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Caledonian Nappe Sequence of Finnmark, Northern Norway, and the Timing of Orogenic Deformation and Metamorphism

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Caledonian Nappe Sequence of Finnmark, Northern Norway, and the Timing of Orogenic Deformation and Metamorphism Caledonian Nappe Sequence of Finnmark, Northern Norway, and the Timing of Orogenic Deformation and Metamorphism B. A. STURT Geologisk Institutt, adv. A, Joachim Frielesgt. 1, Bergen, Norway I. R. PRINGLE Department of Geophysics, Madingley Road, Cambridge, England D. ROBERTS Norges Geologiske Undersakelse, Postboks 3006, Trondheim, Norway ABSTRACT and have a varying metamorphic state, reaching at least into garnet grade (Holtedahl and others, 1960). The exact relation between On the basis of regional, structural, and metamorphic studies these rocks on Mageray and the older Caledonian metamorphic combined with age determinations, it is demonstrated that the in- complex of the Kalak Nappe are, however, not yet established. ternal metamorphic fabrics of the main nappe sequence of Finn- Correlations made by a number of authors equate, with the mark, northern Norway, developed during an Early Ordovician obvious exception of the younger sedimentary rocks of Magertfy, phase of the Caledonian orogeny (Grampian?); the cleavage de- the rocks of the Finnmark nappe pile with the upper Pre- velopment in the autochthon also belongs to this phase. The final cambrian-Tremadoc sequence of the foreland autochthon. This mise-en-place of the thrust-nappe sequence, however, appears to correlation indicates that- the entire Caledonian sequence of the belong to a later phase of Caledonian orogenic development, prob- nappe pile (without Mageroy) and immediately underlying au- ably toward the end of the Silurian Period, as does the deformation tochthon is upper Precambrian, passing conformably up at least and metamorphism of the Silurian sequence of Mageray. The into the lower part of the Tremadoc. The nappe units beneath the geochronological results also give information regarding the posi- Kalak Nappe, although of lower metamorphic grade, have a simi- tioning of the lower boundaries to the Cambrian and Ordovician lar structural sequence. It is demonstrated in this paper that the Systems. Key words: areal geology, orogeny, geochronology, major internal deformation pattern and metamorphic develop- metamorphic rocks, Precambrian, Cambrian, Ordovician. ment, together with folding and cleavage formation in the adjacent autochthon, were broadly synchronous and produced during the INTRODUCTION 500- to 530-m.y. orogenic phase established for the main metamor- phic event within the Kalak Nappe. The geochronological evidence In recent years considerable progress has been made in under- implies that the deformation and metamorphism of the Finnmark standing the Caledonian geology of the northernmost part of Nor- nappe pile was coeval with the Grampian event in the Scottish way. As a result of studies extending over nearly a century, the Dalradian and that this Early Ordovician orogenic phase had con- general sequence and major features of the nappe complex in siderable regional development. It is difficult to ascribe a precise Finnmark and its underlying upper Precambrian—Tremadoc au- age for thrusting in this region, which is probably, in part, of Late tochthon are now well known. Problems exist, however, in estab- Silurian development. lishing precise correlations between the various tectono-strati- This paper represents part of a systematic study designed to graphic units, owing to the general lack of paleontological evidence. elucidate the geochronologic relations within the Finnmark region, Indeed, with the exception of the underlying autochthon and two and it is hoped that work in progress and planned by us will shed fossiliferous localities in the Kalak Nappe, paleontological evidence further light on the remaining problems and perhaps establish the enabling stratigraphic correlation is absent. Lithostratigraphic precise age of emplacement of the various tectonic units. markers such as the glacigene sedimentary deposits of the Varanger ice age enable limited correlations to be established, but facies LATE PRECAMBRIAN-TREMADOC AUTOCHTHON variations and changes in metamorphic state between the various tectonic units, combined with the general lack of fauna, make In eastern Finnmark the autochthonous upper Precambrian and precise stratigraphic correlations a difficult task. Cambrian-Ordovician sedimentary rocks rest unconformably on a Recent studies of the geochronology of this region indicate that Precambrian crystalline basement of Karelian and older age (Fig. the age of the high-grade metamorphism in the Upper or Kalak 1). These rocks form an outcrop belt extending some 200 km from Nappe (F0yn, 1967) is in the region of 500 to 530 m.y. on the basis west to east, with a maximum width of approximately 60 km. In of Rb-Sr isochron studies (Pringle and Sturt, 1969; Pringle, 1974; the west rocks of the outcrop constitute a narrow zone along the Pringle and Roberts, 1973). This pattern finds confirmation in eastern margin of the Caledonian thrust front, continuing into the K-Ar determinations on nepheline from alkaline rocks in West more southerly regions of the fold belt (the Dividal Group or Finnmark (Sturt and others, 1967). Results of K-Ar and Rb-Sr Hyolithus zone). Comparable autochthonous sedimentary deposits studies of mica from the rocks of this nappe also indicate a major overlying the upper Precambrian Raipas rocks have been described climatic event toward the end of the Silurian Period within the from tectonic windows in the Reppafjord-Alta-Kva;nangen region range 384 to 420 m.y. B.P. Evidence of this Late Silurian event is of West Finnmark and North Troms (Fayn, 1964, 1967; Roberts found in the metamorphic rocks of eastern Mager0y, where a diag- and Fareth, 1973). nostic Silurian fauna including monograptids has been collected The maximum thickness of the East Finnmark autochthonous (Henningsmoen, 1961; F0yn, 1966). The Silurian rocks of rocks is between 4,000 and 5,000 m (Banks and others, 1971), Mageray have been strongly deformed (Ramsay and Sturt, 1971) with fresh or marine shallow-water shale and sandstone pre- Geological Society of America Bulletin, v. 86, p. 710-718,1 fig., May 1975, Doc. no. 50S13. 710 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/5/710/3418572/i0016-7606-86-5-710.pdf by guest on 25 September 2021 CALEDONIAN NAPPE SEQUENCE OF FINNMARK, NORTHERN NORWAY 25° 30° 71° 70° Figure 1. Tectono-stratigraphic units of Finnmark, northern Norway. dominating (Table 1). To the north the rocks are truncated by a The sedimentary rocks of the East Finnmark autochthon are major dislocation zone, the Trollfjord-Komagelv fault (Siedlecka bounded to the northwest and west by a succession of nappes and Siedlecki, 1967, 1971; Harland and Gayer, 1972; Roberts, (Table 2). The lowest tectonic unit, the Gaissa Nappe, is virtually 1972), which separates them from the sequences of the Barents Sea unmetamorphosed and is overlain by the Laksefjord Nappe with and Raggo Groups. It has recently been proposed by Siedlecki greenschist-facies rocks. Above these is the high-grade Kalak (1974) that this fault zone can be traced from offshore Mageray Nappe (F0yn, 1960, 1967; Laird, 1972) or Kalak Nappe Complex through northern Norway (Fig. 1) and on into northern Russia (Roberts, 1973). The highest part of the succession in the au- between Kildin and the mainland. This indicates that this is a major tochthon, namely, the Tremadoc, is truncated by the Laksefjord fault of considerable lateral extent, but as yet of unknown amount Nappe on the Digermul Peninsula (Reading, 1965). or type of displacement. Although the sequence north of the The autochthonous succession south of the Trollfjord-Komagelv Trollfjord-Komagelv fault zone is a highly significant element in the fault zone begins with the Older Sandstone Series (Fayn, 1937) and late Precambrian development of northern Norway, its lithostratig- is essentially a sandstone-shale sequence some 1,300 m in thickness raphy will not be discussed in the present account. Descriptions of (Banks and others, 1971). Initial Rb-Sr isochron studies on rocks the rocks of this particular region have been given by Siedlecka and from this area have demonstrated that shale within the Vadsa Siedlecki (1967, 1971), and although these authors make correla- Group (Banks and others, 1974) of the Older Sandstone Series at tions between the rocks north and south of the Trollfjord- Varangerfjord give an age of 825 ± 19 m.y.1 (Prjngle, 1973). This Komagelv fault zone, they are based only on lithostratigraphic particular shale is unmetamorphosed and bears no tectonic cleav- similarities between parts of successions of predominantly clastic age, and the isochron is considered as indicating the age of rocks, essentially of shallow-water origin, from sequences now in diagenesis of these rocks. entirely different tectonic settings. Recent K-Ar studies on basic The Older Sandstone Series is overlain by the Vestertana Group dikes cutting the sedimentary sequence north of the fault indicate with a low-angle but significant unconformity (Table 1). Two tillite that the sequence may be at least 1,000 m.y. old (Beckinsdale and formations occur at the base of the Vestertana Group separated by others, 1974). We do not propose to go further into the problem of the intertillite Nyborg Formation. The latter contains shale con- relations on either side of the fault now but merely state that we find no justification for placing the rocks of the Laksefjord Nappe 1 at the base of an 0st Finnmark Supergroup (Siedlecka, 1973), that All Rb-Sr results use the decay constant Rb"' 1.39 x 10~" yr"'. All quoted Rb-Sr isochron ages have been recalculated on the York model II program (York, 1969), for is, beneath the rocks of the Barents Sea Group that lie north of the the least-squares fitting of a straight line with calculated errors, to achieve compatibil- Trollfjord-Komagelv fault. ity with other age determinations in Norway. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/5/710/3418572/i0016-7606-86-5-710.pdf by guest on 25 September 2021 712 STURT AND OTHERS taining little or no sand-size fraction and is not affected by Caledo- have been identified only 150 m above the base of the formation.
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