Abstract
The Eastern Segment in the Sveconorwegian orogen comprises Paleoproterozoic–Mesoproterozoic magmatic suites, which formed along an active continental margin, and Mesoproterozoic suites emplaced during intracratonic extension. Zn–Pb sulphide and Fe oxide mineralizations in 1.9 Ga metavolcanic rocks form a significant mineral resource cluster in the northeastern part. Deformation and metamorphism under low-pressure (≤5 kbar) and variable-temperature conditions, including anatexis and granulite facies, prevailed during 1.9–1.8 Ga (Svecokarelian) and 1.5–1.4 Ga (Hallandian) accretionary orogenies. Sveconorwegian tectonothermal reworking initiated at c. 0.99–0.98 Ga in structurally lower levels. Crustal shortening, underthrusting with eclogite facies metamorphism (18 kbar), exhumation by eastwards thrusting (D1) during continued shortening and high-pressure granulite (8–12 kbar) to upper amphibolite facies metamorphism prevailed. Anatexis and folding around east–west axial surfaces with west-northwesterly constrictional strain (D2) followed at c. 0.98–0.95 Ga, being consanguineous with crustal extension. Structurally higher levels, northwards and eastwards, consist of high-pressure (10–12 kbar) orthogneisses, not affected by anatexis but also showing polyphase deformation. Sveconorwegian convergence ceased with upright folding along north–south axial surfaces and, in the uppermost frontal part, greenschist facies shearing with top-to-the-foreland normal followed by reverse displacement after 0.95 Ga. The normal shearing detached the upper compartment from the underlying gneisses.
The lithotectonic unit referred to as the Eastern Segment (Berthelsen 1980; Bingen et al. 2008; Fig. 15.1) constitutes the easternmost part of the Sveconorwegian orogen in southwestern Scandinavia. This lithotectonic unit forms a c. 100–150 km-wide crustal segment that trends approximately north–south for more than 600 km. Immediately east and NE of the erosional front of the orogen, it adjoins the previously cratonized part of the Fennoscandian Shield affected by older accretionary orogenic systems. This is referred to here as the cratonic foreland to the Sveconorwegian orogen (Fig. 15.2).
Eastern Segment in the Sveconorwegian orogen, southwestern Sweden, shown in black and addressed in this chapter. The white lines show the boundaries between different lithotectonic units identified in this Memoir (Stephens 2020).
Bedrock geological map of the Eastern Segment and surroundings modified after the 1:1 million national bedrock database (Bergman et al. 2012) at the Geological Survey of Sweden. The spatial distribution of the different compartments inside the Eastern Segment is shown in Figure 15.4a. On account of the need to simplify at the scale adopted here, the different types of high-strain deformation zones (fault, ductile shear zone) have neither been distinguished nor shown with their true width. The rocks included in the Eastern Segment are shown in darker tones and presented in the legend. The remaining part of the bedrock is shown in paler tones without any legend but is addressed in other chapters in this Memoir. The eclogite-bearing terrane is distinguished with the help of the symbol ‘e’ inside an area bounded by a folded deformation zone. This symbol does not show the specific occurrences of eclogite. Sample sites for radiometric age determination (287 samples) and lithogeochemical analysis of magmatic rocks (1020 samples) in the Eastern Segment, based on the respective databases at the Geological Survey of Sweden, are also shown. The radiometric ages in the upper, middle and lower compartments are compiled in Tables 15.1, 15.2 and 15.3, respectively.
Most of the Eastern Segment is exposed in southwestern Sweden, where critical constraints on the tectonothermal evolution of the Sveconorwegian orogeny during the time span 1.0–0.9 Ga are revealed. The Eastern Segment is also unique since it exposes a metamorphic gradient from greenschist to high-pressure (high-P) granulite facies metamorphism that occurred at around 1.0–0.9 Ga in a southwesterly direction across the orogen; a detached slab containing bodies and boudins of eclogite is also present in its southwestern part (Fig. 15.2). The regional geological context of the Eastern Segment was addressed in Chapter 14 (Stephens et al. 2020).
The Eastern Segment is characterized by intense 1.0–0.9 Ga structural and metamorphic reworking of older continental crust (Figs 15.2 & 15.3). Paleoproterozoic (1.7 Ga) magmatic rocks constitute the dominant, pre-Sveconorwegian crustal component. However, magmatic rocks formed at 1.9–1.8 and 1.5–1.4 Ga are also present in the easternmost and southern parts, respectively. Pre-Sveconorwegian metamorphism and deformation in the Eastern Segment were also spatially and temporally associated with magmatic activity (Svecokarelian and Hallandian orogenies, respectively). Dolerites emplaced at 1.57, 1.27 and, together with felsic plutonic rocks, at around 1.2 Ga complete the pre-Sveconorwegian component. Syn-orogenic magmatic activity during the Sveconorwegian orogeny is uncommon, being restricted to the emplacement of late-orogenic c. 0.96–0.93 Ga pegmatitic and granitic dykes. Early Tonian dolerites (c. 0.98–0.95 Ga) are present in the eastern cratonic foreland to the orogen (see Chapter 13, Ripa & Stephens 2020d) as well as in, at least, the frontal part of the Eastern Segment.
Overview of the timing of deposition of supracrustal rocks, emplacement of intrusive rocks, and deformation and metamorphism in the Eastern Segment. The lithological information focuses attention on the character of the protolith and shows only the dominant component in each suite. Dolerites are also present in the magmatic suites emplaced around 1.47–1.44 and 1.2 Ga. The time span for the Svecokarelian orogeny extends back in time to 2.0 Ga in the whole of Fennoscandia, and the time span for the Sveconorwegian orogeny refers to the entire orogen in Norway and Sweden.
Apart from the information published in connection with regional geological mapping studies (Lundegårdh 1995; Wik et al. 2006, 2009; Stephens et al. 2009) and data extracted from various geological and geophysical databases at the Geological Survey of Sweden, the results presented in published scientific papers, addressed specifically below, have been used to compile this overview of the Eastern Segment. In the same manner as in other chapters in this Memoir, the general term ‘deformation zone’ is used for zones showing locally high shear strain. It is consistently used on the geological and geophysical maps, since zones affected by ductile, brittle or combined ductile and brittle deformation have not been distinguished on these maps. Furthermore, both ductile and brittle deformation are present along many of the high-strain shear zones, and solely brittle deformation along others. An evaluation of the complex, polyphase metamorphic and deformational evolution is addressed prior to the presentation of the protolith character of the rocks inside this lithotectonic unit. This chapter is completed with an overview of the mineral and bedrock resources, and some discussion and concluding remarks, including an identification of key issues for future work.
Character of the boundaries to, and the metamorphism and deformation inside, the Eastern Segment
A comprehensive study of the variation in metamorphic grade and style of ductile deformation across the Eastern Segment south of Lake Vänern, identifying different metamorphic zones linked to three different compartments (Möller & Andersson 2018), constitutes the basis for a general subdivision of the entire Eastern Segment using these attributes (Fig. 15.4). This subdivision also corresponds to different patterns on the magnetic anomaly map (Fig. 15.5a). The character of the metamorphism and deformation in the Eastern Segment is addressed compartmentally below. Constraints on the timing of the polyphase orogenic evolution in the Eastern Segment are presented separately in the following section.
Internal subdivision of the Eastern Segment into different compartments based on metamorphic grade and style of ductile deformation. (a) Spatial distribution of the different compartments at the ground surface extracted from the 1:1 million national bedrock database (Bergman et al. 2012) at the Geological Survey of Sweden. Early Tonian dolerites and an outlier of siliciclastic sedimentary rocks are also shown in the eastern cratonic foreland (see Chapter 13, Ripa & Stephens 2020d). Profile lines in (b) and (c) are marked. (b) Schematic WNW–ESE profile across the Eastern Segment immediately north of Vänern. Structural orientation data acquired in the vicinity of the profile line in the western and eastern parts of the upper compartment (see vertical lines connected to stereographic projections) are also shown (modified after Wahlgren et al. 1994). (c) Schematic east–west profile across the Eastern Segment south of Vänern (modified after Möller et al. 2015). Top-to-the-east sense of shear along the Mylonite Zone shown in (b) took place prior to top-to-the-west movement shown in (c). Ductile shear zones in the upper compartment, shown by thicker black lines in both profiles, are drawn schematically without an attempt to correlate with the ductile, brittle–ductile and brittle zones shown on the map in (a). Information on the geometry at depth in the profiles is acquired from reflection seismic data (Juhlin et al. 1989, 2000; Dahl-Jensen et al. 1991; Lundberg & Juhlin 2011).
Geophysical anomaly maps covering the Eastern Segment (darker tone) and its surroundings (paler tone), extracted from geophysical databases at the Geological Survey of Sweden. (a) Magnetic total field after subtraction of the geomagnetic reference field (DGRF 1965.0). (b) Bouguer gravity. Maps generated by Ildikó Antal Lundin (Geological Survey of Sweden).
In order to emphasize their relative geometrical relationship (Fig. 15.4b, c), the three compartments were originally referred to as the upper, middle and lower units (Bergman et al. 2012). Subsequently, they were renamed as the frontal wedge, transitional section and internal section, respectively (Möller et al. 2015). The nomenclature used here maintains a reference to the geometrical aspect. Geological mapping shows that the lower compartment dominates south of Vänern (Figs 15.4 & 15.5), where it is exposed in the core of a major antiformal dome structure plunging gently northwards.
A network of spaced, moderately to steeply dipping deformation zones striking approximately north–south in the upper compartment (frontal wedge) marks the eastern boundary of the Eastern Segment and, by corollary, the entire Sveconorwegian orogen (Gorbatschev 1980; Andréasson & Rodhe 1990; Wahlgren et al. 1994; Wik et al. 2006, 2009). In its eastern part, the network separates the Sveconorwegian orogen from the Blekinge–Bornholm orogen in the south (see Chapter 11, Wahlgren & Stephens 2020b), the Bergslagen and Småland lithotectonic units in the Svecokarelian orogen on both sides of Lake Vättern in the central part (see Chapters 6 (Stephens & Jansson 2020) and 7 (Wahlgren & Stephens 2020a), respectively), and the post-Svecokarelian Paleo- and Mesoproterozoic rocks (see Chapters 9 (Ripa & Stephens 2020a) and 10 (Ripa & Stephens 2020b), respectively) in the north (Fig. 15.2). In its western part, the network cuts across the boundary between the underlying middle and lower compartments (Figs 15.2 & 15.5), where metamorphism during the Sveconorwegian orogeny occurred under high-P and amphibolite or granulite facies conditions. The character of the ductile deformation in the network is presented where the upper compartment is addressed below.
A major west-dipping deformation zone, which, in northern parts, is steeply dipping or subvertical with a northwesterly strike, marks the western boundary of the Eastern Segment, separating it from the Idefjorden terrane (Fig. 15.2). It has traditionally been referred to as the Mylonite Zone (Magnusson 1937; Lindh 1974). The zone has an arcuate shape (Fig. 15.2), which has important impact on the overall kinematic framework. Extensional or transtensional deformation with a top-to-the-west shear component overprinted transpressive, variably developed top-to-the-east and strike-slip shear (Park et al. 1991; Robin & Cruden 1994; Stephens et al. 1996; Berglund 1997; Viola & Henderson 2010; Viola et al. 2011). North of Vänern, this complex zone also cross-cuts the boundary between the middle (to the north) and lower compartments of the Eastern Segment; it is also partly discordant to the large-scale folding in the lower compartment south of Vänern (Figs 15.5a & 15.6a). Furthermore, the structures inside this deformation zone strike sub-parallel to the regional structural grain in the Idefjorden terrane to the west. For these reasons, the character, kinematics and timing of deformation along the western tectonic boundary to the Eastern Segment are addressed in more detail together with the Idefjorden terrane in the following Chapter 16 (Bergström et al. 2020).
Spatial distribution of ductile structures and metamorphic facies inside the Eastern Segment, and their relationship to similar features in surrounding lithotectonic units in the southern part of Sweden. Areas lacking form lines east of the Eastern Segment show little deformation and metamorphism. The Eastern Segment is bounded by thicker black lines in (b) and (c). (a) Form lines showing the trend of the tectonic foliation formed under different metamorphic conditions based on structural field data and the trend of positive magnetic anomalies. (b) Foliation dips. (c) Lineation and fold axis plunges. Information extracted from tectonic and bedrock map databases at the Geological Survey of Sweden.
In general, the form-line pattern for the tectonic foliation inside the Eastern Segment shows an approximately north–south trend in much of the northern and eastern parts (Fig. 15.6a). By contrast, the pattern is strongly disturbed by major folding with axial surface traces trending approximately WNW–ESE and north–south in southwestern parts (Fig. 15.6a). Regional variations in metamorphic grade are also apparent, with an increase towards the SW inside the lithotectonic unit (Möller & Andersson 2018; Fig. 15.6a).
The foliation dips persistently steeply in easternmost areas (upper compartment) and is more variable in dip further west (Fig. 15.6b). Gently plunging lineations, including both mineral lineations and mesoscopic fold axes, dominate inside the Eastern Segment; moderately or steeply plunging lineations being restricted to the easternmost part of the upper compartment (Fig. 15.6c). The cause of the scatter in fold axis orientation, especially in the middle and upper compartments, is not resolved. It is possibly steered by the presence of different generations of folding or to a variable degree of rotation of fold axes into parallelism with the stretching lineation during progressive deformation.
Frontal wedge in the upper compartment including a spaced network of greenschist facies ductile shear zones
As for the entire Eastern Segment, the upper compartment can be followed continuously along strike for more than 600 km from southernmost Sweden in the south into Norway to the north, where it is covered by the thrust sheets belonging to the Caledonide orogen (see the broader context in Chapter 14 (Stephens et al. 2020) in this Memoir). It consists predominantly of Paleoproterozoic (1.9–1.8 and 1.7 Ga) and Mesoproterozoic (1.6, 1.5–1.4 and 1.3–1.2 Ga) magmatic suites reworked during the Sveconorwegian orogeny (see later for the protolith character of these suites).
Ductile deformation that can be ascribed to the Sveconorwegian orogeny in the upper compartment is largely spaced in character, occurring along a network of ductile shear zones that were active under greenschist facies metamorphic conditions. The network of ductile zones corresponds to a banded pattern on the magnetic anomaly map, marked discordances to the magnetic signature in the pre-Sveconorwegian foreland or significant gradients on the Bouguer gravity anomaly map (Fig. 15.5a, b). North of Lake Vättern (Fig. 15.5a), the ductile strain becomes more pervasive to the west, in conformity with a westwards increase in metamorphism to lower amphibolite facies conditions (Fig. 15.6a). South of this lake, the middle compartment is partly cut out and, south of Ljungby, there is an abrupt break between the upper and lower compartments, the latter affected by Sveconorwegian high-P granulite facies metamorphism.
The foliation in the upper compartment strikes predominantly NNW–SSE or north–south and is dominated by either a steep to subvertical or moderate easterly dip (Fig. 15.7a). Mineral stretching lineations vary in plunge down the dip of the foliation surfaces (Fig. 15.7b). There is a large variation in the orientation of mesoscopic, moderately to steeply plunging fold axes, again, down the dip of the foliation (Fig. 15.7c). It is necessary to keep in mind that neither different generations of Sveconorwegian structures nor pre-Sveconorwegian structures have been distinguished in the structural dataset overviewed here (see below for a discussion of the polyphase orogenic evolution).
Equal-area stereographic projections on the lower hemisphere for (a) poles to foliation (N = 7536), (b) mineral lineation orientation (N = 1588) and (c) fold axis orientation (N = 187) using structural data from the upper compartment. Corresponding structural data from the middle compartment are shown in (d) with N = 863, (e) with N = 327 and (f) with N = 37; and from the lower compartment in (g) with N = 5747, (h) with N = 576 and (i) with N = 293. All data have been extracted from the bedrock map database at the Geological Survey of Sweden.
Although the network of spaced ductile shear zones marks the easternmost occurrence of Sveconorwegian ductile strain in the Fennoscandian Shield (Berthelsen 1980; Wahlgren et al. 1994), the ancient pre-Sveconorwegian craton in the eastern foreland to the orogen in central Sweden and southwestern Finland was affected by brittle deformation during the Sveconorwegian orogeny (Drake et al. 2009; Sandström et al. 2009; Viola et al. 2009; Saintot et al. 2011; Mattila & Viola 2014). This deformation occurred either by reactivation of older ductile shear zones or along newly formed faults (Stephens et al. 2015; Skyttä & Torvela 2018). Adularia-quartz mineralization along a calcite–fluorite–sulphide vein deposit in central Sweden has also been interpreted to have formed during a late stage of the Sveconorwegian orogeny (980–950 Ma), prior to younger calcite–fluorite–sulphide mineralization (Saintilan et al. 2017).
North of Vättern
In the Karlskoga–Kristinehamn–Skagern area, north of Vättern (Fig. 15.5a), the upper compartment is c. 40 km wide at the ground surface; two phases of Sveconorwegian ductile shear deformation that took place under similar greenschist facies conditions are present (Wahlgren et al. 1994; Stephens et al. 2009). Zones striking NNW–SSE to north–south that developed earlier in the structural evolution were rotated into a younger set of zones with a more northeasterly strike, the latter referred to as the Sveconorwegian Frontal Deformation Zone in Wahlgren et al. (1994). The overall structural pattern is fan-like (Fig. 15.4b). In eastern parts (Karlskoga), the foliation related to ductile shearing dips westerly and contains a steeply plunging mineral lineation (Figs 15.4b & 15.8a). A vertical or subvertical foliation prevails further west (Fig. 15.8b). In western parts (Kristinehamn), the foliation dips moderately eastwards and contains a gently to moderately, east-northeasterly plunging mineral lineation (Figs 15.4b & 15.8c).
Character of Sveconorwegian ductile strain in the upper compartment (frontal wedge). All photographs from the Karlskoga–Kristinehamn–Skagern area, north of Vättern. Place names on this and all other figures are shown on Figure 15.2. (a) Composite S–C′ fabric showing reverse top-to-the-east movement along retrogressive shear zone in supracrustal rock affected by 1.8 Ga (Svecokarelian) granulite facies metamorphism. S surfaces dip westwards and are rotated into horizontal spaced C′ surfaces. View to the north on vertical surface, Karlskoga. (b) Composite fabric with foliation and shear bands (probably C′-type) showing west-side-up movement in protomylonitic, 1.8 Ga porphyritic granite to quartz monzonite. S surfaces are subvertical; spaced C′ surfaces dip steeply westwards. View to the south on a vertical surface near Karlskoga (after Stephens et al. 2009). (c) Composite fabric with foliation and shear bands (probably C-type) showing normal top-to-the-east movement in protomylonitic 1.7 Ga granite to quartz monzonite. S surfaces are subhorizontal and are rotated into spaced C surfaces dipping eastwards. View to the north on a vertical surface south of Kristinehamn (after Stephens et al. 2009). (d) Dextral rotation of an older foliation (S) into a Sveconorwegian steeply dipping ductile shear zone with a northeasterly strike. View to NNE, near to Skagern (after Stephens et al. 2009). (e) Folding of tectonic foliation in a hinge to a larger Z-asymmetrical, east-verging fold with horizontal axis in deformed 1.7 Ga granite to quartz monzonite. View to the NW on a steep surface close to Kristinehamn. All photographs are by C.-H. Wahlgren, except for (b) which is by M.B. Stephens.
Kinematic indicators including composite planar fabrics (C–S and C′/C–S fabrics) and rotated porphyroclasts show that dip-slip movement with a consistent top-to-the-east sense of shear occurred across the entire fan-like structure, independent of the change in dip or shear-zone set (Fig. 15.8a–c). The older rotated zones show extensional dip-slip shear and the younger set reverse displacement with a conspicuous dextral strike-slip component (Fig. 15.8d). Progressive deformation in the more pervasively deformed western part of the compartment, affected by lower amphibolite facies metamorphism, resulted in the development of asymmetrical, consistently east-verging folding of the foliation and lineation around approximately north–south-trending, subhorizontal fold axes (Fig. 15.8e). These axes are orientated at a high angle to the east-northeasterly plunging mineral lineation (Fig. 15.4b).
By combining the results of various reflection seismic studies from different crustal depths in the area around Lake Vänern (Dahl-Jensen et al. 1991; Juhlin et al. 1989, 2000), it was proposed that the fan-like structure is a major, upper-crustal feature and that the west-dipping reverse dextral zones in the easternmost frontal part of the orogen become listric at depth (Juhlin et al. 2000). The frontal zones were interpreted in previous studies to root westwards in the upper mantle (EUGENO-S Working Group 1988).
South of Vättern
The deformation zone system in the upper compartment south of Vättern, traditionally referred to as the ‘Protogine Zone’ (Gorbatschev 1980; Andréasson & Rodhe 1990), is narrower, being up to c. 25 km in width at the ground surface (Wik et al. 2006, 2009). However, the geometry mimics the fan-like configuration north of Vättern, with a change in the dip of the foliation from steeply west-dipping in the easternmost part of the shear zone system through the vertical into steeply east-dipping in the west (e.g. Wik et al. 2006, 2009; Möller et al. 2015; Fig. 15.4c). The foliation striking north–south along the shear zones (Fig. 15.9a) is commonly also associated with a downdip plunging, steep to subvertical mineral lineation (Andréasson & Rodhe 1990; Wik et al. 2006).
Detailed views of a magnetic anomaly field expressed as a difference between measured and upwards continued data to 600 m to enhance shallow source anomalies. (a) Area in the southern part of the Eastern Segment showing the upper compartment with north–south deformation zones in the east, the eclogite-bearing terrane and polyphase folding in the lower compartment, and faults with a north-northeasterly strike corresponding to narrow and long, low magnetic lineaments. (b) Folded boundary between the lower and middle compartments (LC (IS) and MC (TS), respectively) in the Karlstad area, north of Vänern. The folds belong to the D2 generation and plunge eastwards. Base anomaly map generated by Ildikó Antal Lundin (Geological Survey of Sweden).
Kinematic indicators in the upper compartment south of Vättern show a dominant dip-slip component of movement with top-to-the-east sense of shear (western block-up). However, a dextral component of movement is also indicated by the rotation of the structural grain in the eastern cratonic foreland to the Sveconorwegian orogen into the Eastern Segment (Fig. 15.5a). The rotated structures include late Svecokarelian ductile shear zones with WNW–ESE strike in the Småland lithotectonic unit (cf. Wikman 1998, 2000; Chapter 7, Wahlgren & Stephens 2020a) and the gneissic fabric with similar trend in the Blekinge–Bornholm orogen (cf. Chapter 11, Wahlgren & Stephens 2020b). It is apparent that the greenschist facies Sveconorwegian shear zones in the frontal part of the Eastern Segment overprint and possibly interfere locally with late Svecokarelian zones that formed under similar metamorphic conditions and show similar kinematic characteristics. An interference with Hallandian or late Svecokarelian higher-grade gneissic structures, exposed to the east in the Blekinge–Bornholm orogen (see Chapter 11, Wahlgren & Stephens 2020b), is also apparent along the upper compartment.
Additional complexities in the upper compartment south of Vättern include brittle reactivation of the ductile zones and overprinting brittle zones with similar orientation. These formed in connection with the development of a Neoproterozoic half-graben structure in the Vättern area and later faulting during the Paleozoic (Andréasson & Rodhe 1990; Månsson 1996; see also Chapter 18 (Wickström & Stephens 2020)).
Transitional section in the middle compartment with amphibolite facies metamorphism, pervasive ductile strain and development of orthogneiss
The middle compartment of the Eastern Segment was affected by pervasive deformation and amphibolite facies metamorphism without anatexis (e.g. Wahlgren et al. 1994; Söderlund et al. 1999; Möller & Andersson 2018), and has its largest extent north of Vänern (Fig. 15.4a). The middle compartment is partly cut out southwards (e.g. south of Ljungby: see Fig. 15.4a), so that the upper and lower compartments, affected by markedly different pressure–temperature (P–T) conditions of Sveconorwegian metamorphism, lie directly adjacent to and are detached from each other. The boundaries of the middle compartment to the other two compartments are more transitional in character in the areas where they are exposed (Fig. 15.4a).
South of Vänern, between Värnamo and Varberg (Fig. 15.5a), the boundary between the middle and upper compartments is marked most conspicuously by a change in the style of deformation. Subvertical deformation zones striking north–south in the upper compartment to the east merge into a gneissic foliation with variable strike, due to the effects of regional-scale folding, in the middle compartment to the west; the changeover takes place over a few kilometres (Möller et al. 2015). The boundary is marked also by the change to middle amphibolite facies metamorphic conditions in the middle compartment, assemblages consisting of hornblende and garnet being present in mafic varieties (Möller & Andersson 2018); magmatic textures are only locally preserved in these rocks (Beckman & Möller 2018). Further west, the appearance of leucosome in both felsic and mafic rocks over a distance of 1 km marks the changeover to the internal section of the Eastern Segment in the lower compartment (Möller & Andersson 2018).
In the area around Kristinehamn, north of Vänern (Fig. 15.5a), the boundary to the upper compartment is gradational over a distance of a few kilometres (Wahlgren et al. 1994). It is marked by a change from semi-pervasive ductile deformation under lower amphibolite facies in the east, to pervasive gneissic fabric development and metamorphic recrystallization under middle amphibolite facies in 1.7 Ga felsic intrusive rocks further west (Fig. 15.10a); 1.6 Ga dolerites and gabbros show variable conversion to amphibolite, sometimes with garnet (Fig. 15.10b). Approximately 30 km west of Kristinehamn, north and west of Karlstad, the middle compartment merges transitionally into the lower compartment in the hinge zone of a major synform (Fig. 15.9b and see below). The lower compartment is distinguished by the frequent appearance of granitic leucosome in felsic rocks and the abundance of migmatitic amphibolite.
Character of ductile structures inside the middle compartment (transitional section), west of Kristinehamn. (a) Pervasively gneissic 1.7 Ga granite with fabric defined mainly by orientated biotite grains. (b) Garnet amphibolite derived from a 1.6 Ga mafic protolith. (c) Folded (D2) granitic vein (red line) concordant with the gneissic structure (S1) in 1.7 Ga granite. View to the NNE on the top surface. (d) D2 folds deforming veins in amphibolite derived from a 1.6 Ga mafic protolith. View to the NE on a steep surface. All photographs are by M.B. Stephens, except for (a) which is by M. Ahl.
Geothermobarometric estimates of metamorphic conditions of a garnet amphibolite in the middle compartment, south of Vättern (Fig. 15.5a), yielded pressures at 10–12 kbar and temperatures of 600–630°C (Söderlund et al. 2004). The pressure estimates correspond to those calculated for the high-P granulite facies metamorphism in the lower compartment, but the temperature of metamorphism was significantly lower in the middle compartment (cf. text below). Published P–T data further north in the entire Eastern Segment are lacking.
The gneissic foliation in the middle compartment shows a variable orientation; a moderately to gently eastwards dipping foliation is dominant (Fig. 15.7d). Mineral lineations also show a variable orientation with a dominant set plunging gently east-northeastwards, more or less down the dip of the foliation (Fig. 15.7e). There are relatively few measurements and considerable variation in the orientation of fold axes in the databank used here. The axes plunge moderately to gently eastwards, but scatter in the two easterly quadrants on a lower-hemisphere stereographic projection (Fig. 15.7f). Focus below is placed on the character and evolution of the mesocopic structures north of Vänern, in the area where the middle compartment has its widest areal extension, and, in particular, in the area between Karlstad and Kristinehamn (Fig. 15.5a).
A gneissic foliation with gentle northerly dip that formed during the earliest deformation phase (D1) is a prominent feature in the 1.7 Ga felsic orthogneisses in the Karlstad–Kristinehamn area. Mineral lineations and measured fold axes are gently plunging but vary considerably in azimuth direction mainly inside the northeasterly and westerly quadrants. Variably inclined folds (D2) deform the gneissosity (Fig. 15.10c) as well as felsic veins in amphibolite derived from a 1.6 Ga doleritic or gabbroic protolith (Fig. 15.10d). An inclined D2 major antiform–synform combination, plunging gently eastwards and verging southwards, is inferred by integrating field data with an inspection of the pattern on the total magnetic field anomaly map, most of the antiform being concealed beneath Vänern; these folds rotate the boundary between the middle and lower compartments (Fig. 15.9b). The broad warping of the D2 structures by open, upright folds with axial surface traces trending approximately north–south (Fig. 15.9b) defines a D3 ductile phase.
Regional-scale brittle deformation zones, including faults, transect the ductile structural grain in the middle compartment and constitute the youngest structures. North of Vänern, these zones correspond to narrow low-magnetic linear anomalies with northerly or north-northwesterly trend on the magnetic anomaly map (Fig. 15.9b). The pronounced fault passing through and continuing north of Kristinehamn (Fig. 15.9b), close to the boundary between the middle and upper compartments, continues northwards inside the Eastern Segment into Norway, where it displaces the basal thrusts in the frontal part of the Caledonide orogen (Solli & Nordgulen 2008).
Internal section in the lower compartment including detached slab with eclogites
The lower compartment, corresponding to the internal section of the Eastern Segment, is mostly exposed south of Vänern (Fig. 15.4a). Only a narrow strip along the northern shoreline of this lake and a volume directly beneath the Mylonite Zone west of Karlstad are present north of the lake.
The lower compartment is dominated by migmatitic felsic orthogneisses (Fig. 15.11a), derived from predominantly 1.7 Ga and subordinate 1.4 and 1.2 Ga protoliths (see also later in this chapter). The orthogneisses enclose disrupted bodies of metamorphosed mafic rocks of variable size. Virtually all our understanding of the metamorphic and deformational character of the lower compartment emanates from detailed studies in its southwestern part, between Borås and the coastal area between Halmstad and Varberg (Fig. 15.5a). For this reason, focus is addressed below on metamorphic and deformational aspects there. Some comments on the style of deformation in the subordinate occurrence north of Vänern complete this section.
Character of ductile structures inside the lower compartment (internal section) south of Vänern. (a) Stromatic migmatite with granodioritic or granitic composition. Photographer: C. Möller. (b) Retro-eclogite (above pen) in contact with mylonitic gneiss (beneath pen) along the basal shear zone to the eclogite-bearing terrane. Photographer: M.B. Stephens. (c) Kyanite- and garnet-rich vein in kyanite-bearing eclogite along the basal shear zone to the eclogite-bearing terrane (after Möller et al. 2015). (d) Metagabbro with magmatic texture overprinted by granulite facies metamorphism. View on top surface (after Wik et al. 2009). (e) Garnet amphibolite with leucosome veins of quartz and plagioclase. View on top surface. Photographer: C. Möller. (f) D2 folding of mylonitic gneissic structure (S1) along the basal shear zone to the eclogite-bearing terrane showing a southerly vergence. Note the increased ductile strain along the fold limbs. View at a high angle to the easterly plunging fold axis and mineral stretching lineation. Photographer: M.B. Stephens.
South of Vänern: metamorphic aspects
The lower compartment is distinguishable from the remainder of the Eastern Segment by the occurrence of Sveconorwegian high-P granulite facies metamorphism and migmatization of both felsic and mafic rocks (e.g. Johansson et al. 1991; Möller et al. 2007; Hansen et al. 2015; Möller & Andersson 2018). In its southwestern part, eclogite bodies and smaller boudins hosted by migmatitic felsic orthogneiss occur in a separate metamorphic terrane (Möller 1998, 1999; Austin Hegardt et al. 2005; Möller et al. 2015; Möller & Andersson 2018; see Fig. 15.5a). This terrane tectonically overlies orthogneisses to the south along a northerly dipping, up to 4 km-wide, ductile shear zone consisting of mylonitic orthogneisses and retro-eclogite boudins (Möller et al. 1997; Lundberg & Juhlin 2011; Tual et al. 2015; Fig. 15.11b, c).
A comparison of metamorphic assemblages in Fe–Ti-rich metamorphosed gabbro along a c. 120 km-long transect defines a metamorphic zoning inside and east of the lower compartment (Möller & Andersson 2018). The mafic rocks in this compartment typically occur as garnet amphibolite or high-P mafic granulite, and are variably migmatitic (Fig. 15.11d, e); relicts of primary igneous textures are uncommon. Pervasively amphibolitized mafic rocks without metamorphic pyroxene or garnet also occur within the internal migmatitic parts of the eclogite-bearing terrane (Möller et al. 2015).
The high-P metamorphic rocks in the southwestern part of the compartment coincide with a positive Bouguer gravity anomaly (Fig. 15.5b) indicating an excess of mass at depth. However, similar positive anomalies also extend over surrounding areas: for example, in the Blekinge–Bornholm orogen (see Chapter 11, Wahlgren & Stephens 2020b), where magmatic suites containing mafic rocks constitute a subordinate exposed component inside various lithotectonic units. For this reason, there remains uncertainty to what extent the anomaly is steered by the high-P mafic granulites exposed in the Eastern Segment or by one or more of these magmatic suites occurring in larger volumes at deeper crustal levels.
Geothermobarometry of relict high-P mineral assemblages in eclogites hosted by strongly deformed stromatic orthogneiss indicates metamorphic conditions with pressures exceeding 15 kbar (Möller 1998, 1999; Austin Hegardt et al. 2005; Möller et al. 2015). Recent petrological models estimate the peak metamorphic conditions at c. 870°C and 18 kbar (i.e. at more than 60 km crustal depth: Tual et al. 2018). High-P granulite to upper amphibolite facies metamorphism with anatexis in the Eastern Segment took place after the tectonic emplacement of the eclogite-bearing terrane (Andersson et al. 1999, 2002; Söderlund et al. 2002; Möller et al. 2007, 2015; Hansen et al. 2015; Tual et al. 2017, 2018). Geothermobarometry applied to high-P granulite facies mafic rocks reported temperatures in the range 680–800°C and pressures of 8–12 kbar (Johansson et al. 1991; Wang & Lindh 1996; Möller 1998; Austin Hegardt et al. 2005; Piñán Llamas et al. 2015).
South of Vänern: deformational aspects
The form-line pattern of the ductile planar structures in the lower compartment defines the characteristic regional-scale folding of the migmatitic gneissic foliation (Fig. 15.6a); this folding is also elegantly expressed in the magnetic anomaly map (see, for example, NE of Svenljunga on Fig. 15.9a). Although there is considerable variation in the orientation data, the foliation on a regional scale generally dips gently northwards (Fig. 15.7g). Both mineral stretching lineations and measured fold axes commonly plunge west-northwestwards (Fig. 15.7h, i); a subordinate set of fold axes plunges gently in a northeasterly direction.
The systematic orientation pattern, and the sub-parallel trend of mineral lineations and fold axes are in sharp contrast to the more varied orientation of structures in the remainder of the Eastern Segment (cf. Fig. 15.7a–f). This contrast coincides with the higher grade of metamorphism in the lower compartment. Strain intensity varies in this compartment, being particularly high in sheared fold limbs (Fig. 15.11f). The deformation fabrics in the migmatitic gneisses are generally characterized by strongly constrictional strain (see examples discussed below). L–S- or S-tectonites are mainly found in discrete deformation zones or in strongly stretched fold limbs of large-scale folds.
The polyphase Sveconorwegian structural evolution (D1–D4) has been established by studies inside the eclogite-bearing terrane, along its boundary ductile shear zone and in the orthogneisses along the coast c. 60 km south of this zone (Möller et al. 2015; Piñán Llamas et al. 2015; Tual et al. 2015). The D1 deformation phase was responsible for the emplacement of the eclogite-bearing terrane from crustal depths at more than 60 km to around 35–40 km, and the formation of a gneissic layering and foliation in the surrounding orthogneisses (S1). This exhumation took place under high-P granulite to upper amphibolite facies conditions. The D2 event is characterized by tight to isoclinal folding of the gneissic fabric (S1) along subhorizontal to gently east- or west-plunging axes (Fig. 15.11f) and the development of an S2 foliation. The upper amphibolite facies D2 fabrics are stretched along the fold axes with the development of a conspicuous sub-parallel stretching lineation.
The D1 and D2 structures were subsequently refolded by open to gentle, inclined folds with variable orientation (D3). The latest ductile deformation phase (D4) involved regional-scale, open, upright folding with axial surface traces and axes trending north–south. According to Möller et al. (2015), distinction between the folds related to the D3 and D4 phases is locally difficult. Leucosome veins are present along the axial surfaces of the D2, D3 and D4 folds (Piñán Llamas et al. 2015).
The ductile structures in the lower compartment are cross-cut by a prominent set of subvertical faults striking NNE–SSW. These structures displace both the boundaries between rock units and positive magnetic anomalies; they also correspond to narrow, low-magnetic linear anomalies that can be followed for several tens of kilometres (Fig. 15.9a). The faults are in places spatially associated with quartz infillings and reddish Fe oxide alteration of feldspars in the host rock. The western boundary of the Lower Paleozoic sedimentary cover succession east of Skara (Fig. 15.2) is steered by a fault belonging to this set and shows a west-side-up sense of movement.
North of Vänern
Studies of the metamorphic character of the migmatitic rocks in the lower compartment north of Vänern, including geothermobarometric work, are lacking. Only some remarks on the ductile structural evolution inside the limited occurrence of this compartment north of Vänern (Fig. 15.9b) are presented below.
Migmatitic felsic orthogneisses and amphibolites, defining a gneissic banding structure (D1), are exposed along the northern limb of the major D2 antiform mostly concealed beneath Vänern (see the text above addressing the middle compartment). These rocks can be traced around the westernmost envelope of a complementary D2 synform with an east–west axial surface trace on land to the north (Fig. 15.9b). Recumbent D2 folding of the D1 gneissic structure comprises parasitic folds on the northern limb of the synform, north of Karlstad (Fig. 15.12a). In contrast, intense ductile strain along the southern limb, referred to as the Hammarö Shear Zone (Berglund et al. 1997; Viola et al. 2011), can be traced in an east-northeasterly direction for more than 30 km along strike in the archipelago along the northern shoreline of Vänern (Fig. 15.9b).
Character of ductile structures inside the lower compartment (internal section) immediately north of Vänern. (a) Recumbent D2 folds on the northern limb of a major D2 synform, rotating S1 gneissosity in migmatitic felsic orthogneiss with bands of amphibolite. Gneissic structures and mineral stretching lineations dip or plunge, respectively, westwards; the fold axes plunge gently north-northwestwards. View to the west, close to Karlstad airport. (b) Gently inclined D2 folding of banding in a mylonitic orthogneiss on the southern limb of the synform discussed in (a) and referred to as the Hammarö Shear Zone (Berglund et al. 1997; Viola et al. 2011). The rodded constrictional fabric parallel to the fold axis (parallel to pen) plunges gently eastwards. View to the east along the northern shoreline to Vänern, west of Kristinehamn. (c) Mineral stretching lineation (parallel to pen) on gneissosity deformed by upright D2 folds plunging gently eastwards. View to the east on the top surface along the northern shoreline to Vänern, SW of Karlstad. (d) Amphibolite and mylonitic orthogneiss affected by recumbent D2 folds plunging gently eastwards (close to hammer). The mineral stretching lineation parallel to these folds is rotated around D3 upright folds plunging gently northwards (upper part of photograph). Location along northern shoreline of Vänern, ESE of Karlstad. All photographs by M.B. Stephens.
The orthogneisses and amphibolites along the southern sheared limb of the D2 synform are strongly lineated and rodded (Fig. 5.12b), constrictional strain with sub-parallel mineral stretching lineation and fold axes in an easterly direction being inferred (Viola et al. 2011). Mesoscopic folds along the Hammarö Shear Zone are variably inclined, plunge gently eastwards and rotate the mylonitic gneissic foliation and, locally, even a mineral stretching lineation (Fig. 15.12b, c). The shear zone cannot be traced northwards around the synformal structure. Intense D2 ductile strain, with progressive development of the mylonitic fabric under constrictional conditions and continued folding, is envisaged along this fold limb. There is a strong resemblance to the D2 structures observed on a mesoscopic scale south of Vänern (cf. Fig. 15.11f).
Refolding by open to close, upright folds with axial surface traces trending north–south on both regional (Fig. 15.9b) and mesoscopic (Fig. 15.12d) scales constitute the D3 ductile phase. Such structures can be correlated with the D4 folds south of Vänern. The similarity between the polyphase ductile structural evolution in the northern and southern parts of the lower compartment gains further support.
Timing of metamorphism and ductile deformation in different orogenic systems, and later faulting
Radiometric age data bearing on the timing of metamorphism using U–Pb secondary ion mass spectrometry (SIMS) dating of complex zircon grains are a key source of information, especially in the migmatitic rocks in the part of the lower compartment south of Vänern. Much of the previous work in this compartment primarily involved determination of the timing of emplacement of magmatic rocks using the U–Pb thermal ionization mass spectrometry (TIMS) technique, combined with field studies addressing the spatial relationship between their emplacement and ductile deformation in the migmatitic orthogneisses.
The radiometric age data from the Eastern Segment are compiled separately for the upper, middle and lower compartments in Tables 15.1, 15.2 and 15.3, respectively. Sample sites for the age determination work are shown in Figure 15.2. Visual summaries in Figures 15.13a (combined upper and middle compartments) and 15.13b (lower compartment) show the timing of crystallization of the magmatic rocks, and, of more interest for the discussion in this section, the timing of metamorphism, deformation and later cooling as a basis for the identification of different orogenic events. Evidence for a complex, polyphase orogenic evolution inside the Eastern Segment is apparent (Fig. 15.13).
Summary of radiometric age determinations providing constraints on the timing of the crystallization of magmatic rocks, and the timing of metamorphism, deformation and later cooling. (a) Combined upper and middle compartments. (b) Lower compartment. 40Ar–39Ar ages around or older than 1.0 Ga need to be viewed with care due to probable excess Ar components in the samples. Data extracted from Tables 15.1. 15.2 and 15.3. Time spans for the different tectonic events shown at the top of each figure correspond to those shown in Figure 15.3.
Radiometric age data in the upper compartment of the Eastern Segment modified after the geochronological database at the Geological Survey of Sweden
Radiometric age data in the middle compartment of the Eastern Segment modified after the geochronological database at the Geological Survey of Sweden
Radiometric age data in the lower compartment of the Eastern Segment modified after the geochronological database at the Geological Survey of Sweden
There remains some uncertainty concerning to what extent several of the U–Pb (titanite) and 40Ar–39Ar (hornblende) ages should be interpreted as metamorphic, deformation or later cooling ages. Furthermore, since 40Ar–39Ar (hornblende) ages of around 1.0 Ga overlap with or are even older than the U–Pb (zircon) metamorphic ages in the lower compartment (Fig. 15.13b), there are probably excess Ar components in these samples. For this reason, these ages need to be handled with care. Excess Ar has also been proposed for samples in the upper compartment yielding 40Ar–39Ar (biotite) plateau ages of 1.1–1.0 Ga (Ulmius et al. 2018; Fig. 15.13a).
Historical overview
Larson et al. (1986) presented a ductile structural evolution for the lower compartment of the Eastern Segment in the Ulricehamn–Borås area, interpreting earlier deformation phases to be older than 1.7 Ga, the east–west folding (D2 in the terminology presented above) to be older than 1.4 Ga and the Sveconorwegian orogenic overprint to be restricted to later phases of deformation. On the basis of field studies in the lower compartment combined with U–Pb (zircon) TIMS geochronology, pre-Sveconorwegian tectonothermal events at 1.7–1.6 Ga (referred to as Gothian) and around 1.4 Ga were inferred (Connelly et al. 1996; Christoffel et al. 1999). Considerable debate arose later when field studies combined with U–Pb (zircon) SIMS geochronology emphasized instead the regional significance of the Sveconorwegian orogeny for formation of the gneissic structure in the Eastern Segment (Andersson et al. 1999, 2001; Larson 2001; and other publications referred to below).
The current radiometric age data bearing on the timing of metamorphism and deformation in the Eastern Segment indicate that there are relicts of a Paleoproterozoic (1.9–1.8 Ga) tectonothermal event in its easternmost part (Svecokarelian orogeny), and Mesoproterozoic (1.5–1.4 Ga) metamorphism and gneissic fabric development (Hallandian orogeny) in its southern part (Figs 15.3 & 15.13). However, much of the D1 gneissic fabric development, the D2 east–west folding and the anatexis inside the Eastern Segment occurred at c. 0.99–0.95 Ga (see below), overlapping partly with the Falkenberg phase (Bingen et al. 2008) of the Sveconorwegian orogeny. Each of these three orogenic events is addressed below. Apart from contact metamorphism in the thermal aureoles of some 1.7 Ga plutons in the upper compartment, evidence for regionally significant metamorphism and deformation at 1.7–1.6 Ga (Gothian) is lacking. However, this may simply reflect the level of erosion currently exposed at the ground surface.
Relicts of Paleoproterozoic (1.9–1.8 Ga) deformation and metamorphism (Svecokarelian orogeny) in the upper compartment
The 1.9–1.8 Ga supracrustal and plutonic rocks constitute a major crustal component along the easternmost frontal part of the Eastern Segment in the upper compartment (Fig. 15.2). This crustal material can be traced eastwards into the cratonic foreland of the Sveconorwegian orogen, where polyphase deformation under greenschist, amphibolite or, locally, even granulite facies and low-P (≤5 kbar) metamorphic conditions at 1.9–1.8 Ga affected these rocks (Svecokarelian orogeny; see Chapters 6 (Stephens & Jansson 2020) and 7 (Wahlgren & Stephens 2020a)). Sveconorwegian ductile shear zones active under greenschist facies conditions transposed and reorientated, to a variable extent, the older structures, thereby motivating the inclusion of this crustal segment in the younger orogenic system. Supracrustal and plutonic rocks affected by 1.8 Ga granulite facies and low-P metamorphism, prior to Sveconorwegian greenschist facies retrogression (Andersson et al. 1992, 2006; Stephens et al. 1993; Wikström & Larsson 1993; Andersson 1997), are conspicuous in the Karlskoga area (Figs 15.2 & 15.8a).
The character of the deformation and metamorphism during the 1.9–1.8 Ga orogeny in the Sveconorwegian cratonic foreland was addressed in the part of this Memoir dealing with the Svecokarelian orogen. The reader is referred to Chapters 6 (Stephens & Jansson 2020) and 7 (Wahlgren & Stephens 2020a) for more details concerning this phase of the pre-Sveconorwegian orogenic evolution inside the Sveconorwegian orogen. Switching between retreating and advancing subduction systems during three separate tectonic cycles along a convergent, active continental plate margin has been inferred (i.e. an entirely accretionary orogenic setting).
Relicts of Mesoproterozoic (1.5–1.4 Ga) metamorphism (Hallandian orogeny)
Relicts of pre-Sveconorwegian high-grade metamorphism in the 1.47–1.38 Ga time interval, overprinted by Sveconorwegian tectonothermal processes, occur in all three compartments of the Eastern Segment south of Vänern. Although not well understood, this metamorphic event (or events) has been related to crustal-scale reworking and concomitant magmatic activity attributed to orogenesis (Hallandian) in an accretionary tectonic setting (Hubbard 1975; Christoffel et al. 1999; Söderlund et al. 2002; Möller et al. 2007; Brander et al. 2012; Ulmius et al. 2015). We follow here the time interval generally used for the Hallandian orogeny of 1.47–1.38 Ga (e.g. Möller et al. 2007; Bingen et al. 2008; Ulmius et al. 2015).
Crust formed or reworked during this orogeny but unaffected by younger orogenic processes occur further east in southeasternmost Sweden (Blekinge–Bornholm orogen; see Chapter 11 (Wahlgren & Stephens 2020b)). Thus, it is apparent that the volume affected by Hallandian orogenesis is significant in the southern part of Sweden, both inside and east of the Sveconorwegian orogen (Fig. 15.14). Records of 1.5–1.4 Ga high-grade metamorphism and deformation are absent north of Vänern in the Eastern Segment (Söderlund et al. 1999), as well as in the entire Idefjorden terrane west of the Mylonite Zone (see Chapter 16, Bergström et al. 2020). Furthermore, the pervasiveness of Hallandian metamorphism and structures south of Vänern is unclear.
Approximate extent of the area inferred to be affected by the Hallandian (1.47–1.38 Ga) orogeny in Sweden. This tectonic development is the younger of the two orogenic components in the Blekinge–Bornholm orogen in southeasternmost Sweden and a pre-Sveconorwegian orogenic component in the southern part of the Eastern Segment in southwestern Sweden. Information was extracted from the tectonic map database at the Geological Survey of Sweden.
Where Hallandian metamorphism has been confirmed in the Eastern Segment, it is characterized by migmatization; it is also spatially and temporally associated with magmatic activity (Söderlund et al. 2002; Möller et al. 2007; Brander & Söderlund 2009; Brander et al. 2012; Ulmius et al. 2015). Granulite facies (700–750°C) and low-P (4–5 kbar) metamorphism affected an aluminous gneiss in the southernmost part of the upper compartment at around 1.45 Ga; a spatially associated granite yielded an emplacement age of c. 1.45 Ga (Ulmius et al. 2015). Similar metamorphic conditions dated at c. 1.44 Ga were obtained for a paragneiss in the Blekinge–Bornholm orogen to the east (Bogdanova et al. 2014; see Chapter 11 (Wahlgren & Stephens 2020b) in this Memoir).
The timing of the pre-Sveconorwegian amphibolite facies metamorphism in the orthogneisses inside the upper compartment between Kristianstad and Värnamo is uncertain. This uncertainty matches the same question around the timing of tectonothermal activity inside the Blekinge–Bornholm orogen in the cratonic foreland to the Sveconorwegian orogen. Hallandian (c. 1.47–1.43 Ga) accretionary activity took place in the Blekinge–Bornholm orogen, but the significance of an earlier event during a late stage of the Svecokarelian orogeny (c. 1.77–1.75 Ga) is not fully resolved (see Chapter 11, Wahlgren & Stephens 2020b). Further work is required to resolve this issue even inside the eastern frontal part of the Sveconorwegian orogen.
In the southwestern part of the lower compartment in the Eastern Segment, a younger c. 1.40 Ga phase of Hallandian migmatization was accompanied by the concomitant intrusion of a granite–norite–charnockite magmatic suite, as well as a widespread injection of pegmatitic and granitic dykes (Hubbard 1989; Åhäll et al. 1997; Andersson et al. 1999; Christoffel et al. 1999; Harlov et al. 2006; Rimša et al. 2007; Petersson et al. 2015; Piñán Llamas et al. 2015). The Hallandian migmatitic structures are typically strongly reworked and transposed by Sveconorwegian ductile deformation that also took place under high-grade metamorphic conditions (Fig. 15.15).
Upright to moderately overturned folds deforming leucosome veins formed during Hallandian (1.47–1.38 Ga) migmatization. The folds plunge gently south-southwesterly, contain a high-grade axial surface foliation and are inferred to have formed during the Sveconorwegian orogenic reworking (Möller et al. 2007). View to the north on a vertical surface perpendicular to the fold axis. The coin (25 mm) is for scale. Photographer: C. Möller (after Möller et al. 2007).
Timing of ductile deformation and metamorphism during the Sveconorwegian orogeny
Early-stage crustal convergence, eclogite exhumation and anatexis (c. 0.99–0.95 Ga)
The earliest record of Sveconorwegian orogeny in the Eastern Segment is the high-P metamorphism of mafic rocks under eclogite facies conditions within the large-scale eclogite-bearing terrane in the southwestern part of the lower compartment (Möller et al. 2015; Tual et al. 2015). Metamorphic zircon ages constrain the timing of this metamorphism to c. 0.99–0.98 Ga (Möller et al. 2015; Fig. 15.13b). During subsequent exhumation, the eclogites were metamorphosed under high-P granulite to upper amphibolite facies conditions; anatexis took place at c. 0.98–0.95 Ga both in the orthogneisses hosting the eclogites and in the orthogneisses outside the eclogite-bearing terrane (Andersson et al. 2002; Söderlund et al. 2002; Austin Hegardt et al. 2005; Möller et al. 2007, 2015, Hansen et al. 2015; Piñán Llamas et al. 2015) (Fig. 15.13b).
Pegmatitic and granitic dykes, emplaced in the time interval c. 0.96–0.93 Ga, cut at a high angle of discordance both the D1 gneissosity and the D2 folds (Möller & Söderlund 1997; Andersson et al. 1999; Christoffel et al. 1999; Söderlund et al. 2002, 2008; Möller et al. 2007; Piñán Llamas et al. 2015; Fig. 15.13b). Piñán Llamas et al. (2015) concluded that the D2 (and later D3) folding in the southwestern part of the lower compartment took place at c. 0.97–0.95 Ga. Constraints on the timing of the D4 upright folding are lacking.
U–Pb (monazite) and U–Pb (titanite) metamorphic or cooling ages in the southwestern part of the lower compartment fall in the time span c. 0.96–0.93 Ga (Connelly et al. 1996; Christoffel et al. 1999; Johansson et al. 2001; Fig. 15.13b). Ignoring 40Ar–39Ar (hornblende) ages from samples with probable excess Ar components, it is apparent that the lower compartment south of Vänern had exhumed sufficiently to be affected by greenschist facies metamorphism during the time span c. 0.95–0.90 Ga (Page et al. 1996a; Wang et al. 1996; Möller et al. 2015; Ulmius et al. 2018; Fig. 15.13b). A slow rate of cooling (c. 3°C Ma−1) from peak metamorphic temperatures at c. 0.98–0.95 Ga until around 0.90 Ga was inferred in the southernmost part of the Eastern Segment (Ulmius et al. 2018).
The timing of metamorphism and deformation north of Vänern is poorly constrained. Metamorphic syn-deformational titanites in orthogneisses in the middle compartment have yielded U–Pb ages of c. 0.98–0.96 Ga (Söderlund et al. 1999), older than titanite ages in the lower compartment to the south (cf. Fig. 15.13a, b). 40Ar–39Ar (hornblende) ages from the same compartment north of Vänern fall entirely in the time span c. 1.01–0.98 Ga (Page et al. 1996b; Fig. 15.13a). Further data are required, not least to test, for example, whether there is a diachroneity in the timing of metamorphism and deformation, and not only cooling, from north to south inside the Eastern Segment, with younger ages linked to later exhumation of this lithotectonic unit southwards.
Late-stage deformation along the frontal part of the orogen after c. 0.95 Ga
North of Lake Vättern (Fig. 15.2), dolerites with a north-northwesterly strike were displaced by the younger set of ductile shear zones with a northeasterly strike in the upper compartment of the Eastern Segment (Wahlgren et al. 1994). These dolerites form part of a regionally significant c. 0.98–0.95 Ga dyke swarm in the cratonic foreland to the Sveconorwegian orogen (see Chapter 13, Ripa & Stephens 2020d). Muscovite defining the mylonitic to phyllonitic foliation along three shear zones in this area has yielded 40Ar–39Ar plateau ages of c. 0.93–0.91 Ga (Andréasson & Dallmeyer 1995; Page et al. 1996b; Fig. 15.13a). It is apparent that ductile shearing at temperatures >350°C occurred along the easternmost frontal part of the orogen after emplacement of the c. 0.98–0.95 Ga dyke swarm, and prior to or during the time span c. 0.93–0.91 Ga.
Further south, in the vicinity of Vättern (Fig. 15.2), 40Ar–39Ar dating of muscovite along retrogressive ductile shear zones in the upper compartment show that temperatures of more than c. 350°C were maintained until at least c. 0.94 Ga (Andréasson & Dallmeyer 1995; Fig. 15.13a). By contrast, in the fault-controlled inlier of the upper compartment in the southernmost part of the orogen (Romeleåsen horst), 40Ar–39Ar (muscovite, biotite) ages fall in the time span c. 0.91–0.88 Ga (Ulmius et al. 2018; Fig. 15.13a). Ulmius et al. (2018) inferred a significant rate of cooling (c. 26°C Ma−1) through greenschist facies conditions at around 0.90 Ga. Similar 40Ar–39Ar mica ages, interpreted to be related to an overall transtensional regime, have also been documented in the southern part of the Mylonite Zone (Viola et al. 2011; see Chapter 16 (Bergström et al. 2020) in this Memoir). Once again, when comparing similar compartments, there is a possibility that ages reflecting deformation, metamorphism or later cooling in the Eastern Segment are younger southwards.
Timing of post-Sveconorwegian brittle deformation
The timing of post-Sveconorwegian brittle deformation is poorly constrained. Faulting during Silurian or post-Silurian time is indicated by the fault-bounded contacts between Lower Paleozoic sedimentary cover rocks and the underlying orthogneisses in the Eastern Segment (Fig. 15.2). Since the major fault passing through and continuing north of Kristinehamn with a north-northwesterly strike (Fig. 15.9b) offsets the frontal thrusts in the Caledonide orogen in Norway (Solli & Nordgulen 2008), faulting along this structure occurred during the later stages of or after the Caledonian orogeny (i.e. Silurian or later). Furthermore, on the basis of studies of the system of faults with a northwesterly strike in southernmost Sweden, referred to as the Sorgenfrei–Tornquist Zone (Fig. 15.2), movement along faults with similar orientation northwards in the Eastern Segment may have occurred during the Permian and later during the Mesozoic and Paleogene. Fault tectonics and sedimentation during Neoproterozoic and Phanerozoic time in Sweden is addressed in more detail in several of the following chapters in this Memoir.
Character and timing of crystallization of protoliths in the pre-Sveconorwegian crust
The protoliths to the variably metamorphosed, pre-Sveconorwegian crust in the Eastern Segment are composed of different suites of Paleo- and Mesoproterozoic magmatic rocks. Sample locations for lithogeochemical data from these suites, extracted from the corresponding database at the Geological Survey of Sweden, are shown in Figure 15.2. These data complement similar information in published material and are addressed here with the aim of constraining the composition and tectonic affinity of the different magmatic suites. Only lithogeochemical data from samples in the Geological Survey of Sweden database that could be assigned with confidence to a particular lithological age group are shown in Figure 15.2. Furthermore, only data from the dominant 1.7 Ga magmatic suite and some of the younger suites are discussed in more detail and illustrated below.
As indicated above, the timing of crystallization of the different magmatic rocks inside the Eastern Segment (Figs 15.2 & 15.3), based mainly on U–Pb (zircon) but also some subordinate U–Pb (baddeleyite, monazite, titanite, columbite) and Pb–Pb (zircon) evaporation data, are tabulated for each compartment in Tables 15.1. 15.2 and 15.3, respectively. Visual summaries in Figure 15.13a (combined upper and middle compartments) and 15.13b (lower compartment) provide the basis for the identification of different magmatic suites discussed below.
Paleoproterozoic (1.9–1.8 Ga) magmatic suites in the upper compartment
Paleoproterozic (1.9–1.8 Ga) rocks in the Eastern Segment are restricted entirely to its upper compartment (Fig. 15.2), where they are affected by the network of Sveconorwegian greenschist facies shear zones (see above). These rocks can be mapped continuously eastwards into the cratonic foreland of the Sveconorwegian orogen, where they have been included in different lithotectonic units inside the Svecokarelian (2.0–1.8 Ga) and Blekinge–Bornholm (1.5–1.4 Ga) orogens. As discussed earlier, the rotation of these units into the upper compartment of the Eastern Segment is consistent with a dextral strike-slip component of displacement along the Sveconorwegian shear zones.
North of Vättern (Fig. 15.2), supracrustal rocks (c. 1.91–1.87 Ga), intruded by c. 1.91–1.87, c. 1.87–1.84 and c. 1.81–1.77 Ga plutonic suites (Fig. 15.3), can be mapped (Stephens et al. 2009) into the cratonic foreland of the Sveconorwegian orogen (Bergslagen lithotectonic unit, Svecokarelian orogen). Abundant Fe oxide and base metal sulphide mineralizations (see the text on mineral resources later in this chapter) are present in the predominantly felsic 1.91–1.89 Ga volcanic rocks, identical to the same association eastwards in the Bergslagen unit. Each of the three plutonic suites contains mafic components that mingle or mix with dominant felsic rocks. The suites vary in composition from calc-alkaline to alkali-calcic and are more ferroan in character with time. The two younger suites belong to different magmatic phases in the lithodemic unit referred to as the Transscandinavian Igneous Belt (TIB-0 and TIB-1, respectively, in Gorbatschev 2004).
Magmatic rocks belonging to the c. 1.81–1.77 Ga TIB-1 suite are also present further south in the upper compartment, immediately west and south of Vättern (Fig. 15.2). A similar continuity from the Eastern Segment of the Sveconorwegian orogen into its cratonic foreland (Småland lithotectonic unit, Svecokarelian orogen) is also apparent in this area (Wik et al. 2006; Stephens et al. 2009). The reader is referred to the descriptions of the Bergslagen and Småland lithotectonic units in Chapters 6 (Stephens & Jansson 2020) and 7 (Wahlgren & Stephens 2020a), respectively, for more details bearing on the protolith character of the Paleoproterozic (1.9–1.8 Ga) rocks and their inferred formation along an active continental margin in an accretionary orogenic setting.
Further south, between Kristianstad and Värnamo, amphibolite facies orthogneisses with a predominantly granitic to granodioritic composition form the oldest, pre-Sveconorwegian crustal component inside the upper compartment (Fig. 15.2). SE of Älmhult (Fig. 15.2), this metamorphosed magmatic suite can be traced eastwards into gneissic c. 1.82–1.74 Ga magmatic rocks belonging to the Blekinge–Bornholm orogen in the cratonic foreland to the Sveconorwegian orogen (Wik et al. 2009). Granitic gneisses of uncertain age also occur in the fault-controlled inlier of upper compartment that is tectonically interleaved with Phanerozoic sedimentary strata SE of Lund in southernmost Sweden (Fig. 15.2).
Paleoproterozoic (1.7 Ga) magmatic suite in all compartments
The dominant crustal component in the Eastern Segment consists of felsic plutonic rocks that crystallized during the time span c. 1.71–1.66 Ga (Figs 15.2 & 15.3). This range is more or less coeval with the timing of crystallization of the youngest phase of the Transscandinavian Igneous Belt (TIB-2 in Gorbatschev 2004), exposed in the cratonic foreland NE of the Sveconorwegian orogen in west-central Sweden (c. 1.71–1.68 Ga: see Chapter 9 (Ripa & Stephens 2020a) in this Memoir). Supracrustal rocks belonging to the 1.7 Ga suite in the Eastern Segment are rare (Lundegårdh 1995; Wik et al. 2006, 2009; Piñán Llamas et al. 2015; Ulmius et al. 2015).
Westwards and southwards across the different compartments inside the Eastern Segment (Fig. 15.4a), the 1.7 Ga magmatic suite varies in metamorphic grade and style of deformation in the manner described earlier. The primary relationships between different rock types are commonly completely obscured by the migmatization and deformation in the lower compartment. For this reason, focus on the field and geochemical character of the variably metamorphosed 1.7 Ga magmatic suite is placed here on the better-preserved varieties in the middle and upper compartments. Nevertheless, an inspection of the geochemical data from samples in the lower compartment indicates similar compositional attributes to those shown by the data from the other compartments. On account of the variable timing of emplacement of mafic intrusive rocks in the Eastern Segment during the late Paleo-, Meso- and Neoproterozoic (see also below), as well as the difficulties in determining primary relationships between different rock types and their timing of crystallization, mafic rocks are probably strongly under-represented in the visualization of the geochemical characteristics of the 1.7 Ga magmatic suite.
Two major rock types constitute the 1.7 Ga plutonic rocks: (1) pale red to greyish red, medium- to coarse-grained, commonly equigranular granitic varieties, poor in dark minerals and in places leucocratic (Fig. 15.16a); and (2) coarser-grained, reddish grey to grey varieties with composition varying from granitic to quartz monzodioritic (Fig. 15.16b). Mingling, mixing and hybridization between rocks with different composition are prominent, with mafic enclaves commonly occurring in the felsic varieties (Wik et al. 2009). Coeval mafic rocks are subordinate in volume at the currently exposed ground surface but, nevertheless, occur throughout the Eastern Segment. Contact metamorphism up to granulite facies has been identified locally in the aureoles to 1.7 Ga plutons inside the upper compartment, where the Sveconorwegian tectonothermal overprint was spatially restricted and took place under greenschist facies conditions (Stephens et al. 1993; Appelquist et al. 2008).
Character of the 1.7 Ga magmatic suite in the upper compartment of the Eastern Segment. (a) Equigranular 1.7 Ga granite intruded by dolerites belonging to the 1.6 Ga magmatic suite. (b) Medium-grained granite to quartz monzonite with a weak grain-shape fabric. This lithology is commonly finely porphyritic. Photographs by C.-H. Wahlgren in the Kristinehamn area (after Stephens et al. 2009).
Apart from a few samples showing low (<25) and high (>65) values of the alteration index used here (Ishikawa et al. 1976; Allen et al. 2003), consistent with enrichment of Na, and K or Mg (or both these elements), respectively, the lithogeochemical data show few signs of hydrothermal alteration (Fig. 15.17a, b). An overall trend from granite through quartz monzonite and quartz monzodiorite to gabbro, with subordinate granodioritic, tonalitic and syenitic varieties, is apparent on the P–Q classification diagram (Fig. 15.17c). The 1.7 Ga rocks in the Eastern Segment display a somewhat broader compositional variation than rocks of similar age and character in the Sveconorwegian foreland to the NE (cf. fig. 9.9a in Chapter 9, Ripa & Stephens 2020a), with quartz-rich felsic varieties (granitoids) also occurring in the former.
Pre-metamorphic hydrothermal alteration and geochemical classification of the plutonic rocks belonging to the 1.7 Ga magmatic suite in the middle and upper compartments of the Eastern Segment. Data extracted from the lithogeochemistry database at the Geological Survey of Sweden. (a) Modified alteration index after Ishikawa et al. (1976) and Allen et al. (2003). Samples in the field between the two horizontal lines do not show an alteration that affected the elements K, Na and Mg. (b) Alkali alteration diagram after Hughes (1973). The discrimination lines constrain an igneous spectrum containing a range of normal magmatic compositions. (c) Geochemical classification using the P–Q diagram after Debon & Le Fort (1983). (d) Geochemical classification using the Fe-number after Frost et al. (2001). (e) Geochemical classification using the modified alkali–lime index after Frost et al. (2001).
The 1.7 Ga plutonic rocks in the Eastern Segment straddle the boundary between magnesian and ferroan compositions, and show an alkali-calcic affinity (Fig. 15.17d, e). Initial εNd values from 1.7 Ga rocks in a small area south of Vättern (Fig. 15.2) are mildly depleted in the range +0.3 to +2.6, the magmas being derived mainly from juvenile, pre-existing crust that was increasingly mafic and less alkaline towards the west (Appelquist et al. 2011). A tendency towards slightly more negative initial εNd values is apparent in the coeval plutonic rocks in the cratonic foreland (TIB-2). Based on combined U–Pb and Hf isotope data, Petersson et al. (2015) suggested that there was no significant generation of juvenile crust around 1.7 Ga, but that the rocks formed by the reworking of a mixed crustal reservoir established earlier at around 1.9–1.8 Ga (Svecokarelian orogeny). A high proportion of rejuvenated material in the 1.7 Ga magmatism was also proposed by Åhäll & Connelly (2008).
The patterns on normalized plots for rare earth elements (REEs) show an enrichment in light REEs (LREE) relative to heavy REEs (HREE), with similar trends occurring irrespective of the SiO2 content; the felsic rocks also display a distinctive negative Eu anomaly (Fig. 15.18a, b). Normalized plots for large ion lithophile elements (LILEs) and high field strength elements (HFSEs) show an overall negative slope towards the HFSEs, and negative anomalies for Ta, Nb and Ti irrespective, once again, of the SiO2 content; rocks with >57% SiO2 also show strong negative anomalies for Ba, Sr and P (Fig. 15.18c, d). All of these features resemble the patterns for the 1.7 Ga rocks in the Sveconorwegian cratonic foreland (cf. fig. 9.9c–f in Chapter 9, Ripa & Stephens 2020a).
Geochemical characteristics of the plutonic rocks belonging to the 1.7 Ga magmatic suite in the middle and upper compartments of the Eastern Segment. Data source as for Figure 15.17. (a) Chondrite-normalized REE diagram for samples with SiO2 ≤57%. Normalization after Boynton (1984). (b) Same as (a) for samples with SiO2 >57%. (c) Primitive mantle-normalized multi-element diagram for samples with SiO2 ≤57%. Normalization after Wood et al. (1979). (d) Same as (c) for samples with SiO2 >57%. (e) Rb–Y + Nb and (f) Ta–Yb tectonic classification diagrams after Pearce et al. (1984).
The felsic rocks in the 1.7 Ga plutonic suite in the Eastern Segment straddle the boundary between the volcanic arc and the within-plate granite fields on tectonic discrimination diagrams, mostly lying within the volcanic arc field (Fig. 15.18e, f). An active continental margin setting was also suggested by Appelquist et al. (2011). The more pronounced within-plate affinity for rocks of similar age and character outside the Sveconorwegian orogen in the cratonic foreland (cf. fig. 9.10 in Chapter 9, Ripa & Stephens 2020a) suggests that the 1.7 Ga magmatic rocks outside and within the Eastern Segment represent complementary distal and proximal variants, respectively, of the same active continental margin and accretionary orogenic system.
Mesoproterozoic magmatic suites
1.6 Ga dolerites in the middle and upper compartments
Dolerites, emplaced at c. 1.57–1.56 Ga and affected, to variable extents, by Sveconorwegian metamorphism and deformation, form a volumetrically significant component in the Eastern Segment (Figs 15.2 & 15.3). They are most prominent in the middle compartment north of Vänern (Lindh et al. 1981; Morthorst et al. 1983; Lundegårdh 1995; Söderlund et al. 2005), occur also in the upper compartment east of this lake (Wahlgren et al. 1996) and in both of these units further south (Söderlund et al. 2004; Söderlund & Ask 2006; Beckman et al. 2017). The inhomogeneous metamorphic and deformational overprint accounts for a variation in character from well-preserved magmatic to strongly foliated garnet-bearing amphibolite (Fig. 15.10b). On account of the pervasive high-grade metamorphism and deformation, it is difficult to distinguish mafic rocks of different ages in the lower compartment. Consequently, the 1.6 Ga mafic rocks may also form a significant component in that compartment.
The variably metamorphosed 1.6 Ga dolerites show no strong hydrothermal alteration using the criteria adopted here (Fig. 15.19a, b). They are predominantly gabbroic in composition, span over into the quartz dioritic, monzogabbroic and quartz monzodioritic fields, and plot across the discrimination line between sub-alkaline and alkaline suites (Fig. 15.19c, d; see also Lindh et al. 1981).
Geochemical characteristics of the 1.6 Ga variably metamorphosed dolerites in the middle and upper compartments of the Eastern Segment. Data source as for Figure 15.17. See Figure 15.17a–c for the explanations to (a)–(c) and Figure 15.18a, c for the explanations to (e) & (f). (d) Geochemical classification of rocks according to Cox et al. (1979).
The REE pattern is flat with a tendency for a slight enrichment in LREE relative to HREE (Fig. 15.19e). The pattern in the LILE–HFSE plot shows a slightly negative sloping trend towards the HFSE, a distinctive negative anomaly in the contents of Ta and Nb, and a slightly negative Ti anomaly (Fig. 15.19f). It is possible that the negative anomalies for Ta and Nb, common in subduction-related magmas, are features inherited from the parent sub-crustal material. The 1.6 Ga dolerites show slightly depleted signatures with initial εHf values in the range +1.1 to +2.4, and εNd values between −0.4 and +0.3 (Söderlund et al. 2005). The intrusion of the dolerites has been interpreted to reflect an extensional event behind an active continental margin (Söderlund et al. 2005). This margin had apparently shifted westwards (current geographical coordinates) with respect to the margins active earlier at around 1.9–1.8 and 1.7 Ga, consistent with the oceanwards-stepping growth model for Fennoscandia during the Paleoproterozoic and into the Mesoproterozoic (e.g. Gaál & Gorbatschev 1987; Åhäll & Connelly 2008).
c. 1.47–1.44 Ga intrusive rocks in the upper and middle compartments
A magmatic suite emplaced at c. 1.47–1.44 Ga occurs south of Vättern mainly in the upper but also in the middle compartment of the Eastern Segment (Figs 15.2 & 15.3). This suite is composed of granite to syenitoid rock (Fig. 15.20a), as well as subordinate anorthosite, gabbro and dolerite (Johansson et al. 1993; Lundqvist 1996; Brander & Söderlund 2009; Brander et al. 2012; Petersson et al. 2015; Ulmius et al. 2015). Most samples display slightly negative Eu anomalies, while a few plagioclase-rich samples show positive anomalies; anorthositic rocks are enriched in LREE (Brander & Söderlund 2009). The c. 1.47–1.44 Ga intrusive rocks are interpreted to have been generated from a mixed crustal reservoir with insignificant generation of new juvenile crust (Petersson et al. 2015).
Character of some Mesoproterozoic magmatic rocks in the Eastern Segment. (a) c. 1.45 Ga granite (Romele granite) in the southernmost part of the upper compartment with inclusion of gneissic supracrustal rock. Photographer: J. Andersson. (b) c. 1.40 Ga charnockite close to Varberg in the lower compartment (after Lundqvist & Kero 2008). (c) Undeformed c. 1.37 Ga granite (Tjärnesjö granite) in the lower compartment. Photographer: J. Andersson. (d) Gneissic Tjärnesjö granite in the lower compartment. Photographer: J. Andersson. (e) Syenite belonging to the c. 1.22 Ga Vaggeryd intrusion south of Jönköping in the upper compartment. Photographer: C. Jelinek. (f) c. 1.22 Ga dolerite with large dark plagioclase megacrysts in the southern part of the upper compartment. Photographer: U. Bergström (after Wik et al. 2009).
The c. 1.47–1.44 Ga magmatic suite is coeval with the plutonic rocks emplaced during the Hallandian orogeny in the Blekinge–Bornholm orogen in southeasternmost Sweden (see Chapter 11, Wahlgren & Stephens 2020b), as well as with isolated granitic bodies and dolerite dykes north of this orogen in Sweden (see Chapter 10, Ripa & Stephens 2020b). The continuation of this Mesoproterozoic crustal component westwards and northwards, from the Blekinge–Bornholm orogen in the Sveconorwegian cratonic foreland into the frontal part of the Sveconorwegian orogen south of Vättern, is also consistent with a dextral strike-slip component of displacement along the frontal Sveconorwegian ductile shear zones.
A characteristic feature of the orthogneisses in the Eastern Segment south of Vänern is 1.7 Ga zircon cores enveloped by c. 1.45 Ga zircon rims (e.g. Söderlund et al. 2002; Möller et al. 2007; Rimša et al. 2007; Brander et al. 2012; Ulmius et al. 2015). The 1.7 Ga zircon age has commonly been interpreted to date magmatic crystallization, whereas the c. 1.45 Ga zircon rims have been related to Hallandian migmatization. However, for weakly deformed granites in the fault-controlled inlier of upper compartment rocks in southernmost Sweden (Fig. 15.2), Ulmius et al. (2015) interpreted the 1.7 Ga zircon cores to be xenocrystic, and the 1.45 Ga zircon rims to have formed during emplacement and crystallization of the granite. If this interpretation is more widely applicable to the orthogneisses south of Vänern, the extent of c. 1.45 Ga Hallandian magmatism may be more widespread than that currently proposed in this part of the Eastern Segment (Ulmius et al. 2015).
c. 1.42–1.38 Ga intrusive rocks in the lower compartment
A granite–norite–charnockite magmatic suite, emplaced at c. 1.42–1.38 Ga, forms a prominent feature in the southwestern part of the lower compartment (Figs 15.2 & 15.3). These intrusive bodies include, for example, charnockite and granite (Fig. 15.20b) around and north of Varberg. They form the structural footwall beneath the overthrust eclogite-bearing terrane (Fig. 15.2). By contrast, south of Kinna to the north, variably gneissic c. 1.37 Ga quartz monzonite to granite with a north–south extension (Tjärnesjö granite: Figs 15.2 & 15.20c, d), belonging to the same suite, lies structurally above the same eclogite-bearing terrane in an overturned D2 fold, the contact dipping eastwards (see also profile in Fig. 15.4c).
The charnockitic rocks vary in composition between monzonitic, quartz monzonitic and granitic varieties. Together with the Torpa granite, they display an alkaline compositional trend (Harlov et al. 2013). The charnockitic rocks are depleted in Rb, Th, U and LREE, and enriched in Ba in relation to the Torpa granite (Harlov et al. 2013). The latter is also partly charnockitic, varies in composition between granite and granodiorite, and displays an alkali–calcic trend (Åhäll et al. 1997), similar to the quartz monzonitic to granitic Tjärnesjö granite (Andersson et al. 1999).
The c. 1.42–1.38 Ga granite–norite–charnockite magmatic suite was intruded during the waning stages of the Hallandian orogeny. It varies in character from being well preserved to strongly deformed and metamorphosed under high-grade conditions, with the development of high-P granulite and anatexis during the Sveconorwegian orogeny (Hubbard 1975; Åhäll et al. 1997; Andersson et al. 1999; Christoffel et al. 1999; Harlov et al. 2006, 2013; Rimša et al. 2007). Syn-orogenic Hallandian magmatism also involved the emplacement of granitic and pegmatitic dykes (e.g. Christoffel et al. 1999; Söderlund et al. 2002, 2008; Möller et al. 2007).
c. 1.27 Ga dolerites in the upper compartment
Dolerites with an east–west strike, occurring in the same part of the upper compartment as the c. 1.47–1.44 Ga anorthosites and dolerites SW of Vättern, have yielded an emplacement age of c. 1.27 Ga (Brander et al. 2011; Fig. 15.3). Although occurring much further to the south, similar ages and initial εHf values have provoked a correlation with the 1.27–1.25 Ga Central Scandinavian Dolerite Group in central Sweden (Brander et al. 2011; see Chapter 12 (Ripa & Stephens 2020c) in this Memoir).
1.2 Ga intrusive rocks in the upper and lower compartments
Felsic plutons and subordinate gabbro, anorthositic gabbro and magnetite-rich melatroctolite, the latter spatially associated with a Fe–Ti mineralization at Smålands Taberg (see later in this chapter), form a significant 1.2 Ga magmatic suite south of Vättern inside the upper compartment of the Eastern Segment (Figs 15.2 & 15.3; see also references in Table 15.1). Variably metamorphosed 1.2 Ga dolerites are also present in both the upper and middle compartments. Söderlund & Ask (2006) proposed magma emplacement during two pulses at c. 1.22 and c. 1.20 Ga.
The 1.2 Ga felsic plutons in the upper compartment are generally well preserved (Fig. 15.20e). They form major elongate bodies with a north–south extension (Fig. 15.2), concordant with the structural grain steered by the spaced ductile shear zones active under greenschist facies conditions in this part of the orogen. The 1.2 Ga dolerites (Fig. 15.20f) show a magmatic mineralogy in central parts, but commonly become progressively deformed and metamorphosed under greenschist to amphibolite facies conditions towards the contacts to the surrounding rocks.
On the basis of the alteration index criteria adopted here, plutonic rocks in the 1.2 Ga magmatic suite are unaltered (Fig. 15.21a). They show high total alkali contents, consistent with a dominant trend from granite to quartz syenite and syenite, and a ferroan composition; subordinate quartz monzonite and monzonite are also present (Fig. 15.21b–d). Initial εHf values of around +3 indicate an influx of juvenile magma into the crust inside the Eastern Segment at 1.2 Ga (Petersson et al. 2015), in accordance with an interpretation based on Sm–Nd data from a syenite and foliated granite in the southern part of the upper compartment (Johansson 1990). The initial εHf values fall along the same εHf v. age trend as the Mesoproterozoic magmatic rocks in the Idefjorden, Bamble, Kongsberg and Telemarkia terranes further west inside the Sveconorwegian orogen (Bingen et al. 2011; Petersson et al. 2015).
Geochemical characteristics of intrusive rocks belonging to the 1.2 Ga magmatic suite in the middle and upper compartments of the Eastern Segment south of Vättern. Data source as for Figure 15.17. See Figure 15.17a–d for the explanations to the figures.
In marked contrast, yet consistent with the metamorphic zoning inside the Eastern Segment, pervasively gneissic syenitoid rocks with an emplacement age of 1.2 Ga (Table 15.3) occur further NW inside the lower compartment (Fig. 15.2). They were reworked during the Sveconorwegian orogeny, partly under high-P granulite facies conditions (Berglund 1997; Wik et al. 2009; Petersson et al. 2015).
Character and timing of crystallization of syn-orogenic Sveconorwegian rocks
Syn-orogenic Sveconorwegian rocks constitute a highly subordinate lithological component inside the Eastern Segment. They mainly comprise pegmatitic and granitic dykes in the middle and lower compartments that were emplaced during the time span c. 0.96–0.93 Ga (see references in Tables 15.2 and 15.3).
Dolerites, which have been correlated with the c. 0.98–0.95 Ga dolerite dyke swarm with an approximately north–south strike in the eastern cratonic foreland to the Sveconorwegian orogen (Fig. 15.2; see Chapter 13, Ripa & Stephens 2020d), occur in the upper compartment of this orogen. In the southern and southwestern parts of the lower compartment, metamorphosed mafic dykes with an amphibolite to high-P granulite facies mineral assemblage and an approximately north–south strike locally cut a gneissic foliation in the country rocks (Piñán Llamas et al. 2015). A c. 0.96 Ga age for a ‘felsic mobilizate’, which shows a mingling relationship with amphibolite, has been interpreted to date the timing of crystallization of the protolith to these rocks (Söderlund et al. 2008).
Mineral and bedrock resources
Apart from a cluster of Zn–Pb sulphide, skarn- or carbonate-hosted Fe oxide, and quartz-rich Fe oxide deposits NE of Vänern in the upper compartment, the Eastern Segment in the Sveconorwegian orogen contains few mineral resources and no operating mines (Fig. 15.22). The Zn–Pb sulphide and Fe oxide deposits are hosted by c. 1.91–1.89 Ga metamorphosed felsic volcanic rocks with interlayered carbonate strata, and form the westernmost part of the Bergslagen ore district (see Chapter 6, Stephens & Jansson 2020). The latter extends from the Bergslagen lithotectonic unit in the 2.0–1.8 Ga Svecokarelian orogen to the east, into the easternmost frontal part of the 1.1–0.9 Ga Sveconorwegian orogen to the west (Fig. 15.22). Since the major part of this district and each type of deposit identified above in the Eastern Segment are addressed in Chapter 6 (Stephens & Jansson 2020), the reader is referred to this chapter for more information on their character and genesis.
Spatial distribution of metallic mineral deposits in the Eastern Segment classified on the basis of commodity. Deposits discussed in the text are named in bold letters. Data extracted from the Geological Survey of Sweden's mineral resource database. Background geology is as in Figure 15.2. The cluster of deposits in the northeastern part of the Eastern Segment (north and south of Karlskoga) continues eastwards into the Bergslagen lithotectonic unit, and forms the westernmost part of the Bergslagen ore district (see Chapter 6, Stephens & Jansson 2020).
This section focuses below on the remaining prospects and abandoned small mines of metallic mineral deposits in the Eastern Segment. These consist of scattered Fe–Ti–V and Mn mineralizations close to and south of Vättern in the upper compartment, as well as base metal sulphide and subordinate Fe oxide mineralizations close to the Mylonite Zone north of Vänern in the middle compartment (Fig. 15.22). County compilations of the mineral and bedrock resources in Sweden by the Geological Survey of Sweden provide more detailed information on the resources in the Eastern Segment (Shaikh et al. 1989; Kornfält et al. 1990; Lundegårdh 1995; Lundqvist & Wik 1998; Wik et al. 2002).
Scattered occurrences of industrial minerals in the Eastern Segment, including garnet, quartz and feldspar in pegmatites, and dykes dominated by quartz, are concentrated in the lower compartment. Several of these occurrences have been quarried on a small scale in the past. Calcite or dolomite in carbonate rock interlayered with the metamorphosed volcanic rocks NE of Vänern in the upper compartment has also been quarried.
Exploitation of different lithologies in the Eastern Segment for dimension stone purposes and as rock aggregates for road and railway construction work is currently ongoing. One of the most famous and aesthetically distinct dimension stones in the Eastern Segment is the Hallandia gneiss (cf. Fig. 15.15). This bedrock resource is restricted to the southwestern part of the lower compartment and has been quarried in several places; only a few quarries are currently in operation. Schouenborg et al. (2015) presented a description of the geology, and the historical and current use of this orthogneiss.
Fe–Ti–V orthomagmatic mineralization hosted by 1.2 Ga dolerite in the upper compartment
Fe–Ti–V orthomagmatic mineralization occurs mainly in the upper compartment of the Sveconorwegian orogen south of Vättern. The most well-known occurrence, mined from the Middle Ages until 1960, is the Smålands Taberg deposit, being hosted by a 1.2 Ga dolerite (Fig. 15.22; Sandecki 2000; Larsson & Söderlund 2005; Larsson 2012). Magmatic cumulates in the dolerite consist of approximately 40–60% olivine, 30% magnetite with exsolutions of ilmenite and pleonaste, 5–25% plagioclase, and coronitic orthopyroxene and amphibole (Larsson 2012).
Mn mineralization related to faulting in the upper compartment
Clusters of Mn mineralization are located in the upper compartment of the Sveconorwegian orogen, the most well known occurring in the Spexeryd and Bölet areas immediately south and NW of Vättern, respectively (Fig. 15.22) (Bergman 2012a, b and references therein).
The Spexeryd area contains more than 20 small deposits along a 2 km-long zone with a north–south trend and has been the largest producer of Mn in Sweden. The Bölet area comprises more than 50 deposits that extend 50 km in an approximately NE–SW direction. Braunite is the dominant ore mineral at Spexeryd, whereas manganite and pyrolusite dominate at Bölet. In both areas, the ore minerals are intimately mixed with barite, calcite and fluorite, as well as quartz at Spexeryd. The Mn mineralizations are hosted by breccia and are interpreted to be related to faulting in connection with the development of a post-Sveconorwegian, Tonian–Cryogenian, half-graben structure (Andréasson et al. 1987; Andréasson & Rodhe 1990; see also Chapter 18, Wickström & Stephens 2020).
Base metal sulphide and Fe oxide mineralization in the middle compartment
The base metal sulphide deposits in the middle compartment north of Vänern, in the northwesternmost part of the Eastern Segment, mainly consist of small, abandoned Cu mines and prospects; spatially associated Fe oxide deposits are restricted to prospects and anomalies (Fig. 15.22; Lundegårdh 1995). All these deposits occur inside the southwestern strike-slip duplex structure spatially associated with the Mylonite Zone close to the tectonic contact between the Eastern Segment and the Idefjorden terrane (see Chapter 16, Bergström et al. 2020). Ore minerals in the sulphide deposits include chalcopyrite and bornite, commonly in association with pyrite; covellite, chalcocite, arsenopyrite, cobaltite, pyrrhotite, galena, sphalerite and magnetite are locally present. The spatial relationship between the mineralizations and high-strain zones inside this duplex structure suggests a genetic relationship between mineralizing fluids and ductile to brittle–ductile shear zones, indicative of structurally controlled mineralization.
Discussion and concluding remarks
Following a brief consideration of the pre-Sveconorwegian tectonic evolution inside the Eastern Segment, the discussion and concluding remarks in this section focus most attention on the Sveconorwegian tectonic evolution at 1.0–0.9 Ga. The section closes with an identification of some key issues for future work inside this lithotectonic unit.
Tectonic evolution prior to the Sveconorwegian orogeny
In contrast to all the other Sveconorwegian lithotectonic units to the west of the Mylonite Zone, the Eastern Segment is generally devoid of lithological components generated in connection with the Sveconorwegian orogeny. Exceptions include c. 0.98–0.95 Ga leucosome development in the migmatitic gneisses in the lower compartment, the intrusion of pegmatitic and granitic dykes at c. 0.96–0.93 Ga, and the emplacement of early Tonian dolerite dykes (c. 0.98–0.95 Ga) at least in the upper compartment. Notwithstanding the limited occurrence, the syn-orogenic magmatic evolution involved both crustal and mantle processes. The Eastern Segment is dominated instead by the tectonothermal reworking of already existing continental crust that can be traced with confidence eastwards into the cratonic foreland to the orogen. These crustal components belong to different lithotectonic units in the foreland where, apart from brittle deformation, Sveconorwegian orogenic overprint is lacking.
Foreland lithotectonic units formed during previous orogenic events include the Bergslagen and Småland units of the 2.0–1.8 Ga Svecokarelian orogen in the central part of Sweden (Chapters 6 (Stephens & Jansson 2020) and 7 (Wahlgren & Stephens 2020a), respectively), the 1.7 Ga magmatic province in north-central Sweden (Chapter 9, Ripa & Stephens 2020a), and the 1.5–1.4 Ga Blekinge–Bornholm orogen in the southeasternmost part of the country (Chapter 11, Wahlgren & Stephens 2020b). The crust in these lithotectonic units was generated in connection with accretionary orogenic activity along the western margin (current geographical coordinates) of Fennoscandia that formed part of the pre-Sveconorwegian Paleoproterozoic–Mesoproterozoic supercontinent Columbia (see also Åhäll & Connelly 2008; Johansson 2014; Roberts & Slagstad 2015). Magmatic suites emplaced during intracratonic extension at 1.6 and 1.3–1.2 Ga in the Eastern Segment also have their counterparts eastwards in the cratonic foreland, and are also addressed elsewhere in this Memoir (see Chapters 10 and 12 (Ripa & Stephens 2020b, c), respectively). The reader is referred to these chapters for more details around the pre-Sveconorwegian tectonic evolution of the Eastern Segment.
Tectonic evolution at 1.0–0.9 Ga during the Sveconorwegian orogeny
A three-fold division of the Sveconorwegian 1.0–0.9 Ga tectonic evolution in the Eastern Segment is outlined below. This development involved a changeover from crustal shortening to crustal extension around 0.98 Ga and the development of a renewed phase of crustal shortening with juxtaposition of the frontal part of the orogen after 0.95 Ga. The broader tectonic implications of these three phases are addressed later in Chapter 17 (Stephens & Wahlgren 2020), after presentation of the pre-Sveconorwegian and Sveconorwegian tectonic evolution in the Idefjorden terrane in Chapter 16 (Bergström et al. 2020).
Progressive shortening with crustal thickening and eclogite exhumation at c. 0.99–0.98 Ga
Crustal shortening, including underthrusting of the western part of the pre-Sveconorwegian crust in the lower compartment beneath the Idefjorden terrane, took place at c. 0.99–0.98 Ga; the mafic rocks in this crustal component being transformed to eclogite (Möller 1998, 1999; Austin Hegardt et al. 2005; Möller et al. 2015; Möller & Andersson 2018). The early stage in the exhumation of the eclogite-bearing terrane to crustal depths of c. 35–40 km and high-P granulite to upper amphibolite facies metamorphism followed during the same time interval (Möller et al. 2015; Tual et al. 2017, 2018). According to these authors, this exhumation is reflected by the D1–D2 deformation phase, the structure enveloping the eclogite-bearing terrane being referred to as a ‘hot fold nappe’.
Tual et al. (2015) have shown that the eclogite-bearing terrane was thrust during the D1 phase on top of the remaining part of the lower compartment, where metamorphism took place at crustal depths no deeper than those corresponding to high-P granulite facies. Furthermore, the basal thrust was subsequently folded by the D2 recumbent folding. As discussed below, the tectonic regime in the Eastern Segment had changed to an extensional mode, and anatexis was in progress at 8–12 kbar when the D2 folding took place at around 0.97 Ga (Piñán Llamas et al. 2015). These observations suggest that the driving mechanism for the early stage in the exhumation process was D1 thrusting during progressive crustal shortening rather than the later D2 folding. For this reason, use of the term ‘fold nappe’ creates some misunderstanding and should be used with care.
Anatexis at c. 0.98–0.95 Ga and bulk extension in a west-northwesterly direction
Both the overthrust eclogite-bearing terrane and its structural footwall in the lower compartment south of Vänern were affected by anatexis during the time interval c. 0.98–0.95 Ga (see above). This period was also characterized by emplacement of a sub-alkaline to alkaline dolerite dyke swarm with a northerly strike, in both the cratonic foreland to the orogen (see Chapter 13, Ripa & Stephens 2020d) and in at least the upper compartment of the Eastern Segment. Siliciclastic sedimentation in a Tonian foreland basin also occurred (see Chapter 13, Ripa & Stephens 2020d). The regional extent of the dolerite dyke swarm suggests mantle involvement and an east–westerly crustal extensional component at c. 0.98–0.95 Ga. Questions arise concerning whether the regional positive gravity anomaly in the southwestern part of the lower compartment is linked, at least partly, to early Tonian mafic plutons at depth that formed feeders to the dolerite dyke swarm.
These features suggest that mantle heat related to mafic magmatic activity may have contributed to the development of anatexis (Möller et al. 2015), which was not simply steered by decompression melting or melting due to fluid infiltration in connection with exhumation of the eclogite-bearing terrane (Möller 1998, 1999; Möller et al. 2015; Tual et al. 2017). The bulk extension would have contributed to continued exhumation of the deeper crustal components, supported by the closure of the U–Pb (monazite) and U–Pb (titanite) isotope systems during the c. 0.98–0.95 Ga time span.
The development of bulk extension associated with anatexis at c. 0.98–0.95 Ga raises the question of whether the D2 (and possibly D3) deformational phases south of Vänern, and the D2 phase north of this lake, developed in an overall transtensional tectonic regime. The constrictional character of the D2 strain (Viola et al. 2011; Piñán Llamas et al. 2015), with sub-parallel mineral stretching lineation and fold axes in a regional west-northwesterly direction, is consistent with such an interpretation (Viola et al. 2011). Piñán Llamas et al. (2015) also discussed this possibility but identified the need for further strain analysis to test the hypothesis.
Renewed crustal shortening after 0.95 Ga
Alternative structural models have been proposed for the subsequent exhumation of the high-grade rocks in the Eastern Segment to greenschist facies and lower metamorphic grade, following the emplacement of the dolerite dykes. These models rely strongly on the broader tectonic interpretation of the ductile shear zones with top-to-the-east displacement in the upper compartment (see above), close to the eastern boundary of the Eastern Segment, and complementary top-to-the-west displacement in the Mylonite Zone along its western boundary (see Chapter 16, Bergström et al. 2020).
Andréasson & Rodhe (1990), Viola et al. (2011) and Ulmius et al. (2018) proposed a bulk extensional or transtensional tectonic model; the Eastern Segment comprising an asymmetrical metamorphic core complex. Crust exposed in the southern part was affected by metamorphism partly at deeper levels and mostly under warmer conditions relative to that exposed in the northern part. By contrast, a two-stage compressional model was one of the alternative structural solutions proposed for the fan-like configuration of ductile shear zones in the upper compartment, north of Vättern (Wahlgren et al. 1994). Subsequently, development of these structures in connection with crustal shortening was inferred from seismic imaging (Juhlin et al. 2000).
A structural link is proposed here for the development of the late Sveconorwegian upright folding (D4 south of Vänern and D3 north of this lake) and the earlier ductile shear zones in the upper compartment dipping easterly with a normal sense of movement. The folding included the overall antiformal dome structure plunging gently northwards on a large scale. It is suggested that the doming was accompanied by shearing with top-to-the-east displacement along the eastern limb that detached virtually the entire low-grade, upper compartment from the underlying higher-grade compartments. Top-to-the-west movement along the Mylonite Zone would have constituted a complementary process along the western limb of the dome. Extension is envisaged to have taken place at upper crustal levels, while overall crustal convergence continued to shorten the crust at deeper levels. This late convergence accounted for not only the late upright folding in both the Eastern Segment and the Idefjorden terrane (see Chapter 16, Bergström et al. 2020), but also provides an explanation for the fan-like structure in the frontal wedge.
Some key issues for future work
The completion of this overview has identified several geological issues that require further study in order to improve our understanding of the tectonic evolution of the Eastern Segment, prior to and especially during the Sveconorwegian orogeny. Six key issues are presented below:
The spatial extent of the Mesoproterozoic Hallandian orogenic event inside the Eastern Segment is not well constrained and requires more attention.
Numerical data constraining Sveconorwegian pressure–temperature–time (P–T–t) conditions in the Eastern Segment are mainly restricted to the southern part of the lower compartment. In order to obtain a broader understanding of the tectonic evolution in the orogen during the later stages of its development (1.0–0.9 Ga), similar integrated data are required around and north of Vänern in the lower and middle compartments, as well as eastwards in the upper compartment. In particular, data bearing on the pressure and timing of Sveconorwegian metamorphism are necessary to constrain better the degree of separation of the frontal upper compartment from the underlying compartments. The hypothesis that there is some diachroneity in the timing of metamorphism and deformation, and not only cooling, inside the Eastern Segment, with younger ages southwards, also needs to be tested.
As indicated above, there remains some uncertainty concerning how the established deformational sequence (D1–D4) is related to the inferred broader geodynamic development from crustal shortening to widespread extension and then back again to a shortening phase. In particular, the mechanism of formation of the major D2 recumbent folding and the timing of formation of the late-stage D4 (south of Vänern) and D3 (north of Vänern) phases need attention.
Complementary data are needed to address the timing of deformation along the network of ductile shear zones in the upper compartment, so as to compare with the movement along the Mylonite Zone along the western boundary to the Eastern Segment.
The geological character of the conspicuous positive gravity anomaly in the southern part of the lower compartment remains uncertain and modelling work is necessary to test different hypotheses.
The Eastern Segment and the Idefjorden terrane to the west should be targeted for detailed, near-vertical reflection seismic profiling as carried out in the central part of the Caledonide orogen (see Chapter 19, Gee & Stephens 2020).
Acknowledgements
Acknowledgement to colleagues who have helped in different ways, peer reviewers who provided constructive comments to the manuscript, and companies and institutions who have contributed to the production of the graphics is given in the front matter of this Memoir.
Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
Author contributions
MBS: writing – original draft (equal); CHW: writing – original draft (equal).
- © 2020 The Author(s). Published by The Geological Society of London. All rights reserved
References
Chapter 15 Polyphase (1.9–1.8, 1.5–1.4 and 1.0–0.9 Ga) deformation and metamorphism of Proterozoic (1.9–1.2 Ga) continental crust, Eastern Segment, Sveconorwegian orogen
- Article
- Abstract
- Character of the boundaries to, and the metamorphism and deformation inside, the Eastern Segment
- Timing of metamorphism and ductile deformation in different orogenic systems, and later faulting
- Character and timing of crystallization of protoliths in the pre-Sveconorwegian crust
- Character and timing of crystallization of syn-orogenic Sveconorwegian rocks
- Mineral and bedrock resources
- Discussion and concluding remarks
- Acknowledgements
- Funding
- Author contributions
- References
- Figures & Data
- Info & Metrics