angles between about 20o and 45o. Some flatten out at 17-20 km depth while

others penetrate a conspicuous reflector that has been interpreted as the Moho. At least one of these

inclined reflectors can be attributed to a known thrust fault, the Outer Isles Thrust, that is developed

entirely in Lewisian basement. The others are assumed to represent similar structures. McBride and

England (1994) interpreted them as evidence for thick-skinned thrusting associated with the Moine

Thrust, which suggests that additional thrusts might exist within the Lewisian basement beneath the

Moine Thrust itself. This view is broadly in accordance with that previously suggested by Butler and

Coward (1984) who envisaged that thrust imbrication was developed within the Lewisian along ductile

shear zones.

The scale of crustal shortening attributable to the Moine Thrust has been a key aspect of many

publications, with estimates of between 25 km and 100 km suggested (Elliott and Johnson 1980, Butler

and Coward 1984, Barr et al. 1986, Butler et al. 2006). Many of these figures are based on thin-

skinned models, in which movement has occurred predominantly in the cover rocks (Moine

metasediments and Proterozoic to Lower Palaeozoic sediments) rather than in the Lewisian basement

and, as such, they have little relevance to the question we raise here of how the Lairg crater might have

been translocated westwards.

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However, the Crustal Duplex model of Soper and Barber (1982), based on the LISPB data, envisaged a

sigmoidal form in which the Moine Thrust steepened eastward to a maximum of 40o to 45o before

flattening out at a depth of perhaps 10 km where it joined a floor thrust in the lower crust. This is more

akin to current thinking on thick-skinned tectonics than the steep structure envisaged by Watson and

Dunning (1979), with the latter model rendered untenable if the substantial displacements measured on

the Moine Thrust Zone are accepted. Significantly, Soper and Barber (1982) cited a value of 66% for

Caledonian crustal shortening across the northern Highlands. As such a thick-skinned model such as

this might provide a mechanism by which the crater, or at least its eroded remnant, could be moved

westwards towards the Stoer Group outcrop.

Discussion

Our suggestion that the Lairg Gravity Low represents the central part of a complex peak ring crater has

come from a reanalysis of the gravity data. The most parsimonious interpretation might be that the

Lairg Gravity Low represents a 50 km crater with the outer rim represented by the ring of positive

anomalies surrounding the central negative anomaly, and the peak ring perhaps represented by subtle

changes in the gravity gradients within the central low. However, this is consistent neither with the

gravity signatures for known peak ring craters nor with the specific gravity signature of the Lairg

Gravity Low where a ring of positive anomalies is itself surrounded by slight negative anomalies.

Instead the Lairg Gravity Low compares more closely with the peak ring and central low of a

substantially larger complex crater. In this respect it is interesting to note that Schedl (2015) used the

thickness of the Stac Fada Member and the size of accretionary lapilli within it to arrive at an estimate

of 80 to 160 km for crater diameter and a location 225 to 325 km away.

Meteorite impacts on the scale of that suggested by the extent of the Stac Fada Member will cause

extensive and deep fracturing of the lithosphere. These fractures potentially may persist as major

crustal weaknesses for hundreds of millions of years (Norman 1984). Substantial erosion of the

putative 'Lairg Crater', between its formation in the Mesoproterozoic and its burial in the

Neoproterozoic, may have reduced it from 100 km across to the 50 km diameter remnant now evident

from the gravity data but deep fracturing associated with the impact may have extended significantly

beyond this eroded remnant. These fractures may have acted subsequently as foci for deep-seated

thrust movements during the Caledonian Orogeny. As such a 100 km diameter Mesoproterozoic

impact crater may have been the architect of its own translocation from a site substantially further east

to its current resting place beneath Lairg. This process may have further truncated the outer parts of the

crater, perhaps accounting for the markedly straight western flank of the Lairg Gravity Low that runs

virtually parallel to the strike of the Moine Thrust.

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The deep structure of the crust to the east of the Moine Thrust Belt remains unclear. Present consensus,

based on structures seen at outcrop in the west, favours a thin-skinned model. However, steep thrust

planes descend deep into the crust to the north and west of the mainland outcrop of the Moine Thrust

and demonstrate that thick-skinned tectonics have played an equally significant role in crustal

shortening across this region during the Caledonian Orogeny. As such we suggest that movement on a

similar deep structure, or structures, further east could accommodate an impact structure on this scale

and might account for the seeming mismatch between the inferred scale of the impact, as deduced

from the Lairg Gravity Low, and its proximity to the Stac Fada Member impact deposit.

Other types of geophysical survey across the region have proven inconclusive in understanding the

nature of the Lairg Gravity Low. There is a minor magnetic anomaly in the same area, but it covers a

substantially smaller area than the gravity low and has been attributed to the Rogart-Grudie granite

pluton (Rollin 2009). At sites of proven impact structures magnetic surveys generally have proven less

useful than gravity surveys. For some, such as Chicxulub, there is a distinct magnetic signature

(Rebolledo-Vieyra and Urrutia-Fucugauchi 2004) but for many others the signal is at best equivocal

and varies greatly according to target rock composition, impact-related magnetization, and the effects

of subsequent crater fill (Cowan and Cooper 2005).

Ultimately answers to these various questions, concerning the scale and even the very existence of the

proposed ‘Lairg impact structure’, and the postulated existence of deep-seated thrust faults passing

beneath it, must await a programme of deep drilling. Even if ultimately the ‘Lairg impact hypothesis’

proves to be unfounded, such an investigation could nonetheless throw considerable light on the

structure of northern Scotland east of the Moine Thrust Belt

Acknowledgements

We thank Renegade Pictures and Lairg & District Community Initiatives for their support of fieldwork

associated with this project.

13

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Figure captions

Fig 1. Regional geology of northern Scotland showing the outcrop of the Stoer Group and its relationship to the residual gravity field (contoured at 2mGal intervals) for the Lairg Gravity Low (from Rollin, 2009). Arrows indicate directional azimuths within the Stac Fada Member.

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Fig. 2. Gravity stations of the British Geological Survey in northern Scotland. The rectangle centred with respect to the Lairg Gravity Low (cross) frames the station used for the re-evaluation of the Lairg gravity field.

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Fig. 3. Bouguer anomaly map for the framed gravity stations in Fig. #1 displaying the roughly circular Lairg negative anomaly. Please note that because of the sea the most northwesterly and southeasterly parts of the Bouguer map lack any gravity stations.

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Fig. 4. A regional trend field computed from the Bouguer anomalies in Fig. #2 by radical moving average low-pass filtering.

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Fig. 5. Bouguer residual anomalies for the Lairg gravity field, with contours at 1 mgal intervals. The residual field results from subtracting the regional field (Fig. #3) from the measured Bouguer field (Fig. #2). Note that in the residual field the central negative anomaly is enclosed by a roughly circular ring of relatively positive anomalies. Dashed red line is circle of radius 25 km centred on the gravity low.

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Fig. 6. Three dashed lines selected for diametric gravity profiles across the Lairg gravity minimum (see Fig. #6). From the circle a diameter of nearly 50 km for the ring of positive anomalies can be deduced.

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Fig. 7. The Bouguer gravity profiles taken from the Bouguer map in Fig. #5 revealing roughly similar shape with regard to the central negative anomaly and the surrounding ring of relatively positive anomalies that are only faintly developed on the NNW - SSE profile. Also note the ring of continued gravity lows beyond the suggested peak ring (arrowed) strongly supporting the peak ring character of a much larger Lairg impact structure.

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Fig. 8. Results of 2.5D model calculation of a very simple two-layer density model for the NW - SE and the SW - NE gravity profiles. For reasons of simplicity, and because effectively the gravity evaluation lacks any "true" zero level, a constant regional shift of -10 mgal has been applied to the curves in Fig. 7.

Fig. 9. Rough approximation of the mass deficit related to the central anomaly by a spherical segment of -0.15 g/cm3 density defecit. Model without vertical exaggeration.

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Fig. 10. Gravity residual anomalies of impact structures of various sizes compared with the Lairg residual anomaly. Note the different scales. Sources: Chicxulub (modified from Hildebrand et al. 1998), Manicouagan (modified from Sweeny 1978), Popigai (modified from Pilkington 2002), Lairg (this paper), Chesapeake (digitised from Gravity Map, Earth Impact Database), Rochechouart (modified from Schmidt 1984), Ries (digitised from gravity map in Kahle 1969).

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Fig 11. Some peak-ring impact structures with an approximate double ratio of crater/peak-ring diameter.

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Fig. 12. Gravity-derived mass deficits of terrestrial impact structures as a function of diameter. Modified and supplemented from Ernstson and Fiebag (1992; with data from Pohl et al. [1978] and references therein; Ernstson [1984]).

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Fig. 13. Schematic sequence of the main events allowing accommodation of a 100 km diameter impact crater centred upon the Lairg Gravity Low.

1170 Ma ago. A recently formed ~100 km diameter peak ring impact crater, surrounded by a thick and extensive ejecta deposit blanketing the sediments of the Stoer Group, is filled with lake sediments above a primary fill of impact breccia and impact melt.

1000 Ma ago. Prolonged erosion, prior to deposition of the Diabaig Formation (Neoproterozoic, Torridon Group), removes the outer rim of the crater and much of the ejecta deposit and Stoer Group. Just a remnant survives in a downfaulted block far to the west.

400 Ma ago. During the Caledonian Orogeny thin-skinned thrusting emplaces westwards a thick cover of Moinian metasediments across the region. Thick-skinned thrusts extend deep into the Lewisian basement, perhaps nucleated on fractures associated with the original impact, and translocate the impact crater tens of kilometres westwards to its present position relative to the Stoer Group outcrop.

Today. Post-Caledonian erosion has stripped away the Moinian metasediments in the west to expose the Stoer Group. The impact crater remains deeply buried beneath Lairg.