2. Laurentian foreland sequence

The Hebridean terrane west of the Moine thrust zone comprises the stable foreland of the Laurentian plate and is continuous across to Greenland and North America. The foreland in NW Scotland comprises Archaean (Scourian complex) and Palaeoproterozoic (Laxfordian complex) basement gneisses of the Lewisian gneiss complex, separated in space and time by a major swarm of intrusive basic dykes, the Scourie dykes (Sutton & Watson, Reference Sutton and Watson1951; Park & Tarney, Reference Park, Tarney, Park and Tarney1987; Park et al. Reference Park, Stewart, Wright and Trewin2002; Wheeler et al. Reference Wheeler, Park, Rollinson, Beach, Law, Butler, Holdsworth, Krabbendam and Strachan2010). Grey tonalitic gneisses, with trondhjemites and granodiorites (tonalite–trondhjemite–granodiorite suite), mafic gneisses and ultramafic enclaves show a dominant granulite-facies metamorphism during Scourian metamorphism, exposed in the central part of the Lewisian of NW Scotland. P–T conditions range from 8 to 12 kbar and 800 to 1000 °C, and timing constraints are ∼3.0–2.7 Ga (Cartwright & Barnicoat, Reference Cartwright, Barnicoat, Park and Tarney1987; Whitehouse, Reference Whitehouse1988, Reference Whitehouse1989; Corfu et al. Reference Corfu, Heaman and Rogers1994). A major phase of basic dyke intrusion (Scourie dykes) with compositions ranging from orthopyroxene-bearing picrite, norite and olivine gabbro to quartz dolerite, dated at 2.4–2.0 Ga, generally striking WNW–ESE, intrudes the Archaean granulites. To the north and south of the Scourian terrane, the Archaean gneisses and the Scourie dykes have been overprinted by a Palaeoproterozoic amphibolite-facies metamorphism during the Laxfordian event at 1.8–1.75 Ga (Kinny & Friend, Reference Kinny and Friend1997; Corfu et al. Reference Corfu, Crane, Moder and Rogers1998). Characteristic K-feldspar-rich granite–pegmatite sheets dated at 1.85–1.7 Ga cut the Laxfordian gneisses (Friend & Kinny, Reference Friend and Kinny2001). Major sub-vertical ductile shear zones bound the central Scourian granulite terrane to the north (Laxford shear zone) and south (Canisp shear zone). These shear zones are not terrane boundaries or suture zones separating different plates, but instead represent either metamorphic/fluid infiltration fronts or different crustal levels (lower crust Scourian granulites; middle crust Laxfordian amphibolites) during progressive collision-related Palaeoproterozoic Laxfordian orogenesis.

The Lewisian basement is overlain unconformably by a thick sequence of fluvial, lacustrine and red-bed continental conglomerates, sandstones and siltstones, mainly derived from erosion of high mountain topography dominantly in the west. These include the Stoer and Sleat Groups, which fill in a highly irregular Lewisian topography. Both are unconformably overlain by the Torridon Group, which is up to 6 km thick (Stewart, Reference Stewart2002) and is well exposed on many of the high peaks of NW Scotland, including Suilven, Canisp and Quinag. A series of Precambrian E-dipping normal faults bound several half-grabens. The Torridonian sandstones were either deposited in these half-grabens, or more likely, the faulting occurred after Torridonian sedimentation and prior to the overlying Cambrian (Bonsor et al. Reference Bonsor, Strachan, Prave and Krabbendam2012). These Precambrian normal faults have up to 5 km of throw, with the unconformably overlying Cambrian basal quartzites deposited above thick Torridonian in the half-grabens, and in places deposited directly on Lewisian basement gneisses (Searle et al. Reference Searle, Cornish, Heard, Charles and Branch2019).

A regional unconformity marks the base of the Cambrian quartzites (Eriboll Formation sandstones and quartzites), which are interpreted as marking shallow marine deposition on the stable passive continental margin of Laurentia during the spreading of the Iapetus ocean (Woodcock & Strachan, Reference Woodcock and Strachan2012). The 150–200 m thick quartzites show extensive cross-bedding indicating easterly flow directions, and contain characteristic trace fossils, Skolithos ‘pipes’ or worm burrows. The quartzites are overlain by thin pale brown siltstones (Fucoid beds) and sandstones (Salterella grits), both showing abundant trace fossils. A major regional unconformity above the Cambrian strata marks the base of the Early Ordovician Durness Group limestone–dolomite marine transgression. The ending of shallow marine sedimentation in the Durness Group is coincident with the beginning of ophiolite obduction and accretion of the Midland Valley arc terrane, and subsequent collision-related uplift, crustal thickening and metamorphism in the Moine and Dalradian hinterland (Fig. 4).

3. Moine thrust zone

The Moine thrust zone in the Assynt region comprises at least four major W-vergent thrust sheets, which, from hinterland to foreland include the earliest and deepest Moine thrust, further down-section to the Ben More and Glencoul thrusts, and finally to the Sole thrust (Elliott & Johnson, Reference Elliott and Johnson1980; Butler, Reference Butler, Law, Butler, Holdsworth, Krabbendam and Strachan2010; Searle et al. Reference Searle, Law, Dewey, Streule, Law, Butler, Holdsworth, Krabbendam and Strachan2010 a). All rock units of the foreland occur within the Moine thrust zone including, in the Assynt area, a suite of highly alkaline igneous intrusions ranging from alkaline ultramafic rocks through pseudoleucite (nepheline + muscovite + K-feldspar) + melanite garnet borralanite, to nordmarkite and quartz syenite (Parsons, Reference Parsons1965, Reference Parsons1972; Goodenough et al. Reference Goodenough, Young and Parsons2004, Reference Goodenough, Evans and Krabbendam2006; Searle et al. Reference Searle, Law, Dewey, Streule, Law, Butler, Holdsworth, Krabbendam and Strachan2010 a). The Loch Borralan and Loch Ailsh syenite intrusions have ages between ∼431 and 429 Ma (Goodenough et al. Reference Goodenough, Millar, Strachan, Krabbendam and Evans2011), and are cut by thrusts above and below, providing a maximum age of initiation of the Moine thrust (Searle et al. Reference Searle, Law, Dewey, Streule, Law, Butler, Holdsworth, Krabbendam and Strachan2010 a; Fox & Searle, Reference Fox and Searle2021). One nordmarkite dyke cuts across mylonitic fabrics along the Moine thrust (Law et al. Reference Law, Casey and Knipe1986; Law, Reference Law1987, Reference Law, Snoke, Tullis and Todd1998, Reference Law2014; Thigpen et al. Reference Thigpen, Law, Lloyd and Brown2010 a, Reference Thigpen, Law, Loehn, Strachan, Tracy, Lloyd, Roth and Brown2013), but none appear to intrude the un-mylonitized Moine schists above the Moine thrust zone. Two large composite alkaline syenites do intrude the Moine Supergroup structurally above the Moine thrust. The Loch Loyal syenite in the far north, east of Loch Eriboll, has a poorly constrained single grain ²⁰⁶Pb–²³⁸U zircon age of c. 425 Ma, and the Glenn Dessary syenite in the southern part of the Northern Highlands terrane, east of Knoydart, has a mean ²⁰⁶Pb–²³⁸U age of 445.7 ± 8 Ma and a concordia age of 445.3 ± 1.9 Ma (Goodenough et al. Reference Goodenough, Millar, Strachan, Krabbendam and Evans2011).

The Glencoul and Ben More thrusts, structurally beneath the Moine thrust, bring up slices of Lewisian basement as well as imbricated Cambrian–Ordovician sedimentary strata. The structural geometry suggests a foreland-propagating thrust system from the Moine thrust down-structural section to the Arnaboll thrust (Loch Eriboll), and Ben More, Glencoul, Borralan and Sole thrusts (Assynt) with time (Elliott & Johnson, Reference Elliott and Johnson1980; Coward, Reference Coward1983, Reference Coward1984; Butler, Reference Butler, Law, Butler, Holdsworth, Krabbendam and Strachan2010; Searle et al. (Reference Searle, Law, Dewey, Streule, Law, Butler, Holdsworth, Krabbendam and Strachan2010 a, Reference Searle, Cornish, Heard, Charles and Branch2019). The Moine thrust wraps around the Assynt culmination preserving deeper structural units in the culmination, which enables construction of balanced cross-sections across the entire Moine thrust zone (Elliott & Johnson, Reference Elliott and Johnson1980; Searle et al. Reference Searle, Law, Dewey, Streule, Law, Butler, Holdsworth, Krabbendam and Strachan2010 a; Fox & Searle, Reference Fox and Searle2021). The Moine thrust truncates underlying imbricate structures along the southern part of the Assynt Window (Peach et al. Reference Peach, Horne, Gunn, Clough, Hinxman and Cadell1888; Coward, Reference Coward1983) and rests almost directly on the undisturbed foreland in the south suggesting some late-stage out-of-sequence displacement. The Assynt culmination has been bulged up by deep thrusting exhuming several thrust sheets with Lewisian basement (e.g. Ben More, Glencoul thrusts). Total amounts of shortening across the Moine thrust zone are at least 70–80 km (Elliott & Johnson, Reference Elliott and Johnson1980; Coward, Reference Coward1984, Reference Coward1985; Searle et al. Reference Searle, Law, Dewey, Streule, Law, Butler, Holdsworth, Krabbendam and Strachan2010 a). A maximum age of shearing is given by the 431–429 Ma intrusion ages from the Loch Borralan syenite (Goodenough et al. Reference Goodenough, Millar, Strachan, Krabbendam and Evans2011), and a minimum age on shearing is given by ⁴⁰Ar–³⁹Ar cooling ages on the mylonites of 430–420 Ma (Johnson et al. Reference Johnson, Kelley, Oliver and Winter1985; Kelley, Reference Kelley1988; Dallmeyer et al. Reference Dallmeyer, Strachan, Rogers, Watt and Friend2001).

4. Northern Highlands terrane (Moine Supergroup)

The Northern Highlands terrane, bounded by the Moine thrust zone in the west and the GGF in the SE, is comprised of rocks of the Moine Supergroup, a thick sequence of Neoproterozoic psammitic and semi-pelitic sedimentary rocks intruded by granites and affected by several phases of deformation and metamorphism (Peach et al. Reference Peach, Horne, Gunn, Clough and Hinxman1907; Ramsay, Reference Ramsay1958; Holdsworth, Reference Holdsworth1990; Friend et al. Reference Friend, Strachan, Kinny and Watt2003; Holdsworth et al. Reference Holdsworth, Alsop, Strachan, Ries, Butler and Graham2007; Strachan et al. Reference Strachan, Holdsworth, Krabbendam, Alsop, Law, Butler, Holdsworth, Krabbendam and Strachan2010; Cawood et al. Reference Cawood, Strachan, Merle, Millar, Loewy, Dalziel, Kinny, Jourdan, Nemchin and Connelly2015). Figure 5 shows a metamorphic map of northern Scotland showing the distribution of the Barrovian and Buchan facies metamorphic rocks, granites, syenites and gabbros (after Winchester, Reference Winchester1974; Fettes et al. Reference Fettes, Graham, Sassi and Scolari1976; Fettes, Reference Fettes, Harris, Holland and Leake1979; Tanner, Reference Tanner, Corfu, Gasser and Chew2014). Several inliers of Archaean Lewisian complex gneiss underlie the Moine metasedimentary rocks. Detrital zircon ages from the Moine Supergroup constrain deposition to <1000 Ma (Strachan et al. Reference Strachan, Harris, Fettes, Smith and Trewin2002; Cawood et al. Reference Cawood, Nemchin, Strachan, Kinny and Loewy2004, Reference Cawood, Strachan, Merle, Millar, Loewy, Dalziel, Kinny, Jourdan, Nemchin and Connelly2015), suggesting that the Moine rocks could be time-equivalent to the unmetamorphosed Sleat and Torridon groups in the foreland (Geikie, Reference Geikie1884, Reference Geikie1893; Soper et al. Reference Soper, Harris and Strachan1998; Stewart, Reference Stewart2002; Krabbendam et al. Reference Krabbendam, Prave and Cheer2008). Details of the Precambrian stratigraphy of the Northern Highlands and Grampian terranes are given in Holdsworth et al. (Reference Holdsworth, Strachan, Harris, Gibbons and Harris1994, Reference Holdsworth, Alsop, Strachan, Ries, Butler and Graham2007), Strachan et al. (Reference Strachan, Harris, Fettes, Smith and Trewin2002), Leslie et al. (Reference Leslie, Smith, Soper, Higgins, Gilotti and Smith2008), Chew et al. (Reference Chew, Daly, Magna, Page, Kirkland, Whitehouse and Lam2010) and Chew & Strachan (Reference Chew, Strachan, Corfu, Gasser and Chew2014). Moine sedimentary rocks have been stratigraphically subdivided into three main groups, the oldest Morar Group overlain by the Glenfinnan Group, and the youngest Loch Eil Group. The sedimentary rocks are all shallow marine deposits formed in graben systems, and in general becoming more distal towards the east, consistent with sedimentation on a passive continental margin during Iapetus ocean spreading. Soper et al. (Reference Soper, Harris and Strachan1998) mapped two major half-grabens with E-dipping normal faults, similar to some of the Torridon Group half-grabens on the Lewisian foreland. Detrital zircon ages suggest that the Morar Group was deposited after c. 1000–980 Ma and the Glenfinnan and Loch Eil sediments were deposited after 900–870 Ma (Cawood et al. Reference Cawood, Strachan, Merle, Millar, Loewy, Dalziel, Kinny, Jourdan, Nemchin and Connelly2015). There are no young (Cambrian – Lower Ordovician) protoliths recorded in the Moine Supergroup, as seen in the Dalradian, probably because the Moine rocks were situated in a more hinterland position relative to the more continental margin setting of the Dalradian and Highland Border complex. The youngest Loch Eil Group is intruded by the 870 Ma West Highland granitic gneiss, showing that the Moine sedimentation was older than development of the Iapetus passive margin.

Fig. 5. Metamorphic map of northern Scotland, after Winchester (Reference Winchester1974) and Gillen (Reference Gillen1982), showing facies across the Northern Highlands and Grampian terranes, after Fettes (Reference Fettes1970, Reference Fettes, Harris, Holland and Leake1979), Harte & Hudson (Reference Harte, Hudson, Harris, Holland and Leake1979), Stephenson et al. (Reference Stephenson, Mendum, Fettes and Leslie2013a,b) and Tanner (Reference Tanner, Corfu, Gasser and Chew2014) for the Dalradian, and Thigpen et al. (Reference Thigpen, Law, Loehn, Strachan, Tracy, Lloyd, Roth and Brown2013), Ashley et al. (Reference Ashley, Thigpen and Law2015) and Mako et al. (Reference Mako, Law, Caddick, Thigpen, Ashley, Cottle and Kylander-Clark2019, Reference Mako, Law, Caddick, Kylander-Clark, Thigpen, Ashley, Mazza and Cottle2021) for the Northern Highlands (Moine) terranes.

A suite of felsic igneous rocks was intruded into the Moine sedimentary rocks at ∼870 Ma, and a possible amphibolite-facies metamorphism and deformation event, referred to as the ‘Knoydartian event’ and bracketed between ∼840 and 790 Ma, has been proposed (Vance et al. Reference Vance, Strachan and Jones1998; Rogers et al. Reference Rogers, Hyslop, Strachan, Paterson and Holdsworth1998; Tanner & Evans, Reference Tanner and Evans2003; Cutts et al. Reference Cutts, Hand, Kelsey and Strachan2009, Reference Cutts, Kinny, Strachan, Hand, Kelsey, Emery, Friend and Leslie2010; Cawood et al. Reference Cawood, Strachan, Merle, Millar, Loewy, Dalziel, Kinny, Jourdan, Nemchin and Connelly2015). Late Proterozoic granites such as the Carn Chuinneag and Inchbae granites dated by U–Pb zircon at 594 ± 11 Ma (Oliver et al. Reference Oliver, Wilde and Wan2008) intrude the Moine sedimentary units. U–Pb zircon ages of 503 ± 2 Ma on trondhjemitic gneisses from the calc-alkaline Strathy complex within the Moine Supergroup have suggested an intra-Moine Cambrian subduction zone (Dunk et al. Reference Dunk, Strachan, Cutts, Lasalle, Storey, Burns, Whitehouse, Fowler, Moreira, Dunlop and Pereira2019). It is also notable that some Moine-like sedimentary rocks are present SE of the GGF (Glen Banchor – Dava groups; Stephenson et al. Reference Stephenson, Mendum, Fettes and Leslie2013a,b; Badenoch Group; Cawood et al. Reference Cawood, Strachan, Merle, Millar, Loewy, Dalziel, Kinny, Jourdan, Nemchin and Connelly2015) in the Central Highlands (Grampian) terrane making the separate origin of the Dalradian and Moine supergroups (e.g. Bluck, Reference Bluck1985) questionable.

Figure 6 shows a structural cross-section across the SW part of the Moine, from the Moine thrust in the west to the GGF in the SE, after Powell & Glendinning (Reference Powell, Glendinning, Allison, May and Strachan1988) and Soper et al. (Reference Soper, Harris and Strachan1998), modified by Strachan et al. (Reference Strachan, Harris, Fettes, Smith and Trewin2002), with a simplified restoration of the major folds only (Fig. 6b). The absolute minimum amount of crustal shortening along this profile is ∼25 km, but in reality it is probably considerably more. In the northern part of the Moine, shortening is much greater with exhumation of high-grade kyanite and sillimanite-grade rocks along the hangingwall of the Naver thrust and Sgurr Beag thrust (Ashley et al. Reference Ashley, Thigpen and Law2015; Mako et al. Reference Mako, Law, Caddick, Thigpen, Ashley, Cottle and Kylander-Clark2019, Reference Mako, Law, Caddick, Kylander-Clark, Thigpen, Ashley, Mazza and Cottle2021). As in all high-grade metamorphic terranes characterized by extensive ductile folding and shearing, it is not possible to balance or restore cross-sections with any accuracy, but a restoration of major folds, shear zones and thrusts can provide an estimate of the minimum amount of crustal shortening.

Fig. 6. (a) Simplified cross-section across the Northern Highlands terrane Moine Supergroup, after Powell & Glendinning (Reference Powell, Glendinning, Allison, May and Strachan1988), Soper et al. (Reference Soper, Harris and Strachan1998) and Strachan et al. (Reference Strachan, Harris, Fettes, Smith and Trewin2002). The restoration (b) shows the approximate positions of the Sgurr Beag thrust, Knoydart thrust, and Moine thrust sheets in the Northern Highlands terrane.

The Moine Supergroup sedimentary rocks have been affected by Precambrian (Knoydartian) metamorphism at 820–720 Ma (Vance et al. Reference Vance, Strachan and Jones1998; Kinny et al. Reference Kinny, Friend, Strachan, Watt and Burns1999; Friend et al. Reference Friend, Jones and Burns2000; Cutts et al. Reference Cutts, Hand, Kelsey and Strachan2009, Reference Cutts, Kinny, Strachan, Hand, Kelsey, Emery, Friend and Leslie2010), and by regional Barrovian facies metamorphism during the Ordovician – early Silurian Caledonian orogeny across the Northern Highlands terrane (Mako et al. Reference Mako, Law, Caddick, Kylander-Clark, Thigpen, Ashley, Mazza and Cottle2021). The hangingwall of the Naver thrust in the east consists of sillimanite-bearing migmatites that preserve P–T conditions of c. 8–9 kbar and 600 °C with decompression and heating to conditions of 6–7 kbar and 700 °C (Kinny et al. Reference Kinny, Friend, Strachan, Watt and Burns1999; Friend et al. Reference Friend, Jones and Burns2000; Ashley et al. Reference Ashley, Thigpen and Law2015; Mako et al. Reference Mako, Law, Caddick, Thigpen, Ashley, Cottle and Kylander-Clark2019; Dunk et al. Reference Dunk, Strachan, Cutts, Lasalle, Storey, Burns, Whitehouse, Fowler, Moreira, Dunlop and Pereira2019). In the southern part of the Northern Highlands terrane, the Sgurr Beag thrust may be the southern continuation of the Naver thrust. P–T conditions decrease westwards, and structurally downward, to 4.8–7.2 kbar and ∼550–680 °C in the hangingwall of the Moine thrust (Ashley et al. Reference Ashley, Thigpen and Law2015; Mazza et al. Reference Mazza, Mako, Law, Caddick, Krabbendam and Cottle2018; Mako et al. Reference Mako, Law, Caddick, Thigpen, Ashley, Cottle and Kylander-Clark2019, Reference Mako, Law, Caddick, Kylander-Clark, Thigpen, Ashley, Mazza and Cottle2021). U–Pb monazite and xenotime ages range between 430 and 415 Ma, although two samples are older at 445–440 Ma (Fig. 4; Mako et al. Reference Mako, Law, Caddick, Thigpen, Ashley, Cottle and Kylander-Clark2019). The latter, defined as the ‘Grampian II event’, are similar to Lu–Hf garnet ages in the Moine, Ben Hope and Naver (+ Sgurr Beag) thrust sheets (Bird et al. Reference Bird, Thirlwall, Strachan and Manning2013). Syn-tectonic granite intrusions in the Naver thrust sheet are 430–420 Ma (Fig. 4; Kinny et al. Reference Kinny, Strachan, Friend, Kocks, Rogers and Paterson2003; Strachan et al. Reference Strachan, Alsop, Ramezani, Frazer, Burns and Holdsworth2020), and confirm the presence of high temperatures in the middle and lower crust during this period. These rocks have been affected by ductile shear zones that have emplaced higher grade rocks above lower grade rocks (Ashley et al. Reference Ashley, Thigpen and Law2015). This inverted metamorphism appears to have formed by post-peak metamorphic thrusting (Mazza et al. Reference Mazza, Mako, Law, Caddick, Krabbendam and Cottle2018), rather than by any channel flow type mechanism. The regional structure and metamorphism of the Moine Supergroup is comparable to the Greater Himalayan Sequence metamorphic rocks above the Main Central thrust (Searle et al. Reference Searle, Law, Godin, Larsen, Streule, Cottle and Jessup2008; Streule et al. Reference Streule, Strachan, Searle, Law, Law, Butler, Holdsworth, Krabbendam and Strachan2010). Both regions show extensive staurolite, kyanite and sillimanite-grade metamorphic rocks with sillimanite + K-feldspar migmatites at higher structural levels, and an inverted metamorphic field gradient down-structural section. The Himalaya contain a far greater volume of partial melt migmatites and leucogranites (with garnet, muscovite, tourmaline and cordierite), so the Himalayan channel flow model, lubricated by extensive melting (e.g. Searle et al. Reference Searle, Law, Jessup, Law, Searle and Godin2006, Reference Searle, Law, Dewey, Streule, Law, Butler, Holdsworth, Krabbendam and Strachan2010 a), is unlikely to be applicable to the Moine metamorphism. However, the time scales of metamorphism are similar (lasting ∼45–55 m.y.), in both the Caledonian and Himalayan orogenies.

5. Grampian Highlands terrane (Dalradian Supergroup)

The Grampian terrane is bounded by the GGF in the NW and the Highland Border complex and HBF zone in the south (Bailey, Reference Bailey1922; Stephenson & Gould, Reference Stephenson and Gould1995; Stephenson et al. Reference Stephenson, Mendum, Fettes and Leslie2013a,b). Like the Moine metasedimentary rocks, the lower part of the Dalradian metasedimentary sequence is mostly Precambrian and unfossiliferous. Both the Moine and Dalradian supergroups have similar detrital zircon ages, but the upper part of the Dalradian has fossil evidence that suggest protolith rocks could be as young as Lower Ordovician (Stephenson et al. Reference Stephenson, Mendum, Fettes and Leslie2013a,b; Tanner, Reference Tanner, Corfu, Gasser and Chew2014). Detrital zircon ages from the Dalradian Supergroup show a similar age range as rocks from East Greenland and Newfoundland, confirming a Laurentian plate origin (Cawood et al. Reference Cawood, Nemchin, Smith and Loewy2003, Reference Cawood, Nemchin, Strachan, Prave and Krabbendam2007; McAteer et al. Reference McAteer, Daly, Flowerdew, Whitehouse and Monaghan2014). Five main stratigraphic groups have been defined, the Grampian Group at the base overlain by the Appin, Argyll and Southern Highlands groups, overlain in turn by the partially fossiliferous Trossachs Group, of oceanic affinity, at the top (Cawood et al. Reference Cawood, Strachan, Merle, Millar, Loewy, Dalziel, Kinny, Jourdan, Nemchin and Connelly2015). The latter may be continuous with the Highland Border complex along the southern margin of the Grampian terrane; see Stephenson & Gould (Reference Stephenson and Gould1995), Tanner & Sutherland (Reference Tanner and Sutherland2007), Tanner (Reference Tanner2008, Reference Tanner, Corfu, Gasser and Chew2014) and Stephenson et al. (Reference Stephenson, Mendum, Fettes and Leslie2013a,b) for comprehensive reviews.

Palaeoproterozoic basement units are present in the form of the Rhinns complex syenitic gneisses on Islay. Most clasts within the basal conglomerates of the Iona Group in the Inner Hebrides are Archaean–Palaeoproterozoic gneisses, similar to the Lewisian basement in the foreland and in the Northern Highlands terrane (McAteer et al. Reference McAteer, Daly, Flowerdew, Whitehouse and Monaghan2014). Neoproterozoic rocks at the base of the Dalradian, or sub-Grampian Group (Glen Banchor, Dava Group) rocks, are possible lateral equivalents to the Moine Supergroup in the Northern Highlands terrane. The Appin Group comprises the Ballachulish, Lochaber and Blair Atoll formations, with ages ranging from ∼800 to 660 Ma (Prave et al. Reference Prave, Fallick, Thomas and Graham2009), and is overlain by the Argyll Group, which includes the 600 Ma old Tayvallich volcanics, and intruded by ∼590 Ma granites, notably the Ben Vurich and Portsoy granites (Tanner et al. Reference Tanner, Leslie and Gillespie2006; Tanner, Reference Tanner, Corfu, Gasser and Chew2014). The youngest protolith sedimentary rocks in the Trossachs Group contain trilobite faunas, dated as Arenig, ∼478–468 Ma (Tanner, Reference Tanner, Corfu, Gasser and Chew2014). Sedimentation associated with the Ballantrae ophiolite also continued up to late Arenig time (∼470–466 Ma; Tanner, Reference Tanner, Corfu, Gasser and Chew2014), correlative with the younger Dalradian sediments to the NW. In the southern part of the Grampian Highlands terrane, ophiolite complexes (Bute, Tyrone ophiolites) were obducted from the oceanic Midland Valley terrane NW onto the Laurentian southern passive margin (protolith of the Dalradian metasediments) between ∼490 and 470 Ma (Dewey & Shackleton, Reference Dewey and Shackleton1984; Chew et al. Reference Chew, Daly, Magna, Page, Kirkland, Whitehouse and Lam2010; Chew & Strachan, Reference Chew, Strachan, Corfu, Gasser and Chew2014). The Unst ophiolite in the Shetland Islands has a comparable age with a U–Pb zircon date on the metamorphic sole of 484 ± 4 Ma (Crowley & Strachan, Reference Crowley and Strachan2015), but it is uncertain how the Shetland Islands geometrically fits with the Northern Highlands (Moine Supergroup) and Grampian Highlands (Dalradian Supergroup) structures. The GGF has been correlated northwards to the WBF on Shetland, but the two faults do not appear to link on seismic profiles in the Moray Firth and across the northern North Sea (Stoker et al. Reference Stoker, Hitchen and Graham1993; Chadwick & Evans, Reference Chadwick, Evans and Boldy1995). By 470 Ma Dalradian Supergroup sedimentation had ceased, and crustal thickening, regional metamorphism and S-type granite intrusion began (Fig. 4).

Metamorphism of the Dalradian Supergroup (Figs 4, 5) is complex involving two facies series, a regional Barrovian-type metamorphism characterized by biotite, garnet, staurolite, kyanite and sillimanite in pelitic rocks (Barrow, Reference Barrow1893, Reference Barrow1912), and a localized LP–HT Buchan facies series in the NE characterized by LP–HT andalusite, cordierite and sillimanite index minerals in pelites (Read, Reference Read1923, Reference Read1955; Fettes, Reference Fettes1970; Fettes et al. Reference Fettes, Long, Bevins, Max, Oliver, Primmer, Thomas, Yardley and Harris1985; Harte, Reference Harte, Harris and Fettes1988). Buchan-type metamorphism is spatially associated with a suite of gabbro and diorite intrusions (Fettes, Reference Fettes1970; Harte & Hudson, Reference Harte, Hudson, Harris, Holland and Leake1979; Dempster et al. Reference Dempster, Rogers, Tanner, Bluck, Muir, Redwood, Ireland and Paterson2002), but appears to be continuous with, and overprinted by the high-grade regional Barrovian facies series (Fettes, Reference Fettes1970; Dempster & Harte, Reference Dempster and Harte1986). Dempster et al. (Reference Dempster, Hudson and Rogers1995, Reference Dempster, Rogers, Tanner, Bluck, Muir, Redwood, Ireland and Paterson2002) described the overprinting of low-pressure andalusite-bearing Buchan assemblages with Barrovian facies kyanite-grade metamorphism. The additional heat and lower pressure conditions of Buchan metamorphic rocks are probably accounted for by high-level gabbroic intrusions that are only present in the NE Grampian Highlands (Fig. 4). Johnson et al. (Reference Johnson, Kirkland, Reddy, Evans and McDonald2016) published U–Pb zircon ages of 486 ± 9 Ma and 493 ± 8 Ma for diorites within the Portsoy gabbro, ages that are broadly similar to the youngest preserved fossiliferous sediments in the Trossachs Group (Dalradian), and also to the age of ophiolite obduction (Fig. 4).

Originally thought be a short-lived (<10 m.y.; 470–460 Ma) metamorphism associated with ophiolite obduction (Dewey, Reference Dewey2005), or alternatively with crustal extension (Viete et al. Reference Viete, Oliver, Fraser, Forster and Lister2013; Viete & Lister, Reference Viete and Lister2016), recent radiometric age dating casts doubt on this model. Granitic and gabbroic intrusions in the Dalradian have emplacement ages of ∼493–440 Ma (Fig. 4), dates that are roughly concomitant with the metamorphic ages (Oliver et al. Reference Oliver, Chen and Buchwaldt2000; Appleby et al. Reference Appleby, Graham, Gillespie, Hinton and Oliver2008; Johnson et al. Reference Johnson, Kirkland, Reddy, Evans and McDonald2016). Baxter et al. (Reference Baxter, Ague and DePaolo2002) and Bird et al. (Reference Bird, Thirlwall, Strachan and Manning2013) reported Sm–Nd garnet whole-rock dates of Dalradian pelites starting at 473–467 Ma and lasting at least until 464.8 ± 2.7 Ma, interpreted as dating prograde metamorphism. Bird et al. (Reference Bird, Thirlwall, Strachan and Manning2013) also published Lu–Hf garnet ages, 470.8 ± 3.2 Ma from the Leven schist and 463.2 ± 1.7 Ma from metabasic intrusions within the Dava succession, that confirm the contemporaneous metamorphism of the Moine and Dalradian supergroups during the older Grampian metamorphic episode (Fig. 4). So far, the younger metamorphic ages from Sm–Nd and Lu–Hf garnet (Bird et al. Reference Bird, Thirlwall, Strachan and Manning2013) and U–Pb monazite (Mako et al. Reference Mako, Law, Caddick, Thigpen, Ashley, Cottle and Kylander-Clark2019) Silurian ages from the Moine Supergroup have not been replicated in the Dalradian Supergroup. However, extensive high ⁸⁷Sr/⁸⁶Sr S-type granites in the Dalradian, spanning 475–440 Ma (Oliver et al. Reference Oliver, Wilde and Wan2008), and the Silurian – Early Devonian ‘Newer Granites’ of Pankhurst & Sutherland (Reference Pankhurst, Sutherland and Sutherland1982), attest to temperatures high enough for crustal melting during this time. On the basis of the garnet and monazite ages, as well as the dating of S-type granites, it seems more likely that post-accretion and syn-collision crustal thickening resulted in a single extended period of regional metamorphism, including both the earlier, localized Buchan event, and the overprinting and longer-lasting Barrovian sequences, with dates spanning 478–420 Ma, rather than two separate short-lived ‘events’ (Strachan et al. Reference Strachan, Harris, Fettes, Smith and Trewin2002; Dewey, Reference Dewey2005; Johnson & Strachan, Reference Johnson and Strachan2006; Viete et al. Reference Viete, Oliver, Fraser, Forster and Lister2013; Viete & Lister, Reference Viete and Lister2016). The more localized LP–HT Buchan facies series can be spatially related to the gabbro intrusions in the NE part of the Grampian Highlands (Dempster et al. Reference Dempster, Hudson and Rogers1995, Reference Dempster, Rogers, Tanner, Bluck, Muir, Redwood, Ireland and Paterson2002; Baxter et al. Reference Baxter, Ague and DePaolo2002).

Deformation of the Dalradian Supergroup in the Grampian Highlands terrane is also extremely complex. Figure 7a shows a simplified cross-section across the SW part of the terrane (after Roberts & Treagus, Reference Roberts and Treagus1977; Stephenson et al. Reference Stephenson, Mendum, Fettes and Leslie2013a,b; Tanner, Reference Tanner, Corfu, Gasser and Chew2014), with a simplified restoration of the major structures shown in Figure 7b. The overall structure is a complex one showing a major NW-facing recumbent fold (Islay anticline) in the NW, and a huge SW-facing recumbent fold, the Tay nappe, along the SE margin (Shackleton, Reference Shackleton1958; Krabbendam et al. Reference Krabbendam, Leslie, Crane and Goodman1997). A minimum shortening of ∼65 km occurs simply by restoring the major fold structures (Fig. 7b), although the true amount of crustal shortening is likely double this.

Fig. 7. (a) Simplified cross-section across the Grampian Highlands Dalradian Supergroup, after Roberts & Treagus (Reference Roberts and Treagus1977), Stephenson et al. (Reference Stephenson, Mendum, Fettes and Leslie2013a,b) and Tanner (Reference Tanner, Corfu, Gasser and Chew2014). (b) Restoration of the major fold structures in the Dalradian, showing minimum amounts of crustal shortening.

6. Great Glen fault (mainland Scotland) and Walls Boundary fault (Shetland)

In mainland Scotland, the GGF is the major NE–SW-aligned strike-slip fault that separates the Moine Supergroup in the Northern Highlands terrane to the NW from the Dalradian Supergroup in the Grampian Highlands terrane to the SE (Fig. 1). The first recognition of strike-slip motion along the GGF was by Kennedy (Reference Kennedy1946) who proposed 65 miles (∼100 km) of sinistral offset, based on the offset of the Strontian granite (location 10 on Fig. 8; 425 ± 3 Ma; Rogers & Dunning, Reference Rogers and Dunning1991) and Foyers granite (location 20 on Fig. 8; 424 ± 4 Ma; Oliver et al. Reference Oliver, Wilde and Wan2008), and the offset of metamorphic zones in the Northern Highlands and those in the Grampian Highlands (Winchester, Reference Winchester1974). Since the Devonian Old Red Sandstone is also affected by the fault in places, Kennedy (Reference Kennedy1946) proposed that slip occurred post-Devonian, but pre-late Carboniferous. Both the Strontian and Foyers granites show a zonation from tonalite–granodiorite–granite towards the core, and a broadly similar geochemical evolution and zircon inheritance, confirming a strong affinity, but not necessarily proof that they were once the same pluton (W. E. Stephens, pers. comm. 2021).

Fig. 8. Map of Scotland, northern Ireland, and northern England, showing locations of the Silurian – early Devonian granites. U–Pb ages for each dated granite pluton are shown in the key.

Post-Caledonian motion along the GGF is mainly dextral or extensional, with the GGF and Helmsdale fault clearly cutting Jurassic rocks onshore and Cretaceous rocks offshore in the Moray Firth (Chadwick & Evans, Reference Chadwick, Evans and Boldy1995). Rogers et al. (Reference Rogers, Marshall and Astin1989) documented 25–29 km of dextral displacement along the GGF between late Frasnian and early Permian times based on offset of mapped Old Red Sandstone on the Scottish mainland. Dextral displacement of ∼8 km has been proposed, based on the offset of a Permian–Carboniferous dyke swarm in Argyll (McQuillan et al. Reference McQuillan, Donato and Tulstrup1982). Some authors have proposed a dominant normal faulting component, down-to-SE during the Mesozoic period forming a boundary of the North Sea extensional graben systems to the west (Bacon & Chesher, Reference Bacon and Chesher1975).

Determining older Caledonian motion along the GGF is more difficult. The fault is poorly exposed, running mainly along the deep glaciated valleys of Loch Linnhe and Loch Ness. The GGF cuts metamorphic fabrics in the Moine Supergroup to the NW and those in the Dalradian Supergroup to the SE, as well as cutting several granitic plutons. Phyllonites, mylonites and cataclasites showing sinistral kinematic indicators crop out near Fort William, and provide clear evidence of sinistral displacement (Stewart et al. Reference Stewart, Strachan and Holdsworth1997). Left-lateral shear along the GGF clearly occurred after metamorphism in the Moine (c. 478–420 Ma), and after metamorphism and crustal melt granite formation in the Dalradian (472–435 Ma). The ‘Newer Granites’, intruded into the Dalradian Supergroup between c. 435 and 405 Ma, are present across the whole Grampian terrane, as well as parts of the Northern Highlands terrane, and are not spatially associated with strike-slip faults. The GGF cuts two such granites to the NW along its trace, the Strontian granite (location 10 on Fig. 8) and the Clunes tonalite (location 13 on Fig. 8; 427.8 ± 1.9 Ma; Stewart et al. Reference Stewart, Strachan, Martin and Holdsworth2001), and one granite, the Foyers granite to the SE (location 20 on Fig. 8). The Glenelg–Ratagain granite (location 8 on Fig. 8; U–Pb zircon age 425 ± 3 Ma; Rogers & Dunning, Reference Rogers and Dunning1991; Hutton & McErlean, Reference Hutton and McErlean1991) is cut along the south side by the Strathconon fault, which runs parallel to the GGF within the Moine Supergroup. A swarm of lamprophyre dykes dated at 435–410 Ma (Rock & Hunter, Reference Rock and Hunter1987) is also cut by the GGF. The ductile shear fabrics in the granites were clearly formed after crystallization of the intrusive granite (<425 Ma), not during granite emplacement, as proposed by Hutton (Reference Hutton1988), Hutton & McErlean (Reference Hutton and McErlean1991) and Hutton & Reavy (Reference Hutton and Reavy1992). The ductile shear fabrics associated with, and parallel to, the GGF must have been imposed after 424 Ma, the youngest emplacement age of the granite, or after 410 Ma, the youngest age of lamprophyre dykes cut by the GGF, and likely before c. 405 Ma, the base of the unconformably overlying Old Red Sandstone (Fig. 4). Since the Old Red Sandstone is also offset by limited amounts along the GGF, some limited motion continued after this time, into Devonian time.

The GGF was previously thought to continue NE to connect with the WBF in the Shetland Islands (Flinn, Reference Flinn1992; Watts et al. Reference Watts, Holdsworth, Sleight, Strachan and Smith2007; Dewey et al. Reference Dewey, Dalziel, Reavy and Strachan2015). However, the two faults are not directly connected on seismic reflection profiles across the northern North Sea (Chadwick & Evans, Reference Chadwick, Evans and Boldy1995) and the GGF seems to have a different kinematic history (Armitage et al. Reference Armitage, Watts, Holdsworth and Strachan2021). The Shetland Islands expose Lewisian-type Neoarchaean basement and overlying Precambrian–Cambrian metasedimentary rocks (Sand Voe, Yell Sound and Westing groups and East Mainland succession), in turn overlain by Devonian sedimentary and volcaniclastic successions (Armitage et al. Reference Armitage, Watts, Holdsworth and Strachan2021). Neoproterozoic to Devonian rock units are present both west and east of the WBF. Late Ordovician (c. 465 Ma) to Late Devonian (c. 360 Ma) granites intrude both west and east of the WBF (Lancaster et al. Reference Lancaster, Strachan, Bullen, Fowler, Jaramillo and Saldarriaga2017). On the islands of Unst and Fetlar, a late Cambrian to Early Ordovician ophiolite sequence has a U–Pb zircon age of 484 ± 4 Ma from the metamorphic sole (Crowley & Strachan, Reference Crowley and Strachan2015), similar timing as the Ballantrae ophiolite south of the Midland Valley arc, and the Highland Border ophiolites (Bute ophiolite) (Fig. 4). On Shetland, Armitage et al. (Reference Armitage, Watts, Holdsworth and Strachan2021) documented intense dextral late Carboniferous transpressional fabrics, including a 2 km wide zone of cataclasites, fault gouge and local pseudotachylytes that formed during dextral strike-slip shearing. Dextral motion along the WBF is constrained as being post-370 Ma, the age of granites cut by the fault, and prior to overlying Permian sedimentary rocks at c. 330–300 Ma (Armitage et al. Reference Armitage, Watts, Holdsworth and Strachan2021). Dewey et al. (Reference Dewey, Dalziel, Reavy and Strachan2015) proposed at least 500–700 km of sinistral motion along the GGF–WBF between 428 and 385 Ma. There are, however, no clear pinning points either side of the GGF, so these amounts are only speculation. Dewey & Strachan (Reference Dewey and Strachan2003) proposed that the GGF, together with other similar strike-slip faults, shows up to 1200 km of sinistral displacement, during the period 435–395 Ma, during which the GGF juxtaposed the Dalradian Supergroup rocks in the SE next to the Moine Supergroup rocks in the NW. If this model was correct the obvious question arises: what lay in the space 700–1200 km long, between the Moine and Dalradian terranes prior to slip along the GGF? These very large strike-slip offsets must imply either that the Moine and Dalradian supergroups were separate ‘blocks’ or ‘exotic terranes’, separated by some unknown oceanic tract, or that very large lateral strike-slip motion occurred along the Laurentian continental margin at the end of the Caledonian orogeny.

7. Silurian: Devonian granites and volcanic rocks

A large volume of granites was emplaced into the crust during late Silurian – early Devonian times (435–408 Ma) across northern Scotland on both sides of the GGF, as well as in the Southern Uplands suture zone and northern England Avalonian terrane, the latter called the Trans-suture granite suite (Read, Reference Read1961; Stephens & Halliday, Reference Stephens and Halliday1984; Atherton & Ghani, Reference Atherton and Ghani2002; Fowler et al. Reference Fowler, Kocks, Darbyshire and Greenwood2008; Neilson et al. Reference Neilson, Kokelaar and Crowley2009; Miles et al. Reference Miles, Woodcock and Hawkesworth2016). Figure 8a shows the locations of the Silurian – early Devonian granites across Scotland, northern Ireland and northern England, together with a key showing U–Pb ages and references from sources of data (Figure 8b). Figure 9 is a compilation of all the U–Pb zircon and monazite ages as well as Sm–Nd and Lu–Hf garnet ages of granitoids and metamorphic rocks across the Northern Highlands and Grampian Highlands terranes.

Fig. 9. Lower Palaeozoic time chart showing the age ranges of granites (U–Pb zircon) and metamorphic rocks (U–Pb monazite, Sm–Nd and Lu–Hf garnet) dates. U–Pb zircon age data is compiled from Rogers & Dunning (Reference Rogers and Dunning1991), Kinny et al. (Reference Kinny, Friend, Strachan, Watt and Burns1999, Reference Kinny, Strachan, Friend, Kocks, Rogers and Paterson2003), Dempster et al. (Reference Dempster, Rogers, Tanner, Bluck, Muir, Redwood, Ireland and Paterson2002), Oliver et al. (Reference Oliver, Wilde and Wan2008), Neilson et al. (Reference Neilson, Kokelaar and Crowley2009) and Goodenough et al. (Reference Goodenough, Millar, Strachan, Krabbendam and Evans2011). U–Pb monazite age data from the Moine is from Mako et al. (Reference Mako, Law, Caddick, Thigpen, Ashley, Cottle and Kylander-Clark2019). Lu–Hf and Sm–Nd garnet age data is from Baxter et al. (Reference Baxter, Ague and DePaolo2002), Cutts et al. (Reference Cutts, Kinny, Strachan, Hand, Kelsey, Emery, Friend and Leslie2010), Bird et al. (Reference Bird, Thirlwall, Strachan and Manning2013). Ages are based on the International Chronostratigraphic Chart 2016/04.

The ‘Newer Granites’ (Read, Reference Read1961) have been divided into the compositionally distinct Argyll and Northern Highlands suite, and low-Ba and Sr granites of the Cairngorm suite (Stephens & Halliday, Reference Stephens and Halliday1984). Stephens & Halliday (Reference Stephens and Halliday1984) recognized three different suites, all geochemically distinct: (a) the Argyll suite, (b) the Cairngorm suite, both in the Dalradian, and (c) the South of Scotland suite, or the so-called Trans-suture granites across the Southern Uplands into northern England (Shap, Weardale, Skiddaw granites). The Argyll suite is characterized by hornblende-bearing granites and diorites, with high Ba, Sr, K₂O, Na₂O and La/Yb ratios, and are closely associated, both spatially and temporally, with appinites and lamprophyres, similar to the Silurian plutons north of the GGF (Stephens & Halliday, Reference Stephens and Halliday1984). The Cairngorm suite is composed mainly of silicic granites with high Nb, Rb and Th, and low Ba and Sr (Stephens & Halliday, Reference Stephens and Halliday1984). The Trans-suture granites have U–Pb zircon ages between 426 and 395 Ma, overlapping with ages of the ‘Newer Granites’ in the Dalradian Supergroup (Soper, Reference Soper1986; Miles et al. Reference Miles, Woodcock and Hawkesworth2016).

The younger granites intruding Dalradian rocks of the Grampian Highlands have U–Pb zircon ages spanning 430–405 Ma (Fig. 9). Re–Os molybdenite ages of the Ballachulish (433.5 ± 1.8 Ma) and Kilmelford (425.8 ± 1.7 Ma) igneous suites overlap with the U–Pb zircon ages (Conliffe et al. Reference Conliffe, Selby, Porter and Feely2010). Similar age granites occur in the Donegal pluton in western Ireland (Pitcher, Reference Pitcher1972; Pitcher & Berger, Reference Pitcher and Berger1972) and in northern England (e.g. Shap, Weardale, Skiddaw granites; Miles et al. Reference Miles, Woodcock and Hawkesworth2016). U–Pb zircon ages of granites intruding the Moine Supergroup in the Northern Highlands have a similar span of ages from c. 430–418 Ma. These include the Strath Naver, Klibreck, Clunes tonalite, Ratagain, Strontian and Ross of Mull granites (Rogers & Dunning, Reference Rogers and Dunning1991; Stewart et al. Reference Stewart, Strachan, Martin and Holdsworth2001; Kinny et al. Reference Kinny, Strachan, Friend, Kocks, Rogers and Paterson2003; Oliver et al. Reference Oliver, Wilde and Wan2008; Strachan et al. Reference Strachan, Alsop, Ramezani, Frazer, Burns and Holdsworth2020). The ages of the ‘Newer Granites’ suite are also similar to the large syenite intrusions in the Assynt area (Canisp, Loch Borralan, Loch Ailsh syenites), which are 431–429 Ma (Goodenough et al. Reference Goodenough, Millar, Strachan, Krabbendam and Evans2011; Fox & Searle, Reference Fox and Searle2021).

The Silurian – Early Devonian granites in the Northern Highlands and Central Highlands (Grampian) terranes have been used as evidence that subduction dipped NW beneath Scotland during this (Scandian) period (Atherton & Ghani, Reference Atherton and Ghani2002; Neilson et al. Reference Neilson, Kokelaar and Crowley2009). However, the granites are not restricted to the Moine–Dalradian terranes as would be expected in this Andean-type model, but are also present across the suture zone, and in the Avalon terrane in northern England. Silurian–Devonian granites are not similar to Andean granites, in that the late Caledonian granites are generally not associated with gabbros, or related volcanic rocks, and they have a more alkaline geochemistry (Pitcher, Reference Pitcher1972; Chappell & Stephens, Reference Chappell and Stephens1988; Atherton & Ghani, Reference Atherton and Ghani2002; Neilson et al. Reference Neilson, Kokelaar and Crowley2009). Andean granites have only a minor S-type component (e.g. Cordillera Blanca granites, Peru; Pitcher, Reference Pitcher1972), again unlike the Caledonian granites, and they are not associated with regional lamprophyric magmatism as seen across northern Scotland. The classic I–S classification of Chappell & White (Reference Chappell and White1974, Reference Chappell and White2001) works well for end-members such as typical Andean granites (I-type) or typical Himalayan crustally derived leucogranites (S-type), but the Caledonian granites do not fit either category easily. A-type granite magmatism (syenites, syenogranites, etc) is the most controversial category. These granites include associated ultrapotassic lamprophyres, shoshonites and adakites, and typically occur in late, post-collision settings or anorogenic ‘within-plate’ settings (Pearce et al. Reference Pearce, Harris and Tindle1984; Whalen & Hildebrand, Reference Whalen and Hildebrand2019). Many of the Caledonian granites have compositional and temporal similarities with associated appinite and lamprophyre magmas.

Most of the Silurian – Early Devonian granites in Scotland are medium to high-K calc-alkaline monzodiorite, granodiorite and monzogranite of intermediate to silicic compositions, with a great variation in incompatible trace elements and light rare earth elements (LREEs). They have an enriched lithospheric mantle signature and ⁸⁷Sr/⁸⁶Sr initial ratios of <0.708, and are associated with small-volume appinite–lamprophyre intrusions (Halliday, Reference Halliday1984; Rock & Hunter, Reference Rock and Hunter1987; Shand et al. Reference Shand, Gaskarth, Thirlwall and Rock1994; Neilson et al. Reference Neilson, Kokelaar and Crowley2009; Miles et al. Reference Miles, Woodcock and Hawkesworth2016). Appinites can range from ultramafic to felsic in composition, with extensive metasomatic fluid infiltration resulting in geochemical characteristics indistinguishable from low-K, calc-alkalic arc magmatism (Murphy, Reference Murphy2020). In Scotland, appinites are melanocratic hornblende-rich syenites, monzonites and diorites that span alkaline to calc-alkaline trends. Lamprophyres are small-volume alkaline, silica-undersaturated, mafic and ultramafic rocks derived from deep and hot mantle melting. They include biotite- and hornblende-phyric minettes, vogesites and alkali feldspar-bearing micro-syenites and rhyolites (Rock, Reference Rock1984; Rock & Hunter, Reference Rock and Hunter1987). The appinite–lamprophyre intrusions have high Ni, Cr, V and MgO with high large-ion lithophile elements (LILE) and LREE abundances, consistent with plagioclase-absent fractional crystallization (Neilson et al. Reference Neilson, Kokelaar and Crowley2009). Lamprophyre magmatism is a common late orogenic occurrence, for example across Tibet (Chung et al. Reference Chung, Chu, Zhang, Xie, Lo, Lee, Lan, Li, Zhang and Wang2005), but they do not require crustal thinning or any form of slab break-off to form.

Volcanic rocks, mainly hypabyssal to extrusive basalt–andesite–trachyte–rhyolites, are associated with some of the Silurian – Early Devonian granites and are mainly present across the Central Highlands Grampian terrane (Figs 8, 9). They include the large Rannoch Moor (422.5 ± 0.5 Ma), Ben Nevis, Cruachan, Starav–Etive (414.9 ± 0.7 Ma) and Ballachulish (419.6 ± 5.4 Ma) granites in the Dalradian Supergroup, and the Loch Linne and Strontian granites NW of the GGF in the Moine Supergroup (Neilson et al. Reference Neilson, Kokelaar and Crowley2009). The Caledonian volcanic rocks include the Lorn–Etive volcanic sequence and the Glencoe caldera volcanoes, which are the extrusive equivalents of the large-scale Lochaber batholith (Kokelaar & Moore, Reference Kokelaar and Moore2006). The Lorn lavas (location 26, Fig. 8; U–Pb zircon ages 426–425 Ma) around Oban are andesites and dacites erupted some 5 m.y. before the major Glencoe caldera lavas and the Etive dyke swarm (location 24, Fig. 8; U–Pb zircon ages 420–415 Ma; Neilson et al. Reference Neilson, Kokelaar and Crowley2009). The andesites and trachyandesites in the Grampian Highlands were erupted continuously over a period of ∼22 m.y. from c. 430 to 408 Ma, and were concomitant with emplacement of shoshonites and lamprophyres within the Lorn–Etive lava pile, dated at 430–415 Ma (Kokelaar & Moore, Reference Kokelaar and Moore2006; Neilson et al. Reference Neilson, Kokelaar and Crowley2009). The temporal and spatial association of the volcanic rocks with shoshonites and lamprophyres is similar in many respects to the extensive, but small-volume, post-collisional melts exposed across the Tibetan plateau (Chung et al. Reference Chung, Chu, Zhang, Xie, Lo, Lee, Lan, Li, Zhang and Wang2005; Searle, Reference Searle and Watts2015). The Grampian granite suite, including the appinites and the lamprophyric dykes, were intruded at a similar time as the Assynt alkaline suite (Canisp, Loch Borralan, Loch Ailsh; 431–429 Ma; Goodenough et al. Reference Goodenough, Millar, Strachan, Krabbendam and Evans2011; Fox & Searle, Reference Fox and Searle2021) in the Moine thrust zone, suggesting a far more widespread regional alkaline igneous event during this time.

There are no subduction-related, Andean-type granites emplaced in the Grampian terrane during subduction accompanying the Early to Middle Ordovician closure of the Iapetus ocean along the Southern Uplands suture zone, as would be expected from an Andean analogy (Chappell & Stephens, Reference Chappell and Stephens1988). Miles et al. (Reference Miles, Woodcock and Hawkesworth2016) noted that there was an absence of magmatic activity during Iapetus subduction between c. 455 and 425 Ma. Instead, the ‘Newer Granites’ generally have younger U–Pb zircon ages between c. 430 and 408 Ma, and have an origin related to partial melting of a LILE- and LREE-enriched mantle (Neilson et al. Reference Neilson, Kokelaar and Crowley2009). The ubiquitous lamprophyres associated with the granites may relate to a deeper, hotter, possibly asthenospheric mantle origin. The granites are scattered throughout the Grampian terrane and the southern part of the Northern Highlands terrane, as well as the Southern Uplands and northern England (Trans-suture granites). They are unrelated, either geographically or temporally, to strike-slip faulting, either along the GGF or other strike-slip faults, most of which were active later (<405 Ma). The generation of these granites remains a puzzle, but we can rule out models involving an Andean-type, subduction-related origin and intrusion along strike-slip faults. Their origin appears more related to late or post-collision alkaline A-type syenite, syeno-granite and monzonite suites, ultimately derived from partial melts of enriched (K-amphibole, phlogopite-bearing) sub-continental lithospheric mantle combined with lower crust melting and some possible assimilation (Neilson et al. Reference Neilson, Kokelaar and Crowley2009).

8. Highland Boundary fault

The HBF dividing the Central (Grampian) Highlands to the NW from the Midland Valley to the SE is a major geological feature running from Stonehaven on the east coast to the island of Bute on the west coast of Scotland (Fig. 1), and further SW across northern Ireland (Chew et al. Reference Chew, Daly, Magna, Page, Kirkland, Whitehouse and Lam2010). The Clew Bay complex in western Ireland is the youngest lower Palaeozoic part of the Dalradian Supergroup (Chew et al. Reference Chew, Daly, Magna, Page, Kirkland, Whitehouse and Lam2010) and the Trossachs Group on Arran (Tanner & Sutherland, Reference Tanner and Sutherland2007). It is not clear where the fault runs across the island of Arran, partly because of the intrusion of the Paleocene Arran granite across the westward extension of the fault zone. The Highland Border complex is a zone less than 1 km across that contains a diverse sequence of mudstones, limestones, cherts, pillow lavas and highly altered serpentinites. A series of highly deformed ophiolitic rocks crop out along the Highland Boundary, notably the Bute ophiolite (Ayrshire), Unst ophiolite (Shetland Islands) and the Tyrone ophiolite in Ireland (Chew et al. Reference Chew, Daly, Magna, Page, Kirkland, Whitehouse and Lam2010; Tanner, Reference Tanner, Corfu, Gasser and Chew2014). The Highland Border complex was initially interpreted as an exotic terrane juxtaposed next to the Grampian terrane by strike-slip motion along the HBF (Bluck, Reference Bluck1984, Reference Bluck1985). It has also been interpreted as a major Caledonian sinistral strike-slip fault (Woodcock & Strachan, Reference Woodcock and Strachan2012) and a NW-dipping thrust (Tanner, Reference Tanner2007, Reference Tanner2008). However, it now known that the sub-ophiolite sediments form a continuous succession with the Dalradian rocks, and the Grampian terrane was overthrust by the Highland Border ophiolites during late Cambrian – Early Ordovician times (Tanner & Sutherland, Reference Tanner and Sutherland2007; Chew et al. Reference Chew, Daly, Magna, Page, Kirkland, Whitehouse and Lam2010; Cawood et al. Reference Cawood, Strachan, Merle, Millar, Loewy, Dalziel, Kinny, Jourdan, Nemchin and Connelly2015). The Highland Border ophiolites were initially obducted to the NW during the period 510–485 Ma and later steepened and overturned to dip steeply NW (Fig. 10). The HBF is interpreted here as a steep, NW-dipping (back)thrust fault that cuts structures in the Dalradian Tay nappe to the NW (Harris et al. Reference Harris, Bradbury and McGonigal1976), and also cuts Lower Devonian rocks in the Midland Valley terrane to the SE. Earlier Cambrian ophiolite obduction structures, both in the Highland Border remnant ophiolites and the underlying Dalradian Tay nappe, have been overprinted by large-scale refolding and backthrusting. Lateral displacements are limited, less than 30 km (Tanner, Reference Tanner2008).

Fig. 10. (a) Reconstruction of the Laurentian margin at c. 480 Ma showing geometry of the margin, the Dalradian continental margin, the Highland Border ophiolite suites, and the Midland Valley arc, after Chew et al. (Reference Chew, Daly, Magna, Page, Kirkland, Whitehouse and Lam2010). (b) Reconstruction of the Indian plate continental margin in the western Himalaya, showing the geometry of similar thrust sheets related to obduction of the Late Cretaceous Sapat ophiolite, accretion of the large-scale Kohistan island arc, with suture zone rocks along both margins; after Searle (Reference Searle and Watts2015).

The major point of contention now lies in whether the Buchan and early Barrovian metamorphic facies series in the Dalradian (Grampian phase of the orogeny) was a result of the obduction of ophiolites (Dewey & Shackleton, Reference Dewey and Shackleton1984; Dewey et al. Reference Dewey, Dalziel, Reavy and Strachan2015), or whether the metamorphism was related to a later, Himalayan-style crustal thickening event. Despite some models for Himalayan crustal thickening being related to ophiolite obduction (Spontang ophiolite; Searle & Treloar, Reference Searle, Treloar, Kusky, Zhai and Xiao2010), most U–Pb monazite ages and high-temperature geochronology suggest that Himalayan metamorphism was related to post-obduction, continental collision-related crustal thickening and internal heat generation during middle Eocene to Miocene (∼45–20 Ma) crustal thickening and shortening (Searle, Reference Searle and Watts2015). The main difference is that the Buchan metamorphism in the Dalradian Grampian Highlands was associated with syn-metamorphic gabbroic intrusions, whereas in the Himalaya, LP–HT metamorphism (andalusite, sillimanite, cordierite) was associated with extensive migmatite and crustal melt leucogranite, and followed the regional staurolite–kyanite–sillimanite regional Barrovian-type metamorphism (see Searle, Reference Searle and Watts2015 for a review). In both the Himalaya and the Caledonian orogenies, ophiolite obduction was followed very closely by arc accretion (Kohistan island arc in the Himalaya; Midland Valley arc in the Caledonides), and continental collision, resulting in crustal thickening, regional Barrovian facies metamorphism and lower crustal melting forming S-type granites. In both orogenic belts the suture zone region was steepened and overturned, as seen along the Indus suture in the Himalaya and the Highland Boundary in Scotland (Fig. 11).

Fig. 11. Simplified cross-sections of (a) the Scottish Caledonides from the Laurentian foreland to the Highland Border and Midland Valley, combining structure of the Moine Thrust zone, Northern Highlands (Moine Supergroup) and Grampian Highlands (Dalradian Supergroup) rocks. (b) Simplified cross-section of the Indian plate Himalaya (after Corfield & Searle, Reference Corfield, Searle, Khan, Treloar, Searle and Jan2000; Searle, Reference Searle and Watts2015) from the Indian foreland to the Indus–Tsangpo suture zone. Note the similarities in scale and fold vergence–facing directions across the Tay nappe backfold and the backfolds across the northern margin of Indian and the Indus–Tsangpo suture zone.

9. New model for Caledonian orogeny

Many published tectonic models invoke a SE-directed subduction during the Grampian ‘orogeny’ that was dominated by NW-directed ophiolite obduction, followed by a subduction switch to NW-dipping subduction during Late Ordovician – Silurian times (Fig. 2; Dewey, Reference Dewey1969; Dewey & Shackleton, Reference Dewey and Shackleton1984; Dewey et al. Reference Dewey, Dalziel, Reavy and Strachan2015). An alternative model presented in this paper is that the entire Caledonian orogeny in Scotland, comprising ophiolite obduction (490–475 Ma) and accretion of the Midland Valley terrane, crustal thickening, regional metamorphism and partial melting (480–420 Ma in the Northern Highlands; 480–435 Ma in the Grampian Highlands), can be explained by continuous SE-directed subduction and NW- to WNW-vergent foreland-directed thrusting. The geometry and time scales of orogeny are closely comparable to the Indian plate – Kohistan island arc – Asia collision zone along the Himalaya. The early parts of the orogenic process (Fig. 10) involve accretion of a large island arc (Kohistan in the Himalaya; Midland Valley arc in the Caledonian orogeny) and ophiolite obduction onto a previously passive continental margin (Indian plate; Laurentian plate margin). Following ophiolite obduction, continental collision-related crustal thickening resulted in widespread Barrovian metamorphism (Greater Himalayan Sequence; Dalradian, Moine metamorphism). The latest stages involved foreland-propagating thrusting (Lesser Himalaya and Main Central Thrust zone in the Himalaya; Moine thrust zone in Caledonides). The orogenic wedge from the Moine thrust zone in the WNW to the HBF in the ESE shows a gradual thickening towards the ESE, consistent with this geometric model. In this model, the Tay nappe and the Highland Border ophiolite are in a backthrust zone (Fig. 11), a similar structure as that seen along the Insubric Line and the SE hinterland margin of the Alps (Schmid et al. Reference Schmid, Pfiffner, Froitzheim, Schönborn and Kissling1997), and also similar to the backthrusting and backfolding mapped along the Indus–Tsangpo suture zone in the India–Asia collision today (Searle, Reference Searle and Watts2015; Corfield & Searle, Reference Corfield, Searle, Khan, Treloar, Searle and Jan2000).

The separation of the Moine and Dalradian supergroups seems unnecessary given the similarities in protolith ages, metamorphic and crustal melt granite dates, and structures. Seismic surveys suggest that the GGF extends to the base of the crust (McBride, Reference McBride1994), but there is no evidence from seismic tomography that it is a plate boundary suture zone (Nicolson et al. Reference Nicolson, Curtis and Baptie2014). There are no ophiolites, deep-sea sedimentary rocks, mélanges or (ultra)high-pressure rocks exposed along the GGF. It is therefore suggested here that the Northern Highlands terrane (Moine Supergroup) and Grampian Highlands terrane (Dalradian Supergroup) were all originally part of one contiguous hyper-extended continental margin along the eastern margin of Laurentia during Neoproterozoic to Ordovician times (Fig. 3), and later deformed and metamorphosed during a single long-lived metamorphic event from c. 470 to 420 Ma (Fig. 4). It is also proposed that late Caledonian offsets along the GGF were far less than those widely accepted, and that the fault post-dates late Silurian – Early Devonian granite emplacement in both the Northern Highlands and Central (Grampian) Highlands terranes. The GGF can only be traced offshore north as far as east of the Orkney Islands, and is not directly connected to the WBF on the Shetland Islands.