Laurentian margin evolution and the Caledonian orogeny—A template for Scotland and East Greenland
Published:January 01, 2008
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A. Graham Leslie, Martin Smith, N.J. Soper, 2008. "Laurentian margin evolution and the Caledonian orogeny—A template for Scotland and East Greenland", The Greenland Caledonides: Evolution of the Northeast Margin of Laurentia, A.K. Higgins, Jane A. Gilotti, M. Paul Smith
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The orthotectonic Scottish Caledonides constitute only a small fragment of the Neoproterozoic to Paleozoic margin of Laurentia, albeit one which lies at a prominent bend in that margin. Sequences exposed in the Scottish outcrop include Mesoproterozoic, Neoproterozoic, and Cambrian-Ordovician strata that record sedimentation, volcanism, and deformation related to the latter stages of the amalgamation of Rodinia, the subsequent breakout of Laurentia, and growth of the Iapetus Ocean. Metamorphic and tectonic overprints then record the destruction of that ocean through Ordovician arc accretion and mid-to-late Silurian collision of Laurentia, Baltica, and Avalonia and the final closure of Iapetus by end-Silurian time. New isotopic data and recent advances in the understanding of the late Mesoproterozoic (Stenian) to Cambrian-Ordovician stratigraphic framework now better constrain the sequence and timing of events across the “Scottish Corner” and invite a dynamic comparison with the current research into the East Greenland Caledonides summarized in this volume. Although many broad similarities exist, the comparisons described here reveal for the first time a number of significant contrasts in the spatial arrangement of depocenters, location of rifting, and patterns and timing of magmatism, metamorphism, and contractional deformation. This expanded understanding of the late Neoproterozoic evolution of these adjacent sectors of Laurentia provides an important basis for reconstructions of the subsequent early Paleozoic Caledonian orogenic evolution of the present North Atlantic region.
INTRODUCTION AND REGIONAL COMPARISONS
The Caledonides of East Greenland disappear southward beneath Paleogene flood basalts at Scoresby Sund (70°N). Thereafter, the most proximal sector of the preserved Laurentian margin at the onset of Silurian (Scandian) collision of East Greenland with Baltica is the Scottish Highlands and northern parts of Ireland. Palinspastic reconstructions (Dickin, 1992; Cambridge PalaeoMap, 1998) indicate that during early Paleozoic time, Scotland may have lain as little as 500 km to the south of central East Greenland (Fig. 1).
With a present-day along-strike section of ∼600 km, and as one of the most intensively studied orogens in the world (Strachan et al., 2002), Scotland shares many aspects of Laurentian geology (Fig. 2) with the 1300-km-long East Greenland Caledonides. Nevertheless, several fundamental problems still lack a definitive interpretation despite thousands of publications detailing decades of field and laboratory investigations into the perplexing architecture and history of the Scottish Caledonides. The basin architecture accommodating each of the Stoer and Torridon Groups (the “Torridonian”) and the Moine Supergroup has not been resolved; whether or not parts of these sequences might be correlated within a single depositional system is still a matter of debate. The cause and spatial extent of the Knoydartian tectonothermal event (or events) are likewise unresolved, and we cannot as yet define the age and nature of the base of the Dalradian Supergroup. The glacigenic deposits within the Dalradian Supergroup (tillites) are not yet age-constrained or definitively placed within a global sequence. These problems arise, at least in part, from the difficulty in dealing with the lack of critical exposure in an upland glaciated terrain where there is an extensive cover of superficial deposits.
Thus, there is much to envy in the geological vistas of the fjords and mountains of East Greenland. With the completion of the 1:500,000 mapping and research program by the Geological Survey of Denmark and Greenland (GEUS), it is therefore timely to explore the similarities and contrasts between these two sectors and their roles in the Precambrian to early Paleozoic evolution of the Laurentian margin. We note, however, that while with our broadened perspective we may have moved on from the vestige of a beginning in our understanding of this part of the Laurentian margin, there seems to be no prospect of an end in view!
In this account, we take the stance that the East Greenland and Scottish Caledonides record a shared Neoproterozoic to lower Paleozoic tectono-stratigraphic and tectono-metamorphic Laurentian geological history, albeit with a diachroneity and difference in detail that reflect their individual locations on the margin. An alternative “mobilistic” model, which is not further discussed here but involves a protracted history of major lateral movements and the amalgamation of terranes of quite separate affinities, has been proposed for the Scottish Caledonides (Bluck et al., 1997).
The late Neoproterozoic (ICS [International Commission on Stratigraphy] time scale, Gradstein et al., 2004) position of Scotland has been previously interpreted as occupying a stable promontory in the Archean to Paleoproterozoic Hebridean Shield (Dalziel and Soper, 2001) and ultimately close to an Ediacaran (Vendian) RRR junction (inset to Fig. 1) (Soper, 1994a). The most recent analysis of the paleomagnetic and geological constraints argues that the western Scandinavian margin of Baltica faced the eastern Greenland margin of Laurentia in its right-way-up orientation (Cawood and Pisarevsky, 2006). The reconstruction in Figure 1 adopts this configuration and includes a schematic restoration on the numerous major thrusts identified in the Caledonides of East Greenland and in the north of Scotland. On this basis, Scotland does lie on a corner in the Laurentian margin and potentially in the vicinity of a RRR junction, but the concept of a “Scottish promontory” is not sustained.
In contrast, a general absence of volcanic activity or other evidence of extensive or rapid upper Neoproterozoic extension suggests that the East Greenland and Eastern Svalbard margin of Laurentia lay distant from such an Iapetan RRR locus. East Greenland was also apparently isolated from the active spreading junction that affected northern Greenland, western/central Svalbard, Scandinavia, and Siberia as recorded by ca. 650–600 Ma igneous mafic activity (Gromet and Gee, 1997; Bingen et al., 1998) (Fig. 1). Earlier attempts at rifting, possibly signaling the initiation of continental breakup, are recorded in the ca. 720 Ma Coronation mafic dike swarm of Baffin Island and West Greenland (Shellnutt et al., 2004) and ca. 700 Ma mafic sills in NE Svalbard (Johansson et al., 2004). Abundant mafic volcanics also mark extension and rifting on the Appalachian sector at this time (Bailey and Tollo, 1998; Tollo and Hutson, 1996; Tollo et al., 2004). Such activity is absent from central East Greenland. Thus, over many millions of years, the Neoproterozoic successions of Greenland perhaps record relatively quiescent deposition on a continental margin lying between migratory RRR junctions throughout the evolution of the Iapetus Ocean.
For reference we delineate, in the inset to Figure 2, those fault-bounded crustal terranes identified in Scotland as having a distinctive geological history (cf. Strachan et al., 2002). Where practicable, we will refer to geographical regions (Fig. 1) rather than specific terranes; fault-bounded terranes are not defined in the East Greenland Caledonides, and the full status of some of the Scottish “terrane boundaries” outlined in the inset map (Fig. 2) is still a matter of debate. In general terms, the Neoproterozoic–Ordovician rocks of the northwest Scottish Highlands preserve a record of the Laurentian autochthonous-parautochthonous foreland (Figs. 2 and 3A). These successions correspond stratigraphically and chronologically with rocks of the Laurentian foreland succession preserved in the nunatak region of central East Greenland, the parautochthonous belt of western Kronprins Christian Land in eastern North Greenland, and the parautochthonous successions exposed in tectonic windows along the length of the East Greenland Caledonides (Figs. 3B and 3C). Elements of the geology of the Northern and Grampian Highlands of Scotland may be compared with similar sequences in the Niggli Spids thrust sheet, Hagar Bjerg thrust sheet, and Franz Joseph allochthon of central East Greenland, to the eastern hinterland of Dronning Louise Land, and to the Vandredalen thrust sheet of Kronprins Christian Land (Figs. 3A, 3B, and 3C).
Bearing in mind the relative positions and regional structural trends (Fig. 1), it is then tempting to link the individual major fault structures in Scotland (e.g., Moine thrust, Great Glen fault, Highland Boundary fault) with the major bounding structures of the East Greenland thrust sheets (e.g., Caledonian sole thrust, Fjord region fault, Western fault zone). That said, the differences in present knowledge and understanding of the history of these fault zones mean that such correlations remain speculative.
While such broad similarities invite comparison, intriguing contrasts also exist between Scotland and East Greenland. For example, the thick siliciclastic contemporaneous (early Neoproterozoic) sequences of the Krummedal succession of central East Greenland (Higgins, 1988) and the Moine Supergroup of the Northern Highlands in Scotland (Figs. 3A and 3B) share a similar provenance (Friend et al., 2003; Kalsbeek et al., 2000; Watt et al., 2000) and together, are remarkable for evidence of repeated high-pressure, high-temperature metamorphism. While the metasedimentary rocks from the Northern and Grampian Highlands record evidence for a series of Neoproterozoic tectonothermal events between 820 Ma and 730 Ma (Fig. 4), these are unknown in Greenland. Instead, the Krummedal succession was there affected by a single high-grade metamorphic event that culminated in the generation of voluminous S-type augen granite, at ca. 910 Ma (Leslie and Nutman, 2003).
The younger and well-preserved mid- to late Neoproterozoic and early Cambrian sequences of the Grampian Highlands reveal a history of repeated uplift, rifting, and complex internal basin architecture. Mafic volcanic rocks developed at several levels in the evolving depositional pile, and the transition from rift to drift in the developing Iapetus Ocean occurred at ca. 610–600 Ma (Figs. 3A and 4). In contrast, the succession of mid- to late Neoproterozoic sediments in central East Greenland, the Franz Joseph allochthon, although extremely thick, records sustained subsidence but no active rifting. An apparent depositional thickness of over 14 km of sediment is assigned to the Eleonore Bay Supergroup alone, and a further 1 km is assigned to the Ediacaran Tillite Group (Sønderholm et al., this volume); mafic volcanic rocks are conspicuous by their absence (Fig. 3B). While there may be some limited evidence of stretching and rift shoulder uplift farther inboard on the restored margin (Leslie and Higgins, 1998, this volume), major extension and rifting are only clearly evident much farther north in eastern North Greenland where the late Neoproterozoic Hekla Sund Basin rift-sag sequence dominates the geology (Fig. 3C; Higgins et al., 2001b). Neoproterozoic tillites, if originally deposited, are now absent on the foreland in Scotland, whereas they are present in significant thicknesses, in both the parautochthonous foreland windows and the fjord region allochthon in East Greenland.
The ensuing Neoproterozoic and early Paleozoic deformation and metamorphism across Scotland and Greenland record the final amalgamation then breakup of the ancient supercontinent of Rodinia, followed by convergence, and eventual collision of Baltica with the East Greenland sector of the Laurentian margin in the Caledonian orogeny (Fig. 4). Grampian (Ordovician) orogenesis and arc accretion dominate the structural framework of the Grampian and parts of the Northern Highlands in Scotland but are apparently only expressed in the southernmost extremity of the East Greenland Caledonides. Conversely, the subsequent Scandian (Silurian) orogenesis, which documents the final collision and docking of Baltica with Laurentia, dominates the structure in only the Northern Highlands in Scotland but is the pervasive control on Caledonian structural chronology and architecture throughout East Greenland.
In this chapter, we present a synopsis of the Archean to lower Paleozoic geology of the “Scottish Corner” of Laurentia as a series of time slices summarized in the tectono-stratigraphic template presented in Figures 3 and 4. In this framework, we explore and test the key comparisons and contrasts between Scotland and East Greenland, drawing particular attention to the locations of rifting and spatial arrangements of depocenters, as well as to the key orogenic events. We conclude by presenting a dynamic synthesis of Iapetan rifting and Caledonian orogenesis in the Scottish Caledonides, and, while recognizing that no consensus can currently be reached, we contend that the combined geology of Scotland and Greenland provides real constraints upon the Neoproterozoic to early Paleozoic evolution of Laurentia.
ASSEMBLY OF A STABLE BASEMENT: PRE-STENIAN (1200 Ma) HISTORY
The oldest rocks in Scotland are the crystalline basement rocks of the Lewisian Complex in the Hebrides and Northwest Highlands (Fig. 2; Plates 1A and 1B). This complex is exposed along the mainland coastal strip in the footwall of the Moine thrust zone (Fig. 2; Plate 1B) and extends westwards across the Outer Hebrides and to a broad region (Rockall) on the edge of the UK continental shelf (Dickin, 1992). East of the Moine thrust zone, geophysical data (Trewin and Rollin, 2002) imply that Lewisian, or similar, rocks underlie the Moine rocks in Scotland at least as far as the trace of the Great Glen fault (Fig. 2). The Lewisian Complex is, and has been, the subject of intense scrutiny. Only a brief summary is provided here, and the reader is referred to the comprehensive summary of Park et al. (2002) for further details and the recent review of Park (2005).
The oldest rocks are Archean-age granulite-facies orthogneiss (the Scourian), which had already been reworked by two tectonothermal events prior to the intrusion of the basic and ultra-basic magmas of the Scourie dike suite in mid-Paleoproterozoic time (Park et al., 2002; Park, 2005). The Scourian rocks are typically gray, banded trondhjemite-tonalite-granodiorite (TTG) orthogneisses with rare metasedimentary enclaves; basic enclaves may represent relict oceanic crust (Plate 1B). Studies of the tectonothermal history in the Lewisian Complex have previously assumed that these rocks are broadly correlatable across the Hebrides and Northwest Highlands. However, recent work by Friend and Kinny (2001), Kinny et al. (2005), and Park (2005) has proposed a series of discrete Archean blocks that amalgamated during the Paleoproterozoic, each with a more complicated, but as yet unresolved, early history. Orthogneisses of the mainland central block yield 3.03–2.96 Ga protolith ages and record granulite-facies metamorphism (the Badcallian event) at ca. 2.5 Ga (Whitehouse, 1989; Friend and Kinny, 1995; Corfu et al., 1998). Protolith ages in the northern block range from 2.84 to 2.68 Ga, and in the southern block, ages range from 2.82 to 2.73 Ga (Kinny and Friend, 1997; Corfu et al., 1998).
The ensuing Inverian event postdated a suite of pegmatites (2.49–2.48 Ga) and is marked by retrogression to amphibolite facies and the formation of shear zones that predate the earliest Scourian dikes (Corfu et al., 1998). The Scourian dikes were intruded, predominantly as quartz-dolerites, over a considerable period of time, and the main swarm was emplaced ca. 2.42 Ga (Heaman and Tarney, 1989).
Younger supracrustal strata are represented in the Loch Maree Group, which consists of two structural belts of metasedimentary and meta-igneous rocks (M on Fig. 2). The Loch Maree succession accumulated at ca. 2.0 Ga and includes graywacke, quartzite, carbonate rock, and banded iron formation, along with sheets of amphibolite. Park et al. (2001) interpreted these as graywackes that accumulated close to a continental source; the rare earth element (REE) chemistry of the amphibolite suggests an origin as oceanic plateau lava and subsidiary primitive island-arc basalt. The supracrustal rocks are cut by orthogneiss that yields U-Pb zircon ages of 1.9 Ga, which are interpreted to be the age of emplacement (Park et al., 2001). These crosscutting gneisses have a primitive arc signature and thus provide further evidence of a Paleoproterozoic magmatic arc and subduction of oceanic material (Park et al., 2001).
The Laxfordian refers to tectonothermal events that modify the Scourie dikes. An early phase, dated at 1.86–1.63 Ga, may be related to a network of low-angle transpressional shear zones (Coward and Park, 1987). Later-phase, steep shear zones and refolding in upright structures were accompanied by retrogression to greenschist-facies conditions.
The rocks of the Lewisian Complex exposed on the islands of the Outer Hebrides (Fig. 2) are broadly similar to the northern and southern mainland blocks but do show some significant differences. There are, for example, far fewer Scourie dikes, and although the supracrustal rocks on South Harris have similar lithologies to the Loch Maree Group, they yield younger U-Pb ages, younger than ca. 1.9 Ga (Whitehouse and Bridgwater, 1999). Granulite-facies metamorphism dated at 1.9–1.8 Ga is dominant. These differences led Friend and Kinny (2001) to argue that the Outer Isles basement has more affinity with East Greenland than with the adjacent Scottish mainland.
The pattern of protolith ages in the basement complexes in East Greenland also reflects this widespread Paleoproterozoic activity. Archean protolith (2.8–2.7 Ga) is also evident in the gneisses in Gletscherland and Nathorst Land, which are broadly equivalent, therefore, to the Archean gneisses of the Ammassalik region (Rex and Gledhill, 1981; Thrane, 2002; F. Kalsbeek, 2005, personal commun.) (Fig. 1 in Higgins and Leslie, this volume). Paleoproterozoic (ca. 1.9 Ga) gneissose and granitoid rocks that occur farther north in East Greenland (eastern Frænkel Land and Suess Land; Fig. 1 in Higgins and Leslie, this volume) were accreted around 2.0 Ga (Kalsbeek et al., 1993) and are in many respects similar to the somewhat younger (and better preserved) Ketilidian orogenic belt in South Greenland.
Reconstructions of the Paleoproterozoic belts and Archean cratons of the North Atlantic region (Buchan et al., 2000) show the Lewisian Complex as part of the Paleoproterozoic internal belt linking the eastern Churchill province of the Canadian Shield, the Nagssugtoqidian of Greenland, and the Lapland-Kola belt of Scandinavia (Fig. 5). Wright et al. (1973), Myers (1987), Kalsbeek et al. (1993), and Park (1994) have highlighted the many similarities of the Lewisian Complex to the Nagssugtoqidian, or Ammassalik belt, of South-East Greenland. The latter consists of reworked Archean granitoid gneisses cut by mafic dikes and other intrusions (Kalsbeek, 1989). The central part of the Ammassalik belt contains abundant metasediments with a depositional age of ca. 2.0–1.9 Ga (Bridgwater et al., 1996) cut by subduction-related, 1.9 Ga calc-alkaline granitoid intrusions (Kalsbeek et al., 1993) and by deformed and metamorphosed intrusions of the 1.89 Ga Ammassalik Complex. The Charcot Land and Eleonore Sø supracrustal successions (Higgins et al., 2001a) of central East Greenland share many similarities with the Loch Maree Group rocks of Scotland and should be regarded as directly comparable.
By ca. 1.9 Ga, the ancient basement of Scotland (Lewisian Complex), Greenland (Nagssugtoqidian), and the coeval Lapland-Kola belt formed one continuous accretionary belt composed of various Archean cratonic components welded together during the assembly of Laurentia and Baltica (Fig. 5; Buchan et al., 2000; Dickin, 1992). By ca. 1.84 Ga, calc-alkaline magmatism was concentrated along a new active margin represented by the ca. 1.9–1.85 Ga Makkovik/Ketilidian belt of Labrador and South Greenland and the younger (1.85–1.50 Ga) Labradorian-Gothian belt of NE Canada and SW Scandinavia. In Scotland, the Labradorian-Gothian belt is represented by the Rhinns Complex of Islay (Muir et al., 1994), part of a largely submerged area (the Malin block) of juvenile Proterozoic crust that formed ca. 1.78 Ga (Marcantonio et al., 1988).
MESOPROTEROZOIC BASIN EVOLUTION (Ca. 1200–900 Ma)
There is a paucity of evidence to define the regional extent of the Mesoproterozoic to early Neoproterozoic Grenvillian global orogenic event (ca. 1200–950 Ma) in Scotland and East Greenland. Evidence has yet to be determined in Greenland and, in the Western Highlands of Scotland, is only locally recorded in the Glenelg Inlier, where eclogite- and amphibolite-facies metamorphism is dated at ca. 1000 Ma (Sanders et al., 1984; Storey et al., 2004). This time interval is marked in both East Greenland and Scotland by the accumulation of thick silici-clastic sedimentary successions, and the degree to which syn-depositional crustal extension and active rifting were involved in these accumulations remains unclear.
The Stoer and Torridon Groups (“Torridonian”)
In Scotland, the Lewisian Complex had by late Proterozoic time been deeply eroded and buried by an unconformable succession of red arkosic sandstones, informally referred to as the “Torridonian” (Figs. 2 and 3; Plate 1C). These sediments are facies equivalents of similar red-bed successions in Labrador and the Great Lakes area, and they are interpreted to have occupied rifts that developed peripherally to the eroding and maturing Grenville orogenic belt that extended across Rodinia (Winchester, 1988; Turnbull et al., 1996; Gower, 1988). While a number of lines of sedimentologic evidence support a rift-dominated setting for these deposits in Scotland (Stewart, 2002), evidence of related volcanism is restricted to undersatu-rated mafic volcaniclastic detritus present in the Meso proterozoic (ca. 1200 Ma) Stoer Group (Stewart, 1991). The early Neoproterozoic (ca. 1000–900 Ma) Torridon Group lacks any volcanic association, is unconformable on the Stoer Group, and the relative ages are in accord with paleomagnetic evidence that shows a 90° change in polarity across the unconformity (Smith et al., 1983). This unconformity broadly coincides with the climactic Grenvillian orogenic deformation in North America.
Active syndepositional faulting cannot unequivocally be demonstrated in the Torridon Group since east-facing faults reactivated during Iapetan thrusting (Butler, 1997) may signify Iapetan rather than early Neoproterozoic extension. An alternative model is that the Torridon Group sediments were deposited in fluvial environments by major river systems that drained the foreland to the Grenville orogen (Rainbird et al., 2001). Detrital zircons from the Torridon Group have yielded a minimum U-Pb age of 1060 ± 18 Ma (Rainbird et al., 2001), coeval with the later stages of Grenville orogenic activity. No comparable sediments are preserved in East Greenland.
The Moine Supergroup
In areal terms, the Moine Supergroup dominates the rocks of the Northern Highlands of Scotland (Figs. 2 and 3A). Inliers of similar strata occur southeast of the Great Glen fault (the Dava and Glen Banchor successions) and form the basement to the younger Dalradian Supergroup in the Grampian Highlands (Smith et al., 1999).
The monotonous siliciclastic lithology of the Moine Supergroup provides few distinctive horizons that can be easily correlated over great distance. Three formal lithostratigraphic groups are recognized (Fig. 4): Morar (oldest), Glenfinnan, and Loch Eil (youngest) (Holdsworth et al., 1994). The Morar Group is a 5-km-thick tripartite psammite-pelite-psammite succession. The Glenfinnan Group is characterized by striped units of thinly interbanded psammites, semipelites, pelites, and quartzites (Plate 1D) together with thick pelite formations; estimates of thickness vary from 1 to 4 km, largely as a result of the high levels of ductile strain. The largely psammitic Loch Eil Group may be up to 5 km thick.
Detrital and inherited zircon grains constrain a source age for detritus between ca. 1.8 and 0.9 Ga; Archean sources (ca. 2.9 Ga) are only prominent in basal units just above the inliers of Lewisian basement rocks (Friend et al., 2003; Cawood et al., 2004). The Moine rocks were thus probably derived from erosion of the assembled domains of the ca. 1.1–1.0 Ga Grenville deformed basement and deposited in a distal foreland setting with respect to the relict mountainous hinterland in the core of the orogen. The youngest Moinian detrital zircons detected so far (ca. 900 Ma) have been found in samples from the Loch Eil Group in the eastern part of the Northern Highlands, whereas the minimum-age detrital zircons from the western Morar Group rocks are more in accord with the minimum-age samples from the Torridon Group, at ca. 1.06–1.00 Ga (Rainbird et al., 2001; Cawood et al., 2004).
Determination of the depositional environment of parts of the Morar (regressive tidal shelf) and Loch Eil (shallow marine) Groups has been possible where low strain permits sedimentologic study; paleocurrents in both areas indicate a general flow direction from south to north (Glendinning, 1988; Strachan, 1986). Soper et al. (1998) proposed deposition in two major half-graben basins that were bounded to the west by inferred east-dipping normal faults, schematically represented in Figure 3A. An initial phase of rifting is proposed to have accommodated deposition of the Morar and Glenfinnan Groups; the former displays marked westward thickening in its upper part, which is consistent with deposition in a half-graben (Glendinning, 1988). The same formations appear to become progressively more distal eastward, and the striped and pelitic rocks of the Glenfinnan Group may represent a distal facies of the Morar Group (Soper et al., 1998). In this model, the Loch Eil Group marks the onset of renewed rifting; asymmetrical facies distribution and westward thickening are again consistent with deposition in a second half-graben bounded by an east-dipping normal fault (Strachan, 1986). Dalziel and Soper (2001) suggested that the Moine rocks were deposited within an aborted rift zone that formed during the early stages of Laurentian break-out from the supercontinent of Rodinia as East Gondwana separated from West Laurentia to form the Pacific Ocean.
Metasediments of the Glenfinnan and Loch Eil Groups are cut by the West Highland Granite Gneiss and intruded by metabasic intrusions of tholeiitic affinities both dated at ca. 870 Ma (Friend et al., 1997; Millar, 1999; Fig. 2; Plate 2A). The chemistry of the basic rocks is consistent with intrusion into thinned continental crust. Ryan and Soper (2001) proposed emplacement of the basic intrusions at depth to provide the heat necessary to locally melt the underlying basement and Moine sediments. Granitic melts then migrated through the sedimentary pile to higher levels.
Geikie (1893) first proposed that the “Torridonian” and Moine rocks could represent different elements of the same sedimentary succession foreshortened by Caledonian thrusting. The available isotopic constraints on the age of these deposits (Turnbull et al., 1996), and on the provenance of detritus, suggest that parts of these two extended sequences were broadly contemporaneous, and so it remains a plausible hypothesis—the Torridon Group may have been deposited by a major fluvial system that flowed eastward into a marine setting for the dispersal of the Morar Group sediments (M. Krabbendam, 2006, personal commun.). This cycle of deposition probably spanned the period from ca. 1000 to 900 Ma; accumulation of the youngest parts of the sedimentary pile (Loch Eil Group; Soper et al., 1998) was followed relatively soon after by ca. 870 Ma intrabasinal acid and basic magmatism (Millar, 1999).
In East Greenland, comparable sequences of thick Mesoproterozoic metasedimentary rocks are represented by the Krummedal succession (Higgins, 1988; Leslie and Higgins, this volume), which is widely distributed in both the Niggli Spids and Hagar Bjerg thrust sheets. The Krummedal succession and equivalent successions are exposed over a N-S distance of at least 600 km in East Greenland; the reconstruction of Higgins et al. (2004) suggests that the original depositional basin was at least 300 km wide (with the eastern limit undefined). Ion microprobe analyses of detrital zircon (Kalsbeek et al., 2000; Watt et al., 2000) and electron microprobe analyses of monazite (Gilotti and Elvevold, 2002) are available from Krummedal succession metasediments of both the Niggli Spids and Hagar Bjerg thrust sheets. Like the Moine Supergroup, deposition occurred after ca. 1050 Ma, while Archean and mid-Paleoproterozoic (1800–2000 Ma) detrital zircon grains are rare and indicate that the older rocks of the western foreland region to the Grenville belt were not a significant source region. Watt et al. (2000) and Watt and Thrane (2001) advocated a distant source area.
The widespread occurrence of these marine siliciclastic successions in Greenland, NW Scotland, and in the eastern province of Spitsbergen (Brennevinsfjorden Group, Helvetsflya Formation; Gee and Teben'kov, 1996; Gee et al., 1995) indicates that late Mesoproterozoic sedimentary basins extended over several thousand square kilometers, perhaps in a number of interconnected depocenters. The general lack of Archean detritus in the Moine and Krummedal sequences implies that detritus was derived from more recently active interior areas of the Laurentian hinterland (e.g., the Grenville belt) rather than more parochial Archean or Paleoproterozoic basement. Those sediments were then deposited in marginal settings floored by Archean to Paleoproterozoic continental crust. Since no equivalent of the early Neoproterozoic Torridon Group fluvial red-bed system seems to have been present at the paleolatitude represented by East Greenland, we conclude that it is possible that the Krummedal succession may represent a northward continuation of marine dispersal of parts of the Moine succession.
NEOPROTEROZOIC OROGENESIS (Ca. 900–730 Ma)
In Scotland, the nature and timing of Neoproterozoic tectonothermal activity affecting the Moine Supergroup are problematical and highly contentious. Current debate is focused on whether or not the Moine rocks were affected by one or more high-pressure, high-temperature granulite-facies orogenic event between ca. 820 and 730 Ma. These events both led to extensive migmatization of the Moine metasediments but did not generate significant volumes of granitic melt in the form of discrete plutonic bodies. This contrasts markedly with central East Greenland, where high-pressure, high-temperature granulite-facies metamorphism culminated in the generation of significant volumes of S-type granite at ca. 910–930 Ma.
For Scotland, Ryan and Soper (2001) proposed that the protolith to the 870 Ma West Highland Granite Gneiss (and the contemporaneous basic sheets) was generated in response to crustal thinning and extension affecting an undeformed sedimentary pile. The East Greenland granites were intruded by basic sheets and subsequently affected by sillimanite-grade ductile shear zones, but no precise age constraints are available for these later events (Leslie and Nutman, 2003).
Evidence for a ca. 820 Ma orogenic (Knoydartian) event exists in the Moine rocks of the Northern Highlands (Fig. 4). Bodies of pegmatite generated in situ in localized zones of high strain yield precise U-Pb zircon and monazite ages of 827 ± 2 Ma, and Sm-Nd ages of ca. 820–790 Ma obtained from post-D1 Morar Group garnets apparently date the early metamorphism (Rogers et al., 1998; Vance et al., 1998). Peak pressures of 12.5–14.5 kbar indicate crustal thickening at this time. A further episode of high-pressure, high-temperature metamorphism (ca. 10 kbar, 800 °C) occurred at 730 Ma (Fig. 4) but was seemingly restricted to the eastern part of the Moine outcrop (Tanner and Evans, 2003; Emery, 2005).
Combined, these data from the Scottish and Greenland sectors apparently indicate three periods of orogenesis (ca. 930 Ma, ca. 820 Ma, ca. 730 Ma) in this sector of Laurentia, each separated by ∼100 m.y. The wider significance of these orogenic events is unclear, not least because they occurred at a time when the Rodinian supercontinent was undergoing late-stage amalgamation, even the initiation of fragmentation in some locations (Hoffman, 1991; Dalziel, 1992). It has been proposed that intracratonic movements accommodating extension and compressional deformation may account for these early to mid-Neoproterozoic orogenic events (e.g. Cawood et al., 2004). Repeated high-pressure, high-temperature conditions, however, are largely inconsistent with models of intracratonic (low-pressure, high-temperature) orogenic belts (cf. Petermann and Alice Springs orogenies of Australia; Sandiford and Hand, 1998; Scrimgeour and Close, 1999; Shaw et al., 1992).
We note here that ca. 950 Ma calc-alkaline volcanics have been recorded in the eastern Svalbard Nordaustlandet terrane Kapp Hansteen Group along with 930–960 Ma augen granites that were emplaced synchronously with an episode of deformation and high-pressure, high-temperature crustal anatexis (Johansson et al., 2000). An alternative hypothesis might propose a more marginal continental setting, in which a continental block actively overrode oceanic crust as a consequence of active spreading on an opposing plate margin. Thus, repeated Neoproterozoic tectonothermal events on the eastern Laurentian margin may have some analogy to the patterns of high heat flow and repeated deformation found in subductionzone continental back arcs (Hyndman et al., 2005).
MID-NEOPROTEROZOIC TO EARLY PALEOZOIC SEDIMENTATION
During the late Neoproterozoic, post–Pan African rifting of eastern Rodinia culminated with the formation of the Iapetus Ocean as Baltica and Amazonia drifted away from Laurentia (inset to Fig. 1, Fig. 4; Soper, 1994b; Cawood et al., 2001, 2003, 2004). These rift and rift-drift events were recorded by the formation of widespread progradational passive-margin sedimentary sequences, igneous activity, and oceanic sedimentation along the eastern side of Laurentia that lasted from ca. 700 Ma until the mid-Ordovician.
In Scotland, this geological evolution is recorded in two separate successions, the Cambrian to mid-Ordovician carbonate shelf succession of the Northwest Highlands and the late Neoproterozoic to mid-Cambrian, shallow- and deep-water sediments and volcanic rocks of the Dalradian Supergroup in the Grampian Highlands. The former readily correlates with the foreland succession in East Greenland, and the latter is comparable in age to the Eleonore Bay Supergroup in East Greenland (Soper, 1994b). Thus, we can interpret the Hebridean Cambrian-Ordovician and Grampian Dalradian rocks and their Laurentian correlatives as forming two subparallel sedimentary belts located along the eastern continental margin, where the shelf carbonate rocks were positioned on the landward side of the generally deeper-water marine lithologies. Oceanic crust is presumed to have developed to the southeast of the present Dalradian outcrop Scotland, where the youngest sediments ultimately prograded onto Iapetan oceanic crust.
The Pre-Dalradian Basement
In Scotland, southeast of the Great Glen fault, the stratigraphically, and structurally, oldest known strata within the Grampian Highlands comprise the recrystallized and mainly gneissose to locally migmatitic psammite and semipelite of the Glen Banchor and Dava successions (Fig. 4; Piasecki, 1980; Smith et al., 1999; see also review by Strachan et al., 2002).
Rocks of the Glen Banchor and Dava successions are invariably intensely recrystallized, do not preserve sedimentary structures, and commonly contain intrafolial isoclinal folds that deform the first foliation (usually a gneissosity) (Smith et al., 1999). In contrast, the overlying Grampian Group rocks are variably recrystallized, structurally less complex, and commonly preserve sedimentary structures.
In the Dava and Glen Banchor succession, locally close to the contact with younger rocks, late Neoproterozoic syntectonic blastesis and pegmatite segregation are present within ductile shear zones (Hyslop, 1992; Hyslop and Piasecki, 1999). U-Pb monazite analyses from such pegmatites have provided high-precision ages of 808 +11/–9 Ma and 806 ± 3 Ma, and a concordant age of 804 +13/–12 Ma from the host mylonite matrix (Noble et al., 1996). Recent U-Pb dating of single zircon grains within kyanite-bearing migmatites yielded an age of 840 ± 11 Ma (Highton et al., 1999). These data are interpreted as the effects, in the Dava–Glen Banchor succession rocks, of high-grade metamorphism and migmatization associated with Knoydartian orogenesis recognized farther west in the Moine Supergroup.
In contrast, isotopic evidence for Neoproterozoic tectonothermal events has yet to be recorded in the Dalradian rocks. There is evidence of progressive overstep of various lithologies onto the older strata (Smith et al., 1999; Robertson and Smith, 1999; British Geological Survey, 2002, 2004), and this, combined with 87Sr/86Sr isotopic signatures from the lowermost Dalradian strata (Thomas et al., 2004, see following), provides evidence for a significant stratigraphic and tectonothermal break at the base of the Grampian Group (Smith et al., 1999).
In Greenland, the very thick Neoproterozoic–Ordovician succession recognized within the Franz Joseph allochthon is dominated by the Neoproterozoic Eleonore Bay Supergroup (Higgins et al., 2004). The lower contact of the Eleonore Bay Supergroup against the older gneissose metasedimentary rocks of the Krummedal succession in the Hagar Bjerg thrust sheet is likely to be an unconformity modified during westward Caledonian transport (Leslie and Higgins, this volume).
The Dava and Glen Banchor succession rocks are interpreted to form a Moine-like metasedimentary basement that was affected by a Neoproterozoic Knoydartian tectonothermal event prior to deposition of the overlying Grampian Group. The relationship is thus analogous to that observed at the base of the Eleonore Bay Supergroup in Greenland.
Dalradian Sedimentation (Ca. 730–470 Ma)
The Dalradian Supergroup is a relatively well-differentiated progradational passive-margin sedimentary sequence dominated by marine metasandstones, siltstones, mudstones, and carbonate rocks. They are subdivided into a Cryogenian (Grampian, Appin Groups) and an overlying Ediacaran to mid-Cambrian (Argyll Group and Southern Highland Group) succession (Harris et al., 1978, 1994) (Figs. 4 and 6). The Dalradian Supergroup has an apparent total thickness of at least ∼25 km, but this is unlikely to have been deposited in a single continuous succession. Significant volumes of the middle to upper parts of the succession (e.g., in the southwest Scottish Highlands) are affected only by low-grade metamorphism and/or relatively weak deformation, and here the sedimentologic and basin evolution history can be interpreted with some confidence. When a restoration is made of the distribution of lithostratigraphy prior to Ordovician (Grampian) folding, it seems reasonable to presume that deposition would have migrated broadly southeastward with time.
One key to unraveling the stratigraphy is a number of regionwide events that were probably broadly synchronous across Scotland and Ireland. These include transgressive flooding surfaces (e.g., base of the Ballachulish and Easdale subgroups; Fig. 6), Neoproterozoic glaciation events (e.g., Port Askaig Tillite, base of the Argyll Group; Plate 2C; McCay et al., 2006), and rift-related magmatism (as represented by A2-group granitoids and large volumes of basic volcanic rocks; e.g., Tayvallich Volcanics Formation; Plate 2D) in the Argyll Group (Tanner et al., 2006). These key horizons are used to help constrain the suggested correlation of Figure 6.
Modern sequence stratigraphic concepts have only been applied to the lowermost Grampian Group (Glover and McKie, 1996; Banks, 2005). Previously undetected intrabasinal unconformities and periods of nondeposition are now recognized in the southern and central Scottish Highlands (A.G. Leslie for the British Geological Survey [BGS], unpublished data). One such important unconformity that affects the Appin Group and Argyll Group succession has been added to the detail on Figure 6. Alternative evidence for major stratigraphic and tectonic orogenic breaks (Prave, 1999; Alsop et al., 2000; Hutton and Alsop, 2004) has not been regionally validated and remains speculative.
Age constraints on the initiation of Dalradian sedimentation are poor. The Grampian Group must be younger (<800 Ma) than the pegmatites contained within the basement Dava and Glen Banchor successions, and the youngest deformation event recorded within the equivalent Moine rocks to the northwest of the Great Glen fault is now ca. 730 Ma (Tanner and Evans, 2003; Emery, 2005). The youngest Dalradian detrital zircons yield ages of 900 Ma (Cawood et al., 2003), and 87Sr/86Sr whole-rock isotope data from the lowermost metacarbonate rocks of the Grampian Group are consistent with a global late Neoproterozoic strontium seawater signature younger than 800 Ma and possibly as young as ca. 670 Ma (Thomas et al., 2004). Thus, Grampian Group sedimentation occurred after 800 Ma and could have initiated as late as ca. 700–730 Ma.
Neoproterozoic augen granites such as the Ben Vuirich pluton (BV on Fig. 4) were intruded into Appin-Argyll Group Dalradian strata at 590 Ma (Rogers et al., 1989; Pidgeon and Compston, 1992). These rift-related intrusions have an A2-group chemistry (Tanner et al., 2006) and are probably genetically linked with the Argyll Group mafic volcanism (Tayvallich volcanics) dated at 595 Ma (Dempster et al., 2002). This magmatism was contemporaneous with a second pulse of bimodal magmatism throughout the Appalachians (e.g., Badger and Sinha, 1988; Rankin et al., 1989; Aleinikoff et al., 1995), and it was related to continental breakup of Laurentia (Cawood et al., 2001). Reliable information critical to the biostratigraphic age of the Dalradian Supergroup is only preserved in the uppermost parts of the Southern Highland Group, where locally developed metacarbonate rocks (Leny Limestone) contain topmost Lower Cambrian Pagetia trilobites, indicating an approximate age of ca. 515 Ma for the upper limits of deposition (Fig. 6; Pringle, 1940; Tanner, 1995).
An age of ca. 730 Ma for the base of the Grampian Group has important implications for the ages of the tillite formations in the mid- to upper Dalradian (Port Askaig and Inishowen-Macduff). Brasier and Shields (2000), Condon and Prave (2000), and McCay et al. (2006) proposed that the Argyll Group tillites correlate with the Ghubrah (Sturtian) glacial dated at 723 +16/–10 Ma (Brasier et al., 2000), but deposits of this age would mean that accumulation of both the Grampian and Appin Groups would have overlapped the high-pressure, high-temperature tectonothermal event recorded in the eastern parts of the Moine (Tanner and Evans, 2003; Emery, 2005), which is contradicted by the 87Sr/86Sr isotope data from the Grampian Group metacarbonate rocks (Thomas et al., 2004). Displacements on the Great Glen fault would permit some room for maneuver here, but even the larger estimates of displacement (e.g., Dewey and Strachan, 2003) seem unlikely to solve the space problems inferred from these overlapping ages. The scenario favored here (Fig. 6) envisages two principal preserved glacigenic intervals in the Dalradian. We equate the younger Southern Highland Group glacigenic deposits in Inishowen (Donegal, Eire) and Macduff (NE Scotland) with the Gaskiers Formation (ca. 580 Ma; Bowring et al., 2003) and the Varangerian tillites of Norway (620–590 Ma; Gorokhov et al., 2001; Bingen et al., 2005). The older Port Askaig Tillite Formation (Argyll Group) and the Storelv and Ulvesø Formations (Tillite Group) of East Greenland (Hambrey and Spencer, 1987) are then equated with the Marinoan-Ghaub glacial (ca. 635 Ma; Hoffmann et al., 2004). The Kinlochlaggan Boulder Bed is an isolated set of occurrences restricted to the central Scottish Highlands and assigned to the Lochaber Subgroup (British Geological Survey, 2002) (Fig. 6). It must, on that basis, presumably be significantly younger than 700 Ma and cannot correlate with the Ghubrah glaciation. The Kinlochlaggan Boulder Bed is interpreted as a mature sandy deposit containing glacially rafted dropstones (J.R. Mendum, 2006, personal commun.) rather than a subglacial till and may thus still mark the earliest record of glacial influence in the Dalradian.
Provenance studies summarized here (Fig. 7; Cawood et al., 2003, 2004) provide useful data that track the evolution and denudation history of the hinterland. Probability density distributions of concordant detrital zircon ages show that Grampian Group detrital zircons maintain the earliest Neoproterozoic to late Paleoproterozoic spectrum of the Torridon Group and Moine Supergroup (Cawood et al., 2003, 2004; Friend et al., 2003). However, quite different distributions are apparent in the data currently available for the remainder of the Dalradian Supergroup. After an early (Grampian Group) rifting episode in the Scottish sector of Laurentia, Appin Group shallow-marine-shelf sedimentary facies associations developed during postrift thermal subsidence (Stephenson and Gould, 1995). Flooding at the base of the Ballachulish Subgroup introduced, or just preceded, the arrival of Archean zircons in detritus and an apparent absence of Grenvillian ones. The onset of Argyll Group deposition coincided with supply of a broad spectrum of earliest Neoproterozoic to Archean detritus to an increasingly unstable and volcanically active margin (Stephenson and Gould, 1995). These successions were ultimately overstepped as the margin foundered in response to Iapetan rifting and was inundated with immature siliciclastic detritus from Crinan Subgroup time onward (upper Argyll Group). We see apparently less abundant Paleoproterozoic zircons in the sediment load upward from the base of the Crinan Subgroup, and these data might suggest that any ca. 1800 Ma Rhinnian source became increasingly isolated from the Dalradian basins as Archean supply increased. Iapetan rifting may have removed, or more likely submerged, that sector of the Laurentian margin that had been supplying the Paleoproterozoic detritus, presumably as Amazonia and Baltica broke away.
Thus, the picture emerges in Scotland of a Dalradian succession deposited in an evolving pericontinental environment over a period of ∼180–200 m.y. Fluctuations in water depth accompanied active rifting and the development of second- and third-order subbasins on this sector of the continental margin.
In Greenland, the broadly time-equivalent Eleonore Bay Supergroup (apparently 14.5 km thick) and overlying Tillite Group (0.8 km thick) and Kong Oscar Fjord Group (4.5 km thick) are, by comparison, more poorly constrained chronologically. Here, there is no evidence of contemporaneous volcanic activity, and the available age data loosely bracket deposition in the period ca. 940 to ca. 460 Ma (Sønderholm and Tirsgaard, 1993; Smith et al., 2004). These East Greenland strata record shelf and ramp sedimentation, which was probably punctuated by significant periods of nondeposition on a slowly subsiding passive margin, a setting in stark contrast to the linked Neoproterozoic rift-basin architecture of Scotland. Farther north on the Laurentian margin, the Neoproterozoic Hekla Sund Basin (Higgins et al., 2001b) represents another locus of rifting activity in eastern North Greenland. In such a setting, and based upon the data currently available, we propose that a generalized correlation is possible between the Greenland and Scottish sectors in Laurentia (Figs. 3 and 6).
Cryogenian to Early Ediacaran Basin Evolution (Ca. 730–610 Ma)
The Grampian Group records the initiation of middle to late Neoproterozoic extension and basin development and consists of three main lithofacies associations (subgroups), which are interpreted as representing distinct phases of early and synrift extension followed by a protracted period of postrift thermal subsidence (Fig. 6). Deposition occurred within a series of linked NE-trending rift basins bounded by major crustal lineaments (Glover and Winchester, 1989; Smith et al., 1999; Banks, 2005). Despite regional deformation and metamorphism to amphibolite-facies conditions, the stratigraphic integrity and overall geometry of these basins has been preserved largely intact.
The actual base to the Grampian Group is unexposed, but the oldest unit is the spatially restricted and fault-bounded Glenshirra Subgroup, which is nowhere observed in primary undisturbed contact with the underlying Glen Banchor or Dava succession rocks (Fig. 6). With a maximum exposed thickness of ∼2 km, the Glenshirra Subgroup is composed of stacked shoaling sequences of geochemically distinct, immature arkosic psammite and beds of metaconglomerate (Banks and Winchester, 2004). Banks and Winchester (2004) interpreted the sediments as alluvial-fan and shallow-water sediments deposited within a SE-thinning fan-delta clastic wedge. Progressive thickening and coarsening of the strata toward the west may imply the presence of a basin margin to the west or northwest (approximately coincident with the present trace of the Great Glen fault). The clastic wedge was supplied from an exposed hinterland of mature crust beyond the basin margin (Banks and Winchester, 2004), which, based upon clast populations, was predominantly composed of quartzofeldspathic gneiss and granitic rock. Detrital zircon populations are dominated by 1.8 Ga detritus, with subsidiary 1.2 Ga detritus (Fig. 7), suggesting that “Rhinnian-type” basement was an important source area (Cawood et al., 2003).
The Glenshirra Subgroup is abruptly but conformably overlain by a distinctive and regionally widespread succession of psammite and semipelite assigned to the Corrieyairack Subgroup (Fig. 6). This change records a basinwide flooding event that heralded a period of subsidence and rift-related extension (Banks, 2005). A near-complete sequence is preserved through the main rift cycle in 4–5 km of siliciclastic deposits deposited by prograding turbidite complexes (Banks, 2005). Variations in sediment supply and source area are indicated by changes in the proportions of plagioclase and K-feldspar, whereas variations in bed thickness and form reflect depositional processes. Bouma cycles are well represented, but bottom structures are extremely rare (Banks, 2005). A reduction in sand-grade sediment supply and development of shelf conditions along the tectonically active basin margins and intrabasinal highs are recorded by lateral thickness and facies changes to striped semipelite and psammite. This was followed by a renewed influx of sand-dominated turbidites (Plate 2B) deposited by fan-lobe systems derived from the northwest, passing south and eastward into shelf environments (Glover et al., 1995; Robertson and Smith, 1999; Banks, 2005).
The turbidites of the upper Corrieyairack Subgroup are overlain by shallow-marine sediments of the Glen Spean Subgroup, which prograded into the basin from the northwest and southeast (Fig. 6) after a flooding event (Banks, 2005). Reduced subsidence and relative tectonic stability at this time are interpreted to represent a postrift thermal subsidence phase (Glover et al., 1995). The lithological associations of the Glen Spean Subgroup, combined with well-preserved sedimentary structures, indicate deposition in shallow-marine (tidally influenced) shelf environments with intensive sediment recycling and winnowing of the underlying turbiditic rocks (Banks, 2005). Analysis of detrital zircon populations shows a marked absence of any Archean detritus, peaks at 2.0 Ga and 1.4 Ga in the Corrieyairack Subgroup, and progressive dilution by 1.1–0.9 Ga Grenvillian detritus in the Glen Spean Subgroup (Fig. 7; Cawood et al., 2003; Banks, 2005).
In Greenland, the Nathorst Land Group (Fig. 6; Sønderholm and Tirsgaard, 1993) consists of up to 9 km thickness of silici-clastic sediment that rests in sheared or unexposed contact on the Hagar Bjerg thrust sheet (Higgins et al., 2004). The Nathorst Land succession is informally subdivided into seven formations (NLG1– NLG7; Smith and Robertson, 1999), all of which record persistent fine-grained and shallow-marine shelf sedimentation. Several lithofacies associations of sandstone–dolostone–quartz arenite alternating with heterolithic fine sandstones, siltstones, and mudstone are identified. Carbonate deposits with parallel lamination of possible algal origin are present in the upper part of the group. Beautifully preserved delicate sedimentary structures including desiccation cracks, ripple-lamination, cross-lamination, and heavy mineral bands are characteristic at several levels. Depositional environments include outer-shelf storm-influenced and inner-shelf to tidally influenced shoreface environments. Two major flooding surfaces are identified at the junctions between the Nathorst Land Group formations 3 and 4 and between formations 6 and 7 (Smith and Robertson, 1999), and we tentatively match these with the flooding events that bracket the Corrieyairack Subgroup in the Scottish sector in order to help constrain the suggested correlation in Figure 6.
In Scotland, siliciclastic sedimentation continued up into the lowermost Appin Group (Lochaber Subgroup, see Fig. 6). Although locally conformable, the Lochaber Subgroup has a markedly diachronous base at the basin scale, and, with an overall decrease in thickness of the subgroup to the southwest of its crop, it is interpreted to have been deposited with considerable lateral facies variation during marine transgression (Key et al., 1997). Similarly, above the Nathorst Land Group in East Greenland, the base of the Lyell Land Group is marked by transgression, marked locally in places by an angular unconformity (Smith and Robertson, 1999). The Lyell Land Group is composed of 3 km of siliciclastic shelf and coastal-plain tidal sediments dominated by storm and wave events. Cyclical changes in sea level and shoreward reorganization of facies are linked to large-scale regressions that can be traced along 300 km of inferred paleocoastline (Tirsgaard and Sønderholm, 1997).
Across Scotland, we interpret the base of the succeeding Ballachulish Subgroup as transgressive, and maximum flooding is likely to have coincided with deposition of the Ballachulish Slate Formation in the locally anoxic environments that characterize the lower part of the subgroup (Fig. 6). Subsequent progradation is marked by progressive development of extensive shallow, tidally influenced, shelf sedimentation and a period of stability (Anderton, 1985). A fourfold subdivision at formation level can be traced with remarkable continuity for some 300 km across the Grampian Highlands in Scotland and northwest Ireland. Limestonepelite-quartzite facies associations are characteristic and mark a significant break from the siliciclastic-dominated record of the Grampian Group and Lochaber Subgroup as presently defined. Continuity at this regional scale, almost on a bed-for-bed basis, attests to the widespread stability and relatively uniform nature of the subsidence. Interestingly, available data indicate that Archean detrital zircon grains become evident in the sediment load at this juncture (Fig. 7; Cawood et al., 2003), further emphasizing the change that occurs at the base of the Ballachulish Subgroup.
In Greenland, the base of the Ymer Ø Group may record the same event; there is a sharp break in sedimentary facies association from heterolithic sandstones in the Lyell Land Group below to fine-grained mudstone above. A wide variation in lithology comparable to the Ballachulish Subgroup is also evident—siliciclastic mudstone and sandstone pass upward into black limestone and dolomite with algal biostromes (Sønderholm and Tirsgaard, 1993). A wide range of environments is indicated at this time, including basinal and slope deposits, inner-shelf environments, and horizons of evaporitic sulfate deposition.
The Blair Atholl Subgroup marks the continued diversification of Dalradian lithologies in Scotland with renewed flooding and a change to deeper deoxygenated marine conditions in the Scottish sector (Fig. 6; Stephenson and Gould, 1995). In the type area, the basal slates and phyllites are conformable with the underlying Ballachulish Subgroup, but, importantly, some hints of volcaniclastic detritus and minor tuff horizons are also recorded, pointing to the earliest signs of basin instability on this sector of the Laurentian margin.
In East Greenland, the Ymer Ø Group is succeeded by the Andrée Land Group, which should thus be broadly equivalent to the Blair Atholl Subgroup. The Andrée Land Group is composed of 1275 m of algal limestone and dolomite deposited on a NE-facing storm-influenced carbonate ramp (Frederiksen, 2001). Laterally extensive facies form cyclical stacking patterns in response to sea-level fluctuations and are the basis for subdivision into seven formations. Changes in ramp geometry and transgression have been linked to an episode of extensional faulting that marked incipient stretching on this part of the margin (Frederiksen, 2001).
Lower Argyll Group
Argyll Group Dalradian sedimentation in Scotland as a whole records the rapid onset of instability in the mid- to late Neoproterozoic and the replacement of widespread shallow-marine conditions of the Appin Group by cycles of rapid basin deepening. Initially, the distinctive successions of black graphitic pelite, metacarbonate rock, and quartzite of the Appin Group are succeeded by an equally distinctive glaciomarine tillite (Plate 2C), the Port Askaig Tillite Formation and other correlatives. Above, deeper-water psammites and quartzites comprise the remainder of the Islay Subgroup. The tillite formation is a prominent marker horizon across Scotland and Ireland and marks the onset of cold-climate glaciomarine sedimentation equated here with the Marinoan glacial period at ca. 635 Ma (Fig. 6). The top of the subgroup is located in Ireland by a cap carbonate (the Cranford Limestone Formation; McCay et al., 2006), after which cold-climate conditions apparently ameliorated with no further sign of glaciogenic deposits in the Argyll Group. Thereafter, variable lithofacies of psammite and quartzite and locally thick accumulations indicate that sediment input kept pace with extension in a series of NE-trending basins (Stephenson and Gould, 1995). While detrital zircons from the tillite units are comparable to the enclosing Appin and Argyll Group rocks (Cawood et al., 2003), this change in sedimentation is also marked by increasing volumes of Archean grains above the level of the tillite formations (Fig. 7; Cawood et al., 2003).
The Tillite Group in Greenland is likewise marked by units of diamictite sandstone, carbonate, and shale (Fig. 6). Here, massive bedded diamictites and cross-bedded sandstones of eolian origin mark the base and are overlain by shales and sandstones formed by debris-flow and turbidite events. There is a second horizon of diamictite below tidally influenced dolomites and shales at the top of the group (Hambrey and Spencer, 1987; Moncrieff and Hambrey, 1988). We follow the assessment of Sønderholm et al. (this volume) for a “Marinoan” age for these deposits and thus make the ca. 635 Ma chronostratigraphic correlation with the Dalradian Islay Subgroup used in the construction of Figure 6.
In Scotland and Ireland, the Easdale Subgroup sees a return to a wide range of finer-grained lithologies, including graphitic black pelite, calcareous semipelite, and metacarbonate rock, commonly associated with pebbly quartzite and sheets of basic meta-igneous rock of varying abundance (Stephenson and Gould, 1995). Exhalative saline brines gave rise to a laterally persistent bed of strata-bound sulfide, barite, and vein mineralization (Hall et al., 1991). Taken together with the greater abundance of mafic meta-igneous rocks, these occurrences point to an increased extension in this sector of the Laurentian margin at the end of the Cryogenian.
In Greenland, an erosional break encompassing the later Ediacaran and earliest Cambrian separates the sediments of the Tillite Group from the Cambrian-Ordovician succession of the Franz Joseph allochthon in the central fjord region of East Greenland (Fig. 6). Uplift and erosion in Greenland at this time coincided with the onset of enhanced rifting and mafic volcanism leading up to continental rupture in the Scottish sector.
Ediacaran Basin Evolution (Ca. 610–542 Ma)
Upper Argyll Group
In Scotland, individually thick formations of often immature sediment and deep-water turbiditic facies characterize the succeeding Crinan Subgroup. This pronounced change in sedimentary facies association coincides with regional overstep in the southern and northeast Grampian Highlands, and we suggest that it is this change that indicates the onset of rift-drift transition in the Scottish sector of Laurentia as Iapetan rupture expanded northward. The overlying Tayvallich Subgroup is dominated by carbonates, which are locally accompanied by thick extrusive mafic volcanic rocks (including pillow lavas, Plate 2D), and subvolcanic sills that mark, perhaps for the first time, rupture of the continental crust during rifting. Felsic tuffs within the Tayvallich Volcanic Formation have yielded U-Pb zircon ages of 601 ± 4 Ma (Dempster et al., 2002). Rapid lateral variations in facies and thickness associations with unconformities, overstep relations, and pebbly beds typify this part of the succession.
Southern Highland Group
The uppermost unit of the Dalradian Supergroup is characterized by an ∼4-km-thick pile of coarse-grained turbiditic siliciclastic and volcaniclastic strata that lie immediately above the Tayvallich Subgroup (Fig. 6). These sediments mark rapid basin deepening that persistently stayed ahead of the sedimentary and volcanic fill. The coarse-grained sediments were probably laid down in slope apron or ramp settings with channels on the lower slopes and inner zones of deep-water submarine fans with overbank deposits or as outer-fan facies (Burt, 2003). No apparent match for these immature turbidite-dominated sedimentary facies associations occurs anywhere along the Greenland sector of Laurentia.
In Scotland, volcaniclastic units are a conspicuous component of the Southern Highland Group. They are most prevalent in the lowermost 1 km and are interpreted as recording, in part, the erosion of the underlying basic volcanics, but they may also have resulted from contemporaneous volcanism and ash fall on the hinterland (Pickett et al., 2006). This interpretation is not, however, strongly supported by the detrital zircon data, which are dominated by Archean detritus in both the volcaniclastic “Green Beds” and their siliciclastic Southern Highland Group counterparts. Ages younger than 900 Ma are generally absent, although one grain yielded an age of 553 ± 24 Ma (Cawood et al., 2003). An important glaciogenic deposit is recognized in Inishowen in the north of Ireland (Condon and Prave, 2000). We follow those authors in correlating the Inishowen occurrences with others in the Southern Highland Group Dalradian section at Loch na Cille and Macduff in Scotland and then on a global scale with the Gaskiers glacial at ca. 580 Ma (Fig. 6).
Cambrian-Ordovician Sedimentation (Ca. 540–470 Ma)
No mid- to late Neoproterozoic tillite deposits are preserved on the foreland of the NW Highlands, so the early Neoproterozoic Torridon Group rocks are overlain unconformably by the Lower Cambrian Ardvreck Group (Eriboll and An t-Sron Formations; Fig. 6). This siliciclastic succession is dominated by feldspathic to quartzitic sandstones and subsidiary siltstone and is interpreted as a transgressive sequence passing upward into storm-dominated calcareous siltstones and regressive sands (McKie, 1990). The Ardvreck Group sediments are conformably but sharply overlain by 900 m of Durness Group dolostone with limestone and minor chert (Ghrudaidh to Durine Formations; Fig. 6) that accumulated on a low-energy shelf (Park et al., 2002). From this change onward, sedimentation in peri- and subtidal environments continued into the Middle Ordovician (Wright and Knight, 1995), and, thus, we envision Greenland-style passive subsidence and a broad platformal shelf extending across inboard parts of the Scottish sector (cf. Higgins et al., 2001a). The lower siliciclastic formations contain distinctive Skolithos burrows (Piperock) and pass up into dolomitic siltstone and minor limestone containing diverse macrofaunal assemblages and Planolites burrows (Park et al., 2002).
In the central fjord region of East Greenland (Fig. 6), the Cambrian-Ordovician Kong Oscar Fjord Group is separated from the Tillite Group by a disconformity or erosional unconformity with cross-bedded quartz arenites and rippled bed tops passing upward into shales, thin sandstones, and limestones (Smith et al., 2004). Limestones and dolostones then increase upward to become the dominant facies. The base of the Ordovician is marked by a transgression, above which subtidal carbonate environments dominate (Smith et al., 2004). The Cambrian-Ordovician lithostratigraphy can be traced continuously over many hundreds of kilometers N-S along strike and demonstrates systematic thickening of the succession from inboard to outboard positions on the original depositional margin (Higgins et al., 2001a).
In summary, the NW Highlands Cambrian-Ordovician succession of Scotland apparently represents an intermediate position on the slowly subsiding Laurentian carbonate platform that was located between more inboard and more outboard environments, each of which is represented by Cambrian-Ordovician sedimentary rocks in East Greenland (Smith et al., 2004).
The lower parts of the shallow-water carbonate shelf succession represented by the NW Highlands Durness Group and the Kong Oscar Fjord Group of East Greenland are thought to be contemporaneous with the younger elements of the deep-marine turbidite basins of the southern Highlands of Scotland (Fig. 6; Wright and Knight, 1995; Park et al., 2002). Tanner (1995) made it clear that the lower Paleozoic Leny Limestone Formation occurs in stratigraphic continuity with the uppermost parts of the Southern Highland Group, and thus we have the only reliable biostratigraphic age for the Dalradian Supergroup. These metacarbonate rocks preserve Pagetia trilobites (Pringle, 1940) and constrain the uppermost Dalradian to be topmost Lower Cambrian, i.e., ca. 515 Ma. Acritarchs from the Leny Limestone Formation have been correlated with Greenland but are long-ranging (Downie, 1982).
Fault-bound slivers preserved locally along the Highland Boundary fault and assigned to the Highland Border Complex preserve Arenig carbonate sediments and black shale and pillow lava of Arenig age along with remnants of a fragmented pre-Arenig ophiolite (Tanner and Sutherland, 2007; but see also review in Bluck, 2002). While the provenance of these fault-bound slivers is undoubtedly Laurentian, and their stratigraphic ages overlap with part of the Ordovician Durness Group, their affinity with the Dalradian succession with which they are now juxtaposed has remained equivocal until now. Tanner and Sutherland (2007) reappraised the paleontologic and stratigraphic evidence and argued for a largely autochthonous Highland Border Complex in stratigraphic continuity with the Dalradian, which was overridden by a Highland Border ophiolite early in the arc-accretion process.
THE CALEDONIAN OROGENY
Ordovician Arc Accretion: Grampian Orogenesis (470–460 Ma)
By mid-Ordovician time, the Grampian phase of oro genesis halted passive-margin sedimentation (Fig. 6; Lambert and McKerrow, 1976; Soper et al., 1999; McKerrow et al., 2000). This phase records the convergence of the Laurentian continental margin with an intra-oceanic subduction zone and volcanic arc. The paleogeography of the margins of the Iapetus Ocean is likely to have been complex, and the potential for preservation is low; much of the evidence remaining is fragmentary. Parts of an early Cambrian to Early Ordovician continent-facing mafic to silicic arc and suprasubduction ophiolites are exposed in western Ireland, where it has proved possible to determine the sequence of events in a short-lived continent-arc collision orogeny (Dewey and Ryan, 1990; Dewey and Mange, 1999). There is indirect evidence that such an arc is buried beneath the Devonian-Carboniferous sedimentary cover in the Midland Valley of Scotland (Bluck, 1983, 1984).
Accretion is thought to have resulted in an overthrust ophiolite nappe, perhaps analogous to the Shetland ophiolite complex, which structurally overlies Dalradian rocks on Unst in NE Shetland. However, this particular fragment of Iapetan crust was apparently obducted at ca. 490 Ma (Flinn et al., 1991), some 20 m.y. prior to the peak of Grampian regional deformation and Barrovian metamorphism in Dalradian and Moine sediments. It is this later stage (ca. 470 Ma) that probably represents major collision and arc accretion, although oblique convergence resulted in some diachroneity of development in the regional structural architecture. Geochronologic constraints for Grampian orogenesis indicate that deformation, magmatism, and regional metamorphism and migmatization occurred within an interval of ∼10 m.y. in the Middle Ordovician, between ca. 471 Ma and 462 Ma (Friedrich et al., 1999). Switching subduction polarity at about this time initiated a south-facing arc and an Ordovician-Silurian subduction-accretion complex (Dewey and Ryan, 1990; Dewey and Mange, 1999). The Tyrone ophiolite of Northern Ireland may represent an Arenig-Llanvirn back-arc basin overthrust by the Sperrins nappe, part of the SE-facing and southerly directed nappe complex that includes the Tay nappe in the southern part of the Grampian Highlands (Fig. 8; Krabbendam et al., 1997).
Grampian orogenesis affected all of the Dalradian and older rocks of the Grampian Highlands. Isotopic evidence proves that parts of the Northern Highlands were also affected by a ca. 470 Ma tectonothermal event, which has been correlated with Grampian orogenesis (Kinny et al., 1999; Rogers et al., 2001; Emery, 2005). The effects of this event are most evident in east Sutherland and eastern Inverness-shire, where the effects of the later (Silurian) Scandian reworking are weak. Peak Grampian deformation culminated in the formation of major fold stacks or nappe complexes and associated zones of structural attenuation. Although deformation was superimposed upon a complex stratigraphic template, the gross lateral continuity of the Dalradian lithostratigraphy precludes the existence of any large-scale thrusting at the present exposure level in the Grampian Highlands (see the cross section in Fig. 8). Illustration of the gross architecture of the deformation is best portrayed in the three-dimensional (3D) block diagram reproduced by Stephenson and Gould (1995, after Thomas, 1979).
Several suites of Ordovician plutonic rocks were intruded into the Dalradian rocks of the northeast Grampian Highlands during regional deformation and metamorphism at ca. 470 Ma (Kneller and Aftalion, 1987; Dempster et al., 2002). These include a syn- to late tectonic suite of basic and ultramafic plutons and two suites of syn- to late tectonic diorites and granites (Fig. 8).
There is scant evidence in the East Greenland Caledonides (or in Svalbard; Harland et al., 1997) of these short-lived early Paleozoic marginal arcs and basins that must have accommodated subduction of Iapetan oceanic crust and convergence with Baltica. Rather, the evidence now available suggests that these distinctive orogenic elements were mainly incorporated as thrust sheets into the higher structural levels of the Scandinavian Caledonides (Roberts et al., 2001; Yoshinobu et al., 2002; Andréasson et al., 2003).
The earliest known (Ordovician and Silurian) granitoids in the East Greenland Caledonides are I-type calc-alkaline grano-diorite and quartz-diorite intrusions in the Scoresby Sund region (70°N–72°N), dated by sensitive high-resolution ion microprobe (SHRIMP) U-Pb analyses of zircons to between 466 ± 9 Ma and 432 ± 10 Ma (F. Kalsbeek, 2005, personal commun.). The older date is close to that of the youngest (Middle–Late Ordovician, ca. 460 Ma) sediments preserved in the Franz Joseph allochthon, suggesting that a tectonic control may have brought sediment accumulation to a close in this sector of the Laurentian margin. These I-type granitoids are only known in the southeastern portions of the Hagar Bjerg thrust sheet, which perhaps indicates that this part of the Laurentian continental margin was closest to the site of collision during the Grampian phase of arc accretion on the Laurentian margin. No similar rocks are known in the whole of the East Greenland orogen farther north.
Laurentia-Baltica Scandian Collision (Ca. 430 Ma)
Continued closure of the Iapetus Ocean in the Scottish sector after the Grampian orogenic event was achieved by reversal of the polarity of oceanic subduction (Dewey and Ryan, 1990). The paleogeographic details of these latter stages of convergence and collision are complex and result from the collision and interaction of three continental blocks, namely Laurentia, Baltica, and Avalonia (Soper and Hutton, 1984; Soper et al., 1992; van Staal et al., 1998).
Baltica-Laurentia collision is expressed in Scotland as the Scandian orogeny in the Northern Highlands. Regionally significant ductile thrusting and folding of the Moine rocks and associated basement inliers culminated in the development of the Moine thrust zone at ca. 430 Ma, marking the boundary with the autochthonous-parautochthonous foreland rocks of the Northwest Highlands. The Moine thrust zone is therefore a comparable structure to the Caledonian sole thrust of East Greenland (Higgins and Leslie, 2000).
Scandian Ductile Thrusting and Folding of the Moine Supergroup
Scandian thrust-related folding and fabric development were pervasive within the Moine rocks of west Sutherland but were restricted to localized reworking of migmatites and structures in the east above the Naver thrust (Strachan et al., 2002) (NT on Fig. 8). By analogy, and in the absence of any reliable isotopic evidence to the contrary, a Scandian age may be inferred for some of the movement on similar structures farther south in Rossshire and Inverness-shire, including the Sgurr Beag thrust (SBT on Fig. 8). The total displacement along these thrusts is uncertain, but it is likely to be at least tens of kilometers and conceivably >100 km in the case of the Sgurr Beag thrust (Powell et al., 1981). This places this structure in the same order of magnitude as the Hagar Bjerg thrust in East Greenland, with which it shares a similar structural level in a foreland-propagating system (cf. Higgins et al., 2004). In Scotland, intense upright folding followed internal ductile thrusting and resulted in the structure referred to as the Northern Highland steep belt (Fig. 9; Strachan et al., 2002). Regional deformation was accompanied by amphibolite-facies Barrovian metamorphism (Strachan et al., 2002).
This regional-scale foreland-propagating thrust system was responsible for the major interleaving of Moine rocks with Lewisian-type basement in Sutherland (Strachan et al., 2002). Many basement inliers occupy the cores of sheath and isoclinal folds along the trace of many of the major thrusts (Fig. 8). The trace of the Sgurr Beag thrust through Ross-shire and northern Inverness-shire to Loch Hourn is commonly marked by allochthonous slices of basement (Tanner et al., 1970); these may have been derived from a rift shoulder within the Moine sedimentary basin (Tanner et al., 1970; Soper et al., 1998) in the same style as the modified rift shoulder of the Hekla Sund Basin in eastern North Greenland (Higgins et al., 2001b).
Moine Thrust Zone
The Moine thrust zone is the westernmost and youngest of the system of Scandian thrusts on the Scottish mainland (Fig. 8). Although localized Caledonian displacements may also have occurred along the Outer Isles fault zone farther to the west, the Moine thrust zone is generally taken to define the northwest edge of the Caledonian orogenic belt (Strachan et al., 2002). In this regard, it is most likely to correlate in style and structural level with the Caledonian sole thrust in East Greenland (Higgins et al., 2004; Leslie and Higgins, this volume), providing a suitable analogue for the margin of the orogen in that region.
The Moine thrust zone varies from a relatively simple planar structure to a complex array of interconnected thrust sheets (Plate 1A; Krabbendam and Leslie, 2004). Detailed analysis has shown that the thrusts generally developed in a foreland-propagating sequence, and successively younger and lower thrusts transported older and higher thrusts to the WNW in piggyback fashion (Elliott and Johnson, 1980; McClay and Coward, 1981; Butler, 1982). Early-formed thrusts within the foreland-propagating sequence are commonly folded as a result of the accretion of underlying thrust sheets. This simple pattern is complicated in some areas by later, low-angle “out-of-sequence” faults that cut through previously thrust-and-folded strata (Holdsworth et al., 2006).
Rb-Sr and K-Ar dating of recrystallized micas within Moine mylonites suggests that emplacement of the Moine rocks onto the foreland occurred ca. 435–430 Ma (Johnson et al., 1985; Kelley, 1988; Freeman et al., 1998). This is consistent with the U-Pb zircon age of 430 ± 4 Ma obtained from the syntectonic Loch Borralan Complex within the Moine thrust zone in the Assynt area (van Breemen et al., 1979). However, isotopic ages as young as ca. 408 Ma have been obtained from mylonites in the Dundonell area, leading to the suggestion that, locally at least, thrusting may have continued into the Early Devonian after the main Scandian collision (Freeman et al., 1998).
The direction of regional thrusting was toward 290°N (McClay and Coward, 1981). It is difficult to estimate the displacement on the Moine thrust itself, but its association with a very thick belt of mylonites (up to 100 m) suggests that it is a major displacement zone with a minimum offset of many tens of kilometers. The construction of balanced sections drawn parallel to the direction of thrusting demonstrates a minimum slip across the thrust zone of 77 km (Elliott and Johnson, 1980), and Butler and Coward (1984) showed that the Cambrian shelf sequence can be restored for ∼54 km to the ESE. A total minimum displacement for the Moine thrust zone of around 100 km is therefore commonly accepted.
The Caledonian orogeny in East Greenland was the result of Silurian collision of Baltica with the margin of Laurentia. The structural record and architecture of that part of the orogen preserved onshore record collision in a system of foreland-propagating thrust sheets, which were derived from the Laurentian margin and translated westward across the orogenic foreland (Higgins and Leslie, 2000, this volume; Higgins et al., 2004; Leslie and Higgins, this volume). Restoration of thrusting indicates that the site of collision was probably several hundred kilometers east of the present-day onshore preserved part of the orogen. The thickened orogen was already recording the effects of E-directed thinning and collapse toward the core of the orogen as latest Silurian–Early Devonian orogen-parallel strike-slip deformation began to dominate, dissecting the orogenic welt along its axis.
Mid-Silurian to Early Devonian (ca. 430–400 Ma) subduction-related magmatism is recognized throughout the Highlands of Scotland (Stephenson et al., 1999, and references therein). Recent geochronologic research indicates that emplacement of the majority of these granites in the Northern and Grampian Highlands was focused around 427–425 Ma (Rogers and Dunning, 1991; Oliver, 2001; Fraser et al., 2004), an interval more or less synchronous with the final closure of the Iapetus Ocean in the late Wenlockian (e.g., Stone et al., 1987, 1993; Soper et al., 1992; Torsvik et al., 1992).
Magmatism overlapped Scandian-age deformation, and the emplacement mechanism of many intrusions was structurally controlled either by ductile thrusts or by the later strike-slip brittle faulting. Two main groups are recognized (Stephenson et al., 1999) (Fig. 8): the first is represented by a series of small alkaline intrusions that occur in the northwest of Scotland, mainly in the Assynt area. The second, and volumetrically more important, is commonly referred to as the “Newer Granites” (Read, 1961), although it includes a range of related rock types, including diorite, tonalite, and granodiorite. Members of this group are present on both sides of the Great Glen fault but are particularly common in the Grampian Highlands. The magmatism is represented by mainly I-type, high-K calc-alkaline rocks, some of which are shoshonitic (high-K and high-Mg) in nature. Early Devonian intrusions may have acted as feeders to volcanic sequences (Stephenson et al., 1999).
A subduction-zone setting, where of magmas were derived from the melting of mantle and/or lower-crustal sources, has been considered appropriate for both the alkaline and Newer Granite suites (Stephenson et al., 1999; Strachan et al., 2002). This is reinforced by the calc-alkaline nature of the Newer Granite suite, in particular, the Devonian volcanic rocks (Thirlwall, 1981, 1982). The isotope characteristics of the Newer Granite suite (Halliday, 1984; Stephens and Halliday, 1984; Thirlwall, 1988), as well as the presence of inherited zircons that can only have been derived from older continental basement, indicate some crustal recycling (O'Nions et al., 1983; Harmon, 1983). However, it is also clear that a proportion of magma was derived from the subcontinental lithospheric mantle (Stephens and Halliday, 1984; Tarney and Jones, 1994; Fowler et al., 2001). This mantle-derived magma is represented by the mafic enclaves and appinites associated with some plutons, and also by the calc-alkaline lamprophyre dike swarms. Thus, we conclude that the Newer Granites were derived mainly from the melting of lithospheric mantle and lower-crustal sources and that melting was probably initiated by the introduction of fluids derived from a northward-subducting oceanic slab into an overlying mantle wedge.
It should be noted, however, that the emplacement of the Newer Granites lagged some 30 m.y. after the commencement of NW-directed subduction in the Middle–Late Ordovician at ca. 460 Ma (Oliver, 2001). Onset of plutonism in the late Llandovery at ca. 430 Ma coincided with the time when tectonics in the Southern Uplands of Scotland changed from orthogonal underthrusting to sinistrally oblique underthrusting (Stone, 1995). This change in plate kinematics may ultimately have resulted in development of the crustal-scale sinistral wrench faults that are believed to have acted as fundamental controls on the locus of magma emplacement (Hutton and Reavy, 1992; Jacques and Reavy, 1994).
Kalsbeek et al. (this volume) relate the voluminous Silurian S-type Caledonian granites of the central fjord region of East Greenland to partial fusion of fertile lithologies within a thickening Krummedal supracrustal sequence, most obviously during foreland-directed translation of the Hagar Bjerg thrust sheet. Formation of this granite magma occurred prior to and during emplacement of the major thrust units and subsequent collapse of the thickened orogen between 435 and 425 Ma. Orogenic collapse followed rapidly after foreland-propagating thrusting linked to Laurentia-Baltica collision (Gilotti and McClelland, this volume; Leslie and Higgins, this volume) and may reflect the same change in plate kinematics that triggered emplacement of the Scottish Newer Granites at ca. 425 Ma.
Sinistral Transtensional Faulting in the Northern and Grampian Highlands
The main phase of mid-Silurian Scandian ductile thrusting was followed by sinistral strike-slip displacements along an array of NE-trending structures that dissect the Northern and Grampian Highlands (Fig. 2). Strike-slip faulting and ductile shear zones developed prior to and during the oblique collision of Avalonia and Baltica with Laurentia throughout the Late Silurian to Early Devonian (Soper et al., 1992). Most of these structures developed prior to the onset of post-Caledonian Old Red Sandstone (Devonian) deposition (Watson, 1984; Mykura, 1991). The most prominent structures are the Great Glen–Walls Boundary and Highland Boundary faults, along which hundreds of kilometers of displacement may have occurred (Dewey and Strachan, 2003).
Seismic-reflection studies show that the Great Glen fault is coincident with a subvertical structure that extends to at least 40 km depth (Hall et al., 1984). Silurian mantle-derived lamprophyre dikes appear to have different isotopic signatures on either side of the fault, suggesting that this structure has some expression in the upper mantle (Canning et al., 1996, 1998). Stewart et al. (1999) argued that blastomylonitic rocks preserved in the core of the fault zone may reflect the presence of an exhumed positive flower structure that formed during sinistral transpression along the same zone of weakness. Relationships among fault-zone structures, dated igneous intrusions, and postorogenic sedimentary rocks constrain the main sinistral displacement along the Great Glen fault to the period between ca. 428 Ma and ca. 400 Ma (Stewart et al., 1999). Newer Granite plutonism was thus initiated broadly concurrently with the onset of major sinistral transtensional displacement. A lower age limit of ca. 400 Ma is indicated by the low-strain nature of Old Red Sandstone (upper Emsian?) sedimentary rocks within the fault zone. Post–Old Red Sandstone structures along the fault zone are invariably brittle in style, and fault products are typically incohesive, consisting of clay fault gouge and poorly consolidated fault breccia (May and Highton, 1997; Stewart et al., 1999).
Although the timing of late Caledonian sinistral transtension is relatively well constrained, the magnitude of early displacement along the Great Glen fault is less certain because there is no unambiguous correlation of pre-Devonian features across the fault. The general consensus (Strachan et al., 2002) is that sinistral strike-slip displacements along the Great Glen fault are unlikely to have exceeded 200–300 km, although Dewey and Strachan (2003) later argued that a bare minimum of 700 km displacement is required if no Scandian deformation should be identified in the Grampian Highlands. The lower value is, however, consistent with the most reliable paleomagnetic evidence (Briden et al., 1984), the inferred offset of reflectors within the mantle lithosphere (Snyder and Flack, 1990), correlation between the Moine Supergroup and the Dava–Glen Banchor successions, and similarities in the timing of Neoproterozoic and Ordovician tectonothermal events on either side of the fault (Bluck, 1995; Stewart et al., 1999; Highton et al., 1999; Rogers et al., 2001). The possibility remains that the Great Glen fault does not after all represent a terrane boundary (sensu stricto), and that a greater apparent discontinuity (including a contrast in crystalline basement properties) exists between the Midland Valley and Southern Uplands of Scotland along the trace of the present Southern Uplands fault.
The present Highland Boundary fault is a high-angle reverse fault that emplaced Dalradian rocks onto the Highland Border Complex and Old Red Sandstone rocks of the Midland Valley (Anderson, 1946; Bluck, 1984). Geophysical studies have shown that the fault is broadly coincident with a change in lower crustal structure (Bamford et al., 1978; Barton, 1992; Rollin, 1994), and this implies that the present structure may have reactivated an older and more fundamental structure. Various workers have speculated that this may correspond to the edge of the Laurentian craton (e.g., Soper and Hutton, 1984). Although Late Silurian to Early Devonian sinistral displacements comparable with the Great Glen fault have been commonly assumed (e.g., Harte et al., 1984; Soper and Hutton, 1984; Hutton, 1987; Soper et al., 1992), other workers have argued against major displacements (e.g., Hutchison and Oliver, 1998), and, thus, the regional tectonic significance of this fault is uncertain.
In central East Greenland, mid-Devonian continental sediments were deposited on the eroded Caledonian orogen; sedimentation was controlled, in part, by sinistral displacements along the Western fault zone (Larsen and Bengaard, 1991; Olsen, 1993). Larsen and Bengaard (1991) tentatively linked the Western fault zone with the Storstrømmen shear zone to the north and with the Great Glen fault to the south. Dewey and Strachan (2003) argued that relative motion between Laurentia and Avalonia-Baltica changed from sinistrally transpressive collision at ca. 425 Ma to more orogen-parallel sinistrally transtensional movements, which persisted until ca. 400 Ma and were terminated, in Britain, by the brief Acadian orogeny (Soper and Woodcock, 2003). Dewey and Strachan (2003) argued that the Great Glen fault would have been the principal structure on which sinistral displacement was accommodated in the Scottish sector of the orogen, linked northward via the Walls Boundary fault in Shetland into East Greenland as the Western fault zone.
Similar translations have been proposed to explain the juxtaposition of the separate terranes identified in Svalbard (e.g., Soper et al., 1992; Harland et al., 1997). Gee and Teben'kov (2004) proposed an alternative interpretation which minimizes the scale of strike-slip displacement and, if proven, might argue that displacement on the major strike-slip fault systems transecting the Scottish Caledonides should be of a similar dimension, i.e., <200 km.
A TEMPLATE FOR COMPARISON OF IAPETAN RIFTING AND CALEDONIAN OROGENESIS IN THE SCOTLAND-GREENLAND SECTOR OF LAURENTIA
The Iapetan geological record from Scotland constrains the establishment of the Laurentian passive margin and culmination of the two-stage breakout of Laurentia from the super-continent Rodinia. The uppermost Neoproterozoic to lower Paleozoic Scottish successions record proximity to a RRR junction (Soper, 1994a), and by late Neoproterozoic (Ediacaran) time, the “Scottish Corner” may have resembled a patchwork of marginal plateaus or continental ribbons extending some 1000 km or more along the margin (Fig. 9), similar in style to the paleogeography proposed by Waldron and van Staal (2001) and Cawood et al. (2001) for the Newfoundland sector. The geological map of the United Kingdom–Faroes sector of the present-day continental shelf presents a framework of interlocking depocenters (Hatton, Rockall, and Porcupine Basins) and structural basement highs (Rockall and Porcupine highs) and provides a good modern analogue for the late Neoproterozoic paleogeography of the “Scottish Corner” of Laurentia.
In contrast, the lithostratigraphy of East Greenland can be traced continuously over considerable distances along strike along the original depositional margin. A general absence of volcanic activity suggests that the East Greenland sector may have lain (dependent upon stretching rates) at some distance from any focus of Iapetan volcanism (Fig. 1). There is systematic thickening of the successions from inboard to outboard positions in the sector, and the pattern of N-S facies belts shapes a slowly subsiding but relatively stable continental margin that must have extended beyond and included northeastern Svalbard at this time (Gee and Teben'kov, 2004).
In the following sections, we provide a dynamic synthesis of the geological evolution of the Scottish sector of Laurentia (the “Scottish Corner”) through a complete Wilson cycle that encompasses the opening and closure of the Iapetus Ocean. The synthesis reflects the authors' stance in regard to the wealth of new and archived data available to geologists studying the evolution of the Scottish and East Greenland Caledonides.
Cycle I: Early (Failed) Cryogenian Rifting
Mid-Cryogenian rifting on the “Scottish Corner” probably initiated after 730 Ma and then lasted for 60–70 m.y., with post-rift thermal subsidence extending for another ∼50 m.y. or so toward the end of the Cryogenian. Turbiditic sands and muds of the Grampian Group, derived from a hinterland to the west and northwest, infilled a series of basins (Banks, 2005), the distribution and depositional geometry of which are constrained by the position, and uplift history, of discrete intrabasinal highs, e.g., Robertson and Smith (1999). These localized basins were infilled by the time progradational shelf sands extended from the south and east across these early (presumably failed) rifts and their margins in the upper Grampian Group. The continued sand- and mud-dominated sedimentary facies associations of this first cycle culminated with diminished sediment input into marginal off-shelf to lagoonal, locally emergent, even evaporitic environments as recorded in the lowermost Appin Group (Lochaber Subgroup) (Stephenson and Gould, 1995).
In Greenland, the contemporaneous Nathorst Land Group appears to reflect a more gradual and widely distributed subsidence, and there is no sign of localized depocenters (Sønderholm et al., this volume). The correlation proposed here at significant flooding surfaces (Fig. 6) may signify short-lived enhanced stretching that affected all of this part of Laurentia. Active stretching probably advanced northward from the vicinity of the “Scottish Corner” toward East Greenland, perhaps linking with an active rift system that was encroaching from the north.
Cycle I: Sag
A major marginwide transgression across a flooding surface marks the base of the Ballachulish Subgroup in Scotland and the Ymer Ø Group in East Greenland (Fig. 6). Flooding began the onset and development of wide-ranging and uniform sedimentary lithofacies. Typically, deposition of dark anoxic limestone and mud is followed by shallowing-upward cycles of progradational clean-washed sands and shallow-water muds and limestones (Stephenson and Gould, 1995). Basins at this time were probably wide and shallow, as evidenced by remarkably similar successions along strike extending across Ireland and Scotland and with tentative correlations possible at formation level across Greenland into the Hekla Sund Basin of eastern North Greenland (Fig. 6). Renewed flooding (eustatic change?) heralded the deposition of further muds and limestones (Blair Atholl Subgroup in Scotland and the Andrée Land Group in East Greenland) before “Marinoan” diamictites were deposited during a major lowstand at ca. 635 Ma. In the Southern Grampian Highlands, some parts of these basins were interspersed with sediment starved areas, which are now expressed as major stratigraphic omission and subsequent overstep (A.G. Leslie for BGS, unpublished data).
Cycle II: Renewed Rifting to Rift-Drift Transition in the Ediacaran
Renewed and vigorous extension is recorded by rifting in the Scottish Dalradian in early Ediacaran time. Contemporaneous mafic volcanism marks the transition to Iapetan rift-drift. Sharply defined thickness changes become apparent in the Islay Subgroup depositional record and are most likely fault controlled (Anderton, 1985). The upward-shallowing cyclical behavior recorded previously begins to repeat again in the Easdale Subgroup, albeit in an increasingly unstable, but still essentially shallow-marine, on-shelf environment, with deposition of quartzitic sands, limestones, and muds. Instability is recorded in influxes of pebbly sands (Stephenson and Gould, 1995). Localized volcanogenic centers become a feature of this sector of the margin, with punctuated episodes of mafic volcanism (Goodman and Winchester, 1993; Macdonald et al., 2005).
Sediment-starved sectors in Scotland are only overstepped at this later stage (A.G. Leslie for BGS, unpublished data), and a more rapidly foundering rift system (Crinan Subgroup) then evolved, accumulating debris flows and slumps (Stephenson and Gould, 1995). Incipient volcanic spreading centers located mainly in the southwest (Tayvallich Subgroup, Fig. 6) gave way to organized turbiditic submarine fans (Southern Highland Group) as final rupture occurred and this sector of Iapetus began to widen. The major change to more immature sediment, increasingly dominated by Archean detritus, occurs at the base of the Crinan Subgroup and persists on through the Southern Highland Group as separation occurred (rift-drift transition) and the foundering margin evolved toward a stable passive margin by Cambrian-Ordovician time. Laurentian fauna (Pagetia) contained within the uppermost, youngest Dalradian strata (Tanner, 1995) and conodont assemblages in the Durness Group (Wright and Knight, 1995) place the “Scottish Corner” firmly on the Laurentian margin.
Erosion of the upper formations of the Eleonore Bay Supergroup to provide clasts in the latest Cryogenian to early Ediacaran (Marinoan) Tillite Group indicates pre–Tillite Group uplift, but it is not clear whether the absence of any younger Ediacaran deposits merely records lack of supply and nondeposition or if extensional rifting and block tilting removed any sediment accumulation prior to deposition of the Cambrian succession (Smith et al., 2004; Sønderholm et al., this volume; Smith and Rasmussen, this volume). Any accumulations may have been scavenged from this relatively stable, nonvolcanic marginal platform into the active and volcanic rifts on the “Scottish Corner.”
Figure 9 presents a summary of how the Scottish sector may have looked at this time. Rifting leading to separation occurred ca. 600 Ma; fragments of the continental margin then broke away, and Iapetan spreading ridges became active (off to the lower right of the cartoon). Relative to the central parts of the Grampian and the southwest Scottish Highlands, we speculate that Connemara in western Ireland may have constituted a marginal plateau or ribbon, while the Moray-Buchan area of northeast Scotland may have constituted a subdued marginal platform. The newly established peri-Laurentian trough received a deluge of submarine debris flows (Crinan Subgroup); localized volcanic centers built out volcaniclastic deltas. Carbonate buildups were reworked from the shelf into deeper water as redeposited metacarbonate rocks in the Tayvallich Subgroup (Thomas et al., 2004). Submarine fans prograded from the margin into the trough and may have extended along the trough axis, spilling out onto adjacent marginal platforms. Deltaic volcaniclastic debris was reworked along the margin as “green beds” in the Southern Highland Group (Pickett et al., 2006). The extensional geometry of the various components of this architecture must of necessity be speculative; possible cross-section configurations are incorporated in the model of Figure 9. We can suggest, however, that these geometries subsequently exerted control on the collisional geometry and acted as nuclei for deformation structures during arc accretion in the Grampian orogeny.
The analogue model of Figure 9 can be extended to include the East Greenland sector of the Laurentian margin. Although late Neoproterozoic extension may locally have affected the inboard portion of North-East Greenland (e.g., in the Målebjerg region; Leslie and Higgins, 1998, this volume), the lack of volcanism and persistent shallow-marine shelf depositional environments imply a broad, gradually subsiding continental platform where subsidence rate was broadly matched by sediment influx, at least until the late Cryogenian. The Hekla Sund Basin of eastern North Greenland preserves evidence of the northward change into to a more active, but nonvolcanic, extensional setting on Laurentia, in the Cryogenian at least.
Evolution to a Cambrian-Ordovician Passive Margin
By Cambrian-Ordovician time, a broad carbonate platform was established along the continental margin of Laurentia facing the Iapetus Ocean. Orthoquartzite/carbonate successions that accumulated in this period are recognized all along the length of the Caledonian fold belt in East Greenland (Higgins et al., 2001a), across the far northwest of Scotland, and through Newfoundland and the Appalachians (see compilation in Dewey and Mange, 1999). Whether or not Cambrian-Ordovician shelf, shelf-slope, and rise successions either did (Dewey, 1969) or did not (Bluck et al., 1997) continuously extend across Scotland from the Laurentian foreland into the deeper-water environments of the Cambrian upper Dalradian of the Southern Grampian Highlands has been a matter of debate. Some support for the former position comes from reconstructions of the Newfoundland margin (Waldron and van Staal, 2001) and from East Greenland (Leslie and Higgins, 1998, this volume), and the new work of Tanner and Sutherland (2007) has resolved the matter in the Scottish sector. Analogues for the Cambrian-Ordovician successions of the Highland Border Series are perhaps to be found in later pelagic passive-margin sedimentation on a fragmented continental margin (Stoker et al., 2001), which would have accumulated in the troughs featured in the model of Figure 9.
Grampian Arc Accretion
Figure 10A illustrates our Early Ordovician (Tremadoc–Arenig) reconstruction of the Scottish sector of Laurentia. At this point, Cryogenian siliciclastic detritus had built out a progradational sedimentary prism, drowning in the process such intrabasinal structural features as the Glen Banchor “basement high” (Robertson and Smith, 1999; GB on Fig. 10A). Ediacaran (ca. 600 Ma) intrusion of extension-related bimodal magmatism (Tanner et al., 2006) into the expanding sediment prism is represented by the Ben Vuirich Granite Pluton and the Tayvallich Volcanics Formation (BV and TV on Fig. 10A, respectively). The Cambrian-Ordovician Durness Group represents major marine transgression onto the subsiding continental margin, while deeper-water contemporaneous successions accumulated on the Laurentian continental slope and rise (upper Dalradian Leny Limestone). Intra-oceanic obduction and accretion was under way by this time (Bluck, 2001), and the Midland Valley Arc began to encroach upon the Laurentian continental margin.
By Arenig-Llanvirn time, the Midland Valley Arc (MVA on Fig. 10B) had been accreted onto the continental margin. The arc was partly underthrust on the margin such that a zone of top-to-the-south or -southeast noncoaxial simple shear formed in the lower structural levels of the Tay nappe, as recorded in the southern Scottish Highlands (Krabbendam et al., 1997). SE-facing extensional fault blocks on the ruptured continental margin (Fig. 9) would have rotated and steepened toward the interior of the orogenic wedge underneath the developing nappe (HB on Fig. 10A). Syntectonic mafic and calc-alkaline felsic magmas emplaced into the thickening orogenic pile at ca. 470 Ma (Friedrich et al., 1999) probably formed as a consequence of the suborogenic heating as the subducting oceanic crust detached southeastward beneath the accreting magmatic arc (Dewey and Mange, 1999). The Grampian front clearly extended into the Moinian rocks of the Northern Highlands of Scotland (Dallmeyer et al., 2001), and crustal flexure at this time (ca. 460 Ma) would have ended shallow-water carbonate deposition on the continental platform.
No widespread evidence of Ordovician arc accretion survives in East Greenland; the terranes involved in collision have been transported eastward and incorporated into the higher structural levels of the Scandinavian Caledonides (Yoshinobu et al., 2002).
With rapid uplift and erosion of the thickened orogenic wedge postdating the Grampian metamorphic peak (ca. 465 Ma), Grampian-age metamorphic detritus was dispersed across the accreted arc (or arcs) and supplied to an accretionary wedge facing the narrowing Iapetus Ocean. In Scotland, such detritus appears in the sedimentary record of the Southern Uplands accretionary prism in the Caradoc (Oliver, 2001). However, the system of large-scale strike-slip displacements that affected this sector of the closing Iapetus in the Silurian-Devonian (ca. 425–400 Ma) means that the Late Ordovician–Early Silurian sediment dispersal pathways cannot now be determined but are likely to have incorporated considerable distances along the orogenic belt.
Baltica collided with the East Greenland sector of Laurentia by late Llandoverian–early Wenlockian time. The reconstruction in Figures 10B and 10C speculates that the Scottish sector would also have experienced oblique collision with parts of Baltica. Post-metamorphic peak exhumation of the Grampian orogenic wedge was largely complete, and the lower inverted limb of the Tay nappe had by now formed the flat belt identified in the cross section in Figure 8. In contrast, Scandian nappe stacking dominated the structural architecture of the Northern Highlands of Scotland and the East Greenland Caledonides. One possible view is that the Grampian Highlands may thus have formed a relatively rigid block entrained between the contractional deformation zones at the leading edges of the colliding Laurentian and Baltican plates. At ca. 425 Ma, in the mid-to-late Silurian, large volumes of felsic magma were intruded into the Grampian and Northern Highlands, and partitioned sinistral transtensional stresses began to replace Early Silurian oblique convergence and transpression, as evidenced by movements along the Great Glen (Stewart et al., 1999), Highland Boundary, and Southern Upland faults. Juxtaposition of the present-day Southern Uplands region with the Midland Valley and Scottish Highlands was largely achieved at this time as “Scottish” Laurentia and Avalonia were juxtaposed along WSW-ESE strike-slip systems, expressed today in the Southern Uplands fault (see ca. 410 Ma inset on Fig. 10C). Major pull-apart and extension in the more N-S–trending tracts of the Caledonian suture between “East Greenlandic” Laurentia and western Baltica may have initiated the Early Devonian eclogite exhumation processes in the Western Gneiss Region of Norway and the eclogite-bearing terrain of East Greenland (Krabbendam and Dewey, 1998; cf. Gilotti et al., this volume). Evidence for orogenic collapse in the Scottish sector is possibly represented by the depositional architecture and deformation of the Old Red Sandstone (Middle Devonian) Orcadian Basin in Orkney and Shetland (Seranne, 1992), and the top-to-the-NE–directed shear fabrics recorded in the Shetland ophiolite complex and the metasedimentary successions of Unst and Fetlar in NE Shetland (Cannat, 1989). NE-vergent extension in Shetland is opposite to the SW-vergent extension and exhumation of the Western Gneiss Region in Norway (Krabbendam and Dewey, 1998), which suggests perhaps an internal zone of collapse toward the interior of the Caledonian orogen.
CONCLUSIONS AND FUTURE RESEARCH
A number of key observations and conclusions emerge from these correlations and discussion.
By ca. 1.9 Ga, the ancient basement of Scotland and Greenland lay within a continuous accretionary belt made up of various Archean cratonic components welded together during the assembly of Laurentia and Baltica. Calc-alkaline magmatism concentrated along a new active margin is represented by the ca. 1.9–1.85 Ga Makkovik-Ketilidian belt of Labrador and South Greenland, juvenile Proterozoic crust forming at ca. 1.78 Ga in the Rhinns Complex in Scotland, and the 1.85–1.50 Ga Labradorian-Gothian belt of NE Canada and SW Scandinavia.
Early Neoproterozoic fluvial red-bed successions (Torridon Group) buried deeply eroded Archean-Paleoproterozoic basement rocks in the foreland to the Caledonian orogen in Scotland; no comparable sediments are known in East Greenland.
Late Mesoproterozoic to early Neoproterozoic silici clastic successions are widespread in Greenland (Krummedal supra-crustal succession), NW Scotland (Moine Supergroup), and in the eastern province of Spitsbergen (Brennevinsfjorden Group). These successions are almost entirely marine deposits and indicate that a depocenter, or interconnected depocenters, may have extended over several thousand square kilometers along the Laurentian margin during this time. Since no equivalent of the early Neoproterozoic Torridon Group fluvial red-bed system seems to have been present at the paleolatitude represented by East Greenland, we conclude that it is possible that the Krummedal succession may represent a northward continuation of marine dispersal of parts of the Moine succession.
Three episodes of early to mid-Neoproterozoic orogenesis (ca. 930 Ma, ca. 820 Ma, ca. 730 Ma) are recorded in the Scottish and Greenland sectors of Laurentia, and each is separated by ∼100 m.y. The Rodinian supercontinent was undergoing late-stage amalgamation during this time, and so intracratonic movements accommodating extension and compressional deformation may be required to account for these orogenic events (e.g., Cawood et al., 2004).
In Scotland, the mid-Neoproterozoic to mid-Cambrian Dalradian Supergroup was deposited in an evolving pericontinental environment over a period of ∼180–200 m.y. Fluctuations in water depth accompanied active rifting and the development of second- and third-order subbasins on this sector of the Laurentian margin. In East Greenland, farther north on that same margin, the broadly time-equivalent Eleonore Bay Supergroup and overlying Tillite Group and Kong Oscar Fjord Group record no active rifting and contain no evidence of contemporaneous volcanic activity. East Greenland strata record shelf and ramp sedimentation, which was probably punctuated by significant periods of non-deposition on a slowly subsiding passive margin. Farther north still, the Neoproterozoic Hekla Sund Basin represents another locus of rifting activity in eastern North Greenland. The youngest units of the Dalradian Supergroup in Scotland record rapid basin deepening that persistently stayed ahead of the sedimentary and volcanic basin fill. No apparent match for these immature turbidite-dominated sedimentary facies associations occurs anywhere along the Greenland sector of Laurentia.
The NW Highlands Cambrian-Ordovician succession of Scotland apparently represents an intermediate position on the slowly subsiding Laurentian carbonate platform that was located between more inboard and more outboard environments, each of which is represented by the Cambrian-Ordovician succession in East Greenland.
Mid-Ordovician Grampian orogenesis affected all of the Dalradian and older rocks of the Grampian Highlands as well as parts of the Moine Supergroup in the Northern Highlands. This phase records the convergence of the Laurentian continental margin with an intra-oceanic subduction zone and volcanic arc. There is, in contrast, scant evidence in the East Greenland Caledonides (or in Svalbard) of these short-lived early Paleozoic marginal arcs and basins that must have accommodated subduction of Iapetan oceanic crust and convergence with Baltica.
Baltica-Laurentia collision is expressed in Scotland and East Greenland as the Scandian orogeny. Regional-scale ductile thrusting and folding of the Moine rocks in the Northern Highlands of Scotland culminated in the development of the Moine thrust zone at ca. 430 Ma, marking the boundary with the autochthonous-parautochthonous foreland rocks of the Northwest Highlands. The Moine thrust zone is therefore a comparable structure to the Caledonian sole thrust of East Greenland. Restoration of the system of foreland-propagating thrust sheets derived from the Laurentian margin and translated westward across the orogenic foreland in East Greenland indicates that the site of collision was probably several hundred kilometers east of the present-day onshore preserved part of the orogen.
Mid-Silurian to Early Devonian (ca. 430–400 Ma) subduction-related magmatism is recognized throughout the Highlands of Scotland (Stephenson et al., 1999, and references therein). Kalsbeek et al. (this volume) relate the voluminous Silurian S-type Caledonian granites of this age in the central fjord region of East Greenland to partial fusion of fertile lithologies as a consequence of crustal thickening, most obviously during foreland-directed translation of the Hagar Bjerg thrust sheet.
The present distribution of Caledonian domains across the northern Atlantic region is in part a consequence of the trans-tensional shearing that resulted from relative lateral motion of the two large continental segments (Laurentia and Baltica) after their respective continental margins were in contact and subduction of oceanic crust had ceased. Sinistral wrench faults dissect the Caledonian orogen of both Scotland and East Greenland.
Together, East Greenland and Scotland preserve a partial 350 m.y. record of deposition and rifting leading toward opening and spreading of the Iapetus Ocean, and then of the arc accretion and continent-continent collision that consumed Iapetus. While the “Scottish Corner” is undoubtedly geologically fascinating, and perplexing, we have found it hugely rewarding to work on the bigger scale. We present this overview as a comparative synopsis of our present understanding of the Scottish and East Greenland orthotectonic sectors of the Caledonian orogen and as an encouragement to future researchers to address issues of diachroneity in otherwise similar events and strive for better understanding of the “big picture.” The tentative (tectono)stratigraphic correlations proposed here must be tested further. Many of those answers will lie in systematic analysis of Caledonian geology around the North Atlantic region, linking Scandinavia, Svalbard, Greenland, Scotland, Labrador, and Norway.
Smith and Leslie publish with the permission of the Director of the British Geological Survey. Constructive comments and criticism were gratefully received from P. Cawood and from an anonymous referee.
Figures & Tables
The Greenland Caledonides: Evolution of the Northeast Margin of Laurentia
- Arctic region
- Caledonian Orogeny
- continental margin
- East Greenland
- Great Britain
- plate collision
- plate tectonics
- United Kingdom
- upper Precambrian
- Western Europe