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A non-collisional, accretionary Sveconorwegian orogen – Reply

Trond Slagstad,1 Nick M. W. Roberts,2,3 Mogens Marker,1 Torkil S. Røhr1 and Henrik Schiellerup11Geological Survey of Norway, Postboks 6315 Sluppen, Trondheim 7491, Norway; 2Department of Geology, University of Leicester,

Leicester LE1 7RH, UK; 3NERC Isotope Geosciences Laboratory, Keyworth, Nottingham NG12 5GG, UK

Orogeny involves magmatic, meta-morphic, deformational and erosionalprocesses that are caused by or leadto crustal thickening and the develop-ment of high topography. In general,these processes operate along the mar-gins of continental plates, either as aresult of subduction of oceanic crustor collision between two or more con-tinental plates; referred to here asaccretionary and collisional orogeny,respectively. As pointed out by us ear-lier (Slagstad et al., 2013), many ofthese processes are common to accre-tionary and collisional orogeny, anddo not uniquely discriminate betweenthe two. For example, major crustalshortening and high-grade metamor-phism are hallmarks of any orogen,whereas the composition of syn-orogenic magmatic rocks will differsignificantly between accretionary(mantle-wedge + crustal source) andcollisional (mainly crustal source) oro-gens. Our study in the western Sveco-norwegian province has focused on itsmagmatic evolution, which appears tobe more compatible with accretionaryrather than traditionally held colli-sional orogeny. Our model is clearlyimmature at this stage, and althoughwe do not attempt to explain theobserved metamorphic and deforma-tional record, as pointed out byM€oller et al., this is mainly a short-coming of our article rather than ourmodel. The points raised by M€olleret al. mainly pertain to metamorphicand deformational processes that aregenerally non-unique to accretionaryand collisional orogeny (e.g., DeCelleset al., 2009), and below we show howobserved deformation and metamor-phism within the Sveconorwegian

province can be as well, or better,understood by accretionary processes.

Subduction and high-grademetamorphism at 1050–1020 Ma

The geochronological, geochemicaland isotopic data from the SirdalMagmatic Belt (SMB) are compatiblewith it representing a continental arcbatholith formed between 1050 (pos-sibly as early as 1070 Ma accordingto our new, unpublished data) and1020 Ma. M€oller et al. argue thathigh-pressure metamorphism in theIdefjorden terrane, farther east in theorogen, is incompatible with our pro-posed retreating subduction zoneduring this period. First of all, westated that “[observations] may indi-cate an extensional convergent set-ting at least periodically during SMBarc magmatism, but the evidence forthis is as yet weak”. Whether thesubduction zone was in extension orcompression is in no way crucial toour model, and it may well havealternated between the two.In our view, interpreting the SMB

as having formed by crustal melting ina collision zone does not explain itsgeochemical and isotopic composition,nor does it explain why it started form-ing earlier that the onset of high-grademetamorphism. We note that M€olleret al. do not question our interpreta-tion of the SMB. If the SMB formedin an arc at 1050–1020 Ma, a likelyinterpretation of the coeval high-pres-sure metamorphism recorded in theIdefjorden terrane is that it representsretroarc thrusting, as discussed by De-Celles et al. (2009). As more data,including high-precision geochronol-ogy, become available from the Sveco-norwegian province, we can test thisidea because DeCelles et al. makesome detailed predictions about howarc magmatism responds to events inthe retroarc; but it certainly does notrequire collision per se.

UHT metamorphism at 1010–1005 Ma

Dr€uppel et al. (2013) recently docu-mented that Sveconorwegian-ageUHT metamorphism in SW Norwayinvolved temperatures of c. 1000 °C atpressures around 7.5 kbar. Such PTconditions cannot, as far as we know,be produced in collisional orogens,although both Dr€uppel et al. andM€oller et al. cite modelling by Clarket al. (2011) as evidence that this ispossible simply through crustal thick-ening and internal radiogenic heat pro-duction. However, in Clark et al.’smodels such temperatures were onlyreached at depths of 40 km below alarge orogenic plateau and took 55–60million years to reach following thick-ening. This is in contrast to the 20–25 km depth estimated by Dr€uppelet al. and the 10–15 Myr time lag fromcessation of SMB magmatism to UHTconditions. Obtaining such tempera-tures (at 40 km depth) also requires aheat-production rate of 4 lW m�3,not < 2 lW m�3 as demonstrated bySlagstad (2008) and Slagstad et al.(2009). Modelling by Gapais et al.(2009) shows that c. 1000 °C at depthsbetween 15 and 20 km can be pro-duced in accretionary orogens 15 Maafter the onset of compression. This isvery nearly identical to the time frameimplied by our model.M€oller et al. argue that our model

cannot produce such high tempera-tures because we invoke underthrust-ing of low-heat producing oceaniccrust. This is apparently a misunder-standing, as we have not claimedthat radiogenic heat production isthe heat source. The heat in ourmodel is derived from the precedingto coeval arc magmatism. Also, oce-anic-plateau subduction is just oneway of decreasing slab dip; anotherwould be to subduct an oceanicspreading ridge, thereby allowingdirect contact between asthenosphere

Correspondence: Dr. Trond Slagstad, Geo-

logical Survey of Norway, Leiv Eirikssons

vei 39, Trondheim 7491, Norway. Tel.: +47

73 90 42 29; fax: +47 73 92 16 20; e-mail:

trond.slagstad@ngu.no

© 2013 Blackwell Publishing Ltd 169

doi: 10.1111/ter.12028

and lower crust (e.g., Santosh et al.,2012). The UHT metamorphism inSW Norway can, therefore, beexplained satisfactorily in an accre-tionary, but not a collisional, model.

Post-Sveconorwegian (990–920 Ma) evolution and 980–970 Ma eclogite

We maintain that we find it difficultto understand how 70 Ma of appar-ently continuous ferroan magmatismin the Sveconorwegian provincecould have been fuelled by a delami-nation event following crustal thick-ening. That said, we acknowledgethat the geochronological record offerroan magmatism is scant, and that70 Ma of continuous back-arc exten-sion without evolving into rifting andocean-floor spreading may be equallyunlikely. We are currently dating alarge number of post-Sveconorwe-gian granites in South Norway andhope to establish time-integrated esti-mates of the magmatic activity dur-ing this period. It is likely that wewill find periods of voluminous activ-ity separated by more quiescent peri-ods, possibly related to periods ofextension and compression along themargin, respectively. Periods of com-pression may then explain featuressuch as the 980–970 Ma eclogites inthe Eastern Segment, followed byexhumation during periods of exten-sion. At present we are lacking con-straints to do anything but speculateon the possibility of such a model.That said, the collisional model isclearly problematic in explaining theformation of the Eastern Segmenteclogites as there is no evidence ofhigh-grade metamorphism and defor-mation farther west at this time. It ispossible that the event represents afinal amalgamation of units west ofthe Mylonite Zone (MZ) with unitsto the east, as suggested by M€olleret al., but this does not alter oursuggestion that amalgamation tookplace in an accretionary setting.

Allochthonous, but not byinference exotic

M€oller et al. argue that because therehas been significant movement alongthe MZ between 980 and 968 Ma(Andersson et al., 2002), units west ofthe MZ are allochthonous with respect

to the Fennoscandian Shield. Otherworkers have suggested that units eastand west of the MZ represent continu-ous, Mesoproterozoic continentalmargin growth (Andersen et al., 2004;�Ah€all and Connelly, 2008). It is possi-ble that partial or complete separationbetween units east and west of the MZtook place during the late Mesoprote-rozoic evolution of the SW Balticamargin, and that the eclogite-faciesevent in the Eastern Segment repre-sents a reamalgamation of these units.However, we do not see any reason toassume that units east and west of theMZ are exotic to each other, or thatthere has been significant orogen-par-allel transport. We also fail to see howthis would in any way support a colli-sional model.

Termination of theSveconorwegian orogeny

Our interpretation of the HBG suiteand the Rogaland AMC complex isthat they formed behind an activemargin. As of yet we have no knownrecord of what took place along thismargin after c. 920 Ma, includingwhether or not the orogenic systemactually terminated. There is, forexample, ample evidence of tectono-thermal events along the Baltica andLaurentian margins following theGrenvillian–Sveconorwegian orogeny(e.g., Kirkland et al., 2007; Cawoodet al., 2010) that may be incorporatedin a model involving continued oro-genesis. It is also possible to envisagecontinued southwestard migration ofthe active margin, leaving an essen-tially passive margin behind, as hasbeen proposed for the Terra Australisorogen in eastern Australia (Cawoodet al., 2011). Also, although most pas-sive margins develop from continentalrifting and drifting, examples of activemargins evolving into passive marginsare well documented (e.g., Jabaloyet al., 2003), and cannot be ruled out.

Summary

M€oller et al. do not attempt to inte-grate our data regarding formationof the SMB in their collisionalmodel, and fail to acknowledge thatapparently well-documented Sveco-norwegian UHT metamorphism isproblematic in a collisional orogeny.We also miss a discussion of alterna-

tive interpretations of the features(large-scale thrusting and high-grademetamorphism) that they claimrequire collision. In our opinion,these features are integral in bothcollisional and accretionary orogens.To conclude, we would like to

acknowledge the study carried out bynumerous workers in the Sveconorwe-gian province over the years, includingthat of M€oller and co-authors. How-ever, our recent study shows that sev-eral features in the orogen are difficultto reconcile with a collisional orogenyand that this fundamental premiseneeds to be discussed. We are happyto see that our article has promptedjust such a discussion.

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Slagstad, T., Roberts, N.M.W., Marker,M., Røhr, T.S. and Schiellerup, H.,2013. A non-collisional, accretionarySveconorwegian orogen. Terra Nova,25, 30–37.

Received 7 January 2013; revised version

accepted 21 January 2013

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