J. geol. Soc. London, Vol. 137, 1980, pp. 241-250, 3 figs. Printed in Northern Ireland. Caledonian deformation in Norway 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 Finnmark and probably also developed in western Norway; 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 western districts 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 Scandinavia. serveral hundred km have been indicated for some of Chronostratigraphical and lithostratigraphical correla- these nappes (Kautsky 1946; Oftedahl 1966; Gale & tions, aided by a flow of 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-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 region 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 deposits were laid down (Roberts 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 Norway 243 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