Polyphase Deformation in the Hattfjelldal Nappe, Intemal Zone of the Scandinavian Caledonides, North-Central Norway
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Polyphase deformation in the Hattfjelldal Nappe, intemal zone of the Scandinavian Caledonides, North-Central Norway WINFRIED K. DALLMANN Dallmann, W. K.: Polyphase deformation in the Hattfjelldal Nappe, internal zone of the Scandina vian Caledonides, North-Central Norway. Norsk Geologisk Tidsskrift, Vol. 66, pp. 163-182. Oslo 1986. ISSN 0024-196X. This paper presents a structural investigation of the Hattfjelldal Nappe (part of the Storfjiillet Nappe Complex),Nordland, Norway, leading to a tentative model for the sequence of deformational events within the nappe and its bordering thrust zones. The investigated area belongs to the internalzone of the Caledonian fold-and-thrust belt. The rocks have been subjected to multiple deformational phases. The deformational sequence comprises at !east two generations of isoclinal folds, as well as younger transverse and longitudinal folds. The phases can be correlated with movements in the bordering thrust zones. Finally, some aspects of correlation of fold phases in neighbouring nappes are discussed by comparing the Hattfjelldal Nappe with the underlying Jofjiillet Nappe. W. K. Dal/mann, Institutt for geologi, Universitetet i Oslo, P. B. 1047 Blindern, 0316 Oslo 3, Norway. Published literature conceming the Hattfjelldal these maps were still rather general and lacked Nappe is sparse compared with that elsewhere in structural elements, they have provided a valu the Seve-Koli Nappe Complex in the Scandina able base for further investigations. Finally, the vian Caledonides. The first lithological descrip initiation of the IGCP Nordland project led by tions appeared in the work of Holmsen (1912). l. B. Ramberg (University of Oslo) has increased Regional mapping of the Hattfjelldal district on our understanding of the regional geological situ the scale 1:250,000 was first presented in 1924, ation (Mork 1979, Ramberg 1981, Ramberg & together with a description of the map (Rekstad Stephens 1981, Stephens et al. 1985, Stolen 1985, 1924). More detailed work was started in 1950, in Sverdrup 1985). Fig. l shows the present knowl connection with prospecting for sulphide depos edge of the tectonostratigraphy in this area. its, but was restricted to the area around Mikkel The aim of this paper is to present a structural jord (Færden 1953). Until this date, it was known analysis of the Hattfjelldal Nappe, which is the that the upper boundary of the Hattfjelldal rocks uppermost nappe of the Seve-Koli Nappe Com was defined by a major thrust zone, which could plex in the area between Rossvatnet and Borge be followed northward from the Grong district to fjellet. A detailed description of the structural el the Hattfjelldal area (Foslie 1923, 1924). Strand ements is followed by a discussion of the defor (1953, 1955) published his results of mapping and mational events, which leads to a tentative model stratigraphical investigations in the area between for the structural development of the nappe. Rossvatnet (Fig. l) and Tunnsjoen (Grong dis trict), and made a first attempt at lithostrati graphical correlation. Furthermore, he recog Regional context nized the minor thrust zone at the base of the rocks around Hattfjelldal and introduced the In the western part of Central Scandinavia, both term 'Hattfjelldal Nappe'. continental margins of the early Paleozoic Iape The mapping in the Koli Nappes since the tus Ocean (or a segment of it) are thought to be 1960's has significantly increased our knowledge exposed, the eastern margin being the Baltic of the structure and stratigraphy in this nappe Shield, the western margin probably being the complex. The area north and west of the Borge uppermost thrust nappes (Helgeland and Ro fjellet Window (Fig. l), however, was not investi dingsfjiillet Nappe Complexes; Stephens et al. gated in detail. In the 1970's, L. A. Barkey com 1985). The whole nappe pile in between reflects piled a series of lithological maps on the scale different settings of miogeoclinal and eugeoclinal 1:50,000, which remained unpublished. Though character (Table 1). NORSK GEOLOGISK TIDSSKRIFT 66 (1986) 164 W. K. Dal/mann Helgeland Nappe Complex graphic position of the Hattfjelldal Nappe that here forms the highest thrust unit within the . �pp p x /. Rø�ing:sfJ.allet N e C�m le. Seve-Koli Nappe Complex. Due to this position, Upper: Hattfjelldal and Akfjellet it is thought to represent sediments derived from Nappes a tectonic setting situated between the volcanic _.___.____ __._____.__ __.__._ �-� are (represented in the underlying Middle and Jofjall et Nappe ........_______ _....___.__ _.________._�--'-- � Upper Koli Nappes) and the presumed western :o Krutfjellet Nappe continental margin (represented in the overlying :..::l CV Helgeland Nappe Complex) - before, during or > ..., Middle: Gjersvik Nappe CV •r- � � ---'------ � Vl� after the collision of the two units. �x Leipikvattnet Nappe CV 5 � --"'--'----'- This interpretation requires a Lower Paleozoic c:� � o Q. � �åkko Napp age of the rocks of the Hattfjelldal Nappe. This is � E :o � ..., o _.___,_ �u :..:: Stikke, Atofjallet or discussed in detail in Dallman (in prep.), where u ocv Laxfjallet Nappe an Ordovician to Silurian age can be envisaged � Q. � Q. C:X:IØ via lithostratigraphical correlation with other z: Lower: Bjorkvattnet Nappe s.. Koli Nappes. CV Q. The metamorphism in the investigated Koli Q. Seve unit ::> lithologies is low-grade. The Helgeland Nappe Middle Allochthon Complex, however, where it overlies the Hatt fjelldal Nappe, shows transition from medium- to Lower Allochthon high-grade metamorphism towards the south. autochthonous cover sediments Concerning the terminology on Table l and --------- ------------- --- ---- Baltic Shield Fig. l, one should note that Haggbom (1980) de Table l. Tectonostratigraphic table of the Caledonian Nappes in fined the term 'Jofjallet Nappe' for all the Koli Southern Vasterbotten (Swedish side) and Southern Nordland rocks between the Krutfjellet Nappe and the Ro (Norwegian side). dingsfjallet Nappe Complex. This was modified by Ramberg (1981), who had mapped out the 'Akfjellet Nappe' as an isolated unit with the The Baltic Shield is overlain by an autochtho higher metamorphic Brakfjellet Tectonic Lens at nous cover sequence, succeeded by a series of its base. As a result of field work in this area nappes (Lower and Middle Allochthon) derived (Theisen & Dallmann 1984), I consider it more from areas on or adjacent to the Baltoscandian appropriate to follow the terminology of Ram continent (Gee 1975, Gustavson 1978, Roberts berg, as a tectonic break of the same order can be 1978, Ramberg & Stephens 1981, Stephens et al. observed at the base of both the Akfjellet and 1985, Stephens & Gee 1984). Above them occurs Jofjallet Nappes. Furthermore, the Akfjellet the Seve-Koli Nappe Complex (Upper Alloch Nappe has a similar regional importance to that thon) derived from distal continental, oceanic of the Hattfjelldal Nappe. To follow the first pub and volcanic arc-related settings (Gee 1975, Step lished work by Haggbom would mean to increase hens & Gee 1985), which is succeeded by nappes the complexity of the terrninology of the Koli thought to be derived from a western continental Nappes by finding new terms and building up a margin (Uppermost Allochthon). more complicated system of overlapping collec Based on earlier work, Stephens & Gee (1985) tive names and subunits. interpret the Upper Koli Nappes as being derived from a fore-are related setting, and the Middle Koli Nappes from volcanic are and back-are re Iated settings. Assuming a time relationship be Outline of the stratigraphy tween these complexes, these authors suggest an The reconstruction of the lithostratigraphy of the eastward dipping subduction zone during the Hattfjelldal Nappe is difficult because of strong early Ordovician. After the collision of the west deformation and comparatively low degree of ex ern continental margin with the volcanic are com posure in many areas. Obvious primary rock plex, the sense of vergence was considered to boundaries are restricted to those between rocks have changed to allow the final thrusting east that are weakly foliated (some marbles, conglom ward onto the Baltic Shield. erates, quartzites). The Iithostratigraphic recon Fig. l shows the location and tectonostrati- struction is mostly based on frequently observed NORSK GEOLOGISK TIDSSKRIFT 66 (1986) Polyphase deformation, Hattfjelldal Nappe 165 repetitions or bed sequences in tectonically dif General characteristics of ferent positions and way-up indications from con deformation glomerate pebbles. The establishment of the stratigraphy will be The Hattfjelldal Nappe provides an example of explained in detail in Dallmann (in prep.), and structural development in the internal zone of the only the main division is introduced here. It con Caledonian thrust-and-fold belt. It is character sists mainly of two groups (Fig. 2): ized by syn-sedimentary tectonics (Dallmann, in l. A lower marble group, mostly consisting of prep.) and by later, strongly compressive defor the so-called 'Hattfjelldal limestone' (Strand mation resulting in polyphase folding combined 1955), which is referred to as the 'Hattfjelldal with thrust and shear movements. The structures Group'; of the whole nappe are easterly vergent. 2. An upper phyllitic, quartzitic and conglom Fig. 3 shows a structural map of the Hatt eratic group showing many similarities with the fjelldal Nappe. The main map-scale repetitions 'Limingen Group' of the Gjersvik Nappe (Lutro of the stratigraphy are interpreted as early iso 1979), with which it has been correlated (Dall clines, because alternating upright and inverted mann 1984). stratigraphical sequences can often be observed TECTONOSTRATIGRAPHY of the HATTFJELLDAL AREA 0��5===1�0--�1�5==�20 km LEGEND: Helgeland Nappe Cplx. CJ Tustervatnet Complex lillill R&lngsfjalet�Nappe c Hattfjelldal/ m Akfjellet Napp Upper IITIIIIll Jofjallet Nappe i:ili _ Krutfjellet N. � - .J::. Atofjallet � D Nappe <( 1 Middle Kol i l Bjorkvattnet N� l Lower Kol i) [2] Susendalen Nappe (MiddleAIIochthon) flll Bi:>rgefjellet Window ( Basement) compiled offer work by : BARKEY, L. A RAMBERG, l 8. DALLMANN, WK SMqH-MEYER, 5.