Carl-Henric Wahlgren

Kinematics of a major fan-like structure in the eastern part of the Sveconorwegian orogen, Baltic Shield, south-central Sweden

Abstract The N-S-trending so-called Protogine Zone in the Baltic Shield of south-central Sweden is usually considered to mark a tectonic boundary between the rocks of the Transscandinavian Igneous Belt (TIB) in the east and the Sveconorwegian orogen in the west. Detailed structural mapping in the Karlskoga-Kristinehamn area has shown that an anastomosing network of ductile deformation zones with generally N-S strike extends ca. 40 km east of the traditional “Protogine Zone”. Furthermore, the western boundary of this ductile deformation and the TIB is not constrained in the Kristinehamn area. Reconnaissance studies indicate that they both extend westwards towards the so-called Mylonite Zone. It is suggested that the eastern limit of the Sveconorwegian orogen is located some 40 km east of the present boundary and that the “Protogine Zone” concept is obsolete. The term Sveconorwegian Frontal Deformation Zone (SFDZ) is proposed as a more appropriate alternative in southern Sweden (south of lake Vattern) and to correspond to a younger set of oblique ductile deformation zones with reverse and right-lateral components of movement in the easternmost part of the orogen farther to the north. Ductile deformation zones older than the SFDZ in the Karlskoga-Kristinehamn area display a fan-like geometry in an E-W cross-section, with steep westerly dips in the eastern part of the section, vertical dips farther west and moderate easterly dips in the western part of the section. Kinematic analysis indicates that dip-slip movements predominate with a consistent top-to-the-east sense of movement across the entire fan-like structure. In their present orientation, deformation zones are characterized by reverse movements in the eastern part and normal movements in the western part of the structure. Between Kristinehamn and the Mylonite Zone, the main foliation is gently dipping to subhorizontal, indicating that the regional structure is strongly asymmetric, and that the fan-like structure occurs close to the foreland of the orogen. Deformation zones are spaced to semi-penetrative in the eastern part of the fan-like structure, whereas the deformation is more or less penetrative and the TIB rocks are transformed to orthogneisses west of Kristinehamn. This east to west increase in bulk strain is in accordance with an increase in syn-deformational metamorphic grade across the structure. Younger ductile deformation zones belonging to the SFDZ are responsible for a major change in orientation of the older deformation zones in the easternmost part of the structure. The fan-like structure is best explained by models involving the interference of two separate tectonic events. Deformation occurred after ∼ 1.57 Ga and prior to deposition of Neoproterozoic and younger cover sedimentary rocks. It is not yet clear whether the initial phase of deformation (early Sveconorwegian or older) was related to the build-up of an imbricate thrust stack in a compressional regime, as favoured here, or to regional E-W extension. The younger deformation phase was related to rotation of these older structures into the compressional, late Sveconorwegian SFDZ.

Left-lateral transpressive deformation and its tectonic implications, Sveconorwegian orogen, Baltic Shield, southwestern Sweden

Abstract The Mylonite Zone (MZ) is a major, ductile deformation zone in the Sveconorwegian orogen (Baltic Shield) of southwestern Sweden and southeastern Norway which has a strike length of over 400 km and an across-strike width which often exceeds 5 km. It is an orogen-parallel deformation zone which formed under retrogressive metamorphic conditions relative to the higher-grade structures in the surrounding crustal units. The MZ marks a conspicuous metamorphic break in the area south of lake Vanern and a distinct lithological break in the area north of this lake. Regional metamorphic considerations suggest that its surface exposure represents an oblique section through the crust with deeper levels exposed along the southern parts of the zone and shallower levels exposed farther north. Structural studies in three areas north of lake Vanern (Varmlandsnas, Kil and Torsby-Charlottenberg) suggest that the MZ displays coeval, left-lateral, strike-slip and reverse, dip-slip senses of shear characteristic of a transpressive tectonic regime active under upper greenschist-facies conditions. In the northernmost Torsby-Charlottenberg area, at relatively shallow crustal levels, the MZ splays out to form two left-lateral, contractional strike-slip duplexes which together define a positive flower structure. Regionally, the MZ defines the western flank of an east-verging thrust system which formed in the eastern part of the Sveconorwegian orogen. The structures within this thrust system preceded the MZ, contain a subordinate, left-lateral strike-slip component of movement and were rotated into the Sveconorwegian Frontal Deformation Zone (SFDZ) in the frontal part of the orogen. The MZ is inferred to have formed in an extrusive (dispersive) tectonic environment which developed in connection with a slightly oblique collision and crustal shortening in a WNW-ESE direction. The initial stages of this collision produced the east-verging thrust system and shortening was absorbed, at this stage, by crustal thickening. Late, possibly out-of-sequence thrusting with a right-lateral, strike-slip sense of shear along the SFDZ marked the waning stages of crustal shortening in the Sveconorwegian orogen.

U-Pb ages of Proterozoic metaplutonics in the gneiss complex of southern Värmland, south-western Sweden

Abstract U-Pb analyses on zircons from three meta-plutonics in the polymetamorphic gneiss complex of south-western Sweden in Varmland have yielded minimum ages of 1609, 1563 and 1249 Ma. Although the metaplutonics have been strongly reworked, the apparent U-Pb ages of the zircons are almost concordant. This is probably due to low uranium contents. The ages show that many rocks in the ‘Pregothian’ are age equivalents of better preserved rocks in the Amal tectonic mega-unit. It is also concluded that the last deformation, metamorphism and mobilization in the investigated area took place during the Sveconorwegian.

UPb baddeleyite dating of dolerite dykes in the eastern part of the Sveconorwegian orogen, south-central Sweden

Abstract Two dolerite dykes occurring in semi-penetratively deformed Transscandinavian Igneous Belt (TIB) granitoids, east of the traditional ‘Protogine Zone’ have a composite UPb baddeleyite age of 1568+30−8 Ma. Since the dolerites are deformed themselves, this age sets a maximum time constraint for the ductile deformation in the easternmost part of the Sveconorwegian orogen, northeast of Lake Vanern, west of the Sveconorwegian Frontal Deformation Zone (SFDZ; Wahlgren et al., 1994). The new age agrees, within the error limits, with previously reported but less precise UPb and SmNd ages for petrographically and geochemically indistinguishable dolerites in the orthogneisses west of the traditional ‘Protogine Zone’, and together provides abundant evidence that these dolerites belong to the same generation of mafic magmatism. Therefore, the dolerites straddle the boundary between the orthogneisses and the semi-penetratively deformed TIB, i.e. the traditional ‘Protogine Zone’. Apart from intracrustal deformation, the area north of Lake Vanern from the SFDZ in the east to the Mylonite Zone in the west has constituted a coherent segment of crust from the time of the TIB intrusions (∼ 1850-1650 Ma). The dolerites in this study have intruded as dykes into isotropic host rocks. The present dip and concordance to the schistosity in the surrounding rocks are due to subsequent progressive deformation in the eastern part of the Sveconorwegian orogen. It is suggested that the structural geometry and concordant relationships between the dolerites and the orthogneisses west of the traditional ‘Protogine Zone’ are also mainly a consequence of this subsequent progressive deformation that post-dates the dolerite emplacement.

40Ar/39Ar geochronological constraints on the tectonothermal evolution of the Eastern Segment of the Sveconorwegian Orogen, south-central Sweden

Abstract A 40Ar/39Ar study to constrain the tectonothermal evolution across the Eastern Segment of the Sveconorwegian Orogen has been initiated in the area north and east of lake Vänern, south-central Sweden. This segment of the orogen is confined by two major deformation zones, the Sveconorwegian Frontal Deformation Zone (SFDZ) in the east and the Mylonite Zone in the west. Previous structural work and the prograde character of the metamorphism within the study area suggest that an older (< c. 1.57 Ga), regional foliation was formed by ductile shear deformation in a compressional tectonic regime. The orientation of this foliation was subsequently modified by later rotation along younger ductile shear zones in the easternmost, frontal part of the orogen (SFDZ). The 40Ar/39Ar ages for hornblende suggest that the regional foliation is Sveconorwegian. Furthermore, white mica ages demonstrate that the Sveconorwegian tectonothermal overprint continues at least 40 km east of the traditionally accepted limit situated along the ‘Protogine Zone’. These results also provide age constraints for different phases of Sveconorwegian tectonothermal evolution with an older group of ages from 1009–965 Ma and a younger set from 930–905 Ma. The older ages are inferred to constrain a minimum age for crustal thickening during which the regional foliation and metamorphism developed, while the younger are associated with later compressional movement along the SFDZ.

Mica-lamprophyres in western Värmland, south-western Sweden

Abstract Lamprophyric dikes occur in south-western Varmland (Sweden). They are mainly composed of biotite, pyroxene, olivine, serpentine, chlorite, plagioclase, apatite and ore. One of the dikes has been dated by the K-Ar method and yielded a Sveconorwegian model age of 904 ± 13 Ma.

Seismic imaging in the frontal part of the Sveconorwegian orogen, south-western Sweden

Reflection seismic data have been acquired along a 17 km profile in the eastern frontal part of the Sveconorwegian orogen, south-western Sweden. Receiver spacing was 25 m and nominal shot spacing 100 m. Shots ranging in size from 0.5 to 1.0 kg were fired in 3 m deep shot holes. The data image the structure of the upper part of the crust from depths of a few 100 m down to depths of approximately 10 km. The profile crosses the central axis of an inferred fan-like structure, indicated from earlier, detailed surface mapping results. Deformational fabrics within this structure dip approximately 15–40°E in the western part of the profile and 60–80°W in its eastern part. The seismic data display a bivergent geometry with the central axis situated approximately 8 km east of a major fault (Protogine Zone in earlier studies). Surface structural data predict the hinge axis to be located about 3 km farther east. Despite this discrepancy, there is good general agreement with the structure inferred from the surface data. The image obtained from the reflection seismic data establishes that the fan-like structure is a major, upper crustal feature. The present data in combination with previous deep seismic data sets exclude a single-stage extensional model, with an east-dipping extensional deformation zone protracting eastwards from the western part of the profile. The combined seismic data provide support for a two-stage compressional model for the development of the fan-like structure, with crustal thickening and stacking prior to 955 Ma and later compression with a stronger horizontal component of displacement during 930–905 Ma. The latter gave rise to the development of the complex system of retrogressive deformation zones referred to as the Sveconorwegian Frontal Deformation Zone (SFDZ), a tectonic analogy to the Grenville Front in the eastern part of North America. The SFDZ dips westwards, may become listric at depth and possibly sole in a zone of lower crustal reflectivity west of the present profile. The kinematics and geometry of the SFDZ suggest that reverse displacements (thrusting) may be responsible, at least in part, for the exhumation of the medium- to high-grade rocks in the western part of the fan-like structure. Both the seismic data and the surface structural geology show many similarities to the Grenville Front.

A cratonic ultrabasite in south-western Sweden

Abstract An ultrabasic intrusion is described from south-western Varmland, Sweden. It is tentatively inferred to be of Sveconorwegian age. The rock is mainly composed of olivine, which is almost completely altered to serpentine, and secondary minerals, such as amphibole and chlorite. The ultrabasite is suggested to have been tectonically emplaced in its present position.

40Ar–39Ar biotite age of a lamprophyre dyke and constraints on the timing of ductile deformation inside the Idefjorden terrane and along the Mylonite Zone, Sveconorwegian orogen, south-west Sweden

A sharply cross-cutting lamprophyre dyke inside the Idefjorden terrane and along the Mylonite Zone in the Sveconorwegian orogen, Sweden, yields a plateau 40Ar–39Ar biotite age of 914.6 ± 1.2 Ma. This result confirms a published K–Ar age and is interpreted to record magmatic intrusion of the dykes. The lamprophyres are coeval to the similarly oriented but less potassic 916 ± 11 Ma noritic to anorthositic Hakefjorden Complex exposed west of the Göta Älv Shear Zone in the Idefjorden terrane. The lamprophyres are characterized by high magnetic susceptibility, chilled margins, biotite phenocrysts and deuteric alteration of the groundmass. They classify as kersantite and minette, and are interpreted to represent rare, minor explosive mantle-derived magmas, intruded along deep-reaching, east-west trending, extensional structures related to the waning stages of the Sveconorwegian orogeny. Since the host bedrock, at the current level of erosion, was responding to brittle deformation prior to the intrusion of the lamprophyre dykes, the age of 914.6 ± 1.2 Ma sets a minimum limit for the termination of Sveconorwegian ductile deformation inside the Idefjorden terrane and along the Mylonite Zone, north of Lake Vänern. This contrasts to the situation in the westernmost Rogaland–Vest Agder sector of the Sveconorwegian orogen, in Norway, where 930–920 Ma anorthosite to granite plutonism was spatially related to high temperature-low pressure metamorphism and ductile deformation during the waning stages of the Sveconorwegian orogeny. The new age underscores the diachronic exhumation of the orogen.