The Uppermost Allochthon in the Scandinavian Caledonides: From a Laurentian ancestry through Taconian orogeny to Scandian crustal growth on Baltica
Published:January 01, 2007
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David Roberts, Øystein Nordgulen, Victor Melezhik, 2007. "The Uppermost Allochthon in the Scandinavian Caledonides: From a Laurentian ancestry through Taconian orogeny to Scandian crustal growth on Baltica", 4-D Framework of Continental Crust, Robert D. Hatcher, Jr., Marvin P. Carlson, John H. McBride, José R. Martínez Catalán
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Thrust sheets of the Uppermost Allochthon in the Caledonides of Scandinavia are distinguished by lithological assemblages and magmatic units that are, in many ways, quite different from those in subjacent nappe complexes. Supracrustal successions are derived mainly from platformal, shelf-edge and basinal-slope environments and are characterized in particular by extensive developments of carbonate rock units that range in age from Late Riphean to Early Silurian. Metasedimentary iron ore formations are also present. Another prominent feature is the Ordovician, arc-type, granitoid plutons and batholiths that dominate the geology in certain parts of the allochthon. In addition to these lithological elements, the Uppermost Allochthon carries an Ordovician tectonothermal record and early Caledonian, NW-vergent thrust polarity that is unique in Norway. Taken together, these features are indicative of a history of development and crustal growth along the eastern margin of Laurentia, involving an outboard magmatic arc, or arcs, and Taconian accretionary orogenesis. This was followed by recycling and deposition in Late Ordovician to Early Silurian successor basins prior to Laurentia-Baltica collision and the onset of the Scandian orogeny. The Taconian thrust sheets were then detached from their Laurentian roots and incorporated into the Siluro-Devonian, Scandian orogenic wedge on the Baltoscandian margin of Baltica. Taking into account the widely reported sinistral megashear arising from the Scandian, oblique collision and plate rotation, the rock units that constitute the Uppermost Allochthon are likely to have originally been located closer to the northern Appalachian segment of the margin of Laurentia, in view of the strikingly similar lithostratigraphic, magmatic, and tectonothermal histories of these two, now widely separated terranes.
The Scandinavian Caledonides form part of a once extensive, but now fragmented, early Paleozoic orogenic belt that fringes the North Atlantic Ocean from the southeastern United States and northwest Africa via the British Isles to the Barents Sea. The Scandinavian segment, contiguous with East Greenland prior to the Cretaceous-Tertiary opening of the Atlantic, is characterized by its innumerable thrust sheets that carry stratigraphic and magmatic evidence of diverse and disparate origins. These thrust sheets, or nappes, were transported roughly from west to east onto the Baltoscandian margin of the paleocontinent Baltica in Siluro-Devonian time, during the Scandian orogeny (Gee, 1975).
It had long been known that in the foreland, along the Caledonian front in the east, thin sedimentary successions of Vendian-Cambrian age lie unconformably upon older Precambrian crystalline basement (Petersen, 1878), and these are succeeded, above a prominent sole thrust, by folded and imbricated, low-grade sedimentary rocks of similar character to those in the autochthon. Farther west, rocks of higher metamorphic grade are involved, in places with slices of basement rocks, and these relationships eventually convinced most nineteenth-century geologists, led by Törnebohm (1888, 1896), that only thrusting over vast distances could account for the juxtaposition, above sharp mylonitic contacts, of markedly different rock complexes.
Close to a century later, following extensive, detailed mapping and structural studies, geologists realized that many of these thrust sheets and nappes, in some cases already grouped together in “nappe complexes,” could be combined into higher-rank “allochthons” based on a variety of lithological, magmatic, and geochemical criteria. Recognition of shelf and miogeoclinal successions, and more outboard oceanic- and arc-type terranes succeeded by even higher, exotic, thrust rock complexes, led to the establishment of a common tectonostratigraphy composed of Lower, Middle, Upper, and Uppermost Allochthons (Fig. 1) (Gee and Sturt, 1985; Roberts and Gee, 1985). Successions in the Lower and Middle Allochthons are clearly indigenous to the Baltoscandian continental margin, whereas the Upper Allochthon contains elements of the continent-ocean transition zone (the Seve Nappes; Andréasson, 1994) and the overlying, exotic, Iapetus Ocean terranes (the Köli Nappes). Faunas of mainly Laurentian affinity in one of the highest Köli Nappes clearly point to a foreign provenance (Bergström, 1979; Bruton and Bockelie, 1979, 1980; Spjeldnæs, 1985). In the Uppermost Allochthon, rock assemblages from a variety of environments, including extensive carbonate formations and voluminous granitoid plutons that do not occur in subjacent thrust sheets, led workers in the 1980s to suggest possible derivation of these exotic assemblages from either Laurentia or unknown microcontinents (Stephens and Gee, 1985; Roberts et al., 1985). These early ideas were taken further and elaborated upon (Grenne et al., 1999), and a Laurentian ancestry has been substantiated by chemostratigraphic, structural, and isotopic dating work during the last few years (e.g., Melezhik et al., 2001a, 2001b, 2003; Roberts et al., 2001, 2002; Yoshinobu et al., 2002).
In this contribution, we present a short description of the constituent units of the Uppermost Allochthon, concentrating on its more exotic elements and noting features of its tectonothermal history that are different from those recorded in the Baltoscandian and Iapetan allochthons. This, in turn, leads into a brief discussion of the accretionary development and crustal growth processes involved in the Uppermost Allochthon in particular, and within the Scandinavian Caledonide orogen as a whole. In addition to the emphasis on crustal growth and orogenic accretion, a main aim of this contribution has been to present a modern review of this highest allochthon and exotic terrane in the Scandinavian Caledonides.
REGIONAL EXTENT OF THE UPPERMOST ALLOCHTHON
The Uppermost Allochthon is almost entirely confined to north-central Norway in the counties of Troms, Nordland, and part of Nord-Trøndelag (Fig. 1), covering six degrees of latitude over a strike length of more than 700 km. It extends into Sweden in only one small area, southeast of Mo i Rana. The more-or-less continuous exposure of the Uppermost Allochthon is broken in just one part of northern Nordland, where the “Tysfjord window” of crystalline basement rocks forms a bridge that links to the Precambrian rocks of the Lofoten archipelago and thus disrupts the continuity of the overlying thrust sheets.
Since the time of the initial map compilation (Gee et al., 1985), additional detailed mapping, structural investigations, and geochemical studies have led to minor modifications to the general geographic extent of the Uppermost Allochthon. Later field studies have given rise to suggestions that outliers or fault-bounded units of typical Uppermost Allochthon complexes may also be present farther to the southwest in the coastal areas west of Trondheim, notably on the islands of Hitra (Nordgulen et al., 2002; Tucker et al., 2004) and Smøla (Fig. 1), where Arenig-Llanvirn faunas of North American aspect have been recorded (Bruton and Bockelie, 1979). In northern Norway, in the Ofotfjorden-Lyngen district, parts of the successions in the Uppermost Allochthon were relegated to the Upper Allochthon by Anderson (1989), Steltenpohl et al. (1990), and Anderson et al. (1992), but more recent regional mapping and compilations (Zwaan et al., 1998; K.B. Zwaan, 2005, personal commun.) still favor the original assignments of Gee et al. (1985).
Even farther north, in the county of Finnmark, recent studies in the highest turbidite formation of the tectonostratigraphic succession on Sørøya (Fig. 1) suggest that this unit is more likely to be Laurentian rather than Baltican and should preferably be assigned either to an exotic, high-level Köli Nappe or to the Uppermost Allochthon (Kirkland et al., 2005; Slagstad et al., 2006). Recent data also suggest that the Magerøy Nappe on the island of Magerøya (just off Fig. 1, northeast of Sørøya) may have been derived from the Laurentian margin, and could also possibly be regarded as an outlier of the Uppermost Allochthon (Kirkland et al., 2005; Corfu et al., 2006). Clearly, as new results and interpretations appear from different areas, further modifications to the allochthon hierarchy will be required.
Over the greater part of its exposure, south of the Tysfjord window, the Uppermost Allochthon is dominated by two tectonic mega-units, the Rödingsfjället Nappe Complex (Gavelin and Kulling, 1955) and the overlying Helgeland Nappe Complex (Ramberg, 1967) (Fig. 1). The basal contact of the allochthon, against the subjacent Köli Nappes of the Upper Allochthon, is characterized by a thick zone of mylonites and blastomylonites with some phyllonites, commonly imbricated into slices and lenses. Over large parts of its outcrop, the thrust-related mylonites are overprinted by late Scandian, ductile extensional structures (Osmundsen et al., 2003).
Each of these two nappe complexes is essentially a major, composite unit composed of several thrust sheets, which, in turn, are composed of diverse supracrustal rocks, basement gneisses, and a variety of intrusions. In the Helgeland Nappe Complex, no specific geographical names have been introduced for these separate thrust sheets, but their distinctive histories have been recognized in certain areas by the adoption of informal terms such as Lower, Middle, and Upper nappes (Thorsnes and Løseth, 1991; Yoshinobu et al., 2002).
One thrust sheet that was long thought to be a correlative of parts of the Helgeland Nappe Complex, but is geographically separated from it, north of Mo i Rana, is the Beiarn Nappe (Rutland and Nicholson, 1965) (Fig. 1). Later field work and regional map compilations have favored the view that the Beiarn Nappe is best considered to be a higher thrust unit within the Rödingsfjället Nappe Complex (Gustavson and Gjelle, 1991; Gustavson, 1996). Internal structural features of the Beiarn Nappe, however, are more akin to those in the Helgeland Nappe Complex (Stephens et al., 1985a) than the Rödingsfjället, such that the question of correlation is still open to discussion. A similar problem of thrust sheet affiliation, and one that highlights the rather artificial nature of allochthon subdivision, concerns the Fauske Nappe (Nicholson, 1974). On some map compilations, this has been interpreted as a Köli unit (Upper Allochthon) (e.g., Gjelle, 1988), but later work has placed the Fauske Nappe firmly at the base of the Rödingsfjället Nappe Complex (Gustavson, 1996; Roberts et al., 2002) (Fig. 1), in line with its original tectonostratigraphic position (Gee et al., 1985).
In the region north of the Tysfjord window, a variety of named and unnamed thrust sheets have been assigned to the Uppermost Allochthon (Binns, 1978; Andresen et al., 1985; Barker, 1986; Steltenpohl, 1987; Andresen and Steltenpohl, 1994; Zwaan et al., 1998; Melezhik et al., 2003), and specific correlations have been suggested (Steltenpohl et al., 1990). Based on modern regional map compilations, the subdivisions of Zwaan et al. (1998) for the northernmost areas (Lyngsfjell, Nakkedal, and Tromsø Nappes), and Barker (1986) and Melezhik et al. (2003) for the Ofotfjorden region (Bjerkvik, Evenes, Gratangen, and Niingen Nappes) are currently considered to be the most appropriate for these separate districts (Fig. 1). It should be noted, however, that the more recent compilations show that the Tromsø and Gratangen Nappes would appear to be one and the same thrust sheet.
Although there are differences in the lithological character of the many thrust sheets that constitute the Uppermost Allochthon, notably in those that are juxtaposed, there are also features that can be followed intermittently or, in some cases, almost continuously over distances of 300 km or more. In general terms, a distinction can be made between supracrustal successions, which are widespread, gneissic “basement” slices of more sporadic occurrence, and plutonic bodies that are most extensively developed in the south, in the Helgeland Nappe Complex (Barnes et al., 1992; Nordgulen et al., 1993). Depositional ages of the metasedimentary rocks are believed to range from Riphean to Early Silurian (Stephens et al., 1985a; Binns and Matthews, 1981; Bjørlykke and Olaussen, 1981; Melezhik et al., 2002a, 2002b).
The ubiquitous supracrustals are mostly composed of diverse, medium- to high-grade, psammitic, pelitic, and calcareous schists, calcite and dolomite marbles, and amphibolites of inferred Late Riphean to Cambrian age, with subordinate quartzites, polymict and monomict conglomerates, local diamictites, and laterally persistent stratabound iron formations (Gustavson, 1966; Nicholson and Rutland, 1969; Stephens et al., 1985a; Steltenpohl et al., 1990) (Fig. 2). Interestingly, true tillites, i.e., rocks of undisputed glaciogenic origin, have not been recorded in the Uppermost Allochthon. An appreciation of the diversity and detail of these thick supracrustal successions can be gained by studying many of the map-sheet compilations from this extensive region (e.g., Gustavson, 1981, 1996; Gustavson and Gjelle, 1991; Zwaan et al., 1998). Eclogites, eclogitized gneisses, rare ultramafite lenses, and diverse migmatized rocks have been described from the Tromsø Nappe (Krogh et al., 1990; Selbekk et al., 2000; Corfu et al., 2003), and dismembered ophiolites occur in a few places, both in northern (Boyd, 1983; Barker, 1986; Andresen and Steltenpohl, 1994; Furnes and Pedersen, 1995) and in southern areas (Prestvik, 1980; Thorsnes and Løseth, 1991; Heldal, 2001). The thick and laterally extensive marble units, as well as the iron formations, are special features of the Uppermost Allochthon and are described separately in the following.
Rocks considered to represent a crystalline basement to many of these successions occur in a few thrust sheets, notably in the Helgeland and Rödingsfjället Nappe Complexes. These take the form of various orthogneisses and some paragneisses, locally migmatized. It has been suggested that these units represent an original “basement-cover” situation (Riis and Ramberg, 1979), although no definite traces of unconformable contacts have yet been found (Heldal, 2001). Another form of “basement-cover” relationship is that of the generally low- to medium-grade, sedimentary successions that unconformably overlie deformed and eroded, fragmented ophiolites in the Lyngsfjell and Bjerkvik Nappes (Binns and Matthews, 1981; Boyd, 1983; Tull et al., 1985; Andresen and Steltenpohl, 1994; Zwaan et al., 1998), and the Helgeland Nappe Complex (Sturt et al., 1985; Thorsnes and Løseth, 1991; Heldal, 2001). Basal conglomerates to the successions are dominated by mafic clasts that are clearly derived from an ophiolite pseudostratigraphy (Fig. 2G). Conglomerates above this level are thoroughly polymict, including abundant clasts of psammites, schists, and some marbles from the older, Late Riphean to Cambrian cover sequences. Both the ophiolites and their sedimentary cover are discussed in a following section. A separate section is also devoted to the important plutonic component of the Uppermost Allochthon.
Although variably metamorphosed carbonate formations occur within most of the major nappe complexes in the Scandinavian Caledonides, they are particularly abundant in the Uppermost Allochthon. In the Beiarn, Fauske, and Ofotfjorden areas (Fig. 1), the carbonate formations compose larger parts of the supracrustal complexes, whereas in other areas, they occur less commonly, either as laterally extensive, thick units or a series of lenses interbedded with various types of schist. Most of the carbonate formations are composed of calcite. Dolomite marbles are also present, although they are subordinate and occur as either thick coherent beds (i.e., the Fauske area) or thick lensoidal bodies embedded in calcite marble units (i.e., the Ofotfjorden area).
In general, the carbonate formations have been deformed and metamorphosed at medium to high grade and are mostly devoid of fossils. Therefore, until recently, very little was known about their true depositional ages. The only reported fossils, which enable us to date the sedimentation, are Llandovery-age corals, brachiopods, and gastropods (Binns and Matthews, 1981; Bjørlykke and Olaussen, 1981), which occur in lower-grade meta-limestone successions in the Balsfjorden area (Fig. 1). Recently, several specially targeted, isotope-chemostratigraphic studies have been carried out in an attempt to constrain the depositional ages of these carbonate formations. Within the limitations of isotope chemostratigraphy, particularly for Neo- and Mesoproterozoic time (Melezhik et al., 2001a), Late Ordovician–Early Silurian, Cambrian, and several different age groups of Neoproterozoic carbonate formations (Table 1) have been distinguished with a relatively high level of confidence. The isotope data also suggest that certain stratigraphic correlations and age assignments made in earlier literature are incorrect (see discussion in Melezhik et al., 2002a, 2002b), and also that several thick marble formations do not represent coherent stratigraphic units (Melezhik et al., 2002a). The isotopic data have revealed that in the Ofotfjorden area, the polymetamorphosed and polydeformed carbonate formations, of suggested Neoproterozoic, Cambrian, and Early Silurian age, were tectonically imbricated in nonstrati-graphic order prior to final emplacement onto Baltica during the Scandian orogeny. This complex pattern of thrust-sheet stacking implies that an earlier, pre-Scandian, orogenic episode should be invoked to explain the tectonic juxtaposition of these assemblages, most likely the Middle to Late Ordovician Taconian event (Roberts et al., 2001, 2002; Melezhik et al., 2002a, 2002b).
For the most part, the carbonate formations lack primary depositional features, although in those areas where bedding and related sedimentary structures are preserved, various depositional settings have been reconstructed (Fig. 2E). In the Beiarn and Ofotfjorden area, Neoproterozoic, amphibolite-grade, calcite marbles represent one of the most exceptional examples of retention of original bedding and depositional chemistry (Melezhik et al., 2001b, 2005a). Trace-element changes in these marbles are less profound than those in Phanerozoic carbonates that have undergone only low-temperature diagenesis (e.g., Denison et al., 1994). The high Sr content and high preservational potential are indicative of an aragonitic lime mudstone precursor that could have easily escaped a diagenetic exchange. The Scandinavian Neoproterozoic marbles show close similarities in terms of their trace element and isotope geochemistry to the extensive Neoproterozoic shelf facies consisting of shales and massive limestones reported from East Greenland (Fairchild et al., 2000). Carbonate formations in both regions show evidence for primary arago-nite mineralogy, high Sr contents, negligible Mn concentration, 87Sr/86Sr ratios around 0.70635, and similar carbon isotope values ranging between +4.3‰ and +7.5‰.
The dominant primary structure of the marbles of apparent Neoproterozoic age is rhythmic bedding, where the bedding cycles are of >10 cm thickness prior to tectonic thinning (Melezhik et al., 2001b), which is an important feature of nearly all pelagic carbonate sequences (Scholle et al., 1983). Numerous marble beds exhibit well-preserved graded bedding, and some can be clearly identified as turbidites, suggesting that the carbonate particles were transported by turbidity flows and were deposited within a pelagic system (Melezhik et al., 2001b). Thick units of siliciclastic rocks associated with the Neoproterozoic marbles may also show well-preserved, thin, rhythmic bedding characteristic of distal turbidites (Fig. 2A).
Marble formations of suggested Cambrian age provide evidence of diverse depositional settings. Those in the Ofotfjorden area do not exhibit any primary sedimentary features. However, their massive appearance and form of occurrence resemble buildups of reefal origin (Melezhik et al., 2003). In the Fauske area, a rather rare case of a clastic carbonate formation is locally developed and well exposed in a large quarry just 2 km north of the town. This is a 60-m-thick unit consisting of numerous 5-cm- to 3-m-thick beds, all of which show rapid lateral facies changes. A rapid vertical transition from basal carbonate debris (Fig. 3) through carbonate breccias to a calcarenite-graywacke turbidite lithofacies is accompanied by southeast-directed clast fining. Blocks in the basal facies are up to 6 × 4 m in size and represent fragments detached from the immediate substrate. Channelling and cross-bedding indicate southeast-directed paleocurrents. The depositional model involves a tectonically unstable, carbonate-shelf edge and southeast-directed transport of carbonate clasts, with finer carbonate debris accumulating on a basinal slope deepening to the southeast (Melezhik et al., 2000a) (Fig. 4), i.e., the opposite direction with respect to the Baltoscandian margin of Baltica, thus suggesting a foreign and inferred Laurentian paleogeographical ancestry (Roberts et al., 2001, 2002). Interestingly, the shelf-edge breccia facies (Figs. 3, 4A, and 4B) shows many similarities to less-deformed, Cambrian, bank-edge carbonate breccias in the Appalachians of the northeast United States (Rodgers, 1968) and Canada, e.g., the Cow Head breccia in New-foundland (James and Stevens, 1986).
Carbonate formations of known or apparent Late Ordovician–Early Silurian age vary in lithofacies and metamorphic grade. In several places, these formations contain dolomite-hosted and dolomite collapse breccia–hosted, stratabound, 34S-rich (+20‰ to +31‰), Zn (±Cu-Pb) occurrences (Fig. 2B) (Melezhik et al., 2000b). These have been interpreted as likely Mississippi Valley–type deposits (Bjørlykke and Olaussen, 1981; Grenne et al., 1999). Among diverse lithofacies, a “tripartite unit” has been described (Melezhik et al., 2002a, 2003). This is a succession composed of isotopically unusual (Melezhik et al., 2005b), dark gray (Fig. 2C), variegated (Fig. 2D), and white marbles that is developed discontinuously over a distance of 450 km from the Balsfjorden area (Bjørlykke and Olaussen, 1981) in the north via Ofotfjorden (Steltenpohl et al., 1990) to near Hattfjelldalen in the south (Fig. 1). Near Balsfjorden, the tripartite marble unit is a chemostratigraphic and lithostratigraphic equivalent of the lower-grade meta-limestone successions (Melezhik et al., 2002a). There, the dark gray and white carbonate rocks contain a varied Llandovery-age fauna assemblage (Binns and Matthews, 1981; Bjørlykke and Olaussen, 1981), whereas the strongly 13C-depleted, thinly banded, and, in places, mylonitic, variegated unit is apparently nonfossiliferous. On a global basis, however, variegated carbonate rocks with comparable, extremely low δ13Ccarb values were deposited in the time interval 600–540 Ma (Melezhik et al., 2005b). Thus, although the precise depositional age of the banded, variegated marbles in Nordland and Troms is not known, a Vendian age is currently in favor. Whatever the case, the tripartite unit serves as a prominent chronostratigraphic marker horizon in the north-central Norwegian Caledonides.
In the far north in western Finnmark, a high-grade marble unit in a tectonostratigraphic succession on Sørøya has yielded isotope-chemostratigraphic data indicating a depositional age between 760 and 710 Ma (Slagstad et al., 2006). These carbonate rocks structurally underlie a turbidite formation that contains a tuffaceous psammite and two cross-cutting granite sheets that have yielded almost identical U-Pb zircon ages of ca. 438 Ma (Kirkland et al., 2005). Both the marble unit and the turbidite formation are now considered by Kirkland et al. to represent parts of either high-level Köli Nappes or the Uppermost Allochthon and to have been derived from the Laurentian margin.
Metasedimentary oxide iron ores, in some places manganiferous, are numerous in the Uppermost Allochthon and are widespread over a distance of more than 500 km from the Mosjøen area of Nordland to the Tromsø district (Fig. 1). In the Mo i Rana area, where iron ores are most abundant, calcareous mica schists and marbles of the Dunderland Group are the principal host rocks (Bugge, 1948; Søvegjarto, 1977). Similarly, in other areas farther north, schists and marbles are hosts to the iron ores. Although the host-rock lithology is rather uniform, the ores vary in P and Mn content, which show stratigraphic and geographic controls. In the Mo i Rana ore district, the lower ore horizon is composed of magnetite, apatite, carbonate, and amphibole and contains 0.8–1.0 wt% P (Bugge, 1948; Søvegjarto, 1977), whereas the upper magnetite-hematite-quartz horizon has only 0.15–0.30 wt% P. Both ore types have low Mn contents, which contrast with Fe-spessartine ores from the Ofotfjorden area that contain up to 15 wt% Mn (Foslie, 1949).
The two ore horizons in the Dunderland Group show marked differences in gangue mineralogy, notably with a dominance of amphibole minerals in the lower, P-rich ores. These features and the presence of amphibolitic host rocks in the case of the lower horizon suggested to Grenne et al. (1999) the possibility of a volcanogenic source for these particular magnetite-rich ores. Bugge (1948) had originally concluded that both ore types were of sedimentary origin.
The isotope-chemostratigraphic approach applied to the dating of carbonate sedimentation, and thus the deposition of iron ores, suggests a Neoproterozoic age within the range 650–595 Ma (Melezhik et al., 2002b, 2003). This time interval overlaps with the onset of speculative “snowball Earth” conditions through the Neoproterozoic and the last appearance of banded iron formations on Earth after their long absence since 1800 Ma (Hoffman and Shrag, 2002). Their reappearance was assigned to the melting event of an ice-covered, stagnant ocean and transport of reduced Fe to oxic surface–shelf environments. However, there are several controversies involved with the snowball Earth hypothesis itself, and, consequently, this also applies to the genesis of the Neoproterozoic iron formations (Young, 2002).
In the far north, east of Tromsø, the Lyngen Magmatic Complex dominates the lower parts of the Lyngsfjell Nappe, and it constitutes the largest dismembered ophiolite in the Norwegian Caledonides (Boyd and Minsaas, 1984; Furnes et al., 1985). Gabbros, mafic volcanites, and scattered mafic dikes are the principal components. The complex is divided into western and eastern suites separated by a ductile, oceanic shear zone (Furnes and Pedersen, 1995). In a part of the volcanite section to the east, a coeval tonalite body has yielded a U-Pb zircon age of 469 ± 5 Ma (Oliver and Krogh, 1995). In brief, the geochemical signature of the gabbros of the western suite indicates that the parental magmas were tholeiites of back-arc basin origin. Eastern suite gabbros, on the other hand, are cumulates that are believed to have crystallized from ultradepleted, high-Ca boninitic magmas akin to those in forearcs (Furnes and Pedersen, 1995; Kvassnes et al., 2004). The Lyngen Magmatic Complex can be traced for more than 200 km to the southwest in a series of disconnected, tectonic lenses or thrust sheets of sheared mafic and ultramafic rocks (Boyd, 1983; Tull et al., 1985; Barker, 1986; Andresen and Steltenpohl, 1994).
A major unconformity separates the deformed, uplifted, and eroded Lyngen ophiolite from a fossil-bearing sedimentary succession (locally with a volcanic component) known as the Balsfjord Group, which is of latest Ordovician to Early Silurian age (Bjørlykke and Olaussen, 1981; Binns and Matthews, 1981; Minsaas and Sturt, 1985). This succession, with a basal mafic-pebble conglomerate, dominates the Lyngsfjell Nappe southwest of Lyngen (Minsaas and Sturt, 1985; Zwaan et al., 1998). A carbonate-evaporite tidal sequence that shows diverse sedimentary structures (Fig. 2E) is prominent within parts of the succession. The Lyngsfjell Nappe thins dramatically to the southwest, but it can be followed southward into the Narvik district as thin lens-oid thrust sheets of sheared ophiolite and suprajacent mafic-clast conglomerate (Boyd, 1983; Steltenpohl et al., 1990; Andresen and Steltenpohl, 1994) (Fig. 2G). One such sheared ophiolite fragment, on Hinnøya, contains a tonalite gneiss that has provided a U-Pb zircon age of 479 ± 1 Ma (Northrup, 1997). In the Ofotfjorden district, carbonate rocks reappear in the thrust-dissected Evenes Nappe (or nappe complex), units of which have been correlated with those in the Balsfjord Group by Steltenpohl et al. (1990).
In southern areas of the Uppermost Allochthon, dismembered and fragmented ophiolites are important components in some of the thrust sheets of the Helgeland Nappe Complex (Thorsnes and Løseth, 1991; Nordgulen et al., 1993; Heldal, 2001). The Leka Ophiolite, which includes a spectacular exposure of harzburgite tectonite (Prestvik, 1980; Furnes et al., 1988) and which evolved in a suprasubduction-zone setting, is now considered to lie within the structurally lowermost part of the Helgeland Nappe Complex (Yoshinobu et al., 2002). A minor felsic sheet within the gabbro/sheeted dike unit on Leka has provided a U-Pb zircon age of 497 ± 2 Ma (Dunning and Pedersen, 1988). Fragmented ophiolites that occur farther north on several small islands (e.g., Bolvær complex; Heldal, 2001) are considered to lie at the same structural level as Leka, but there are also lensoid ophiolite remnants at higher levels in the tectonostratigraphy (Thorsnes and Løseth, 1991; Nordgulen et al., 1993). A common feature is that the ophiolite complexes were deformed and metamorphosed and overlain unconformably by mafic breccias and conglomerates at the base of low-grade, continental sedimentary successions of unknown but inferred Ordovician age (Gustavson, 1981, 1988; Sturt et al., 1985; Thorsnes and Løseth, 1991; Heldal, 2001). A minimum age for the thrusts beneath the ophiolite slices and their sedimentary cover has been provided by a U-Pb zircon date of 444 ± 11 Ma for the transecting Heilhornet Pluton (Nordgulen and Schouenborg, 1990). More recent studies in this coastal region have shown, however, that the various nappes and thrust sheets were imbricated during the time interval 477–447 Ma, prior to Late Ordovician exhumation (Yoshinobu et al., 2002). The low-grade cover successions in the Helgeland Nappe Complex, which probably predate 470 Ma (see the following), and their subjacent ophiolitic basements are therefore quite different in age from their counterparts in the Lyngen region in the far north.
A variety of mainly calc-alkaline granitoid batholiths and intrusive complexes constitutes an important component of the Uppermost Allochthon. They were recognized early as a significant feature of the mountain belt, but prior to the 1980s, very few geochronological, geochemical, and structural studies had been undertaken. A major review of the igneous activity in the Scandinavian Caledonides as a whole is that of Stephens et al. (1985b). At that time, however, little information was available on the most prominent granitoid batholiths in Norway. Thus, attempts to reconstruct the igneous history of the mountain belt were based essentially on studies of the tectonostratigraphy and geochemistry of mafic volcanic rocks, whereas the granitoids were largely neglected (e.g., Stephens et al., 1985b).
Granitoids are particularly abundant within the Helgeland Nappe Complex, where the compositionally varied Bindal Batholith (Nordgulen et al., 1993) includes more than 50 individual plutons and composite intrusive complexes (Fig. 5). The batholith spans an age range from 477 to 430 Ma. An older group (477–468 Ma) of peraluminous to strongly peraluminous granites was intruded during a regional migmatization event that affected rocks in the western part of the nappe complex (Yoshinobu et al., 2002). These older plutons have isotopic compositions indicating mainly crustal sources (Birkeland et al., 1993). They include tourmaline granites and the major Vega pluton (Fig. 5), which is composed of heterogeneous anatectic granite.
A younger and texturally varied group of plutons was emplaced from 448 to 430 Ma (Nordgulen and Schouenborg, 1990; Nordgulen et al., 1993; Eide et al., 2002). They range in composition from olivine gabbro to leucogranite; however, tonalitic to granitic metaluminous rocks are clearly the most abundant. Most of the rocks are calc-alkaline, but there are significant occurrences of alkali-calcic gabbro to monzodiorite and syenite (Nordgulen, 1992; Barnes et al., 1992; Barnes and Prestvik, 2000). Detailed studies of some plutons show that emplacement was assisted by stoping and ductile flow in the wall-rock aureole (Barnes and Prestvik, 2000; Dumond et al., 2005). In some areas, hot, mafic plutons interacted with their pelitic and calcareous host rocks causing assimilation, mixing and mingling, and generation of hybridized rocks (Barnes et al., 2003, 2005; Dumond et al., 2005). Calc-alkaline plutons with U-Pb ages of 456 ± 2 and 458 ± 3 Ma have also been reported from the highest nappe of the Upper Allochthon in the Gjersvik area, directly beneath the Helgeland Nappe Complex (Roberts and Tucker, 1991; Meyer et al., 2003).
About 250 km farther southwest, the Smøla-Hitra Batholith (Gautneb and Roberts, 1989; Nordgulen et al., 1995, 2001) occupies large parts of the islands of Smøla, Hitra, and Frøya, and a narrow coastal zone on the adjacent mainland (Tucker et al., 2004), and it represents an intrusive complex that is probably correlative with that at Bindal. Apart from the crustally derived rocks of the Helgeland Nappe Complex, the chemistry and isotope composition of both batholiths (i.e., Bindal and Smøla-Hitra) are consistent with derivation from mixed crustal and mantle sources (Nordgulen and Sundvoll, 1992; Birkeland et al., 1993; Barnes et al., 2002). A common feature of the batholiths is their high Sr content (and Ba); many plutons may have Sr contents as high as 1000–1800 ppm (Nordgulen and Sundvoll, 1992; Barnes et al., 1992).
Granitoids also occur in the Rödingsfjället Nappe Complex, particularly in southern areas, although mainly as dike swarms that cross-cut the foliation in host rocks, which consist mainly of metasedimentary units interlayered with minor metavolcanic rocks (Ramberg, 1967; Bjerkgård et al., 1997). The Sr-isotope composition of marbles suggests Neoproterozoic ages (600–590 Ma) comparable to those of the Helgeland Nappe Complex (Bjerkgård et al., 1997). A Late Ordovician age for the dikes is suggested by a Rb-Sr whole-rock isochron (447 ± 7 Ma), i.e., similar to granites in the underlying Krutfjellet Nappe of the Upper Allochthon (Stephens et al., 1993). Little is known in terms of composition, but the generally evolved nature of the rocks suggests a mixed source for the magmas.
Large granitoid intrusions are abundant in the Beiarn Nappe (Fig. 1) between Mo i Rana and Bodø (e.g., Gjelle, 1980; Brattli and Tørudbakken, 1987; Solli, 1990). Porphyritic and equi-granular granites and granodiorites are common in addition to subordinate gabbro, diorite, and tonalite. The plutons intrude medium-grade, foliated metasedimentary rocks of assumed Proterozoic to Cambrian age (Tørudbakken and Brattli, 1985). Age determinations on granitoids include Rb-Sr whole-rock dates of 495 ± 14 Ma for a quartz-monzonitic gneiss at Harefjell (Cribb, 1981), and 440 ± 30 Ma for the Høgtind Granite (Tørudbakken and Brattli, 1985). The only U-Pb dates so far reported are from a granite on an island ∼5 km south of Bodø. These are on zircon and monazite and fall in the range 432–429 Ma (Bingen et al., 2002). Although compositional data and other precise U-Pb dates are lacking, the rocks appear to be analogous, in many ways, to the Bindal Batholith, and they probably were formed in a similar tectonic environment.
In the northernmost part of the Uppermost Allochthon, granitoids occur in the Ofotfjorden area within the stack of structural units (Gustavson, 1969; Steltenpohl et al., 2003), each of which contains early plutons that are texturally and chemically specific to each thrust sheet. The composition of these particular granitoids is relatively complex; most are S- and I-type, whereas early phase Snaufjell granitoids have A-type characteristics (Steltenpohl et al., 2003). The youngest granitoids have ages of ca. 430 Ma and cut nappe boundaries. This suggests that they are genetically related to processes that occurred during nappe stacking, which, according to Steltenpohl et al. (2003), was a product of the Scandian orogeny. The early granitoids, including the Snaufjell granite, remain undated, however, but Steltenpohl et al. (2003) suggested that they might record igneous activity related to the Taconian event.
In the far north, variably peraluminous to metaluminous granitic plutons, sheets, and dikes in the allochthons on Sørøya and Magerøya have yielded U-Pb zircon ages in the range 438–434 Ma (Kirkland et al., 2005; Corfu et al., 2006). These authors presented arguments favoring granite emplacement into successions draping the margin of Laurentia. The host rocks and intrusions were subsequently incorporated into either the Upper (Köli) or the Uppermost Allochthon during Scandian orogenesis.
The tectonic setting and geodynamic significance of the plutonic rocks that occur throughout the Uppermost Allochthon, and in some of the very highest Köli thrust sheets, have been considered on the basis of petrological and tectonostratigraphic evidence. The Early to Middle Ordovician peraluminous rocks of the Bindal Batholith originated during collision of microcontinental fragments and, possibly, concomitant ophiolite obduction in a peri-Laurentian setting (Yoshinobu et al., 2002). These conclusions are in general agreement with those derived from studies of ophiolites and arc sequences within the Upper Allochthon in western Norway (e.g., Pedersen and Dunning, 1997) and the Canadian Appalachians (e.g., Whitehead et al., 2000; Waldron and van Staal, 2001). The pattern of continued plutonism through the Ordovician and into the Silurian is also represented in various segments of the Appalachian belt (e.g., Kerr, 1997; Drummond et al., 1997; Whalen et al., 1997; Aleinikoff et al., 2002, Hollocher et al., 2002). The variable composition of the granitoids reflects derivation from multiple crustal and mantle sources, and it is, in general, related to a complex evolution of arc terranes, and eventual collisions between arcs, ribbon micro-continents, and the Laurentian margin during the final stages of the Taconian orogeny.
Until the last few years, comparatively few precise geochronological data existed on the rock complexes of the Uppermost Allochthon. Together with a meager faunal record, restricted to one area in Troms, this hampered most attempts to interpret the likely Caledonian evolution of this major, composite thrust complex. Most workers considered the principal deformation and metamorphism, including thrust emplacement, to relate to the Scandian event (e.g., Nicholson, 1974; Gustavson, 1978). The discovery of fossils in a succession correlative with the Balsfjord Group (Olaussen, 1976; Bjørlykke and Olaussen, 1981), and the realization that clasts in a basal conglomerate above a major unconformity derived from the Lyngen ophiolite, led several workers to speculate about evidence for an earlier, pre-Scandian event (Minsaas and Sturt, 1985; Steltenpohl et al., 1990; Sturt and Roberts, 1991; Andresen and Steltenpohl, 1994). At that time, this early Caledonian event was considered to be Finnmarkian (Late Cambrian–Early Ordovician), based mainly on work in northern Norway but also on the belief that almost all Norwegian ophiolites were of Cambrian to earliest Ordovician age (Roberts and Gale, 1978; Furnes et al., 1985). Subsequently, publication of the minimum age (469 Ma) for the Lyngen ophiolite (Oliver and Krogh, 1995) opened up the probability that a younger Ordovician event was likely to be represented in the thrust sheets of this northern part of the Uppermost Allochthon. The main thrusting, however, is Scandian, with a peak metamorphism dated to 432 Ma (Northrup, 1997; Coker-Dewey et al., 2000).
Earlier, Rb-Sr isotopic investigations in areas farther south had, in fact, revealed loosely constrained evidence of Middle Ordovician tectonothermal activity. From the small area of Rödingsfjället Nappe Complex exposed in Sweden, Claesson (1979, p. 356) reported a Rb-Sr isochron age of 447 ± 7 Ma for a granitic dike that cut the regional schistosity and concluded that the penetrative deformation in the nappe “is probably Middle Ordovician in age.” Comparable Rb-Sr studies in the Beiarn Nappe yielded a 440 ± 30 Ma age for a small granite pluton that transects both S2 schistosity and the mylonitic contact between internal thrust sheets (Tørudbakken and Brattli, 1985). These authors also concluded that the earlier D1 deformation in the Beiarn Nappe likely predated the inferred crystallization age of a quartz monzonite dated at 495 ± 14 Ma (Cribb, 1981).
In a study of a Zn-Pb deposit at Bleikvassli (Fig. 1), in a part of the Rödingsfjället Nappe Complex, Skauli et al. (1992) reported a Rb-Sr isochron age of 464 ± 22 Ma for a microcline gneiss, which they interpreted as a metamorphic age; and a nearby syn- to postmetamorphic granite pegmatite yielded a Rb-Sr date of 457 ± 5 Ma. Thus, metamorphism in this particular thrust sheet was believed to be of Middle Ordovician age. Rb-Sr studies in a part of the Helgeland Nappe Complex just west of Bleikvassli (Tørudbakken and Mickelson, 1986) indicated that the main D2 tectonometamorphic event there preceded intrusion of a granite dike dated to 433 ± 11 Ma. A common thread in all these studies, however, was that a metamorphic overprint, and emplacement of the Uppermost Allochthon upon the Köli Nappes, occurred during the main, Scandian orogeny.
From this brief review, it is clear that there were strong indications of a pre-Scandian, inferred Middle Ordovician event in the Uppermost Allochthon well before the advent and application of more precise U-Pb and Ar-Ar dating methods. Dallmeyer and Andresen (1992) presented 40Ar-39Ar data from hornblende concentrates in the Nakkedal and Tromsø Nappes that pointed to tectonothermal activity at some stage during the Ordovician. Later, Selbekk et al. (2000) applied the U-Pb method to titanites from migmatite leucosomes and an associated anorthosite dike in the Nakkedal Nappe and produced ages of 456 ± 3 and 456 ± 4 Ma, respectively, which were interpreted as dating migmatization and dike intrusion. Eclogites in a passive-margin succession in the Tromsø Nappe were investigated by Corfu et al. (2003), who provided an isotope dilution–thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon age of 452.1 ± 1.7 Ma, which was interpreted as dating the high-pressure metamorphic event. This was supported by 451–450 Ma ages of titanite from eclogite and rutile in neosome veins. Within the same nappe, a trondhjemitic orthogneiss yielded a U-Pb zircon age of 493 +5/–2 Ma, reflecting Late Cambrian magmatic activity (Corfu et al., 2003).
By far the most extensive U-Pb isotopic dating study of rocks in the Uppermost Allochthon is that from the southwestern parts of the Helgeland Nappe Complex (Nordgulen and Schouenborg, 1990; Nordgulen et al., 1993; Yoshinobu et al., 2002). The plutons and batholiths from this Velfjorden-Bindal region (Fig. 5) have already been described here. It is therefore sufficient to state that all of the thrust sheets in this coastal district are cut by the 447 Ma Andalshatten granodiorite. Thrust imbrication, moreover, is west-vergent, and superposed extensional structures, relating to exhumation, also predate 447 Ma (Yoshinobu et al., 2002). The oldest pluton affected by west-vergent thrusting is dated at 470 Ma, thus providing a minimum age for the low-grade successions that lie unconformably upon the ophiolites. In some thrust sheets, pulses of regional migmatization occurred during the time intervals 477–468 Ma and 448–445 Ma. Thus, most of Ordovician time between 477 and 445 Ma in this part of the Helgeland Nappe Complex was taken up by crustal growth processes—plutonism, migmatization, and contractional west-vergent deformation—and the emplacement of major plutons even continued up to 430 Ma, into the Early Silurian (Nordgulen et al., 1993). This was followed by further imbrication, metamorphism, and eastward thrusting during the Scandian orogeny. The significance of the earlier west-vergent structures is discussed later.
Structural information supporting the multiphase and polyorogenic evolution of the Uppermost Allochthon has so far come from two main areas: (1) the Fauske district, in the basal part of the Rödingsfjället Nappe Complex, and (2) the Velfjord-Bindal region of the Helgeland Nappe Complex. In the Fauske Nappe, barren calcite marbles and carbonate conglomerates/breccias have been accorded an Early to Middle Cambrian (ca. 520 Ma) depositional age based on C and Sr isotope stratigraphy (Melezhik et al., 2000a). Sedimentological studies have revealed that deposition of the conglomerates occurred at the edge of an unstable carbonate shelf, with finer debris and turbiditic facies accumulating on a basinal slope deepening to the southeast (present-day coordinates) (Fig. 4). The earliest tectonic structures are NW-vergent, ramp-and-flat thrust faults and fault-bend folds (Fig. 6) with an associated S1 schistosity (Roberts et al., 2001, 2002, 2004). These early structures and schistosity are over-printed by SE-verging folds and small-scale thrusts, and a penetrative, NW-dipping, S2 cleavage. The shelf-to-basin geometry and paleocurrent data, coupled with the NW-verging, staircase thrusts and early folds, have been interpreted as recording initial deposition and subsequent Taconian deformation, respectively, along the eastern margin of Laurentia (Roberts et al., 2001). On the contrary, the diverse, later SE-verging structures and fabrics are interpreted as Scandian, relating to nappe emplacement onto Baltica. Although we have, as yet, no geochronology from the Fauske Nappe to confirm the age of the early, tectonic phase, the several indications of a Middle Ordovician tectonothermal event recorded elsewhere in the Rödingsfjället Nappe Complex do lend support to this conclusion.
In the Velfjorden-Bindal region, the west-vergent structures noted previously and described by Yoshinobu et al. (2002) are the earliest recorded tectonic structures in that part of the Helgeland Nappe Complex. Westward vergence is recognized in the entire stack of four, separate, internal thrust sheets, two of which include fragmented ophiolite complexes and their low-grade cover successions. This early deformation, which started at or just before ca. 470 Ma, and Ordovician magmatism have been interpreted by Yoshinobu et al. (2002) to be related to Taconian orogenesis close to the eastern margin of Laurentia. Subsequent thrust emplacement of the entire Helgeland Nappe Complex occurred during the Scandian orogeny, juxtaposing this allochthon against the Rödingsfjället thrust sheets and, in turn, above the Ordovician-Silurian successions of the Köli Nappes.
In the far north, in Troms, direct evidence for west-directed structural polarity at the mylonitic bases of thrust sheets carrying internal proof of Middle Ordovician metamorphism is lacking, but little detailed work has yet been done on these contacts. In the Ofotfjorden district, isotope-chemostratigraphical studies on many barren carbonate formations in the tectonically dissected Evenes and Gratangen Nappes have pointed to several likely depositional ages ranging from late Riphean to Silurian (Melezhik et al., 2002a, 2002b). While much of this imbrication is probably Scandian, it cannot be ruled out that juxtaposition of some Neoproterozoic and Cambrian units, as well as ophiolite obduction, might have occurred during the Taconian event along the eastern margin of Laurentia (Melezhik et al., 2002a). The minimum age of the Lyngen ophiolite, noted previously, also supports such an interpretation.
In this review, we have tried to show how the Uppermost Allochthon differs from the other major allochthons in the Caledonides of Scandinavia, principally in terms of its characteristic lithologies, which have disproportionately high abundances of platformal carbonate successions, granitoid batholithic complexes, and, in places, stratabound banded iron formations. In addition to these features, which are clearly unlike those that make up the thrust sheets that are indigenous to the Baltoscandian margin of Baltica, the Uppermost Allochthon carries a tectonothermal record and early Caledonian thrust polarity that is unique in Norway. The exotic assemblages alone led workers, over twenty years ago, to suggest that the suspect terranes represented by these high-level thrust sheets might have derived from the continental margin of eastern Laurentia or from an unknown microcontinent (Stephens and Gee, 1985, 1989; Roberts et al., 1985; Roberts, 1988). The nature of Caledonian metallogeny also favors such a notion (Grenne et al., 1999).
In more recent years, the revelations of the isotope-chemostratigraphic investigations on carbonate formations, structural studies in selected areas, and increasing geochronological evidence of a complex plutonic record and Ordovician, tectonothermal history support the earlier suggestions for a Laurentian provenance. This holds not only for Neoproterozoic to earliest Silurian sedimentation and volcanism, and the prolific Ordovician granitoid magmatism, but also for the pre-Scandian deformation, metamorphism, and migmatization. It has been suggested, in fact, that a large proportion, if not most, of this pre-Scandian, Ordovician deformation in the Uppermost Allochthon was actually an integral part of the Taconian orogenesis recorded along the Laurentian margin (Roberts et al., 2001, 2002; Yoshinobu et al., 2002; Roberts, 2003). Some of the highest Köli Nappes of the Upper Allochthon also provide evidence of this Ordovician event (Hall and Roberts, 1988; Stephens et al., 1993; Meyer et al., 2003). It cannot be ruled out, however, that a slightly earlier deformation, possibly equivalent to the Tremadoc-Arenig Penobscot arc and obduction event in the northern Appalachians (Colman-Sadd et al., 1992; van Staal et al., 1998), may be represented, e.g., in the Beiarn Nappe. The precise age of obduction of the Leka ophiolite is also unknown, maximum 497 Ma, with a younger constraint of 477 Ma. Whatever the case, subsequent emplacement of these Ordoviciandeformed elements onto the oceanic Köli and subjacent Baltoscandian nappes first occurred during the Scandian orogeny at the time of collision between Baltica and Laurentia.
In the context of the theme of the 2004 International Basement Tectonics Association Conference, i.e., integrating our field and laboratory data toward a better understanding of the causative processes in crustal growth and its ultimate modification, we can examine briefly the overall development of the Uppermost Allochthon from the time of supracrustal accumulation in Neoproterozoic time and point to critical stages of crustal evolution. Continental crustal growth is known to occur by episodic accretion in diverse forms, such as contractional, accretionary orogeny, terrane amalgamation, and the addition of juvenile crust produced in arc systems. In the case of the Uppermost Allochthon of the Scandinavian Caledonides, crustal growth involved two major accretionary events, the earlier of which actually contributed to growth along the Laurentian continental margin. Later, during the Scandian orogeny, some of these very same, single or composite thrust sheets were transported onto the Baltoscandian margin, effectively contributing to crustal growth for a second time in their history.
Developments along the Laurentian margin in Late Neoproterozoic to early Paleozoic time have been described in many publications (e.g., Swett and Smit, 1972; Stanley and Ratcliffe, 1985; Williams, 1995; Smith et al., 2006; Higgins et al., 2004, and references therein). A passive, platformal margin with ensialic shelf basins hosted diverse siliciclastic successions and an extensive carbonate bank, with shelf-edge breccias passing oceanward into an expanding continental rise prism, outboard of which was the oceanic realm of Iapetus. Laurentia was persistently located at low latitudes during this late Riphean to Cambrian time interval, a position conducive to carbonate precipitation along its shelves. On the Baltican continent, on the other hand, which was positioned at high to intermediate southerly latitudes (Torsvik et al., 1996), carbonate formations are not so common, but tillites are well represented in the early to middle Vendian stratigraphic record (e.g., Reading and Walker, 1966; Banks et al., 1971; Edwards, 1984; Kumpulainen and Nystuen, 1985; Nystuen, 1985; Halverson et al., 2005). Initial oceanic contraction in Late Cambrian to Early Ordovician time partly coincided with the first stages of development of magmatic arcs. Sedimentation ultimately ceased in the platformal domain as the thrust sheets containing ophiolites, island-arc complexes, and slope-to-basin facies were successively incorporated, diachronously, and transported from east to west, in the developing Taconian orogenic wedge (Stanley and Ratcliffe, 1985; Williams, 1995). Following this major, crustal accretionary phase that culminated in Middle to early Late Ordovician time, uplift and erosion heralded a period of sedimentation and local volcanism in “successor basins,” in Late Ordovician to Early Silurian time, representing recycling of the accretionary orogen. In certain terranes, however, magmatism continued into Early Silurian time. Several successor basins are known from the Appalachians (Robinson et al., 1998; Bream et al., 2004), and in Norway, in the Uppermost Allochthon, the Balsfjord Group succession, floored by debris from the Lyngen ophiolite, is considered to be one such example (Roberts et al., 2001). Some of the basinal successions overlying ophiolite fragments in the Helgeland Nappe Complex may possibly fall into the same category.
The collision between Baltica and Laurentia in Silurian-Devonian time that produced the Scandian orogeny and formation of the Caledonides in Scandinavia, Britain, and Ireland is known to have been oblique, as Baltica rotated counterclockwise and Laurentia drifted slowly southward (Torsvik et al., 1996; Cocks and Torsvik, 2002). This scissor-like collision inevitably led to a certain variation in the timing of peak metamorphism in the Scandinavian Caledonides, both transversely and laterally. A common thread, however, is one of vast magnitudes of thrust translation, in terms of hundreds of kilometers, particularly in the middle and higher nappe complexes (Gee and Sturt, 1985; Gayer et al., 1987). Subducted Baltican upper crust reached depths of 125 km at around 407 Ma (Terry et al., 2000) in a thrust sheet in part of northern west Norway. At the same time, Devonian sedimentation was proceeding at the surface in small extensional basins (Steel et al., 1985). Such recycling is also inferred to have occurred in an extensive foreland basin, now eroded, far to the east in Sweden (Samuelsson and Middleton, 1998; Larson et al., 1999; Huigen and Andriessen, 2004), although Hendriks and Redfield (2005) have argued for a measure of caution on this issue. During this period of Scandian contractional and accretionary orogeny, and crustal growth in the Baltican realm, the platformal successions, magmatic arc, and fragmented ophiolitic complexes were detached from their Laurentian roots and retransported onto the Baltoscandian margin—as the Uppermost Allochthon.
Although the subsequent late Scandian, extensional deformation, in the shape of many low-angle detachment faults (e.g., Hossack, 1984; Séranne, 1992; Braathen et al., 2000; Osmundsen et al., 2003, 2006), is strictly outside the topic of this contribution, the coeval link to ductile, sinistral shear along major vertical faults is an interesting facet of this story that requires elaboration from the point of view of terrane restoration. This Silurian-Devonian, left-lateral megashear has been described from many areas in East Greenland, Svalbard, Norway, the British Isles, and Ireland, and also from the northern Appalachians of New England and Newfoundland (Harland, 1971; Murphy, 1985; Grønlie and Roberts, 1989; Larsen and Bengaard, 1991; Soper et al., 1992; Strachan et al., 1992; Dewey and Strachan, 2003). East Greenland has no record of Taconian orogeny, and in its northernmost areas, carbonate sedimentation in fact persisted into Llandovery time (Higgins et al., 2004), just prior to Scandian deformation. The only record of Middle Ordovician arc magmatism is from an area in southern East Greenland (Hansen and Tembusch, 1978; Henriksen and Higgins, 1988). Thus, the present positions of Norway and Greenland in terms of Caledonide geology, arising from the opening of the North Atlantic, are somewhat anomalous. However, if we restore Baltica and the Scandinavian Caledonides southwestward to their likely position prior to the major, sinistral megashear, this would place Norway and the Uppermost Allochthon much closer to the northern Appalachians, near ter-ranes that have similar magmatic and tectonothermal histories. The shelf-edge carbonate breccias of the Fauske Nappe would then have immediately recognizable counterparts in Newfoundland (Cow Head), Quebec, Vermont, and Maryland.
The magmatic arc granitoids described here have correlatives in the northern Appalachians that compare well in terms of both time of intrusion and geochemistry. In New England, for example, isotope geochronology has shown that arc-related plutons were emplaced during the time interval 480–442 Ma (Robinson et al., 1998). In detail, two linear arcs have been proposed, an earlier one (Shelburne Falls arc) above an ocean-facing subduction zone at 485–470 Ma, and a second, calc-alkaline arc (Bronson Hill arc) in late to post-Taconian time, 454–442 Ma (Karabinos et al., 1998). Clearly, this general situation has much in common with that recorded in the Helgeland Nappe Complex in Norway.
In East Greenland, Ordovician granodiorites and diorites occur only in the extreme south, in the Scoresby Sund region (Hansen and Tembusch, 1978; Higgins et al., 2004). It has been suggested that this occurrence may represent the northernmost tip of a Taconian arc (Roberts et al., 2004). The plutons are of I-type, calc-alkaline character and have been interpreted as indicative of landward subduction (Higgins et al., 2004).
The highest nappe complexes in the Caledonides of Scandinavia, collectively termed the Uppermost Allochthon, are distinguished by their characteristic lithological assemblages and magmatic units that, in many ways, set them apart from the subjacent, Baltoscandian, platformal and continental-rise successions. Sandwiched between the two are the Iapetan, oceanic ter-ranes of the Köli Nappes, which have Early Ordovician, North American faunas in one of the highest thrust sheets and sparse Baltican faunas at the lowest levels.
Principal features of the Uppermost Allochthon include an extensive outcrop of carbonate rocks (marbles, banded meta-limestones, carbonate conglomerates, and local breccias) that can be followed for >600 km along strike, and the volumetrically significant granitoid plutons and batholiths that dominate the geology of some of the larger nappe complexes. Less prominent yet equally significant are the metamorphosed sedimentary iron formations that can also be traced throughout the length of the Uppermost Allochthon. All three of these main elements have no direct counterparts in the subjacent nappes and thrust sheets that derive from the Iapetus oceanic terranes and the Baltoscandian margin of Baltica. On the contrary, many of the features of the Uppermost Allochthon are highly reminiscent of those preserved in the thrust sheets and terranes of the northern Appalachians and East Greenland, derived from the Neoproterozoic to early Paleozoic continental margin of eastern Laurentia, and the magmatic arcs that developed just outboard of that margin. The tectonothermal record and evidence of early, NW- to W-vergent thrusting in the Uppermost Allochthon also point to the preservation of a Taconian history. Thus, taken as a whole, the Uppermost Allochthon appears to represent segments of the Laurentian margin and its conjoined arcs, and it carries a tectonothermal history of Taconian (and possible earlier Caledonian?) accretion onto that margin. Subsequently, during the Silurian-Devonian Scandian orogeny, the thrust sheets that now compose the Uppermost Allochthon were detached from their North American roots and incorporated into the Scandian orogenic wedge.
Crustal growth along the Laurentian margin during the early Paleozoic coincided with Taconian accretion and was augmented during this tectonomagmatic cycle by the emplacement of large volumes of granitoid plutonic rocks. Recycling of the accretionary orogen began in Late Ordovician time with accumulation in successor basins. This basin sedimentation and calc-alkaline magmatism continued into Early Silurian time. Scandian orogenesis followed, with the Taconian-deformed assemblages playing a major part in crustal growth on the Baltoscandian margin of Baltica.
Accepting the evidence for the major sinistral shear that has been widely reported to have accompanied and, in part, postdated the oblique, Scandian, continent-continent collision along the length of the Caledonian-Appalachian orogen, this would imply that the Scandinavian Caledonides lay at some unknown distance to the southwest, relative to their current position opposite Greenland. Similarly, the Appalachians would have been farther northeast, in relative terms, prior to the onset of left-lateral megashearing. It is therefore quite possible, given the similarities of their lithostratigraphic, magmatic, and tectonothermal histories, that the supracrustal rocks that now constitute the thrust sheets of the Uppermost Allochthon and the terranes of the northern Appalachians were much closer together before the final Scandian collision, nappe accretion, and initiation of major, sinistral, crustal movements along deep-seated, vertical faults.
In completing this summary of our knowledge of the highest allochthon in the Scandinavian Caledonides, we are especially indebted to the Geological Survey of Norway for supporting our diverse research and mapping projects over the last two decades. Contributions from several research teams from American, British, and Norwegian universities have also provided significant advances in our understanding of these thrust complexes. The senior author is particularly grateful to Robert Hatcher Jr. for providing the financial support that enabled him to present this summary and overview at the International Basement Tectonics Association Conference in Oak Ridge. We are also grateful to our colleagues Peter Robinson and Arne Solli for their comments and suggestions on an early version of the manuscript. The final manuscript (formally accepted by the volume editors on 16 November 2005) has been improved following the constructive reviews of Robert Hatcher Jr. and Mark Steltenpohl. Translation of the Queen's English to the American version has been expertly performed by one of the Geological Society of America's desk editors.
Figures & Tables
4-D Framework of Continental Crust
- absolute age
- Caledonian Orogeny
- carbonate rocks
- continental crust
- island arcs
- metamorphic rocks
- metasedimentary rocks
- Narvik Norway
- Nordland Norway
- Scandian Orogeny
- sedimentary basins
- sedimentary rocks
- shear zones
- Taconic Orogeny
- thrust faults
- Western Europe
- zircon group
- Helgeland Complex
- Fauske Nappe
- Tromso Nappe
- Rodingsfjallet Complex
- Niingen Nappe
- Gratangen Nappe
- Evenes Nappe
- Bejarn Nappe
- Nakkedal Nappe
- Lyngsfjell Nappe
- Mosjoen Norway