CommentA non-collisional, accretionary Sveconorwegian orogen –Comment

Charlotte M€oller,1 Bernard Bingen,2 Jenny Andersson,3 Michael B. Stephens,3,4 Giulio Viola2,5 andAnders Scherst�en11Department of Geology, Lund University, S€olvegatan 12, SE-223 62 Lund, Sweden; 2Geological Survey of Norway, Postboks 6315 Slup-

pen, 7491 Trondheim, Norway; 3Geological Survey of Sweden, Box 670, SE-751 28 Uppsala, Sweden; 4Department of Civil, Environmental

and Natural Resources Engineering, Division of Geosciences, Lule�a University of Technology, SE-971 87 Lule�a, Sweden; 5Department of

Geology and Mineral Resources Engineering, Norwegian University of Science and Technology, Sem Sælands veg 1, 7491 Trondheim,

Norway

In a stimulating contribution, Slag-stad et al. (2013) propose to interpretthe 500-km-wide Sveconorwegianorogen in Scandinavia as a non-colli-sional, accretionary orogen. Theychallenge the more widely held viewthat it is the product of a late Meso-proterozoic to early Neoproterozoiccontinent–continent collision, formedduring welding of supercontinent Ro-dinia. We question the model pro-posed by Slagstad et al. (2013) as itfails to integrate the lithotectonicand tectonometamorphic frameworkof the entire orogen, in particular thepresent-day eastern parts where theprincipal records of high-pressuremetamorphism and collisionaltectonics have been retrieved.Currently, the late Palaeoprotero-

zoic to Mesoproterozoic evolutionalong the Grenvillian and Sveconor-wegian margins of proto-Laurentiaand proto-Baltica is pictured as along-lived accretionary system,affected by cycles of variably retreat-ing and advancing subducting slabs(the “tectonic switching” model ofCollins (2002); e.g. Rivers and Corri-gan, 2000; Karlstrom et al., 2001;Hermansson et al., 2008; �Ah€all andConnelly, 2008). Accretion suppos-edly ended when an unknown conti-nent, possibly Amazonia, interactedwith the subduction system, resultingin continent–continent collision.Slagstad et al. (2013) report new

geochemical and geochronologicaldata on orthogneisses from the Ro-galand–Vest-Agder sector in the west-

ernmost Sveconorwegian orogen.They demonstrate a wider distributionthan previously recognized of compar-atively juvenile, calc-alkaline pluto-nism in the 1050–1020 Ma timeinterval (the Sirdal Magmatic Belt;their Fig. 2), and propose an Andeanvolcanic arc setting. In our view,problems arise when they propose toextend the accretionary model (theirFig. 5) to the entire time intervalbetween 1050 and 920 Ma, and to useit as an orogen-scale tectonic model.Their suggestion implies that proto-Baltica did not meet another conti-nent during its wandering between atleast 1.9–1.8 Ga (Svecokarelian oro-geny) and c. 430 Ma (Caledonianorogeny).

Lithotectonic framework in theeastern part of the orogen

The major lithotectonic units insidethe eastern part of the orogen arethe Eastern Segment and the Idefjor-den terrane (Bingen et al., 2008).The Eastern Segment consists pre-dominantly of reworked 1.8–1.7 Gafelsic and subordinate mafic intrusiverocks, identical to the Transscandi-navian Igneous Belt exposed farthereast, outside the orogen (e.g. Wahl-gren et al., 1994; S€oderlund et al.,1999; M€oller et al., 2007). This litho-tectonic unit can be directly linkedto the remainder of the Fennoscan-dian Shield and is thus parautoch-thonous.An up to 5-km-thick shear belt,

the “Mylonite Zone” separates theEastern Segment from the tectoni-cally reworked, predominantly 1.6–1.5 Ga sedimentary and calc-alkalinemagmatic rocks of the Idefjorden ter-rane (e.g. �Ah€all and Connelly, 2008).

The Mylonite Zone can be traced forat least c. 450 km and shows a com-plex Sveconorwegian structural evo-lution. At an early stage, the shearbelt accommodated transpressivedeformation with reverse, top-to-the-ESE displacement (Stephens et al.,1996; Berglund, 1997; Viola andHenderson, 2010). This was followedby top-to-the-W extension (Berglund,1997; Viola et al., 2011). The Idefjor-den terrane and other lithotectonicunits farther west have been dis-placed out of their original tectoniccontext, and are thus allochthonouswith respect to the FennoscandianShield (Andersson et al., 2002).Any orogen-scale tectonic model

requires critical evaluation of thestructural evolution within andbetween the constituent lithotectonicunits and of the allochthoneity ofthese units. This is totally missing inSlagstad et al. (2013).

Tectonometamorphic framework

Slagstad et al. (2013) divide the Sveco-norwegian orogeny into three timeslices, 1050–1020 Ma, 1020–990 Maand 990–920 Ma (their Fig. 5). Below,we discuss each of these time slicesfrom a tectonometamorphic perspec-tive, as this aspect – especially pressure– is critical in a geodynamic context.

1050–1020 Ma: High-pressuremetamorphism in the Idefjordenterrane

Slagstad et al. (2013) interpret volu-minous, mainly felsic, calc-alkalineplutonism in the Rogaland–Vest-Agder sector as resulting from aretreating slab and the connectedback-arc extension in the entire

Correspondence: Charlotte M€oller,Department of Geology, Lund University,

S€olvegatan 12, SE-223 62 Lund, Sweden.

Tel.: +46 46 222 78 78; fax: +46 46 222

44 19; e-mail: charlotte.moller@geol.lu.se.

© 2013 Blackwell Publishing Ltd 165

doi: 10.1111/ter.12029

orogen (their Fig. 5A). However,high-pressure granulites and kyanite-bearing gneisses in the Idefjordenterrane (their Fig. 3) record high-pressure metamorphism at 1.0–1.5GPa (35–55 km depth) and 700–740 °C, at 1050–1020 Ma (Bingenet al., 2008; S€oderlund et al., 2008).This metamorphism took place con-comitant with the Ottawan orogenicphase in the Grenville orogen (Ind-ares et al., 1998; Carr et al., 2000;Rivers, 2009; Rivers et al., 2012).Thus, during the 1050–1020 Ma timeperiod, the Idefjorden terrane under-went compression, crustal imbrica-tion, deep burial and (at leastpartial) exhumation. This needs to beintegrated in an orogen-scale tectonicmodel – a simple retreating slabmodel will not suffice.

1020–990 Ma: Ultrahigh-temperaturemetamorphism in the Rogaland–Vest-Agder sector

The Rogaland–Vest-Agder sector ischaracterized by ultrahigh-tempera-ture granulite facies metamorphism.The most recent data by Dr€uppelet al. (2013) indicate that this meta-morphism took place at 1010 Maand peaked around 1000 °C and0.75 GPa, following a clockwisepressure–temperature path. Thus,uncommonly high mid-crustal tem-peratures were attained during thisphase of regional Sveconorwegianmetamorphism. Slagstad et al. (2013)interpret the 1010 Ma metamorphismas a result of compression and flatslab subduction of an oceanic pla-teau (their Fig. 5B). Underthrustingof a cold and relatively unradiogenicoceanic plateau would, however, iso-late the overlying continental crustfrom the underlying asthenosphericmantle and prevent major heating ofthe crust. We also note that ultra-high-temperature metamorphism inthe Rogaland–Vest-Agder sector iscoeval with amphibolite facies meta-morphism in the Telemark sectorand Idefjorden terrane to the east(Bingen et al., 2008); widespreadmetamorphism is difficult to recon-cile with inversion of a narrow back-arc basin. Furthermore, although thegeodynamic setting leading to ultra-high-temperature metamorphismremains poorly constrained, we notethat recent thermo-mechanical mod-

elling by Clark et al. (2011) favoursa long-lived collisional phase withhigher than normal radioactivedecay, and low erosion rates.

990–920 Ma: Eclogite and high-pressure granulite metamorphism ofthe Eastern Segment and subsequentevolution

Slagstad et al. (2013) interpret wide-spread ferroan granite plutonism (990–920 Ma) and anorthosite-mangerite-charnockite plutonism (950–920 Ma),extending from the Rogaland–Vest-Agder sector to the Idefjordenterrane, as a record of renewedretreating of a subducting slab andback-arc extension (their Fig. 5C).The interpretation is tentative as thiskind of magmatism can stem fromother genetic processes (e.g. VanderAuwera et al., 2011). Importantly, theparautochthonous Eastern Segmentunderwent kyanite eclogite (>1.5 GPa,>55 km depth) and high-pressuregranulite (0.9–1.2 GPa, 30–45 kmdepth) metamorphism at 990–970 Ma(M€oller, 1998, 1999; Johansson et al.,2001; Hegardt et al., 2005), broadlycoeval with the Rigolet phase in theGrenville orogen (Rivers, 2009; Riverset al., 2012). This eclogitization atteststo westward subduction and under-thrusting of Fennoscandian continen-tal lithosphere, concomitant withreverse top-to-the-ESE kinematicsalong the Mylonite Zone. This evolu-tion is compatible with collisional tec-tonics, but not with a retreatingsubducting slab and extension.The post-980 Ma tectonic evolu-

tion in the Eastern Segment includedcrustal-scale anatexis at 980–960 Ma(Andersson et al., 1999, 2002; S€oder-lund et al., 2002; M€oller et al., 2007),and the intrusion of a dolerite dykeswarm at 980–945 Ma (within and tothe east of the Eastern Segment) inresponse to E-W extension (S€oder-lund et al., 2005; Stephens et al.,2009). Contrary to Slagstad et al.(2013), who state that “There is noevidence of large-scale extensionalstructures that would be expected ifthe orogen had undergone orogeniccollapse, …”, we note that a recordof late orogenic extensional tectonicshas been described (Berglund, 1997;Viola et al., 2011). Extension wasfollowed by renewed regional top-to-the-east shortening, in the eastern-

most Eastern Segment dated at 930–905 Ga (Wahlgren et al., 1994; Pageet al., 1996), overlapping or post-dat-ing the 950–920 Ma anorthosite-mangerite-charnockite plutonism inthe Rogaland–Vest-Agder sector andlate orogenic magmatism at 930–920 Ma in the Idefjorden terrane(Eliasson and Sch€oberg, 1991;Scherst�en et al., 2000, 2004; Hell-str€om et al., 2004).

Conclusion

The facts and reasoning outlinedabove lead us to consider the accre-tionary model proposed by Slagstadet al. (2013) problematic. Its simplic-ity and elegance cannot serve as analibi to overlook both the record ofwhat is most readily interpreted ascollisional tectonics and the similari-ties to (rather than differences from)the Grenville orogen. We summarizeour considerations by addressing thefollowing key questions to Slagstadand coauthors:1 Why was the structural evolutionwithin and between different litho-tectonic units ignored and what arethe consequences of allochthoneity,including possible orogen-paralleltransport, for the orogen-scaleaccretionary model?

2 How are the high-pressure meta-morphism and the implica-tions thereof reconciled with theproposed accretionary tectonicmodel, including the inferred solelyeast-directed subduction polarityover the entire time period 1050–920 Ma?

3 How is the termination of the pro-posed 1050–920 Ma accretionaryorogenic system envisaged?

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Received 2 December 2012; revised version

accepted 21 January 2013

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