J. geol. Soc. , Vol. 137, 1980, pp. 241-250, 3 figs. Printed in Northern Ireland.

Caledonian deformation in

D. Roberts & B. A. Sturt

SUMMARY: Geological research and mapping over the last decade has ledto a reassessment of the Caledonian orogen in Norway. Two principal stages of orogenesis are now established: (1) a late Cambrian to early Ordovician Finnmarkian or Grampian event recognized mainly in and probably also developed in ; and (2) the widespread Middle- Upper Silurian Scandinavian stage of central and southern areas. Another significant finding, aided by radiometric studies, is that of incorporation of Precambrian crystallines as discrete tectonic slices within the Caledonian nappe complexes, reminiscent of the Lewisian-within- Moinerelationship of Scotland. In theextensive gneiss terrain of westerndistricts and of Lofoten the effects of Caledonian tectono-metamorphic reconstitution have been minimal. Biostratigraphicand geochronological indicators provide evidence that diachroneity is a majorfeature common to boththe above evorogenic stages, with deformation younging progressivelytowards thesoutheastern quadrant. In S Norway,folding and local thrusting continued into the Devonian. In terms of geometry and strain, individual nappes are generally extensive, thin, sheet or westward-thinning elements in which protracted high strains in basal zones have produced complex, banded mylonitic and pseudo-psammitic lithologies. Indications of an overall, primary, progressive simple shear deformation are ubiquitous, commonly with evidence of fold rotation into the stretching lineation trend in areasof high ductile strain; this is masked by important episodic incrementsof flattening and extensional strain producing lensoid, mega-boudin structures and leading to excision of some nappe units. This vertical shortening and gravitational spreading is recognized in both the Cambrian and the Silurian stages of the orogenesis and affects calculationsof nappe translation, estimated in hundredsof km for central and southern areas. In conclusion, some comparisons are outlined between the Scandinavian and East Greenland Caledonides; in these two segments of the orogen, nappe displacement is in opposite directions.

During the pasttwo decades manysignificant advances parautochthonous units to far-travelled allochthonous havebeen made in understandingthe Caledonian units, and in the latter case eastward displacements of orogenand its deformation history in . serveral hundred km have been indicated for some of Chronostratigraphical and lithostratigraphical correla- these nappes (Kautsky 1946; Oftedahl 1966; Gale & tions, aided by a flowof radiometric age detennina- Roberts1974; Gee 1975, 1978a; Hossack 1978). tions, are now morereliable andhave allowed a Evidencehas also accumulatedto indicate that the radical reassessment of what constitutes truly Caledo- evolution of this complex nappe sequence was in itself niancover and Precambrian basement material re- theproduct of twomajor Caledonian orogenic spectively. As aresult, vast areaswithin the Nor- episodes, the first or Finnmarkian (Ramsay & Sturt wegian Caledonides where the rocks had previously 1976;Roberts & Gale1978) occurring in late been considered as having undergone their tectono- Cambrian-early Ordovician times, and the second or thermal evolution entirely during the development of Scandinavianoccurring in Middle-Upper Silurian the Caledonianorogen, are now seento bear the time.Both these evorogenic episodes involve poly- imprint of a varied pre-Caledonian orogenic history phasedeformation, igneous intrusion and metamor- and subsequently to have been variably reworked and phic reconstitution, and the deformation history overprinted during the Caledonian cycle. Further, it can beseparated into several fold phases with can be shown that Caledonian tectono-thermal evolu- isoclinal folding usually characterizing the earliest tion was a complex process thatwas both episodic and structures. diachronous, involving successive periods of ocean Duringthe major part of theOrdovician and floor andisland arc creation, destruction of former Silurian, the Finnmarkian nappe complex acted as a ocean basins and -continent collision. basement to this later Lower Palaeozoic coverin some Since the pioneer studyof Tornebohm (1896),Scan- areas, thusfurther complicating the pattern of dinaviahas been a classic regionfor the study of basement/coverrelations in thissegment of the thrust-nappe tectonics, and Caledonian nappe config- orogen. The internal strains within individual nappes urations are now well established throughout this seg- differ markedly from place to place, related in part to ment of the orogenic belt. The nappes have long been the effect of regionalmega-boudinage on theem- understood in terms of relativelyshort-travelled placed nappe pile. As a result, one may pass laterally

0016-7649/80/0500-0241$02.00 @ 1980 The Geological Society

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/3/241/4886242/gsjgs.137.3.0241.pdf by guest on 25 September 2021 242 D. Roberts & B. A. Sturt from relatively low strain regimes to extreme exten- sionalductile strains where mylonitic fabrics are widely developed and which may wipe out the mem- ory of previous deformation history. In this paper we review some of the features which we consider importanttothe understanding of Caledoniandeformation patterns in both space and time within the Norwegian Caledonides. Considerably moredetail on specific areas can befound in the recent volume of regionalsyntheses onthe Caledonian-Appalachian orogen in a Geological Sur- vey of Canada Paper (Gustavson 1978; Roberts 1978; Sturt & Roberts1978; Sturt & Thon1978a). In common with other segments of the orogen it may be noted that theNorwegian sector has also had its share of plate tectonic reconstructions (e.g. Gale & Roberts 1972,1974; Ramsay 1973;Gee 1975; Robins & Gardner1975; Fumes et al. 1976).Space doesnot permit a discussion of this topic here; a synthesis in a wider orogenic perspective was given by Roberts & Gale (1978).

Timing of Caledonian deformation It is now generallyaccepted thatthe Caledonian

orogenicdeformation in Scandinavia occurredin 3 l main phases (Fig. l), the first 2 being accompanied in the internalthe orogenic zone by deep-seated metamorphism: FIG. 1. Zonal distribution of the times of principal tectonic deformation and metamorphism within the Scandinavian Caledonides (after Roberts & Gale (i) Finnmarkian (Grampian): Late Cambrian-early 1978, fig. 6). For the sake of clarity, Precambrian Ordovician. elements incorporated in the nappe pile have been (ii) Scandinavian: Mid-late Silurian. ignored. Overprinting of one event by another is (iii) Late Devonian (Svalbardian, ?Acadian). difficult to portray on such a map ; in W Norway, for example, the Ordovician event has been high- Innorthernmost Norway the Finnmarkianphase lightedat the expense of the equallyimportant produced a number of major nappes, each of which Silurian deformation. 1: Devonian deformation. 2: usually comprises a couplet of Precambrian basement Scandinavian(Silurian) deformation event. 3: andlate Precambrian-Cambriancover (Sturt et al. Finnmarkian (late Cambrian-early Ordovician)de- 1975, 1978; Sturt& Roberts 1978; Zwaan & Roberts formation phase. 4: Baikalian/Cadomian (late Pre- 1978). These are overlain by nappe units containing cambrian) deformation. 5: Sveconorwegion (Gren- sequences with Upper Ordovician-Lower Silurian villian), Sveconfennian and earlier Precambriande- formations. faunas. Inthe Finnmarkiannappes a well differen- tiated succession of metasediments of late Precambrian-Cambrian age occurs. The type sequence The stratigraphic age of theforeland sequence within the Kalak Nappe Complex has been defined on ranges continuously up from latest Riphean through S@r@y(Ramsay 1971a, b) and can becorrelated Vendian and Cambrian into the Tremadoc. Within the throughout the of Finnmarkian evorogenic ac- mobilebelt the lowerstratigraphic age limit of the tivity (Fig.2). The sequence restswith stratigraphic sequence is unknown as, except in the less far travel- unconformity on a heterogeneous Precambrian base- led Gaissa and Laksefjord Nappes, the sediments are ment which shows significant variations in the different entirely marine and no tillite or drop-stone horizons nappe units. Owing to theintensity of the Finnmarkian have been observed. Fairly high in the succession of deformation, the precise unconformity is only rarely allochthonous units, archaeocyathids have been found preserved(Ramsay & Sturt1977) and in many in- which indicate an age near the boundary of Lower and stances the cover sequence has been uncoupled from Middle Cimbrian (Holland & Sturt 1970). Above this its substrate to form separate tectonic units. level, thick flysch depositswere laid down (Roberts

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1968) and are assumed to be of Middle/Upper Cam- (1978~) to datea relatively late stage in the obduction brian age. It is dfiult to place an upper stratigraphical cycle. If this is the case, much of the tectono-thermal constraint on the Finnmarkian orogenic phase for, as development of this older sequence may be broadly yet, no unconformity has been discovered beneath the coeval with Finnmarkian/Grampianorogenic evolu- Upper Ordovician-Silurian sequences of the Scandina- tion. There are indications, however, that part of this vian nappes. The only stratigraphic indicator of this schist belt may be older, in that preliminary dating of orogenic phase comes from boulder material in con- cross-cutting granites in the N of Stord has given ages glomerates on Mageray and on theLyngen peninsula, c. 670-700Ma (Riheim & Thon, pers. comm. 1978). which can readily be correlated with lithologies in the Thus, until faunal evidence is acquired or a more Finnmarkian complex. detailed geochronology becomes available, the precise The major evidence for the age of the Finnmarkian affinity of this oldersequence wiIl remain deformation comes from geochronological studies, problematical. summarized by Sturt et al. (1978). The most reliable A somewhatanalogous situation obtains in the agedeterminations are those of Rb-Sr whole-rock allochthonous assemblages of the region, isochrons onthe syn-tectonicigneous rocks of the where a younger Caledonian cover sequence of vol- SeilandProvince (550-500 Ma). The resultsbracket canics and metasediments (the StarenNappe) lies with the tectono-thermal evolution of thc Finnmarkian into tectonic contact upon a more complex succession of the period late Cambrian-early Ordovician and indi- generallyhigher grade, amphibolite-facies metasedi- cate that the duration of the Finnmarkian phase was ments with a variety of acidic to ultramafic intrusives probably in the order of 50 Ma. As Rb-Sr whole-rock which constitute theGula Group (Roberts 1967, isochrons determined on late Precambrian slates give 1978; Wolff 1967) (Figs 2, 3). Theage of the Gula is a early Ordovician ages, ithas been assumed that matter of some debate; Precambrian ages have been Finnmarkian deformation commenced in the internal obtainedfrom itswesternmost development in the zone in Upper Cambrian time, and that the progres- Surnadal Syncline (Riheim1977) lateand siveeastward translation of thenappes only en- Cambrian/early Ordovician ages from certain trondh- croached on to the foreland sequences in the earliest jemites (Berthomier et al. 1972) and from K-Ar min- Ordovician. Finnmarkian D, structures in the foreland eral (hornblende) data (Wilson et al. 1973).The zone are thus broadlytime-equivalent to Dz of the minimum age of the last metamorphic phase is Middle Kalak Nappe Complex. Silurian. These features suggest either that the Gula On Mager~y,a thrust marked by amphibolite facies Group represents Precambrian materials which have blastomylonites separatesthe Upper Ordovician- undergone a complex sequence of orogenic evolution, Siluriancover from rocks of the Kalak substrate or that the group includes rocks of more than one (Ramsay & Sturt 1976). The latter comprises a thick cycle. The tectonic position of the Gula, beneath the unit of migmatites developed from the basal Klubben Staren Nappe, is comparable to that of the high-grade Psammite Group of the Kalak Nappe, and bears the Seve unit which underlies the low-grade K~liNappe in imprint of 2 major deformation cycles prior to thrust- Sweden (Zachrisson 1969); this broad correlation is ing. Above the thrust plane, Silurian greywackes pre- adopted in Fig. 2. The age of the St~renNappe serve flattened bedding, and the thrusting is seen to sequence ranges fromthe Tremadocian to assumed have been coeval with the first major (Scandinavian) lower Llandoverian. Numerous mineral ages indicate deformation suffered by these rocks. that evorogenicdeformation occurred in Middle- In SW Norway a major unconformity is recognized Upper Silurian times (Wilson et al. 1973), but there is separatingan Upper Ordovician/Silurian sequence also evidence suggesting that an early Ordovician from a substrate comprising a polyphasally deformed metamorphic event has affected the tholeiitic basalts metamorphic complex traditionally considered as pre- of the Stf~ren Group (Sturt et al. 1967). Middle Ordovician (Kvale 1960; Sturt & Thon 1976, Further indications of some form of link with SW 1978a, b, c; Faerseth et al. 1977) (Fig. 1). No fauna Norway are found on the island of Sm~la.There, an has, as yet, been recovered from this older complex Arenig-Llanvirn volcano-sedimentary sequence was and hence noprecise stratigraphic age can be assigned folded and weakly metamorphosed prior to intrusion to it. The older sequence, the Samnanger Complex of by quartz diorites dated (Rb-Sr) as Ashgillian the region (Faerseth et al. 1977), bears many (Sundvoll & Roberts 1977; Bruton & Bockelie 1979). lithostratigraphicresemblances tothe Dalradian of In the Trondheimregion as a whole, an upper limit for Scotland. Sturt & Thon (1978~)showed that this older the, main, Scandinavian event is imposed by the sequence contains a major ophiolite (the Karm~y occurrence of late-orogenic, intermontaneOld Red Ophiolite) to which the substantialgreenstone/ Sandstone sediments, mainly inwestern areas. The gabbro/serpentinite complexes of the western Nor- oldestformations in this molasse succession are of wegian Caledonides may relate. Granitic rocks, of Downtonian or possibly Ludlovian age (Siedlecka & crustal derivation, intrude the ophiolite and their dat- Siedlecki 1972; Gee & Wilson 1974). ing at c. 450Ma was assumed by Sturt & Thon Inthe area (Bjarlykke 1978),the effects of

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Mageroy SCANDINAVIAN CALEDONIDES .

Geology and main tectonicunits

-, 0 :o ‘\ 0-

Ndppe complexes

B Jotun, Bergen

m Helgeland, Beiarn m Rodingfjell,,Mager m St@ren,Trondhei.m (Koli) SS3 Kalak,Reisa,Gula (Seve) m Laksefjord,Valdres (Sarv m Kvitvola (Offerdal) m Lower thrust units and - autochthon m Permian, Oslo region I3d Devonian sediment s m Region

0 200km m Precambrian basement l 1

~~~ FIG. 2. The Scandinavian Caledonides, showing the principal tectonic units and geological subdivisions, and the approximate ‘stratigraphy’ of the nappe sequence. The map is, much simplified and is compiledfrom several of the sources cited in the text. Strict correlation is not intended in all cases; in some instances other solutions are equally probable (e.g., the Mager0y-Lyngen unit could well equate with the St0ren in terms of tectonic position and the age of its lithologies). Swedish nappe names are shown in parentheses. The cross-sections, I-X, are presented in Fig. 3. D, Dombls; L, Laksefjord; J, Jel0ya; T, Trondheim; S, Solund.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/3/241/4886242/gsjgs.137.3.0241.pdf by guest on 25 September 2021 Caledonian deformation in Noway 245 Caledonian uplift within the main orogenic belt were expressed by an unconformity in early Wenlock times followed by extensiveprograding deltaic sedimenta- tion through into Ludlovian time. The youngest rocks present in the Oslo, Lower Palaeozoic, pre-orogenic sequence occur in theextreme S, onthe island of Jel~ya,and contain a Downtonianfauna. In other areas, e.g. in W Norway and at Roragen (SE Trond- heim region), extensive late-orogenic clastic sequences were deposited in Lower to Middle Devonian times (Fig. 2). These sediments represent the molasse to the Scandinavianorogenic phase, though they also bear evidence of orogenicdeformation, in places polyphasal, and very low grade metamorphism. Two of the W Norwegian Devonian outliers, at Solund and Kvamshesten, show a thrustrelationship with their substrate (H~isaether 1971; Indrevaer& Steel 1975). The precise age of these orogenic movements is not known but they are widely considered to be of Sval- bardian (Frasnian) development. Roberts (1974) con- sidered that the main folding of the Lower Palaeozoic sequence of theOslo region also belonged to this stage. The lack of Carboniferous strata in Norway means that an upper stratigraphical constraint cannot be given for this deformation. It is possible, however, thatthe dating of a tropicalweathering profile on i VI Andprya, in N Norway, at close to the Devonian/Carboniferous boundary, may provide such an upper limit (Sturt et al. 1979). To summarize, although the Norwegian Caledonides showgood evidence of 3 major deformation cycles during Caledonian development, 2 of which represent periods of major orogenic, or evorogenic, evolution, it must be stated that thestratigraphic limits are in some cases imprecisely known and much work is necessary before meaningful correlations can be made through- out the belt.

Deformation during Caledonian evolution The existence of a complex pattern of Caledonian deformation in relation to time implies thatmajor Caledonianstrains may gave occurredduring either X the polyphasal Finnmarkian orthe Scandinavian evorogenic cycles. Also, with the lack of chronostratig- raphic controlin certain parts of the belt, it is not 0 100 km unlikely that some deformation patterns hitherto as- sumed to be of Caledonian development may be found to represent strains related to Precambrian events. In someparts of Scandinavia, geochronological dating FIG.3. Geological cross-sections across the Scan- has, in fact, already led to reassessment of the tem- dinavian Caledonides. The section lines are indi- poral aspects of the episodicdeformation and cated in Fig. 2. Note the increased horizontal scale. Also, to helpdistinguish thedifferent units the metamorphic patterns. Examples of the recent recog- vertical scale is greatly exaggerated. The ornament nition of pre-Caledoniantectono-metamorphic com- is the same as in Fig. 2; the blank areas represent ponents in traditional'Caledonian' nappe sequences Precambrian basement. are found in the Kalak Nappe Complex of Finnmark

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and (Ramsay & Sturt 1977; Sturt & Roberts In the interrelationships between 1978; Zwaan & Roberts 1978); the nappes of Nord- a variably caledonizedbasement and the cover are land (W. Griffin, pers. corn.); the Seve unit of the extremely complex. Aided by Rb-Sr age determina- Seve-K@li NappeComplex of Sweden(Zachrisson tions, it is increasingly clear that some of the nappe 1969; Zwart 1974; Gee 19786); the nappe complexes superposition is itself pre-Caledonian, of Sveconor- of the Trondheim-Dombis region (Riheim 1977; Sol- wegian(Grenvillian) age or older(Naterstad et al. heim1977; Gukzou 1978; Roberts 1978); and the 1973;Solheim 1977; Sturt & Thon1978a). These allochthon of Hardangerviddaand western Norway Precambrian nappes were subsequently dissected and (Andresen et al. 1974). In the present accountwe will incorporatedin the Caledonian nappes, mainly in confineour attention to Caledonian deformation in Silurian time. In the westernbasal gneiss district, W of segments of the belt where either the chronostratig- theTrondheim region, ‘caledonization’ of thebase- raphic or the geochronological controls have reason- ment rocks is thought to have been minimal and is able confidencelimits. largely seen in retrogrademineral reactions, K-Ar mineral ages, andshear zones in the gneisses. Moreover, augen gneiss and blastomylonitezones pre- Basement/cover relations viously thought to be Caledonian are giving Precam- Theproblem of basementand cover has already brian dates (Riheim 1977; Solheim 1977). beendiscussed, and it is clearthat basement/cover Withinthe nappes of theorogen as a whole, relationships are difficult to elucidate within the Scan- Caledonian reworkingof basement can range from the dinavian Caledonides. The basement varies from area simple development of thin, basal, mylonite zones to to area from Sveconorwegian, Svecofennian and older major zones of strongly blastomylonitized gneisses and Precambriancrystallines, to little-metamorphosed ultramylonites.In some nappes the basement/cover younger Precambrian rocks, to intra-Caledonian base- interface became a locus of high strains, and an effec- ment produced in response to the early, Finnmarkian tive decoupling of cover from basement occurred. In phase of theorogeny. Furthermore, the basement such instances the cover sequence was further trans- materials range from those which are readily correla- latedas an independent nappe, thus obscuring the tive with the Baltic Shield crystallines to those which original basement/cover relationships. The effects of are considered to represent long-transportedalien ele- Caledonian metamorphic overprinting further compli- ments. The latter contain rocks which in many cases cate this relationship, particularly where the basement havebeen affected by orogenicdeformation and has undergone advanced tectonic reworking (Ramsay metamorphism, of which little or no record is seen in & Sturt 1976, 1977). the Baltic Shield. Although in many areas within the ScandinavianCaledonides a reasonably clear under- standing of localbasement/cover relations has been The nappes reached, there are still extensive tracts where correla- tions are based on doubtful or ambiguouslithostratig- The Norwegian thrust nappes haveevolved during 3 raphic similarity and outdated concepts. independent phases of Caledonian orogenic activity, A very important problem is that of the nature and though deep-seated metamorphism is associated with extent of Caledonian reworking of pre-existing base- onlythe 2 earliestphases, i.e. theFinnmarkian/ ment complexes. The basement beneath many of the Grampianand Scandinavian stages. An additional greattransported nappes, e.g., theJotun and complication is that of the pre-Caledonian nappe jux- Hardanger-Ryfylke nappe complexes (Naterstad et al. taposition.Over wide areas, particularly along the 1973;Sturt & Thon 1978a) andthe Kalak Nappe more marginal parts of the orogen, the Caledonian- Complex(Sturt et al. 1975,1978), generally shows transportednappes form relatively flat-lying thrust littleevidence of majorCaledonian disturbance, al- sheets (Fig.3),where parautochthonous and al- though western areas are sometimes strongly affected lochthonousunits can be readily identified (Strand locally, particularly along the western sidesof tectonic 1972). In the various the nappe ‘stratigraphy’ windows. This difference in the degree of ‘caledoniza- has acquired its own local terminology. For details the tion’ of the Precambrian basement-locally pervasive reader is referred to tables and maps in the syntheses in the W to minimal or lacking in the E-has also been of Gustavson (1978), Roberts (1978), Sturt& Roberts describedfrom by Nicholson & Rutland (1978) and Sturt & Thon (1978a); the nappe sequ- (1969) and Wilson& Nicholson (1973). Aninteresting ences in Sweden are described by Kulling (1972) and situation is provided by the Archaean to Proterozoic Gee (1978b). Nappe correlationin general is discussed complex of theLofoten-Vesterilen region (Fig. 2) in Strand(1972), Gayer (1973), Nicholson (1974), which, with theexception of local thrustfaulting, Binns (1978) and Zwaan & Roberts (1978). appears to havebeen largely untouched by the Interms of geometryand strain, individual al- Caledonian deformation or metamorphism (Griffin et lochthonous units are generally extensive, thin, sheet al. 1978). or wedge-shapedelements which commonlyshow

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westward thinning (Nicholson & Rutland 1969; Zach- phases. Incremental vertical shorteningwith concomit- risson 1969; Gayer 1973) and evidence of protracted ant E-SE-directed stretching has therefore greatly in- high strains, especially in basal zones. Indicationsof an fluenced the developing nappe pile, sometimes pro- overall primary progressive simple shear deformation ducing an extreme thinning, boudinage, or even exci- are ubiquitous, with evidence of fold rotation into a sion of certain nappe units. This mechanism of nappe stretching lineation trend in areas of moderately high stretching was alsoinvoked by Gee(1978a), while ductilestrain, but this is oftenmasked by episodic pure shear alone has been advocated for nappe thin- increments of flattening and extensional strain (Ram- ning and total displacement of some of the nappes in say 1971b; Olesen et al. 1973; Ramsay & Sturt 1973; southern Norway (Hossack 1978). Rhodes & Gayer 1977) which have assistedin produc- Quantitativestudies on the strain involved in ing lensoid, mega-boudin geometries (Ramsay 1971b; Caledonian deformation in Norway are few, and gen- K. B. Zwaan, pers. comm. 1975; Gee 1978a) as well erally apply torather restrictedtectonic situations. as increasing the estimateof total nappe translation. A Hossack's (1978)application of astrain integration later stage of gravity collapse of the nappe pile has technique is animportant step, although his irrota- alsoproduced regionally developed flat-lying meso- tional, vertical thinning model is likely to prove con- scopic foldsin central areas of Norway (Roberts 1971). troversial.Calculated displacements of nappeshere The fabrics of thrust zones often truncate polyphase include a minimum of 290 km for the Jotun Nappe structures in the nappes, which were developed in a (now revised to 400km, Hossack, pers. comm.); and moreinternal setting within theorogen. The more the shorteningof the cover nappesin the Valdres area westerly nappes show evidenceof protracted emplace- correspondsto a 58% contraction with a maximum menthistories, andearly thrust planes have been pebble elongation of 390% (Hossack 1978). strongly folded, often polyphasally, e.g., the Bergen InFinnmark, finite strain data are scattered and nappes (Sturt & Thon 1978a) and the Storen Nappe largely unpublished (except for Ramsay& Sturt 1970). ofthe Trondheim region (Gale & Roberts1974; Investigations by one of us (D. R.) on a conglomerate Roberts 1978). As a result, complex geometries were in theLaksefjord Nappe have revealed a marked developed and the strain histories are difficult to un- flattening strain towards the basal thrust with pebble ravel. It was traditionally believed that thrusting was a elongation of closeto 300%, and 85% shortening relatively latephenomenon in Caledonianorogenic across the schistosity. More detailed work by G. Wil- evolution,but evidence is accumulatingfavouring liams, T. Chapman & N. Milton (in prep.) on this same episodic thrusting throughout orogenic development. conglomerate zone confirms this picture, though with An excellentexample of this is seen in theKalak indications of a slightly moreconstrictional strain Nappe Complex of N Norway, where thrusting with along the thrust sole. associated mylonite development occurred during the firstFinnmarkian deformation (DlF), pre-dating the regionalmetamorphic peak,alsoand syn- Comparison with the East metamorphically during the second Finnmarkian de- GreenlandCaledonides formation (DzF); later brittle thrusting occurred after the thermal maximum had passed, and also during the The E-SE-directed nappe displacements in Norway, Silurian orogenic stage in bringing the nappe complex involving basement slices, aremirrored on the E into its present position (Sturt et al. 1975, 1978). In Greenland side of the orogen in westward translation contrast, thesyn-metamorphic thrusting of the upto 100 km ormore of nappe unitswhich are Mageroy Nappe occurred during thefmt Scandinavian composed of much pre-Caledonian crystalline material deformation (Dls). (Henriksen & Higgins 1976).There, middle Pro- Estimates of nappetransport distances for the terozoicisotopic ages are assigned torocks which Caledonian allochthon in Scandinavia are numerous, were previously thought to belong to the late Precam- andvary from few a kmforsome eastern brian stage of the Caledonian cycle, and the intensity parautochthonousunits to 200-5OOkm ormore for of Caledonianorogenesis is far less thanformerly highernappes (Strand 1972; Gale & Roberts1974; imagined (Higgins 1976). The picture, therefore, has Gee 1975). Figuresof around 1000 km have also been much in common with the situation in Norway. proposed (Kautsky 1946; Gee 1978a). Difficulty ar- Considering the horizontal component of thrust dis- ises, however, in quantifying the sum displacement of placement in the E Greenland and Scandinavian seg- nappeunits in viewof thecomplexity of the total ments, this must clearly have diminished to zeroin the strain. Our own work in Finnmark and TrBndelag over central collisional or mobile zoneof the orogen. There two decades has revealed a pattern of episodic rota- the continent-continent collision produced a thickened tional deformation with important componentsof flat- crust and emergent topographic high from where the teningand extensional strain in thelate stages of nappes are considered to have derived andflowed-in individualfold-producing episodes; this applies to opposite directions, to the E and to the W(cf. Gale & both the Finnmarkian and the Scandinavian orogenic Roberts 1974, fig. 4e)-partly under the influence of

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/3/241/4886242/gsjgs.137.3.0241.pdf by guest on 25 September 2021 248 D. Roberts & B. A. Sturt gravity. In thismanner, higher grade nappes may Proterozoic ‘block’ has been very little affected by the overlie ones characterized by low-grade metamorphic Caledonian deformation and metamorphism and was assemblages, and basementmay ultimately override its apparently part of the European or Baltic plate during cover.However, whether the incorporation of Pre- thismajor tectonic cycle(Tull 1977; Griffin et al. cambrian crystallines in the Scandinavian and Green- 1978). Indications are that the Scandinavian nappes, landnappe piles derived wholly fromthe impacted including their slices of Precambrian crystalline base- continentalplate margins or in partfrom micro- ment,were derived from a source W of Lofoten continental prisms or ‘islands’ in theCaledonian during Silurian times. IapetusOcean (Roberts & Gale1978, p. 323) is a problemrequiring solution through future detailed International Geological Correlation Programme research. The situation in the Lofoten-Vester%len reg- Norwegiancontribution No. 18to ProjectCaledonide ion is of interest in this connection. This Archaean- Orogen

References

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ZWART,H. J. 1974. Structureand metamorphism in the metamorphic nappes. Centenaire Soc. Geol. Belg. Geol. Seve-Koli Nappe Complex (Scandinavian Caledonides) Dom. Crist. Liege. 129-44. and its implications concerning the formation of

Received 30 May 1979. B. A. STURT,Geologisk Institutt, Universitet i Bergen, 5014 Bergen, Norway. D. ROBERTS,Norges Geologiske Unders~kelse, 7001Trondheim, Norway.

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