Evidence for an Early Silurian Synorogenic Basin

Tectonics

RESEARCH ARTICLE Evidence for an Early Silurian Synorogenic Basin Within the 10.1029/2018TC005050 Metamorphic Hinterland of the North Atlantic Caledonides: Key Points: Insights From the U-Pb Zircon Geochronology of the • U-Pb zircon dating in combination with published mineral age data Funzie Conglomerate, Shetland, Scotland shows that the deformed and metamorphosed Funzie S. Biejat1 , R. A. Strachan1 , C. D. Storey1 , and P. J. Lancaster1 Conglomerate in Shetland, Scottish Caledonides, was deposited in the 1School of Earth and Environmental Sciences, University of Portsmouth, Portsmouth, UK Early Silurian between ca. 440 and 430 Ma • The clast population was derived from easterly (offshore) sources Abstract The spectacular kilometer-thick Funzie Conglomerate is a deformed and metamorphosed comprising Neoproterozoic high-energy sedimentary deposit that occurs in Shetland (Scottish Caledonides). Its age and tectonic metasedimentary rocks and the significance in relation to Early Ordovician (485–475 Ma) and Late Ordovician (450–445 Ma) accretionary northern extension of the Ordovician – Midland Valley Arc that formed orogenic events and the culminating Silurian (430 425 Ma) continental collision and closure of the Iapetus during closure of the Iapetus Ocean Ocean have hitherto been unknown. U-Pb zircon dating indicates that the clast population of the • The tectonic setting is interpreted as conglomerate was derived from easterly sources comprising Neoproterozoic metasedimentary rocks and the a synorogenic basin that received detritus from, and was buried by, northern extension of the Ordovician-Silurian Midland Valley Arc that formed during Iapetus closure. A lower advancing thrust nappes during the age limit of 440 Ma is indicated by the ages of the youngest matrix detrital zircon grains and the youngest collision of Laurentia and Baltica granitic clast. Deformation and metamorphism of the conglomerate is attributed to sinistrally oblique extension at ca. 430 Ma that excised at least 10 km of crustal section. Sedimentation of the conglomerate is Supporting Information: therefore constrained to a 10-myr time span (440–430 Ma). The tectonic setting is interpreted as a synorogenic • Supporting information S1 basin that received detritus from, and was buried by, advancing thrust nappes during the Early Silurian • Table S1 collision of Laurentia and Baltica. Tectonic burial was followed rapidly by ductile deformation during oblique Correspondence to: extensional unroofing of the metamorphic pile and accounts for preservation of the Funzie Conglomerate as a R. A. Strachan, rare example of a synorogenic basin within the metamorphic hinterland of the North Atlantic Caledonides. [email protected] 1. Introduction Citation: Biejat, S., Strachan, R. A., Storey, C. D., & Orogenesis, uplift, increased erosion rates, and the rapid deposition of thick successions of often Lancaster, P. J. (2018). Evidence for an coarse-grained, proximally derived sediments are inextricably linked processes. Such sedimentary Early Silurian synorogenic basin within successions are most commonly preserved in foreland basins formed along the margins of mountain belts. the metamorphic hinterland of the North Atlantic Caledonides: Insights These have a high preservation potential due to incorporation into foreland-propagating thrust systems from the U-Pb zircon geochronology of and will often preserve a valuable detrital record of the now eroded segments of an orogen. However, the Funzie Conglomerate, Shetland, although coarse wedges of synorogenic continental sediments will presumably accumulate within orogenic Scotland. Tectonics, 37. https://doi.org/ 10.1029/2018TC005050 hinterlands during early stages of collision, their preservation potential (especially in pre-Mesozoic orogens) is reduced because of the deeper levels of erosion. The potential for such successions to be preserved is Received 12 MAR 2018 increased where orogenic tracts have undergone extensional thinning either during or after crustal Accepted 27 JUL 2018 thickening. The upper crustal expression of such extension is often the rapid accumulation of continental Accepted article online 3 AUG 2018 sedimentary rocks within an “extensional collapse basin” (e.g., Andersen et al., 1994; Dewey et al., 1993; Krabbendam & Dewey, 1998; Malavieille, 1993; Séguret et al., 1989). However, distinguishing between synconvergent and synextensional sedimentary successions within orogens is problematic because of the following: (1) processes may occur too rapidly to be constrained easily by paleontological and/or geochronological data; (2) the true kinematic significance of low-angle faults may be difficult to establish; and (3) there may be overlap between convergence and extension at different crustal levels, and thus, the distinction may in any case be semantic (e.g., Andersen, 1993, 1998; Andresen et al., 2007). In this context, we use U-Pb zircon geochronology to assess the age, provenance, and tectonic setting of the deformed and metamorphosed Funzie Conglomerate within the Shetland Islands, Scottish Caledonides (Figure 1a). The new evidence indicates that this enigmatic succession represents the fill of a synorogenic basin that developed during Early Silurian collisional orogenesis. Caledonian orogenesis in the North Atlantic region resulted from the closure of the Iapetus Ocean and the — ©2018. American Geophysical Union. sinistrally oblique collision during the Silurian-Devonian of three continental blocks Laurentia, Baltica, and All Rights Reserved. Avalonia (Figure 1a; Pickering et al., 1988; Soper et al., 1992; Chew & Strachan, 2014). Shetland formed part

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Figure 1. (a) Relative positions of sectors of the Caledonides prior to Mesozoic rifting and opening of the North Atlantic Ocean, with location of Unst and the Norwegian ophiolites and Ordovician-Silurian volcanic and intrusive arc rocks mentioned in the text. Abbreviations for Britain and Ireland: GGF = Great Glen Fault; HBF = Highland Boundary Fault; SUF = Southern Upland Fault; IS = Iapetus Suture; MTZ = Moine Thrust Zone; NHT = Northern Highland Terrane; CB = Clew Bay. Abbreviations for Norway: K = Karmoy ophiolite; SB = Sunnhordland Batholith; G = Gullfjell ophiolite; S-S = Solund- Stavfjord ophiolite; S-H = Smøla-Hitra Batholith; Lk = Leka ophiolite; BB = Bindal Batholith; Ly = Lyngen ophiolite. (b) Simpliﬁed geology of Fetlar and Unst, showing the main geological units, the extent of the Funzie Conglomerate (modiﬁed from British Geological Survey, 2002) and sample site for RS-03. LHF = Lamb Hoga Fault. (c) Detailed structural map of Fetlar and Unst showing locations of major oblique extensional detachments and thrusts, and L1 and L2 lineations (data from R. A. S. and British Geological Survey, 2002). HT = Hevda “Thrust”; FB = Funzie Bay.

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of Laurentia. Synorogenic to late-orogenic sedimentary basins formed in a variety of tectonic settings. Silurian foreland basin successions that accumulated during collision are preserved in eastern North Greenland (Hurst & McKerrow, 1985) and northern England (Kneller, 1991). A change in relative plate motions during the Devonian resulted in a diachronous switch from sinistrally oblique collision to sinistrally oblique transtension and the development of late- to post-orogenic continental sedimentary basins along the length of the orogen (Dewey & Strachan, 2003). In East Greenland and Norway extensional structures formed by reactivation of low-angle Caledonian thrusts and by development of hinterland-dipping shear zones such as those that underlie the extensional collapse basins of SW Norway (Andersen et al., 1994; Séguret et al., 1989).

2. Geological Setting 2.1. Tectonic Overview Closure of the Iapetus Ocean occurred over a protracted time period from the Late Cambrian to the Late Silurian. From the Early Ordovician onward, the Laurentian margin was active, with development of arc/back-arc and supra-subduction zone ophiolite complexes that were accreted to the continent during Taconic/Grampian orogenic events (e.g., Dewey & Ryan, 1990; Roberts, 2003). In contrast, the Baltican margin was largely passive during this period, as was the margin of Avalonia during the Silurian. The west directed subduction of oceanic lithosphere beneath Laurentia during the Silurian culminated in the collision of the three continents. The Avalonia-Laurentia collision was relatively “soft” (Soper & Woodcock, 1990; Torsvik & Rehnström, 2003), and consequently, there was no more than a few tens of kilometers of orthogonal over- thrusting of peri-Laurentian terranes onto the foreland of the subducting Avalonian plate. In contrast, the “hard” collision of Laurentia and Baltica resulted in substantial crustal thickening (the Scandian orogeny) and the emplacement of a major stack of allochthonous thrust nappes derived from Laurentia and the Iapetus Ocean onto the Baltican margin (Corfu et al., 2014; Gasser, 2014, and references therein). In the Scottish-Irish sector, oceanic closure commenced with the development of east or southeast dipping (present reference frame) subduction zones in the Iapetus Ocean during the Late Cambrian to Early Ordovician. This was followed by accretion of ophiolites and magmatic arcs to the Laurentian margin to result in the 480- to 470-Ma Grampian orogenic event (Chew et al., 2010; Lambert & McKerrow, 1976; Oliver et al., 2000; Tanner, 2014). Supra-subduction zone ophiolites are exposed in Clew Bay and Tyrone (Ireland), Bute (SW Scotland), Ballantrae (S Scotland), and Shetland (Figure 1a; e.g., Gass et al., 1982; Prichard, 1985; Spray & Dunning, 1991; Kawai et al., 2008; Chew et al., 2010). In Ireland, magmatic arcs are exposed south of Clew Bay (Dewey & Mange, 1999) and in Tyrone (Hollis et al., 2012, 2013) but, in Scotland, are thought to be buried beneath the Upper Paleozoic cover successions of the Midland Valley Terrane (Bluck, 2002). Following a ﬂip in subduction polarity and development of the Southern Uplands accretionary prism in Scotland and Ireland (Figure 1a; Leggett et al., 1979; Stone & Merriman, 2004), renewed deformation and metamorphism at ca. 450–445 Ma may correspond to terrane accretion (the “Grampian II” event of Bird et al., 2013) and/or to ﬂat-slab subduction (Dewey et al., 2015). To the northeast of Shetland, early Caledonian accretionary orogenic events are preserved within the thrust nappes that were emplaced eastward onto the Baltica margin during the Scandian orogeny. The structurally highest Uppermost Allochthon (Figure 1a) is believed to represent part of Laurentia (Roberts, 2003; Roberts et al., 2007). This is underlain by thrust nappes that consist of exotic oceanic terranes comprising Paleozoic ophiolitic and island arc sequences. These are thought to have a peri-Laurentian origin (e.g., Pedersen et al., 1992; Roberts et al., 2002) and may therefore be directly analogous to similar-aged sequences along strike in Scotland and Ireland. The oldest oceanic terranes are Late Cambrian to Early Ordovician ophiolites (e.g., Karmoy, Gulfjell, Leka, Lyngen, Figure 1a) and island arc sequences (Dunning & Pedersen, 1988; Pedersen & Dunning, 1997; Pedersen & Hertogen, 1990). They record several phases of pre-Scandian deformation and are overlain unconformably by Late Ordovician to Early Silurian successions (Andersen & Andresen, 1994). The younger Late Ordovician to Early Silurian Solund-Stavfjord ophiolite and its associated volcanic and intrusive arc rocks in SW Norway (Figure 1a) also developed within an intra-Iapetus marginal basin (Andersen et al., 1990; Furnes et al., 2012). These oceanic terranes underwent a protracted evolution between ca. 495 and ca. 430 Ma and were emplaced as composite entities during the Scandian orogeny (Andersen & Andresen, 1994; Corfu et al., 2006, 2011; Pedersen & Furnes, 1991). Shetland is likely to form the lateral correlative of some segment of the Laurentian-derived Norwegian Caledonides, although given

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Figure 2. (a) Geological cross-section across Fetlar along line AB in Figure 1c (modiﬁed from Flinn, 2014). WG = Westing Group; EMS = East Mainland Succession; LN = lower ophiolite nappe; MP = Muness Phyllite; UN = upper ophiolite nappe; FC = Funzie Conglomerate; LHF = Lamb Hoga Fault; AF = Aith Fault. (b) Simpliﬁed geological map of SE Fetlar (see Figure 1b for location; data from R. A. S. and British Geological Survey, 2002) showing sample locations. FB = Funzie Bay; other abbreviations as for (a).

the likelihood of late orogenic sinistral strike-slip displacements of potentially up to 500–700 km between Laurentia and Baltica (Dewey & Strachan, 2003), it is difﬁcult to identify a precise match.

2.2. Geology of Shetland and the Funzie Conglomerate Shetland is mainly underlain by Laurentian metasedimentary successions: the ca. 1,100- to 930-Ma Yell Sound and Westing groups (Cutts et al., 2009, 2011) and the ca. 700- to 550-Ma East Mainland Succession (Strachan et al., 2013). The latter is broadly correlative with the Dalradian Supergroup of the Grampian Terrane in mainland Scotland (Figure 1a; Flinn, 2007; Strachan et al., 2013). Rb-Sr mineral ages of 480–440 Ma constrain the main metamorphic assemblages and deformation fabrics to have formed during Grampian orogenic events (Walker et al., 2016). The earliest stage of Grampian orogenic activity corresponds to the obduction of the Unst ophiolite (Figure 1b; Flinn, 1985, 2000, 2014; Spray & Dunning, 1991; O’Driscoll et al., 2012, 2018). The ophiolite has been duplicated by thrusting and is exposed in two thrust nappes separated by low-grade metasedimentary rocks, the Muness Phyllite and an associated metatuff unit (Figures 1b and 2a; Flinn, 1958, 1985, 2014, and references therein). The Muness Phyllite is dominated by fine-grained, strongly deformed metamorphosed siltstones with occasional conglomeratic layers up to 10 m thick. The latter contain centimeter-scale clasts of plagiogranite, leucotonalite, gabbro, and volcaniclastic material (Flinn, 2014) suggestive of derivation from ophiolitic-plutonic sources and deposition in an active marginal basin setting. An Early Ordovician age is assumed in the absence of any geochronological constraints (Crowley & Strachan, 2015). In southeast Fetlar, the lower ophiolite nappe is overlain by the deformed and metamorphosed Funzie Conglomerate (Figures 1b and 2a), which was made internationally famous by Derek Flinn, whose structural analysis of its quartzite clasts formed the basis of a classic paper on three-dimensional strain (Flinn, 1956). The Funzie Conglomerate has a structural thickness of ~1km and is dominated by pebble- to boulder-sized clasts of orthoquartzite, up to a meter in length, set in a fine- to medium-grained matrix of quartz-muscovite phyllite (Figures 3a and 3b; Flinn, 1956, 2014). Subordinate clast lithologies include phyllite, similar mineralogically and texturally to the matrix, granitic lithologies ranging from coarse-grained granitic gneiss to fine-grained, undeformed microgranite, and rare occurrences of marble (Flinn, 2014). Lithological layering is defined by occasional psammite horizons, up to 30 cm thick (Figure 3c). However, no sedimentary structures are preserved, so the direction of younging is unknown. On the north coast of Fetlar, bedding dips moderately to the east-southeast. However, within 2–300 m to the east it steepens to pass through the vertical and dips moderately to steeply westward across much of the outcrop of the unit. Bedding is transected by a pervasive tectonic fabric that is defined by the shapes of the deformed clasts. Measurements of the clasts indicate that the conglomerate mainly records plane strain deformation, although locally the clasts may be either mark- edly elongate or flattened (Flinn, 1956). The longest axes of the clasts are aligned in a north-northeast direction, defining a prominent linear L2 fabric (Figures 1c and 3d). In the northern outcrop of the unit, L2 is

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Figure 3. (a, b) Deformed clasts of orthoquartzite varying in size from pebbles to boulders, set in a matrix of ﬁne-grained muscovite-quartz phyllite; hammer lies parallel to foliation. (c) Fine-grained psammite layer within the conglomerate (above hammer). (d) Elongation of clasts parallel to L2 lineation. All images taken in Funzie Bay; length of hammer is 25 cm.

horizontal or plunges a few degrees to the north-northeast (Figure 1c). Further to the south L2 steepens and may plunge at angles of up to 20–30° to the south-southwest (Figure 1c; British Geological Survey, 2002). On the north coast of Fetlar (Figure 1c), the Funzie Conglomerate is faulted against underlying ultramaficrocks of the lower ophiolite nappe (e.g., HU 6670 9270). The contact is sharp, relatively planar, and dips moderately to the east-southeast (Figure 2a). Although there has clearly been some displacement across the contact, there is no evidence that the base of the Funzie Conglomerate is defined by a regionally significant fault. Hence, although possibly tens of meters of stratigraphy may have been excised, the most reasonable interpretation is that the unit was deposited unconformably on the ophiolite (Flinn, 2014). The Muness Phyllites, which overlie the lower ophiolite nappe further west on Fetlar and on Unst (Figures 1b and 2a), locally contain thin conglomerate layers (<1- to 10-m scale) but in contrast to the Funzie Conglomerate the clast suite is dominated by igneous lithologies, sourced in part from the underlying ophiolite (Flinn, 1952, Flinn, 2014). The upper ophiolite nappe structurally overlies the Funzie Conglomerate on the south coast of Fetlar (Figure 2a). 2.3. Caledonian Evolution of the Unst Ophiolite and Associated Metasedimentary Rocks Isotopic, metamorphic, and structural data provide the following constraints on the Caledonian evolution of the Unst ophiolite and its associated metasedimentary rocks: 1. Formation of the ophiolite in a fore-arc setting is dated by a magmatic crystallization age of 492 ± 3 Ma (U-Pb zircon; Spray & Dunning, 1991). Obduction onto the Laurentian margin is bracketed by the following: (a) a U-Pb zircon age of 484 ± 4 Ma (Crowley & Strachan, 2015) and (b) K-Ar hornblende ages of between 479 ± 6 and 465 ± 6 Ma (Spray, 1988), both obtained from the metamorphic sole (“Norwick hornblende schists” in Figure 1b), which is preserved at the structural base of both ophiolite nappes. 2. “Grampian I” regional deformation and amphibolite facies metamorphism of footwall metasedimentary successions at ca. 485–475 Ma accompanied ophiolite obduction (Cutts et al., 2011). The transport direction is thought to have been toward the west, parallel to an L1 lineation only preserved in northwest Unst (Figure 1c; Cannat, 1989; Flinn & Oglethorpe, 2005; Flinn, 2014). This was followed by the reworking of low- angle thrust-related fabrics into a regionally steep orientation across Yell and Mainland Shetland (Walker et al., 2016). Peak pressure-temperature conditions were approximately 10 kbar and 775 °C (Cutts et al., 2011).

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3. “Grampian II” amphibolite facies metamorphism of the footwall metasedimentary successions at ca. 450–445 Ma (Walker et al., 2016). 4. Development of a greenschist facies foliation and associated NNE trending L2 mineral lineation forms the single and dominant deformation fabric within the Funzie Conglomerate (Figure 1c). Top-to-the-NNE kinematic indicators parallel to L2 are well developed (Cannat, 1989; R. A. S., unpublished data). L2 is commonly >5–10° clockwise of strike, suggesting a sinistrally oblique sense of extensional shear. The metamorphic contrast between the ophiolite and footwall mineral assemblages suggests that at least ~10 km of crustal section is missing (Cutts et al., 2011). The basal tectonic contact of the lower ophiolite nappe cannot therefore be the original obduction thrust, and the metamorphic break could be accounted for by out-of-sequence thrusting and/or extensional displacements (Cutts et al., 2011). The tectonic contacts at and near the base of the lower ophiolite nappe are thus depicted as “low-angle detachment faults” (Figure 1c). Rb-Sr white mica ages produced by Walker et al. (2016) include an Ordovician age of ca. 465 Ma from immediately west of Burrafirth (Figure 1c) and is interpreted to relate to ca. 485–475 Ma Grampian I orogenesis, and Silurian ages of ca. 440–430 Ma from the greenschist facies fabric, which are thought to broadly date L2 (see below). 5. Thrusting of the upper ophiolite nappe onto the Funzie Conglomerate and various components of the lower ophiolite nappe (Figure 1b). Reexamination of the western margin of the Skaw Granite confirms the view of Read (1934) that this has been strongly modified by faulting (hence shown as a thrust in Figure 1) and is not a straightforward intrusive contact as interpreted by Flinn and Oglethorpe (2005). The 40Ar/39Ar mineral ages of ca. 425 Ma obtained from northeast Unst by Flinn and Oglethorpe (2005) date late muscovite growth immediately adjacent to the tectonized margin of the Skaw Granite and could therefore correspond to the timing of thrusting, presumably during the Scandian orogeny. Despite the detailed framework outlined above, the age, tectonic setting, and provenance of the Funzie Conglomerate are unknown. U-Pb geochronology carried out on zircons from clasts and the matrix of the conglomerate provides insights into these issues and the basis for our interpretation of the tectonic setting of the basin.

3. U-Pb Zircon Dating U-Pb zircon dating was undertaken on 3 orthoquartzite clasts, 2 samples of the matrix of the metaconglome- rate, and 14 granitic clasts. Most samples were obtained from Funzie Bay (HU 665 896; Figure 2b). Some granitic clasts were also sampled ~1.5 km further south (HU 6624 8824; Figure 2b), and one (RS-03) was obtained from the north coast of Fetlar (HU 6665 9260; Figure 1b). Of the granitic clasts analyzed, eight yielded abundant zircon and are described in detail in section 3.4. Six samples contained relatively few zircons, most of which were either inherited or in poor condition, and these are discussed only brieﬂy in section 3.5.

3.1. Analytical Methods Three- to four-kilogram samples were crushed, milled, and sieved to separate the <355-μm fraction. Heavy minerals were extracted using a Wilfley density separating table and sorted on a Frantz magnetic separator, isolating the nonmagnetic cut. Zircons were handpicked under binocular microscope, mounted in epoxy resin, ground to half thickness, and gold coated for cathodoluminescence (CL) imaging. Detrital zircons were imaged on a JEOL JSM-6060LV, while zircons from granitic clasts were imaged on a Phillips XL 30CP, both at the University of Portsmouth. All zircons of sufficient size were analyzed, avoiding cracks or inclusions and adhering to representative sampling principles (Sláma & Košler, 2012). U-Pb ages were measured by laser ablation inductively coupled plasma mass spectroscopy at the University of Portsmouth after Jeffries et al. (2003), using a New Wave UP-213 Nd:YAG laser system attached to an Agilent 7500cs quadruple inductively coupled plasma mass spectroscopy. Background signals were measured for 30 s, followed by 60 s of ablation using a 25- to 30-μm spot rastered along a 40- to 60-μm line. The amount of 204Pb in these analyses was below the detection limit, and no common Pb correction was undertaken. However, in order to test the validity of the discordant analyses (where an array of discordant analyses propagating away from relatively few concordant analyses was apparent), a common lead estimation anchored through a 207Pb/206Pb of 0.9 ± 0.1 was used with this uncertainty added in quadrature to the final value.

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Ratios for detrital zircons were calculated using LAMTRACE (Jackson, 2008), while ratios for igneous zircons were calculated using an in-house spreadsheet derived from LamTool (Košler et al., 2008). Both reduction methods employed sample-standard bracketing, normalized to GJ-1, and measuring Plešovice as a secondary standard to monitor accuracy. Average 207Pb/206Pb and 206Pb/238U ratios of GJ-1 were 0.0601 ± 0.0013 and 0.0975 ± 0.0017 (n = 362, 2SD), respectively, while those of Plešovice were 0.05345 ± 0.0016 and 0.05456 ± 0.0013 (n = 88, 2SD), respectively consistent with published values (Jackson et al., 2004; Sláma et al., 2008). The exception was sample SH-21, where Plešovice was the primary standard and GJ-1 secondary. Average 207Pb/206Pb and 206Pb/238U ratios of Plešovice were 0.05319 ± 0.0015 and 0.05394 ± 0.0011, (n = 18, 2SD), respectively, and of GJ-1 were 0.0602 ± 0.0020 and 0.0950 ± 0.0021 (n = 12, 2SD), respectively. Previous work under the same conditions, using 91500 as the secondary standard before it was polished away, yielded 206Pb/238U and 207Pb/206Pb ratios of 0.1764 ± 0.0040 and 0.0747 ± 0.0024 (n = 37, 2SD), respectively. The resulting average 207Pb/206Pb age of 1,059 ± 97 Ma provides conﬁdence that using such a young primary standard yielded correct 207Pb/206Pb ages on Archaean samples, albeit with large uncertainties. Isoplot 3.07 (Ludwig, 2003) was used for all Concordia plots and age calculations. 207Pb/206Pb ages are reported for zircons older than 1.0 Ga, and 206Pb/238U ages are reported for zircons younger than 1.0 Ga. Analyses whose probability of concordance was greater than 0.001 were considered concordant and are discussed further. All individual grain ages are given at 2σ, while Concordia ages are given at 95% unless indicated due to the typically small number of analyses available for these calculations.

3.2. Quartzite Clasts The three quartzite clasts sampled (SH-13, SH-14, and SH-15) are petrographically very similar. They are mostly ﬁne to medium grained and comprise polycrystalline quartz (>90%) with minor amounts of plagioclase and mica (muscovite ± biotite ± chlorite) and accessory apatite, zircon, titanite, rutile, magnetite, and ilmenite. Quartz grains typically display undulose extinction and moderately sutured grain boundaries. A tectonic fabric deﬁned by aligned micas and ribbons of epidote is interpreted to have developed during regional deformation and greenschist facies metamorphism of the conglomerate. Zircon populations vary from large (~300 μm) orange, pink, and yellow grains that are highly rounded to smaller (150 μm) colorless and subrounded zircons. CL imaging reveals various internal structures including oscillatory zoning, both concentric and parallel; common sector zoning; and some extensively recrystallized cores. Some grains, however, show no internal zoning. A total of 108 analyses from the three samples is <15% discordant and yields 207Pb/206Pb ages between ca. 1,000 and ca. 3,000 Ma. These are presented as kernel density estimate plots (Vermeesch, 2012) and histograms in Figures 4a–4d. The dominant populations are ca. 1,100–1,000, ca. 1,700, and ca. 2,700 Ma.

3.3. Matrix The two matrix samples (SH-16 and SH-23) are petrographically very similar. Both are ﬁne-grained phyllites, dominated by quartz and muscovite in approximately equal proportions, with subordinate biotite, plagioclase, and chlorite and accessory zircon, apatite, tourmaline, rutile, titanite, and opaque minerals. Zircons range from rounded to subhedral in form, displaying sector zoning in CL imaging, with or without oscillatory overprinting (Figure 5a). Where overgrowths are present, they most often display more intense oscillatory zoning than the core. Some large subeuhedral grains display strong oscillatory zoning in all growth zones. A total of 133 analyses from the two samples is <15% discordant and presented as kernel density estimate plots and histograms in Figures 4e–4g. The dominant populations are ca. 450–400, ca. 1,000, ca. 1,700, and 2,700 Ma. The youngest grains yield overlapping Concordia ages of 447 ± 4 Ma (SH-16) and 442 ± 5 Ma (SH-23; Figures 6a and 6b).

3.4. Group 1 Main Granitic Clasts Each sample provided sufﬁcient analytical data to constrain an age of crystallization. Most samples contain a variably developed protomylonitic foliation deﬁned by aligned mica (muscovite ± biotite ± chlorite) and aggregates of dynamically recrystallized quartz that is thought to have developed during regional deformation and greenschist facies metamorphism of the conglomerate. However, some samples carry a coarse centimeter-scale gneissic fabric that is inferred to have resulted from

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Figure 4. Adaptive kernel density estimates and histograms of analyses of zircons from (a–c) orthoquartzite clasts SH-13, SH-14, and SH-15; (d) the three samples aggregated; (e–f) matrix samples SH-16 and SH-23; (g) the two matrix samples aggregated; (h) the East Mainland Succession on Shetland (Strachan et al., 2013); (i) “Group 1 Main Granitic Clasts”; and (j) “Group 2 Subordinate Granitic Clasts.” All plotted using Density Plotter v.6.2 of Vermeesch (2012) and a bin width of 80 Ma; errors are 2σ. Only 207Pb/206Pb ages older than 1 Ga that are <15% discordant and 206Pb/238U ages whose error ellipses overlapped the Terra-Wasserburg Concordia are included.

high-temperature recrystallization and deformation and hence must predate incorporation into the conglomerate. 3.4.1. FZ-02 This is a gray-pink granitic gneiss dominated by quartz, plagioclase, and orthoclase (porphyroclasts up to 7 mm), with subordinate muscovite, biotite, and chlorite, and accessory zircon, epidote, titanite, amphibole, and opaque minerals. Zircons are colorless, 100–250 μm in size with medium aspect ratio, and euhedral with rounded terminations (Figure 5b). Low-U, CL-bright xenocrystic cores are distinguishable from darker rims with variable degrees of concentric oscillatory zoning. Of 13 analyses, two cores give ages >1 Ga and are interpreted as inherited. The remaining analyses on rims and mantles produce ages between ca. 510 and ca. 540 Ma, giving a weighted average of 525 ± 6 Ma. However, of these, only three are concordant and used in the Concordia age calculation of 524 ± 16 Ma (Figure 6c). A Pb correction estimation on discordant samples

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Figure 5. Typical cathodoluminescence images of zircons from this study and locations of laser spots. U-Pb analyses indicated by continuous outlines. Data presented as grain number, U-Pb age ± 2σ.

intercepted Concordia at 523 ± 4 Ma (see supporting information). There is no distinguishable difference between rim, mantle, and core ages, which are all within error of each other. 3.4.2. RS-03 This is a leucotonalite dominated by plagioclase (often sericitized) and quartz with accessory zircon, apatite, tourmaline, opaque minerals, and epidote. The sample appears undeformed, but there is abundant evidence for metamorphic recrystallization of what are presumed to have been relatively coarse quartz grains into ﬁner-grained granoblastic aggregates. Most zircons are 100–150 μm in length and retain a prismatic habit, with oscillatory or sector-zoned rims surrounding dark, unzoned cores (Figure 5e). A 25-μm laser spot was used on narrow rims on this sample. Fifteen of 25 analyses are concordant and provide a Concordia age of 511 ± 6 Ma (Figure 6d). Six rims and one whole unzoned grain give a Concordia age of 504 ± 6 Ma or a weighted mean of 501 ± 6 Ma, while four analyses of mantle give a Concordia age of 510 ± 9 Ma and a weighted mean of 509 ± 10 Ma. Two cores and two unzoned grains are older with a Concordia age of 526 ± 9 Ma or a weighted average of 524 ± 10 Ma. Analysis R3-3 recorded the youngest Concordia age of 481 ± 17 Ma.

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Figure 6. Terra-Wasserburg Concordia plots for Paleozoic zircons from (a, b) matrix samples SH-16 and SH-23 (c–j) clasts of “Group 1 Main Granites.” Filled blue ellipses indicate Concordia age based on stated number of analyses (solid black line ellipses); dashed black line ellipses indicate concordant analyses excluded from this calculation. Dashed lines where present through discordant analyses (gray outline ellipses) indicate intercept calculated by common lead estimation on discordant analyses anchored through a 206Pb/204Pb of 0.9 ± 0.1 (shown only to indicate validity of discordant analyses). Error ellipses are shown at 2σ, and ages are given at 95% unless indicated as 2 s (*).

3.4.3. FZ-08 This sample is a ﬁne-grained, weakly foliated metagranite dominated by K-feldspar and quartz with subordinate mica (mainly muscovite with occasional biotite), chlorite, and accessory zircon, titanite, and epidote. Two distinct zircon populations are present. One is dominated by relatively large (~250–350 μm) grains that are honey-colored, anhedral, and very well rounded with high sphericity and irregular but predominantly oscillatory zoning. The second group is characterized by smaller (~150–250 μm) grains that are colorless to

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pale yellow, idiomorphic with high aspect ratio, and generally exhibiting regular concentric oscillatory zoning (Figure 5c). All analyses from the ﬁrst group are older than 1.0 Ga and interpreted as inherited, while half of the second group are concordant and give a Concordia age of 487 ± 7 Ma (Figure 6e). 3.4.4. FZ-14 This sample is a pink-green granitic gneiss comprising plagioclase, quartz, and hornblende with rare occurrences of relict pyroxene, now largely replaced by actinolite and chlorite. Accessory minerals include epidote and titanite. Zircons are colorless, euhedral with angular terminations, and of medium aspect ratio (Figure 5d). There are two distinct populations based on size. The smaller zircons (~150–200 μm) form an older group of analyses, yielding a Concordia age of 487.2 ± 3.0 Ma, while the larger zircons (~250– 300+ μm) give a younger Concordia age of 469 ± 5 Ma. Twenty-six of 48 analyses are concordant and produce a combined Concordia age of 482 ± 3 Ma (Figure 6f). 3.4.5. FZ-10 This sample is a pink-gray granitic gneiss dominated by quartz, plagioclase, and K-feldspar with subordinate mica (muscovite with minor biotite), chlorite, and accessory zircon, epidote, and titanite. Zircons are far less abundant in this sample and often in poor condition with cracks and inclusions. Grains are typically colorless, small (<150 μm), subhedral to anhedral, and generally display concentric oscillatory zoning (Figure 5f). Four of 17 analyses are older than 1 Ga and interpreted as inherited, while another six analyses are concordant and produce a Concordia age of 464 ± 6 Ma (Figure 6g). 3.4.6. SH-18 This sample is a pink and green-gray granitic gneiss with K-feldspar augen up to 5 mm in length. It is dominated by quartz and K-feldspar with subordinate mica (muscovite and biotite), chlorite, rare plagioclase, and accessory zircon and opaque minerals. Zircons are 100–200 μm in length, vary from well rounded to euhedral (Figure 5g), and are typically pale yellow to colorless and often cloudy. Six of 38 analyses give ages older than 1 Ga and are interpreted as inherited. Twenty-three of the remaining 32 analyses are concordant, with a weighted mean age of 457 ± 5 Ma. Nineteen of these yield a Concordia age of 458 ± 4 Ma (with the two oldest and two youngest analyses excluded from the calculation; Figure 6h). 3.4.7. SH-21 This sample is a granodioritic gneiss dominated by plagioclase and quartz with subordinate biotite and muscovite, secondary chlorite, and accessory zircon, apatite, and opaque minerals. Zircons are mainly 150–200 μm in length, colorless to pale yellow, and euhedral with medium aspect ratio and oscillatory concentric zoning (Figure 5h). Twenty-six of 34 analyses are concordant and give a Concordia age of 467 ± 3 Ma (Figure 6i). The youngest four rims give a Concordia age of 460 ± 9 Ma. 3.4.8. FZ-06 This is a gray-pink granitic gneiss dominated by quartz, plagioclase, and orthoclase with subordinate muscovite and chlorite and accessory zircon, apatite, epidote, titanite, and opaque minerals. Zircons are colorless, medium sized (~100–200 μm), and euhedral with concentric oscillatory zoning (Figure 5i). Rims are frequently cracked and contain multiple inclusions of apatite. Of 24 zircons analyzed, 23 are cores, only one of which provides an older age of 1684 ± 41 Ma and is interpreted as inherited. Thirteen analyses are concordant and give a Concordia age of 448 ± 3 Ma (Figure 6j). The three youngest grains give a Concordia age of 436 ± 13 Ma.

3.5. Group 2 Subordinate Granitic Clasts These six samples were all of leucotonalitic composition and undeformed with the exception of FZ-03 and only varying slightly in grain size and mica content. Although all samples in this group provided zircons for analysis, there was no distinct population from which to calculate a meaningful crystallization age. Zircon yield was low, and most grains are in a poor condition, with multiple inclusions or cracks and fractures or partially metamict. The combined data from these samples have age peaks at ca. 520–440, ca. 1,100, ca. 1,500, and ca. 2,800 Ma (Figure 4j). Zircons from the youngest peak are present in each sample, and thus, the Precambrian ages are interpreted as inherited. The exception was sample FZ-09, where zircons are deep brown, fragile, and of rusty appearance, and CL imaging reveals no zoning. No old ages are observed, but a single concordant analysis yields an age of 496 ± 16 Ma, while a common Pb regression anchored through the >20% discordant analyses gives an intercept of 503 ± 16 Ma.

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4. Discussion and Conclusions 4.1. Age of the Funzie Conglomerate: Stratigraphic and Regional Tectonic Implications The Funzie Conglomerate must have been deposited after the age of the youngest clast(s) and/or the youngest population of detrital zircons. The age of the youngest clast is 448 ± 3 Ma (FZ-06), and the youngest population of detrital zircons is 442 ± 5 Ma (SH-23). The latter indicates a lower limit of ca. 440 Ma for deposition. Provided that the NNE trending L2 lineation on Unst and Fetlar formed during a single orogenic event, and is not the result of superimposed events, an upper limit on deposition could be provided by Rb-Sr muscovite ages obtained from lithologies that carry this fabric in northeast and southwest Unst (Walker et al., 2016). However, these span a range of ages, and some analyses are of uncertain significance. An age of ca. 440 Ma from just east of Burrafirth may not be robust because of the low 87Rb/86Sr ratio. Younger 2-point ages of ca. 435 and ca. 430 Ma were obtained from a sample in southwest Unst. Combining the data from the two analyses from this sample yields a 3-point isochron of 433 ± 14 Ma (mean square weighted deviation [MSWD] 5.5). The error and relatively high MSWD suggest that there is some geological scatter due to mixing of multiple mica populations, which would account for the contrasting 2-point ages (Walker et al., 2016; S. Walker et al., personal communication, May 2018). Based on these data, an age of ca. 430 Ma is taken as a conservative upper limit on deposition of the Funzie Conglomerate. Given that the closure temperature of muscovite is likely to be greater than the temperature at which the fabric formed (Walker et al., 2016), the age is likely to be dating closely deformation. The new data, in combination with published mineral ages, therefore indicate the following: (1) that the Funzie Conglomerate was deposited, deformed, and metamorphosed within a period of ~10 million years in the Early Silurian (ca. 440–430 Ma); and (2) an Early Silurian age for L2 and sinistrally oblique extensional unroofing of the Grampian metamorphic pile. The latter is significant because elsewhere in Shetland the dominant structures and metamorphic mineral assemblages are thought to have formed during the Ordovician, apart from the Early Devonian Uyea Shear zone in northwest Shetland (Walker et al., 2016). 4.2. Provenance of the Clasts: Sourced From the West or East? The pebble to boulder sizes of the clasts within the Funzie Conglomerate implies a relatively proximal source, perhaps no more than a few tens of kilometers distant at most. A single source is assumed, although cannot be proven unequivocally. A key question is whether the clasts were sourced locally within Shetland or to the east where they could have been derived from basement either submerged under the North Sea or exposed onland in Norway. An easterly provenance is preferred for three main reasons. First, there is no obvious source of orthoquartzite within any of the exposed metasedimentary successions in Shetland to account for the quartzite clasts. Although areas of quartzite are shown on published maps within the lowest parts of the East Mainland Succession on Unst and Fetlar (British Geological Survey, 2002), these are relatively impure and noticeably more feldspathic than the great majority of orthoquartzite clasts within the Funzie Conglomerate. Second, three of the granitic clasts that yielded U-Pb zircon ages of <465 Ma are characterized by gneissic fabrics consistent with high-temperature recrystallization and deformation. However, none of the granitic intrusions on Shetland that were emplaced after 465 Ma contain gneissic fabrics (Lancaster et al., 2017), ruling out a local source. Third, the matrix of the Funzie Conglomerate shows no sign of derivation from a medium- to high-grade Barrovian metamorphic terrane. The Yell Sound and Westing groups, as well as the East Mainland Succession, contain common garnet within pelitic and mafic lithologies, and monazite, kyanite, and staurolite are also locally common (Flinn, 2007, 2014; Read, 1934). If the Funzie Conglomerate had been derived from a source within Shetland, it would be reasonable to sup- pose that these minerals would be present as detritus within the matrix, but instead they are entirely absent. 4.3. Age, Provenance, and Depositional Location of the Sedimentary Protolith of the Orthoquartzite Clasts The sedimentary protolith of the orthoquartzite clasts must have been deposited after the youngest significant population of detrital zircons, therefore after ca. 1,100–1,000 Ma (Figure 4). Correlation of the age-frequency plots for the detrital zircons with the main crustal age provinces in eastern Laurentia and Baltica (Figure 7) might be expected to provide information on provenance. The age-frequency plots for the detrital zircons have broadly similar peaks at ca. 1,100–1,000, ca. 1,700, and ca. 2,700 Ma (Figures 4a–4d), which compare closely with detrital zircon data sets from the East Mainland Succession (Figure 4h; Strachan et al., 2013). However, clear identification of source areas is problematic because of

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Figure 7. Paleocontinental reconstruction showing the relative positions of Laurentia, Amazonia, and Baltica during the late Neoproterozoic (<700 Ma), the main age provinces that relate to major orogenic and/or magmatic events, and locations of late Neoproterozoic and Cambrian igneous suites/intrusions mentioned in the text (modiﬁed from Cawood et al., 2007). Areas of crust subsequently affected by Caledonian orogenesis are mostly left blank. S = Shetland Islands; SV = Sveconorwegian Orogen; AMCG = anorthosite-mangerite-charnockite-granite; RSI = Rondonian-San Ignacio Province; CC = Carn Chuinneag-Inchbae granite; BV = Ben Vuirich granite; TV = Tayvallich Volcanics; T = Tyrone Igneous Complex; SIP = Seiland Igneous Province; SN = Seve Nappe dykes; ST = Storen Nappe; E = Egersund dolerites; F = Fen carbonatite complex.

the similarities in the Neoarchaean to Mesoproterozoic crustal evolution exhibited by eastern Laurentia and Baltica (Figure 7; Cawood et al., 2007). The most conservative interpretation is that the sedimentary protolith was deposited on the margin of Laurentia in a high-energy middle-to-outer continental shelf setting, consistent with the highly mature nature of the sediments. A <1,000-Ma age would permit broad correlation with either the Westing Group or the East Mainland Succession. The latter is preferred as there is no evidence within the clasts of the high-grade gneissic fabrics that might be expected had the protolith undergone the 930- to 920-Ma metamorphic event recorded by the Westing Group (Cutts et al., 2009).

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An alternative scenario is that the sedimentary protolith was deposited on Baltica and later transferred as clasts onto Laurentian crust (Shetland) during the Caledonian continental collision. However, given that southern Norway is the most likely part of the Baltica paleocontinent to have collided with Shetland, this can be ruled out as this region does not contain the Archaean crust necessary to account for the oldest detrital grains analyzed in the present study.

4.4. Provenance and Significance of the Lower Paleozoic Zircons From the Granitic Clasts and Matrix 4.4.1. Cambrian Zircon Ages Samples FZ-02 and RS-03 yielded Cambrian U-Pb zircon ages of 524 ± 16 and 511 ± 6 Ma, respectively, but these are of uncertain significance because they overlap contrasting tectonic events. There is general agree- ment that igneous activity at ca. 615–580 Ma relates to continental breakup, separation of Baltica and Laurentia, and opening of the Iapetus Ocean. This includes in Norway the ca. 616-Ma Egersund dolerites (Bingen et al., 1998), ca. 606-Ma sheeted dykes within the Seve Nappes (Svenningsen, 1994), and the ca. 583-Ma Fen carbonatite province (Meert et al., 1998) and in Scotland the Tayvallich Volcanic Formation and the Ben Vuirich, Portsoy, Keith, Carn Chuinneag-Inchbae, and East Sutherland augen granites (Dempster et al., 2002; Kinny et al., 2003; Oliver et al., 2008; Rogers et al., 1989; Strachan et al., 2002; Figure 7). However, the younger ca. 580- to 560-Ma Seiland Igneous Province in Norway (Figure 7) (Roberts et al., 2010), which contains late pegmatites dated at ca. 531–523 Ma (Pedersen et al., 1989), has also been attributed to continental extension. Rather than this being related to the opening of Iapetus, it might have resulted from an inboard switch of the locus of rifting associated with the separation of microcontinental fragments from Baltica (see Andersen et al., 2012; Jakob et al., 2017). Conversely, subduction may have been initiated as early as ca. 505–500 Ma (Seve Nappe eclogites; Mørk et al., 1988; Essex et al., 1997). The oldest felsic igneous rocks that have been attributed to subduction have yielded U-Pb zircon ages of 495 ± 3 Ma (Støren Nappe, central Norway; Roberts & Tucker, 1998) and 493 ± 2 Ma (Tyrone Igneous Complex, Ireland; Draut et al., 2004; Figure 7). Accordingly, there would appear to be two main possibilities for the Cambrian ages reported here: Either (1) they date a relatively young phase of continental extension or, and perhaps more likely, (2) subduction and closure of the Iapetus Ocean was initiated significantly earlier than generally supposed. Further geochemical and isotopic studies are necessary to resolve this issue. 4.4.2. Ordovician to Silurian Zircon Ages In the context of tectonic models for the early stages of Iapetus closure in the North Atlantic region (Dewey & Ryan, 1990; Dewey & Mange, 1999; Chew et al., 2010), it is likely that the oldest Ordovician ages (ca. 487 and 482 Ma) relate to the development of a magmatic arc above an east dipping (oceanward) subduction zone (Figure 8a). The collision of the magmatic arc with the Laurentian margin at ca. 485–470 Ma resulted in ophiolite obduction and the Grampian I orogenic event that was followed by a change in subduction polarity to westward and toward Laurentia (Figure 8b; Dewey & Ryan, 1990). The other four dated granitic clasts (ca. 466–448 Ma) and the youngest zircon populations in the two matrix samples (ca. 447 and 442 Ma) are presumed to have been derived from plutons that were emplaced during this younger phase of subduction. The Midland Valley Terrane is thought to extend along strike under the North Sea to underlie the Utsira High and the East Shetland Basin (Figure 9; Fichler et al., 2011;Lundmark et al., 2014 ; Slagstad et al., 2011), both close potential source areas for the Funzie Conglomerate. Calc-alkaline plutons sampled in offshore bore- holes yielded U-Pb zircon ages in the range ca. 482 to ca. 421 Ma (Slagstad et al., 2011; Lundmark et al., 2014) and could potentially account for some of the igneous-derived detritus in the Funzie Conglomerate. Another point of similarity is that both data sets contain inherited zircons consistent with derivation of melts from older (Mesoproterozoic to Archaean) basement. An older crustal component, if present, would mean either that subduction was continent directed or that a continental ribbon had become detached from Laurentia and served as the basement for a Cambro-Ordovician arc that was subsequently accreted back to the continental margin. However, the presence of inherited zircon grains does not, on its own, necessarily imply proximity of the arc to the Laurentian margin. Numerous studies have provided evidence of long- distance transport of continentally derived sedimentary rocks into the oceanic realm prior to their incorporation into a subduction zone (e.g., Dewey & Casey, 2015, and references therein; Shao et al., 2015). Potential onshore sources for the Ordovician-Silurian detritus in the Funzie Conglomerate occur along much of the length of the Scandinavian Caledonides. The closest to Shetland are exposed in SW Norway where allochthonous oceanic-derived nappes include major ophiolites and associated volcanic and intrusive arc

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Figure 8. Schematic model for Cambrian to Early Silurian events within the Caledonides of Shetland. (a) Development of an intra-oceanic arc above a subduction zone within the Iapetus Ocean (dots indicate Neoproterozoic to Cambrian sedimentary rocks). (b) Collision at ca. 485–470 Ma of the arc with the Laurentian margin, ophiolite obduction, deformation, and metamorphism of footwall successions. Note the change in direction of subduction following polarity ﬂip and the dashed line that indicates the amount of crustal section removed subsequently by erosion. (c) Collision at ca. 440–430 Ma of Baltica and Laurentia, resulting in ① renewed thrusting. The Funzie Conglomerate is thought to have been derived from, and ultimately overridden by, thrust sheets composed of East Mainland Succession Dalradian (Neoproterozoic) metasedimentary material, igneous lithologies that formed part of the oceanic arc accreted at 485–470 Ma, and younger plutons emplaced into both units. This was followed by ② sinistrally oblique extension (movement direction into the page) that excised ~10 km of crustal section, juxtaposing the Unst ophiolite with the metamorphic rocks in its present footwall. “U” in (b) and (c) corresponds to the approximate location of Unst within the two thickened crustal sections.

rocks that have yielded U-Pb zircon ages in the range ca. 495–460 Ma. These include the Karmoy (493 + 7/- 4 Ma) and Gullfjell (489 ± 3 Ma) ophiolites (Figure 1a; Pedersen & Dunning, 1997), the Vardefjell Gabbro (472 ± 2 Ma), a rhyolite from the Siggio Complex (473 ± 2 Ma), a granite (474 ± +3/À2 Ma) and a quartz diorite (479 ± 5 Ma) from the West Karmoy Igneous Complex, a rhyolite from the Kattnakken volcanics (476 ± 4 Ma; Pedersen & Dunning, 1997), and the Rolvsnes Granodiorite of the Sunnhordland Batholith (Figure 1a; 466 ± 3 Ma; Scheiber et al., 2016). The continuation of arc magmatism into the Silurian is demonstrated by U-Pb zircon ages obtained from a diorite that intrudes the Solund-Stavfjord ophiolite (443 ± 3 Ma; Dunning & Pedersen, 1988) and, further aﬁeld, from the Smøla-Hitra Batholith (ca. 445– 440 Ma; Tucker et al., 2004) and the Bindal Batholith (ca. 447–430 Ma; Nordgulen et al., 1993; Figure 1a).

4.5. A Synorogenic Basin Setting for Deposition of the Funzie Conglomerate The new data reported here in combination with the pressure-temperature data of Cutts et al. (2011) and isotopic data of Walker et al. (2016) indicate a complex Ordovician-to-Early Silurian evolution as follows. Grampian I orogenesis between 485 and 470 Ma resulted from the collision of the Laurentian margin with a magmatic arc (the Midland Valley arc and its along-strike continuation in the North Sea and within the

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Figure 9. Sketch map of basement units in the northern North Sea and surrounding areas (redrawn from Lundmark et al., 2014). North Atlantic oceanic crust omitted.

allochthonous oceanic terranes of west Norway). Arc-continent collision resulted in obduction of the Unst ophiolite (Figure 8b). This was followed at ca. 450–445 Ma by Grampian II metamorphism and localized deformation, although the tectonic driver is poorly understood. Alternative solutions are either the collision of a small microcontinental fragment or arc with Laurentia (Figure 8b; Bird et al., 2013) or flat-slab subduction (Dewey et al., 2015). Initial collision of the outboard sectors of Laurentia and Baltica is likely to have started at some time after ca. 440 Ma (Grimmer et al., 2015), resulting in early thrust stacking on both continental margins (Figure 8c). On the Laurentian side of the collision (i.e., northeast Shetland) that formed the retrowedge of the orogen, the advancing thrust nappes were composed largely of greenschist facies Neoproterozoic metasedimentary rocks derived from the East Shetland Platform (= protolith of the orthoquartzite clasts) as well as granitic rocks derived from lapetan arcs underlying the East Shetland Basin and/or sources in west Norway. It is envisaged that rapid erosion of these nappes generated the Funzie Conglomerate during west directed thrusting and nappe stacking (Figure 8c). This was followed at ca. 430 Ma by sinistrally oblique extension along low-angle detachments, culminating in final juxtaposition of the ophiolite against its footwall basement units (Figure 8c). The tectonic drivers for oblique extension are uncertain. One possibility is that it resulted from instability in the orogenic wedge arising from deeper-level (but currently unexposed) thrusts. An alternative is that it marks the early onset of relative sinistral displacements between Laurentia and Baltica following oblique continental collision (Dewey & Strachan, 2003), although it was postdated by final thrust emplacement of the upper ophiolite nappe in Unst and Fetlar. One key observation suggests that the Funzie Conglomerate accumulated in a synorogenic setting rather than an extensional collapse basin, and this relates to the pervasively metamorphosed nature of the at least 1-km-thick succession at temperatures broadly equivalent to the middle greenschist facies. It is difficult to understand how these temperatures could have been generated during extension and/or without generat- ing an unfeasibly thick sedimentary overburden. Sedimentary successions within extensional collapse basins elsewhere such as the Norwegian Caledonides (Osmundsen et al., 2003, 2006; Osmundsen & Andersen, 2001; Séguret et al., 1989) and the French Variscides (Echtler & Malavieille, 1990) are largely unmetamorphosed and only locally deformed adjacent to the basin-bounding detachment faults. For this reason, the most likely scenario is that the Funzie Conglomerate was deposited within a synorogenic basin that received the detritus from, and was buried by, advancing thrust nappes during the initial stages of crustal thickening resulting from the collision of Laurentia and Baltica (Figure 8c). This provides a feasible mechanism for the tectonic burial to elevated background temperatures, followed rapidly by pervasive ductile deformation during the

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oblique extensional unrooﬁng of the metamorphic pile. It is this phase of extension that accounts in part for the preservation of the Funzie Conglomerate as the sole occurrence of what was presumably a regionally extensive synorogenic basin within the metamorphic hinterland of the North Atlantic Caledonides.

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