University of Aberdeen

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 Butler, Rober W. H.; Alsop, G. Ian

Published in: Journal of the Geological Society

DOI: 10.1144/jgs2019-168

Publication date: 2019

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Citation for published version (APA): Butler, R. W. H., & Alsop, G. I. (2019). 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. Journal of the Geological Society , 177, 443-446. https://doi.org/10.1144/jgs2019-168

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1 A reassessment of the proposed ‘Lairg Impact Structure’ and its potential 2 implications for the deep structure of northern Scotland: a discussion 3 4 Robert W.H. Butler and Ian Alsop 5 6 School of Geosciences, University of Aberdeen, Kings College, Aberdeen AB24 7 3UE, United Kingdom. 8 9 [email protected] 10 11 The interpretation of the Stac Fada Member of the Stoer Group of NW Scotland as a 12 proximal ejecta blanket of late Mesoproterozoic age (Amor et al. 2008) has led to the 13 search for the impact crater from which it originated. One prominent suggestion has 14 been that this crater lies buried beneath east-central Sutherland, broadly centred on 15 the village of Lairg (Fig. 1; Simms 2015). Simms & Ernstson (2019) develop this 16 suggestion by interpreting the well-known negative anomaly that dominates the 17 Bouguer gravity map of northern Scotland to represent the roots of this crater. 18 However, they qualify their interpretation, noting that the geology at the Stac Fada 19 outcrops implies a more distant crater location, and go on to propose hitherto 20 unreported “crustal shortening structures” to explain this discrepancy. In doing so 21 they challenge current understanding of structural evolution in the Moine Thrust Belt 22 and for crustal structure in NW Scotland. The purpose of this discussion is to 23 demonstrate not only that the Simms & Ernstson (2019) proposals are incompatible 24 with the known structural geology of northern Scotland, but that the geology of the 25 Moine Thrust Belt renders the Lairg site to be one of the very few places that can be 26 regarded as a highly unlikely source for the Stac Fada impact rocks. Internal Earth 27 processes, rather than extra-terrestrial interventions, should be sought as 28 explanations for the “Lairg gravity low”. A more general aim is to show, through this 29 discussion, that making small changes to tectonic models commonly carry 30 unintended consequences for geological interpretations that can go unrecognised if 31 only illustrated on unscaled sketches. 32 33 The Moine Thrust Belt as a crustal dredging machine

1 34 As recognised by Simms & Ernstson (2019) amongst many others, any 35 Precambrian structure, whether or not an impact crater, that lies beneath east- 36 central Sutherland, has been buried by the Moine thrust sheet during Scandian 37 tectonism (c. 430-410 Ma; see Thigpen et al. 2013, and references therein). These 38 interpretations require the Moine Thrust, which carries its eponymous thrust sheet, to 39 follow a relatively shallow-dipping trajectory beneath northern Scotland. In this 40 fashion, the Precambrian basement (chiefly the Lewisian complex) that outcrops to 41 the west of the surface trace of the thrust could continue to depth. It is within this 42 ancient metamorphic basement that any crater would need to be preserved. That the 43 Moine Thrust follows such a trajectory (Fig. 2a) relies on (1) the restoration of thrust- 44 stacked rocks within the Moine Thrust Belt, (2) the deduction that the Moine Thrust 45 Sheet was emplaced onto these rocks before they were significantly stacked and 46 that (3) the Moho beneath northern Scotland has not been reset. If these 47 requirements are accepted, then Lewisian rocks preserved in thrust sheets in the 48 Moine Thrust Belt of the Assynt district have been derived directly from the proposed 49 impact site. As Amor et al. (2019) note, they contain no such evidence. 50 Simms & Ernstson (2019) suggest that structural interpretation in the Moine 51 Thrust Belt is unimportant for their deliberations. Actually, it is critical. The Moine 52 Thrust Belt contains Lewisian rocks, unconformably overlain both by Cambrian and 53 Torridon Supergroup strata, within thrust sheets that were translated to the WNW. 54 The Lewisian thrust sheets never exceed thicknesses of 1 km, and are generally 55 significantly less. In Assynt, the main slices of allochthonous Lewisian basement 56 constitute the Glencoul and Ben More Thrust Sheets (Fig. 1). All workers since their 57 discovery and mapping by Peach et al. (1907; Coward et al. 1980; Elliott & Johnson 58 1980; Butler & Coward 1984, Butler et al. 2006) restore these thrust sheets of 59 Lewisian basement to several tens km to the ESE of their current location. This is 60 based on offsets of Precambrian structures from their position in the Caledonian 61 foreland. Ramsay (1969) proposed that the Kinlochewe Thrust Sheet, in the 62 southern part of the thrust belt, experienced at least 40 km displacement towards 63 WNW. So, the structural style, of the accretion and translation of thin sheets of 64 basement with its authochthonous cover beneath the over-riding Moine Thrust, is 65 common along the thrust belt. 66 Displacements of the magnitudes implied by the offset sheets of Lewisian 67 rocks are supported by restorations of the highly imbricated stacks of Cambrian

2 68 strata which typify much of the northern part of the Moine Thrust Belt. The Foinaven 69 transect (located on Fig. 1) restores to a pre-thrusting width in excess of 50 km 70 (Butler & Coward 1984). A similar value has been estimated for imbricate systems in 71 southern Assynt (located on Fig. 1; Coward 1985). That these imbricate systems 72 only contain Cambro-Ordovician sediments and collectively record substantial lateral 73 displacements, requires that the crust upon which they lay before thrusting, must 74 remain at depth and ESE from the outcrop trace of the thrust belt. It is a 75 demonstration of thin-skinned tectonics not, as Simms & Ernstson (2019) state, an 76 assumption of this tectonic style. Furthermore, it is a requirement of their proposal 77 that the “Lairg gravity low” resides in a structure of Precambrian (and therefore pre- 78 thrusting) age in the crust of northern Scotland. Adopting the foreland crustal 79 thickness of 28km and assuming that the Moho beneath northern Scotland 80 originated in the Precambrian and has not been “reset” since, requires northern 81 Scotland to be underlain by the crustal roots of the Moine Thrust Belt. 82 Simms & Ernstson (2019) argue that a crater of the size and location they 83 infer from the “Lairg gravity low” is too close to the outcrops of its ejecta blanket on 84 the modern west coast (Fig. 1) given their facies and deformation state. But there are 85 more fundamental problems. The western edge of the Lairg gravity low, as defined 86 by Simms & Ernstson (2019), abuts the eastern edge of the Assynt culmination (note 87 that their fig 5 miss-places the Moine Thrust through the Assynt district, confusing its 88 trace with that of the Sole thrust, up to 10 km further west). A displacement of less 89 than 10 km on the Ben More thrust sheet places its origins on the flanks of their 90 putative crater. A more plausible displacement of 30-40 km restores the Ben More 91 sheet to ground zero. Yet within these Lewisian thrust sheets of Assynt, of impact 92 structures there is not a trace. 93 Consider the internal structure of the Ben More sheet. It largely comprises 94 gneisses that preserve granulite facies metamorphic assemblages, with complex 95 metaigneous structures cross-cut by mafic dykes that preserve barely deformed 96 primary intrusive relationships and orientations equivalent to the early 97 Mesoproterozoic “Scourie” dyke swarm. Locally these structures are deformed by 98 narrow shear zones and faults that also deform overlying Cambrian strata. So the 99 Ben More Thrust Sheet preserves, in exquisite detail, the geological relationships 100 typical of the “Central Block” of the Lewisian of the foreland (Coward et al. 1980). 101 The striking preservation of geological relationships in allochthonous thrust sheets

3 102 that can be found in the foreland is undisputed. Beyond being translated in a thrust 103 sheet, the Lewisian rocks have not been deformed in any significant fashion since 104 the early Mesoproterozoic, let alone show any evidence of impact structures or 105 solidified impact melt, prior to Scandian thrusting. The crustal dredging machine of 106 the Moine Thrust Belt found only intact basement without impactites or their 107 structures. 108 109 Deep-rooting thrusts beneath northern Scotland? 110 To overcome the current proximity of the proposed impact site at Lairg and 111 the Stac Fada outcrops on the west coast (Fig. 1), to which we must also add the 112 present-day outcrops of the Ben More Thrust Sheet, Simms & Ernstson (2019) 113 propose a hitherto unrecognized crustal-scale thrust. On this structure they envisage 114 displacements sufficient to telescope the distance between crater and ejecta blanket 115 and, if it is to address the preservation of Lewisian geology, contain the restored 116 position of the Ben More Thrust Sheet. They suggest several tens km lateral 117 displacement on a late thrust, rooting beneath, and carrying, the “Lairg gravity low”. 118 Obviously, no such structure exists at outcrop so it would need to have been 119 truncated by the Moine Thrust. And to avoid the Ben More Thrust Sheet sampling 120 impact rocks, the Moine Thrust would have to post-date the Ben More and other 121 structures in the Moine Thrust Belt. Although local parts of the Moine Thrust show 122 evidence for these late movements, they do not offer sufficient continuity to have 123 entirely decapitated a crustal-scale thrust sheet (e.g. Butler 2004). Indeed, the trace 124 of the Moine Thrust around Assynt, where it is folded into a broadly antiformal 125 structure, strongly suggests that it pre-dates thrust sheet stacking (including the Ben 126 More sheet) in this area (e.g. Elliott & Johnson 1980, and others since). That 127 emplacement of the Moine Thrust Sheet predates thrust-sheet stacking and 128 imbrication in the rocks of the underlying Moine Thrust Belt is supported by the 129 evolution of fault rocks (from mylonitic to increasingly cataclastic) from east to west 130 across the structures. 131 But is a crustal scale thrust, active after the emplacement of the Ben More 132 Sheet but before final motions on the Moine thrust, even plausible? Simms & 133 Ernstson (2019) cite the crustal duplex model of Soper & Barber (1982; and its 134 precursor propositions by Watson & Dunning 1979) to support their late thrust model. 135 However, for Soper & Barber, the Moine Thrust represents the western limit of crust

4 136 that experienced substantial ductile deformation during the Caledonian orogeny. 137 Certainly, any Proterozoic structures in this crust would have been thoroughly 138 reworked and therefore would not be expected to have retained the original crater 139 shapes. However, while the crustal duplex, in which thrusts cut down to the Moho, 140 may be appropriate elsewhere in the orogen (e.g. Brewer & Smythe 1984), 141 understanding of the structural evolution in the Moine Thrust Belt, published after this 142 model was proposed and fully discussed elsewhere (Butler & Coward 1984; Butler 143 2010), render this model highly unlikely for mainland Scotland. Simms & Ernstson 144 (2019) are correct to note that thin-skinned thrusting is not the only way that the crust 145 deforms in contraction, either globally or elsewhere within the Caledonian system. 146 However, to achieve significant lateral translations (heaves) on steeply-dipping 147 reverse faults requires those structures to have substantial throws. To achieve lateral 148 translations of several tens km, as postulated by Simms & Ernstson (2019), would 149 uplift almost an entire crustal thickness (Fig. 2b). This thrust would need to lie east of 150 the existing outcrops of the Moine Thrust Belt to avoid appearing at outcrop, and for 151 it later to be decapitated by the Moine Thrust itself. Rather than sketch this 152 schematically (c.f. Simms & Ernstson 2019), we can model the geometry to test its 153 plausibility. 154 Consider a rotational thrust climbing a crustal ramp (Fig. 2c). Using the 155 general ramp geometries proposed by Fischer & Coward (1982, and many others 156 since), simple trigonometry relates the width of an “exclusion zone” for upper crust 157 preserved after thrusting and lying level with the position in the footwall, to the height 158 and inclination of the ramp. A ramp of 25 km height and at an angle of 35° requires 159 an “exclusion zone” in excess of 43 km wide. So, any preserved Lewisian upper 160 crust, within which a putative crater may lie, cannot be closer than 43km of the trace 161 of the thrust ramp. In our specific case, crustal thrusts cannot bring a crater within 43 162 km of the trailing edge of the Moine Thrust Belt. Notwithstanding the absence of any 163 outcrop geology to support the notion of large-displacement crustal ramps, had the 164 crustal shortening structures invoked by Simms & Ernstson (2019) existed below the 165 Moine Thrust Sheet, they would have destroyed their proposed crater site. 166 167 “Lairg gravity low” is an unlikely late Meosproterozoic impact crater 168

5 169 Collectively, the geological evidence argues strongly against the crust beneath the 170 village of Lairg containing a Mesoproterozoic impact crater. Indeed, other studies 171 argue that the Stac Fada rocks were derived from a broadly NW direction (e.g. Amor 172 et al. 2019). So what is the geological explanation of the negative Bouguer gravity 173 anomaly in central Sutherland? Butler & Coward (1984) argue that there are slightly 174 thickened Moine Rocks here, preserved in the excavated footwall ramp from which 175 the Lewisian sheets of the Moine Thrust Belt were derived. Leslie et al. (2010) 176 developed this proposal to model the gravity anomaly, adding a buried late 177 Caledonian granite. This explanation satisfies both gravity data and our discussions 178 above. It is interesting to note that the Lairg area would certainly contain the roots of 179 the Borrolan igneous complex (Fig. 1), now translated into the Moine Thrust Belt of 180 southern Assynt (e.g. Elliott & Johnson 1980; Coward 1985). While the site is a 181 highly unlikely location for the crater from which the Stac Fada ejecta blanket was 182 sourced, we do concur with Simms & Ernstson (2019) that further investigations of 183 crustal structure in northern Scotland, and indeed elsewhere in the British Isles, is 184 sorely needed. 185 186 References 187 188 Amor, K., Hesselbo, S.P., Porcelli, D., Thackrey, S. & Parnell, J. 2008. A 189 Precambrian proximal ejecta blanket from Scotland. Geology, 36, 303–306. 190 191 Amor, K., Hesselbo, S.P., Porcelli, D., Price, A., Saunders, N., Sykes, M., 192 Stevanović, J. & MacNiocaill, C. 2019. The Mesoproterozoic Stac Fada proximal 193 ejecta blanket, NW Scotland: constraints on crater location from field observations, 194 anisotropy of magnetic susceptibility, petrography and geochemistry. Journal of the 195 Geological Society, 176, 830-846. 196 197 Brewer, J.A. & Smythe, D.K. 1984. MOIST and the continuity of crustal reflection 198 geometry along the Caledonian-Appalachian orogeny. Journal of the Geological 199 Society, London, 141, 105-120. 200

6 201 Butler, R.W.H. 2004. The nature of ‘roof thrusts” in the Moine Thrust Belt, NW 202 Scotland: implications for the structural evolution of thrust belts. Journal of the 203 Geological Society, London, 161, 849-859. 204 205 Butler, R.W.H. & Coward, M.P. 1984. Geological constraints, structural evolution 206 and the deep geology of the NW Scottish Caledonides. Tectonics, 3, 347-365. 207 208 Butler, R.W.H., Holdsworth, R.E. & Matthews, S.J. 2006. Styles of basement 209 involvement in the Moine thrust Belt, NW Scotland. In: Mazzoli, S. & Butler, R.W.H. 210 (eds) Styles of continental contraction. Geological Society of America Special Paper, 211 414, 133-150. 212 213 Coward, M.P. 1985. The thrust structures of southern Assynt, Moine thrust zone. 214 Geological Magazine, 122,1-13. 215 216 Coward, M.P., Kim, J.H. & Parke, J. 1980. The Lewisian structures within the Moine 217 thrust zone. Proceedings of the Geological Association, 91, 327-337. 218 219 Elliott, D. & Johnson, M.R.W., 1980. Structural evolution in the northern part of the 220 Moine thrust belt, NW Scotland. Transactions of the Royal Society of Edinburgh, 71, 221 69-96. 222 223 Fischer, M.W. & Coward, M.P. 1982. Strains and folds within thrust sheets: the 224 Heilam sheet NW Scotland. Tectonophysics, 88, 291 - 312. 225 Leslie, A.G., Krabbendam, M., Kimbell, G.S. & Strachan, R.A. 2010. Regional-scale lateral variation and linkage in ductile thrust architecture: the Oykel Transverse Zone, and mullions, in the Moine Nappe, NW Scotland. In: Law, R.D., Butler, R.W.H., Holdsworth, R.E., Krabbendam M. & Strachan, R. (eds) Continental Tectonics and Mountain Building. The Legacy of Peach and Horne. Special Publications of the Geological Society of London, 335, 359-381.

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7 227 228 Peach, B.N., Horne, J., Gunn, W., Clough, C.T., Hinxman, L.W. & Teall, J.J.H., 1907. 229 The geological structure of the NW Highlands of Scotland. Memoirs of the 230 Geological Survey of Great Britain, pp 653. 231 232 Ramsay, J.G., 1969. The measurement of strain and displacement in orogenic belts. 233 In: Kent, P.E., Satterthwaite G.E. & Spencer, A.M. (eds) Time and Place in Orogeny. 234 Special Publications of the Geological Society of London, 3, 43-79. 235 236 Simms, M.J. 2015. The Stac Fada impact ejecta deposit and the Lairg Gravity Low: 237 Evidence for a buried Precambrian impact crater in Scotland? Proceedings of the 238 Geologists’ Association, 126, 742–761 239 240 Simms, M.J. & Ernstson, K. 2019. A reassessment of the proposed ‘Lairg Impact 241 Structure’ and its potential implications for the deep structure of northern Scotland. 242 Journal of the Geological Society, 176, 817-829. 243 244 Soper, N.J. & Barber, A.J. 1982. A model for the deep structure of the Moine Thrust 245 Zone. Journal of the Geological Society, 139, 127-138. 246 247 Thigpen, J.R., Law, R.D., Loehn, C.L., Strachan, R.A., Tracy, R.J., Lloyd, G.E., Roth, 248 B.L. & Brown, S.J. 2013. Thermal structure and tectonic evolution of the Scandian 249 orogenic wedge, Scottish Caledonides: integrating geothermometry, deformation 250 temperatures and conceptual kinematic‐thermal models. Journal of Metamorphic 251 Geology, 31, 813-842. 252 253 Watson, J. & Dunning, F.W. 1979. Basement–cover relations in the British 254 Caledonides. In: Harris, A.L., Holland, C.H. & Leake, B.E. (eds) Caledonides of the 255 British Isles – Reviewed. Geological Society, London, Special Publications, 8, 67–92 256 257 258

8 259 260 Figures 261 262 Fig. 1 Simplified geological map of northern Scotland, with locations discussed in the 263 text, together with the location of the contested “Lairg impact crater”. X-Y is the 264 location of the section line for Fig. 2a. 265 266 Fig. 2 a) Scaled crustal cross-section through northern Scotland (X-Y on Fig. 1). b) 267 Geometry of a crustal scale thrust, as envisaged and schematically illustrated by 268 Simms & Ernstson (2019) but here drawn to scale to illustrate the separation of 269 upper crustal rocks across an elevated crustal section. c) The geometric model from 270 which the width of an “exclusion zone” can be calculated – within which a putative 271 impact crater would be lost by deformation, erosion or decapitation by (hypothetical) 272 late motion on the Moine Thrust. Collectively these diagrams illustrate the 273 implausibility that the “Lairg gravity low” represents a Precambrian impact crater. 274

9 Fig 1 Click here to access/download;figure;Fig 1 NWlocationmap.pdf

Durness Thurso Moine Thrust Sheet with Lewisian Lewisian of Foinaven Glencoul transect Tongue Moine Thrust Belt Thrust Sheet t with Lewisian sheets s ru Cambrian of foreland h Scourie T e Torridonian of foreland in Glencoul sheet o Lewisian of foreland X M came from here Stac restored Lewisian Fada thrust sheets proposed extent A S S Y N T Lewisian of of impact crater Ben More Thrust Sheet Lairg south Assynt Borrolan transect suite GRAVITY LOW N Ullapool Ben More Y sheet came Laide t from here s u r h T Gairloch e n i Kinlochewe o Cromarty Basin M Thrust Sheet Torridon Devonian and younger rocks

syn-late Caledonian Applecross 25 km intrusions Fig 2 Click here to access/download;figure;Fig 2 crustal set-up.pdf

restored width of Cambrian imbricate thrust stacks X Y Stac restored position of Lewisian in thrust Fada sheets WITHOUT impact structures impactite Cromarty basin outcrops LAIRG (Devonian-Mesozoic

Caledonian “foreland” Moine Thrust Sheet Moine extent of crater proposed by Simms & Ernstson (2019) Thrust Belt crust upon which rocks of Moine Lewisian (Mesoproterozoic and older) crust Thrust Belt lay continues to at least here 10 km Moho - assumed to be Precambrian and not “reset” after Silurian... M M M M M a) M M M M M M M late Caledonian intrusions not shown “exclusion zone” D d1 d2 trailing edge upper crust cannot approach a of Moine thrust belt closer than this Thrust Belt impact crater translated west

outcrops by crustal-scale thrust Stac Fada “exclusion zone”

tilted crustal section ramp height (h) ramp a a/2 height ramp angle of c) crustal thrust d1 = h d2 = h . tan (a/2) b) tan (a) half ramp-angle