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Geological Society of America Special Paper 230

1989

Terranes and polyphase accretionary history in the Scandinavian Caledonides

Michael B. Stephens Geological Survey of Sweden, Box 670, S-751 28 Uppsala, Sweden David G. Gee Department of Geology, University of Lund, Solvegatan 13, S-223 62 Lund, Sweden

ABSTRACT

Thrust-emplaced tectonic units dominate the structure of the Scandinavian Caledonides. The lower units made up the Baltoscandian margin of the early Paleozoic continent Baltica. The thrust sheets that probably formed the outer part of this margin are treated here as suspect terranes. Three such terranes (1 through 3) are distinguished. The higher tectonic units, occurring as far-transported (>400 km) thrust sheets in the Upper and Uppermost Allochthons, are composed of exotic terranes derived from out-board of Baltica. Terranes containing, at least in part, oceanic sequences (4 through 9) have been distinguished from those comprising metamorphic complexes of less-certain affinity (10 through 12); a terrane (13) composed predominantly of exotic continental lithosphere forms the highest unit in the tectonostratigraphy.

A complex polyphase accretionary history is apparent for the outboard terranes. Late Cambrian to Early Ordovician collision of the Baltoscandian margin with an outboard arc complex resulted in the Finnmarkian orogenic activity. Erosion and, there-by, loss of outboard units emplaced by early Caledonian thrusting on the outer margin of Baltica is thought to have played a major role during later uplift. Middle Ordovician clastic sequences in one of the outboard terranes (4) and in a foreland basin are related to the uplift and erosion associated with the Finnmarkian event.

Several of the outboard terranes (5, 6, and 11) are inferred to have accreted to Laurentia (terrane 13?) during Early Ordovician continent-arc collision. This Laurentian accretionary episode resulted in the early Caledonian orogenic activity in these terranes and preceded rifting, opening of one or more marginal basins, and closure of Iapetus; proximity to Baltica was achieved only during Late Ordovician time. The early Caledo-nian tectonothermal activity recorded in several of the outboard terranes was, therefore, unrelated in space and possibly also in time to the Finnmarkian episode along the Baltoscandian margin. Subsequent continent-continent collision during the Silurian and Devonian resulted in the Scandian orogenic activity, both along the Baltoscandian margin and in the outboard terranes, and birth of the megacontinent Laurasia.

INTRODUCTION

In recent years, the terrane concept has been successfully applied to the Circum-Pacific orogenic belts (Coney and others, 1980) and has provided a considerable stimulus to tectonic inter-pretation elsewhere. In the Scandinavian Caledonides, the con-cept was born with the application of nappe theory nearly 100 years ago (Tornebohm, 1888). Distinct, low- to medium-grade, lower Paleozoic, volcano-sedimentary associations (Koli Nappes),

together with underlying, generally higher grade complexes (Seve Nappes), were shown to override the very low grade Baltoscan-dian platformal successions by distances of at least 100 km. A more radical hypothesis (Hogbom, 1910; Holtedahl, 1936; Asklund, 1938; Kautsky, 1953) claimed that these higher alloch-thonous terranes were derived from west of the present Scan-dinavian land area. Although widely contested, this hypothesis

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18 Stephens and Gee

was established beyond reasonable doubt during the 1970s, im-plying that the Baltoscandian margin (platform and miogeocline) of the early Paleozoic continent Baltica extended to at least some 400 km west of the present thrust front of the orogen.

The several independent lines of evidence available in the early 1970s favoring vast thrust displacements (Gee, 1975) have now been complemented by a variety of critical studies of the tectonic evolution. Much of this information was presented re-cently in Gee and Sturt (eds., 1985) and the accompanying 1:2,000,000-scale tectonostratigraphic map of the orogen (Gee and others, 1985a). The latter provides the basis for a regional perspective, often so illusive in national presentations. Such a perspective allows thrust sheets derived from the Baltoscandian margin of continent Baltica to be distinguished from overlying units of suspect (probable outer margin) and exotic (outboard) character (Stephens and Gee, 1985; Fig. 1). The latter, occurring in the Koli Nappes of the Upper Allochthon and in the Upper-most Allochthon, contain relicts of ocean-floor and arc-basin systems; the highest units are dominated by ensialic material showing an Ordovician and Silurian active continental margin evolution. The Koli Nappes and higher tectonic units were in-ferred to have originated from within an ocean (Iapetus of Har-land and Gayer, 1972) and probably its western (Laurentian) margin. Having obtained a basis for separating the indigenous continental margin associations from the exotic terranes, future work must concentrate on a closer analysis of the different out-board terranes along the orogen and their accretionary history both to each other and the Baltoscandian margin.

Terrane analysis provides essential constraints on plate-tectonic models for the orogen, especially as far as the Late Cam-brian to Early Devonian convergent-plate history is concerned. Interpretation of this evolution has increased in complexity, from earlier ideas that solely involved B-subduction followed by continent-continent collision (see, for example, Gale and Roberts, 1974; Gee, 1975), to those involving B-subduction, early Caledo-nian continent-arc collisions, B-subduction flip, marginal basin development, and ultimately, continent-continent collision (Brekke and others, 1984; Stephens and Gee, 1985; Dallmeyer and Gee, 1986; Stephens, 1988). Nevertheless, there remains today a notable lack of consensus over the interpretation of the tectonic evolution of the orogen. Some authors (Gale and Rob-erts, 1974; Fumes and others, 1976; Brekke and others, 1984; Roberts and others, 1985) claim that all the ocean-derived ter-ranes were incorporated along the Baltoscandian margin during Late Cambrian-Early Ordovician orogeny (Finnmarkian); some of the ophiolites were obducted during this orogeny, and others were "rooted" (Sturt, 1984) in back-arc basins shortly thereafter. We present evidence for an alternative interpretation, that most of these outboard terranes were not proximal to the Baltoscan-dian margin until the Late Ordovician; their accretion to Baltica occurred during Silurian and Early Devonian time, i.e., by Scan-dian collision (Gee, 1975; Bruton and Bockelie, 1980; Stephens and Gee, 1985; Stephens, 1988). Rigorous terrane analysis should be able to resolve these different interpretations.

This chapter first presents the Baltoscandian margin of con-tinent Baltica as it is preserved in the Autochthon and lower thrust sheets (Parautochthon, and Lower and Middle Alloch-thons). This is followed by accounts of the tectonostratigraphic status, principal characteristics, and constraints on accretionary history of those tectonic units, which are suspected to have formed the outer part of the Baltoscandian margin (terranes 1 through 3). Terranes that lay outboard of this margin and are thought to be exotic in character are treated thereafter (terranes 4 through 13). The paper finishes with a discussion of the poly-phase accretionary history of the orogen and a model for its development. The analysis is presented with greater confidence for the central part of the orogen (latitudes 61°40'N to 67°50'N), since it is within this region that our own detailed work has been concentrated. The segmentation of the mountain belt by major transverse culminations, exposing lower thrust sheets, introduces uncertainty into the correlation of overlying units along the oro-genic belt. Future work may well allow correlation of terranes in southwestern Norway and north of latitude 67°50'N with those in the central part of the orogen. Breaking several of the terranes described here into smaller units is also likely. For these reasons, we have avoided using formal names for terranes except in those areas where a more rigorous analysis has already been presented. It has often proved convenient to refer to the relative location of tectonic units, etc., in terms of their present geographic orienta-tion (e.g., the platformal sequence extends westward into the miogeocline). This practice should not influence interpretation of their latest Proterozoic and Paleozoic paleogeographic relationships.

In this paper, we refer to a variety of isotopic age-determination data and their relationship to the latest Proterozoic and Paleozoic timescale (Snelling, 1985). All isotopic ages have been recalculated on the basis of the decay constants recom-mended by Steiger and Jäger (1977).

BALTOSCANDIAN MARGIN OF CONTINENT BALTICA

The Baltoscandian margin of continent Baltica occurs within the Autochthon, Parautochthon, and Lower Allochthon and has been recognized with confidence also in the Middle Allochthon throughout most of the mountain belt (Fig. 1). In northernmost areas, the status of the highest unit (Söröy Nappe) in the Kalak Nappe Complex, referred to the Middle Allochthon in Gee and others (1985a), is less secure; it is therefore treated here as suspect.

In the Autochthon, thin Vendian to Cambrian sandstones and overlying black shales rest on the Precambrian basement of the Baltoscandian platform. The basement rocks are predomi-nantly Early and Middle Proterozoic in age. However, an Ar-chean cratonic core passes under the Caledonian cover in northernmost Norway, and the Proterozoic basement is intensely reworked by the Sveconorwegian (Grenvillian) orogeny south of latitude 61°N in southern Norway. As recorded in the Parautoch-



Terranes and polyphase accretionary history 19

Oslo Palaeor i f t (Permian)

Old Red Sands tone bas ins (S i l u r i an -Devon ian )

15° E

EXOTIC TERRANES DERIVED FROM OUTBOARD OF BALTICA

Cont inenta l l i thosphere der ived f rom Laurent ia? (UmA)

Metamorphic complexes (UA) 10-12

Oceanic sequences (UA.UmA)

SUSPECT TERRANES PROBABLY COMPRISING THE OUTER TECTONICALLY SHORTENED MARGIN OF

•:<3:-j S0r0y Nappe (MA or UA)

Seve Nappes (UA)

Barents Sea and Lokv i k f j e l l G roups (A,P)

•\2\

MX

TECTONICALLY SHORTENED MARGIN OF BALTICA

pyyyi P lat formal and mioaeocl ina l sed iments , and ' • " • I P recambr ian c rys ta l l ine rocks (P,LA,MA)

"1 Precambr ian c rys ta l l ine rocks (Por LA)

CRATONIC BALTICA

f v T ] P la t fo rmal sed iments (A)

[ ] Precambr ian crysta l l ine rocks (A)

20° E

60° N 60° N"—

Thrus t f au l t

Dext ra l s t r i ke - s l i p f a u l t ^

H i g h - a n g l e faul t

Pr imary con tac t

68° N

66° N

6 4 N

62° N —

Figure 1. Terrane map of the Scandinavian Caledonides. 1 through 13 identify the individual terranes. A, P, LA, MA, UA, and UmA refer to different tectonostratigraphic units, the Autochthon, Parautoch-thon, and the Lower, Middle, Upper and Uppermost Allochthons, respectively.



2 0 Stephens and Gee

thon, and Lower and Middle Allochthons, the platformal se-quence extends westward into the miogeocline where the Vendían to Cambrian successions thicken and are underlain by Vendian tillites as well as Late Proterozoic dolomites and fluvial to shallow-marine sandstones, several kilometers thick (Kumpu-lainen and Nystuen, 1985). Extensive sheets of Middle Protero-zoic crystalline rocks, locally dominating the Middle Allochthon, suggest that promontories of elevated basement may have charac-terized the Baltoscandian passive margin. Intrusion of a tholeiitic, rift-related dyke swarm (Solyom and others, 1979a) into sand-stones and tillites in the outer part of the miogeocline occurred in the Vendian (Claesson and Roddick, 1983). Disseminated Pb-Zn sulfide deposits (Bjdrlykke and Sangster, 1981) are hosted by the Vendian to Cambrian platformal sandstones in the Autochthon and Lower Allochthon.

In the Lower Allochthon, Ordovician platformal limestones pass westward into a clastic wedge derived from a westerly source. The detritus in these clastic rocks has a composition sim-ilar to the sandstones and crystalline rocks of the Middle Alloch-thon. Radiometric age-determination evidence (Claesson, 1980) favors thrust assembly of the Middle Allochthon in the Early Ordovician; thus, the deposition of a clastic wedge in a foreland basin was probably related to this early Caledonian (Finnmark-ian) orogenic activity along the Baltoscandian margin. Only in the far north, in the Finnmarkian type area, where successions younger than Tremadoc have not been found, is there isotopic age-determination data on shales and slates (Pringle, 1973), sug-gesting that nappe emplacement onto the platform occurred in the Early Ordovician (Sturt and others, 1978). We restrict our use of the term "Finnmarkian" in this paper to this early Caledo-nian tectonothermal activity along the Baltoscandian margin.

In central and southern areas, Late Ordovician regression was followed by Silurian deposition of thin shallow-marine sand-stones and limestones prior to deepening of the basin and passage into black shales and graywackes in the late Llandovery and Wenlock. Regression into nonmarine sandstones in the Wenlock (Bassett and others, 1982; Worsley and others, 1983) occurred along the mountain front apparently in response to the advance of nappes during the Scandian phase of Caledonian orogenesis. An Old Red Sandstone facies was deposited in intermontane basins during the final stages of Scandian nappe emplacement.

Stratigraphic correlation of these successions in the Autoch-thon, Parautochthon, and Lower and Middle Allochthons al-lows palinspastic reconstruction of the Baltoscandian platform and miogeocline as outlined above. This passive margin extended at least 400 km west of the present thrust front during the Cam-brian. Westward, across the strike of the orogen, rocks derived from the platform and miogeocline are exposed beneath the higher nappes; grade and intensity of deformation increases, and in southwestern Norway, the Precambrian crystalline rocks in the windows have been metamorphosed at temperatures of at least 800°C and at depths of over 70 km (Bryhni and others, 1977; Griffin and others, 1985). Radiometric age-determination data on eclogites and host-rock gneisses indicate pervasive Silurian (ca.

425 Ma) high-pressure metamorphism of Middle Proterozoic protoliths (Griffin and Brueckner, 1980). The Scandian deforma-tional and metamorphic evolution leading to partial mobilization of the basement was apparently related to westerly directed sub-duction during underthrusting of Laurentia by Baltica (Gee, 1975).

SUSPECT TERRANES, PROBABLY COMPOSING THE OUTER MARGIN OF BALTICA

A variety of tectonic units that probably were integral parts of the outer margin of the Baltoscandian miogeocline are treated here as suspect terranes, pending further analysis. These include the fault-bounded low-grade rocks in the Autochthon of Varan-gerhalvOya, northernmost Norway, the highly metamorphosed Seve Nappes of the Upper Allochthon, and the Surety Nappe of the Kalak Nappe Complex of Finnmark, northernmost Norway.

Terrane 1. Tectonostratigraphic status: Autochthon and Parautochthon.

In northernmost Norway (Fig. 1), north of the Trollfjord-Komagelv Fault, there occurs a thick sedimentary succession of Late Proterozoic age (Siedlecka and Siedlecki, 1971; F0yn, 1985; Vidal, 1985). A lower unit, the Barents Sea Group, is composed of basal turbidites passing up into shallow-marine sandstones, shales, and subordinate dolomites, together about 8.5 km thick. This succession is unconformably overlain by about 6 km of mostly coarse, shallow-marine sandstones and conglomerates of the LokvikQell Group (Siedlecki and Levell, 1978). Detailed stratigraphic correlation with successions of the Baltoscandian margin has not proved possible, and major (minimum about 500 km) dextral strike-slip displacement has been suggested for the Trollfjord-Komagelv Fault (Kjode and others, 1978). This dis-placement is thought to have occurred in the Vendian or Cam-brian prior to Finnmarkian nappe emplacement (Roberts, 1985). The terrane is therefore treated as an independent unit of the Caledonian Autochthon. It was probably derived from the outer part of the Baltoscandian miogeocline during rifting and opening of the Iapetus Ocean.

Terrane 2. Tectonostratigraphic status: Seve Nappes, Upper Allochthon.

Medium- to high-grade, mostly quartzo-feldspathic schists, gneisses and subordinate marbles, rift-related metabasites (Sol-yom and others, 1979b; Hill, 1980), and occasional solitary gabbros and ultramafites make up the Seve Nappes (Zachrisson, 1973; Williams and Zwart, 1977; Fig. 1). These Seve rocks occur in thrust slices showing contrasting metamorphism under intermediate- and high-pressure conditions (van Roermund and Bakker, 1983; Zachrisson and Stephens, 1984; Andreasson, 1986). Mineral assemblages related to high-pressure metamor-phism have been recognized in metabasites, ultramafites, and host metasediments, the metabasites occurring as corona dolerites, pil-low basalts (E. Zachrisson, oral communication, 1982), and eclo-gites (van Roermund, 1985; Andreasson and others, 1985), locally with glaucophane and crossite (Stephens and van Roer-mund, 1984).




This terrane has been inferred to compose the western out-board edge of the Baltoscandian margin on the basis that the metasediments and rift-related metabasites resemble those in the underlying Sarv Nappes of the Middle Allochthon (Stromberg, 1969; Gee, 1975); further stratigraphic control is lacking, and protoliths of Middle Proterozoic age occur, at least locally (Rey-mer and others, 1980; Claesson, 1982). Uplift, following high-pressure metamorphism, the latter related to oceanward-directed subduction, was in progress during Early Ordovician time (Dall-meyer and others, 1985; Dallmeyer and Gee, 1986). Emplace-ment onto the Baltoscandian platform followed during Scandian thrusting.

Terrane 3. Tectonostratigraphic status: Sdrtfy Nappe, the uppermost unit of the Kalak Nappe Complex, in the Middle Allochthon or basal Upper Allochthon; underlies the Vaddas Nappe.

A variety of thrust sheets in northernmost Norway compose the Kalak Nappe Complex; they have in common a stratigraphy dominated by thick unfossiliferous psammites and are interca-lated with sheets of penetratively deformed Precambrian crystal-line rocks (Ramsay and others, 1985). Locally, the psammites can be shown to have been deposited on the crystalline rocks prior to nappe assembly. Tholeiitic dolerites occur extensively in these tectonic units. Thus, most of the Kalak Nappe Complex compares closely with units of the Middle Allochthon farther south in the orogen. However, the highest thrust sheet in the complex, the Sorely Nappe (Fig. 1), is characterized by wide-spread intrusion, not only of dolerites but also of layered gabbros and ultramafites together with diorites, monzonites, nepheline syenites, and carbonatites (Robins and Gardner, 1975). These early tholeiitic and later alkaline to peralkaline intrusions com-pose the Seiland Igneous Province.

Until recently (Debrenne, 1984), the metasedimentary suc-cession was thought to reach into the Early or Middle Cambrian (Holland and Sturt, 1970), and Finnmarkian deformation, met-amorphism, and nappe assembly were considered to be synchronous with the intrusive activity that was dated by the Rb/Sr whole-rock method to ca. 540 to 490 Ma (Sturt and others, 1978). However, the Rb/Sr data are far from unambigu-ous; ages of ca. 600 Ma (Brueckner, 1973, 1975) may be closer to the time of intrusion. Although most authors (e.g., Roberts and others, 1985, and references therein) have interpreted the magma-tism to be subduction-related and Finnmarkian in age, we prefer the hypothesis that the Seiland Igneous Province intruded the extending Baltoscandian passive margin (Bergstrom and Gee, 1985; Krill and Zwaan, 1988) about 100 m.y. prior to Finnmark-ian tectonothermal activity. However, relationships within the Stfrtfy Nappe are sufficiently complex to favor separate suspect treatment pending the results of new investigations.

EXOTIC TERRANES DERIVED FROM OUTBOARD OF THE BALTOSCANDIAN MARGIN

Whereas terranes 1 to 3 above are suspected to have been related to the Baltoscandian margin prior to onset of Finnmark-

ian tectonothermal activity, several of those described below are inferred, mainly on the basis of faunal evidence, to have existed proximal to Baltica only by the Late Ordovician. The presence in several of the outboard terranes of Late Ordovician faunas of Baltoscandian affinities suggests that Iapetus had essentially closed as a barrier to faunal migration by ca. 450 Ma, prior to Early Silurian collision of Baltica and Laurentia and the begin-ning of the Scandian tectonothermal episode. The use of faunal evidence to define oceanic separations and closures is based on the principles outlined in Cocks and Fortey (1982).

The outboard terranes, occurring in the Upper and Upper-most Allochthons, are divided here into three groups (Fig. 1). Terranes 4 through 9 contain, at least in part, volcano-sedimentary sequences deposited in various oceanic settings, 10 through 12 are metamorphic complexes of less-certain affinity, and terrane 13 is dominated by a fragment of exotic continental lithosphere with an Ordovician and Silurian active continental margin evolution.

The locations of terranes 4 through 13 in relation to Baltica have in general been interpreted on the basis of their position in the tectonostratigraphy. Thus, terrane 4 (Virisen) of the Lower Köli Nappes is thought to have come into proximity with the Baltoscandian margin earlier than terrane 6 (Hölonda) of the Stören Nappe in the Upper Köli. This interpretation finds support in the Early Ordovician faunas in the Holonda terrane. However, where such evidence is lacking, the possibility exists that late imbrication may be superimposed on earlier thrust emplacement of the terranes. This potential complexity may account for some of the problems of terrane correlation, particularly in the northern parts of the mountain belt (terranes 8, 12, and 13) referred to above.

Terranes with oceanic sequences

Terrane 4. Tectonostratigraphic status: Upper Allochthon, including 4a Lower Köli Nappes between latitudes 64°20' and 67°50'N, and 4b upper part of Otta Nappe.

The Lower Köli Nappes in the Swedish Caledonides (4a on Fig. 1) and their probable extension southward through eastern Tröndelag into the Otta area of southwestern Norway (4b on Fig. 1), have a similar lithostratigraphy and locally contain diagnostic fossils. They are thought to represent a single, possibly composite terrane (the Virisen terrane of Stephens and Gee, 1985). The oldest units identified in the type area in the Swedish Caledonides (Fig. 2) comprise Early Ordovician (ca. 490 Ma, Claesson and others, 1983), bimodal volcanic, and high-level intrusive rocks, related to ensimatic rifted-arc development (Stephens and others, 1985a). These are associated with various phyllites and solitary as well as detrital ultramafites. The general monomict character of the latter suggests an isolated origin within or foreign to the volcano-sedimentary association, thus allowing the possibility that this terrane is composite. Faunas of early Llanvirn age, oc-curring in the detrital serpentinites at Otta, are of both North




Terrane 6 (Haiondai i r

Terrane 5 (Gjersvik) i r

Terrane 4 (Virisen)

Limestone, shale, andesite

© Ocean f loor (spreading axis)

( B ) Instabi l i ty and basin infi l l proximal to Laurentia

Calcareous turb ld i tes. basal volcanlc last lc conglomérala, mafic volcanic rocks

Mafic overlain by bimodal volcanic and Intrusive rocks

Malic and le ls lc

««oSï Calcareous turb ld i tes, mafic and occas ional ly te ls ic Intrusions

and Intrusive rocks

Pellte, r ibbon che r t , mafic volcanic rocks, serpent lnl te

© Core of r i f ted arc

© Instabil i ty and basin infi l l

(A ) O f f - a x i s oceanic mater ial , shal lowing upwards

© Distal part of r i f ted arc

© Basin infi l l

Calcareous turb ld i tes

l imestone, peine Q u a r t z - r i c h turbidl tes, conglomerate

Pellte. serpentinite. ( A detr i ta l serpent lnl te J

© O f f - a x i s oceanic mater ial

© D i s t a l part of r i f ted arc

© Basin inf i l l prox imal to Balt ica shal lowing upwards

(D ) Transgression, bas in deepening and infi l l

Figure 2. Stratigraphic sections from outboard terranes 4 through 6 in the central part of the Scan-dinavian Caledonides and their implications for the tectonic evolution during Ordovician, and, locally, Silurian time.

American and Baltoscandian affinities (Bruton and Harper, 1981).

The volcano-sedimentary sequence is overlain by quartz-rich turbidites and conglomerates of Middle Ordovician age. The detritus in these clastic rocks indicates erosion of a continental margin with local influx of material from a mafic volcanic and ultramafic source. They regress upward into shallow-marine quartzites and limestones containing an Ashgill brachiopod and coral fauna (Kulling in Strand and Kulling, 1972) similar to that on the Baltoscandian platform (Bruton, 1986). Resurgence of clastic sedimentation is heralded by the deposition of Early Silu-rian black phyllites associated with subordinate, rift-related, mafic volcanites (Stephens and others, 1985a), which pass upward into calcareous turbidites and conglomerates.

The ensimatic character of the rifted-arc complex implies that the Virisen terrane cannot have been part of the uplifting Baltoscandian margin in the Early Ordovician, as suggested by Sturt (1984). An alternative model has coupled the Early Ordo-vician arc volcanism with subduction of the outer part of the Baltoscandian margin (terrane 2) during continent-arc collision (Dallmeyer and Gee, 1986). Oceanward-directed subduction po-larity, indicated by the margin sequence, has been inferred. Age-dating results (Claesson and others, 1983; Dallmeyer and Gee, 1986) suggest, however, that subduction of the margin started at least 20 m.y. prior to the development of the Virisen rifted-arc complex. Evidence relating this complex to the Baltoscandian margin in the Early Ordovician is therefore lacking, and the subduction polarity of the arc volcanism remains unclear. Based on the character of the post-arc clastic material, proximity to the

Baltoscandian margin was achieved by the Middle Ordovician. Thrust emplacement onto this margin occurred during Scandian orogenesis.

Terrane 5. Tectonostratigraphic status: Upper Allochthon, including 5a Middle Koli Nappes between latitudes 64°20' and 67°N, and 5b Meraker Nappe.

Terrane 5 in its type area (5a on Fig. 1) contains Early Ordovician (ca. 490 Ma, Claesson and others, 1988) bimodal volcanites and intrusions, related to ensimatic rifted-arc develop-ment (Stephens, 1982; Reinsbakken 1986; Fig. 2). These are overlain, in western areas unconformably, by volcaniclastic con-glomerates and calcareous turbidites interlayered with predomi-nantly mafic volcanites and high-level intrusions. It is possible that at least some of these intrusions are Late Ordovician in age (Claesson and others, 1988). The post-arc basinal development and igneous activity have been related to the rifting (Stephens, 1981; Stephens and Gee, 1985). A presumed pre-arc sequence, consisting of fine-grained clastic rocks, cherts, conglomerates, and mafic volcanites, is inferred to have been deposited in an intra-plate, ensimatic setting (Stephens and others, 1985a; Stephens and Gee, 1985). Farther south, in the Meraker Nappe (5b on Fig. 1), a rifted-arc complex was deformed and metamorphosed, probably during the Early Ordovician, and is unconformably overlain by a post-arc sequence also related to a rifting episode (Grenne and Lagerblad, 1985). The latter passes upward into black phyllites of Early Silurian age and a younger, coarse, clastic sequence of calcareous turbidites and conglomerates (Hardenby, 1980). Cu-Zn massive sulfide deposits occur in the pre-arc mafic volcanites and Zn-Cu(-Pb) deposits in both the volcanites of the



Terranes and polyphase accretionary history 2 3

rifted-arc and the clastic rocks of the post-arc sequence (Stephens and others, 1984).

A lithostratigraphic connection between terrane 5 and the metamorphic complex in terrane 11 (see below) has been pro-posed, and a single Early Ordovician volcanic arc/fore-arc basin couple, related to east-directed subduction, has been in-ferred (Stephens and Gee, 1985). For this reason, terranes 5 and 11 were included together in the Gjersvik terrane of Stephens and Gee (1985). The possibility exists, however, that segments of different arc systems may be present in terranes 5 and 11. Pending results of further investigations, separation of these terranes is favored here, and we restrict use of the term "Gjersvik" to terrane 5 alone.

The distribution of major intrusions and relicts of pre-Scandian tectonothermal activity argue for accretion of terranes 5, 6, and 11 some time during the Early Ordovician (Stephens and Gee, 1985; Roberts and others, 1985). On the basis of faunal assemblages in terrane 6 (see below), proximity to the Laurentian margin during Arenig-Llanvirn time has also been inferred (Ste-phens and Gee, 1985). Early Caledonian accretion is also sup-ported by the occurrence of a Middle(?) Ordovician clastic sequence containing calcareous turbidites in all three terranes. An Early Ordovician accretionary model involving easterly (ocean-ward) directed subduction away from Laurentia followed by continent-arc collision has been proposed (Stephens and Gee, 1985). The subsequent tectonic evolution of the composite ter-rane (5, 6, and 11) was dominated by rifting and opening of one or more basins marginal to Laurentia. Evidence for proximity to Baltica only becomes apparent in the Late Ordovician (faunal assemblages in terrane 6, see below) to Early Silurian (lithostra-tigraphy in terrane 5). Thrust emplacement onto terrane 4 occurred during Scandian orogenesis.

Terrane 6. Tectonostratigraphic status: Upper Allochthon, Stören Nappe.

Terrane 6 (Fig. 1), referred to by Stephens and Gee (1985) as the Hölonda terrane, is characterized by a variety of thrust sheets containing ophiolites with Cu-Zn massive sulfide deposits (Grenne and others, 1980). The ophiolites are, at least partly, of pre-late Arenig age. They are overlain, locally unconformably, by Early to Middle Ordovician conglomerates, volcaniclastic and calcareous turbidites, black shales, limestones, and volcanites (see, for example, Roberts and others, 1984; Fig. 2); Late Ordo-vician limestones are the youngest fossiliferous rocks in the suc-cession, which is also intruded by major felsic and mafic igneous bodies. Although much of this sequence is interpreted to have accumulated in a marginal basin (Roberts and others, 1984), attempts have been made to separate ophiolites in the different thrust sheets, both as regards their stratigraphic position (Roberts and others, 1984) and their relation to either a major ocean or more restricted marginal-basin setting (e.g., Sturt and others, 1984). However, both the stratigraphic and geochemical con-straints for this distinction are inadequate (Stephens and Gee, 1985). Post-ophiolite, mafic, intermediate, and felsic volcanites show a calc-alkaline, ensialic-arc affinity. An inferred easterly

subduction polarity (Roberts, 1980), relying heavily on major-element compositions, is in some doubt due to the generally high mobility of several of these elements.

Early Ordovician (Arenig-Llanvirn) limestones in the post-ophiolite sequence contain faunas showing an unequivocal North American affinity (Bruton and Bockelie, 1980). By contrast, fau-nas in Late Ordovician limestones are similar to those on the Baltoscandian platform (Bruton, 1986). A paleogeographic posi-tion along the Laurentian margin during the Early Ordovician has been inferred and has been accommodated in several tectonic models (Gee, 1975; Bruton and Bockelie, 1980; Stephens and Gee, 1985; Stephens, 1988). Proximity to the Baltoscandian mar-gin becomes compelling in the Late Ordovician, and accretion to Baltica occurred during the Silurian. Some authors (e.g., Gale and Roberts, 1974; Sturt, 1984; Roberts and others, 1985), however, have rejected the faunal evidence and claimed that all the ophio-lites in the Holonda and even other terranes were either obducted onto the Baltoscandian margin in the Early Ordovician or were formed in back-arc basins along this margin in the Early to Mid-dle Ordovician. The criteria supporting such an interpretation remain obscure. More details of the accretionary history of ter-rane 6 relative to the tectonostratigraphically underlying terranes 4, 5, and 11 have been provided above (see terrane 5).

Terrane 7. Tectonostratigraphic status: Upper Allochthon, Sunnhordland Nappe Complex.

The Sunnhordland Nappe Complex (terrane 7 on Fig. 1) is characterized by several different ophiolite sheets, at least partly of Early Ordovician age (ca. 490 Ma, Dunning and Pedersen, 1988), unconformably overlain by conglomerates, Late Ordovi-cian quartzites and limestones, and Early Silurian black shales; Middle to Late Ordovician granitoids intrude the various se-quences (Bryhni and Sturt, 1985). In some areas, volcanic complexes with a substantial felsic component separate the ophio-lites from the Late Ordovician and Silurian succession. One such complex (Btfmlo, latitude 59°40'N) comprises Cambrian or Early Ordovician-ensimatic, arc volcanites (535 ± 46 Ma, Furnes and others, 1983) in its basal part. These are overlain unconformably by Middle to Late Ordovician, mafic, intermediate, and felsic volcanites, mostly of subaerial origin; similar volcanites asso-ciated with clastic rocks are present in the Silurian part of the sequence. The Middle Ordovician to Silurian mafic volcanites on Bomlo show geochemical affinities to mafic rocks from ensialic magmatic arcs and continental basalts of the Basin and Range type, related to an extensional tectonic regime (Brekke and oth-ers, 1984).

The ophiolites (and overlying ensimatic-arc volcanites) were obducted onto a continental margin, assumed to be Baltica, prior to the Late or, locally, the Middle Ordovician (Sturt and others, 1984). Biostratigraphic control is limited to the Late Ordovician and Early Silurian successions, and faunal affinities suggest prox-imity to Baltica by the Late Ordovician (Bruton, 1986). Imbrica-tion, together with thrust sheets containing the marginal sequences of the continent Baltica, occurred during Scandian orogenesis.




Terrane 8. Tectonostratigraphic status: Uppermost Alloch-thon, Lyngen Nappe.

A thick layered gabbro makes up the lower part of terrane 8 (Fig. 1); other parts of a presumed fragmented ophiolite (Sturt and others, 1984) are reported from immediately above the basal thrust, which separates rocks derived from this terrane from the higher-grade units of the Nordmannvik Nappe, tentatively in-cluded in terrane 13 (see below). A major unconformity has been described to separate the gabbro from overlying conglomerates, phyllites, limestones, and mafic volcanites (Minsaas and Sturt, 1985). Much of the clastic sequence is apparently fluvial and deposited on the deeply eroded gabbro after emplacement of the latter at a high structural level on a continental margin. The clasts in the conglomerates are dominated by ophiolite debris in the lower parts; higher units contain an increasing amount of material derived from a metamorphosed terrane composed of psammitic and pelitic schists and marbles. A variety of other exotic clasts, particularly unmetamorphosed igneous rocks, testify to the prox-imity of another source that is not represented in the Lyngen Nappe. The overlying phyllites and limestones contain a Late Ordovician and/or Early Silurian coral-brachiopod fauna; this part of the sequence is also associated with rift-related mafic volcanites (Bjarlykke and Olaussen, 1981).

As in terrane 7, biostratigraphic control is limited to the Late Ordovician and/or Early Silurian part of the succession. The affinity of the continental margin, upon which the presumed ophiolite in terrane 8 was emplaced, prior to the Late Ordovician, remains obscure. Likewise there is little control on the timing of thrust emplacement onto the underlying Nordmannvik Nappe. On the basis of faunal affinities, proximity to Baltica is compell-ing by the Late Ordovician and/or Early Silurian (Bassett, 1985; Bruton, 1986).

Terrane 9. Tectonostratigraphic status: Upper Allochthon, Magenay Nappe.

Terrane 9 (Fig. 1) is composed of a coarsening-upward sequence of basinal turbidites, overlain by shelf-facies sediments of Early Silurian age (Henningsmoen, 1961; Andersen, 1981). The latter consist of shales and fine-grained sandstones cut by channel conglomerates as well as lenses of calcareous fine-grained clastics, intraformational conglomerates, and fossiliferous lime-stones. The shelf-facies sediments pass transitionally upward into a second basinal turbidite sequence (Andersen, 1981). Granites and mafic/ultramafic plutonic complexes, the former yielding at one locality a Late Silurian age (ca. 410 Ma), intrude this stratig-raphy (Andersen and others, 1982). Thrust emplacement onto the Baltoscandian margin occurred during Scandian orogenesis.

Metamorphic complexes

Terrane 10. Tectonostratigraphic status: Upper Allochthon, Tannforsen Nappes.

The Tannforsen Nappes form an isolated klippe in the cen-tral part of the orogen (Fig. 1). The structurally lowest units occur

in two separate tectonic lenses above the Seve Nappes (Beckhol-men, 1982). One of these lenses is dominated by mica schist but also contains an association of ultramafites, serpentinite- and jasper-bearing conglomerates, metagabbros, dolerites, and green-stones, which may represent a fragmented ophiolite (Handòl ophiolite of Gee and Sjòstròm, 1984). The other lens contains conglomerates with mafic rock fragments that pass up into turbi-dites and conglomerates of continental derivation, and then quartzites, pelmatozoan limestones, and phyllites. The latter se-quence has been compared (Beckholmen, 1978) with the Middle and Late Ordovician succession in the Virisen terrane (4 above). Calcareous psammites of higher metamorphic grade and with an Early Ordovician (ca. 475 to 510 Ma) tectonothermal history (Dallmeyer and others, 1985) occur in higher thrust sheets, which dominate Tannforsen; these age-determination data deny earlier correlation (Beckholmen, 1982) of these psammites with the post-arc, Ordovician clastic rocks in terrane 5. It is probable that the Tannforsen Nappes comprise several separate terranes, the relationship of which, both to the underlying Seve Nappes and other outboard terranes, remains obscure; accretion of the Handòl ophiolite could have occurred during the Finnmarkian event (Sjòstròm, 1986).

Terrane 11. Tectonostratigraphic status: Upper Allochthon, including 1 la Upper Kòli Nappes between latitudes 65°30'N and 66°20'N l i b Gula Nappe and 11c Stipok allochthon and its equivalents at Sulitjelma (latitude 67°10'N).

The principal component in terrane 11 is a metamorphosed volcano-sedimentary succession intruded by granites, trondhje-mites, and locally, ultramafites, gabbroic to dioritic plutons, and dolerites. Intrusive activity occurred after an early tectonothermal episode had influenced the host rocks and prior to later deforma-tion; on this basis, the intrusions are inferred to be syntectonic in character. Clastic metasediments, including turbidites, dominate the succession; conglomerates with volcanic and plutonic rock fragments, graphitic pelites and cherts, marbles, and predomi-nantly mafic volcanites are also present. Metamorphic grade is variable; in the central part of the orogen (11a and 1 lb on Fig. 1), medium- to high-grade rocks lie structurally beneath low-grade rocks.

The tectonic affinity of terrane 11 is poorly constrained. Based on geochemical data from mafic volcanites and the litho-logical association, the sequence in the Upper Kòli Nappes (11a) is thought to have been deposited in a fore-arc basinal setting (Stephens and Senior, 1981). In the upper part of the Gula Nappe ( l ib) , a clast-bearing facies has been interpreted as a mélange (Home, 1979), developed during obduction of ophiolite in ter-rane 6. Geochemical data from some of the younger mafic intru-sions are consistent with an extensional tectonic regime (Otten, 1983). Scanty fossil control and various radiometric age-dating results indicate that the volcano-sedimentary succession is partly Ordovician and that deformation and metamorphism had com-menced during the Early Ordovician (Gee and others, 1985b). Pre-Scandian accretion has been inferred (Stephens and Gee, 1985).




In the northerly continuation of terrane 11 (1 lc on Fig. 1), a second component formed of continental crustal material is con-spicuous. It comprises quartzo-feldspathic gneisses with subordi-nate marbles and amphibolites, intruded by syntectonic gabbroic and ultramafic plutons, dolerites, and trondhjemitic dykes (Ste-phens, 1986; Sundblad, 1986). This component is structurally interleaved with a volcano-sedimentary sequence also containing syntectonic intrusions and similar to that in terrane 11 farther south (11a and l ib) . At Sulitjelma (Boyle and others, 1985), mafic dykes with occasional screens of metasedimentary rocks (Stephens, 1986) pass stratigraphically upward into pillow lavas, calcareous turbidites with mafic high-level intrusions, black phyl-lites, and limestones with Middle or Late Ordovician fossils. Cu-Zn massive sulfide deposits occur close to, or at the contact between, the volcanites and the overlying clastic rocks. The mafic complex beneath the turbidites has been inferred to be part of an ophiolite (Boyle, 1980; cf. Stephens, 1986).

Terrane 11, in its northerly extension, is composite in char-acter. The two components accreted to each other prior to the "stitching" intrusive activity and deposition of the mafic volcan-ites and calcareous turbidites (Stephens, 1986). Further com-ments on an accretionary model, involving early (pre-Scandian) accretion of terranes 5, 6, and 11 to Laurentia, later rifting and marginal basin development, and final (Scandian) accretion to Baltica, have been provided above (see terrane 5).

Terrane 12. Tectonostratigraphic status: Upper Allochthon, including 12a Vaddas and Kafjord Nappes, and 12b upper part of Rombak Group.

In northern Scandinavia, terranes 2 and 3, both suspected to be related to the outer margin of Baltica, are overthrust by meta-morphic complexes dominated by psammites, pelites, and mar-bles with subordinate amphibolites. These occur in the upper part of the Rombak Group (Gustavson, 1966) north of the Rombak-Sjangli Window, and farther north in the Vaddas and Kafjord Nappes (Zwaan and Roberts, 1978) of eastern Troms and south-ern Finnmark (Fig. 1). These generally low- to medium-grade successions are themselves overlain by thrust sheets of higher metamorphic grade, the Narvik Group in the south and the Nordmannvik (or Dyrfly) Nappe farther north, both tentatively included here in terrane 13 (see below).

Within the Vaddas Nappe, Binns and Gayer (1980) de-scribed Late Ordovician and/or Early Silurian fossils from limestones in association with quartzites, phyllites, conglomer-ates, and pillow basalts near the base of the thrust sheet. Underly-ing psammites were correlated by Ramsay and others, (1985) with units in the S<3r0y Nappe, and a major unconformity was inferred to be present separating an older metasedimentary suc-cession from the fossiliferous units. The unconformity remains to be confirmed (Zwaan in Ramsay and others, 1985); if correct, it implies that terrane 12 was probably accreted to the Baltoscan-dian margin during the Finnmarkian orogenic episode.

The metasediments of the Vaddas Nappe are locally mig-matized and intruded by gabbro. The overlying Kafjord Nappe is dominated by a thick succession of psammitic schists, which were

deformed and metamorphosed prior to Late Ordovician (ca. 450 Ma) intrusion of granite (Dangla and others, 1978). Thus, terrane 12 is a complex unit, part of which underwent early Caledonian deformation and metamorphism. This tectonothermal activity may have been Finnmarkian or it may have occurred outboard of and unrelated to Baltica. Thrusting of the Kafjord onto the Vad-das Nappe occurred during Scandian orogenesis.

Continental lithosphere

Terrane 13. Tectonostratigraphic status: Uppermost Al-lochthon, including 13a Rodingsfjallet and Helgeland Nappe Complexes, 13b Fauske and Beiarn Nappes, 13c Narvik, Salangen and Niingen Groups, and 13d Nordmannvik and Troms0 Nappes.

The type area of terrane 13 is situated in the central part of the mountain belt (13a and 13b on Fig. 1) where it is composed of tectonic units belonging to the Uppermost Allochthon. Tec-tonostratigraphic correlation of these units north of latitude 68°N is highly uncertain (see discussion in Stephens and others, 1985b, p. 151-154), and the status of 13c and 13d (Fig. 1) in the present terrane analysis thus remains an open question. The possibility exists that late imbrication may be superimposed on earlier thrust emplacement of the terranes in northern Scandinavia, thus lead-ing to repetition of units characteristic of lower nappes (e.g., Seve) at higher tectonostratigraphic levels (e.g., Nordmannvik).

Terrane 13, along the whole Norwegian coastal belt, is dom-inated by medium- to high-grade schists and gneisses; thick do-lomite and calcite marbles are conspicuous. These successions are intruded, particularly south of 68°N in 13a and 13b, by syntec-tonic gabbros and granitoids (Gustavson, 1978; Stephens and others, 1985b). In some cases, the latter are of batholithic dimen-sions. Pb-Zn massive sulfide (Stephens and others, 1984) and magnetite-hematite deposits, hosted by metasediments, occur both in the type area and north of latitude 68°N in the Salangen Group (13c on Fig. 1). Tull and others (1985) have described a possible unconformity between the Salangen and Narvik Groups, and correlated the former with the succession of terrane 8, which contains Late Ordovician and/or Early Silurian faunas. In the southern part of the Helgeland Nappe Complex (13a on Fig. 1), a few ultramafic and mafic-ultramafic complexes have been inter-preted as ophiolite fragments (Sturt and others, 1984).

Radiometric age dating of some of the gneisses tentatively suggests a Middle Proterozoic protolith and a resetting event during the Devonian (Cribb, 1981; Brattli and others, 1982). The granitoids reveal a suite of inferred intrusion ages ranging from Early Ordovician (ca. 495 Ma) to Silurian (Priem and others, 1975; Claesson, 1979; Cribb, 1981; Torudbakken and Brattli, 1985; Ttfrudbakken and Mickelson, 1986). Initiation of the Caledonian tectonothermal history during the Cambrian or Early Ordovician in at least some units has been inferred. Eclogites occurring in the Tromse) Nappe yield a K/Ar secondary-amphibole age of ca. 420 Ma (Krogh and others, 1982), but the




age of high P/T metamorphism remains uncertain (Griffin and Brueckner, 1985).

The fragment of continental lithosphere that dominates this terrane displays an affinity to a continental margin that was tec-tonothermally active at least during the Ordovician and Silurian. On the basis of lithologic characteristics, metallogenetic signature, and the complex igneous history, it is exotic with respect to Baltica. The North American faunas in terrane 6 suggest the possibility that this terrane represents a piece of Laurentia (Ste-phens and Gee, 1985). Since the inferred ophiolite fragments were in thrust contact with gneisses, marbles, etc., prior to intru-sion of "stitching" granitoid plutons (T. Thorsnes, oral communi-cation, 1986), the terrane is apparently composite in character. Deformation of the Early Paleozoic plutons and thrusting onto a variety of structurally underlying terranes occurred during Scan-dian orogenesis.

SUMMARY AND DISCUSSION

Within the Scandinavian Caledonides, the Baltoscandian platform and miogeocline make up the critical reference margin to the Early Paleozoic continent Baltica. This margin occurs within the Autochthon, Parautochthon, and the Lower and Mid-dle Allochthons. Precambrian crystalline basement and a thin platformal cover succession compose the Autochthon; thicker platformal to miogeoclinal sequences and Precambrian crystalline rocks compose the tectonically shortened extension of this mar-gin, in parautochthonous and allochthonous units.

Three tectonic units that probably formed the outer part of the Baltoscandian miogeocline are treated here as suspect terranes (1 through 3). Detailed stratigraphic correlation with successions of the Baltoscandian margin has not yet been established in two of these terranes, and a unique igneous province is present in the third. They occur in the Autochthon-Parautochthon of Varanger-halvflya, north of the dextral strike-slip Trollfjord-Komagelv Fault, in the rifted continental margin of the Seve Nappes in the lower part of the Upper Allochthon, and in the Set rely Nappe of Finnmark, northernmost Norway, situated in either the Middle or basal Upper Allochthon. It is in the remaining tectonic units of the Upper and Uppermost Allochthons that truly exotic terranes, derived from outboard of the Baltoscandian margin, have been recognized. Ten such terranes, all bounded by thrusts, have been distinguished here on the basis of their specific stratigraphic char-acter; in some cases, parameters such as tectonothermal evolu-tion, igneous characteristics, metallogenetic signature, and faunal provincialism have aided the distinction. Terranes containing, at least in part, oceanic sequences (4 through 9) have been separated from those comprising metamorphic complexes of less certain affinity (10 through 12); a fragment composed predominantly of exotic continental lithosphere occurs as the highest unit in the tectonostratigraphy (13).

A n early collisional event involving Baltica and an outboard arc complex is recorded by the Late Cambrian to Early Ordovi-cian (Finnmarkian) orogenic activity along the Baltoscandian

margin and in the suspect terranes 2 and 3 probably composing its outer edge (Fig. 3). Although some of the outboard terranes (7, 8, 10, and 12) could have been involved in this Finnmarkian accretion, loss of terranes accreted to Baltica at this early stage by subsequent erosion is thought to be of major significance. Middle Ordovician clastic sequences composed of quartz-rich turbidites and conglomerates in one of the outboard terranes (4—Virisen) and in a foreland basin testify to the uplift and erosion associated with the Finnmarkian event (Fig. 3). There is some doubt con-cerning definition of the Finnmarkian orogeny in the type area (S0r<3y Nappe), but recent age-determination studies from other areas suggest that tectonothermal activity related to this event influenced most of the Baltoscandian margin.

Several tectonic models for the mountain belt relate the accretion of all ocean-derived terranes with Baltica to this Finn-markian event (see, for example, Sturt, 1984; Roberts and others, 1985). Such models postulate a belt-length unconformity that links these terranes to the Baltoscandian margin and forms the base of a younger Ordovician and Silurian volcano-sedimentary sequence. Subsequent Silurian to Early Devonian (Scandian) or-ogeny merely sculptured the final tectonostratigraphic assembly in connection with elimination of Iapetus and continent-continent collision. We reject such an accretionary history for three critical reasons. First, stratigraphic and faunal evidence for proximity of outboard terranes to Baltica was not compelling until the Late Ordovician; in only one of these terranes (Virisen) is proximity indicated at an earlier stage, during the Middle Ordovician (Fig. 3). Second, key Early Ordovician faunas above the postulated belt-length unconformity in one of the outboard terranes (6— IMonda) belong to the North American rather than the Balto-scandian biogeographical province, and in another—at Otta in the Virisen terrane—these faunas are, at least in part, foreign to Baltica. Third, Early Ordovician (ca. 490 Ma) ensimatic rifted-arc complexes in terranes 4 (Virisen) and 5 (Gjersvik) must have been situated outboard of Baltica. We maintain that the accre-tionary history in the context of Baltica-Iapetus-Laurentia space is considerably more complicated than these models allow.

Evidence for an early Caledonian tectonothermal evolution in several of the outboard terranes and the composite nature of at least two of them (11 and 13) indicate that outboard terranes were involved in one or more early accretionary events. The distribution of major plutons and clastic sequences containing calcareous turbidites also provides evidence for proximity of out-board terranes to each other prior to the Late Ordovician. Thus, in the central part of the mountain belt, accretion of the different components in terrane 11, much of its internal deformation and metamorphism, and accretion to terranes 5 (Gjersvik) and 6 (Htalonda) are thought to have occurred some time during the Early Ordovician (Fig. 3). For convenience, this is shown on Figure 3 to be more or less contemporaneous. The critical faunal evidence in the Hdlonda terrane indicates that this early accretion occurred marginal to Laurentia, not Baltica (Fig. 3). A possible relict fragment of Laurentia is preserved in terrane 13, which also displays pre-Scandian deformation and metamorphism (Fig. 3).




Figure 3. Time-space diagram for the central part of the Scandinavian Caledonides displaying the tectonic evolution of the margin of Baltica and a model for the accretionary history of outboard terranes 4 through 6, 11 (v = volcano-sedimentary component, c = continental crustal component), and 13. The various orogenic episodes are related to major accretionary events in the mountain belt. More confident estimates of the time of initiation of these episodes are marked by a continuous line. Such estimates are based on radiometric and/or stratigraphic time constraints, key references to which may be found in the text under the respective terrane. Less confident estimates are marked by a discontinuous line. The time-scale is adopted after Snelling (1985). Terrane 4 is inferred to have been proximal to Baltica (light grey tone) by Middle Ordovician time. By contrast, terranes 5, 6, 11, and 13 are inferred to have accreted to each other (medium grey tone) some time during the Early Ordovician; these terranes only became proximal to Baltica by the Late Ordovician. Accretion of all these outboard terranes to Baltica during Silurian time gave rise to the megacontinent Laurasia (dark grey tone).

TIME 3 4 0 - ,

SCALE (Ma) WEST

Carboniferous

EAST

Devonian

Late Silurian

Early Silurian

Late Ordovician

3 6 0 -

3 8 0 -

4 0 0 -

4 2 0 -

4 4 0 -

4 6 0 - Mid Ordovician

4 8 0 -

5 0 0 -

5 2 0 -

5 4 0 -

5 6 0 -

5 8 0 -

6 0 0 -

6 2 0 -

Early Ordovician

Cambrian

Precambrian

-Ordov ic ian and Si lur ian p lu tons

OUTBOARD TERRANES BALTICA LAURENTIA

Black sha le /Sha l low marine, s a n d s t o n e - l i m e s t o n e sequence

1» i With ca lcareous tu rb id i tes

„ „ - r f W i t h c o n t i n e n t - d e r i v e d , q u a r t z - r i c h tu rb id i tes

Ordov ic ian and Early Si lur ian c l a s t i c - d o m i n a t e d sequences

Initiation of this tectonothermal history during the Cambrian or Early Ordovician has been inferred.

Following early-stage accretion to Laurentia, subsequent evolution involved rifting and opening of one or more marginal basins (including that in the Htflonda terrane), active continental margin development along the inferred Laurentian margin (ter-rane 13), and finally, proximity of terranes, 5, 6, 11, and 13 to terrane 4 and Baltica during the Late Ordovician; Scandian accre-tion and birth of the megacontinent Laurasia then followed (Fig.

3). For convenience, this is also shown on Figure 3 to be more or less contemporaneous in the outboard terranes. Constraints on the position of other outboard terranes (7 through 10, and 12) in Baltica-Iapetus-Laurentia space, prior to the Late Ordovician, are lacking. Thus, a complex polyphase accretionary history in-volving both the margins of Laurentia and Baltica is suggested; the early Caledonian deformation and metamorphism in several of the outboard terranes is unrelated to the Finnmarkian event in space and possibly also in time.




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Michael B. Stephens and David G. Gee Terranes and polyphase accretionary history in the Scandinavian Caledonides Geological Society of America Special Papers

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