Lithologies and metamorphism in the Nordfjord-Stadlandet area

MAARTENKRABBENDAM&ALlCEWAIN NGU- B UL L 432 ,19 97 - PA G E 127

Late -Caledonian structures, differential retrogression and structural position of (ultra)high-pressure rocks in the Nordfjord - Stadlandet area, Western Gneiss Region

MAARTEN KRABBENDAM&ALlCE WAIN

Krab bendam,M. & Wain,A.1997.Lat e-Caled o ni an str uc t ures,diffe rentialretr og ressionandst ructura l position of (ult ra)hi g h-pressu rerocksinthe Nordfjo rd-Stadlandetarea,Weste rn GneissReg io n.Norgesqeoloqiske undersekelse Bulletin432,127-139.

Late-Caledo nia n, amph ibolit e-facies st ru ctu res in the Nordfjord-Sta d landetarea com pris ean 8 km-widezoneof non -coaxial shear belowtheNo rdfj ord -SognDeta chm ent (NSD).Belo wthis zone,coaxia lstruc t ures dom in at e.The gneissic layerin gwit hin the coaxialzone,in itspresent for m,result edmainlyfrom pervasive late-Caled oni an east- westext en sion . East-westtrend in gfo ld hingesand prol atefeldspar auge nind icate substantialnort h-southshort- ening durin g east-westext ension.This pervasive deform ati onplayedasignificantro le in the diffe rent ial retrogressi- onofeclog ite-faciesassemb lages, sothatmostfelsicrocksnowhave am p hibo lite-faciesassem b lage s. Integration of petrologicaldata withthe struct ural data showsthat at tenuationof the crustisunevenl ydistribu t ed andthatlocal- ly ult rahig h-pressure(UHP)eclogitesoccurst ruct u rallyabo ve HP(P- 20-24 kba r)eclog ites.So m e15 kmofcrust has beenexhumedfromabovethe NSD.S7km of crust isnowcutout bythe NSD, buton lyabo uthalfof thathasbeen exhu med by non -coaxialst rain.Below the NSDat tenu at io nto72- 86%occurred. Lo cally,how ever, atte n uat io n to 93-96%has occur redas sugges te dbythe occurr enceof HPand UHP eclogiteslessthan1 km apa rt . Even so,nodis- tincthigh-strain zonesoccu rin the vicinity of UHPeclogite pod s.

Maarten Krabbendam & A/iceWain,Department ofEarthSciences,UniversityofOxford,OxfordOX1 3PR,UK.

Email:martin k@earth.ox.ac.uk. (Addressfor M.K.from1/7-98:Depar tment ofEarth Sciences,MonashUniversity,Clayton, Victoria3768,Australi a).

Introduction

The Nordfjord- Stadlandetarea is a critical areawit hin the Scandin avianCaledonide sbecause it contains some of the deepest buriedrocks of theWestern GneissRegion (WGR), as indicated by the occurrence of coesite-eclog ite bodies (Smit h 1984,1995)and alsothe most shallow rockswhich were deformed duringthe Devonian,asexemplified by the Devonian Hornelen Basin (Steel et al. 1985). Bryhni (1966) provides a very usefuldescription of rock typesandst ruct u- res of the southern part of the study area, whereasLappin (1966)madea thorough study of the northern part. Since then,mostwor k has concentratedon petrologicaland mine- ralogical studies of eclogite pod s(e.g.Bryhn i &Grimstad 1971,Smith 1988,1995).Thepresence of coesitein eclogite in thearea,indicati ng peak meta morphic pressuresof~28 kbar (ult rahigh pressureor UHP hereaft er)was reported by Smith(1984) and Smith&Lappin(1989). Wain (in press)des- cribesmany new coesite or coesite-pseudomorph-bearing eclogiteoccurrencesin the area,which canbeusedto deli- neate adistinct ultrah igh-pressure (UHP) zoneand a high- pressure(P<25kbar) zone.

The purpose of this paperisto describ ethe amphibolite- facies, late-Caledon ian struct ures wit hin the Nordfjord- Stadland et area, therole ofamphi bolite-faciesdeforma tion in diffe renti al retrogression of eclogi te-facies rocksduring exhu mat io n,and the relative structu ralposit ionof thecoe-

site-eclog itepodswit h respectto non-coesitebearing eclo- gites,granul iti cbodies and theDevonianHornelenBasin,and to discuss the uneven distibution ofatte nuati on throughout the crust duringtheexh umat ionof(U)HProcksinthe area.

Geological setting

The study area is located in the western part of the WGR,a large basement window of Proterozoic gneisses,reworked during the CaledonianOrogeny(Diet ler et al. 1985, Kullerud et al. 1986).The occurrenceofeclogite and coesite-eclogite indicates thatat least the western partof the WGR has been buriedto greatdepth(>100 km,Smith 1984,Waininpress).

In theeast and southeast,the WGRis overlainby Scandian thrust nappes(e.g.Roberts &Gee 1985); in the northe ast these thrustnapp eshavebeen traced indeep synclinesinto the WGR(e.g.Robinson1995).

In the west,theWGR is separated from allochthonous rocks and Devonian sediments by the Nordfjord-Sogn Detachment (NSD), whichhas a top-to-the-westshear sense and hasplayedanimport ant rolein the exhumationof the WGR and in the development of the Devonian basins (Norton1987,Seranne&Sequret 1987, Andersen&Jamtveit 1990, Andersen et al. 1991). A second detachment,the Hornelen Detachment,lies st ruct urally abovethe NSD,sepa- ratin g the'Midd lePlate'from the'Upper Plate' (Andersen&

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Jamtveit 1990,Dewey et al. 1993). The'Upper and Middl e Plate'terminology describesthe tectonostratigraph icposi- tion of rock unitsaft er late-Caledon ian extension, whereas the more classical'Upper andMiddle Allocht hon'terminol- ogy (e.g. Roberts &Gee 1985) describes thetecton ostrati- graphicposition of thrust nappesafterCaledonianthrusting butbefore late-Caledonian exten sion .The expo sedporti on of the Upper Plate comp rises mostly,but not exclusively, rocksof the UpperAllochtho nand the Hornelen Basin itself;

the Middle Plate comprisesmostly,but not exclusively,rocks ofthe Middle Allochthon. Noneof the allochthon ousrocks in the west have experi enced Caledonian HP metamor - phism.OnBremangerland et, in theSWof the st udyarea,the HornelenBasinunconformablyoverliestheKalvaq Melange (Bryhni & Lyse 1985, Steen & Andresen 1997) and a Caledoniangabbro-norite-granod iorite intrusion.This intru- sion hasbeendatedat 380±26 Ma(Srn-Nd)and390±29 Ma(Rb-Sr) (Furnesetal. 1989),thusprovid ing a rat herloose- lyconst rained, maximumage fortheonset of deposition in theHornelenBasin.

On the northernsideof the Hornelen Basin,theNSDand the Hornelen Detachm ent convergeandcannot bedistin- guished;herethe Devonian rocks are juxt aposed almost directl y againstWGRrocks,separated by a mylonitezone,a few hundred metres thick(Plate 1,Dransfield 1994).West of Hornelen Mountain, the two detachment s diverge (Andersen & Jamtveit 1990, Hartz et al. 1994) and the Dalsvatn Fault can be taken as a continuation of the Hornelen Detachme nt,whereastheVetvikaShear Zone can be regarded asthe continuationof the lower NSD.Both the NSD and the Hornelen Detachm ent are folded with the Devonianrocksoccurringin theHornelen Synform(Seranne

& Sequret 1987, Andersen &Jamt veit 1990,Chauvet &

Serann e1994), so that, in the study area, thedetachments are south dipp ing and haveadextral shear sense(Plate 1, Hartz et al. 1994,Dransfield 1994).

Lithologies and metamorphism in the Nordfjord-Stadlandet area

The Nordfjord-Stadlandetareaof the Western Gneiss Region contains a wide variety of lith ol ogi cal-metamorphic units withparageneses corresponding topre-Caledonian(granu- lite),peak-Caledonian (eclogite) and late-Caledonian (am- phibol it e-facies)stages,which canberelated via progressive deformation and recrystallisation. Bot h ultrahigh-pressure (UHP) and non-ultra high-pressure (t ermed HP throughout this paper)eclogites are found in this area(Smith 1995,Wain in press) and the relationships between these and their country rocks can be exami ned. About 90% of the study area consistsof pervasivelydefo rmed,composit ionally hete- rogeneous, amphibolite-facies gneisseswith minor metaba- site, meta-anorthosite and/or ultrabasite. Eclogit e or pre- Caledonian granulitic material ispreserved in low-strain zo- nes,surroundedby amphiboli te-faciesrocks.

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Pre-Caledonian granulite enclaves The Flatraket-U/vesund HP granulitecomplex

Granulite-facies meta-igneous rocksof quartz-monzon it ic, anorthosit icto basic composit ion(t helatter commonlypre- sent as cross-cutting sheets or dykes)arepreservedwith in the Flatraket and Ulvesund bodies (Plate 1),mapped as

'mangerite' by Bryhni (1966) and as 'quartz-syenite' by

Lappin(1966).In all those areasunaltered by later eclogite- or amphibo lite-fac iesoverprint,these rockspreservea high- pressure granuliteassemblage,of probableProterozoicage, wit hgarnet+c1ino pyroxene+plagioclase as the criticalas- semblag e, and additio nal megacrystic K-feldspar, biotite, hornblendeand accessoryrutileor Fe-Tioxides,depending on lithology.Rare orthopyroxeneis a relicof anearlier(pos- sibly igneous?)assemblage.Eclogit e-facies and,more com- monly, amphibolite-fac ies assemblages are found in Caledonian zones of fluid-infil tration and/orhighstrain,par- ticularly onthe margins of the Flatraket body. Eclogitesand relics of eclogite-facies mineralog y in amphibolite-fac ies meta-anorthosite have been reported from Seljeneset ,on the eastern marginof the Flatraketbody(Cotkinet al. 1988, Cot kin 1997). Onthe western margin of the Flatraket body, a cleartemporal progressionfrom granulitethrough eclogi- teto amphibo lite-fac iesassemblages is seenin anorthos itic and basicrocks.Anorthosit icand mafic granu lite istransfor- med to eclogite in shear zones(consisting ofkyanite,zoisite, quartz,omphacite,garnet,phengite for anorthos it ic rocks, and garnet, omphacite,quart z,rutile +/- phengite for mafic rocks).These assemblages may havea later,stati c,symplecti- tic amphibo lit e-faciesoverprint or are cut by amphiboli te- faciesshear zones.Amphibol ite-fac iesmaterial is rich infine granularplagioclase,c1inozo isite,biot ite,white micaandam- phibole wit h relict K-feldspar or garnet porphyroclasts in som elit holog ies. TheFlatraket and Ulvesund bodies have experienced granulite-fac ies, eclogite -facies and subse- quent amphibolite-faciescondit ions;the metastablepreser- vationof pre-or peak-rog enicassemblages isrelated to the absence of subsequent deformationandfluidinfilt ration in unaltered areas.

Peak-Caledonianenclaves: eclogite-faciesrocks Eclogitessensa-str icto

Eclogite pods occur throughoutthe areaandarecommo nly concentrated in trailsalong specific structura l levels,best seenwherecoasta loutcrops areparallel with the struc tural grain(Plate1).The Gangeskardeneseteclogit etrail(Lappin 1966)is exposedfor some 500mwhereas a15km lon g trail stretchesfrom Almenn ingentoBryggjaparallel to the north - ern shore of Nordfjord. The eclogitesat Selj e and Arsheim- neset oneitherside ofStadlandet probablyalso form part of a trail.Eclogite pods vary in sizefrom 1 m or lessto up to several tens of met res across,with several bodiesreaching >500m length in the Nordfjord area. Eclogites occur:(1) withi n amphibolite-facies layered gneisses,possessing a foli ated amphibolite-faciesrim isofacial to the enclosinggneisses; (2)

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withinpackages,up to 1 km in thickness,ofeclogite-facies mater ial including eclog ite-facies garnetiferou s gneissor meta-anorth osite e.g.at Krokkenakken,Risnakken, Drage (Fig.1) and Otnheim.

The mineralog y of thedifferent eclog ite pod sisvery va- riable(see Smit h 1988 foranexte nsiveoverview). In addi - tion togarnetandomphaciti c clin op yroxene,eclogi tes con- tainvariabl e amo untsofeither (1)quartz,phengi te,kyanite, (cIino-) zoisit e,rutile, carbon ate and/or amphibo le or (2) quartz,phlogopite, ort ho pyroxene,rut ileand/oram phibo le.

Thesecorrespondwit h the kyanit e andorthopyroxeneeclo- gitelineages (e.g. Smith 1988).Bot h type s occur inassociat i- on at Gryt t ing, Arsheimn eset and Gangeskardeneset.

Com positi onal heterogeneity is found wit hin some eclo- gites wit h mineral segregat ions forming layers or bou din s wit hinan individu al eclog ite body, e.g.phengi te-clino pyro - xenelayerson the isletof Falken, nearAlmenningen (Plate 1). Both UHPand HPeclogitesarefoun din the Nordfjo rd- Stadland et area(Smi t h 1988,Wain inpress) and belong to bot h the orthopyroxeneandkyaniteeclogite lin eages.They arenot dist ingu ishable in hand specimen or by field rela- tion shipsbut can be disti nguished by microstructu re and mineralogy (Wainin press).

Ret rogr ession of eclogite to amphibolite occursviaa symplecti t icstage that is observab lein the field.Inthe smal- lerpods,the core consistsof the least retrogressedeclogite wit hincreasing ret rogressiontowards themargin.Long tails ofamphibol itewit hin enclosing amphibolite-faciesgneisses are commo n. In the larger mafic bodies,however,several 'fresh' eclogite cores occur, separated by a netwo rk of amphibo lite shear zones (e.g. at Drage (Fig.1 ),Grytting) (Plate1).

Ec/ogite-faciesgneissesand meta-anorthosite

Felsic eclogite-facies rocks,previously unreport ed from the area,areinvariably associated wit heclog itebodiesand oc- curcommonly along the northernshore of Nordfjord andlo- callyonStadlandet . These occurwrapping around orinter-

NGU-BU L L432,1997 -PAGE 12 9

Fig. 1. Field drawing of an UHP eclogi te occurrence,illust rating the co-existe nce of UHP-eclogite and eclogite-facies garnet - phengi te gneiss and thepreferentialretr o- gression towards amphibo lite-facies ass- em blages in high-strain zones. Locat ion: 2.5 kmwestof Drage,LP 015915.

layered wit hbot h UHP and HPeclogi tesas part ofeclogite- facies packages on the10m tokmscale(Fig. 1).

Garnet-phe ngi te-q uartzgneiss cont ain sup to 20%gar- net and hasaneclog ite-faciesmineralogywit haccessoryky- anite, zoisite, clinozoisite, rutil e, carbo nate and/or zircon.

Theserocks appearto have experienced the samecondit i- onsasthe inte rlayeredeclogites,and quart zpseudomorph s after coesite are observed in suchgneiss interlayered wit h UHPeclogiteat Totl and and Drage (Wain in press).During amphibo lite-facies metamorph ism, phengite recrystalli sed to bioti te-plag ioclasesymplectite,whilstot her phases broke down to biot ite, plagi oclase, epidote and hornb lende.

Preservati on of eclog ite-fa cies mineralogy or texturesisfa- vouredonlyinamp hiboli te-facies low-st rain zonesassocia- ted wit heclogi t epods(e.g.atKrokenakkenandDrage). In domainsof amphibo lite-facies high st rain, the eclogite-faci- es assemblage recrystallised to a foliate d biotite- plagio- clase-quartz rich gneisswit h relict garnet,and accessory sphene,Fe-Tioxides,epidoteor amphibo le.

In additio nto eclogite-facies meta-anor t hosite inshear zonesfrom the Flat raket-Ulvesund granul ite complex des- cribe dabove, eclogi te-facies meta-anort hositeisfound in- terlayered wit h coesite-eclog ite at Otnheim, Stad landet.

Here, eclog ite-fac ies meta-anor t hosite conta ins garnet, quartz, clinozoisite and minor kyanite and phengite,and quart zpseudo morphsaft er coesite arefound in garnet and clinopyroxene,attesti ng to an UHP paragenesis.Thisis part of alarger(1km')anor thosi t ic-felsic-basiccomplex on the east Stadlandetcoast(Plate1),largely defor medandrecryst- allisedatamphibo lite facies.

Amphibolite-facies material Layered granodioriticgneiss

The dominantrocktype,comprisingabout90%of the st udy area,islayered gneissof broadly grano diori ticcomp ositi on with amp hiboli te-facies parageneses. The gneisscontains predo minant ly plagioclase and quartzwit h 10-20%biot it e and/orwhite mica. Inplaces,up to10%epido te,K-feldspar

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Fig.2.Field photogra phs, illustrating st rong deformation of layered gneiss. Much ofthe layer-formingflatteningand all the folding are as- sociatedwith late-Caledonianextensionand constrict ion.

(a)Layered,granodi oriti cgneiss,withFafolding.Viewto west;notebook is 20 cm long.Location:coastal point,250 msout h of the Verpeneset eclogitepod,LP 0066 6920. (b)Steep,near concentric Fb fold inlayered gneiss.View to WNW,notebookis20 cmlong. Locat ion :small,disused quarry,600 m WNW of Asrnundsvaq,KP 9972 7875. (cl Recumbent.

near-concentric Fb folds insharply layered gneiss. View toward sthe east;notebook is20 cmlong. Location:along track,2 km NNWfrom Almenn ingen,LP02217120.

and/orblue-greenhornblende are present.Opaques,sphene and zircon are common accessoryminerals.

Thegneissic layeringin thestudyareais commonlydefi- nedby distinct pale,felsiclayers and dark-grey,moremafic (bioti te-muscovite-epidote-amphibole rich)layers onamm to dm scale(Fig. 2a).Sing lelayersof amphibo lit icorgranit ic composit ionalso occur,ranging in thickness from 1 -10 cm.

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Epidote-rich layers alsooccur.The boundariesare sharp or more gradual(t he'w ispy layering'of Lappin 1966). In this type of layering, there are,generally,no grain-sizedifferen - cesbet ween layers of differentcomposition.

Thisisin contrast to stromaticlayering,whichis obser- vedin placesonStadlandet,around Sorpollenand betwee n Gangeskardneset and Flatraket (Plate1,Fig.3). Stromatic layering typ ically comprises leucocrati c segregations of trond hjemitictograno dioritic composition 5 - 30 mmwide, bounded bydark melanosomelayers,<1- 4mm thick,rich in biot it e and, in places, garnet, amphibo les or epidote.

Mesonomelayers ofintermediat ecomposition canvaryin widt h(2-10cm)and relativeamo unt. Thelayering is typi- callywavy, commonlywit h pinch- and-swellstructures.The grain size inthest romati c gneisses is relativ ely coarse(0.5-2 mm,up to4mm inleucosome).The stromaticlayering isin- terpreted as formed byanatect ic migmatisation,basedon the occur renceof melanosome,the differen t feld spargrain sizesin the leucosome (3-4 mm) and in the mesosome

« 1 mm)and onpert hiticexsolution features(indicative of format ion at high temperatures) observed in many feld- spars, in particularnearSelje.

The gneissic layering commonly wrapsarou nd eclog ite pods,truncating eclogite-faciesstructures. No evidenceof eclogi te-faciesmetamorphism is foundin these gneisses in

Fig.3.Stromatic migm atit icgneiss. Migmatitic layeringis folded by tight -isoclinalFafolds(e.g.justrightof the hand-lens)and byclose, steep,south-dip ping Fb folds.ViewtowardsENE,hand-lens is5cm long.

Location: road-cut alongthe Selje -Arsheimneset road,2km ENEof Selj e,230 m altit ude. LP109S8S12.

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the studyareabut pervasivelate-Caledonian am p hi bolit e- facies deformation may have led to completere-equilibrati- on of possible former HPassem b lages.

Granitic gneiss

In several places, notably between Mal0Y andVerpeneset, large bodies of granitic augengneissoccur(Plate 1).Theto- tal contentof maficminerals (bioti te,zoisite,muscoviteand chlorite) is 5%or less,muchlessthanin thelayeredgranod i- oriticgneiss. Most of thegranit icgneiss contai ns arat her uniform feldspar augen texture and layering is generally onlyverypoorly develop ed,some layersenriched in K-feld- spar occur in places.Undeformed pegmat it icpatchesoccur within the graniticgneiss. An igneous orig infor the auge n gneisses seems probable, but the metamorphic mineral assemb lage indicates, at most, amphibolite-facies meta- morphism.

Garnetiferou s gneiss

Differentvariet iesof garn et iferous gneisscanbedistinguis- hed in the study area. Amphibolite-faciesgarnet-biot ite- plagioclase rich gneisses are associated commonly wit h eclogites (e.g.at Drageor Krokken akken,seePlate1). The transition from eclogite-facies garnet-phengite gneiss to amphibol ite -facies garnet-biotite-pl agioclase gneiss via a sym p lectiticstage is thought to be related to deformation and re-crystallisation, as outlined above. The presence of garnet is the main distinguishi ng feature of this rock from the more common layeredgranodioritic gneiss. Finalbreak- down of eclogite-faciesgarnetmay render this gneiss ind is- tinguishablefrom the latter. However,itisnot clear whet her allgarnet ife rousgneisses represent relics of HPassem bla- ges. Some garnet-K-feldspar-epidote richgneisses appear to havepure ly amphi boli te -facies parage neses.

Quartzit e

Layers of quartziteoccur in particularbetweenBort nenand Endal,south of Nordfjord(Plat e 1, Bryhni1966)and east of the studyarea inHj elmelandsdalen(Bryhni 1974).Quartzite layers aregenerally 1 to 10 m thick but canext end for seve- ral hundredsof metres(Plate 1). Thequartzitelayersattest to a sedimenta ry orig infor at least some ofthe lit hologie s occurring in the WGR.

Anorthosite

In addition to the granu lit e or eclogi t e-fac iesoccurrences describ ed above, anor th ositic rocks are found more com- monly as layerson a metr eto 10 mscalewit hin amphibolite- facies layeredgneissin theNordfjo rd area.These invariab ly have an amphibolite-facies assem blage consistingof plagio- c1ase with minor epidote,clinozoisite, zoisite,margarit e,par- agonite,chlorit e,sphene,carb on ate and rarely K-fe ldspar.

They attest to an igneousorig infor partof theWGR andoc- cur,in part icu lar,betwe enBort nen andDavik(Plat e 1,Bryhni

1966),

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in Hj elme landsd alen and to the south of Nordfjorde id (Bryhni 1966,1972, 1974).

Ultramafics

Large bodiesof ultramaficrocksoccur to the east of the stu- dy area in Sunndalen,Almk lovda lenand Bjerkedalen (Bryhni 1966, Lappin 1966,Jamtveitetal. 1991).These arepartlyser- pent iniseddunitebodies and containgarn et-py roxeniteand eclogite pods.Thetecton ic implicati on of thesebodie s isa matt erof disput e(Jamt veitetal.1991). Inthe studyarea, a fewsmallerbodi es occur(Plate 1);theyareusuallylargelyser- pent init ised(e.g. Bryh ni1966) andare readil y identified by their sandy-brown weat her ing. Veins,normalto the regional east-west ext ension trend are filled wit h acicularserp ent ine minera lswith thefibres parallelwith theextensiontrend.

Metamorphic evolution

Asoutlin ed abo ve, thetransition bet w een pre-Caledon ian (granulit e-facies),peakCaledon ian (eit herUHP or HP eclo- gite-facies) and late-Caledon ian(am phibolite-facies)assem- blagescan be seen onthe localscaleacross various lit hologi- cal groups.Pre-and peak-Caledonianassemblages are pre- servedin locallow -strainzones, whil st pervasivelydeformed amp hibolite- facies materialmakesup~90%ofthearea.

Pre-eclogit ic granulite-facies rocks are restricted to the Flatraket-Ulvesund body,corresponding toP-T conditionsof 10- 12 kbar,-800QC (Wain,in prep.). These plagioc lase-bear- ing rocks werepreserved metastably during eclogite-facies metamorphism. The age of granulite metamorphism is poorly constrained but predatesthe Caledo nian eclogi t e- facies eventand probablypostdate sthe 1520±10 Ma U-Pb upper inter cept age of the Flat raket megacrystic quartz - monzon it ic gneiss(Lappinetal.1979).

Eclogit e-faciesP-T condition s,regarded to beCaledon- ian in age (Griffi n & Brueckne r 1980, Gebauer et al. 1985, Krogh&Carswell1995),show markedvariation through the study area,asshown on Plate 1. Wain(in press)suggeststhe presenceof threezones withdiffere ntpeakpressures(Plates 1 and 2): (1) an UHP province,centredon Stadlandet,where eclogi t es contain coesiteand/or coesite-pseudomorphs as part ofapeak-pre ssure assem blage(P>28kbar)andwhere no evi dence of prograde text ures and mineralog ies has been found;(2)aHP zone,st retch ing fro mtheNSDto the nor thern shore of Nordfj ord, where eclogit es com monly preserve prograde zoned garn et s wit h amphibolit e-facies cores(Bryhni &Grimstad 197 1,Krogh 1982)and noevidence of(former) coesitehas been found; a maximumpressureof 20-24 kbar at T- 650-800QC has beencalcu latedfor these eclogites (Martin 1994,Wain, in press),(3) an - 8 km wide bimodal zone in betw een,stretching from thenorthernshore ofNordfj ord to Rundarheim,where both UHP and HP eclogi- tesoccur,someti me s lessthan 1 km apart. Eclogite-facies gneissesor meta-anorthosit e ap pear to be isofacial wit h associated HP orUHPeclogite s.The Flat raket andUlvesund

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granulit icbodies,metastabl eat HP,occurwithin the bimodal zone.Eclogite-facies shearzones withinthis complexcorre- spond to HPmetamorphism(20-22 kbar,-750QC, Wain in prep.) but show no evidence of having experienced UHP metamorphism.

Amphibolite-facies metamorphismispervasive inthe re- maining rocksthroughout the area, correspondi ngtorelati- vely uniform conditions of 8-12 kbar,-650 QC (Oransfield 1994).Otherauthors have estimated condit ions of 11kbar, 700-750QC(Cut hbert 1991).and between 4.3-8.2kbar,520- 610QC(Cot kin et al. 1988),forpoint s inthe exhumationpat h ofpart icularrocks.

A very significant correlation exists bet ween late- Caledonian deformation and amphiboli te-faciesassembla- ges. All the rocks that have experienced late-Caledonian deformation show stable amphibolite-facies assemblages.

However,domains where late-Caledo nian deforma t ion is absent or very weak show almost invariably eit her Caledonian peak eclogite assemblagesor pre-Caledonian granulite assemblages or relicsthereof.

Late-Caledonian structures

Most late-Caledonian structures are expressed by a defor- mation of thegneissiclayering.The generation of the gneis- siclayeri ng itself isthe resultofa longandcomplexhistory but an important part of this history is related to late- Caledonian extension. Only the late-Caledonianstructures, formed under amphibolitetogreenschist-facies conditions, aredescribed indetail in thispaper;structureswithineclogi- tepod s and granulitebodies will be described elsewhere.

Andersen et al.(1994)described eclogite-faciesstructures from Sunnfj ord in the southwes tern part of the WGR;some of these observations are valid for the Nordfjord area (e.g. Oransfield1994).

Structures related to east-west extension

The now classic sequence in extensional detachment zones, described from metamorphi c core complexesin the Basin and Range Province,USA(e.g.Lister&Davis 1989),ofcataclasi- tes,greenschistto amphibol ite-faciesmylon itesto mylon itic gneiss is characteristicof theNSD.However,unlikemost Basin and Range metamorphiccore complexes,in whichthe cores themselvesexperienced relativelylittl e st rain during exten- sion,theWGR belowthe detachment zone has also beensub- jected to high strain.Andersen & Jamtveit(1990)demonstra- ted that this zone has been subjected to bulk coaxialeast- west exte nsion;Krabbendam&Dewey(in press)expanded on this idea and showedthat thebulk finitestrain ellipse is con- strictional(rat her than planestrain),wit h north-south andver- tical shortening coevalwit h east-west extension.

Structur es rela ted to theNSDzone, indicatin g stro ng ly non - coaxialshear

In the study area,the NSD/HO is poorlyaccessible.In the far

MAARTENKRABBENDAM

s

southeastern part of the study area,the detachment has been studied in detail(Plate1).Here,st eepSSE-dipp ingmy- lonites havea well-devel ope d,sub-horizontal WSW-plung- ing lineat ion and contai n abunda nt dextr al / top -to-t he- westshear senseindicators.Thepenetrativemyloni ti c folia- tion and lineati on are oriented 10-20⁰clockw isefrom the trace ofthe brittle detachment itself.Sout h ofBorntepollen, the detachme nt zonecan be studied only some 2-300 m belowthe base oftheOevonian.Here,sout h-dipp ingmylon- ites occur withabundant a-ty pefeldsparporphyrocl astsin- dicating adext ral(top-to-t he west)shearsense(Oransfield 1994). Astron glineati on,mainlydefined byquartzrods,is sub-ho rizon tal east-west.The metamorphic gradeof these mylonitesis uppergreenschistfacies;the protoli t his grano - dior it ic epidote-biotite gneiss. North of Borntepollen,the rocks grade intomylonitic,mica-rich gneisses with sub -hori- zontal east-west lineations,defined by quartz-roddi ng and prolat efeld sparaugen(aspect ratio 1:3:10). Shearsensein- dicato rsare abundant andinclude shear bands,extensional crenulation cleavages, internal oblique shape fabric in quartz-rich layers, a-type feldspar porhyroclasts and an asymme tr icwrappin g of micaaround garnet(see the revi- ewsofSimpson &Schmid 1983,Hanmer&Passchier 1991).

all indicating a top-to-the- west shear sense. In the Borntepollen - Leirgulen area,most shearingoccurre dunder amphibolite-fa cies condit ion sbut cont inuedduringgreen- schist-faciesretro gre ssionas shown bychloritegrowthalong certainshear planes.On Biskjelneset,on the north shore of Nordfjord,a highlysheared,graniticaugen gneiss cont ains abundant a-type feldspar porphyroclasts and asymmetri c extens ionalshear band s, whichcutan older gneissiclayer- ing. Thus,shear senseindi cators,indicat ing strongly non- coaxial,top-to-the-west shear under amphiboliteto green- schist-faciesconditions,are abundant in a 5 km-wde zone north of the NSD,alt hou gha penet rative mylonitefoliati on is only developed in thetop 2-400m.

Structuresin the coaxia lzone belo wthe NSD

North of Nordfjord,struc turesindicativeof non-coaxial shear are rare and absent over large areas.However,the grano- dior itic graniticgneissesare highly and pervasively defor- med by near-coaxial strain. East-west trending lineations can be found at almosteveryoutcrop(Plate 1,Fig. 4)andin- cludebiot it eminerallineat ion s and quartz-roddin gin gra- nodiorit icgneissesand amphibo lemineral lineations in am- phibo lites.The granit icaugengneissnear Verpeneset con- tains a fabricof symmetric,stronglyprolate feldsparaugen with an aspectratioin the orderof 1:3:8-10to 1:1.5:8,imply- ing general const rict ionalst rain,similarto the strainregi me observed along Fordefjorden further south in the WGR (Krabbendam & Oewey,in press).

An amphibo lite -facies mineral foliation is commonly very well developed and isparallel with thecomp osit ional layering,exceptin the hinge zones oftightfolds(seebelow).

Thisfoliat ion,termed Sa,is defined by a preferred orienta-

. .

MAARTEN KRABBENOAM&ALICE WAIN

. ~

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o^o01'1⁰¹¹⁰m^~o

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00 0

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a)Stadl and et

• Foliations:n=76

o Lineation s:n=30

b) Serpo llentoFlatraket

• Foli ations:n=166

e Lineations:n

=

NGU-BULL432, 19 9 7 - PAG E 13 3

c)Ulvesund

• Foliations: n=76

o Lineat ions:n

=

*

d)Verpeneset - Almen ningen

• Foliation s:n

=

o Lineation s:n

=

e)Bortnepoll en

• Foliati ons:n

=

*

f)Nordfjord toFlatr aket +Fb fold axes,n=33

*

=

Fig.4.Equal-areadiagramsof str uct uraldata,separated in differentdomains.Foliation saregneissiclayerin g andamp hibolite-faciesfoliation.Great cir- cles are1tplanesoffoliat ion data inFigs. 1a-d andofFbaxialplanesinFig.1f.Thegrey squaresarerrpolesof thesame planardata.

tion of biotite,white mica and amphiboles and epidote (where present )butalso bythe shape orient ationof quart z and feldspars and by quartzribbon s.

Orientationofgneissiclayerin g,fol iationandlinea tio n Because the gneissic layeringand themineralfoliation are parallel,except in thehingezonesof small tight folds,they canbe treatedtogetherin orient ati on analysis. The gneissic layering and foliation st rike E-Wbut havevariabl edip s(Fig.

4). The dip variation is attr ibute d to lateFbfolding (see below). Closeto the NSD,lineationsplun ge gent ly west, whereas aw ayfrom theNSD,east- plunging lineation s be- comedominant wit hplunges up to 30°to the east(Plat e 1, Fig. 4). Thischangefrom west to east plunging lineations occurssome 3 km north of theNSD and isprob ablyrelated to a plunge culmination located structu rally a few kmbelow the NSD, also reported by Milnesetal.(1985, 1997)along the Sognefjord transectfurther sout h in the WGR. Not ethat the plunge culmination does not coincide with thetransit ion from thenon-coaxial tothe coaxialzone.

Structures indicat ing N-S shortening :E-W folds Structu res relatingto two types of late-Caledoni anfolding, differi ng bot h in style and in relat ive timing,can be obser- vedin the st udyarea.Becausealongand presumablycom- plextectonichistory preceded thesefolds, wedo not usean Fl , F2 order; rather, we refer to the folds asFa and Fb.

Althou ghFb overprints Fa inevery locality where both fold s occur(e.g.Fig.3),it is not necessarilythe casethat all Fb folds are youngerthan allFa folds in the areaas a whole.

Early folds(Fa)

Fa foldsare tightto isoclinal folds,foldingthegneissiclayer- ing(Fig.2a).Fafolds are commonlynear-similar and rootless Fa foldsare common.The strongly developedamphibolite- faciesmineral foliation (Sa) is axial planarto the Fa folds,in- dicating that Fa foldsdeveloped under amphibolit e-facies conditions. Because Sa and the gneissiclayering appear to be (sub)parallelin the Fa fold limbs,theaxial planar fabric relationship of Sa and Fa can commonly only be reliab ly established in the Fa foldhinges.Thewavelengthof Fa folds is generally 1-20 cm and rarelyexceeds1m;thefoldampli-

NGU-BULL43 2,19 97 -PAGE134

tude,however,can be severalmet resor more. TheFa fold hing es are either east- or west-plungi ng and everywhere parallel wit h the mineral lineati ons(La),indicating that the E-W extension (responsible for the strong lineat ions) was coevalwit hN-Sshortening. Although Fa folds aregenerally strong ly asymmetric,no regionally dominant vergence is observed,attest ingto a near-coaxial strain regim e.

Latefolds(Fb)

Fb folds are open to tight,commonly near-concentric folds, foldin gthe gneissiclayering,the earlier Fa foldsandtheSa minera lfabrics(Figs.2b-c,3).This deform ation has common- ly result ed inan inte nse crenulation ofmicas inthe tighte r parts ofFbfold hing es.The axialplanar fabricof Fb foldsis thereforea crenulationcleavage, which tend s to be wellde- velopedonlyinthe foldhingesof mica-richrocks. Themeta- morp hic gradeofthisdeform ationstage issomew hatlower than theamphibolite-facies metamorphism that prevailed during the formationof Fafolds, asnosig nificant newmine- ral growth isassociated wit h Fb.Fbfold hinges aregeneral- ly E-Wtrending and are grossly parallel with the regional mineral lineati on and theFa fold hinges (compare Fig.4f wit h Figs.4a-d).At many localiti es,however,Fbcanbe seen to foldthemineral lineations(La)andangles up to 15°bet- weenFb andLa occurlocally,forinsta ncein road-cutsnorth of Alm enningen(cf.Chauvet &Seranne 1994).

Two sets of Fb folds occur:recumbentand uprightfolds (Figs.2b&c,4e&f).The upright Fb folds,mapped wit hin the WGR gneisses,have amplitudes and waveleng t hs varyin g from 1 m to 1 km(Plate 2). If the synform s containingthe fourDevonian basinsintheWGR arealsoFbfolds,whichis likely,then theFb wavelength under thesebasinsis several kilo met res.Recumb ent foldsare less common and general- lysmaller witha wavelengthofafew metre sat most. Along Borntepollen,recumbent folds have wavelengths of up to 10 m. No overprint ing relationsbetween recumbent and upright Fbfolds have been observed and it is not clear whetherthey were formed synchronously ornot. If thefor- meristhecase,Fb folds may haveformedduring bulk east- west generalconstriction.

Inan8-9 km-w idezone,from the NSDtothenorth shore of Nordfjord,upright Fb folding is intense with numerous close to tight folds resulting in steep,vertical and locally overt urned gneissic layering (Plate 2,FigAd). North-sou th shorte ning byFbfolding in this zone is estima tedat about 50%by line balancing.

AlongthecoastfromFalkevik to Almenningen,many re- cumbent folds occur.Itis,however,notclear whether these folds are Fb foldsorpossib ly earlierfoldsrelatedto accom- modationof strain around thenumerous eclogi te bod ies at this struc turallevel, whichactedaslow-st rainzones.

Along Sorpollen,Fb foldingis rat her open andthe gneis- sic layering is gent lyeastdipp ing(Fig.4b). However, structu- rally above the Flatraket granulite,east and southeast of Nordpo llen,Fbfolding isagain abun dant and steep atti tu-

MAARTENKRA88ENDAM

s

des are common(Plates1and 2). These differencesmaybe attributed possibly to the Flatraket granulite body,having inhibited homogeneous N-S shortening.

Between Flatraket and Rundarheim,Fb folding is also common and attitudes are steep. On large parts of Stadlandet,Fb folding is rare and the gneissic layering is sub-horizontal (Fig.4 a) withthe except ionof a 2-3km-wide stee p zone near Selje,bounded by two moderately steep south-dipp ing monoclines (Plates 1 and 2).The classic Grytting coesite-eclog ite pod occurs in this steep zone.

North-south shortening byFb onStad land et isestim ated at 10%maximumby linebalancing.

Structu ral positions of eclogite and granulitic bodies

The layeredgranodioriticgneisses,forming the bulk of the WGR,recordonly late-Caledonian metamorphic conditions, whereas peak-metamor phic condit ion sare onlyrecordedin the relative ly rare eclogite pods. Therefore, the relative stru ctural posit ions of eclogite pods, recording different peak-pressures,are importantinassessing thetecton icrela- tionships and evolution of eclog it e-facies rocks in the Western GneissRegion. Overall,there is a gross increasein recorded peak pressuresfrom the NSD toward sthe nort h.

Indetail,however,complicat ionsoccur,asindicated by the close occurrence of UHP and HP rocks wit hi n the bimoda l zone(see section on MetamorphicEvolut ion).

The Ulvesund and Flat raket granulite bodies are most likelypart s ofthe same body,connect ed via a N-Strend ing, openanticlinealong Sorpollen(see alsoBryhni 1966). Ina N-Scross-section,thisbody hasa rath erflat-lyin g attit ude (Plat e 2). Eclogite-facies shear zones suggest that the body has experienced HP(20-22 kbar)but it apparentlyconta ins no evidenc efor UHP metamorph ism.The Straumencoesite- eclogite pod in Sorpo llen (Smith & Lappin 1989) occurs structurallybelowtheFlat raket body,as does the new lydis- coveredFlat raket coesite -eclog itepod,the latt er only200m away from the Flatraketgranulite(Plates 1 and 2);this repre- sentsaninvertedmetamorphiczonation.The coeslte-eclo- git e pod in Nordpollen is structurally above the Flatraket Body.Thus,the FlatraketbodyhasUHP eclog it esboth struc- turally belowand above, indic atin g bothinvertedandright- way-up metamorphiczonations.Other close occurrencesof UHP and HP eclogites include (Plates 1 and 2): Gange- skardeneset (HP)- Flatraket (UHP); UHP and HP eclogites along the northern shore of Nordfjo rd,and at Straumen, where HPand UHP eclogitesare prob ablyseparated byless than 1 km althoughthe exact locati on of theUHPeclogiteat Straumen (Smit h & Lappin 1989)could not be found. UHP and non-UHPeclogi tesarealwaysseparatedby>100 m of pervasively deformed amphibo lite -faciesgneisses wit h no evidence,however,for discrete shearzonesseparating the two suites. The exact tectonic relati on ship betwee n UHP and non-UHPeclogites in the UHP-HP bimoda l boundary

MAARTEN KRABBENDAM&ALICEWAIN

zone is obscured by the amphibolite-faciesoverpr int ing, but the P-Tjump in thisarea suggests zonesof ext reme attenuat ionor adistincttectonicbreak,pre-datingregional amphibo lit e-facies metamorphism and deforma ti on and separat ing an UHP province fromtherestofthe WGR(Wain in press). Itisunclearfrom our cross-section whet her the UHP province asa wholeis structu rally above orbelowthe HP zoneof theWesternGneissRegion .

Discussion

Three issues will bediscussed below :theformationand mod- ification of gneissic layering by pervasive late-Caledon ian strain;thein-situversus foreigneclogitemetamorphismmo- dels from a deformational point of view and the heteroge- neous attenuation of the crust causing differential exhuma- tion and tectonic juxtapositionof HP and UHP rocks.

Formation and modificationof gneissiclayering by pervasive late-Caledonian strain

Some element sof thegneissic layering may beof prim ary origin, such asoriginalsedimentarylayeringorlayer-parallel int rusions. However,most of the gneissiclayering in the WGR is of tecton icorigin,formed by flattening of originally discordant rock cont acts.Passchieret al.(1990)have descri- bed this process in some detailandargue thatthis isa very common originof gneissiclayering.That thetecton icflatt en- ing,which played such a major role inthe formation and modification of gneissic layeri ng inthe studyarea(andin- deedin most ofthe western partof theWGR),isCaledo nian rather than Proterozoic,can be arguedasfollow s.First, inthe eastern andsout hern part of the WGR,many discordantcon- tacts of igneousrockscan be seen concomit ant wit h low strain, for instance at the excellent exposures below Nigardsbreen (part of Jostedalsbreen)and also thoserepor- ted by Milnes et al.(1988, 1997)in the Sognefjordentrans- ect. Such highly discordant contacts are not present in the western part of the WGR,where the compositional differen- ces arenow planar, except in those domains which show ol- der deformation and metamorphism,e.g.near Flatraket and on Bardsholmen in Sunnfjord (Andersen et al. 1994). Thus, from east to west, contacts become moreconcordant,con- comitant wit h thewestward increaseof Caledonianstrain (Diet ler et al.1985,Miln eset al. 1988). Secondly,thelate- Caledonian mineral foliation (bioti te, muscovite,epidote, amphibo les) is generally parallel withthe compositi onal lay- ering . Thirdly, amphibolit e-facies material wraps and/or truncateseclogite-facies structu res,andthinlayers ofdefor- med amphiboli te (ret rogressive after eclogi te) containing layer-parallelamphibolite-facies fabrics are continuouswith eclogi te pods, indi cating that these layerswhere formed by deformation that post-dated the eclogite-forming event.

The last twoargume nts also indicatethata substan ti al part of the strong deformatio n responsiblefor theformation of the gneissiclayering was late-Caledonian. In the southwest-

NGU-BULL 432 , 199 7 - PAGE 135

ern part of the WGR,bulk near-coaxialstrain belowthe NSD has been takenup by anastomosing shear zones, surroun- ding low- strain zone s(Andersen&Jamtveit 1990), whereas in the st udy areabulknear-coaxial st rain has beentakenup in a far morepervasive and mesoscopically homogeneous way. This pervasivelate-Caledon iandefo rm at ion and rela- ted pervasive amphiboli te-facies retrogression may accou nt forthescarcity of relics of HP-assemblagesin thelayered granod io rit icgneissin thest udy area; elsewhere in the WGR, such relics appear to be more common (Griffin 1987, Andersenetal. 1994).

Late-Caledonian deformation and the in-situ versus foreign eclogitemodels

Two models of eclogite formation in West Norway have been proposed,an'in-sit u'and a 'foreign' model. The'foreign model' involves the tectonicintroduction of eclogites,for- med at mantle depths, into the host gneisses, which never experiencedHP or UHP(Lappin 1966,Lappin&Smith 1978, Smith 1984).Smith(1988, 1995) proposed avariationon this theme, theFIF(foreig n, in-situ,foreign)model. This model envisages tectonicintr oduct ion of UHP rocks into HP rocks and subsequent tectonicintroduction of HP rocksinto LP rocks.

Many aut hors,includi ng Bryhni (1966),Cuthbert et al.

(1983)and Griffin &Carswell (1985),have argued that the eclogitesformed in-situ in their present host gneisses,so that all rocks were subj ect ed to the same P-Tcondit ions.

Compatibl eP-Tconditionsforsomeeclogit esandgneisses (wit h high -pre ssuregranulit eassembl ages)were previous ly argued (Krogh 1980)butitisnow widely accepted thatthere is a significant pressurediffer encebetweenthe dominantly amphibol it e-faciesgneissesand both HPand particularly UHP eclogites (as argued by Smith 1988). Thus,an in-situ origin for the eclogites must imply either metastability of felsic rocks at depth or differential retrogression whereby retrogression was favoured in felsic rocks, ora combination of those processes.

In the study area,evidence for metastability of felsic rocks at eclogite facies is restrictedto the Flatraket-Ulvesund granulite complex,which has clearly experienced eclogite- facies conditions but preservespre-orogenic granulite-faci- es assemblages in low-strain zones. The extremely strong correlation of amphibolite-facies assemblages with late- Caledonianextensiona ldeformation suggeststhat differen- tialretrogression has been operative in the majorityof the count ry rocks,so that mainly undeformed and mafic rocks tend to retain their eclogi t e-facies assemblages, whereas strongly defo rmed, main ly felsic rocks are preferent ially retrog ressed. The pervasive,late-Caledonian, extensiona l deform ation wit hinthefelsic gneisseswould greatlyfavour retro gressionof HPassem blages. Aneclogi te-faciesassem- blage with a granodiori ti c composition is more hydrous than an amp hiboli te-facies assemblage of the same com- position(e.g. Bousquet et al.1997), so that the retrogressive

NGU-BULL432, 1997 -PAGE 136

reaction from HPto LP in a rock wit h granodioritic com- position iskineticallyfavourable, as the rocktends to retain its most dehydrated state (e.g. Heinr ich 198 2). The preser- vation of eclogite-facies garnet-pheng ite gneiss in late- Caledo nian low-strain zones wit h a direct correlation be- tween retrogression and late-Caledonian deformation, provides strong evidence for in-sit u metamorphism, for bot h HPand UHPeclogites(e.g.Fig.1).

Thereis no eviden ceforspecific high-strainzones,lin k- ingUHPor HPeclogites,as proposed by Smith(1988,1995), inexisten ce today.The steep zone,link ingthe Gryttingand Arsheimnes et UHP eclogitepods, is the resultofFb folding (Plate2)ina verylate stage ofexhumation and is not a spe- cifichigh-strain zone(cf.Smith 1988). Therefore,field evi- dence in this area isnot compatiblewithsimplesubvertical solid int roducti onof foreign rock bodies(cf.Smith 1995).

It remainspossible, however,that suchshear-zones have existed butthattheirfabrics have been obliterated during the pervasive late-Caledonian amphibol ite-fac iesmetamor- phism. Thiswould imply that anyforeign intr od ucti on,ifit did occur,must have happened priorto thisamphibo lite - facies event. Thus,the second stage of tectonicint roduct ion ofSmit h's(1988)FIFmodel,that of HP rocks intoLProcks, is incompatiblewith the data presentedin thispaper.The first stage of introduction,that of UHP rocks into Hp,canno t be ruledout,although all field and metamorphicrelationship s can be explained by metastabilit yor differential retr ogres- sion with late-Caledonian deformation playing akey role in the latter and in the juxtapos ition of HP and UHP rocks. Another argument againstforeign int rod ucti onis that the gneisses withinthe UHPprovince are effectivelythe same as those in theHPzone;in other words,lithologicallythe UHP prov ince isnot exoti c.

Differential amountsofexhumation

The exhu matio nofthe UHP rocksin the WGRcan be parti- tioned into three dom ains: exhumation from above the NSD,atten uati on acrossthe NSD by non-coaxialstrain,and attenuation by pervasive coaxial strain below the NSD. By comparing the vertical distance between indivi du al rock samples prior to exhumation (as constrained by peak- pressure estimate s) with the current structural distance betweenthose samples,we can const rainthecontr ibution of each of thesedomains to the total attenuati on of the crust. This comparison indicat es that attenuat io n is not distributeduniformlythroughout the crust(Fig.5).

The amountof exhumation which occurredfrom above the NSDis constrained by the highest Caledon ianpressure estimateinthe allochthon.Cut hbert(199 1)calculateda Pof 4.1± 1.8kbarand T = 510± 40°C forasillimanit e-garnet hornfe lsin theKalvag melange,within the thermalaureole of the granod iorit ic intrusion on Bremangerlandet. This pressure estim ate can betaken as Caledonian,as therelated Gaseyintrusion hasbeendated at 380 ±26 Ma(Sm-Nd)I 390±29Ma(Rb-Sr).(Furnes etal. 1989).The calculatedmax-

MAARTENKRABBENDAM&ALICEWAIN

imum P of 9.2 ± 1.4 kbar for a mica schi st on Marey (Cuthbert 1991)at a lowertect onostratig raphic level than the Kalvagmelange,isregard ed by som eas pre-Caledo nian (T.B.Andersen,pers.comm. 1997).Thus,thepeakCaledo nian pressureofthe allochth onou sunits is taken as 4.1 kbar;the occurren ce of andalu siteand cordierit e inthethermal aure- ole of the granodi oritic intrusion on Bremang erland et (Bryhni & Lyse 1985) suppo rt sthis low-pressureestim ate.

Such a pressure suggeststhat some 15km of crust wasre- moved from above the NSD/Hornelen Detachment. The Kalvagmelangeand the grano dioriteint rusionareoverlain unconformablybythe DevonianHorne lenBasin(Plat e1).A number of norma lfault s occur belowthisunconformityand are truncated by it(Hartz et al.1994,Hartz& Andersen,in press)indicat ingthat the 15kmof crust was removed bya combinationof erosionandexte nsion(Fig. 5),part lypriorto deposit ion ofthe Devon ian sediments,very similar to the tecton ic evoluti on of the allochthonous unit s in the Sunnfjord area(Osmundsen & Ander sen1994).

Theeclog itepodnearest tothe NSDand theallochthon occursalong Borntep ollen,structurallylessthen 1 kmbelow the NSD. Apressureof20kbar has beencalculated for this eclogite (Martin 1994). A pressure differenceof 16 kbar, equivalent to57kmof crustal sectio n, is nowrepresentedby about1 km ofstru ctural thickness acro ss the NSD, implying an attenuation of 98% (defi ned as shortening: (1-1/1₀₎ x 100%). It isdoubtful,however,if all this exhumation was achieved solelyby simple shearalong the NSD.The maxi- mumhorizon taldisplacemen tofthe NSDis equaltothe dis- tancebetweenthe easternedge of the NSDand the western edge of the Jot un Nappe(50-60km),plusthedistance which the Jotun Nappe has moved back to the west along the thrust plane (est imated at 20-35 km,Fossen &Hoist 1995), plusthe horizontal displacement alo ng the Laerdal-Gjende Fault Zone(-35 km,Milnes etal.199 7);that is,a maxim um total displacementof 105- 130km. Ifexhumatio n('vertical displacement')of 57 kmwasachi evedsolely bysimple shear along the NSD this wouldrequirea dipof 26-33⁰of the NSD anda subseque ntrotation of16-23⁰to thecurrentdipof10^0• Such asteep origina ldip of the NSDand later rotati on is unlikelybecaus:-

•the dip ofthe NSDis very constantover adistanceof - 80 km,making bot h wholesalerotation orarolling hingerot ationhardto conceive;

•the SSOCophioli tewasobductedalong ashallowdip- pingplane,asis the case for most ophiolites(J.F. Dewey, pers.com m.1997);

•a substa ntial differ encein metamorphicgrade would be expected betweentheDevo nian rocksinthe east- ernand in the weste rn partofthe Hornelen Basin;

•themetamorph ic grade ofthe most westerly alloch- thonous rocks would be expect ed to be substant ially higher.

It is likely,therefore,that theNSD originated wit hvery much itscurrent dip(seealso Andersen&Jamtvei t1990,Deweyet

MAARTEN KRABBENDAM

s

Fig 5.Differential exhumationandattenuation of isobars.a)Schematic map, not to scale,showing domains with different peak pressures.

b)Diagram showing differential exhumationand atte nuationof isobars.

K~ Kalv';y melange.pressureafter Cut hbert (19 9 1). Other pressure dataafterWain(in press).Forfurtherexplanation: seetext.

al. 1993, but see Fossen 1992),that the exhumation taken up by non-coaxialdeformation along the NSDprobably did not exceed25 km,and that a substantial part of the exhumation was taken up by coaxialverticalshortening below the NSD (e.g. Andersen & Jamtveit 1990, Dewey et al. 1993, Krabbendam & Dewey in press)with the highest level secti- ons of the crust thus attenuated subsequently cut out by the NSD.

The most southerly andstructurally highest UHPeclogi- tes occur along the north shore of Nordfjord near Totland (Wain inpress,Plate 1).Thisleveloccurs about 8 km from the NSD.How ever,becauseof Fb fold repetition,thetrue struc- turalthickn essofthe'HP zone'may beaslittle as 4 km(Plate 2 and Fig.5 ).With theminimum pressureofcoesite-eclogi- tes taken as28 kbar,the differencein recordedpeakpressu- re from Bortnepollento Totlandis~8 kbar,equivalent to at least29 km of crustaI section, which isnow attenuatedto 4-8km. Therefo re, an overall at tenuat ion of 72-86 %took place acrosstheHPzone.This attenu atio n, however,is very unevenly distributedthrough out, asHP andUHPeclogites occurvery close together along the northNordfjord coast . A similar situationoccurs withinthe 8 km-w idebimodal UHP- HPzone,where UHP eclogites (P~28 kbar)and HP eclogites (P - 20-24 kbar)occur as close as1km from each other. With a minimum pressure difference of 4-8 kbar,equivalent to

~ 14-28 km of crust, the local attenuation between such eclogite occurrences amounts to~93-96%.

Thus,attenuationof the crust was not homogeneous but varies locally by a factorof 8 or more. Even so,no specific high-strain zones have been discerned between occurren- ces of HP and UHP rockswith the exception of the high- st rainzonethatbound sthe Flatrak et granulite body separa- ting itfromthecoesit e-eclog it e just to the west. Therefor e, both themodel of the WGR as a coherentunit during sub- ductionandexhumat ion(e.g.Fossen1992,Wilk s &Cuthbert 1994) and theforeign int roduct ion model (e.g. Smith 1988) are too simple. A possibl e scenario for producing the close juxtap osit ion ofHP and UHP rocksand thelocally inverte d metamorph ic zonati on may necessitate thejuxtapositionof anUHP domain closetoanHP domain(bot hforming partof the same terrane butone part more deeplyburied than the other)during the first stages of exhumation. Furtherexhu- mation,by continuing E-W extensionconcomitant with N-S shortening(Fa folding)underamphibolite-faciesconditions (Krabbendam &Dewey in press)broughtUHP and HProcks closer together butalsofoldedthe already attenuatedmeta- morphic pile,so that locally the UHProcks occur at struc- turally higher levelsthan the HP rocks(Plat e 2, Fig.5)within the bimodal UHP-HP boundary zone north of Nordfjord.

Summary and conclusions

Amph ib olite-facies,late- Caledonia n deformat ion in thefel- sicgneisses in theNordfjord-Stadl and et area isremarka b ly pervasive,and hasbeen responsib le fortheformat ion and UHPzone

K

• ' time

NSD HPzone

attenuatedIexcised to-1km in NSD;only in part bynon-coaxial strain, total verticalshortening:-98%

vertical shortening: -72-86%;

locally up to96%

t removed by erosion :and extension

1 1

peak -Scand ia n

o

20

30 10

60 50 40

70

80

90

100

.

! ^IUHPI

110

b

N

i

a

UHPprovince

- - -

- - - - - -- - - I

28+kbar. ... - - - - _

~ ---- -~----

___ J

r, 1 - - - - _ _ _ _ : ) I "

... • -22kbar HP-granulite '

----~ , UHPIHP

- - - -. __ " bimod al

--

_22 kbar - - - fzone,6-8km

---;---

I I I

I HPzone

4-8 km

---