Abstract
The Eastern Segment in the Sveconorwegian orogen, southwestern Sweden, is dominated by 2.0–1.8, 1.7 and 1.5–1.4 Ga crust; and the overlying Idefjorden terrane by 1.6–1.5 Ga crust. Assuming reorganization of a subduction system prior to 1.5–1.4 Ga and applying a sinistral transpressive component of disruption during the subsequent Sveconorwegian orogeny (1.1–0.9 Ga), the Idefjorden terrane is inferred to be indigenous outboard rather than exotic with respect to the continental plate Fennoscandia (Baltica). The geological record then records successive westwards shift of accretionary orogens along a convergent plate boundary for at least 500 million years. Sveconorwegian foreland-younging tectonic cycles at c. 1.05 (or older)–1.02 Ga (Idefjorden terrane) and at c. 0.99–0.95 Ga (Eastern Segment) prevailed. Crustal thickening and exhumation during oblique convergence preceded migmatization, magmatic activity and a changeover to an extensional regime, possibly triggered by delamination of continental lithosphere, in each cycle. Convergence after 0.95 Ga involved antiformal doming with extensional deformation at higher crustal levels (Eastern Segment) and continued magmatic activity (Idefjorden terrane). An overriding plate setting is inferred during either accretionary orogeny or, more probably, protracted continent–continent collision. Continuity of the erosional fronts in the Grenville and Sveconorwegian orogens is questioned.
The Sveconorwegian orogen in southwestern Scandinavia is dominated by older continental crust that was tectonically reworked during a 1.1–0.9 Ga mountain-building event. Due to the deep level of erosion, the Sveconorwegian orogen provides an insight into the character of reworked crystalline basement in younger orogenic systems, where pre- or synorogenic supracrustal successions more commonly form the dominant component. This chapter completes Part V in this Memoir, which concerns the part of the orogen in southwestern Sweden (Fig. 17.1).
Lithotectonic units inside the 1.1–0.9 Ga Sveconorwegian orogen, southern Norway and southwestern Sweden, and internal subdivision of the two eastern units exposed mainly in Sweden. Solid rock formed prior to and during this orogeny is distinguished in each lithotectonic unit. Early Tonian dolerites and an outlier of siliciclastic sedimentary rocks are also shown in the eastern cratonic foreland (see Chapter 13, Ripa & Stephens 2020d). There remains uncertainty as to what extent the window in the Caledonide orogen of southwestern Norway, marked as part of the Sveconorwegian orogen, is an allochthonous unit inside the Caledonide orogen (cf. Chapter 19, Gee & Stephens 2020). Map modified after Piñán Llamas et al. (2015), and including additional information from Park et al. (1991), Koistinen et al. (2001) and Bergman et al. (2012).
The purpose of this chapter is to address three key tectonic issues that were identified initially in the opening chapter to Part V (Chapter 14, Stephens et al. 2020). Following a short review of the internal lithotectonic framework, the chapter focuses attention firstly on the question around the exotic or indigenous outboard character of the Idefjorden terrane with respect to the cratonic foreland, east of the Sveconorwegian orogen. This issue is coupled to the tectonic character of the boundary between the Idefjorden terrane and the Eastern Segment: crustal suture or intracratonic shear zone? This chapter continues with a focus on the tectonic evolution during the Sveconorwegian orogeny at 1.0–0.9 Ga, as inferred from an integrated evaluation of the relevant information in the Eastern Segment and Idefjorden terrane, Sweden. A key geodynamic question in this context concerns whether the orogenesis involved a continent–continent collision or took place along a convergent plate boundary where solely subduction-related accretionary processes prevailed. This chapter then closes with a brief discussion of the broader tectonic context of the Sveconorwegian orogen, more specifically the Grenvillian–Sveconorwegian connection.
The discussion and conclusions presented in this chapter build on the detailed overviews of the Eastern Segment and the Idefjorden terrane provided in the foregoing Chapters 15 (Stephens & Wahlgren 2020) and 16 (Bergström et al. 2020), respectively; and the regional geological context of these lithotectonic units presented in the introductory Chapter 14 (Stephens et al. 2020). The reader is referred to these chapters for more extensive reference to previously published work. Current geographical coordinates are used when referring to different directions in space.
Internal lithotectonic framework
The Sveconorwegian orogen shows an internally variable but overall north–south structural trend (Fig. 17.1). It has been divided into five major lithotectonic units, referred to from east to west as the Eastern Segment and the Idefjorden, Kongsberg, Bamble and Telemarkia terranes (Bingen et al. 2008a). The Eastern Segment and the Idefjorden terrane are mostly exposed in southwestern Sweden and are addressed in this Memoir; the Bamble, Kongsberg and Telemarkia terranes are restricted entirely to southern Norway (Fig. 17.1). The five units differ on the basis of the character and timing of the Sveconorwegian orogenic overprint, and the character of their pre-Sveconorwegian crustal components (e.g. Bingen et al. 2008b).
The lithotectonic units are separated by zones or complex belts with enhanced ductile strain. These zones or belts show different kinematic signatures, established at different times and under different metamorphic conditions. Contrasts in metamorphic grade and style of ductile deformation inside the Eastern Segment, and steeply dipping, ductile shear zones with north–south strike in the Idefjorden terrane have provided the basis for the identification of separate compartments or tectonic segments inside each of these lithotectonic units (Fig. 17.1). The boundary shear zone between the Eastern Segment and the Idefjorden terrane (Mylonite Zone, see Fig. 17.1) is markedly arcuate, and also shows a variable sense of movement along its strike length. Limitations in our knowledge concerning to what extent the Sveconorwegian orogen continues northwards beneath the Caledonide orogen (Fig. 17.1) provides an uncertainty concerning which structural data (see below) are the more representative on a larger regional scale.
From south of Lake Vänern northwards to the erosional front of the Caledonide orogen, a distance of more than 350 km, the Mylonite Zone varies in strike from NNE–SSW to NW–SE (Fig. 17.1), and shows a sinistral strike-slip and top-to-the-east reverse dip-slip component of movement consistent with sinistral transpression (Park et al. 1991; Robin & Cruden 1994; Stephens et al. 1996; Viola & Henderson 2010). The strike-slip displacement is far more distinctive along the steeply dipping portion striking NW–SE (nearly 175 km), and is also conspicuous along the segment north and south of Vänern with NNE–SSW to NNW–SSE strike. In these areas, the western boundary of the zone is more steeply dipping than the internal parts and shows by a strong sinistral strike-slip sense of movement.
Along a strike length of approximately 100 km in southernmost parts, the Mylonite Zone dips gently, varies in strike from north–south to ENE–WSW and shows either dip-slip or dextral strike-slip movement (Viola & Henderson 2010; Möller et al. 2015). It is here, in the footwall to the Mylonite Zone, that a detached slab containing bodies and boudins of eclogite is preserved (Möller 1998, 1999; Austin Hegardt et al. 2005; Möller et al. 2015; Möller & Andersson 2018; see also Chapter 15 (Stephens & Wahlgren 2020) and Fig. 17.1). Crustal shortening along the Mylonite Zone took place prior to top-to-the-west normal movement, most distinctive close to and south of Vänern (Berglund 1997; Viola & Henderson 2010; Viola et al. 2011).
Regionally significant steeply dipping zones with north–south strike inside the Idefjorden terrane (Park et al. 1991) and along the boundary between the Kongsberg and Telemarkia terranes in southwestern Norway (Scheiber et al. 2015) also contain a sinistral transpressive component followed by extension. The dominance of a broadly north–south structural trend inside the exposed Sveconorwegian orogen, and the sinistral transpressive component of shear deformation along significant parts of shear zones with this orientation, suggest that oblique convergence prevailed during this orogeny (Park et al. 1991). Partitioning of the strain into approximately north–south sinistral strike-slip and east–west thrust components resulted. Bulk crustal shortening in a WNW–ESE or NW–SE direction is inferred, with displacement of the Idefjorden terrane southwards and eastwards relative to the cratonic foreland to the east (Fennoscandia part of Baltica).
Idefjorden terrane: exotic or simply an indigenous outboard component of Fennoscandia (Baltica)?
There is a confident correlation between the pre-Sveconorwegian crustal components in the Eastern Segment of the Sveconorwegian orogen and the continental part of the pre-Rodinia plate, including Fennoscandia (Baltica), exposed in the more easterly cratonic foreland to the orogen (Figs 17.1 & 17.2). In particular, the tectonic developments at 2.0–1.8 Ga, around 1.7 Ga and at 1.5–1.4 Ga (see Chapters 8 (Stephens 2020), 9 (Ripa & Stephens 2020a) and 11 (Wahlgren & Stephens 2020), respectively), related to the subduction of oceanic lithosphere and accretion to the older Paleoproterozoic and Archean crust in Fennoscandia (Baltica), are preserved in both these parts of the shield.
Comparison of the timing of deposition of supracrustal rocks and emplacement of intrusive rocks in the Idefjorden terrane and Eastern Segment, Sveconorwegian orogen, and the eastern cratonic foreland to this orogen. Only pre-Sveconorwegian crustal components are shown. The lithological information focuses attention on the character of the protolith and shows only the dominant component in each suite. Dolerites are also significant in the magmatic suites emplaced around 1.47–1.44 and 1.2 Ga.
Such a correlation is more problematic where it concerns the Idefjorden terrane (Fig. 17.2) and, by corollary, the other lithotectonic units (Bamble, Kongsberg and Telemarkia) further west in the Sveconorwegian orogen. Several authors have proposed that the Idefjorden terrane was separated from (i.e. was exotic with respect to) Fennoscandia (Baltica) prior to the Sveconorwegian orogeny, and the Mylonite Zone separating the Eastern Segment from the Idefjorden terrane represents a Sveconorwegian orogenic suture (e.g. Cornell et al. 2000; Andersson et al. 2002; Austin Hegardt et al. 2005; Möller et al. 2015; Petersson et al. 2015; Möller & Andersson 2018).
One argument used for the exotic model concerns contrasting εHf values in the magmatic suites in the Idefjorden terrane and the Eastern Segment (Petersson et al. 2015). However, this contrast can be explained by variability in the content of more juvenile mantle-derived magma in their genesis; the more juvenile magma in the Idefjorden terrane being coupled to a more outboard, younger and thinner continental crust. Initial εHf values for 1.2 Ga plutonic rocks in the Eastern Segment actually fall along the same εHf v. age trend as the Mesoproterozoic magmatic rocks in the Idefjorden, Bamble, Kongsberg and Telemarkia terranes (Bingen et al. 2011; Petersson et al. 2015). Bearing in mind also the allochthonous character of the Idefjorden terrane relative to the Eastern Segment, the argument using isotopic data is judged to be equivocal. The critical argument for the exotic model concerns the contrast in tectonic evolution in these two lithotectonic units during the Mesoproterozoic era, prior to the Sveconorwegian orogeny.
There is no evidence for the 1.66–1.52 Ga (Gothian) orogeny in the Eastern Segment and the cratonic foreland to the Sveconorwegian orogen (Fig. 17.2). Furthermore, the 1.47–1.38 Ga (Hallandian) orogeny, which affected both the Eastern Segment and, in part, the cratonic foreland, is absent in the Idefjorden terrane (Fig. 17.2). Magmatism related to subsequent Ectasian intracratonic rifting took place in both lithotectonic units but at different times (Fig. 17.2). Notwithstanding these contrasts, closer comparison of the 1.7–1.5 Ga tectonic evolution in both the cratonic foreland of the Sveconorwegian orogen and the Idefjorden terrane has some bearing on the question around the exotic character of the latter. This comparison addresses possible distal effects of orogeny in adjacent cratonic areas.
A large part of the episodic magmatism related to intracratonic rifting during 1.7–1.5 Ga in the cratonic foreland to the Sveconorwegian orogen, including emplacement of the classic rapakivi granite complexes, is coeval with the Gothian orogeny to the west (see the overview in Chapter 10 (Ripa & Stephens 2020b; and Figs 17.2 & 17.3). Westwards younging of this magmatic activity from westernmost Russia into Finland and Sweden took place up to 1.56 Ga, with a pause at 1.62–1.59 Ga. Following a second pause at 1.56–1.55 Ga, a significant eastwards shift in this activity back into westernmost Russia took place at 1.55 Ga. It was followed one more time by westwards younging; intracratonic magmatism and crustal extension taking place until around 1.50 Ga, and even later at 1.47–1.44 Ga in Sweden (Fig. 17.3). Åhäll et al. (2000) proposed that the 1.7–1.5 Ga intracratonic magmatic activity was tectonically steered by a distal response to the Gothian orogeny along an active continental margin; in particular, recurring subduction of oceanic lithosphere in an extensional arc-retreating mode.
Comparison of the timing of magmatic activity associated with late Paleoproterozoic–Mesoproterozoic intracratonic rifting in the cratonic foreland to the Sveconorwegian orogen, and crustal shortening events during the Gothian orogeny (based on geochronological information provided in Chapters 10 (Ripa & Stephens 2020b) and 16 (Bergström et al. 2020), respectively, in this Memoir).
The pause in the intracratonic magmatism during the period 1.56–1.55 Ga correlates with the crustal shortening event during the Gothian orogenic evolution (DG2 in Chapter 16 (Bergström et al. 2020); Fig. 17.3). Furthermore, the eastwards shift of renewed intracratonic magmatic activity into westernmost Russia at 1.55 Ga can be explained by a renewed arc-retreating mode in the outboard orogenic setting, and a more gently dipping subduction zone relative to that in operation during the arc-advancing mode at 1.56–1.55 Ga. The earlier pause at 1.62–1.59 Ga also overlaps partly with the earlier shortening event in the Idefjorden terrane (DG1 in Chapter 16 (Bergström et al. 2020); Fig. 17.3). These considerations support the tectonic model linking the intracratonic rift-related magmatism in the east to the Gothian orogenic activity in the Idefjorden terrane in the west (Åhäll et al. 2000). The magmatic activity at 1.47–1.44 Ga in the cratonic foreland to the Sveconorwegian orogen (Fig. 17.3) is also inferred to have been coupled to the Hallandian orogeny further west.
Assuming the same tectonic link between outboard, arc-retreating subduction during accretionary orogeny and inboard intracratonic rifting as at 1.7–1.5 Ga, the intracratonic rifting in the Idefjorden terrane at 1.5–1.4 Ga (Fig. 17.2) is inferred to represent an easterly inboard response to the outboard Telemarkian and Hallandian orogenies. In order to concur with the current spatial distributions of these orogenic systems following Sveconorwegian disturbance, a significant change in the trend of the active continental margin prior to the Hallandian event is required in a pre-Sveconowegian palinspastic reconstruction (Fig. 17.4). A reorganization of the subduction system prior to the Hallandian event was also proposed by Roberts & Slagstad (2015). The Sveconorwegian sinistral strike-slip component of displacement along the Mylonite Zone, conspicuous around and north of Vänern (Fig. 17.1), would have displaced the northern boundary of the Hallandian orogenic overprint in the Telemarkian and Gothian orogens, located west of this shear zone, an unknown distance southwards. It is tentatively assumed to be buried beneath Phanerozoic sedimentary cover rocks south of Norway and Sweden.
Palinspastic reconstruction of northern Europe shortly after 1.4 Ga, showing the Svecokarelian orogen (2.0–1.8 Ga), the successively westwards younger late Paleoproterozoic–Mesoproterozoic orogens (1.7–1.4 Ga) and the late Paleoproterozoic–Mesoproterozoic magmatic rocks (including rapakivi granites) emplaced into the cratonized 2.0–1.8 orogen.
This model can explain the disparity in the orogenic evolution between the Idefjorden terrane and the Eastern Segment, without invoking an exotic origin for the former with respect to the continental plate Fennoscandia (Baltica). By corollary, it is inferred that the Mylonite Zone does not represent a suture but, as proposed by Åhäll & Connelly (2008), a ductile shear zone that developed in an intracratonic position during the Sveconorwegian orogeny. A critical component in the pre-Sveconorwegian palinspastic model proposed here (Fig. 17.4) is the successive shift westwards of accretionary orogenic systems along a long-lived convergent plate boundary during the time interval 1.9 Ga to at least 1.4 Ga, close to the western active continental margin to Fennoscandia (e.g. Gaál & Gorbatschev 1987; Åhäll & Connelly 2008; Roberts & Slagstad 2015).
Sveconorwegian tectonic evolution at c. 1.05–0.90 Ga in the Idefjorden terrane and Eastern Segment
A four-phase tectonothermal model bracketed between 1.14 and 0.90 Ga has been proposed for the Sveconorwegian orogeny (Bingen et al. 2008a, b; and see Chapter 14 (Stephens et al. 2020)). Sveconorwegian amphibolite to granulite facies metamorphism at 1.14–1.08 Ga (Arendal phase), affecting early Mesoproterozoic (1.6–1.5 Ga) continental crust, has been identified in the central part of the orogen in southern Norway (Kongsberg and Bamble terranes). Emplacement of alkaline ultrabasic dykes, including carbonatites and lamprophyres, in the cratonic foreland in northeasternmost Sweden (see Chapter 12, Ripa & Stephens 2020c) is coeval with the earliest part of the Arendal tectonothermal phase. There is no other possible trace of this phase in Sweden and it is not addressed further here. This section focuses instead on the effects in Sweden of the younger tectonothermal evolution, referred to by Bingen et al. (2008a, b) as the Agder, Falkenberg and Dalane phases (Fig. 17.5).
Comparison of the timing and character of Sveconorwegian deformation, metamorphism and syn-orogenic magmatic activity in the Idefjorden terrane and Eastern Segment.
The main Sveconorwegian tectonothermal event in the Idefjorden terrane reworked 1.6–1.5 Ga and younger Mesoproterozoic crust at c. 1.05–1.02 Ga (Fig. 17.5). The subsequent development at c. 0.99–0.98 Ga took place in the Eastern Segment (Fig. 17.5), where the pre-Sveconorwegian components are composed of late Paleoproterozoic (1.7 Ga) and even older (2.0–1.8 Ga) continental crust, and subordinate Mesoproterozoic magmatic rocks. The youngest tectonothermal phase occurred throughout the time span c. 0.98–0.92 Ga in both the Idefjorden terrane and the Eastern Segment (Fig. 17.5). The broad extent of this youngest phase (>600 km), across and east of the orogen, is illustrated by the emplacement of a 0.98–0.95 Ga dolerite dyke swarm with northerly trend adjacent and parallel to the Sveconorwegian orogen in the eastern cratonic foreland (Söderlund et al. 2005; Fig. 17.5), and c. 0.99–0.92 Ga magmatic activity west of the Idefjorden terrane in southwestern Norway (Slagstad et al. 2013, 2017; Roberts & Slagstad 2015; Fig. 17.1).
Multiphase crustal thickening and tectonothermal response
Stage 1 (c. 1.05–1.02 Ga)
At c. 1.05 Ga and at >50 km depth, high-pressure (high-P) mafic granulites developed in the footwall to the Göta Älv shear zone that separates the two internal lithotectonic units inside the Idefjorden terrane (Median and Western segments in Fig. 17.1; see Park et al. 1991; Söderlund et al. 2008). Underthrusting in connection with the sinistral transpressive shear along this zone (Park et al. 1991) is suggested to have operated at this time (Fig. 17.6a). Bearing in mind that crustal thickening occurred prior to the establishment of high-P granulite facies metamorphism in the Eastern Segment (see below), the c. 1.05 Ga age can be regarded as a minimum age for the underthrusting and thickening in the Idefjorden terrane.
Schematic illustration of the c. 1.05–0.91 Ga deformational evolution in the Idefjorden terrane and Eastern Segment. Sinistral transpressive deformation took place along the Göta Älv shear zone and along c. 75% of the strike length of the Mylonite Zone during the earlier stages of their development. For this reason, a sinistral strike-slip component of movement is shown along these zones at c. 1.05 and c. 0.99–0.98 Ga, respectively. By contrast, dip-slip and dextral strike-slip movement occurred along the gently dipping southernmost part of the Mylonite Zone. The blue diamond shows the inferred movement of a point in space during the deformational evolution.
Exhumation of the high-P rocks in the Median segment to crustal depths of c. 35 km with fabric development (Fig. 17.6b), as well as anatexis and emplacement of crustal-derived melts in both the Median and Western segments, occurred at c. 1.05–1.02 Ga (Romer & Smeds 1996; Söderlund et al. 2008; Austin Hegardt & Cornell 2010). The west-vergent extensional shear component along the Göta Älv shear zone (Park et al. 1991) is assigned to this stage in the tectonothermal evolution (Fig. 17.6b), downthrowing higher crustal levels in the Western segment and possibly explaining why the pre-Sveconorwegian 1.66–1.52 Ga (Gothian) accretionary orogenesis is preserved in this segment (see Chapter 16, Bergström et al. 2020). The c. 1.05–1.02 Ga tectonothermal evolution in the Idefjorden terrane was more or less coeval with c. 1.06–1.01 Ga magmatic activity, with both felsic and mantle-derived mafic components (Slagstad et al. 2013, 2017; Coint et al. 2015; Roberts & Slagstad 2015), as well as c. 1.03–1.01 Ga high- or ultrahigh-temperature (ultrahigh-T) metamorphism (Drüppel et al. 2012) further west in the orogen (Telemarkia terrane; see Fig. 17.1).
Stage 2 (c. 0.99–0.95 Ga)
The ductile tectonothermal evolution switched focus to the Eastern Segment at c. 0.99–0.95 Ga. Underthrusting along the boundary Mylonite Zone (Robin & Cruden 1994; Stephens et al. 1996; Viola & Henderson 2010; Fig. 17.1) resulted in the formation of eclogites at c. 0.99–0.98 Ga and at >60 km depth in the footwall to this zone (Möller et al. 2015; Tual et al. 2015; Fig. 17.6c); exhumation to depths of c. 35–40 km was steered by easterly-directed thrusting in a part of the Mylonite Zone where strike-slip movement was lacking (Möller et al. 2015; Fig. 17.6d). West-vergent folding of the gneissic fabric in the Idefjorden terrane is assumed here to be coeval with the thrusting in the Eastern Segment (Fig. 17.6d); the Mylonite Zone forming a positive flower structure (Stephens et al. 1996).
Anatexis under high-P granulite to upper amphibolite facies and major folding (e.g. Andersson et al. 1999, 2002; Möller et al. 2007, 2015; Hansen et al. 2015; Fig. 17.6e), combined with the emplacement of a dolerite dyke swarm in both the Eastern Segment and the cratonic foreland to the orogen (Söderlund et al. 2005) associated with east–west bulk crustal extension, all took place at c. 0.98–0.95 Ga. Decompression melting in connection with exhumation, or melting due to fluid infiltration possibly accompanied by mantle heat related to mafic magmatic activity, has been proposed (e.g. Möller et al. 2015). Initiation of west-vergent extension along the Mylonite Zone (Berglund 1997; Andersson et al. 2002; Viola et al. 2011) is also feasible at this time (Fig. 17.6e). The c. 0.99–0.95 Ga tectonothermal evolution in the Eastern Segment was again coeval with magmatic activity further west in the orogen (Vander Auwera et al. 2011; Slagstad et al. 2013, 2017; Roberts & Slagstad 2015).
Stage 3 (after c. 0.95 Ga)
Emplacement of quartz jotunite, norite–anorthosite, granite and dolerite at c. 0.95–0.92 Ga in the Western segment of the Idefjorden terrane (Årebäck & Stigh 2000; Eliasson et al. 2003; Hellström et al. 2004; Årebäck et al. 2008) marked a continuation of the extensional phase established during Stage 2; lamprophyre dykes were also emplaced into the Median segment at c. 0.92 Ga (Wahlgren et al. 2015). This magmatic activity was partly coeval with emplacement of a volumetrically more significant c. 0.93–0.92 Ga anorthosite–norite–mangerite–charnockite complex and ultrahigh-T granulite facies metamorphism in the southwestern part of the orogen (Telemarkia terrane: Schärer et al. 1996; Vander Auwera et al. 2011), again attesting to the spatially widespread thermal response during the orogenic activity.
Eastwards in the orogen, the latest deformational part of the orogeny included orogen-parallel upright folding, comprising a major synformal structure in the Median segment of the Idefjorden terrane and an antiformal dome-like structure in the Eastern Segment (Fig. 17.6f). The antiformal structure plunges northwards, exposing crustal levels deformed and metamorphosed at deeper crustal levels in the southern part of the Eastern Segment, compared with that further north between Vänern and the Caledonian erosional front in central Norway. Greenschist facies rocks lie tectonically on top of orthogneisses along east-dipping deformation zones with a top-to-the-east sense of shear in the eastern part of the antiformal structure (Andréasson & Rodhe 1990; Wahlgren et al. 1994; Fig. 17.6f). This shearing took place after emplacement of the early Tonian dolerites, prior to or during the time span c. 0.93–0.91 Ga, or slightly later in the southeasternmost part of the orogen (Andréasson & Dallmeyer 1995; Page et al. 1996b; Ulmius et al. 2018). Antithetic top-to-the-west shearing also occurred at c. 0.92–0.86 Ga along the southernmost part of the west-dipping Mylonite Zone (Johansson & Johansson 1993; Page et al. 1996a; Berglund 1997; Viola et al. 2011).
The features described above have been interpreted as forming in a crustal extensional regime, with a metamorphic core complex model for the southern part of the Eastern Segment after 0.95 Ga (Andréasson & Rodhe 1990; Viola et al. 2011; Ulmius et al. 2018). However, the shear zones associated with top-to-the-east movement in the eastern frontal part were rotated into a younger set of zones with reverse dextral kinematics, and the overall structural pattern along the eastern erosional front of the orogen is fan-like (Fig. 17.6f). Reflection seismic studies (Dahl-Jensen et al. 1991; Juhlin et al. 1989, 2000) show that the fan-like structure is a major crustal feature and that the west-dipping zones in the easternmost frontal part of the orogen become listric at depth (Juhlin et al. 2000; Fig. 17.6f). It is proposed in this Memoir (see Chapter 15, Stephens & Wahlgren 2020) that extension took place at higher crustal levels, while overall plate convergence continued to shorten the crust. This shortening could account for the late upright folding in both the Idefjorden terrane and Eastern Segment, and the development of the fan-like structure.
Tectonic cycles
The orogenic evolution in the Idefjorden terrane and Eastern Segment involved eastwards- and foreland-younging cycles of tectonic activity during stages 1 and 2. Each cycle initiated with crustal shortening involving underthrusting and high-P metamorphism. Metamorphic conditions at around 1.05 Ga (or possibly earlier) in the Idefjorden terrane and at around 0.99–0.98 Ga in the Eastern Segment suggest that the crust had shortened and thickened to more than 90 km and, subsequently, to more than 100 km in these lithotectonic units, respectively. Shortening and thickening were rapidly followed by exhumation of deeply buried crust, high-T metamorphism with anatexis, magmatic activity, folding and a changeover to an extensional tectonic regime. Continued plate convergence, bulk shortening and extensional deformation at higher crustal levels completed the Sveconorwegian orogenic evolution after 0.95 Ga during Stage 3. This stage was also accompanied by magmatic activity in the Idefjorden terrane and further west in the orogen (Telemarkia terrane), and the ultrahigh-T metamorphism in the latter.
Variation in crustal thickness in the eastern part of the Sveconorwegian orogen
There is a marked east–west thinning of the crust in southwestern Sweden, especially between the lakes Vättern and Vänern in the Eastern Segment (Fig. 17.7). Crustal thicknesses of more than 52 km east of the erosional front of the Sveconorwegian orogen rapidly decrease to around 40 km west of the Mylonite Zone inside the Idefjorden terrane (Fig. 17.7). A spatial relationship between the location of this change in crustal thickness and the reworking of the crust by the Sveconorwegian orogeny is apparent.
Variation in crustal thickness in parts of northern Europe (after Grad et al. 2009). The major lithotectonic framework in southwestern Scandinavia is also shown. There remains uncertainty as to what extent the window in the Caledonide orogen of southwestern Norway, marked as part of the Sveconorwegian orogen, is an allochthonous unit inside the Caledonide orogen (cf. Chapter 19, Gee & Stephens 2020).
Most of the crustal thickness variation in the Fennoscandian Shield east of the Sveconorwegian orogen is concentrated in a mafic, lower-crustal layer (Korja et al. 1993; Kozlovskaya et al. 2004). Such high-density material could provide a necessary condition for the foundering and break-off of lower continental crust and mantle lithosphere from the upper continental crust during orogeny. Thus, support is provided for a model involving delamination of the continental lithosphere, triggered by crustal thickening during the Sveconorwegian orogeny (Bingen et al. 2018).
Crustal thickness continues to decrease even further west in the shield to around 35 km west of the Oslo Rift (Fig. 17.7). However, thinning steered by crustal extension during the younger part of the Caledonian orogeny or during development of the Permo-Carboniferous Oslo Rift (Stratford & Thybo 2011) could also have contributed to crustal thickness variation close to and west of the Oslo Rift.
Sveconorwegian orogeny:subduction-related accretion or continent–continent collision?
Previous geodynamic models
Various models involving a convergent plate-boundary setting have been proposed for the 1.1–0.9 Ga tectonothermal evolution inside the Sveconorwegian orogen. Both accretionary orogenic processes along an active continental margin (e.g. Falkum & Petersen 1980; Slagstad et al. 2013, 2017; Coint et al. 2015) and collision of different continental plates along an orogenic suture (e.g. Berthelsen 1980; Gower et al. 1990; Karlstrom et al. 2001; Bingen et al. 2005, 2008a, b; Cawood et al. 2010; Möller et al. 2015) have been envisaged. The model with solely accretionary orogenesis involves subduction of oceanic lithosphere, comparable to, for example, the current Cordilleran mountain belts in the Americas. The model invoking collisional orogeny does not exclude the operation of accretionary processes during the earlier part of its evolution. However, it is characterized by subsequent continent–continent collision with subduction of continental lithosphere, comparable to the situation in the current Himalaya–Tibet orogen after c. 55 Ma.
In the solely accretionary orogenic model for the orogen, all the different lithotectonic units were assigned to the westerly margin of the continent Fennoscandia (Baltica) and stationed in the overriding plate of a subduction system with easterly polarity (e.g. Falkum & Petersen 1980; Slagstad et al. 2013, 2017; Coint et al. 2015). Much focus was placed on the voluminous, syn-orogenic magmatism in the Telemarkia terrane at c. 1.06–0.92 Ga, its tectonic affinity to subduction-related processes and its relationship to high-T or ultrahigh-T metamorphism. Two periods of slab roll-back and extension (1.06–1.02 and 0.99–0.92 Ga), encompassing a period of accretion to the Fennoscandian cratonic margin with compression and reduced magmatic activity (1.02–0.98 Ga), were envisaged (Slagstad et al. 2017). This accretionary orogenic scenario involves a continuation of the orogenic processes that had prevailed along this continental margin during the Paleoproterozoic and earlier Mesoproterozoic (e.g. Falkum & Petersen 1980; Åhäll & Connelly 2008; Roberts & Slagstad 2015; see Chapters 8 (Stephens 2020), 9 (Ripa & Stephens 2020a) and 11 (Wahlgren & Stephens 2020) in this Memoir).
Several tectonic scenarios for the continental plate Fennoscandia (Baltica) have been proposed in the context of the continent–continent collisional model (Möller et al. 2015). Downgoing plate solutions for Fennoscandia (Baltica) were envisaged in the models labelled 1 and 2 by Möller et al. (2015). In model 1, the high-P metamorphism in the Eastern Segment was related to the subduction of Fennoscandia (Baltica) following the descent of oceanic lithosphere along a west-dipping subduction zone. This model resembles the Silurian–Devonian continent–continent collision of Baltica and Laurentia in the Caledonide orogen (see especially Chapter 23 (Gee 2020) in this Memoir). In model 2, the high-P metamorphism was related to crustal thickening, and compared to the current situation in southern Tibet where the downgoing Indian Plate constitutes the lithotectonic assemblage (e.g. Tapponnier et al. 2001). Both models 1 and 2 indicate collision of the continent Fennoscandia (Baltica), preserved inside the Eastern Segment, with a separate continent represented in the Idefjorden terrane and other lithotectonic units further west in the orogen (Sveconorwegia continent of Brueckner 2009). The Mylonite Zone corresponds to an orogenic suture in both models. Möller et al. (2015) favoured a tectonic setting for the Eastern Segment corresponding to model 2 described above.
Alternative overriding plate solutions for all the lithotectonic units in the Sveconorwegian orogen with intracratonic tectonic burial were also envisaged in the continent–continent collisional scenario (models labelled 3 and 4 in Möller et al. 2015). These solutions compared the setting of the Eastern Segment to the northern or lateral eastern part of the Himalaya–Tibet orogen, respectively, with its transpressive deformation (Tapponnier et al. 2001). They permitted all the lithotectonic units in the Sveconorwegian orogen to belong to Fennoscandia (Baltica), and proposed that the downgoing colliding continent was not preserved on land in Scandinavia but was located further west.
Overriding plate setting along a convergent plate boundary
Relicts of oceanic lithosphere are absent along the regionally significant shear zones in the Sveconorwegian orogen. Furthermore, an intracratonic setting for the Mylonite Zone along the boundary between the Eastern Segment and the Idefjorden terrane, rather than an orogenic suture, has been inferred above (see also Åhäll & Connelly 2008). A tectonic setting where the different lithotectonic units belonged to the same continental plate prior to the onset of the Sveconorwegian orogeny, as envisaged, for example, by Slagstad et al. (2013, 2017), is proposed.
As discussed above, the high-T metamorphism with crustal melting at c. 1.05–1.02 Ga in the Idefjorden terrane overlapped temporally with syn-orogenic magmatic activity in the Telemarkia terrane in the western part of the orogen (Slagstad et al. 2018). High-T metamorphism with anatexis at c. 0.98–0.95 Ga also took place after crustal thickening in the Eastern Segment, while syn-orogenic magmatic activity occurred throughout the orogen (Slagstad et al. 2018). The Tonian activity even extended east of the orogen's current erosional front, with the emplacement of a swarm of dolerite dykes in the cratonic foreland (see Chapter 13, Ripa & Stephens 2020d). Polyphase high-grade metamorphism in the Telemarkia terrane has been linked to long-term underplating of mafic rocks derived from the mantle, resulting in melting of the lower crust (Slagstad et al. 2018).
These considerations suggest an overriding rather than a downgoing continental plate setting after c. 1.06 Ga for Fennoscandia (Baltica). For this reason, doubts are raised about the collisional models labelled 1 and 2 for the orogenic evolution in Möller et al. (2015), involving a downgoing continental plate situation for Fennoscandia (Baltica). An accretionary orogenic model for the Sveconorwegian orogen, as presented in, for example, Slagstad et al. (2017), or an overriding plate setting during protracted continent–continent collision, as discussed in the models labelled 3 and 4 in Möller et al. (2015), remain as more feasible alternatives. The sinistral transpressive component of shear deformation along zones with north–south strike inside the orogen is by no means inconsistent with the former and is actually supportive of the latter. Furthermore, high-P metamorphism is not a prerequisite for a downgoing plate in a continent–continent collisional system (Gilotti & McClelland 2007).
Accretionary or continent–continent collisional orogeny?
The proposed affinity of the syn-orogenic magmatism to subduction of oceanic lithosphere and its link to high-grade metamorphism in the Telemarkia terrane provide the key arguments for subduction-related accretionary orogenic activity at around and after 1.06 Ga (e.g. Falkum & Petersen 1980; Slagstad et al. 2013, 2017, 2018; Coint et al. 2015). Furthermore, the occurrence of Neoproterozoic (0.98–0.91 and 0.83–0.71 Ga) accretionary orogenic episodes to the north in the Arctic region (Valhalla orogen of Cawood et al. 2010) indicate that the orogeny, in a broader regional context, had not terminated at 0.9 Ga, opening the possibility that even the Sveconorwegian orogeny was accretionary. However, the Sveconorwegian orogeny shows two features distinctive from the earlier orogenic systems in the Fennoscandian Shield in Sweden, inferred to have formed in response to solely accretionary processes (see especially Chapters 8 (Stephens 2020), 9 (Ripa & Stephens 2020a), 11 (Wahlgren & Stephens 2020), 15 (Stephens & Wahlgren 2020) and 16 (Bergström et al. 2020) in this Memoir).
The eastwards- and foreland-younging of at least two separate cycles of orogenic activity in the eastern part of the Sveconorwegian orogen (Idefjorden terrane and Eastern Segment) resembles the developments in the Caledonide orogen during Silurian–Devonian time, when the continents Baltica and Laurentia collided. Deformation and metamorphism in the Caledonian allochthons derived from a setting closer to the cratonic foreland followed after similar tectonic events in the higher allochthons, which were derived from more outboard locations to the west. It is in marked contrast to the earlier Proterozoic orogenic systems in Sweden, where the tectonothermal activity developed successively westwards and outboard with time.
Secondly, the Sveconorwegian metamorphism associated with crustal thickening in the Idefjorden terrane and Eastern Segment occurred during high-P conditions, differing markedly from the consistently low-P and variable-T conditions associated with shortening during earlier Proterozoic accretionary orogenesis in Sweden. Although crustal thickening and high-P metamorphism can be related to continental subduction and accretion of a magmatic arc to an ancient continental nucleus, as proposed for the late Cambrian–Early Ordovician tectonic evolution in the Caledonide orogen in Sweden (see Chapter 21, Gee et al. 2020), we consider that the regionally significant contrast in metamorphic conditions signals significant differences in the orogenic processes.
Bearing in mind the two features discussed above, an overriding plate setting during protracted continent–continent collision around and after 1.06 Ga is tentatively favoured here. Subduction and accretion during the Sveconorwegian orogeny would have been restricted to an earlier phase, if this collisional hypothesis applies. The input of turbidites in the upper part of the Mesoproterozoic succession in Sweden (Dalsland Group: Fig. 17.2), and the continental back-arc setting proposed for the basaltic rocks in its middle part (Brewer et al. 2002), open the question of whether at least these parts of the succession formed in a retreating subduction mode during the earliest phase of the Sveconorwegian orogeny. Uncertainty remains concerning the identification of the colliding continental plate, and to what extent the strike-slip shear component along zones with north–south strike is related to oblique collision or to lateral extrusion related to crustal thickening, similar to that occurring today in the northern part of the Tibetan Plateau.
In order to shed more light on the key question addressed in this section, more studies are needed. An integrated evaluation of the tectonic setting of the spatially voluminous and temporally extensive magmatic activity around and after 1.06 Ga, in different lithotectonic units in the Sveconorwegian orogen and even in its eastern foreland at 0.98–0.95 Ga, is necessary. Instead of being steered by the subduction of oceanic lithosphere, the magmatism and associated crustal anatexis were possibly triggered by delamination of continental lithospheric mantle, caused by crustal thickening at different times during the protracted orogeny (Bingen et al. 2018). Continental lithosphere slab break-off could have resulted in asthenosphere uprise, the generation of mafic magma and mafic underplating with heat supply to the crust. Such a mechanism would also be consistent with the evolution in each of the tectonic cycles identified in the Swedish part described above.
Grenvillian–Sveconorwegian connection
Advocates of the collisional model for the Sveconorwegian orogeny (e.g. Berthelsen 1980; Gower et al. 1990; Karlstrom et al. 2001; Bingen et al. 2005, 2008b; Möller et al. 2015) have compared its tectonic evolution with Grenvillian orogenesis. The latter was related to the collision of the continents Amazonia and Laurentia in connection with the assembly of the supercontinent Rodinia (e.g. Rivers 1997, 2008; Gower et al. 2008; Li et al. 2008; Hynes & Rivers 2010). Collision took place diachronously, starting southwards and moving northwards along the Laurentian margin (e.g. Chew et al. 2011). Continuation of the northwestern erosional front of the Grenville orogen into the eastern front of the Sveconorwegian orogen is commonly proposed (e.g. Falkum & Petersen 1980; Romer 1996; Karlstrom et al. 2001; Li et al. 2008). However, the presence of orogenic fronts on opposite sides of the Grenville–Sveconorwegian orogen (e.g. Gower 1990; Park et al. 1991) has also been suggested.
The spatial relationship between Laurentia, Amazonia and Baltica, in the area where the three continental plates meet (Fig. 17.8), is complex. Firstly, instead of the Grenville orogen linking directly with the Sveconorwegian orogen in Baltica, a major dextral lateral ramp structure has been identified in northeasternmost Canada (Gower et al. 2008). Deformation with a strike-slip component of movement is present on a regional scale in this part of the late Mesoproterozoic–early Neoproterozoic global orogenic system (Fig. 17.8). Secondly, an interplay between collisional (e.g. Grenvillian and Sveconorwegian) and accretionary (Valhalla) orogenic systems has been inferred to occur close to the meeting point of the three continental plates (Fig. 17.8; see also Cawood et al. 2010). Finally, Amazonia collided with Laurentia and caused Grenvillian orogenesis around 1.10 Ga in the northeastern part of Laurentia (Hynes & Rivers 2010), earlier than the current estimate for the onset of collision in the Sveconorwegian orogen at around 1.06 Ga in Fennoscandia (Baltica).
Late Mesoproterozoic–early Neoproterozoic orogens shown on a 0.9 Ga palinspastic reconstruction of the continents Laurentia, Baltica and Amazonia inside the supercontinent Rodinia, based on Cawood et al. (2010) and Johansson (2014). The black arrow indicates the approximate direction of Sveconorwegian crustal shortening in Fennoscandia (Baltica). Dextral strike-slip movement took place along the southernmost tip of the Mylonite Zone, where it strikes ENE–WSW.
In combination with the inferred overriding plate setting for Baltica during the Sveconorwegian orogeny, the points raised in the previous paragraph open questions around the continuity of the northwestern and eastern erosional fronts of the Grenville and Sveconorwegian orogens, respectively. A non-continuous relationship, in which the two orogenic fronts represent opposite sides of the Grenvillian–Sveconorwegian orogenic system (e.g. Gower 1990; Park et al. 1991) and where NW–SE convergence prevailed in the Sveconorwegian part (Park et al. 1991), requires renewed consideration. Slagstad et al. (2019) even questioned the proximity of Laurentia and Baltica during the Grenvillian–Sveconorwegian orogenesis, favouring a tectonic model where these two continental plates were unrelated between 1.5 and 0.45 Ga.
Conclusions
Development of the Idefjorden terrane in an indigenous outboard location to the continental plate Fennoscandia (Baltica), north of its current position in southwestern Scandinavia, is favoured over a more exotic heritage. The orogen-parallel, regionally significant ductile shear zones, showing a sinistral transpressive component of deformation during the Sveconorwegian orogeny, can account for the contrast in the pre-Sveconorwegian orogenic evolution between this allochthonous lithotectonic unit and the crust further east (Eastern Segment and cratonic foreland). By corollary, the shear zone separating the Eastern Segment from the Idefjorden terrane (Mylonite Zone) is suggested to have developed in an intracratonic position during the Sveconorwegian orogeny (Åhäll & Connelly 2008), and does not represent an orogenic suture.
An overriding rather than downgoing continental plate setting for both the Idefjorden terrane and the Eastern Segment, during either an accretionary orogeny or protracted continent–continent collision, is inferred. The progressive foreland development of the tectonothermal evolution and the character of the metamorphism differ from that observed in the earlier accretionary orogenic systems in the Fennoscandian Shield in Sweden, providing a support to the protracted continent–continent collision hypothesis. Some questions arise concerning the continuity of the exposed erosional fronts of the Grenville and Sveconorwegian orogens.
There is an urgent need for integrated research on the tectonic evolution in the entire Sveconorwegian orogen, rather than isolated studies of certain aspects of this evolution in different parts. Attention should be addressed on the potential tectonic relationship between 1.6–1.5 Ga rapakivi magmatism in the cratonic foreland and the 1.6–1.5 Ga orogenic activity in the lithotectonic units west of the Mylonite Zone. Secondly, was the Sveconorwegian magmatic activity around and after 1.06 Ga related to the subduction of oceanic lithosphere or to the effects of break-off of continental lithospheric slabs following different stages of crustal thickening? Finally, seismic imaging in both the Eastern Segment and the Idefjorden terrane is required to resolve the deeper crustal structure in this part of the orogen with a focus on the late-orogenic (<0.95 Ga) tectonic evolution.
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 (lead); CHW: writing – original draft (supporting).
- © 2020 The Author(s). Published by The Geological Society of London. All rights reserved
References
Chapter 17 Accretionary orogens reworked in an overriding plate setting during protracted continent–continent collision, Sveconorwegian orogen, southwestern Sweden
- Article
- Abstract
- Internal lithotectonic framework
- Idefjorden terrane: exotic or simply an indigenous outboard component of Fennoscandia (Baltica)?
- Sveconorwegian tectonic evolution at c. 1.05–0.90 Ga in the Idefjorden terrane and Eastern Segment
- Grenvillian–Sveconorwegian connection
- Conclusions
- Acknowledgements
- Funding
- Author contributions
- References
- Figures & Data
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