Neoproterozoic Torridonian and Cambrian- 55 Ordovician Sedimentary Rocks (Park Et Al

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Deposited in DRO: 27 May 2021 Version of attached le: Accepted Version Peer-review status of attached le: Peer-reviewed Citation for published item: Strachan, R.A. and Alsop, G.I. and Ramezani, J. and Frazer, R.E. and Burns, I.M. and Holdsworth, R.E. (2020) 'Patterns of Silurian deformation and magmatism during sinistral oblique convergence, northern Scottish Caledonides.', Journal of the Geological Society, 177 (5). pp. 893-910. Further information on publisher's website: https://doi.org/10.1144/jgs2020-039

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1 Patterns of Silurian deformation and magmatism during sinistral oblique convergence, 2 northern Scottish Caledonides.

3 R.A. Strachan1, G.I. Alsop2, J. Ramezani3, R.E. Frazer3, I.M. Burns4 & R.E. Holdsworth5

4 1. School of the Environment, Geography and Geosciences, University of 5 Portsmouth, Burnaby Rd, Portsmouth, PO1 3QL, UK.

6 2. Department of Geology and Geophysics, School of Geosciences, Kings College, 7 University of Aberdeen, AB24 3UE, UK.

8 3. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts 9 Institute of Technology, 77, Massachusetts Avenue, Cambridge, MA 02139-4307, 10 USA.

11 4. 6, Mackenzie Crescent, Bettyhill, By Thurso, Sutherland, KW14 7SY, UK.

12 5. Department of Earth Sciences, The University, South Rd, Durham, DH1 3LE, UK.

13 Abstract

14 Regional ductile thrusting and syn-kinematic granitic magmatism within the Caledonides 15 of northern Scotland occurred within a sinistrally-oblique convergent tectonic setting 16 during the Silurian closure of the Iapetus Ocean. The highest thrust nappes are dominated 17 by structures of probable Grampian (Ordovician) age, and Scandian (Silurian) deformation 18 dominates the underlying thrust nappes. Deformation was overall foreland-propagating 19 but the nappe stack was modified by out-of-sequence thrusting and probable 20 synchronous development of thrusts at different structural levels. Localised dextrally- 21 transpressive deformation is related to an inferred lateral ramp located offshore. New U- 22 Pb (CA-IDTIMS) zircon ages from syn-tectonic granites indicate that the internal Naver 23 Thrust was active between c. 432 Ma and c. 426 Ma. This is consistent with other data 24 sets that indicate that contractional deformation and high-grade metamorphism, and by 25 implication displacements in the Moine Thrust Zone, may have lasted until c. 420-415 Ma.

26 The synchroneity of thrusting and strike-slip movements along the Great Glen Fault 27 implies that partitioning of transpressional strain occurred above a regional basal 28 decollement. The short duration of the Scandian orogen in Scotland (c. 437-415 Ma?) is 29 consistent with only moderate crustal thickening and a location on the periphery of the 30 main Laurentia-Baltica collision further north.

31 [End of abstract]

32 The Caledonian-Appalachian orogen in the North Atlantic region resulted from the closure 33 of the early Palaeozoic Iapetus Ocean and the Silurian collision of Laurentia, Baltica and 34 peri-Gondwanan microcontinents including Ganderia and Avalonia (Fig. 1; Soper & 35 Hutton 1984; Soper et al. 1992; van Staal et al. 1998). The style and intensity of Silurian 36 tectono-magmatic activity varies along the length of the orogen. In the northern 37 Appalachians, Gander-Laurentia collision resulted in the Salinic orogenic event that was 38 characterised by major crustal thickening and kyanite grade metamorphism (e.g. Cawood 39 et al, 1994). In contrast, the coeval but ‘soft’ collision across the Iapetus Suture in the 40 British Caledonides was not associated with significant crustal thickening or 41 metamorphism (Soper & Woodcock 1990). Further north, the Scandian collision of East 42 Greenland and NW Scotland (Laurentia) with Norway (Baltica) resulted in substantial 43 crustal thickening, eclogite-facies metamorphism and a complex history of syn- 44 convergent exhumation that lasted into the early Devonian (e.g. Andersen & Jamtveit 45 1990; Andresen et al. 2007; Gilotti & McLelland 2007). The Northern Highland Terrane 46 (NHT) of Scotland (Fig. 1) represents a fragment of the Laurentian retro-wedge of the 47 orogen and the southernmost part of the Scandian collision zone. However, in contrast to 48 the main Laurentia-Baltica collision zone to the north, the NHT appears to only record 49 moderate crustal thickening that occurred over a relatively restricted period in the mid- 50 to late Silurian (Kinny et al. 2003a; Johnson & Strachan, 2006; Goodenough et al. 2011). 51 The easterly-dipping Moine Thrust Zone forms the northwestern limit of the 52 exposed Scandian orogen in Scotland (Fig. 2). To the west, the Hebridean Foreland 53 comprises Archaean-Palaeoproterozoic basement of the Lewisian Gneiss Complex,

54 overlain unconformably by Meso- to Neoproterozoic Torridonian and Cambrian- 55 Ordovician sedimentary rocks (Park et al. 2002). To the east and structurally above the 56 Moine Thrust, the NHT is dominated by the early Neoproterozoic Moine Supergroup 57 which is disposed in a stack of east-dipping Scandian thrust nappes (Fig. 2; Holdsworth et 58 al. 1994; Strachan et al. 2002, 2010). In north Sutherland these are, from structurally 59 lowest to highest, the Moine, Naver, Swordly and Skinsdale thrust nappes (Fig. 2; Barr et 60 al. 1986; Moorhouse & Moorhouse 1988; Strachan & Holdsworth 1988; Kocks et al. 2006). 61 We note that Thigpen et al. (2013) and Ashley et al. (2015) recognise an additional Ben 62 Hope nappe on the basis that the eponymous thrust (Fig. 2) appears to represent an 63 important thermal break. However, because it does not define a significant lithological 64 difference, we incorporate the rocks in its hangingwall within the Moine nappe as defined 65 here. Further south in Ross-shire and Inverness-shire, the main structural break is the 66 Sgurr Beag Thrust (Fig. 2 inset; Tanner et al. 1970; Rathbone & Harris 1979). Syn-thrusting 67 metamorphic grade increases progressively eastwards and up-section from greenschist 68 to amphibolite facies (Soper & Brown 1971; Johnson & Strachan 2006; Thigpen et al. 69 2013; Ashley et al. 2015; Mazza et al. 2018; Mako et al. 2019). Syn- to late-tectonic granitic 70 intrusions were emplaced during ductile thrusting and have yielded Silurian crystallisation 71 ages (U-Pb zircon or monazite; Kinny et al. 2003a; Kocks et al. 2006, 2014; Alsop et al. 72 2010; Holdsworth et al. 2015). However, the inconsistency of some of the employed 73 radioisotopic techniques and insufficient age resolution means that the precise timing 74 and duration of Scandian thrusting and associated Barrovian metamorphism in the NHT 75 remains somewhat uncertain. 76 The structural evolution of the lower to middle levels of the Scandian nappe stack 77 in Sutherland has been well documented (e.g. Holdsworth 1989; Strachan & Holdsworth 78 1988; Alsop & Holdsworth 1993; Alsop et al. 1996; Holdsworth et al. 2001, 2006, 2007; 79 Thigpen et al. 2010a & b, 2013). In this paper, we synthesise the detailed structure of the 80 less well-known middle to upper structural levels to provide a complete section across 81 this part of the NHT. We distinguish between Scandian and older structures and mineral 82 assemblages, assess the kinematic significance of orogen-parallel lineations developed at

83 the highest structural levels, and investigate the emplacement history of associated felsic 84 melts. We also present the results of new high-precision U-Pb zircon geochronology 85 obtained from syn-kinematic granitic intrusions using the chemical abrasion isotope 86 dilution thermal ionization mass spectrometry (CA-ID-TIMS) method. This provides new 87 constraints on the timing of thrusting in the central part of the nappe stack and enables 88 us to draw conclusions concerning the kinematic significance of the regional variation in 89 Scandian transport directions as well as the duration and wider tectonic context of 90 Silurian orogenesis in the NHT.

91 Geological framework and synthesis of the Scandian thrust nappes in Sutherland

92 The Moine rocks of Sutherland comprise mainly psammites with subordinate pelites. 93 Psammites within the Moine and Skinsdale nappes locally preserve sedimentary features 94 such as cross-bedding, slump folds and gritty to conglomeratic layers (Holdsworth 1989; 95 Holdsworth et al. 2001; Kocks et al. 2006; Alsop et al. 2010). In contrast, the intervening 96 Naver and Swordly nappes are dominated by migmatitic gneisses where all sedimentary 97 features have been obliterated by high strain and intense metamorphic recrystallisation 98 (Moorhouse & Moorhouse 1988; Kinny et al. 1999). The lower Naver Nappe is largely 99 psammitic, whereas the upper Swordly Nappe is dominated by pelitic lithologies. 100 Concordant sheets and pods of garnet amphibolite up to 10 m thick are present in all 101 nappes and are interpreted to be metamorphosed mafic intrusions which have 102 undergone most of the tectonic history of their host rocks. The Moine rocks are 103 additionally interfolded and inter-thrust with Archaean orthogneisses which represent 104 their depositional basement marked by locally preserved unconformities (Fig. 2; Peach et 105 al. 1907; Holdsworth 1989; Holdsworth et al. 2001; Friend et al. 2008). 106 The Moine rocks were affected by Neoproterozoic and Ordovician orogenic events 107 prior to Scandian nappe stacking (e.g. Kinny et al. 1999; Friend et al. 2000; Cutts et al. 108 2010; Cawood et al. 2015; Bird et al. 2013, 2018). Neoproterozoic tectonothermal activity 109 is thought to be related to development of the accretionary Valhalla orogen when the 110 Moine rocks were located on the margin of Laurentia and close to the edge of Rodinia

111 (Cawood et al. 2010). Late Neoproterozoic supercontinent breakup was followed by 112 opening of the Iapetus Ocean during the Cambrian (Cocks & Torsvik 2002). Ocean closure 113 then followed the development of intra-oceanic subduction zones and collision of island 114 arcs with Laurentia. This resulted in ‘Grampian I’ orogenesis at c. 480-470 Ma and 115 metamorphism and deformation of the Moine rocks and the younger Dalradian 116 Supergroup of the Grampian Terrane located SE of the Great Glen Fault (Dewey & 117 Shackleton 1984; Dewey & Ryan 1990). U-Pb zircon ages of c. 470-460 Ma date 118 migmatisation within the Naver and Swordly nappes (Kinny et al. 1999). The magmatic 119 arc that collided with Laurentia lies south of the Highland Boundary Fault (Fig. 1), although 120 in Scotland is largely covered by Devonian-Carboniferous successions (Dewey & Ryan 121 1990). A switch in subduction polarity to northwest-directed resulted in the development 122 of the Southern Uplands accretionary prism between Caradoc times and the final closure 123 of Iapetus (e.g. Leggett et al. 1979; Stone & Merriman 2004). A younger ‘Grampian II’ 124 metamorphic event at c. 450-445 Ma resulted in substantial garnet growth (some syn- 125 tectonic) in the Moine Nappe, although the tectonic driver of this episode is uncertain 126 (Bird et al. 2013). 127 A more complex Grampian tectonic model has been proposed recently by Dunk et 128 al. (2019) arising from a new U-Pb zircon protolith age of c. 503 Ma determined for the 129 calc-alkaline Strathy Complex in Sutherland (Fig. 2). Isotopic and geochemical evidence 130 (Burns et al. 2004; Dunk et al. 2019) indicate that this developed as a juvenile magmatic 131 arc in a distal setting from the Laurentian margin. The complex is interpreted as 132 allochthonous and located along a buried suture that formed during the ‘Grampian I’ 133 orogeny. Dunk et al. (2019) propose that a microcontinental ribbon was detached from 134 Laurentia during Iapetan rifting; the intervening oceanic tract closed by subduction during 135 the late Cambrian and formed a juvenile arc, the protolith of the Strathy Complex. The 136 microcontinental ribbon was then re-attached to Laurentia during ‘Grampian I’ 137 orogenesis which transported the Strathy Complex as an allochthonous slice within a 138 nappe stack. In this model, at least the initiation of the Naver and Swordly thrusts (or their 139 precursor structures) would be Ordovician (Grampian I) in age.

140 Structural domains and relative intensities of Scandian deformation

141 The approach taken here in the analysis of the regional structure is to firstly summarise 142 those structural features that are well constrained as having formed during the Scandian 143 orogeny, and then to trace these eastwards into the structurally higher levels which are 144 less well understood. The metasedimentary rocks of the Moine and Naver nappes record 145 a similar Scandian deformational history involving two sets of overprinting and broadly 146 foreland-propagating structures (described in detail below). These structures have also 147 been traced structurally downwards into the belt of foreland-derived mylonites that 148 forms the uppermost part of the Moine Thrust Zone (Holdsworth et al. 2006, 2007).

149 Although they are referred to locally as ‘D2’, and ‘D3’, they developed diachronously and

150 so D3 at a high level in the thrust stack might be temporally equivalent to D2 at a lower

151 structural level (see also Butler 2010; Leslie et al. 2010). In addition, D2 and D3 in a single 152 thrust sheet may have formed during a single progressive ductile thrusting episode (e.g. 153 Alsop & Holdsworth 1993). Prior to ductile thrusting, the Moine rocks contained older 154 composite structures and fabrics of probable Neoproterozoic and Ordovician age (Kinny

155 et al. 1999; Bird et al. 2013, 2018). These are grouped as ‘D1’ with an ‘M’, ‘N’ or ‘S’ suffix 156 depending on their location in the Moine, Naver or Swordly nappes to emphasise the 157 potential lack of correlation.

158 Scandian structures and deformation sequences in low to middle parts of the nappe stack

159 Structures that are widely described as ‘D2’ and ‘D3’ have been well documented from the 160 Moine Nappe and the upper part of the Moine Thrust Zone (Fig. 2; Strachan & Holdsworth 161 1988; Holdsworth 1989, 1990; Holdsworth & Grant 1990; Alsop & Holdsworth 1993, 1999, 162 2002, 2004; Alsop et al. 1996, 2010; Holdsworth et al. 2001, 2006, 2007, 2015). Reclined,

163 tight to isoclinal D2 folds with southeasterly-dipping axial planes are ubiquitous between 164 the Moine and Naver thrusts and developed on all scales. The largest basement inliers

165 occupy the cores of west-vergent D2 folds commonly modified by ductile thrusting.

166 Regional D2 ductile thrusting and folding resulted in development of an east- to southeast-

167 dipping S0-S1M/N-S2 (=Sn) foliation which intensifies into mylonitic rocks associated with

168 the D2 Moine, Ben Hope and Naver ductile thrusts. S2 carries a mineral extension and

169 rodding lineation (L2) which is sub-parallel to the axes of local F2 folds. L2 gradually 170 changes in orientation from a SSE azimuth (~170°) in the vicinity of the Naver Thrust to 171 an ENE trend (~110°) close to, and within, the Moine Thrust Zone (Fig. 3; Phillips 1937;

172 Kinny et al. 2003a; Law & Johnson 2010). Sections viewed normal to S2 and parallel to L2 173 contain minor structures (e.g. rotated porphyroclasts, S-C fabrics) that demonstrate a top- 174 to-the-NNW to W sense of shear (Holdsworth & Grant 1990; Holdsworth et al. 2001). 175 Sheath-fold geometries are locally common on all scales. Within the Moine Nappe, the

176 widespread parallelism of hornblende with L2 in mafic rocks implies that D2 was 177 accompanied by at least low amphibolite facies metamorphism, consistent with local

178 occurrences of syn- to post-D2 staurolite, kyanite and sillimanite (Burns 1994; Holdsworth

179 et al. 2001; Ashley et al. 2015). D1M structures are restricted to a strong S1M foliation which

180 is only confidently recognised where it is folded by F2 folds, and a narrow belt of north-

181 south trending L1M lineations developed either side of the Kyle of Tongue (Fig. 2). No

182 convincing examples of F1M folds have been identified and facing analyses of D2 structures

183 in the Moine rocks within the Moine Nappe show that they were right way-up after D1M 184 (Holdsworth 1988, 1989).

185 The D2 structures described above are deformed by local F3 buckle folds developed 186 on all scales (Alsop & Holdsworth 1993, 2007; Alsop et al. 1996; Holdsworth et al. 2006,

187 2007). F3 fold axes and associated axial surfaces are variably oriented with respect to L2 188 and have been related to the development of flow perturbations during differential

189 displacements along underlying D2 ductile thrusts (Holdsworth 1990; Alsop & Holdsworth

190 1993; Alsop et al. 1996). F3 folds typically crenulate S2 and fold L2 and are not associated 191 with a new elongation lineation. 192 Structural evidence indicates that deformation was broadly foreland-propagating.

193 This is shown by the way in which major F2 folds and D2 ductile thrusts (Naver, Ben Hope,

194 Achininver and Moine) are folded by underlying F3 structures, which root downwards into

195 D2 ductile thrusts at lower structural levels. (Holdsworth et al. 2001, 2006, 2007; Alsop & 196 Holdsworth 2007; Alsop et al. 2010; Leslie et al. 2010). However, out-of-sequence

197 deformation can be demonstrated at two structural levels. Firstly, within the central 198 Moine Nappe, the Ben Blandy Shear Zone (Fig. 2) comprises a belt of platy blastomylonites 199 (Holdsworth et al. 2001). These are similar to those developed along ductile thrusts 200 elsewhere, but: a) it does not follow thrust ‘rules’ as it juxtaposes younger Moine rocks

201 over older basement; b) it coincides with a sharp 10-15° switch in L2 direction; c) a major

202 F3 fold pair roots downwards into the shear zone (Alsop et al. 1996). These are all features 203 consistent with out-of-sequence thrusting. Secondly, within the Moine Thrust Zone, the 204 base of the mylonite belt is defined by the out-of-sequence Lochan Rhiabach Thrust (Fig. 205 2) which truncates Scandian structures in its footwall (Holdsworth et al., 2006) and is 206 associated with a metamorphic break (Thigpen et al. 2010a, 2013).

207 Evidence for a Scandian age for the Swordly Thrust

208 Detailed mapping in central Sutherland has shown that F2 and F3 folds and associated 209 structures dominate the lower parts of the Naver Nappe and extend east of the hitherto 210 poorly-documented Swordly Thrust (Fig. 4). The Swordly Thrust is a sharp contact within 211 a c. 50m thick high-strain zone, separating interbanded psammitic and semi-pelitic gneiss 212 in the footwall from semi-pelitic gneiss in the hanging-wall (Fig. 4). In contrast to the 213 Naver Thrust, there is little to distinguish the Moine rocks either side of the structure as 214 all lithologies are migmatitic. The case that this contact represents a significant tectonic 215 break rests on the presence of two thin sheets of strongly reworked Archaean basement 216 (Fig. 4). The lithological asymmetry either side of these inliers requires that a tectonic 217 break must lie along either their upper or lower boundaries. In central Sutherland, we 218 interpret the lower boundary as a tectonic break (the Swordly Thrust) and the upper 219 contact as a tectonically modified unconformity. In contrast, the Farr basement inlier on 220 the north coast section (Fig. 5) lies well below the Swordly Thrust within uniform 221 sequences of psammitic gneisses and most likely occupies the core of a large-scale 222 isoclinal fold of uncertain structural age.

223 The orientations of Sn and L2 are essentially the same on both sides of the Swordly

224 Thrust (Fig. 4). Sn dips moderately to the southeast and L2 plunges to the south-southeast

225 (Fig. 4; stereonets from sub-areas 1 and 2). F2 fold hinges are rare, but where present,

226 plunge parallel to L2. Importantly, L2 can be traced continuously from the dated 227 Strathnaver Granite (U-Pb zircon, 429 ± 11 Ma; Kinny et al. 2003a) structurally upwards

228 across the Swordly Thrust and into its hanging-wall (Fig. 4). D2 structures are deformed by

229 tight to open F3 folds which are broadly co-planar and co-linear with the D2 folds. The F3 230 folds are developed on all scales, forming a large-scale, composite reclined SSW-vergent

231 structure that folds the Swordly Thrust (Fig. 4). Adjacent to the Swordly Thrust, F3 folds 232 plunge gently towards the southeast and display moderately-dipping east to southeast- 233 dipping axial surfaces (Fig. 4, stereonets from sub-areas 1 and 2). Associated minor

234 structures include a tight crenulation of S2 and an L3 intersection lineation that plunges

235 sub-parallel to L2. The manner in which F3 folds deform the Swordly Thrust replicates the

236 structural pattern observed at lower levels within the nappe pile, whereby F3 folds also 237 deform the Naver, Ben Hope, Achininver and Moine thrusts (Fig. 2; Alsop & Holdsworth, 238 1993; Alsop et al. 1996; Holdsworth et al. 2006, 2007). The structural framework 239 established previously for the Moine Nappe can therefore now be extended to 240 structurally higher levels within the Naver Nappe, and the Swordly Thrust is interpreted

241 as a D2 structure.

242 Age and nature of orogen-parallel lineations above the Swordly Thrust

243 Regionally, the dominant mineral and stretching lineation within the NHT rotates 244 anticlockwise down-structural section from north-south in the Swordly and Sgurr Beag 245 nappes to east-southeast near the Moine Thrust (Fig. 3; Phillips 1937; Kinny et al. 2003a; 246 Law & Johnson 2010). Whether this regional variation results from one or more orogenic 247 events has not been clear. In east Sutherland, the north-south trending lineation and 248 associated folds are best developed in the Moine rocks above the Swordly Thrust and in 249 the Strathy Complex (Figs. 4 & 5). The Moine rocks here were migmatised during the 250 Grampian I orogenic event (Kinny et al. 1999; Bird et al. 2013). The gneissic foliation is

251 designated S1S, although older (Neoproterozoic?) structures and mineral assemblages 252 may be present.

253 The S1S fabric and its associated structures are best preserved above the Swordly 254 Thrust between Loch Strathy and Loch Crocach (Fig. 4, stereonet for sub-area 3). In this 255 area, the regional foliation dips gently to the east-southeast and is associated with tight

256 to isoclinal, commonly intrafolial, F1S minor folds which have an axial-planar mica fabric. 257 These folds commonly deform the migmatitic layering, but are themselves cut on all 258 scales by gently discordant metre-decametre scale sheets of weakly-foliated leucogranite 259 which are inferred to represent large accumulations of late-tectonic partial melt (Fig. 4). 260 The folds are therefore viewed as having formed synchronous with regional 261 migmatization. Associated with the foliation is a north-south-trending mineral extension

262 and rodding lineation (L1S) (Fig. 4, stereonet from sub-area 3). The lineation is defined by 263 aligned amphiboles in mafic lithologies and by elongate quartz-feldspar aggregates in 264 siliceous rocks. The lineation is also well developed within the grey gneisses of the Strathy

265 Complex (Fig. 5, stereonet k). The lineation is commonly parallel with the axes of the F1S 266 folds which may display ‘eye structures’ indicative of sheath fold geometries when viewed

267 on surfaces perpendicular to L1S [e.g. NC 7503 6534]. Although the lineation is inferred to 268 lie parallel to the direction of tectonic transport during regional deformation, there are 269 no consistently developed kinematic indicators present that might establish the sense of 270 shear.

271 Three lines of evidence are consistent with a Grampian (Ordovician) age for L1S

272 and associated F1S folds. Firstly, the observation (above) that L1S is cut by sheets of

273 leucogranite that do not carry the lineation. Secondly, L1S is most strongly developed in 274 mafic and siliceous lithologies. These are typically less migmatised than pelitic lithologies

275 within which L1S is often absent. It is suggested that L1S was largely obliterated in pelitic 276 rocks as a result of grain-size coarsening associated with the migmatisation and which 277 outlasted deformation. Thirdly, Lu-Hf dating of garnets within an amphibolite in the 278 Strathy Complex yielded an age of 447 ± 15 Ma (Bird et al. 2013). Although the error is 279 large, a late Ordovician age seems most likely. Importantly, the garnets (locally up to 7-8

280 cm size) appear to have statically overgrown an older S1S gneissic fabric (Fig. 6a). In 281 summary, field and isotopic evidence suggests that the dominant structures and

282 metamorphic assemblages within the Swordly Nappe formed during Grampian I 283 orogenesis. The regional lineation pattern within Sutherland is therefore likely to be a 284 composite of Grampian and Scandian orogenic events.

285 The Skinsdale Thrust – an out-of-sequence Scandian thrust?

286 The Skinsdale Thrust (Fig. 2) corresponds to a 300 m thick, southeast-dipping high-strain 287 zone that forms a sharp eastern limit to the migmatitic rocks of the Swordly Nappe (Kocks 288 et al. 2006). The overlying Moine rocks of the Skinsdale Nappe are generally 289 unmigmatised psammitic and quartzitic lithologies that locally preserve sedimentary 290 structures (Strachan 1988). Blastomylonites associated with the thrust carry a SE-plunging

291 L2 mineral and stretching lineation and asymmetric feldspar porphyroclasts indicate a top- 292 to-the-NW sense of shear parallel to the lineation (Kocks et al. 2006). At structurally

293 higher levels further east, the dominant L1 lineation trends approximately north-south 294 where unaffected by later cross-folds, but kinematic indicators are rare and do not

295 provide a consistent sense of tectonic transport (Strachan 1988; Kocks et al. 2006). L1 here 296 must have formed during the Caledonian orogeny because it deforms c. 590 Ma augen 297 granites (Kinny et al. 2003b). The Naver and Swordly nappes appear to be progressively 298 excised towards the south and are presumed to be cut out completely underneath 299 Devonian cover by the Skinsdale Thrust (Fig. 2) which would therefore be an out-of- 300 sequence structure. Whether or not the Skinsdale Thrust correlates with the Sgurr Beag 301 Thrust south of the Dornoch Firth (Fig 2) as proposed by Kocks et al. (2006) remains to be 302 demonstrated.

303 Transpressional reworking of Scandian thrusts within the Torrisdale Steep Belt

304 Along the north coast of Sutherland, all ductile structures in the Moine and Naver nappes 305 are reworked in a large zone of transpressional deformation, the Torrisdale Steep Belt 306 (TSB, Fig. 5; Holdsworth et al. 2001). The TSB has a broadly triangular map pattern and 307 increases in width northwards to c. 9 km (Fig. 5). West and south of the TSB, in both Moine

308 and Naver nappes, Sn and L2 have broadly the same orientation as in central Sutherland

309 (Fig. 5, stereonets a & d). F2 and F3 folds are relatively common within the Moine nappe,

310 axial surfaces and axes are coplanar and colinear with, respectively, Sn and L2 (Fig. 5, 311 stereonets b & c). A northeastward traverse into the TSB reveals that the north-south-

312 trending composite regional foliation, the Naver and Swordly thrusts, and F2 and F3 fold 313 axial planes all steepen and become rotated anticlockwise into a NNW-trend (Fig. 5,

314 stereonets e & f). L2 is progressively overprinted by a strong mineral and rodding lineation

315 (local L4) which plunges gently to the south-southeast, colinear with F2 and F3 fold axes 316 (Fig. 5, stereonets e & f). A steep foliation is generally pervasive, although local zones of

317 low strain preserve relic F2-F3 folds and lineations. Metamorphic temperatures during

318 development of L4 were at least c. 500°C because it is defined by aligned hornblende and 319 recrystallized aggregates of garnet (Burns 1994; Holdsworth et al. 2001). It therefore 320 seems unlikely that there was any significant temporal break between the development 321 of the TSB and the main phase of regional ductile thrusting. Ubiquitous shear band and S- 322 C fabrics within the Moine and basement rocks consistently indicate a dextral sense of

323 shear parallel to L4 (Figs. 6b and 6c).

324 Traversing eastwards and out of the TSB, the steep orientation of Sn is preserved,

325 due to the tight, upright Kirtomy synform, but the characteristic L4 is only rarely present 326 (Fig. 5, stereonet g). The dominant folds within the Moine rocks on both limbs of the

327 synform are tight-to-isoclinal D1S structures, with steep to sub-vertical, NW-trending axial 328 surfaces and steeply-plunging axes (Fig. 5, stereonet h). In contrast, further east within

329 the Strathy Complex, poles to the Sn foliation define a broad east-west girdle (Fig. 5,

330 stereonet i). The dominant lineation on Sn surfaces is assigned to L1S and plunges gently

331 to the south. D1S axial surfaces and axes are, respectively, coplanar and colinear with Sn

332 and L1S (Fig. 5, stereonet j). Large-scale open-to-close folds have broadly upright axial 333 surfaces and gently NNW-plunging axes (Fig. 5, stereonet k). Within the context of north 334 Sutherland, the spatial coincidence of the TSB with the upright folds that are prominent 335 east of Kirtomy Point, but die out a few kilometres to the south, suggests that these

336 structures are probably of similar (D4) age.

337 The overall kinematic significance of the TSB and associated late folds to the east 338 is uncertain because the northern limit of this deformation zone lies offshore. However, 339 it is suggested here that it resulted from the development to the north of an east-west 340 trending lateral ramp or transfer zone within the Caledonian nappes (Fig. 7). Two other 341 lines of evidence also point to the presence of offshore structures trending at a high angle 342 to regional strike. Firstly, the prominent aeromagnetic anomaly coincident with the 343 Strathy Complex terminates against an east-west trending lineament assumed to be a 344 normal fault (Moorhouse & Moorhouse, 1983), Secondly, the analysis of on- and offshore 345 structures is consistent with later development of a large-scale transfer zone (North Coast 346 Transfer Zone) that was active during post-Caledonian basin formation in the Devonian 347 and Permian (Wilson et al. 2010). It is suggested here that these brittle faults were 348 localised along, and reactivated, an older ductile lateral ramp or transfer zone. Within the 349 Torrisdale Steep Belt, the large-scale anticlockwise rotation and steepening of the 350 regional foliation and pre-existing structures is consistent with a sinistral sense of shear 351 across such a structure, although the former may have been partly inherited from earlier

352 large-scale bending of S2 around the northern termination of the major basement infold 353 (Borgie inlier) lying immediately to the west of the TSB (Fig. 7). Distributed ‘domino-style’ 354 foliation-parallel displacements within that zone of rotation can account for the dextral 355 sense of shear shown by kinematic indicators. The focusing of the TSB within the Nappe 356 may reflect the strong planar anisotropy of its constituent lithologies (mainly banded 357 psammitic gneisses) in contrast to the more homogeneous migmatites of the Swordly 358 Nappe and grey gneisses and amphibolites of the Strathy Complex.

359 Magma emplacement during Scandian thrusting and deformation in the Torrisdale 360 Steep Belt

361 In central and southeast Sutherland, the Naver Nappe and structurally high levels of the 362 Moine Nappe contain numerous concordant igneous intrusions (Read 1931; Brown 1967, 363 1971; Holdsworth & Strachan 1988; Holdsworth et al. 2001; Kinny et al. 2003a; Kocks et 364 al. 2014). These are mostly felsic, including leucogranites, granites s.s. and granodiorites,

365 but are locally dioritic. The leucogranites are likely crustally-derived (Brown 1967), 366 whereas some of the more mafic bodies are comparable in their chemistry to the end- 367 Caledonian Northern Highland high Ba-Sr granitoids which were in part mantle-derived 368 (Fowler et al. 2008). They generally range from c. 10 cm to c. 30 m in thickness, although 369 the Strathnaver Granite and Creag Mhor Quartz Monzodiorite (Fig. 2) are much larger. 370 Some intrusions contain xenoliths and contacts with host rocks are invariably sharp, so it 371 is clear that the sheets have migrated and have been emplaced into their present

372 locations and were not generated in situ. Many intrusions cut D2 folds but carry the S2

373 schistosity and the L2 linear fabric and show evidence for high-temperature (>450-500°C) 374 solid-state recrystallisation (Holdsworth & Strachan 1988; Kinny et al. 2003a; Kocks et al.

375 2014). These observations are consistent with emplacement during D2, broadly 376 synchronously with displacements along the Naver Thrust. U-Pb zircon geochronology by

377 secondary ion mass spectrometry (SIMS) performed on four syn-D2 intrusions (Creag nan 378 Suibheag, Strathnaver and Vagastie granites and the Klibreck Sill; Fig. 2) produced ages

379 of c. 429-415 Ma, which form the basis for assigning D2 (and younger) structures to the 380 Scandian event (Kinny et al. 2003a; Alsop et al. 2010). 381 The proportion of intrusions increases towards the north coast section where the 382 Naver Nappe and uppermost c. 200 m of the Moine Nappe contain voluminous amounts 383 of variably deformed granite and felsic pegmatite (Burns 1994; Holdsworth et al. 2001). 384 The foliated Clerkhill Intrusion (Fig. 5) is more variable, comprising diorite and 385 granodiorite augen-gneisses with subordinate appinitic amphibolites. [e.g. NC 7145 6365] 386 (Burns 1994). Around Torrisdale Bay and on Neave Island (Figs. 5 and 6d), granite and 387 pegmatite intrusions locally form up to 50% of the outcrop, varying from veins a few cm 388 thick to large sheet-like, anastomosing bodies up to 100 m thick and traceable laterally 389 for up to 600 m [e.g. NC 694 608]. Most have broadly concordant, sharp contacts with 390 host Moine/basement lithologies. There is a complete spectrum from relatively rare pre-

391 to syn-D2 intrusions that are foliated and carry L2, to more common, generally unfoliated,

392 sheets that cross-cut F3 folds and L4 (Holdsworth et al. 2001). Within the Torrisdale Steep 393 Belt, various intrusions are tightly to isoclinally folded but show little evidence for internal

394 deformation, which implies that they had not fully crystallised at the time of deformation, 395 and so were injected syn-tectonically (see also Butler & Torvela 2018). 396 The proportion of granitoid sheets reduces structurally upwards within the 397 regional nappe pile and the Swordly Thrust is not associated with any spatial density or 398 focusing of these intrusions. This suggests that it was not as significant as the Naver Thrust 399 in providing an ascent pathway for melts during Scandian thrusting. In contrast, at higher 400 structural levels, the Strath Halladale Granite cuts discordantly at a low angle across the 401 Skinsdale Thrust (Fig. 2). The pluton is an east-dipping sheeted complex dominated by 402 granite and granodiorite with minor diorite and ultramafic components and associated 403 with the Reay Diorite to the north (Fig. 2; Read 1931; McCourt 1980). Various lines of

404 evidence support a late-D2 structural age for intrusion, specifically: a) a magmatic foliation 405 was reworked by solid-state deformation at high to moderate temperatures, b) shear 406 zones within the pluton display top-to-the-NW sense of shear similar to that deduced for

407 the Skinsdale Thrust, and c) granite sheets were deformed by curvilinear D3 folds (Kocks 408 et al. 2006). The U-Pb monazite age of 426 ± 2 Ma obtained from the pluton thus

409 approximately dates late-D2 in this eastern part of the Scandian nappe stack (Kocks et al. 410 2006). 411 In summary, the structural and field evidence indicates that the central Sutherland

412 and Strath Halladale granites were emplaced during D2. The parallelism of the intrusions 413 to the regional easterly-dipping foliation suggests that they were emplaced as sills. The 414 spatial coincidence of the intrusions with the Naver and Skinsdale thrusts further suggests 415 that these thrusts acted as gently-inclined channel ways that focused the migration of the

416 melts (Holdsworth & Strachan 1988; Kocks et al. 2006). In contrast, the syn- to post-D4 417 age of granite and pegmatite intrusions within the steeply-inclined TSB suggests that 418 these were instead channelled upwards as dykes within the developing transpressional 419 shear zone. The increase in density of these pegmatites towards the north coast section 420 further suggests that the proposed lateral ramp that controlled the development of the 421 TSB was also acting as a focus for melt transport.

422 Precise U-Pb zircon dating of syn- to late-tectonic granitic intrusions

423 We conducted high-precision U-Pb geochronology by the CA-ID-TIMS method on zircons

424 separated from three syn-D2 intrusions that had previously been dated by the U-Pb SIMS 425 technique (Vagastie Bridge Granite, the Klibreck Sill, the Creag nan Suibheag Granite) and

426 one previously undated late-D2 intrusion (Creag Mhor Quartz Monzodiorite) (Fig. 2). 427 Details of analytical procedures, complete U-Pb isotopic data and methods of U-Pb age 428 calculation and error reporting are given in the Supplementary Materials. Figure 8 429 summarizes the geochronological results.

430 Sample descriptions 431 432 A sample of the syn-D2 Vagastie Bridge Granite (RS-14-19) was collected at [NC 5350 433 2825]. It occurs as a series of anastomosing concordant sheets, up to 500 m long and 50 434 m thick, that intrude Moine Nappe psammites (Holdsworth & Strachan 1988; Kinny et al.

435 2003a). The intrusion cuts F2 folds but carries the S2 foliation and L2 lineation; the latter 436 plunges on a bearing of 140° (Kinny et al. 2003a). It is a coarse-grained, pink gneissic 437 granite with abundant augen (up to 1.25 cm size) of perthitic orthoclase. The augen are 438 wrapped by fine- to medium-grained matrix of dynamically recrystallised plagioclase, K- 439 feldspar, quartz, hornblende, biotite, with secondary chlorite and accessory titanite, 440 zircon and magnetite.

441 A sample of the syn-D2 Klibreck Sill (RS-14-18) was collected from [NC 5815 3110] 442 where it intrudes psammitic gneisses of the Naver Nappe (Fig 2; Kinny et al. 2003a). It is 443 traceable for c. 2 km as a concordant sheet no more than c. 30 m thick. The intrusion cuts

444 F2 folds but carries the S2 foliation and L2 lineation; the latter plunges on a bearing of 170° 445 (Kinny et al. 2003a). It is a pink, equigranular, medium-grained meta-granite, comprised 446 of plagioclase, K-feldspar, quartz and biotite, with accessory titanite, magnetite and 447 zircon.

448 The syn-D2 Creag nan Suibheag Granite (RS-14-20) was collected at [NC 3881 2926] 449 where it intrudes psammites of the Moine Nappe (Fig 2; Alsop et al. 2010). It is c. 4 m 450 thick and can be traced laterally for c. 25 m. The intrusion is concordant and carries the

451 S2 foliation and L2 lineation; the latter plunges on a bearing of 125°. It is a fine- to medium-

452 grained, pink, equigranular meta-granite comprised of quartz, plagioclase (albitic), K- 453 feldpsar, muscovite and biotite, with accessory titanite, zircon and magnetite.

454 The late-D2 Creag Mhor Quartz Monzodiorite (RS-14-26) was collected at [NC 7315 455 0869]. The intrusion occurs as a concordant sheet, c. 6 km long and up to c. 150 m thick, 456 emplaced into Moine Nappe psammites and leucogranites in the immediate footwall of 457 the Naver Thrust (Fig. 2; Kocks et al. 2014). It comprises a medium- to coarse-grained 458 assemblage of plagioclase, quartz, biotite and hornblende, with minor K-feldspar and 459 accessory titanite, magnetite and zircon. Aligned euhedral plagioclase and hornblende

460 define a magmatic fabric, the planar component of which is parallel to S2 in the country

461 rocks, and the linear component to L2. Tiling of hornblende grains shows that magmatic 462 flow was directed towards the west (Kocks et al. 2014). 463 464 Age results and interpretations 465 466 Vagastie Bridge Granite (sample RS-14-19) Four analysed zircons from this sample form a 467 statistically coherent cluster without any outliers and produce a weighed mean 206Pb/238U 468 date of 432.35 ± 0.10/0.21/0.51 Ma and a mean square weighted deviation (MSWD) of 469 2.0 (Fig. 8). The latter best represents the emplacement age of the intrusion coeval with

470 D2 deformation within the Moine Nappe.

471 Klibreck Sill (sample RS-14-18) Analysed zircons from this sample yielded a range of 472 Silurian 206Pb/238U dates from ~437 Ma to 426.18 ± 0.26 Ma. The presence of a discordant 473 Precambrian analysis (z4) suggests that some of the observed age scatter in this sample 474 might be due to xenocrystic zircon cores. However, the bulk of Scandian age analyses are 475 concordant (Fig. 8) and are best interpreted as protracted zircon crystallization (c. 430-

476 426 Ma) during D2 deformation.

477 Creag nan Suibheag Granite (sample RS-14-20) Analysed zircons from this sample range 478 in their 206Pb/238U dates from 432.93 ± 0.75 Ma to 428.34 ± 0.29 Ma. The older analyses 479 (z5 and z6) are relatively imprecise (low U and radiogenic Pb) and discordant and may 480 reflect an inherited component, whereas the younger two (z2 and z3) are concordant, but

481 do not overlap with uncertainty (Fig. 8). These suggest a protracted, syn-D2, zircon 482 crystallization history for the intrusion.

483 Creag Mhor Quartz Monzodiorite (sample RS-14-26) Similar to that in the Klibreck Sill 484 sample, the zircon analyses here produced a range of 206Pb/238U dates that, with the 485 exception of one distinctly older analysis (z6), cannot be explained by zircon inheritance 486 (Fig. 8). The 426.76 ± 0.25 Ma to 425.72 ± 0.21 Ma range of dates from this sample

487 represent protracted zircon crystallization during late stages of D2 deformation. 488

489 Discussion

490 New U-Pb geochronology

491 Previously published radioisotopic geochronology from the northern Scottish 492 Caledonides includes age data of different vintages, produced by a variety of techniques 493 and calculated using different chronometers, which often makes age comparisons 494 problematic. Much of the existing U-Pb ID-TIMS geochronology from the region was 495 based on analyses of multi-grain (µg size) aliquots of zircon, with or without any pre- 496 treatment (e.g., van Breemen et al. 1979; Halliday et al. 1987; Strachan & Evans 2008; 497 Kocks et al. 2014). These age data are in general of questionable accuracy by modern 498 standards due to the possibility of open system behaviour (inheritance and/or Pb loss) in 499 zircon and the manners in which the dates were calculated (e.g., mean 207Pb/206Pb or 500 concordia intercept dates). Caution should therefore be exercised in constructing 501 geologic histories based on compilations of inherently incompatible age results. Only U- 502 Pb ages derived from 206Pb/238U dates of chemically abraded, single-zircon analyses (e.g., 503 Goodenough et al. 2011) and especially those produced using the EARTHTIME tracers and 504 analytical protocols have the precision and reproducibility to be directly compared to the 505 U-Pb results of this study. 506 The enhanced precision of modern U-Pb analyses by the CA-ID-TIMS method helps 507 unravel the complexities of zircon age populations in magmatic rocks that often go

508 undetected by the lower precision in-situ dating techniques. The set of 206Pb/238U dates 509 presented here have analytical uncertainties as low as ± 0.20 m.y. (0.09%), which can 510 easily resolve the observed age scatters of 1 m.y. (RS14-26) to 4.6 m.y. (RS14-20) of this 511 study. In comparison, even the most precise zircon SIMS analyses from the area have had 512 2σ analytical uncertainties of at least ± 10 m.y. (Alsop et al. 2010). 513 A limitation of the in-situ U-Pb dating techniques in terms of accuracy is their 514 inability to perform chemical abrasion on zircon and thus mitigate the effects of Pb loss 515 (e.g. Wu et al., 2016), especially when analysing zircon rims. Both the Vagastie Bridge 516 Granite and Klibreck Sill have produced CA-ID-TIMS weighted mean dates that overlap 517 only with the upper margin of their respective published SIMS dates (424 ± 8 Ma and 420 518 ± 6 Ma, respectively; Kinny et al. 2003a). None of our CA-ID-TIMS analyses from the Creag 519 nan Suibheag Granite are as young as the reported U-Pb SIMS date of this intrusion. This 520 suggests that the distinctly younger published age of the Creag nan Suibheag granite (415 521 ± 6 Ma: Alsop et al. 2010) should be viewed with caution. 522 Our new geochronology indicates a minimum c. 6 myr period of syn-tectonic 523 zircon crystallization between c. 432 Ma (Vagastie Bridge Granite) and c. 426 Ma (Klibreck 524 Sill and Creag Mhor Quartz Monzodiorite) associated with peak- to late-D2 deformation 525 along the Naver Thrust. This period should be regarded as a minimum, as additional data 526 from these and other intrusions may expand the age spectrum of zircons and thus the 527 duration of deformation. Taken at face value, the data suggest that the deformation was 528 older in the footwall of the Naver Thrust (Vagastie Bridge Granite), whereas it continued 529 for another c. 6 myr above the thrust (Klibreck Sill). At present, our results do not seem 530 to support a simple model of foreland-propagating, westerly younging, Scandian 531 deformation. Additional high-precision geochronology, particularly from the western 532 Sutherland, is necessary to resolve the timing of Scandian ductile deformation across this 533 sector of the orogen.

534 Ductile thrust evolution of the Scandian thrust nappes

535 The Scandian nappe stack in northern Scotland developed from the ductile reworking of 536 Moine rocks and their associated basement that had already been deformed and 537 metamorphosed during the Neoproterozoic and the Ordovician. A critical question relates 538 to the extent to which the major ductile thrusts and folds are composite structures that 539 were initially formed during pre-Scandian orogenic events. Bird et al. (2013) noted curved 540 (i.e. syn-tectonic) inclusion trails within 450-445 Ma (= Grampian II) garnets and 541 suggested that some of the major folds within the Moine Nappe may have been initiated 542 at this time, but only attained their present highly curvilinear sheath-fold geometry as a 543 result of Scandian reworking. Dunk et al. (2019) invoked a Grampian I age for the Port 544 Mor Thrust (Fig. 4) and ‘proto’ Naver and Swordly thrusts. However, the systematic 545 eastward increase in deformational temperatures that is apparent from integrated

546 microstructural and geothermometry studies related to proven Silurian fabrics (the L2 547 lineation) demonstrates the reality of a Scandian orogenic wedge that resulted from 548 crustal-scale ductile thrusting (Thigpen et al. 2013; Ashley et al. 2015; Mazza et al. 2018; 549 Mako et al. 2019). 550 Curvilinear regional lineation trends of the type preserved in northern Scotland 551 (Fig. 3) could result from one or more of three possible scenarios: 1) different generations 552 of lineations associated with separate deformation events, 2) heterogeneity in the 553 magnitude and direction of shear combined with strain partitioning during regional 554 transpression, and 3) reorientation of a linear fabric due to a later deformation event. The 555 findings reported here indicate that the first two of these are most relevant. On the 556 regional scale the lineation pattern is probably a composite of Grampian and Scandian 557 orogenic events. Scandian reworking was pervasive in the Naver and Moine nappes and 558 associated with broadly northwest-directed transport. In contrast, Grampian structures 559 may predominate in the structurally higher Swordly and Skinsdale nappes, characterised 560 by north-south trending lineations, although the tectonic transport direction is unknown. 561 The dominance of potentially older structures in the highest nappes in northern Scotland 562 compares with the peri-Laurentian-derived thrust sheets of western Norway. These 563 acquired their main deformational and metamorphic characteristics during the

564 Ordovician to early Silurian and were then emplaced southeastwards onto the Baltica 565 margin as composite entities during the Scandian orogeny (Andersen & Andresen 1994; 566 Corfu et al. 2014). 567 Within this regional framework, there is, however, still a considerable variation in

568 the azimuth of the Scandian L2 lineation that requires explanation (Fig. 3). This has been 569 interpreted previously in different ways: either a gradual rotation in the strain field due 570 to changes in the direction of plate convergence (e.g. Soper et al. 1992) or a progressive 571 change in kinematic partitioning of deformation into coeval thrusting and strike-slip 572 across the Scottish Caledonides (e.g. Dewey & Strachan 2003). Additional insights are 573 provided by the isotopic dating of syenite intrusions thought to have been emplaced 574 during thrusting within the Assynt culmination of the Moine Thrust Zone (Fig. 2; 575 Goodenough et al. 2011). U-Pb CA-ID-TIMS zircon ages bracket their emplacement 576 between 430.7 ± 0.5 Ma and 429.2 ± 0.5 Ma (Fig. 9; Goodenough et al. 2011) and are 577 readily comparable with data in the present paper as they were obtained using essentially 578 the same analytical procedures. The two data sets taken together therefore indicate that 579 NW- to NNW-directed ductile thrusting along the Naver Thrust in the central part of the 580 nappe stack overlapped WNW-directed thrusting within the Moine Thrust Zone. The

581 regional lineation swing defined by L2 therefore appears to reflect essentially 582 contemporaneous deformation at different crustal levels, and is consistent with sinistrally 583 oblique convergence. 584 Early interpretations of structural sequences in the external thrust belts of 585 orogens suggested that thrusts tended to develop in a foreland-propagating manner (e.g. 586 Bally 1966; Elliott & Johnson 1980) and this model was applied to the ductile thrust sheets 587 of northern Scotland (Barr et al. 1986). While this model may well still apply in many cases, 588 a more nuanced approach may be necessary to understand examples where thrusts 589 appear to have moved simultaneously (e.g. Butler 2004) as well as out-of-sequence (e.g. 590 Morley 1988). In northern Scotland, we envisage that the early widely-distributed shear 591 at mid- to upper crustal levels indicated by the geochronological data was followed by the 592 localisation of strain along discrete thrusts that broadly propagated towards the foreland.

593 This is consistent with folding of the Naver and Ben Hope thrusts by F3 folds that 594 developed in their footwalls (Holdsworth 1989; Alsop et al. 1996), and the passive folding 595 of both thrusts above the Assynt culmination in the Moine Thrust Zone (Fig. 2). However, 596 recognition that the Skinsdale Thrust is probably out-of-sequence (this study) as well as 597 the Ben Blandy Shear Zone (Alsop et al. 1996) and the Lochan Rhiabach Thrust and its 598 likely continuation in Assynt (Holdsworth et al. 2006; Thigpen et al. 2010a) suggests more 599 widespread modification of the nappe stack by such structures than understood 600 previously.

601 Magma emplacement during Scandian deformation

602 The focusing of intrusions that are partly mantle-derived along the Naver and Skinsdale 603 thrusts demonstrates the crustal scale of these structures. In contrast, there is a marked 604 lack of thrust-related Caledonian intrusions further south, in Ross-shire and Inverness- 605 shire along the trace of the Sgurr Beag Thrust. One reason for this might be that 606 Sutherland represents a deeper crustal level, which is consistent with the considerably 607 greater amount of Archaean basement exposed within the Moine Nappe (Fig. 2). The 608 association of the ‘early’ Moine Thrust north of central Assynt with a belt of greenschist- 609 facies mylonites in its footwall and hangingwall (Thigpen et al. 2010a & b; 2013) might 610 also be indicative of deformation at a deeper crustal level than further south where the 611 Moine Thrust is a ‘late’ brittle structure (Law & Johnson 2010 and references therein). 612 Given apparently continuous subduction of Iapetan oceanic lithosphere beneath 613 the Laurentian margin from c. 455 Ma to c. 420 Ma, it would be reasonable to invoke 614 melting of mantle wedge sources during subduction to produce these magmas, as well as 615 the c. 430 Ma syenite plutons of the Assynt Culmination in the Moine Thrust Zone and 616 the Loch Loyal Syenite Complex (Fig. 2; Goodenough et al. 2011). Various arguments have 617 been employed to explain the general lack of magmatism within the Northern Highland 618 and Grampian terranes between 455 Ma and 430 Ma. This might be due either to low- 619 angle subduction or erosional removal of a volcanic arc (Oliver et al. 2008; Miles et al. 620 2016). Alternatively, subduction at a high-angle to a continental margin could also

621 suppress magma emplacement if active deformation was not producing the crustal-scale 622 channel ways necessary to facilitate melt transport (Glazner 1991). The onset of 623 magmatism at c. 430 Ma could therefore be directly linked to the Scandian collision and 624 generation of the crustal-scale thrusts that are best developed in Sutherland. Caledonian 625 plutons dated at between c. 465 Ma and c. 438 Ma have been recently recognised along- 626 strike in the Shetland Islands c. 200 km north-northeast of mainland Scotland, perhaps 627 suggesting an intervening change in subduction angle and/or distance between the 628 subduction zone and the Laurentian margin (Lancaster et al. 2017).

629 Timing and duration of the Scandian event in northern Scotland

630 The results of the present study indicate that the Naver Thrust was active (at amphibolite- 631 facies) between c. 432 Ma and c. 426 Ma (Fig. 9) and have implications for regional 632 tectonic models. This is because it has generally been believed that the Scandian event in 633 northern Scotland was terminated by c. 430 Ma given that: a) ductile thrusts within the 634 hinterland are folded passively above culminations within the underlying, and therefore 635 younger, thin-skinned Moine Thrust Zone, and b) dated syenite intrusions in the Assynt 636 area (see above) apparently truncate thrusts in the central part of the Moine Thrust Zone 637 (Woolley 1970; Goodenough et al. 2011; but see Searle et al. 2010). However, no field 638 relationships preclude the continuation of thrusting post-430 Ma along the Sole Thrust 639 and associated structures, and the Moine Thrust clearly truncates the 430.6 ± 0.3 Ma Loch 640 Ailsh syenite (Fig 2; Goodenough et al. 2011). 641 A c. 430 Ma termination to the orogeny is difficult to reconcile with the results of 642 other isotopic studies which also suggest a more protracted evolution. These include: a) 643 Rb-Sr white mica ages (closure T = ~550°C) from Moine Thrust Zone mylonites which 644 indicate that although the main cessation of deformation occurred at c. 430 Ma, there 645 was evidence for strain localisation until c. 410 Ma (Freeman et al. 1998); b) Rb-Sr 646 muscovite ages of c. 428, 423 and 421 Ma from the lower Moine Nappe (Dallmeyer et al. 647 2001); c) U-Pb titanite ID-TIMS ages (closure T = ~550-600°C) of 413 ± 3 Ma and 416 ± 3 648 Ma obtained from the Vagastie Bridge and Kilbreck Sill respectively (Fig. 9; Kinny et al.

649 2003a), d) monazite-xenotime thermometry and U-Pb geochronology that demonstrate 650 that the Naver Nappe experienced peak temperatures of 700°C at c. 425 Ma (Mako et al.

651 2019), e) a U-Pb monazite age of 426 ± 2 Ma for the late-D2 emplacement of the Strath 652 Halladale Granite (see above) (Kocks et al. 2006), and f) a U-Pb concordia age of 426 ± 3 653 Ma obtained from multi-grain fractions of air-abraded zircon from the Glen Scaddle 654 metagabbro in Inverness-shire to the south (Fig. 2), which predated regional scale upright 655 folding at amphibolite-facies grade (Strachan & Evans 2008). In summary, isotopic studies 656 suggest that high-grade metamorphism and contractional deformation within the 657 Scandian nappes persisted until c. 420-415 Ma which necessarily implies a more 658 protracted evolution of the Moine Thrust Zone than considered previously. The Moine 659 Thrust is conventionally assumed to predate the Ross of Mull Granite, (Fig. 2; 418 ± 5 Ma, 660 U-Pb SIMS zircon age, Oliver et al. 2008) which on its eastern boundary intrudes Moine 661 rocks, and on its western boundary thermally metamorphoses rocks on Iona that are 662 assigned to the Caledonian foreland (Potts et al. 1995). However, a para-autochthonous 663 setting for Iona within the Moine Thrust Zone cannot be excluded, and hence the duration 664 of upper crustal thrusting along the margin of the orogen is poorly constrained. 665 A previous estimate for the duration of the Scandian event in northern Scotland 666 suggested that it occurred between 443 Ma and 425 Ma, and lasted <18 myr (Johnson & 667 Strachan 2006). Conservatively, this can be amended to between 437 Ma, the oldest of 668 the white mica 40Ar/39Ar ages recorded from Moine Thrust Zone mylonites (Freeman et 669 al. 1998), and a lower limit possibly as young as 415 Ma (see also Mako et al. 2019), 670 suggesting a duration of potentially up to 22 myr.

671 Partitioned thrusting and strike-slip displacements within the Scandian orogenic wedge

672 The late Silurian to mid-Devonian interval in the Caledonides has been interpreted in 673 terms of a transition from sinistral transpression to strike-slip and then transtension, 674 reflecting relative plate motions between Laurentia and Baltica-Avalonia (Dewey & 675 Strachan 2003). If the Moine Thrust Zone was active until c. 420-415 Ma (see above), then 676 brittle, upper crustal thrusting along the margin of the orogen must have overlapped early

677 strike-slip displacements along the Great Glen Fault and related structures. The timing of 678 the latter has been constrained to c. 430-420 Ma by the isotopic dating of syn-kinematic 679 plutons emplaced along major faults (Rogers & Dunning 1991; Stewart et al. 2001; 680 Kirkland et al. 2008; Kocks et al. 2014; Holdsworth et al. 2015). Many of these plutons 681 have a mantle-derived component to the melts, indicating that the strike-slip faults along 682 which they were emplaced must have been crustal-scale structures (Hutton & Reavy 683 1992; Reavy & Jacques 1994). 684 Synchronous movement of the Moine Thrust Zone and the Great Glen and related 685 faults implies a large-scale partitioning of transpressional strain above a basal 686 decollement (Stewart et al. 1999). Similar partitioned structural regimes have been 687 demonstrated from other orogenic tracts such as the Qilan Shan (northeast Tibet Plateau) 688 (Allen et al. 2017), the Canadian Cordilleras (Oldow et al. 1990), and the Caledonides of 689 NE Greenland (Holdsworth & Strachan 1991; Smith et al. 2007). In Scotland, the overall 690 tectonic regime was likely dominated by strike-slip displacements from 420-415 Ma 691 onwards because the Great Glen Fault appears to truncate mantle reflectors that have 692 been interpreted as Caledonian thrusts (Snyder & Flack 1990). Dating of syn-kinematic 693 granites within the exhumed high-grade core of the Great Glen Fault demonstrated that 694 sinistral movements continued until at least 399 Ma (Mendum & Noble 2010).

695 Regional Appalachian-Caledonian linkages

696 Silurian deformation and metamorphism along the Appalachian-Caledonian orogen 697 between Newfoundland and northern Greenland and Norway resulted from the 698 sinistrally-oblique collision of Laurentia, Baltica and peri-Gondwanan terranes (e.g. 699 Avalonia and Ganderia) and consequent closure of the Iapetus Ocean (Fig. 1; Soper et al. 700 1992; Dewey & Strachan 2003). However, there are considerable differences in the 701 intensities of this event (Fig. 10). In Newfoundland, ‘Salinic’ (c. 435-420 Ma) crustal 702 thickening and metamorphism up to kyanite grade resulted from the collision of peri- 703 Laurentian and peri-Gondwanan arcs across the Red Indian Line (Fig. 10; Cawood et al. 704 1994). Along strike in western Ireland, the slightly younger ‘Erian’ event (c. 424-416 Ma)

705 was associated with sinistrally transpressive cleavage development and folding of the 706 South Mayo Trough sedimentary succession at sub-greenschist facies (Fig. 10; Dewey et 707 al. 2015). The easterly-younging and diachronous nature of the Silurian collision across 708 the Solway Line between western Ireland and south Scotland has been well-documented 709 (Soper & Woodcock 1990). Sinistrally-oblique collision at c. 420 Ma in south Scotland was 710 not associated with significant crustal thickening or metamorphism. The main structures 711 are low-grade NW-directed thrusts, possibly developed as the Southern Uplands 712 accretionary prism was thrust onto the southern margin of the Midland Valley (Bluck 713 2002). There is no evidence of Silurian deformation in the Midland Valley itself where 714 there is continuity of sedimentation across the Silurian-Devonian boundary (Phillips et al. 715 2004), nor within the Grampian Terrane to the north of the Highland Boundary Fault. The 716 differences in the intensity and timing of Silurian orogenic events between Newfoundland 717 and south Scotland can be attributed partly to irregularities in the shapes of colliding 718 continental margins, and also to a gradual slowing of Avalonia-Laurentia convergence 719 rates following initial ‘Salinic’ collision in Newfoundland (Soper & Woodcock 1990). 720 The relatively short-lived nature of the Silurian events in Newfoundland, Ireland 721 and south Scotland contrasts with a much longer duration in East Greenland (Laurentia) 722 and Norway (Baltica). Continental collision here was underway by c. 435 Ma, which 723 corresponds in East Greenland to the oldest S-type granites (Kalsbeek et al. 2001), and in 724 Norway to the oldest mineral ages that relate to development of a regional-scale 725 extrusion wedge (Grimmer et al. 2015). Crustal thickening culminated in Devonian 726 ultrahigh-pressure metamorphism at c. 415-390 Ma in East Greenland (e.g. McClelland & 727 Gilotti 2003; Gilotti et al. 2004; Corfu & Hartz 2011) and c. 405-400 Ma in Norway 728 (Carswell et al. 2003; Tucker et al. 2004). Unroofing of the Greenland-Norway orogenic 729 wedge was at least in part achieved by sinistrally-oblique transtensional displacements 730 along low-angle shear zones through the early to middle Devonian (Osmundsen & 731 Andersen 2001; Osmundsen et al. 2003). The long history of convergence and ‘hard’ 732 collision through the Silurian and the early Devonian contrasts markedly with the ‘soft’ 733 Avalonia-Laurentia collision in south Scotland, and this was presumably accommodated

734 by decoupling of Avalonia and Baltica along the Tornquist Line (Fig. 10; Dewey & Strachan, 735 2003). 736 The much shorter duration of the Scandian event in northern Scotland (<22 myr) 737 relative to East Greenland and Norway (c. 45 myr) is consistent with a likely location on 738 the periphery of the Laurentia-Baltica collision. This is thought to have been followed by 739 sinistral strike-slip displacement of c. 500-700 km along the Great Glen Fault that 740 juxtaposed the northern Scotland crustal fragment against the Grampian Terrane to the 741 southeast which shows no record of Silurian ductile deformation and metamorphism (Fig. 742 10; Coward 1990; Dallmeyer et al. 2001; Dewey & Strachan 2003; Kinny et al. 2003a). It is 743 envisaged that during the initial phase of collision at c. 435 Ma, northern Scotland was 744 located opposite the southernmost part of the Baltica plate and that this drove regional 745 ductile thrusting and development of the Moine Thrust Zone (Dewey & Strachan 2003). 746 Continued sinistrally-oblique movement of Baltica relative to Laurentia moved the main 747 locus of plate collision away from Scotland further north to East Greenland. The 748 Caledonian thrust front in East Greenland is Devonian (c. 400-390 Ma; Dallmeyer et al. 749 1994) in contrast to the end Silurian age of the Moine Thrust Zone, which may account 750 for the bend necessary to link the two structures (Fig. 10). 751 The lateral continuations of the Moine Thrust and the Great Glen Fault to the 752 southwest and into Newfoundland are uncertain and it may be that there are no direct 753 linkages. Given the magnitude of the sinistral displacement that has been proposed for 754 the Great Glen Fault, it would be surprising if a correlative structure were not present in 755 Newfoundland, even if its magnitude had diminished along strike. The steep Baie Verte 756 Line-Cabot Fault (Fig. 10) is a potential candidate which has a long-lived and complex 757 history and along which significant orogen-parallel displacements may have occurred (Lin 758 et al. 2013). However, the main documented ductile displacements are dextral (Brem et 759 al. 2007). The Appalachian Deformation Front (Fig. 10) is often shown as being continuous 760 with the Moine Thrust on reconstructions but this is unlikely given the different tectonic 761 drivers. In northern Newfoundland, White & Waldron (2019) have demonstrated that the 762 frontal Appalachian thrusts formed during the mid-Devonian Acadian Orogeny. Although

763 displacements are modest (<10 km), these or equivalent structures offshore may truncate 764 or rework the lateral continuations of the Moine Thrust and the Great Glen Fault 765 somewhere on the intervening continental shelves, adding to the difficulties in 766 correlation.

767 Conclusions

768 1) The regional lineation pattern in the Caledonian thrust nappes of northern Scotland 769 (Laurentia) is likely to be a composite of Grampian and Scandian orogenic events. The 770 highest structural levels (eastern Swordly and Skinsdale nappes) are dominated by 771 orogen-parallel lineations. Field and isotopic evidence suggests that these formed 772 during the Grampian (Ordovician, c. 470-460 Ma) orogeny, but further geochronology 773 is required to test this hypothesis. Pervasive ductile deformation during the Scandian 774 (Silurian, c. 435-425 Ma) orogeny is characteristic of the underlying thrust nappes 775 (Moine, Naver and western Swordly), associated with NNW- to WNW-directed 776 thrusting. Localised dextrally-transpressive deformation within the Naver and Swordly 777 nappes (the Torrisdale Steep Belt) is interpreted as the ductile expression of a lateral 778 ramp structure located offshore. 779 2) Ductile thrusts at all structural levels are mostly deformed by folds developed in their 780 footwalls and which root downwards into structurally lower ductile thrusts, suggesting 781 an overall foreland-propagating sequence of deformation. However, evidence for out- 782 of-sequence thrusting at three different structural levels suggests a more complex 783 sequence of thrust stacking than previously supposed and that thrusts may have 784 developed synchronously at different levels within the nappe stack. 785 3) Two of the structurally higher thrusts (Naver and Skinsdale) as well as the dextral 786 transpression zone associated with the inferred lateral ramp acted as conduits for syn- 787 tectonic granitic intrusions that have a mantle-derived component. Given the paucity 788 of magmatism prior to 430 Ma in this part of Scotland, its onset at that time could 789 therefore be directly linked to the Scandian collision and development of crustal-scale 790 thrusts synchronously with slab-break-off.

791 4) New high-precision U-Pb zircon ages from syn-tectonic granitic intrusions indicate that 792 the NNW- to NW-directed Naver Thrust was active (at amphibolite facies grade) 793 between c. 432 Ma and c. 426 Ma. This overlaps the emplacement of previously dated 794 syn- to late-tectonic syenite plutons within the WNW-directed Moine Thrust Zone. The

795 regional arcuate anticlockwise swing in the L2 lineation therefore appears to reflect 796 contemporaneous deformation at different crustal levels and is consistent with 797 sinistrally oblique convergence. We envisage that early distributed shear was followed 798 by localisation of strain along discrete foreland-directed thrusts. 799 5) The results of the present and previous isotopic studies suggest that high-grade 800 metamorphism and contractional deformation within the Scandian nappes persisted 801 until possibly c. 420-415 Ma which implies a more protracted evolution of the Moine 802 Thrust Zone than previously considered. 803 6) If the Moine Thrust Zone was active until the latest Silurian to earliest Devonian, brittle, 804 thrusting at relatively shallow crustal levels along the margin of the orogen overlapped 805 sinistral strike-slip displacements along the Great Glen Fault and related structures 806 which are known to have been initiated at c. 425 Ma. The synchroneity of thrusting 807 and strike-slip movements implies a large-scale partitioning of transpressional strain 808 above a regional-scale basal decollement. 809 7) The relatively short duration of the Scandian orogen in Scotland (c. 437-415 Ma?) is 810 consistent with the only moderate amount of crustal thickening recorded and a 811 location on the periphery of the main Laurentia-Baltica collision further north which 812 continued until c. 390 Ma. During initial collision, Scotland was located opposite the 813 southernmost part of the Baltica plate. The loci of crustal thickening then moved 814 progressively along strike to the north during the sinistrally-oblique convergence of 815 Baltica and Laurentia, partly accommodated by sinistral shear along the Tornquist Line.

816 Acknowledgments

817 Fieldwork by RAS, REH and GIA was carried out in part during the remapping of BGS 818 Scotland sheets 114E (Tongue), 108E (Loch Naver), 109E (Kildonan) and 109W

819 (Badenloch) and funded under the NERC-BGS Academic Collaboration Programme. IMB 820 acknowledges a PhD studentship funded by Oxford Brookes University (1990-93). Mark 821 Witton is thanked for drafting. Rick Law, Fernando Corfu, Calvin Mako and Kathryn 822 Goodenough contributed detailed comments that resulted in significant improvements 823 to the paper. Stephen Daly is thanked for efficient editorial handling. 824 825 Figure captions 826 827 Figure 1. Map of the Caledonide-Appalachian orogen in the North Atlantic region prior to 828 Mesozoic rifting (modified from Waldron et al. 2014). NHT, Northern Highland Terrane; 829 GGF, Great Glen Fault; HBF, Highland Boundary Fault.

830 Figure 2. Regional geology of north Sutherland (modified from British Geological Survey 831 1997, 2002, 2003, 2004a & b). Inset map shows location in northern Scotland. 832 Abbreviations of structures: AT, Achininver Thrust; ACT, Achness Thrust; BBSZ, Ben Blandy 833 Shear Zone; BHT, Ben Hope Thrust; LRT, Lochan Riabach Thrust; MT, Moine Thrust; NT, 834 Naver Thrust; PMT, Port Mor Thrust; ST, Swordly Thrust; SKT, Skinsdale Thrust; SoT, Sole 835 Thrust; TT, Torrisdale Thrust. Abbreviations of intrusions: CM, Creag Mhor Quartz 836 Monzodiorite; CSG, Creag Suilbheag Granite; G, Grudie Granite; GSM, Glen Scaddle 837 Metagabbro; HG, Helmsdale Granite; KG, Klibreck Granite; LA, Loch Ailsh Syenite; LB, Loch 838 Borrolan Syenite; LL, Loch Loyal Syenite Complex; LS, Loch Shin Granite; RIC, Rogart 839 Igneous Complex; ROM, Ross of Mull Granite; SHG, Strath Halladale Granite; SNG, 840 Strathnaver Granite; VBG, Vagastie Bridge Granite.

841 Figure 3. Regional trends of the dominant mineral and extension lineations within the 842 Caledonian thrust sheets of northern Scotland (modified from Law & Johnson 2010).

843 Figure 4. Detailed geological map of the Swordly Thrust in central Sutherland (see Fig 2 844 for location) together with structural data presented as lower hemisphere, equal area 845 stereographic projections (see text for discussion). Data for sub-areas 1 and 2 taken from 846 above and below the Swordly Thrust. AB, Archaean basement.

847 Figure 5. Map, cross section and structural data from North Sutherland (see Fig. 2 for 848 location) together with structural data presented as lower hemisphere, equal area 849 stereographic projections (see text for discussion). Abbreviations: FBI, Farr basement 850 inlier; NT, Naver Thrust; PMT, Port Mor Thrust; ST, Swordly Thrust; TT Torrisdale Thrust; 851 TSB, Torrisdale Steep Belt. Subsurface structure from British Geological Survey (1997) 852 (western section) and Dunk et al. (2019) (eastern section).

853 Figure 6. a) post-D1 garnets within the Strathy Complex (Lu-Hf age is 447 ± 15 Ma) are

854 statically overgrowing an S1S fabric that is folded (NC 7982 6590); remaining images are

855 all from the Torrisdale Steep Belt: b) view of a horizontal surface showing L2 folded around

856 an F3 hinge, and D4 dextral shear bands at Swordly Bay (NC 7354 6355); c) view of a 857 horizontal surface at Swordly Bay showing dextrally-sheared lozenges of granitic gneiss 858 (NC 7354 6355); d) view northwestwards to Aird Torrisdale from NC 6885 6285, showing 859 steeply-dipping and foliation-parallel granite and pegmatite sheets.

860 Figure 7. Simplified map showing the location and main components of the D4 Torrisdale 861 Steep Belt and the inferred lateral ramp/transfer zone located offshore. The inset shows 862 how foliation-parallel displacements within a zone of overall sinistral transpression would 863 result in the widespread dextral shear sense indicators observed at outcrop. 864 Abbreviations are as in Fig 5 with addition of: BI, Borgie inlier; SC, Strathy Complex.

865 Figure 8. Conventional concordia plots of the analysed zircons. Error ellipses are plotted 866 at 2 sigma. Dashed lines represent uncertainties in U decay constants displayed as the 867 concordia error envelope.

868 Figure 9. Summary of U-Pb (CA-IDTIMS) zircon ages (dark arrows; Goodenough et al. 2011 869 and this study) and U-Pb (TIMS) titanite ages (grey arrows; Kinny et al. 2003a). CNSG, 870 Creag nan Suibheag Granite; CMQM, Creag Mor Quartz Monzodiorite; KS, Klibreck Sill; 871 MT, Moine Thrust; NT, Naver Thrust; ST, Swordly Thrust; VBG, Vagastie Bridge Granite. 872 See Fig. 2 for locations.

873 Figure 10. Summary map showing the distribution, timing and varying intensities of 874 Silurian (Scandian) deformation and metamorphism (stippled areas) in the North Atlantic 875 Caledonides and northern Appalachians (see text for discussion)

876 References

877 Allen, M.B., Walters, R.J., Song, S., Saville, C., De Paola, N., Ford, J., Hu, Z. & Sun, W. 878 2017. Partitioning of oblique convergence coupled to the fault locking behaviour of fold- 879 and-thrust belts: Evidence from the Qilan Shan, northeastern Tibet Plateau. Tectonics, 880 36, doi: 10.1002/2017TC004476.

881 Alsop, G.I. & Holdsworth, R.E. 1993. The distribution, geometry and kinematic 882 significance of Caledonian buckle folds in the western Moine Nappe, northwestern 883 Scotland. Geological Magazine, 130, 353-362, doi: 10.1017/S0016756800020033.

884 Alsop, G.I. & Holdsworth, R.E. 1999. Vergence and facing patterns in large-scale 885 sheath folds. Journal of Structural Geology, 21, 1335-1349, doi: 10.1016/S0191- 886 8141(99)00099-1.

887 Alsop, G.I. & Holdsworth, R.E. 2002. The geometry and kinematics of flow perturbation 888 folds. Tectonophysics, 350, 99-125, doi: 10.1016/S0040-1951(02)00084-7.

889 Alsop, G.I. & Holdsworth, R.E. 2004. The geometry and topology of natural sheath folds: 890 a new tool for structural analysis. Journal of Structural Geology, 26, 1561-1589, doi: 891 10.1016/j.jsg.2004.01.009.

892 Alsop, G.I. & Holdsworth, R.E. 2007. Flow perturbation folding in shear zones. In: Ries, 893 A., Butler, R.W.H. & Graham, R.H. (eds) Global Tectonic Processes: The Legacy of Mike 894 Coward. Geological Society, London, Special Publications, 272, 77-103, doi: 895 10.1144/GSL.SP.2007.272.01.06.

896 Alsop, G.I., Holdsworth, R.E. & Strachan, R.A. 1996. Transport-parallel cross folds 897 within a mid-crustal Caledonian thrust stack, northern Scotland. Journal of Structural

898 Geology, 18, 783-790, doi: 10.1016/S0191-8141(96)80012-5.

899 Alsop, G.I., Cheer, D., Strachan, R.A., Krabbendam, M., Kinny, P.D., Holdsworth, R.E. & 900 Leslie, A.G. 2010. Progressive fold and fabric evolution associated with regional strain 901 gradients: a case study from across a Scandian ductile thrust nappe, Scottish 902 Caledonides. In: Law, R.D., Butler, R.W.H., Holdsworth, R.E., Krabbendam, M. & 903 Strachan, R.A. (eds) Continental Tectonics and Mountain Building: The Legacy of Peach 904 and Horne. Geological Society, London, Special Publications, 335, 255-274, doi: 905 10.1144/SP335.12.

906 Andersen, T.B. & Jamtveit, B. 1990. Uplift of deep crust during orogenic collapse – a 907 model based on field studies in the Sogn-Sunnfjord region of western Norway. 908 Tectonics, 9, 1097-1111, doi: 10.1029/TC009i005p01097.

909 Andersen, T.B. & Andresen, A. 1994. Stratigraphy, tectonostratigraphy and the accretion 910 of outboard terranes in the Caledonides of Sunnhordland, West Norway. 911 Tectonophysics, 231, 71-84, doi: 10.1016/0040-1951(94)90122-8.

912 Andresen, A., Rehnström, E.F. & Holte, M. 2007. Evidence for simultaneous contraction 913 and extension at different levels during the Caledonian orogeny in NE Greenland. 914 Journal of the Geological Society, London, 164, 869-880, doi: 10.11144/0016-76492005- 915 056.

916 Ashley, K.T., Thigpen, J.R. & Law, R.D. 2015. Prograde evolution of the Scottish 917 Caledonides and tectonic implications. Lithos, 224-225, 160-178, doi: 918 10.1016/j.lithos.2015.03.011.

919 Bally, A.W., Gordy, P.L. & Stewart, G.A. 1966. Structure, seismic data, and orogenic 920 evolution of southern Canadian Rocky Mountains. Bulletin of Canadian Petroleum 921 Geology, 14, 337-381.

922 Barr, D., Holdsworth, R.E. & Roberts, A.M. 1986. Caledonian ductile thrusting in a 923 Precambrian metamorphic complex: the Moine of north-western Scotland. Geological

924 Society of America Bulletin, 97, 754-764, doi: 10.1130/0016- 925 7606(1986)97<754:CDTIAP>2.0.CO;2.

926 Bird, A. F., Thirlwall, M. F., Strachan, R. A. & Manning, C. 2013. Lu-Hf and Sm-Nd dating 927 of metamorphic garnet: evidence for multiple accretion events during the Caledonian 928 orogeny in Scotland. Journal of the Geological Society, London, 170, 301-317, doi: 929 10.1144/jgs2012-083.

930 Bird, A.F., Cutts, K.A., Strachan, R.A., Thirlwall, M. & Hand, M. 2018. First evidence of 931 Renlandian (c. 950 Ma) orogeny in Mainland Scotland: implications for circum-North 932 Atlantic correlations and the status of the Moine Supergroup. Precambrian Research, 933 350, 283-294, doi: 10.1016/j.precamres.2017.12.019.

934 British Geological Survey. 1997. Tongue. Scotland. British Geological Survey, Keyworth, 935 Nottingham.

936 British Geological Survey. 2002. Loch Eriboll. Scotland. British Geological Survey, 937 Keyworth, Nottingham.

938 British Geological Survey. 2003. Kildonan. Scotland. British Geological Survey, Keyworth, 939 Nottingham.

940 British Geological Survey. 2004a. Loch Naver. Scotland. British Geological Survey, 941 Keyworth, Nottingham.

942 British Geological Survey. 2004b. Badenloch. Scotland. British Geological Survey, 943 Keyworth, Nottingham.

944 Bluck, B. 2002. The Midland Valley terrane. In: Trewin, N. (ed) Geology of Scotland (4th 945 edition). Geological Society, London, 149-166, ISBN: 1862391262 9781862391260.

946 Brem, A.G., Lin, S., van Staal, C.R., Davis, D.W. & McNicoll, V.J. 2007. The Middle 947 Ordovician to Early Silurian voyage of the Dashwoods Microcontinent, West

948 Newfoundland; based on new U/Pb and 40Ar/39Ar geochronological, and kinematic 949 constraints. American Journal of Science, 307, 311-338, doi: 10.2475/02.2007.01.

950 Brown, P.E. 1967. Major element composition of the Loch Coire Migmatite Complex, 951 Sutherland, Scotland. Contributions to Mineralogy & Petrology, 14, 1-26, doi: 952 10.1007/BF00370983.

953 Brown, P.E. 1971. The age of the granitic sheets and veins in the Loch Coire migmatites. 954 Mineralogical Magazine, 38, 446-450.

955 Burns, I.M. 1994. Tectonothermal evolution and petrogenesis of the Naver and Kirtomy 956 nappes, north Sutherland. PhD thesis, Oxford Brookes University.

957 Burns, I.M., Fowler, M.B., Strachan R.A. & Greenwood, P.B. 2004. Geochemistry, 958 petrogenesis and structural setting of the meta-igneous Strathy Complex: a unique 959 basement block within the Scottish Caledonides? Geological Magazine, 141, 209-223, 960 doi: 10.1017/S0016756804009070.

961 Butler, R.W.H. 2004. The nature of ‘roof thrusts” in the Moine Thrust Belt, NW Scotland: 962 implications for the structural evolution of thrust belts. Journal of the Geological Society, 963 London, 161, 849-859, doi: 10.1144/0016-764903-131.

964 Butler, R.W.H. 2010. The role of thrust tectonic models in understanding structural 965 evolution in NW Scotland. In: Law, R.D., Butler, R.W.H., Holdsworth, R.E., Krabbendam, 966 M. & Strachan, R.A. (eds) Continental Tectonics and Mountain Building – The Legacy of 967 Peach and Horne. Geological Society, London, Special Publications, 335, 293-320, doi: 968 10.1144/SP335.14.

969 Butler, R.W.H. & Torvela, T. 2018. The competition between rates of deformation and 970 solidification in syn-kinematic granitic intrusions: Resolving the pegmatite paradox. 971 Journal of Structural Geology, 117, 1-13, doi: 10.1016/j.jsg.2018.08.013.

972 Carswell, D.A., Tucker, R.D., O’Brien, P.J. & Krogh, T.E. 2003. Coesite micro-inclusions 973 and the U/Pb age of zircons from the Hareidland eclogite in the Western Gneiss Region 974 of Norway. Lithos, 67, 181-190, doi: 10.1016/S0024-4937(03)00014-8.

975 Cawood, P.A., Dunning, G.R., Lux, D. & van Gool, J.A.M. 1994. Timing of peak 976 metamorphism and deformation along the margin of Laurentia in Newfoundland: 977 Silurian not Ordovician. Geology, 22, 399-402, doi: 10.1130/0091- 978 7613(1994)022<0399:TOPMAD>2.3.CO;2.

979 Cawood, P.A., Strachan, R.A., Cutts, K.A., Kinny, P.D., Hand, M. & Pisarevsky, S. 2010. 980 Neoproterozoic orogeny along the margin of Rodinia: Valhalla orogen, North Atlantic. 981 Geology, 38, 99-102, doi: 10.1130/G30450.1.

982 Cawood, P.A., Strachan, R.A., Merle, R.E., Millar, I.L., Loewy, S.L., Dalziel, I.W., Kinny, 983 P.D., Jourdan, F., Memchin, A.A. & Connelly, J.N. 2015. Neoproterozoic to early 984 Palaeozoic extensional and contractional history of East Laurentian margin sequences. 985 the Moine Supergroup, Scottish Caledonides. Geological Society of America Bulletin, 986 127, 349-371, doi: 10.1130/B31068.1.

987 Cocks, L.R.M. & Torsvik, T.H. 2002. Earth geography from 500 to 400 million years ago: a 988 faunal and palaeomagnetic review. Journal of the Geological Society, London, 159, 631- 989 644, doi: 10.1144/0016-764901-118.

990 Corfu, F. & Hartz, E. 2011. U-Pb geochronology in Liverpool Land and Canning Land, East 991 Greenland – the complex record of a polyphase Caledonian orogeny. Canadian Journal 992 of Earth Sciences, 48, 473-494, doi: 10.1139/E10-066.

993 Corfu, F., Andersen, T.B. & Gasser, D. 2014. The Scandinavian Caledonides: main 994 features, conceptual advances and critical questions. In: Corfu, F., Gasser, D. & Chew, 995 D.M. (eds) New Perspectives on the Caledonides of Scandinavia and Related Areas. 996 Geological Society, London, Special Publications, 390, 9-43, doi: 997 dx.doi.org/10.1144/SP390.15.

998 Coward, M.P. 1990. The Precambrian, Caledonian and Variscan framework to NW 999 Europe. In: Hardman, R.F.P. & Brooks, J. (eds) Tectonic Events Responsible for Britain’s 1000 Oil and Gas Reserves. Geological Society, London, Special Publications, 55, 1-34, doi: 1001 10.1144/GSL.SP.1990.055.01.01.

1002 Cutts, K.A., Kinny, P.D., Strachan, R.A., Hand, M., Kelsey, D.E., Emery, M., Friend, C.R.L., 1003 & Leslie, A.G. 2010. Three metamorphic events recorded in a single garnet: coupled 1004 phase modelling with in situ LA-ICPMS, and SIMS geochronology from the Moine 1005 Supergroup, NW Scotland. Journal of Metamorphic Geology, 28, 249-267, doi: 1006 10.1111/j.1525-1314.2009.00863.x.

1007 Dallmeyer, R.D., Strachan, R.A. & Henriksen, N. 1994. 40Ar/39Ar mineral age record in 1008 North-East Greenland: implications for tectonic evolution of the North Atlantic 1009 Caledonides. Journal of the Geological Society, London, 151, 615-628, doi: 1010 10.1144/gsjgs.151.4.0615.

1011 Dallmeyer, R.D., Strachan, R.A., Rogers, G., Watt, G.R. & Friend, C.R.L. 2001. Dating 1012 deformation and cooling in the Caledonian thrust nappes of north Sutherland, Scotland: 1013 insights from 40Ar/39Ar and Rb—Sr chronology. Journal of the Geological Society, 1014 London, 158, 501-512, doi: 10.1144/jgs.158.3.501.

1015 Dewey, J.F. & Shackleton, R.M. 1984. A model for the evolution of the Grampian tract in 1016 the early Caledonides and Appalachians. Nature, 312, 115-121, doi: 10.1038/312115a0.

1017 Dewey, J.F. & Ryan, P.D. 1990. The Ordovician evolution of the South Mayo Trough, 1018 western Ireland. Tectonics, 9, 887-903, doi: 10.1029/TC009i004p00887.

1019 Dewey, J.F. & Strachan, R.A. 2003. Changing Silurian-Devonian relative plate motion in 1020 the Caledonides: sinistral transpression to sinistral transtension. Journal of the 1021 Geological Society, London, 160, 219-229, doi: 10.1144/0016-764902-085.

1022 Dewey, J.F., Dalziel, I.W.D., Reavy, R.J. & Strachan, R.A. 2015. The Neoproterozoic to 1023 Mid-Devonian evolution of Scotland: a review and unresolved issues. Scottish Journal of 1024 Geology, 51, 5-30, doi: 10.1144/sjg2014-007

1025 Dunk, M., Strachan, R.A., Cutts, K.A., Lasalle, S., Storey, C.D., Burns, I.M., Whitehouse, 1026 M.J., Fowler, M. & Pereira, I. 2019. Evidence for a Late Cambrian juvenile arc and a 1027 buried suture within the Laurentian Caledonides of Scotland: comparisons with the 1028 Appalachians. Geology, 47, 734-738, doi: https:/doi.org/10.1130/G46180.1.

1029 Elliott, D. & Johnson, M.R.W. 1980. Structural evolution in the northern part of the 1030 Moine thrust belt, NW Scotland. Transactions of the Royal Society of Edinburgh: Earth 1031 Sciences, 71, 69-96, doi: 10.1017/S0263593300013523.

1032 Fowler, M.B., Kocks, H., Derbyshire, D.P.F. & Greenwood, P.B. 2008. Petrogenesis of high 1033 Ba-Sr plutons from the Northern Highland Terrane of the British Caledonian Province. 1034 Lithos, 100, 129-148, doi: 10.1016/j.lithos.2008.03.003.

1035 Freeman, S.R., Butler, R.W.H., Cliff, R.A. & Rex, D.C. 1998. Dating mylonite evolution: an 1036 Rb—Sr and K—Ar study of the Moine mylonites, NW Scotland. Journal of the Geological 1037 Society, London, 155, 745-758, doi: 10.1144/gsjgs.155.5.0745.

1038 Friend, C.R.L., Jones, K.A. & Burns, I.M. 2000. New high-pressure granulite facies event 1039 in the Moine Supergroup, northern Scotland: implications for Taconic (early Caledonian) 1040 crustal evolution. Geology, 28, 543-546, doi: 10.1130/0091- 1041 7613(2000)28<543:NHGEIT>2.0.CO;2.

1042 Friend, C.R.L., Strachan, R.A. & Kinny, P.D. 2008. U-Pb zircon dating of basement inliers 1043 within the Moine Supergroup, Scottish Caledonides: implications of Archaean protolith 1044 ages. Journal of the Geological Society, London, 165, 807-815, doi: 10.1144/0016- 1045 76492007-125.

1046 Gilotti, J.A., Nutman, A.P. & Breuckner, H.K. 2004. Devonian to Carboniferous collision in 1047 the Greenland Caledonides: U-Pb zircon and Sm-Nd ages of high-pressure and ultrahigh-

1048 pressure metamorphism. Contributions to Mineralogy and Petrology, 148, 216-235, doi: 1049 10.1007/s00410-004-0600-4.

1050 Gilotti, J.A. & McLelland, W.C. 2007. Characteristics of, and a Tectonic Model for, 1051 Ultrahigh-Pressure Metamorphism in the Overriding Plate of the Caledonian Orogen. 1052 International Geology Review, 49, 777-797, doi: 10.2747/0020-6814.49.9.777.

1053 Glazner, A.F. 1991. Plutonism, oblique subduction and continental growth: An example 1054 from the Mesozoic of California. Geology, 19, 784-786, doi: 10.1130/0091- 1055 7613(1991)019<0784:POSACG>2.3.CO;2.

1056 Goodenough, K.M., Millar, I., Strachan, R.A., Krabbendam, M. & Evans, J.A. 2011. Timing 1057 of regional deformation and development of the Moine Thrust Zone in the Scottish 1058 Caledonides: constraints from the U-Pb geochronology of alkaline intrusions. Journal of 1059 the Geological Society, London, 168, 99-113, doi: 10.1144/0016-76492010-020.

1060 Grimmer, J.C., Glodny, J. Drüppel, K., Greiling, R.O. & Kontny, A. 2015. Early- to mid- 1061 Silurian extrusion wedge tectonics in the central Scandinavian Caledonides. Geology, 43, 1062 347-350. doi: 10.1130/G36433.1.

1063 Halliday, A.N., Aftalion, M., Parsons, I., Dickin, A.P. & Johnson, M.R.W. 1987. Syn- 1064 orogenic alkaline magmatism and its relationship to the Moine Thrust Zone and the 1065 thermal state of the lithosphere in Northwest Scotland. Journal of the Geological 1066 Society, London, 144, 611-617, doi: 10.1144/gsjgs.144.4.0611.

1067 Holdsworth, R.E. 1989. The geology and structural evolution of a Caledonian fold and 1068 ductile thrust zone, Kyle of Tongue region, Sutherland, northern Scotland. Journal of the 1069 Geological Society, London, 146, 809-823, doi: 10.1144/gsjgs.146.5.0809.

1070 Holdsworth, R.E. 1990. Progressive deformation structures associated with ductile 1071 thrusts in the Moine Nappe, Sutherland, N. Scotland. Journal of Structural Geology, 12, 1072 443-452, doi: 10.1016/0191-8141(90)90033-U.

1073 Holdsworth, R.E. & Strachan, R.A. 1988. The structural age and possible origin of the 1074 Vagastie Bridge granite and associated intrusions, central Sutherland. Geological 1075 Magazine, 125, 613-620, doi: 10.1017/S0016756800023426.

1076 Holdsworth, R.E. & Grant, C.J. 1990. Convergence-related ‘dynamic spreading’ in a mid- 1077 crustal ductile thrust zone: a possible orogenic wedge model. In: Knipe, R.J. & Rutter, 1078 E.H. (eds) Deformation Mechanisms, Rheology and Tectonics. Geological Society, 1079 London, Special Publications, 54, 491-500, doi: 10.1144/GSL.SP.1990.054.01.45.

1080 Holdsworth, R.E. & Strachan, R.A. 1991. Interlinked system of ductile strike-slip and 1081 thrusting formed by Caledonian sinistral transpression in northeastern Greenland. 1082 Geology, 19, 510-513, doi: 10.1130/0091-7613(1991)019<0510:ISODSS>2.3.CO;2.

1083 Holdsworth, R.E., Strachan, R.A. & Harris, A.L. 1994. Precambrian rocks in northern 1084 Scotland east of the Moine Thrust: the Moine Supergroup. In: Gibbons, W. & Harris, A.L. 1085 (eds) A Revised Correlation of the Precambrian Rocks in the British Isles. Geological 1086 Society, London, Special Report, 22, 23-32, doi: 10.1144/SR22.3.

1087 Holdsworth, R.E., Strachan, R.A. & Alsop, G.I. 2001. Geology of the Tongue District. 1088 Memoir of the British Geological Survey, HMSO, ISBN-10: 0118845489.

1089 Holdsworth, R.E., Strachan, R.A., Alsop, G.I., Grant, C.J. & Wilson, R. 2006. Thrust 1090 sequences and the significance of low-angle, out-of-sequence faults in the northernmost 1091 Moine Nappe and Moine Thrust Zone, NW Scotland. Journal of the Geological Society, 1092 London, 163, 801-894, doi: 10.1144/0016-76492005-076.

1093 Holdsworth, R.E., Alsop, G.I. & Strachan, R.A. 2007. Tectonic stratigraphy and structural 1094 continuity of the northernmost Moine Thrust Zone and Moine Nappe, Scottish 1095 Caledonides. In: Ries, A., Butler, R.W.H. & Graham, R.H. (eds) Global Tectonic Processes: 1096 The Legacy of Mike Coward. Geological Society, London, Special Publications, 272, 124- 1097 144, doi: 10.1144/GSL.SP.2007.272.01.08.

1098 Holdsworth, R.E., Dempsey, E., Selby, D., Darling, J.R., Feely, M., Costanzo, A., Strachan, 1099 R.A., Waters, P. & Finlay, A.J. 2015. Silurian to Devonian magmatism, molybdenite 1100 mineralisation, regional exhumation and brittle strike-slip deformation along the Loch 1101 Shin Line, NW Scotland. Journal of the Geological Society, London, 172, 748-762, doi: 1102 10.1144/jgs2015-058.

1103 Hutton, D.H.W. & Reavy, R.J. 1992. Strike-slip tectonics and granite petrogenesis: 1104 Tectonics, 11, 960-967, doi: 10.1029/92TC00336.

1105 Jacques, J.M. & Reavy, R.J. 1994. Caledonian plutonism and major lineaments in the SW 1106 Scottish Highlands. Journal of the Geological Society, London, 151, 955-969, doi: 1107 10.1144/gsjgs.151.6.0955.

1108 Johnson, M.R.W. & Strachan, R.A. 2006. A discussion of possible heat sources during 1109 nappe stacking: the origin of Barrovian metamorphism within the Caledonian thrust 1110 sheets of NW Scotland. Journal of the Geological Society, London, 163, 579-582, doi: 1111 10.1144/0016-764920-168.

1112 Kalsbeek, F., Jepsen, H.F. & Nutman, A. P. 2001. From source migmatites to plutons: 1113 tracking the origin of the ca. 435 Ma S-type granites in the East Greenland Caledonian 1114 orogen. Lithos, 57, 91-109, doi: 10.1016/S0024-4937(00)00071-2.

1115 Kinny, P.D., Friend, C.R.L., Strachan, R.A., Watt, G.R. & Burns, I.M. 1999. U—Pb 1116 geochronology of regional migmatites, East Sutherland, Scotland: evidence for crustal 1117 melting during the Caledonian orogeny. Journal of the Geological Society, London, 156, 1118 1143-1152, doi: 10.1144/gsjgs.156.6.1143.

1119 Kinny, P.D., Strachan, R.A., Rogers, G.R., Friend, C.R.L., Kocks, H. & Paterson, B.A. 2003a. 1120 U—Pb geochronology of deformed meta-granites in central Sutherland, Scotland: 1121 evidence for widespread Silurian metamorphism and ductile deformation of the Moine 1122 Supergroup during the Caledonian orogeny. Journal of the Geological Society, London, 1123 160, 259-269, doi: 10.1144/0016-764901-087.

1124 Kinny, P.D., Strachan, R.A., Kocks, H. & Friend, C.R.L. 2003b. U-Pb geochronology of late 1125 Neoproterozoic augen granites in the Moine Supergroup, NW Scotland: dating of rift- 1126 related, felsic magmatism during supercontinent break-up? Journal of the Geological 1127 Society, London, 160, 925-934, doi: 10.1144/0016-764902-148.

1128 Kocks, H., Strachan, R.A. & Evans, J.A. 2006. Heterogeneous reworking of Grampian 1129 metamorphic complexes during Scandian thrusting in the Scottish Caledonides: insights 1130 from the structural setting and U-Pb geochronology of the Strath Halladale Granite. 1131 Journal of the Geological Society, London, 163, 525-538, doi: 10.1144/0016-764905-008.

1132 Kocks, H., Strachan, R.A., Evans, J.A. & Fowler, M.B. 2014. Contrasting magma 1133 emplacement mechanisms within the Rogart igneous complex, NW Scotland, record the 1134 switch from regional contraction to strike-slip during the Caledonian orogeny. 1135 Geological Magazine, 151, 899-915, doi: 10.1017/S0016756813000940.

1136 Lancaster, P.J., Strachan, R.A., Bullen, D., Fowler, M., Jaramillo, M. & Saldarriaga, A. 1137 2017. A. U-Pb zircon geochronology and geodynamic significance of the ‘Newer Granite’ 1138 plutons in Shetland, northernmost Scottish Caledonides. Journal of the Geological 1139 Society, London, 174, 486-497, doi: 10.1144/jgs2016-106.

1140 Law, R.D & Johnson, M.R.W. 2010. Microstructures and crystal fabrics of the Moine 1141 thrust zone and Moine nappe: history of research and changing tectonic interpretations. 1142 In: Law, R.D., Butler, R.W.H., Holdsworth, R.E., Krabbendam, M. & Strachan, R.A. (eds) 1143 Continental Tectonics and Mountain Building – The Legacy of Peach and Horne. 1144 Geological Society, London, Special Publications, 335, 443-503, doi: 10.1144/SP335.21.

1145 Leggett, J. K., McKerrow, W.S. & Eales, M.H. 1979. The Southern Uplands of Scotland; a 1146 Lower Paleozoic accretionary prism. Journal of the Geological Society, London, 136, 755- 1147 770, doi: 10.1144/gsjgs.136.6.0755.

1148 Leslie, A.G., Krabbendam, M., Kimbell, G.S. & Strachan, R.A. 2010. The Oykell Transverse 1149 Zone: linking mullions, regional gravity and large-scale lateral variations in ductile thrust

1150 architecture in the Moine Nappe, Northern Highlands, Scotland. In: Law, R.D., Butler, 1151 R.W.H., Holdsworth, R.E., Krabbendam, M. & Strachan, R.A. (eds) Continental Tectonics 1152 and Mountain Building – The Legacy of Peach and Horne. Geological Society, London, 1153 Special Publications, 335, 359-381, doi: 10.1144/SP335.17.

1154 Lin, S., Brem, A.G., van Staal, C.R., Davis, D.W., McNicoll, V.J. & Pehrsson, S. 2013. The 1155 Corner Brook Lake block in the Newfoundland Appalachians: A suspect terrane along the 1156 Laurentian margin and evidence for large-scale orogen-parallel motion. Geological 1157 Society of America Bulletin, 125, 1618-1632, doi: 10.1130/B30805.1.

1158 Mako, C.A., Law, R.D., Caddick, M.J., Thigpen, J.R., Ashley, K.T., Cottle, J. & Kylander- 1159 Clark, A. 2019. Thermal evolution of the Scandian hinterland, Naver nappe, northern 1160 Scotland. Journal of the Geological Society, London, 176, 669-688, doi: 1161 10.1144/jgs.2018-224.

1162 Mazza, S.E., Mako, C., Law, R.D., Caddick, M.J., Krabbendam, M. & Cottle, J. 2018. 1163 Thermobarometry of the Moine and Sgurr Beag thrust sheets, northern Scotland. 1164 Journal of Structural Geology, 113, 10-32, doi: 10.1016/j.jsg.2018.05.002.

1165 McClelland, W.C. & Gilotti, J.A. 2003. Late stage extensional exhumation of high- 1166 pressure granulites in the Greenland Caledonides. Geology, 31, 259-262, doi: 1167 10.1130/0091-7613(2003)031<0259:LSEEOH>2.0.CO;2.

1168 McCourt, W.J. 1980. The Geology of the Strath Halladale – Altnabreac District: Report of 1169 the Institute of Geological Sciences, Environmental Protection Unit, 80-1, ASIN: 1170 B0018040IO.

1171 Mendum, J.R. & Noble, S.R. 2010. Mid-Devonian sinistral transcurrent movements on 1172 the Great Glen Fault: the rise of the Rosemarkie Inlier and the Acadian Event in 1173 Scotland. In: Law, R.D., Butler, R.W.H., Holdsworth, R.E., Krabbendam, M. & Strachan, 1174 R.A. (eds) Continental Tectonics and Mountain Building – The Legacy of Peach and

1175 Horne. Geological Society, London, Special Publications, 335, 161-187, doi: 1176 10.1144/SP335.8.

1177 Miles, A.J., Woodcock, N.H. & Hawkesworth, C.J. 2016. Tectonic controls on post- 1178 subduction granite genesis and emplacement: the late Caledonian suite of Britain and 1179 Ireland. Gondwana Research, 39, 250-260, doi: 10.1016/j.gr.2016.02.006.

1180 Moorhouse, V.E. & Moorhouse, S.J. 1983. The geology and geochemistry of the Strathy 1181 complex of north-east Sutherland, Scotland. Mineralogical Magazine, 47, 123-137, ISSN: 1182 0026-461X.

1183 Moorhouse, S.J. & Moorhouse, V.E. 1988. The Moine Assemblage in Sutherland. In: 1184 Winchester, J.A. (ed) Later Proterozoic Stratigraphy in the Northern Atlantic Regions. 1185 Blackie, Glasgow, p. 54-73, ISBN: 978-1-4615-7344-9.

1186 Morley, C.K. 1988. Out-of-Sequence Thrusts. Tectonics, 7, 539-561, doi: 1187 10.1029/TC007i003p00539.

1188 Oldow, J.S., Bally, A.W. & Lallemant, H. 1990. Transpression, orogenic float, and 1189 lithospheric balance. Geology, 18, 991-994, doi: 10.1130/0091- 1190 7613(1990)018<0991:TOFALB>2.3.CO;2.

1191 Oliver, G.J.H., Wilde, S.A. & Wan, Y. 2008. Geochronology and geodynamics of Scottish 1192 granitoids from the late Neoproterozoic break-up of Rodinia to Palaeozoic collision. 1193 Journal of the Geological Society, London, 165, 661-674, doi: 10.1144/0016-76492007- 1194 105.

1195 Osmundsen, P.T. & Andersen, T.B. 2001. The Middle Devonian basins of western 1196 Norway: sedimentary response to large-scale transtensional tectonics. Tectonophysics, 1197 332, 51-68, doi: 10.1016/S0040-1951(00)00249-3.

1198 Osmundsen, P.T., Braathen, A., Nordgulen, Ø., Roberts, D., Meyer, G.B. & Eide, E. 2003. 1199 The Devonian Nesna shear zone and adjacent gneiss-cored culminations, North-Central

1200 Norwegian Caledonides. Journal of the Geological Society, London, 160, 137-150, doi: 1201 10.1144/0016-764901-173.

1202 Park, R.G., Stewart, A.D. & Wright, D.T. 2002. The Hebridean terrane. In: Trewin, N. (ed) 1203 Geology of Scotland (4th edition): Geological Society, London, p. 45-80, ISBN: 1862391262

1204 9781862391260.

1205 Peach, B.N., Horne, J., Gunn, W., Clough, C.T. & Hinxman, L.W. 1907. The geological 1206 structure of the NW Highlands of Scotland. Memoirs of the Geological Survey of 1207 Scotland, HMSO, Glasgow.

1208 Phillips, F.C. 1937. A Fabric Study of some Moine Schists and Associated Rocks. 1209 Quarterly Journal of the Geological Society, London, 93, 581-620, doi: 1210 10.1144/GSL.JGS.1937.093.01-04.19.

1211 Phillips, E.R., Barron, H.F., Smith, R.A. & Arkley, S. 2004. Composition and provenance of 1212 the Silurian to Devonian sandstone sequences of the southern Midland Valley. Scottish 1213 Journal of Geology, 40, 23-42, doi: 10.1144/sjg40010023.

1214 Potts, G.J., Hunter, R.H., Harris, A.L. & Fraser, F.M. 1995. Late orogenic extensional 1215 tectonics at the NW margin of the Caledonides in Scotland. Journal of the Geological 1216 Society, London, 152, 907-910, doi: 10.1144/GSL.JGS.1995.152.01.04.

1217 Rathbone, P.A. & Harris, A.L. 1979. Basement-cover relationships at Lewisian inliers in 1218 the Moine rocks. In: Harris, A.L., Holland, C.H. & Leake, B.E. (eds) The Caledonides of the 1219 British Isles – Reviewed. Geological Society, London, Special Publications 8, 101-107, doi: 1220 10.1144/GSL.SP.1979.008.01.09.

1221 Read, H.H. 1931. The Geology of Central Sutherland: Memoir of the Geological Survey of 1222 Great Britain, ASIN: B00AOB0Q7C.

1223 Rogers, G. & Dunning, G.R. 1991. Geochronology of appinitic and related granitic 1224 magmatism in the W Highlands of Scotland: constraints on the timing of transcurrent

1225 fault movement. Journal of the Geological Society, London, 148, 17-27, doi: 1226 10.1144/gsjgs.148.1.0017.

1227 Searle, M.P., Law, R.D., Dewey, J.F. & Streule, M.J. 2010. Relationships between the Loch 1228 Ailsh and Borralan alkaline intrusions and thrusting in the Moine Thrust Zone, southern 1229 Assynt culmination, NW Scotland. In: Law, R.D., Butler, R.W.H., Holdsworth, R.E., 1230 Krabbendam, M. & Strachan, R.A. (eds) Continental Tectonics and Mountain Building: 1231 The Legacy of Peach and Horne. Geological Society, London, Special Publications, 335, 1232 383-404, doi: 10.1144/SP335.18.

1233 Smith, S.A.F., Strachan, R.A. & Holdsworth, R.E. Microstructural evolution within a 1234 partitioned midcrustal transpression zone, northeast Greenland Caledonides. Tectonics, 1235 26, TC4003, doi: 10.1029/2006TC001952.

1236 Snyder, D.B. & Flack, C.A. 1990. A Caledonian age for reflectors within the mantle 1237 lithosphere north and west of Scotland. Tectonics, 9, 903-922, doi: 1238 10.1029/TC009i004p00903.

1239 Soper, N.J. & Brown, P.E. 1971. Relationship between metamorphism and migmatisation 1240 in the northern part of the Moine Nappe. Scottish Journal of Geology, 7, 305-325, doi: 1241 10.1144/sjg07040305.

1242 Soper, N.J. & Hutton, D.H.W. 1984. Late Caledonian sinistral displacements in Britain: 1243 Implications for a three-plate model. Tectonics, 3, 781-794, doi: 1244 10.1029/TC003i007p00781.

1245 Soper, N.J. & Woodcock, N.H. 1990. Silurian collision and sediment dispersal patterns in 1246 northern Britain. Geological Magazine, 127, 527-542.

1247 Soper, N.J., Strachan, R.A., Holdsworth, R.E., Gayer, R.A. & Greiling, R.O. 1992. Sinistral 1248 transpression and the Silurian closure of Iapetus. Journal of the Geological Society, 1249 London, 149, 871-880, doi: 10.1144/gsjgs.149.6.0871.

1250 Stewart, M., Strachan, R.A. & Holdsworth, R.E. 1999. Structure and early kinematic 1251 history of the Great Glen Fault Zone, Scotland. Tectonics, 18, 326-342, doi: 1252 10.1029/1998TC900033.

1253 Stewart, M., Strachan, R.A., Martin, M.W. & Holdsworth, R.E. 2001. Dating early sinistral 1254 displacements along the Great Glen Fault Zone, Scotland: structural setting, 1255 emplacement and U—Pb geochronology of the syn-tectonic Clunes Tonalite. Journal of 1256 the Geological Society, London, 158, 821-830, doi: 10.1144/jgs.158.5.821.

1257 Stone, P. & Merriman, R.J. 2004. Basin thermal history favours an accretionary origin for 1258 the Southern Uplands terrane, Scottish Caledonides. Journal of the Geological Society, 1259 London, 161, 829-836, doi: 10.1144/0016-764903-170.

1260 Strachan, R.A. 1988. The metamorphic rocks of the Scaraben area, East Sutherland and 1261 Caithness. Scottish Journal of Geology, 24, 1-13, doi: 10.1144/sjg24010001.

1262 Strachan, R.A. & Holdsworth, R.E. 1988. Basement-cover relationships and structure 1263 within the Moine rocks of central and southeast Sutherland. Journal of the Geological 1264 Society, London, 145, 23-36, doi: 10.1144/gsjgs.145.1.0023.

1265 Strachan, R.A. & Evans, J.A. 2008. Structural setting and U-Pb geochronology of the Glen 1266 Scaddle Metagabbro: evidence for polyphase Scandian ductile deformation in the 1267 Caledonides of northern Scotland. Geological Magazine, 145, 361-371, doi: 1268 10.1017/S0016756808004500.

1269 Strachan, R.A., Smith, M., Harris, A.L. & Fettes, D.J. 2002. The Northern Highland and 1270 Grampian terranes. In: Trewin, N. (ed) Geology of Scotland (4th edition): Geological Society, 1271 London, p. 81-147, ISBN: 1862391262 9781862391260.

1272 Strachan, R.A., Holdsworth, R.E., Krabbendam, M. & Alsop, G.I. 2010. The Moine 1273 Supergroup of NW Scotland: insights into the analysis of polyorogenic supracrustal 1274 sequences. In: Law, R.D., Butler, R.W.H., Holdsworth, R.E., Krabbendam, M. & Strachan,

1275 R.A. (eds) Continental Tectonics and Mountain Building: The Legacy of Peach and Horne. 1276 Geological Society, London, Special Publications, 335, 233-254, doi: 10.1144/SP335.11.

1277 Tanner, P.W.G., Johnstone, G.S., Smith, D.I. & Harris, A.L. 1970. Moinian Stratigraphy 1278 and the Problem of the Central Ross-shire Inliers. Geological Society of America Bulletin, 1279 81, 299-306, doi: 10.1130/0016-7606(1970)81[299:MSATPO]2.0.CO;2.

1280 Thigpen, J.R., Law, R.D., Lloyd, G.E. & Brown, S.J. 2010a. Deformation temperatures, 1281 vorticity of flow, and strain in the Moine thrust zone and Moine nappe: Reassessing the 1282 tectonic evolution of the Scandian foreland-hinterland transition zone. Journal of 1283 Structural Geology, 32, 920-940, doi: 10.1016/j/jsg.2010.05.001.

1284 Thigpen, J.R., Law, R.D., Lloyd, G.E., Brown, S.J. & Cook, B. 2010b. Deformation 1285 temperatures, vorticity of flow and strain symmetry in the Loch Eriboll mylonites, NW 1286 Scotland: implications for the kinematic and structural evolution of the northernmost 1287 Moine Thrust Zone. In: Law, R.D., Bulter, R.W.H., Holdsworth, R.E., Krabbendam, M. & 1288 Strachan, R.A. (eds) Continental Tectonics and Mountain Building: The Legacy of Peach 1289 and Horne. Geological Society, London, Special Publications, 335, 623-662, doi: 1290 10.1144/SP335.26.

1291 Thigpen, J.R., Law, R.D., Loehn, C.L., Strachan, R.A., Tracey, R.J., Lloyd, G.E., Roth, B.L. & 1292 Brown, S.J. 2013. Thermal structure and tectonic evolution of the Scandian orogenic 1293 wedge, Scottish Caledonides: integrating geothermometry, deformation temperatures, 1294 and conceptual kinematic-thermal models. Journal of Metamorphic Geology, 31, 813- 1295 842, doi: 10.1111/jmg.12046.

1296 Tucker, R.D., Robinson, P., Solli, A., Gee, D.G., Thorsnes, T., Krogh, T.E., Nordgulen, Ø., & 1297 Bickford, M.E. 2004. Thrusting and extension in the Scandian hinterland, Norway: New 1298 U-Pb Ages and Tectonostratigraphic Evidence. American Journal of Science, 304, 477- 1299 532, doi: 10.2475/ajs.304.6.477.

1300 Van Breemen, O., Aftalion, M. & Johnson, M.R.W. 1979. Age of the Loch Borrolan 1301 complex, Assynt, and late movements along the Moine Thrust zone. Journal of the 1302 Geological Society, London, 136, 489-496, doi: 10.1144/gsjgs.136.4.0489.

1303 Van Staal, C.R., Dewey, J.F., MacNiocaill, C. & McKerrow, W.S. 1998. The Cambrian- 1304 Silurian tectonic evolution of the northern Appalachians: history of a complex, 1305 southwest Pacific-type segment of Iapetus. In: Blundell, D.J. & Scott, A.C. (eds) Lyell: the 1306 Past is the Key to the Present. Geological Society, London, Special Publications, 143, 1307 199-242, doi: 10.1144/GSL.SP.1998.143.01.17.

1308 Waldron, J.W.F., Schofield, D.I., Murphy, J.B. & Thomas, C.W. 2014. How was the Iapetus 1309 Ocean infected with subduction? Geology, 42, 1095-1098, doi: 10.1130/G36194.1

1310 White, S.E. & Waldron, J.W.F. 2019. Inversion of Taconic extensional structures during 1311 Paleozoic orogenesis in western Newfoundland. In: Wilson, R.W., Houseman, G.A., 1312 McCaffrey, K.J.W., Doré, A.G. & Buiter, S.J.H. (eds) Fifty Years of the Wilson Cycle 1313 Concept in Plate Tectonics. Geological Society, London, Special Publications, 470, 311- 1314 336, doi: 10.1144/SP470.17.

1315 Wilson, R.W., Holdsworth, R.E., Wild, L.E., McCaffrey, K.J.W., England, R.W., Imber, J. & 1316 Strachan, R.A. 2010. Basement-influenced rifting and basin development: a reappraisal 1317 of post-Caledonian faulting patterns in the North Coast Transfer Zone, Scotland. In: Law, 1318 R.D., Butler, R.W.H., Holdsworth, R.E., Krabbendam, M. & Strachan, R.A. (eds) 1319 Continental Tectonics and Mountain Building: The Legacy of Peach and Horne. 1320 Geological Society, London, Special Publications, 335, 795-826, doi: 10.1144/SP335.32.

1321 Woolley, A.R. 1970. The structural relationships of the Loch Borrolan complex, Scotland. 1322 Geological Journal, 7, 171-182, doi: 10.1002/gj.3350070110.

1323 Wu, Q., Ramezani, J., Zhang, H., Wang, T. T., Yuan, D. X., Mu, L., Zhang, Y. C., Li, X. H. & 1324 Shen, S. Z. 2017. Calibrating the Guadalupian Series (Middle Permian) of South China.

1325 Palaeogeography Palaeoclimatology Palaeoecology, 466, 361-372, doi: 1326 10.1016/j.palaeo.2016.11.011.

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1329

1330 1331 1332 1333 1334

Laurentia Baltica Deformed margin Deformed margin Peri-Laurentian Peri-Baltic Terranes Terranes G n GONDWANA e IA og L T or Peri-Gondwanan AUREN n terranes Nova Scotia dia Scotland can Ganderia NHT S Avalonia Ireland GGF HBF A Megumia New- N IC T Carolinia foundland L Peri-Gondwanan arcs A B Map area

SKT N

Moine thrust 0 5 km

Sole Moine Nappe thrust MT Faraid Head

errane PMT T BHT Great Glen Fault LRT Tongue Sgurr Beag Nappe

50km TT Kyle of ST Loch Eriboll T Grampian A Fig. 5 GSM Thrust LRT Fault MT ROM BHT NT ST Fig. 4

SoT Strath Halladale AT LL

SHG SNG

NT SHG

BBSZ

NT CSG VBG

KG SoT

Assynt SKT

MT ACT HG

BHT ST LA LB 58° N RIC CM LS 5° W 4° W G

Post-Caledonian Rocks Strathy Complex Cambrian Granite Skinsdale Nappe Diorite Silurian Swordly Nappe Neoproterozoic Intrusions Moine Syenite } Naver Nappe Supergroup Moine Nappe } Thrust Fault Basement Archaean Moine Thrust Zone Zone Foreland 00 50 00

N Loch Eriboll

Durness

MT Tongue 0 kilometres 20 BHT ST TT 50 SWT 50

BHT SKT

ST NT A T HF

MT AT Lairg

00 Ullapool 00

Dornoch

SBT

SBT SBT Dingwall LMF

SBT

GGF 50 50

Thrusts: Inverness GU SBT Sgurr Beag thrust SCF SBT SBT SKT Skinsdale thrust SWT Swordly thrust

MT TT Torrisdale thrust NT Naver thrust Strike slip faults: AT Achness thrust LMF Loch Maree fault BHT Ben Hope thrust SCF Strathconon fault MT Moine thrust SBT lineation GGF Great Glen fault ST Sole thrust

00 50 00 70 Sub-area 1 Sub-area 2 Sub-area 3 75 80 N 50 50 12 37 8 8 0 1 2km 10 21 33 283 AB 20 42 Spot height River Naver 9 17 25 14 18 8 15 50 25 75 22 12 12 19 48 15 32 28 13 21 13 22 16 30 38 18 12 30 22 15 40 25 B871 10 20 15 30 23 42 48 21 16 20 18 65 20 Loch 26 26 18 26 17 15 10 17 35 15 20 10 Strathy 22 42 20 17 15 20 32 28 62 28 5 32 22 25 27 42 67 26 30 12 26 20 15 22 55 12 20 20 20 10 15 23 30 50 30 12 53 25 25 12 60 25 29 30 12 20 12 35 30 40 52 20 17 15 33 26 25 16 10 30 20 SNG 22 37 72 30 30 45 45 24 20 20 48 42 20 20 20 57 25 33 20 42 22 40 20 76 t 22 s 70 52 20 u 20 40 20 r 32 30 35 h 12 T 15 20 15 44 20 y 32 l 35 24 22 d 38 r 40 25 25 o 30 20 30 40 17 w 40 S 33 38 22 18 Loch 12 42 15 15 42 18 25 Cròcach 26 15 20 23 23 26 32 48 37 35 50 60 22 25 26 25 23 20 24 20 10 18 40 48 36 Rhifall 30 20 32 Loch 68 27 48 22 50 25 40 40 36 33 40 50 24 26 37 45 20 60 30 27 23 40 18 32 403 20 30 B871 55 28 18 18 22 30 20 75 60 Loch Druim 44 68 80 a’Chliabhain 28 37 Loch Coire 45 nam Mang 9 32 38 84 70 40 40 75 48 80 70 42 40

Sub-area 1 N Sub-area 2 N Sub-area 3 N Devonian

Strathnaver Granite

Leucogranite

Pelitic Gneiss

Interbanded Psammitic & Semi-pelitic Gneiss

Semi-pelitic Gneiss Moine

Swordly Thrust Sn + L2 Sn + L2 Sn + L1S N N Interbanded Psammitic & Semi-pelitic Gneiss }

Sn (foliation) Archean Basement

Thrust L1S or L2 Normal Fault

Major F folds with F3 Axial Planes 3 direction of closure

Foliation F3 Axes L1S lineation

L 2 lineation

F3 AP + axes F3 AP + axes Naver nappe SW of TSB Torrisdale Steep Belt Swordly nappe (Kirtomy Point to Port Mor) d Sn(•) + L2(▲) e Sn(S1-S4)(•) + L4(▲) f F2+F3 axes(■) + AP(□) g Sn(S1-S4)(•) + L4(▲) h F1axes(◆) + AP(◇)

Moine nappe SW of TSB 65 70 75 80 Strathy Complex Devonian c F3 axes(■) + AP(□) i Sn(•) + L1(▲) Clerkhill Intrusion } F4 folds Strathy Complex N L4

70 Psammitic & semi-pelitic gneiss L2 70 Swordly Strathy nappe L1 10 Point

Semi-pelitic gneiss } Moine Supergroup Foliation 82 Swordly Thrust (ST) 46 41 5 8 40 Psammitic & semi-pelitic gneiss 0 2 km 6 72 60 Torrisdale Thrust (TT) Naver X nappe 16 36 1 Psammitic gneiss } 11 15 T 37 B 30X Naver Thrust (NT) 24 Moine Port Mor 32 5 Psammite X } nappe } Kirtomy 85 13 7 70 5 61 Point 77 78 83 63 X 11 51 50 7 Archean basement 81 75 81 16 59 74 45 F axes(■) + AP(□) 80 53 F1axes(◆) + AP(◇) b 2 65 24 70 77 82 34 65 j 31 6 8 67 12 75 52 84 Neave Island 3 2 75 6910 X 80 68 62 79 X 68 82 66 81 30 61 67 80 70 85 60 14 41 52 46 11 60 69 80 22 69 65 Farr 14 7 80 68 12 76 17 75 65 29 Torrisdale 82 ST 60 14 24 5 11 Bay 6 56 12 79 85 50 72 82 Bay 7 12 33 80 76 X 19 22 19 40 83 10 75 75 10 66 65 20 15 76 82 80 Approximate 20 10 14 75 20 4 70 western limit 35 60 8 18 14 64 62 67 坾 20 74 77 71 12 10 10 40 of the TSB 12 T 20 72 67 82 38 68 T 11 40 70 80 47

28 72 20 22 73 8 86 85 22 T 20 72 20 17 70 40 61 70 75 3 16 60 23 72 A NT 10 58 15 20 8 14 72 40 42 Loch 80 78 68 7 67 47 48 60 80 45 39 31 19 12 Mor 32 12 Loch 12 20 5 2 60 70 10 Eileanach 51 18 15 12 40 85 PMT 60 60 10 56 63 22 36 21 48 56 23 15 17 38 26 20 50 12 13 19 31 23 16 85 30 5 44 27 25 25 47 21 14 55 40 15 25 22 FBI 70 16 13 32 12 10 65 22 Loch 15 Loch 28 12 34 46 6 20 22 MeadieST 12 14 80 Buidhe Sn(•) + L2(▲) T 30 36 70 Mor F4axes(◆) + AP(◇) a 30 35 k T 32 40 30 8

29 21 NT 27 21 42 Approximate 32 45 10 82 20 45 43 eastern limit 35 22 37 30 22 12 of the TSB 16 46 60 47 60 26 38 25

X A PMT B

X X X Sea level

X X X

X ST eastern Inferred NT TT limit of TSB suture a) b) L2 wrapped Elongate garnets with N around F3 fold S straight inclusion trails

F3 fold hinge

D4 dextral shear bands

Folding of gneissose fabric ~ 20 cm ~ 25 cm c) N S d) SW NE

Aird Torrisdale Dextrally sheared lozenges of granitic gneiss Steeply-dipping foliation in gneisses

~ 30 cm Pink pegmatite sheets intruded ~ 50 m in to Torrisdale Steep Belt F synform N 4

F4 antiform

L Likely oﬀshore location 4 of Naver Thrust L2 Thrust 5 km Fault

Inferred ductile lateral ramp

Foliation-parallel 'bookshelf-sliding’

SC TSB

PMT

ST NT 432 0.0695 433.2 0.0692 RS14-18 RS14-19 431 Klibreck Sill Vagastie Bridge 433.0 z6 0.0690 430 Granite 432.8 z3

U 432.6 429 z2 0.0688 z5 0.0694 z1 238 750 z4 432.4 0.12 428 700 z4

Pb/ 0.0686 432.2 650 0.10 206 427 z2 600 432.0 206 238 0.0684 z3 550 0.0693 Pb/ U weighted mean age: 500 area of 431.8 432.35 Ma ± 0.10/0.21/0.51 426 z1 0.08 main figure MSWD = 2.0, n = 4 450 z7 0.0682 431.6 425 0.6 0.8 1.0 1.2 0.510 0.515 0.520 0.525 0.530 0.535 0.540 0.529 0.530 0.531 0.532 0.533 0.534

428.5 RS14-20 434 RS14-26 0.0696 z5 Creag Mhor Quartz 428.0 Creag nan 433 z6 0.0686 Monzodiorite 0.0694 Suibheag 427.5 Granite 432 U 427.0 z2 0.0692 z1 238 431 z6 0.0684 426.5 0.074 460

Pb/ 0.0690 430 z5 426.0 450 0.072 z2 206 425.5 z4 0.0688 429 440 0.0682 0.070 z3 area of 425.0 430 main figure 428 0.0686 0.068 424.5 0.52 0.54 0.56 0.58 0.60 427 0.520 0.525 0.530 0.535 0.540 0.545 0.550 0.518 0.520 0.522 0.524 0.526 207Pb/ 235 U 207Pb/ 235 U Ma 410

412 VBG 414

416 KS 418

420

422

424 CMQM 426 CNSG 428 KS 430

432 Assynt syenites VBG 434 G 435-420 Ma: Salinic event, folding and metamorphism up to kyanite grade. 435-415 Ma: Regional ductile thrusting and folding, 435-390 Ma: 'Hard' metamorphism up to 730C collision, regional nappe and 9 kbar. stacking and metamorphism ADF to eclogite facies.

CF MT GGF RIL DBL N DF HBF SUF No SL

MSL End Silurian: sinistrally 430-420 Ma: 'Soft' collision across the transpressive folding and low- Solway Line, late NW-vergent folds in grade cleavage development Southern Uplands accretionary prism, in South Mayo Trough. metamorphism at sub-greenschist facies.