Southwest Scandinavian Domain Research Articles

Crustal structure beneath southern Norway imaged by magnetotellurics

We use data from two magnetotelluric profiles, ToSca10 and ToSca'09, over the Scandinavian Mountains to study the crustal structure in southern Norway. The profiles cross the major tectonic structures of the Caledonian orogen as well as the western margin of the Precambrian Baltica. Dimensionality and strike analyses indicate generally 3-D behavior of the data. However, the majority of the used data distinguishes a preferable strike direction, which is supported by the geology of the region. Hence, we employ 2-D inversion and choose to invert the determinant of the impedance tensor to mitigate 3-D effects in the data on our 2-D models.Magnetotelluric data from both profiles are inverted using a damped least squares solution based on a singular value decomposition. We improved the solution by defining the inverse model covariance matrix through gradient or Laplacian smoothing operators.The two-dimensional inversion models of the ToSca'09 and ToSca'10 field data from southern Norway derived from the damped least squares scheme with the Laplacian inverse model covariance matrix are presented. Resistive rocks, extending to the surface, image the autochthonous Southwest Scandinavian Domain and the allochthonous Western Gneiss Region. Near-surface conductors, which are located between the resistive Caledonian nappes and Precambrian basement, delineate highly conductive shallow-sea sediments, so called alum shales. They exhibit a decollement along which the Caledonian nappes were overthrust. A deeper, upper to mid-crustal conducting layer in the Southwest Scandinavian Domain may depict the remnants of closed ocean basins formed during the accretions and collisions of various Sveconorwegian terranes. In ToSca'10, the Caledonian nappes, the conducting alum shales and the deeper conductor are terminated in the west by the Faltungsgraben shear complex which represents a crustal scale boundary between the Western Gneiss Region in the west and the Southwest Scandinavian Domain in the east.

Seismic structure and composition of the crust beneath the southern Scandes, Norway

New results on P and S-wave seismic velocity structure in southern Norway indicate that the crust has an average Poisson's ratio of 0.25, is predominantly of felsic-intermediate composition and lacks a significant mafic lower crust. A crustal scale refraction seismic study (Magnus-Rex — Mantle investigations of Norwegian uplift structure, refraction experiment) acquired data along three 300 to 400 km long active source seismic profiles across the Southwest Scandinavian Domain in southern Norway, the youngest section of the Fennoscandian shield. Moho depths in the Domain are 36–40 km, thinning towards the continental shelf and Oslo Graben. The high Vp lower crust beneath the Southwest Scandinavian Domain (Vp > 7 km/s) is around 4 km thick. Crustal structure in the adjacent Svecofennian Domain differs significantly; Moho depths reach ~ 50 km and an up to 24 km thick high Vp lower crust is present. Strong P and S-wave arrivals are recorded from Magnus-Rex, from which Poisson's ratio for the crust is calculated. The unusually strong S-wave arrivals allow rare insight into crustal Poisson's ratio structure that is not commonly available from active source data. The thinness of the mafic lower crust in southern Norway is in significant contrast to other parts of the Fennoscandian shield and delamination or convective removal of the lower crust is proposed. Tectonic events that may have triggered detachment of the lower crust include extensional collapse after the Caledonian orogeny and Permian extension.

Provenance of Late Ordovician clastic sedimentary rocks in Jämtland, central Sweden

Zircon grains extracted from three sedimentary units within the Caledonian Lower Allochthon in Jämtland, central Sweden, were investigated for provenance studies using LA-ICP-MS. 207Pb/206Pb ages fall into the intervals: 3.1–1.7 Ga (Rönnöfors Conglomerate), 3.2–1.4 Ga (Ede Quartzite), and 2.6–1.1 Ga (Kyrkås Quartzite). The age distribution of the Rönnöfors and Kyrkås units vary sufficiently to suggest different source areas. The Ede Quartzite constitutes a mixture of the Kyrkås and Rönnöfors provenance groups. The zircon age spectrum of the Kyrkås Quartzite reflects the typical age group present in the Jämtland basin in late Ordovician. In comparison, most zircon grains from the Rönnöfors Conglomerate were derived from a local source, and only a small portion of these grains is characteristic for the basin. The Ede Quartzite contains a mixture of the typical basinal and, to a lesser extent, locally derived material. The variation in age distribution between the Ede and Kyrkås Quartzites reflects different dominating source areas. Tectonic activities along a shear zone (a continuation of SEDZ), which runs through the Ede and Kyrkås areas may be responsible for the noted differences in source areas. A derivation from older sedimentary rocks to the present west-southwest and/or the Southwest Scandinavian Domain is suggested for the younger parts (<1.5 Ga) of the zircon populations. A substantial portion of the zircon grains (Rönnöfors Conglomerate 49%, Ede Quartzite 31%, Kyrkås Quartzite 12%) fall in the 2.6–2.1 Ga interval. There is no evidence for any felsic crustal components from this time interval in the Fennoscandian Shield, which suggests a local, unidentified, source with 2.6–2.1 Ga old zircon grains to be in erosional position in the vicinity of Jämtland in late Ordovician times.

Terrane analysis, regional nomenclature and crustal evolution in the Southwest Scandinavian Domain of the Fennoscandian Shield

Tectonostratigraphic terrane analysis of an orogenic belt involves the use of a well-defined nomenclature which is related to the internal structure and relative displacement of blocks in the orogenic “puzzle”. The tectonostratigraphic terrane is the central concept in terrane analysis, and the term should be restricted to units fulfilling the definition, i.e. to fault-bounded crustal blocks which have distinct lithologic and stratigraphic successions and which have geologic histories different from neighbouring terranes. Application of terrane analysis to a complex, poly-orogenic region such as the Southwest Scandinavian Domain of the Fennoscandian Shield (SSD) must include information on the entire crustal evolution history of potential terranes, including petrological, geochemical and other non-geochronological information. At the present state of knowledge, no crustal block within the SSD fulfil the criteria as a Gothian or Sveconorwegian tectonostratigraphic terrane. Until basic evidence of terrane status is established for a given block of crust, the use of a descriptive regional nomenclature is therefore preferred. A revised, non-genetic nomenclature is proposed, which does not assign terrane status to any of its units, and which will hopefully help eliminating confusing differences in usage, and be helpful in future analysis of Gothian and Sveconorwegian tectonics and crustal evolution in the region. To reach a stage where terrane analysis of the Gothian and Sveconorwegian evolution of the SSD becomes meaningful, it is necessary to carry out both structural studies and isotopic dating of the ductile deformation zones which are potential terrane boundaries.

New constraints on the timing of deposition and metamorphism in the Bamble sector, south Norway: Zircon and titanite U-Pb data from the Nelaug area

In order to further constrain the deposition of supracrustal suites and metamorphism in the Bamble sector, south Norway, zircon and two generations of titanite from the granitic to granodioritic Nelaug Gneiss have been dated. The Nelaug Gneiss, situated along the Porsgrunn-Kristiansand Fault separating the Bamble from the Telemark sector, cuts the supracrustal Selås Banded Gneisses. Four discordant zircon fractions from the Nelaug Gneiss yield an upper intercept age of 1460±21 Ma, interpreted as the age of magmatic emplacement of the protolith of the Nelaug Gneiss, underlining the importance of the 1.5-1.4 Ga interval in the formation of the crust of the western part of the Southwest Scandinavian Domain. The presently available data show that there were two periods of sedimentation in the Bamble sector, an older around 1.45 Ga related to sedimentation in two distinct settings, and a younger period postdating 1.37 Ga. The 1091±2 Ma age of subhedral orange-red titanite is in agreement with previously obtained ages for peak Sveconorwegian metamorphism in the coastal part of the Bamble sector. The 994±30 Ma age of anhedral beige titanite correlates with the p-T path of the Nelaug area which, in contrast to the p-T path of the coastal area, reveals a late increase in metamorphic conditions in the latter area.

Geochemistry and age of two orthogneisses in the Proterozoic Mjøsa-Vänern ore district, southwestern Scandinavia

Two orthogneisses in the southwestern part of the Fennoscandian Shield have been investigated with respect to geochemistry, age and lead isotopic composition in feldspar. Both orthogneisses host gold-bearing quartz veins in the Mjøsa-Vänern ore district. The alkali-calcic Brustad augen gneiss of granitic composition is situated within the Mylonite Zone (MZ), close to Mjøsa in southeastern Norway. The calc-alkaline Harnäs gneiss of granodioritic composition is situated 50 km west of the MZ, northwest of Vänern in southwestern Sweden. U-Pb zircon dating yielded protolith ages of 1674±10 Ma for the Brustad augen gneiss and 1595+24 −17 Ma for the Harnäs gneiss, indicating affinity to the Transscandinavian Igneous Belt (TIB) and the Gothian Åmål granitoid suite, respectively; the extension of TIB units to the northwestern margin of the MZ is thus confirmed. Both rocks were deformed during Sveconorwegian orogenesis (c. 1.15–0.9 Ga). Their feldspar lead isotopic compositions are significantly more radiogenic than expected for initial lead and no vestiges of more primary lead could be detected in pre-deformational grains. Resetting of feldspar lead is suggested to have taken place at around 1000 Ma, in conjunction with Sveconorwegian ductile deformation. A revision of the domain boundaries within the Fennoscandian Shield is proposed, such that the Torsby and Borås Segments are conceptually excluded from the Southwest Scandinavian Domain. Thereby, the eastern boundary of the Southwest Scandinavian Domain is outlined by the MZ. It is further proposed that the Southwest Scandinavian Domain be renamed as the Gothian Domain.

Detrital Zircon Geochronology: New Evidence for an Old Model for Accretion of the Southwest Baltic Shield.

Disruption of the original crustal architecture as a result of tectonic events often complicates studies of the crustal evolution of Precambrian cratons. Ion-probe U-Pb dating of detrital zircons is one way to circumvent this problem. It yields a "fingerprint" of the source of the metasedimentary host that can be used to tie the host to a counterpart elsewhere. In this study, ion-probe U-Pb dating of detrital zircons from segments of the late Mesoproterozoic Southwest Scandinavian Domain (SSD) of the Baltic Shield provides new evidence for an old model of crustal evolution and tectonic development. Zircon ages for both the pebble and matrix fraction of the Seljord conglomerate, Telemark sector, Norway, show a 1.9-1.7-Ga frequency maximum, as was previously recognized for metasediments from the adjacent Bamble sector. However, the Telemark metasediments lack evidence for significant input of 1.4-1.6-Ga-old crustal material, characteristic of some of the Bamble metasediments. A maximum depositional age of ca. 1450 Ma is inferred for the Seljord conglomerate and a quartzite from the Bamble sector. Definite evidence for significant amounts of pre-1.6-Ga crust in the SSD is absent. Instead, provenance from the early Mesoproterozoic Svecofennian Domain for the 1.9-1.7-Ga crust is proposed, supported by the omnipresence of small amounts of Archean zircons and the distribution of Nd model ages of these and other SSD metasediments. It is proposed that the SSD originally formed a single, continuous belt, fringing the Baltic Shield, with the northernmost segments transported to their current position west of the Oslo rift along north-south-trending shear zones, i.e., at odds with the commonly held view of progressive westward growth of the Baltic Shield. The detrital zircon ages from the Faurefjell quartzite, Rogaland-Vest Agder sector, cluster around 1.65 Ga and reveal a maximum depositional age of 1244+/-40 Ma. Its deviating depositional setting indicates that the Rogaland sector evolved in a different setting, shielded from the crust of Svecofennian age. Instead, its ages allow for a source terrain within the Mesoproterozoic crust of South Norway.

Constraints for a structural subdivision of the Southwest Scandinavian Domain in Sweden

Differences in structural overprinting of rocks east and west of a Sveconorwegian shear zone, the Mylonite Zone, Värmland County, SW Sweden, reveal that this part of the Southwest Scandinavian Domain should be subdivided into different structural domains. We here distinguish the Southwest Scandinavian Domain of eastern Värmland to be affected by Sveconorwegian ductile deformation mainly, where the most prominent structural elements are a foliation and mineral lineations, one of which is associated with open, roughly upright, N–S trending folds. Rocks of western Värmland have a pre-Sveconorwegian deformation history characterized by stromatic veining and isoclinal folding. Subsequent Sveconorwegian thrusting overprints these structures in localised shear zones. One of these is the Mylonite Zone, which roughly marks the eastern border of Gothian folding in this area.

Sveconorwegian extension‐parallel deformation structures; an example from the Hammarö Shear Zone, SW Sweden

On the northern shore of Lake Vänern, southern Värmland, southwestern Sweden, various orthogneisses are affected by a major, steeply dipping shear zone, the Hammarö Shear Zone. It is several kilometres wide and comprises a network of individual shear zones (less than 100 m wide), which strike approximately E‐W across the Eastern Segment of the Southwest Scandinavian Domain in the Fennoscandian Shield, from the major Mylonite Zone in the west to the northeastern shore of Lake Vänern. Three fold phases, extensive dynamic and static recrystallisation, and a conspicuous, gently plunging lineation are the main features of the Hammarö Shear Zone. Early deformation occurred under amphibolite‐facies conditions, but narrow zones with a green‐schist‐facies assemblage demonstrate a lower grade of metamorphism during the late stages. Orthogneisses are variably overprinted by the shear deformation. No indisputable supracrustal rocks are found adjacent to the zone, and within the zone there is an extensive tectonic overprinting on primary and pre‐shear zone structures. The shear zone is intimately related to large‐scale regional folding (F4) which also occurs south of Lake Vänern. The steep mylonitic foliation and the horizontal stretching lineation are interpreted to be the result of the final stages of horizontal E‐W extension of the crust during the Mesoproterozoic D4 event. Berglund, J., Larson, S.Å. & Vinnefors, A., 1997: Sveconorwegian extension‐parallel deformation structures; an example from the Hammarö Shear Zone, SW Sweden. GFF, Vol. 119 (Pt. 2, June), pp. 169–180. Stockholm. ISSN 1103–5897.

Geochronology and structural setting of the 1.38 Ga Torpa granite; implications for charnockite formation in SW Sweden

In the Varberg region of the Southwest Scandinavian Domain in SW Sweden, the megacrystic Torpa granite forms a sheet‐like, partly charnockitic intrusion in intensely migmatised gneisses. The granite has been interpreted as part of an igneous suite, termed the Varberg Charnockite‐Granite Association. The gneisses are made up of ca. 1.75–1.55 Ga old crust that has been subjected to high‐grade Sveconorwegian‐Grenvillian metamorphism. A U‐Pb zircon age of 1380±6 Ma for a charnockitic portion of the Torpa granite is interpreted as the emplacement age and is also suggested to represent the age for the entire Varberg Charnockite‐Granite Association. Together, this age and the field relationships demonstrate that the regional gneiss‐forming event(s) in the Varberg region is older than 1.38 Ga, and that the pervasive deformation of the Mylonite Zone, a major N‐S trending shear zone and tectonic boundary in the region, is younger than 1.38 Ga and therefore probably Sveconorwegian in age (1.1–0.9 Ga). The similarity of the Torpa granite age to other megacrystic granites in SW Sweden confirms the existence of extensive anorogenic, bimodal magmatism in SW Sweden at 1.38–1.36 Ga. This magmatism is coeval with abundant anorogenic granite magmatism in SE Sweden and wide‐spread volcanic‐plutonic activity elsewhere in Laurentia‐Baltica. The charnockitic assemblages of the 1.38 Ga Torpa granite are interpreted to be primary in origin, and Sveconorwegian metamorphism is not evident in the U‐Pb systematics of the analysed zircon fractions. In contrast, granulite‐facies rocks from several localities in the Varberg region, including rocks belonging to the Varberg Charnockite‐Granite Association, have yielded Sveconorwegian isochron ages. Consequently, models based on two discrete events of charnockite‐granulite formation must be considered for the Proterozoic evolution of SW Sweden. Åhall, K.‐I., Samuelsson, L. & Persson, P.‐O., 1997: Geochronology and structural setting of the 1.38 Ga Torpa granite; implications for charnockite formation in SW Sweden. GFF, Vol. 119 (Pt. 1, March), pp. 37–43. Stockholm. ISSN 1103–5897.

Sources of lead in the Mjøsa‐Vänern gold quartz veins, Southwest Scandinavian Domain

Sources of lead in the Mjøsa‐Vänern gold quartz veins, Southwest Scandinavian Domain

Crustal units and role of the Mylonite zone system in the Varberg‐Horred region, SW Sweden

Abstract Late Gothian (c. 1.58 Ga) and Sveconorwegian (1.1–0.9 Ga) structures outline a 35 km long, NNE‐oriented, open gneiss synform in the Varberg‐Horred region of SW Sweden. This is a region of the Southwest Scandinavian Domain, within which a major shear zone and tectonic boundary, the Mylonite Zone, forms a branching shear zone system which converges in the eastern part of the synform. A subdivision between the Gothian and Sveconorwegian events is made by using the intervening anorogenic intrusions as structural markers. This, and the non‐recognition of a previously assumed orogenic event, results in a geodynamic model which is similar for the crustal segments on both sides of the largely N‐S trending Mylonite Zone, except for the higher grade Sveconorwegian metamorphism to the east. The evolution is characterised by one or more major Gothian gneiss‐forming events, followed by intermittent anorogenic magmatism and a later Sveconorwegian development that, outside discrete shear zones, gave rise to mod...

Westward accretion of the Baltic Shield: implications from the 1.6 GaÅmål-Horred Belt, SW Sweden

Three new U-Pb zircon age determinations are reported from the Horred region, south-southeast of Göteborg, SW Sweden. This is a region of the Southwest Scandinavian Domain, within which a major N S trending shear zone and tectonic boundary, the Mylonite Zone, juxtaposes comparatively weakly migmatised lithologies in the west against more intensely migmatised gneisses in the east. West of the Mylonite Zone, a metavolcanic rock (the Mjösjödacite) yields an age of 1643 ± 29 Ma, whereas a cross-cutting plutonic rock (the Idala tonalite) has an age of 1584 ± 15 Ma. Together with a recent age for a volcanic rock from theÅmål region farther north (1.61 Ga, Lundqvist and Skiöld, 1992), these ages help to establish the existence of a coherent calc-alkaline igneous belt of 1.6 Ga age for which the nameÅmål-Horred Belt is proposed. East of the Mylonite Zone, a presumably metavolcanic rock (the Grimmared gneiss) yields an age of ∼ 1.61 Ga. The obtained age and the compositional similarity of rocks on each side of the Mylonite Zone indicate that more deformed and more strongly metamorphosed equivalents of the rocks in theÅmål-Horred Belt may occur also to the east of the Mylonite Zone in what is termed the Eastern Segment of the Southwest Scandinavian Domain. The new results establish theÅmål-Horred Belt as a major geological unit younger than most other crustal components in southern Sweden such as theÖstfold-Marstrand Belt (∼ 1.76 Ga), the Eastern Segment gneisses (> 1.66 Ga) and the three age groups of the Transscandinavian Igneous Belt (∼ 1.81 – 1.65 Ga; Larson and Berglund, 1992). The configuration of the crustal units in SW Sweden appears to necessitate more complex Proterozoic models than those with a persistent younging from the present east to the west. The present concept of the “Gothian orogeny” must be revised since at least two different orogenic episodes at ∼ 1.7 and 1.6 Ga can now be distinguished.

Isotopic constraints on the timing of crustal accretion of the Bamble Sector, Norway, as evidenced by coronitic gabbros

This paper presents the first SmNd data for coronitic gabbros (“hyperites”) from the high-grade Proterozoic Bamble Sector, Norway. SmNd whole-rock isochron ages of the Vestre Dale Gabbro, 1.11±0.14 Ga, the Jomåsknutene Gabbro, 1.77±0.19 Ga, and the Flosta Gabbro, 1.64±0.23 Ga indicate that, contrary to earlier ideas, intrusion of these gabbros is not exclusively bound to the Sveconorwegian orogeny (1.2-0.9 Ga). Some of the gabbros were already emplaced at the onset of or during the Gothian Orogeny (1.75-1.5 Ga). Initial ϵ Nd values of about +5.5 of Jomåsknutene and Flosta Gabbros provide clear evidence of a depleted mantle reservoir beneath this part of the Southwest Scandinavian Domain. This highly positive value differs from the Nd-isotope evolution curve for the Southwest Scandinavian mantle as postulated by Mearns et al. (1986), which assumes that depletion of the mantle beneath this part of Scandinavia started at c. 1.74 Ga. However, the initial ϵ Nd value of the Vestre Dale Gabbro plots close to this curve, whereas 143Nd 144Nd ratios of the mantle sources of the Tromöy and Arendal Gabbros evolved with approximately chondritic 144Sm 144Nd ratios; their ϵ Nd (1.1 Ga) values range from +5.1±0.3 to +6.0±0.3. Gothian metasomatism of a depleted mantle source by LILE and LREE-bearing fluids provides a mechanism to obtain low Nd-isotope ratios in magmas represented by the Vestre Dale Gabbro. Examples of low Nd-isotope magmas have also been found in the Telemark Sector, and in southern Sweden. The marked depletion in Nb indicates that the metasomatic fluids were possibly related to Gothian subduction.