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The Terrane Concept and the Scandinavian Caledonides: a Synthesis

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The Terrane Concept and the Scandinavian Caledonides: a Synthesis The terrane concept and the Scandinavian Caledonides: a synthesis DAVID ROBERTS Roberts , D. 1988: The terrane concept and the Scandinavian Caledonides: a synthesis. Nor. geol . unders . Bull. 413. 93-99. A revised terrane map is presented for the Scandinavian Caledcnldes. and an outline is given of the principal suspect and exot ic terranes and terrane-complexe s identified outboa rd from the Baltoscand ian miogeocline. The outermost part of the Baltoscandian continental margin is itself suspect , in the terrane sense. since the true palaeogeographical location s of rocks now represented in the Seve and serey-seuano Nappes, while inferred, are not known. The orogen -internal exotic terranes embrace the oceanic/eugeoclinal elements of the Caledonides, represented by the mag­ matosed imentary assemblages of the Koli Nappes, including ophiolite fragments and island arc products. Even more exot ic terranes occur in the highest parts of the tectonostratigraphy, inclu­ ding units which are thought possibly to derive from the Laurentian side of lapetus . D. Roberts. Norges geologiske uruierseketse, Postboks 3006. Lade, N-7002 Trondbeim , Norway . Introduction Project 233 has been to prepare a preliminary Earlier in this decade much of the research terrane map' at 1:5 M scale (Roberts et al. effort in the Caledonides of Scandinavia was 1986) for a larger, circum-Atlantic compilation. channelled through the highly successfu l IGCP This map, much simplified, is really one of Project 27 The Caledonide Orogen ' (Gee & palaeo-environments (marginal basins, vol­ Sturt 1985). An important aspect of the collabo­ canic arc comp lexes, overstep sequences , rative work in this project was that of map etc.), and not of terranes in the true sense. comp ilation, in particular the consensus rea­ Another terrane map, in somewhat simplified ched at establishing an overall tectonostrati­ form appeared in Stephens (1988), while simi­ graphy for the orogen (Gee et al. 1985, Rob­ lar broad-scale subdivisions of the orogen are erts & Gee 1985). Since then, many Caledon­ found in Hossack & Cooper (1985) and Barker ian workers have maintained the international & Gayer (1985). connection through the current IGCP Project The aim of this short contribution is to pre­ 233 "Terranes in the Circum-Atlantic Paleozoic sent a revised terrane map of the Scandina­ Orogens'; and the tectonostra tigraphic sub­ vian Caledonides (Plate 1) and to outline brief­ division of the Caledonides in Norway and ly the principal terranes and terrane-comp lexes Sweden into 4 major allochthons, each compri­ (Fig. 1) which can be identified. It is not sing an assemblage of nappe-complexes and the purpose, at this stage, to present detailed thrust-sheets, has indeed paved the way for descr iptions of each terrane ; these must await an assessment of the orogen in terms of geolo­ further Norweg ian-Swedish collaboration gical terranes . within Project 233. The map compilation and Since terranes are, by definition (see be­ terrane synthesis forms the basis of a presen­ low), character ised by an internal homogene ity tation given at an IGCP Project 233 sympos ium of geology and as their boundaries are of a in Nouakchott, Mauritania (Roberts 1987). tecton ic nature, any terrane map of the Scandi­ navian Caledonides will necessarily reflect the basic patterns of tectonostratigraphy. An ini­ The terrane concept: principles tial attempt at terrane division and map comp i­ and definitions lation, at 1:2'/, M scale, prepared towa rds the Recognition of the existence of geological 'ter­ end of IGCP Project 27 (D.R., manus. map ranes' in orogenic belts hinges on the initial 1983), identified some 15 terranes outboard identification of the relevant passive continen­ of the Baltoscandian miogeocline. As yet, the tal margins or miogeoclines. As the affinity only intra-Nordic collaboration within IGCP of these miogeocl inal sequences is known , 94 David Roberts GU- BULL. 413. 1988 TERRA NE M AP ( TERRA NE COMPLEXES ) ' .. ' . .. o 300km LEG E 0 t:2J PERt"lIA • OSL O RI F 1;0:0:01 OLD RED SA oSTO E TERRA ES OF U CERTAI ~ ORIG I - Laurent i an, oce an i c ~ or unknown aff il iation OROGE - I TER AL TERRA ES - oceanic/ eug eoc li nal affil ia ion OROGE - EXTER AL ELEME TS Outermost suspect terranes SERGE Baltoscandia n m iog eoc l inal sediments Platf or m s edim ents BALTOSCA NDIAN CRYSTALLINE BASEME NT Allochthonousl parautochthonous Mainly autochthonous DR- 87 Fig. 1. Simplified map showing the principal terrane-complexes in the Scandinavian Caledonides. Within eash of the hree main terrane-complex zones there are several, separate, distinctive terr anes. More detailed subdivision is shown in Plate 1. NGU· BULL. 413. 1988 The terra ne conceptand the Scandinavian Caledonides 95 Le. they can be linked directly to, and derive nental margin. These constitute composite from, the nearby craton, then their palaeotec­ terranes and may carry a common sedimenta­ tonic setting is not really questioned. Out­ ry overstep sequence (Keppie 1985). Several board of these continental rise prisms, doma­ separate or composite terranes may form ins which constitute parts of what has general­ what has been termed a terrane-complex (Rob­ ly been termed the eugeocline may be viewed erts 1987). with varying degrees of suspicion or uncertain­ Constraints on the time of accretion of conti­ ty as to their original palaeogeographical set­ guous terranes may be recognised in a varie­ ting with respect to the cratonic margin. They ty of factors (Fig. 2): thus constitute suspect terranes. Orogen­ (a) Strata overstepping two or more adjacent parallel and transverse strike-slip displace­ terranes may carry fossils and thus pro­ ments of many hundreds of kilometres may vide a fairly reliable upper limit on terrane have been involved, prior to or coeval with accretion (Fig. 2a). post-metamorphic thrusting; and post-accre­ (b) The age of thrusting of one terrane upon tionary strike-slip and extensional fault move­ another (Fig. 2b) may be determined isoto­ ments have added to the complexity. pically or, less precisely, by the age of This, then, was the essence of the terrane the youngest fauna in either terrane. concept which evolved as an explanation for (c) Plutons or dyke swarms which straddle the complex Mesozoic and early Cenozoic or 'stitch' two or more terranes (Fig. 2c) accretionary history of the North American may be dated isotopically, thus providing Cordillera (Monger et al. 1972, Coney et al. an upper limit for terrane accretion. 1980). There, rock sequences in large-scale (d) Regional metamorphism which affects two domains, blocks or slices which are now juxta­ or more terranes or terrane-complexes posed may differ dramatically in terms of fau­ (Fig. 2d) may also be dated radiometrical­ nal age and provenance, palaeomagnetic re­ ly. This, coupled with other timing eviden­ cord or tectonothermal history, and bear evi­ ce within the terranes themselves, would dence of different ages of docking or accre­ help in dating accretion. tion. Application of the concept, involving sus­ pect and exotic terranes, spread to the Appala­ chian orogen (Keppie 1981, 1985, Williams & Hatcher 1982, Williams 1984), and helped lay the foundations for IGCP Project 233. The Baltoscandian margin and Terranes, as such, are defined primarily on their stratigraphy but also on a variety of featu­ terranes in the Scandinavian Cale­ res which render them distinctive from their donides neighbours. A modern definition is as follows Since terrane boundaries are generally faults (Keppie, in press): «aterrane is an area charac­ of one form or other, Caledonide tectonostra­ terised by an internal continuity of geology (inc­ tigraphy thus provides a ready scenario for luding stratigraphy, fauna, structure, meta­ attempts at terrane discrimination. In the map morphism, igneous petrology, metallogeny, compilation (Plate 1), a threefold grouping of geophysical properties and paleomagnetic re­ terranes has been made (see also Fig. 1). (1) cord) that is bounded by faults, melanges re­ Orogen-external suspect terranes. (2) Orogen­ presenting a trench complex, or cryptic suture internal exotic terranes. (3) Exotic terranes of zones across which neighbouring terranes (1) uncertain origin. may have a distinct geological record not ex­ Units of Baltoscandian miogeocline affilia­ plicable by facies changes (Le. exotic terra­ tion, in part with slices of Precambrian crystal­ nes), or (2) may have a similar geological re­ line basement, do not rank as terranes in the cord (i.e. proximal terranes) that may only be strict sense, and have been grouped together distinguished by the presence of the terrane as 'orogen-external elements'. Many of these boundary representing telescoped oceanic Baltoscandian elements, as well as the terra­ Iithosphere.» nes proper, carry geographical names (Plate Analysis of the accretionary histories of ter­ 1), some of which derive from their original ranes may show that two or more may have designation for nappes or nappe-complexes. amalgamated at some time before final accre­ These names, as used here, are strictly in­ tion on to the miogeoclinal prism of the conti- formal. 96 David Roberts NGU-8ULL.413,1988 Fig. 2. Diagrammatic sketches showing features or criteria which help to provide upper limits on the time of accretion of juxtaposed terranes. (a) An 'overstep' sequence with basal unconformity blankets two disparate terranes which are separated by a major fault. The type of ornament or fold pattern used in this and subsequent sketches is me­ ant simply to portray the different geological histories in the contiguous terranes. (b) One terrane is thrust upon another. Such a situation could also possibly be taken to represent an original. vertical. strike-slip fault which has subsequently been reactivated and transposed into a flat lying thrust. (c) Two terranes have been juxtaposed along a fault surface, and then transected by a major pluton (or plutonic complex or dyke swarm).
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