Central Swedish Caledonides Research Articles

Magnetic properties of pseudotachylytes from western Jämtland, central Swedish Caledonides

Abstract. Fault kinematics can provide information on the relationship and assembly of tectonic units in an orogen. Magnetic fabric studies of faults where pseudotachylytes form have recently been used to determine direction and sense of seismic slip in prehistoric earthquakes. Here we apply this methodology to study magnetic fabrics of pseudotachylytes in field structures of the Köli Nappe Complex (central Swedish Caledonides), with the aim to determine fault kinematics and decipher the role of seismic faulting in the assembly of the Caledonian nappe pile. Because the pseudotachylyte veins are thin, we focused on small (ca. 0.2 to 0.03 cm3) samples for measuring the anisotropy of magnetic susceptibility. The small sample size challenges conventional use of magnetic anisotropy and results acquired from such small specimens demand cautious interpretation. Importantly, we find that magnetic fabric results show inverse proportionality among specimen size, degree of magnetic anisotropy and mean magnetic susceptibility, which is most likely an analytical artifact related to instrument sensitivity and small sample dimensions. In general, however, it is shown that the principal axes of magnetic susceptibility correspond to the orientation of foliation and lineation, where the maximum susceptibility (k1) is parallel to the mineral lineation, and the minimum susceptibility (k3) is dominantly oriented normal to schistosity. Furthermore, the studied pseudotachylytes develop distinct magnetic properties. Pristine pseudotachylytes preserve a signal of ferrimagnetic magnetite that likely formed during faulting. In contrast, portions of the pseudotachylytes have altered, with a tendency of magnetite to break down to form chlorite. Despite magnetite breakdown, the altered pseudotachylyte mean magnetic susceptibility is nearly twice that of altered pseudotachylyte, likely originating from the Fe-rich chlorite, as implied by temperature-dependent susceptibility measurements and thin-section observations. Analysis of structural and magnetic fabric data indicates that seismic faulting occurred during exhumation into the upper crust, but these data yield no kinematic information on the direction and sense of seismic slip. Additionally, the combined structural field and magnetic fabric data suggest that seismic faulting was postdated by brittle E–W extensional deformation along steep normal faults. Although the objective of finding kinematic indicators for the faulting was not fully achieved, we believe that the results from this study may help guide future studies of magnetic anisotropy with small specimens (<1 cm3), as well as in the interpretation of magnetic properties of pseudotachylytes.

The continent-ocean (Seve-Köli) boundary in the Sarek-Padjelanta Mts. revisited: Swedish Caledonides

ABSTRACT In the Swedish Caledonides, the continent-ocean boundary is represented by the basal thrust of the Iapetus-derived Köli Nappe Complex (KNC) which overlies the Seve Nappe Complex (SNC) derived from the outer rifted margin of Baltica and the continent-ocean transition (COT). Current maps and literature subdivide the Seve teconostratigraphy in northern Sweden (Sarek-Padjelanta region) into three mega-lenses, the lowest of which (Vaimok lens) is composed of quartz-feldspathic gneisses and subordinate marble, all hosting eclogites. It is overlain by the Sarek lens composed of a COT dyke complex and, along its western margin, a narrow belt of phyllites, schists, quartzite, marbles and greenschists. Overlying this belt is Tsäkkok lens of the same composition as the Vaimok lens. The situation with an intermediate-pressure and often excellently preserved sheeted-dyke complex intercalated between two lenses of complexly deformed high-pressure rocks invited to intricate models of accretion, including out-of-sequence stacking of slices of the continental margin during early Ordovician subduction. Based on a detailed study of a select area and examination of the evolution of previous concepts, this study reinterprets the narrow belt along the western margin of the Sarek Lens as typical of the lower KNC. The favoured explanation of the presence of intermediate-pressure Köli rocks below high-pressure Seve rocks is recumbent folding and/or mega-sheath folding of the Seve-Köli nappe sequence during Scandian (Siluro-Devonian) accretion, by analogy with the situation in places in the central Swedish Caledonides. Previous models of early out-of-sequence stacking during subduction of the continental margin need reconsideration.

3D interpretation by integrating seismic and potential field data in the vicinity of the proposed COSC-1 drill site, central Swedish Caledonides

Abstract The scientific drilling project COSC (Collisional Orogeny in the Scandinavian Caledonides), designed to study key questions concerning orogenic processes, aims to drill two fully cored boreholes to depths of c. 2.5 km each at carefully selected locations in west-central Sweden. The first of these, COSC-1, is scheduled for start late spring 2014 and will target the Seve Nappe Complex, characterized by inverted metamorphism and with parts that have evidently been subjected to hot ductile extrusion. In this study available seismic sections have been combined with surface geology to produce a 3D interpretation of the tectonic structures in the vicinity of the COSC-1 borehole. Constrained 3D inverse gravity modelling over the same area supports the interpretation, and the high-density Seve Nappe Complex stands out clearly in the model. Interpretation and models show that the maximum depth extent of the Seve Nappe Complex is less than 2.5 km, consistent with reflection seismic data. The gravity modelling also requires underlying units to comprise low-density material, consistent with the Lower Allochthon, but the modelling is unable to discern the décollement separating the allochthons from the crystalline Precambrian basement.

Early Cambrian phytoplankton diversification and appearance of trilobites in the Swedish Caledonides with implications for coupled evolutionary events between primary producers and consumers

The newly examined Lower Cambrian strata in the Laisvall-Storuman area, central Swedish Caledonides, yield diverse and stratigraphically significant phytoplanktic organic-walled microfossils (acritarchs) associated with the olenellid trilobites of Holmia sp. that appear to be at the lowermost horizon ever recorded in the Caledonides. The acritarchs, recovered throughout the Grammajukku Formation, are taxonomically reviewed in the context of regional and global trends of phytoplankton diversity. The assemblages are assigned to acritarch zones Skiagia ornata-Fimbriaglomerella membranacea and Heliosphaeridium dissimilare-Skiagia ciliosa, corresponding to the Schmidtiellus mickwitzi and Holmia kjerulfi trilobite zones. The records of acritarch radiations and appearances of trilobites and other faunas in Baltica, and on a more interregional scale, are biochronologically correlated, revealing tightly coupled evolutionary events among primary producers and consumers. The timing of early diversifications of trilobites in various faunal provinces is discussed and the relative age of the oldest known non-mineralized arthropods from the Zawiszyn Formation in Poland is estimated.

Late Ordovician – early Silurian transgressive sedimentation in the Jämtland basin, central Swedish Caledonides

Abstract The sedimentary facies of the Kyrkås Quartzite Formation, Ede Quartzite Formation, and Rönnöfors Conglomerate from the meta‐sedimentary sequences of the Lower Allochthon of Jämtland, central Swedish Caledonides are described and interpreted. The biostratigraphical correlation, which includes new graptolite evidence from the Kyrkås Quartzite, indicates that the transgressive sedimentation represented by these coarse clastic units was initiated in late Ordovician times. Debris flow conglomerates of the Rönnöfors Conglomerate, and slumping in the Kyrkås Quartzite, indicate basement uplift and instability in the Jämtland basin. A local metamorphic source contributed much of the matrix and clasts of the Rönnöfors Conglomerate, but both the Kyrkås and Ede formations were derived from sedimentary source areas. Palaeocurrents that flowed from the west, and the probably wide inshore platform areas of carbonate mud flats fringing Baltica towards the east, mean that much of the coarse clastic sediment for the Kyrkås and Ede formations may have been derived from longshore transport, e.g., from late Precambrian or Cambrian sandstones. Cherns, L. & Karis, L., 1995: Late Ordovician – early Silurian transgressive sedimentation in the Jämtland basin, central Swedish Caledonides. GFF, Vol. 117 (Pt. 1, March), pp. 23–30. Stockholm. ISSN 1103–5897.

Polyorogenic 40 Ar/ 39 Ar mineral age record within the Kalak Nappe Complex, Northern Scandinavian Caledonides

The Kalak Nappe Complex is composed of variably retrogressed, middle Proterozoic basement and imbricated, polydeformed, metasedimentary cover successions which, at least in part, appear to have originated within the late Proterozoic-early Palaeozoic Baltoscandian miogeocline. Mafic-alkaline plutons (Seiland Igneous Suite) occur within uppermost structural units of the nappe complex. 40 Ar/ 39 Ar incremental-release mineral ages suggest local differences in tectonothermal evolution within the Kalak Nappe Complex. Post-metamorphic cooling dates of c. 490 Ma are locally recorded by hornblende within the structurally highest tectonic unit suggesting significant orogenic activity occurred during and/or immediately prior to the Arenig. A distinctly later metamorphic overprint occurred in the Silurian and was of sufficient magnitude to everywhere rejuvenate Ar systems in muscovite and nepheline and locally in hornblende. This was followed by relatively rapid post-metamorphic cooling through hornblende and muscovite Ar closure temperatures between c. 425 and 415 Ma. The complex tectonic history suggested for the Kalak Nappe Complex is generally similar to that previously outlined for the central Swedish Caledonides. Together, 40 Ar/ 39 Ar mineral ages from these widely separated areas indicate that significant tectonothermal activity occurred along the Baltoscandian margin prior to the Silurian. This activity appears, at least in part, to have been associated with formation of a Late Cambrian-Early Ordovician accretionary wedge over a westward-dipping subduction zone during initial closure of the Iapetus Ocean.

Mineral chemistry and crystallization conditions of the Artfjället gabbro and dolerites (Central Swedish Caledonides)

On the basis of plagioclase zoning and other considerations, the compositions of the silicate minerals in the Artfjället gabbro are judged to be close to their original magmatic compositions. They can be used, therefore, to estimate the crystallization conditions. Pyroxene temperatures are 1280-1210°C for picrites, 1240-1180°C for olivine gabbros, and 1180-1050°C for norites. Olivine-spinel-augite thermometry yields ca. 1200°C for the picrites. Comparison with data from melting experiments on basalts suggests that the Artfjället gabbro is one of the few intrusions where pyroxene compositions still record magmatic temperatures. The pressure is constrained between 6 and 10 kbar by the country-rock metamorphism and the presence of plagioclase in the gabbro. No combination of the anorthite-Ca-Tschermakite (An-CaTs) and forsterite-enstatite reaction formulations gives a correct pressure, most yielding overestimates. The An-CaTs reaction is unreliable, especially for low-alumina clinopyroxenes, because of the small CaTs activities and consequent large errors (∼ 5–10 kbar). During crystallization of the gabbro the silica activity probably was nearly constant and the oxygen fugacity near the QFM buffer. For the dolerites, which occur in and are related to the gabbro, temperatures are considered to reflect their liquidus-solidus interval: pyroxene ca. 1200-950°C, brown hornblende 1025-975°C, and biotite roughly 1000-900°C. In quartz-dolerites, temperatures for hornblende (820-660°C) and biotite (ca. 800°C) are lower. The primary water content of the gabbro and dolerite magmas is estimated to have been between 0.5 and 1.5 wt.%.

Occurrence of a uraniferous-vanadiniferous graphitic phyllite in the Köli Nappes of the Stekenjokk area, central Swedish Caledonides

Abstract The Cambro-Ordovician alum shales in Scandinavia have anomalously high contents of certain trace elements, among them U, V and Mo, when compared to other black shales in the world. Variations in the contents of these elements within the formation show a clear correlation with the stratigraphy. The geochemical signature of the different levels of the stratigraphy is apparent over vast areas of the Baltoscandian Platform but has also proved to be valid for fossiliferous black phyllites in the Caledonian autochthon, the lower nappes and one of the higher nappes. Investigation of the content of U, V and Mo in graphitic phyllites of the Stikke Nappe in the Seve-Köli Nappe Complex of the Stekenjokk area has identified the presence of anomalously high contents of these elements compared to average black shales. The content of these elements in the graphitic phyllites correlates clearly with that found in the Early Ordovician Dictyonema-bearing shales in eastern Trøndelag and on the Baltoscandian Platform; it is, therefore, suggested that these metasediments in Stekenjokk are of the same age.

Isotopic composition of ore lead in the Pb-Zn deposits of the Dorotea district, central Swedish Caledonides

Abstract Galena in drill core samples from sandstone-lead deposits in the Dorotea district in the Caledonian Front was analysed for its lead isotope composition. Six samples were taken from Bellvikserg and three from the minor deposit of Ormsjö. The composition of all samples was highly radiogenic with 206Pb/204Pb-ratios between 22 and 23, and 207Pb/204Pb-ratios in the range 16.0 to 16.2. Plotted on a207Pb/204Pb versus 206Pb/204Pb diagram, the Dorotea lead follows a linear trend together with lead from Laisvall and Vassbo, suggesting their radiogenic component to be derived from source rocks of Svecokarelian age.

Petrography, petrochemistry and regional significance of the Saxvallklumpen metabasite, Köli Nappe Complex, Tännforsfältet, central Swedish Caledonides

Abstract The Köli rocks in Tännforsfältet mainly consist of calcareous phyllite and its higher grade metamorphic equivalent – garbenschiefer. In the uppermost, northwestern part of Tännforsfältet the Saxvallklumpen metabasite occurs within the garbenschiefer. It has an elongate lensoid form, extends for 11 km along strike and is up to 500 m thick. The metabasite mainly occurs as a fine-grained amphibolite with both aphyric and plagioclase phyric texture. In its lower part the metabasite contains nodules of epidotite. It was deformed and metamorphosed to lower amphibolite facies together with the surrounding garbenschiefer, the matrix being converted to hornblende and plagioclase phenocrysts to aggregates of epidote, basic oligoclase, quartz and paragonite. Chemical analyses of the metabasite demonstrate a tholeiitic basalt composition. The metabasite follows the trend of abyssal tholeiites. The Ti-Zr-Y, Ti-Cr and Zr/Y-Zr discriminant diagrams indicate an ocean-floor-basalt origin. The calcareous phyllite-garbenschiefer of Tännforsfältet is divided by thrusts into different tectonic units, the Saxvallklumpen metabasite occurring in the uppermost one. The Köli Nappes of northern Jämtland-southern Västerbotten show a similar picture with basic rocks, gabbros, occurring within the Blåsjön calcareous phyllite. The petrochemistry of these gabbros is similar to that of the metabasite in Tännforsfältet, the Saxvallklumpen metabasite being slightly more primitive.

Mylonites and pseudotachylites associated with thrusting of the Köli Nappes, Tännforsfältet, central Swedish Caledonides

Abstract The Gevsjon Formation (calcareous phyllite — garbenschiefer) of the Tannforsfaltet Koli is divided into three tectonic units, separated by two important mylonite zones. These mylonite zones exhibit an earlier ductile phase of deformation and a later brittle phase with brecciation of the mylonitic foliation. Associated with this brecciation pseudotachylites have been recognized at four localities. A fifth locality is so far not directly related to any mylonite zone, but occurs close below the upper mylonite zone. The change from mylonite formation (ductile deformation) to the development of breccias and pseudotachylites represents a change in physical conditions during translation of the nappes. Early movement occurred at higher and the later movement at lower temperatures, implying that the thrusting involved progressive translation of the Koli Nappes to higher structural levels.

Greenstones related to rifting and ocean basin opening in the Jofjället area, central Swedish Caledonides

Abstract Two greenstone formations in the Jofjället area, central Swedish Caledonides, have been analysed for major and trace elements in order to establish their petrology and tectonic environment of origin. The formations — the Ruffe Greenstone and the Brackfjället Greenstone — are situated in the uppermost part of the Seve-Köli Nappe Complex. The enrichment in volatiles, depletion in SiO2, redistribution of CaO and alkali contents are certainly caused by post-eruptive alteration processes. The Ruffe Greenstone is a tholeiite of a transition type between ocean-floor and within-plate basalt, while the Brackfjället Greenstone is an alkaline basalt showing a clear within-plate basalt affinity. The basalts are possibly formed during the later stages of rifting and ocean basin opening in a near-continental environment. The basalts can probably be correlated with the Støren greenstone in Norway.

Superposed folding and metamorphism in the Seve nappe around Åreskutan in the Swedish Caledonides

Abstract The earliest recognisable folds (F1) in the Seve nappe in the Åre area in the central Swedish Caledonides include a phase of co-axial refolding (F1A and F1B). These isoclinal, tight reclined or recumbent folds are parallel to the principal mineral lineation and mylonitic striation. There is a set of tight, reclined folds (F2) which refold the mineral lineation and F1 hinges at large angles. The open F3 folds refold the F2 hinges at small angles. In addition to the above structures, there is also a set of open, upright folds (F1) with axes parallel to the mineral lineation (and to F1) which is younger than F1. The time-relation of these F1 folds to the F2 folds is not clear. The field evidence indicates that F1B and later folds have developed during different stages of thrusting. The period of migmatization and high-grade metamorphism within the nappe has overlapped with the period of development of the F1 and F2 structures.

Geology of the Nordhallen—Duved—Greningen area in Jämtland, central Swedish Caledonides

Abstract A lithotectonic sequence has been studied in the Nordhallen—Duved—Greningen area and found to consist of a Precambrian basement overlain by four distinct thrust-nappe units—the Kallsjon, Quartzite—Mylonite, Seve and Köli Nappes. This nappe sequence is folded by the Mullfjället Antiform, the western limb of which composes much of the investigated area. An important tectonic feature, recognized during the investigation, is the lenticular geometry of these nappe units, as well as individual formations within the highest tectonic unit—the Köli Nappe. The stratigraphy within the Köli Nappe of the Tännforsen area is described in detail and correlated with the classic Köli stratigraphy in Västerbotten. Study of the minor structures within the Köli Nappe reveals a polydeformational sequence (F1, F2, F3). Isoclinal to tight folding occurred prior to (F1) and during (F2) the development of the main foliation (S2). The third-phase structures (F3) deformed the main foliation and are open, asymmetric kink folds and box folds. The peak of the metamorphism of the Köli rocks, represented by garnet and biotite porphyroblastesis, was established prior to the formation of S2.

A probable glaciogenic sediment in the Särv Nappe, central Swedish Caledonides

Abstract A diamictite has been located near the base of the Sarv Nappe. The locality is situated 3 km NNW. of the village of Tannas in the province of Harjedalen. The rock is thought to be of glacial origin and probably of Varangien age. On this assumption the Ottfjall dolerites are not older than Varangien.

A summary of Caledonian front stratigraphy, northern Jämtland/southern Västerbotten, central Swedish Caledonides

Abstract The sedimentary rocks of the Caledonian front in northern Jämtland and southernmost Västerbotten described here include late Pre-Cambrian, Cambrian and Ordovician sequences occurring beneath the metamorphic allochthon of the Seve-Köli Nappe Complex. Three groups are recognized. The lowest, the Risbäck Group (c. 700 m) includes four formations in the type area, dominated by alluvial sandstones. Tillites (Varangian) and a quartzite-shale formation compose the overlying Sjoutälven Group (c. 300 m), which ranges into the Middle Cambrian. The overlying Tåsjön Group (c. 250 m) contains a lower black shale and an upper greywacke-dominated formation, ranging in age from the Middle Cambrian into the Middle Ordovician. The full succession, represented in the west, thins eastwards, and in the autochthon only the uppermost part of the Sjoutälven and the Tåsjön Group are present. The paper briefly discusses the correlation of the three groups elsewhere in the Caledonides.

The structural setting of the Stekenjokk ore bodies, central Swedish Caledonides

Two elongate copper-zinc deposits composing single linear ore body, geometry controlled by two major phases of folding