Basement-cover relations and Caledonian tectonostrati- graphy of Sandseya, Gryteya, Akereya, and Kjetta, Western Gneiss Region, North Norway

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Basement-cover relations and Caledonian tectonostrati- graphy of Sandseya, Gryteya, Akereya, and Kjetta, Western Gneiss Region, North Norway

STEVEW.VAN WINKLE,MARKG.STELTENPOHL&ARILDANDRESEN

VanWinkle,S.W.,Stelt enpo hl, M.G.&And resen,A.1996: Baseme nt-coverrelat ionsandCaledo niantecto nost rat i- graphyofSandsoya,Grytoya,Akereya.and Kjo tta,Western Gneiss Regi on,NorthNorway.Nor. geol.unders.Bull.431, 67-89.

Rocks ofSandseya,Gryto ya, Akereya.and Kje tta,represent fourCaledoniantectonicsett ings:(1)pre-Caledo nian Balt iccrystallinebasementofthe Weste rnGneissRegion, includingintrusivelyenclosedsupracrustalrocks; (2) an autocht honous/parautocht hono us Vendia n/Cambrian (?) metasedim ent ary cover; (3) the Lower/Middl e Alloc hthonsand granitoid slivers of Balticaffin ity;and(4)thesuspector exot ic terranesof theUpp~rAlloc ht ho n.

Detai ledgeologicmappingof theseislandsindicatesUpper Alloc ht ho n unitshavebeeninco rrectlyassign ed to the Lower/Middl e Alloc ht ho nsonearliermaps.Detailedlithologic observat ionsallowforregi on al correlationsthat bet- terdefin ethest ruct uralconfigu rat ionof thewest limb ofthe Ofote nSynfo rm.Sliversof myl onitized granitegneiss overlainbyort hoquartzi te,graphitic schist, mica schist,anddolomi tic/calcitic marblehave been recogn ized. The repetition of these unitsdocume nts st ructural imbrication of Precambri an basement and itsattached cover.

Amph ibo lite-facies (kyanite grade)mineral assemblagesoccurwit hin theimbricat ed basement-cover,documen- tingint ermed iate dept hsoffo rmation. Late-to post-nap peemp lacement left-slipplastic shearzonesin thebasement complex arecommo nlyassociated wit h supracrustalenclaves that parallelthebasement- cover contact.

Sense-of-shearstudies of thebaseme ntshear zones comb inedwit hshear indicators observed inphy llon it iccover units docum ent top-to- t he-no rt heast, left-slip transport along the shallow sout heast-dip ping shear zones.

Horn blende andmuscovite4°Arf'9Arcooling agesof 408and391Ma,respectively,from cover-unitscombi nedwith microstructuralandfabri c observat io nsbracketthe timeof left-slip movement toLateSilurian-EarlyDevonian.The basement-covercontactand the plasticshear zonesaremod ifi edbyvert ical,f1uorit e-bearing microbrecc iazones that reachthicknessesofover 100m.

5teveW.Van Winkle,and MarkG.5teit enpo hl, Depa rtmentof Geology,210PetrieHall, Auburn University,Auburn, Ala bama36849-5305USA.

Arild Andresen,Departmentof Geology,Universityof Oslo,P.o.Box1047,Bli ndern,0316Oslo3,Norway.

Introduction

The Ofoten region of North Norway is one of the optimal places on Earth to investigate orogenic basement-cover relationships. Recent glaciation and uplift have exposed enormous tracts of deep- to middle-crustallevel rocks so that near-equal proportionsof basement and cover are exposed (Fig.1).The basement-cover contact is exposed in three different orogenic settings, the foreland (sout h- east part of Fig.1),the Rombak window,and the more internal Western Gneiss Region exposedalong the coast.

In the foreland, unmetamorphosed parautochthonous cover rocks lie unconformably on autochthonous Precambrian basement of the Baltic Shield. West of the foreland isthe Rombak window,a large (250 km')tectonic window exposing Precambrian basement gneiss and its attached cover through the eroded Caledonian allochthons. Further west along the present day coast is the WesternGneiss Region,another exposureof Precambrian basement gneiss (Foslie 1941, 1942, Vogt 1942,Andresen

& Tu 11 1986). Rocks of the Rombak window and Western Gneiss Region are thought to best ruct urally continuous beneath the allochthons lyingwithin the core of the regi-

onal Ofotensynform (Fig.1: Gustavson 1972). These relations, therefore, permit the direct examination of strati- graphic, metamorphic, and deformational variations along the contact between the basement and itsautoch- thonous/parautochthonous/allochthonous cover across nearly the entire width of the orogen.

The evolution ofthe Western Gneiss Region basement- cover contact in western Ofoten (Fig.1)is controversial.

Three different interpretations have been reported for the same segment of the contact on Hinney. Bartley (1984) reported large, >10 km amplitude, basement- cored, Alpine-style ductile folds that he interpreted to have formed at middle- to deep-crusta I levels(kyanite- grade conditions) during A-type subduct ion between Baltica and Laurentia (Hodges et al. 1982). Bjorklund (1987), on the other hand, reported greenschist-facies rocks caught up in a duplexalong the basement-cover contact that,he argued, refutes the idea that this contact is the ancient subduction zone boundary. More recently, Andresen & Rykkelid (1991) and Rykkelid (1992), suggested that the basement-cover contact on Hinney is an out-of-sequence thrust that brought greenschist-facies rocks into contact with the higher grade amphibolite- facies units.The basement-cover contact on Hinnoy pro-

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68 SieveW.VanWinkle,MarkG.Stetter ooh!&Arild Andresen

16°

68°

16°

~ MIDDLEALLOCHTH ON

~ LOVVER ALLOCHTH ON

1 ,,-0=:22:-=-.

AUTOCHTHO NOUS COV ER

18°

~ UPPERMOST ALLOCHTHON

c::::::::::J

UPPER ALLOC HTHON

m~mmmm ^{OPHIOLl TE}

20°

GU-BULL 431.1996

68^C

PRECAMB RIANBASEMENT ~ ^OFOTEN^{SY NFO RM}

Fig.1.Generalized tectonic mapof the Ofatenregion illustratingthe loca ti o n of thestudyarea(mod ified from Gusta vson 1972,Andersoneral.1992).

jectsdirect lynorthward onto the islandsof Grytoyaand Sandseya (Fig.1),whichare the areas considered in this study. Herein wepresent:1)a detailedgeological map of Sandsoya and it s adjacentislands;2)ourint erpretive cor- relat ion sof units on these islands with rocks throughout the Ofoten region;3)detailed structural and fabric analy- ses ofrocks along the basem ent -covercontact as well as fromthe overlyingout board exoticterranesofthe Upper Allochthon; 4) our interpretati o n of the deformational st yle along thebasement -cover contact;and 5)4°Ar/39Ar cooling date son hornblendeand muscovite that,combined with observed microstructural and fabric relations,

place constraintson thetim ing of tectonothermaldeve- lopmentof rocks andstru ctures inthis area.

Tectonostratigra phy

The tectonostrat ig raph yisdescribed from the st ruct urally lowest unit,the Tysfjord granitegneiss,sout heastward acrossstrike to the str ucturally hig hest unit expo sedon the northeastshoreof Kjetta.the Kobbevik agneiss(Fig.

2). Mappabl erockunit sareinform allynamed after local geog raphic features . Orig inal, primary thicknesses of

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NGU-BULL431, 1996 Steve WVan Winkle,Mark G. Steltenpohl&ArildAndresen 69

SANDS0YA

, , , ,

I 0

/: AKER0YA

I

t>

"

I :

I

J I

f>

11/ ^~

?

<~ KJE~YA

^?

A

N

?

} Rodmelasenmarbleandamphibolite sequ e nce }Sa ndvikagarn et two-micasch1St

} K0bbevika gne iss

A' -:

2 Km

I

o

I Strike and dipof S1/S2 Brittle fault

*

Thrustfault

o

Inferred contact _ __ _ _ _ Mappablecontact

''Ar/'Ar Sample Locations

Fig.2.Lithotectonicmapofthe studyarea.

units have been modified by thrusting, localized shear- ing, isoclinal folding and boudinage,so that these are now apparent thicknesses only. The rocks are described beginning with the dominantrock type in each unit pro- gressing to lessvoluminous ones.A generalized NW-SE cross-section of the study area(A-A') is shown as Fig.3.

Basement complex

Rocks of the basement complex are more orless continu- ously expo sedalong the west shore of Sandseya and the northeast shore of Gryteya. The basement complex includes the Tysfjord granite gneiss,supracrustal xeno-

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70 SteveWVanWinkle,MarkG.Steltenpohl&ArildAndresen GU-BULL431,1996

~ THRUSTFAUlT - - l'APP.AB..ECONTAC --_. - - - - ItriFERREDCONTACT - - - - - - INFERREDTHRUSTCONlACT

JI~

^.^RJm ^{E FAULT}

^lOO ^m ^]

Om Verticalscale

1 km

Fig.3.GeneralizedNW-SEcross-section fromSandseya to Kjotta(A-AJ.For line ofsection,seeFig.2.Verticalscaleisexaggerated(SX).

Iit hs and enclaves, sulfide-bearing metavolcanics and metasedime nt ary rocks of the Krakenesetsequence,and olde rplutonic units.

Tysfjordgranite gneiss. The Tysfjord granite gneissisthe most voluminou sunitin thest udy area. It hasast ruct ural thicknessof at least 400 m onSandseyaand constitutes mostof Grytoya, Regionally,this unit has beendescribed by Foslie(194 1),Gustavson (1966),and Andresen&Tull (1986). The Tysfjord granite gneissonSandsoya ranges from a homogeneous, coarse-grained, slight ly foli ated, microcline-rich granite in the southwest portion of the study area to a recrystallized, fine-grained, quartz-rich biotite mylonitic gneiss, directl y below the basement - covercont act on thenortheastside of the island. Biot ite conte nt increases from the southwest to the northeast portionofthe basement complexand istyp ically accom- paniedbyareducti onin grain sizeand adecrease in the modalabundance of microcline feldspar. In zonesofduc- tileshearing,the Tysfjord granite gneissisdeformedto a strongly foliated (S-tect onite)orthomylonite. Cross-cut- ting the gneiss are numerou s,thin « 15cm wide).fine- grainedquartz monzonitedikes.

Coarse-grained Tysfjord granite gneiss is generally massive,containing aweak foliation definedbythe parallel alignment of bioti tegrains. Pink microcline « 3cm in leng th) characterizesthegneiss,which istyp ically light pin kto gray in color. Modal mineralog yof the gneiss is estimated asmicrocline(60%),plagioclase(15%).biot ite (2-5%),quartz(10-15%),horn blend e(4-8%),andaccessory fluorit e,garnet,apatite,titanite,andepido te.

Ort ho mylonit esident ifi ed in the basement complex containessent ially the same mineralog y as theTysfjo rd granitegneiss.In areas approaching shear zones,aparal- lel alignment of biotite and flattened and exte nded microcline porphyroclasts together define a mylonitic foliation,givingthe rocka gneissic appearance. In hand

sample the rockis medium - to coarse-g rainedwith10to 50% microclineporphyrocl asts« 1cm across). In thin- section, the ort homylonite contains microcline (40%), plagioclase(25%),biotite(15%),quartz(10%),epidote(3- 5%).andaccessory hornblend e,chlori te,calcite,clinozoisite,zoisite,allanite,andapat ite. Epidote is concentrated in som eshear bandswhereit maycomposemorethan 60% of the rock,imparting an apple-g reen colo rto the rock.

Inthe northernpart ofthe basement complex,struc turally below the basement-cove r contact,outcropsof a fine-gr ained,quartz-rich biotite gneissareinterpretedas dyna micallyrecrystallizedTysfj ord granite gneiss. Hand sam ples of this rock are fine- to medium-grained and dark gray incolor. In thin-sectionthe rock hasawell- developedgrain-shapepreferredorientatio n(GSPO)defined by biot ite (40%), plagioclase (35%), quartz (10%), microcline(8%),and epidote(5%).

In severallocations inthe basement complex,quartz monzonite dikes cross-cut the Tysfjord granite gneiss.

The dikesrange in thickness from 3 cm to 15cm and vary in color from gray,tan,to pink-g ray. Metamorphism of the dikesand surround ingTysfjor d granitegneissis interpreted to have been simult aneouson the basisof the parallel alignment of biotite grainsthat extend through thecontactinto both units.

Supracrustalunits. Supracrustal rockscon stitute only a small vol umet ricport ion of the basement com plexon the west shore of Sandseya and the nort heast shore of Grytoya.They areenclosed inthe Tysfjord granit egneiss asxeno lit hs and large enclaves. Becausethese unitsare metasedimen taryorvolcanic inorig in,and because their origi naldepo sitionalrelationsareunknown,theyarecal- led supracrustal rocks by Caledo nian geologists (e.g., Gustavson1969).The different type sofsupracrust alrocks recogni zedin thefield areainclude a fine-g rainedbiot it e

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NGU-BULL431, 1996 SteveW Van Winkle, MarkG.Steltenpohl&Arild Andresen 71

Fig.4.Outcro pphotographof biotiteamphibo lite(dark) of thebasement complex.Note apop hysesof Tysfjordgranite(light) inupper andright-cen- tral por tion ofthephotograph and a xenoli thofbiot it eamphibolitetothe rightofthe hammer.

amphibolite and a banded meta-arkose.

Biotite amphibolite has a structural thickness of appro- ximat ely 120 m where it crops out directly beneath the basement-cover contact on northern Sandsoya. Locally the amphibolite is intruded by the Tysfjord granite gneiss (Fig.4). Outcrops of the biotite amphibolite are penetra- tively deformed and contain a well-developed schistosity defined by biotite and hornblende.Quartz veins

«5

^{m in}

strike length) are common in this rock and parallel the dominant foliation. Hand samples are fine- to medium- grained,finely laminated, and black to green in color.

The dominant minerals include hornblende(85%),biotite (5%),plagioclase(2-5%),and quartz(3-5%).

Banded meta-arkose occurs as blocky or sheared,lens- shaped xenoliths within the Tysfjord granite gneiss (Fig.

5). Compositional banding defined by centimeter-scale, light and dark layers give the rock a striped appearance.

In hand sample the meta-arkose is fine-to medium-grained and light gray to brown in color. In thin-section the lighter colored bands contain microcline (40%),plagio- c1ase(30%),quartz(10- 15%),and biotite(2-5%).The dar- ker bands contain the same minerals as above with the addition of hornblende (25%)and epidote group minerals(10%).

Kraken e set seq uen ce.TheKrakenesetse q u e n ce isarough-

ly tabular, approximately 60 m thick unit of hornfels, amphibolite, epidote-bearing amphibolite, meta-quartz monzodiorite, quartzite, garnet-magnetite-hornblende- bearing biotite phyllonite, epidote-biotite schist, and marble that cropsout within the Basement complexon northwest Sandseya(Fig.2). These units commonlyare

Fig.5.Outcroppho tog raphof bandedmeta-arkose,bottom of photograph, within Tysfjord Granite. Notexenoli thofmeta-arkoseincenterofphoto- graph.

bordered by thin, less than 2 m thick, ductile shear zones that juxtaposeTysfjord granite gneiss.

The most abundant rock in the Krakeneset sequence is approximately 15m of hornfels. In outcrop the hornfels is massive to weakly foliated and locally weathers to a dark gray or black color. Hand samples are fine-grained, st ruct ureless, and dark gray in color. Within the hornfels is a well-foliated,fine- to medium-grained epidote-bearing amphibolite

« 3

^{m thick).}The amphibolite unit contains thin quartzite lenses

«5

cm long) that parallel the dominant foliation defined by hornblende and biotite.

Apophyses of the Tysfjord granite gneiss intrude the amphibolite.

Older igneous rocks.

The older igneous rocks of the Basement complexare rare and are of two types, a fine- grained metagabbro and a younger, coarse-grained granodiorite. Both rock types occur as xenoliths in the Tysfjord granite gneiss on southwestSandseyaand northeast Gryteya.Several xenoliths of the metagabbro are encapsulated withinthe granodiorite. Apophyses of the Tysfjord granite gneiss commonly wedge into and cross- cut the granodiorite but not the metagabbro, thus esta- blishingthe relative age of these plutonic rocks.

Caledonian allochthons

Sersand schist sequence .

Directly above the basement- cover contact on Sandsoyais a175to 350 m thick structurally interleaved sequence of orthoquartzite,graphite- bearing phyllonitic schist, biotite-quartzofeldspathic schist, garnet-biotite-quartz schist, biotite-quartz schist,

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72 SieveW.Van Winkle.MarkG.Steltenpohl&ArildAndresen

Fig.6.Outcropphotograph ofthephyllon itestruc tu rally abovethebase- ment -covercontact. Noteshearbands(dipping moderately totheleft) extending the schistosity(subhorizont al in photograph)and quartzite len- ses.View istowardthesoutheast, lookin gdown dipofthephyllonitic cleav- age andparalleltothe elongation lineation.Senseof shearis left-slip,top- to-the-northeast.

feldspathic quartzite, and microcl ine-bearing biotite - quartz meta-arkosewhich herein isinformallycalled the Sorsand schistsequence.

A 0.5 m thick basal quart zite rests directly upon the mylonit ized basement gneissand marks the basem ent- cover contacton Sandsoyaand Grytoya. Thequartzit eis fine-to medium-grained,tan to buff colored,and has a moderately well-develo ped micaceous cleavage along whichtherocksplitsintoslabs3-5 cmthick.

Overlying the basal quartzite isapproximately30 m of interleavedphyllonit icschistand biotit e-quartzofe ldspat- hic schist. In outcrop,the phyllonit icschistcontains large micaceous'buttons' «3 cm long)and weathers darkgray to medium brown. Quartz lenses « 6 cm long) parallel the phyllonitic cleavage which isdefined by a shear-band foliation along which muscovite and biotite 'fi sh' are extended.The phyllonit ic cleavage itself is crenulated, folded,andlocallytransposed(Fig. 6). Inthin-sectio nthis rock contains bioti te,muscovite,quart z,andminor plagioclase. Muscovite'fish'cont ain fine-grained'dusty' grap- hiteinclusions.

The biotite -quartzofeldspathicschist containsgeneral- ly the same mineralogy as the phyllonit icschist although the former has more quartz and plagioclaseandless muscovite. In outcrop thisrock weathers to a dullgraycolor and commonly has a pitted appearance due to thedisso- lution of carbonate grains. Mica'butt ons' «2cm long) are common.

Structurally above the biotite-quartzofelds pathicschist is a 140 meter thick section of garnet-biotite-qu artz schist. In outcrop this unitweathers medium-bluishgray.

NGU-BULL 43' 1996

Hand samp lesare fine- to medium-grained and contain visible garnet «3 mm in diameter). Inthin-sectionthe rockcontainsgarnet,biot ite,muscovite,quartz, plagio- c1ase, and accessory chlorite,tourmaline,graphie,and calcite. Poikilob lastic garnets commonly have curved inclusion trails of quartz, biotite, and plagioclase that defineanSifabric thatis discordantwith the surroun ding Se fabric of the matrix, indi catin g post-crystallization rotati on.

A biotite-q uartz schist crops outon south ern Sand soya andisinform ally called a'flaser'schistonthe basisof large,dark graybiot ite -rich lenses«3 cmlong)with a distinctive phacoidal shape that resembleflaser structures found in sediment ary rocks. Theflaserschist isapproximately 8 m in thickness and weath ers to a gray-black color. In thin-sectio n,the dom inant mineralog y isbiot ite, muscovit e, microcl ine, plagioclase, and quartz with accessory calcite,apatite,titanite,andopaques.

A very distinctive,flaggy feldspath icquartzite occurs within the structurally upper parts of the flaser schist.

The feldspath icquartzite is generallylessthan 10m thick and characteristically parts in slabs rough ly 1to 2 cm thick.Some slabs areitacolumi te quartzite,meaningthat whenhit witharockhammer it hasaspringy,elasticfeel instead of ringing.

A microcline-bearing biot ite -quartzofeldspathicmeta- arkose crops out stru cturallyabove the'flaser' schist and the feldspathic quartzite . It hasa structuralthicknessof approximately 10 m andweathers to a darkgray color.

Rounded,pinkpotassiumfeldspar porp hyroclasts(<1cm long)are sparse,butwhere present,givethe rocka spot- ted appearance that is diagnostic of this unit. Some of the feld sparc1asts areelongatedparallelto the dominant foliat io n.

Hallevikamarblesequence. The Hallevikamarb lesequen- ce crops out structurally above the Sorsand schi st sequence. The best outcrops of thisunitare foundalo ng the south and northwest shoresof Sandsoya(Fig. 2).This sequencecontainsapproximately400 m of dolomiticand calcitic marble s inter bedded with dolomitic schist and sparse two -m ica schist, garnet-muscovite-biotite schist, quartzit e, graphitic schist, and at its base, a banded amphibo lite.

Dolomitic marbles range in thicknessfrom 1to 25 m.

Outcrops are generally massiveto moderately well banded and weather dark brown-gra yto tan in color. Fresh outcrops are usually white,cream,or medium light gray.

Hand sam ples are fine- to coarse-grained, characteristically sugary textured, and locally contain fine-gr ained muscovite.Thin-section sof the dolomit ic marble contain up to 98%dolom iteand accessorycalcite,muscovite,plagioclase,andquartz.

Outcropsof the calcite marbles commonly weather from cream to dark-gray in massiveor moderatelyban- ded variet ies. Thicknessof this unit ranges fro m 1to 25 meters. Hand samplesarewhite to blue-gray in color, medium -to coarse-gra inedand locallyhave thinlami na

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NGU-BULL431,1996

« 1 mm thick)of concentrated and preferentially aligned muscovite. In thin-sectionthese rocks contain calcite(90- 95%), with minor amounts of muscovite (3-5%) and assessory plagioclase,quartz, biotite, and opaques.

Dol omitic schist, two-mica carbonate schist, garnet- muscovite-biotite schist, quartzite, amphibolite, and graphitic schist constitute only a small volumetric pro- portion of the Hallevika sequence. All but the dolomite schist form thin « 10 m thick)discontinuous lensesinter- calated within the different marbles.The dolomiticschist ranges from 20 to 60 m in thickness. In outcrop,quartz lenses form narrow,thin ridges «5 mm thick)giving the unit a ribbed appearance.

The banded amphibolite unit marking the base of the Hallevika sequence ranges in thickness from 0.5 to 2 m and commonly weathers to grayish olive green in moderately banded varieties. Hand samples are fine- to medium-grained and contain a schistosity and minerallineati- on defined by the parallel alignment of amphibole grains

«3 mm long). Locally these fabrics are completely trans- posed by a strong mylonitic fabric.

Altevik schist sequence.

The Altevik schist sequence lies structurally above the Hallevika marbles and structurally below the basement sliver that cuts Sandsoya nearly in half (Fig. 2). This sequence is exposed alongthe southern and northern shores of Sandseya but is not traceable along strike inland where outcrops are few. Some confu- sion exists in trying to correlate rocks in this sequence because minor Iithologic differences occur in outcrops along the south and north shores of the island.

Traversing structurally upwards toward the basement sliver along the southern shore of Sandseya,the Altevik sequence is composed of calcite-bearing hornblende and biotite schist, biotite quartz-rich schist, amphibolite, c1i- nozoisite-bearing ultramylonite, minor sulphide-bearing schist, quartzofeldspathic schist, and calcite marble.

The dominant rock type along the southern shore is a calcite-bearing hornblende-biotite schist. Outcrops of the rock type are observable along the shoreline and have an approximate combined thickness of 55 meters.

The rock weathers from a gray to a dark-gray color and locally has a pitted appearance due to the dissolution of carbonate grains.

Along the northern shore of Sandseya, the Altevik sequence is composed of amphibolite, calcite-biotite quartzofeldspathic schist, garnet-biotite quartz schist, and subordinate sulfide-bearing schist and quartzite.

Amphibolite ranges in thickness from 5 to 20 m and commonly weathers grayish dark greento blackin moderate- to well-banded samples. Garnet porphyroblasts« 4 cm in diameter)are common in outcrop.

Basement Sliver.

A strongly mylonitized coarse-grained granite gneiss lying structurally above the Altevik sequence is interpreted to be a sliver of Tysfjord granite gneiss of the basement complex. Structurally overlying the basement sliver is a f1aggy quartzite assigned to the

SteveW.Van Winkle,MarkG.Steltenpohl& Arild Andresen 73

Fig.7.PhotomicrographofS-C fabricsin granitic basement sliver. Viewin orientedthin -section isperpendiculartomyloniticfoliation and parallelto the elongation lineation. Senseofshear isleft -slip,top-to-the-northeast.

Fieldofview is 0.2mm.

Fig.8.Outcrop photographofthecontact betweenthegranitic basement sliver (dark rock)andstructu rallyoverlying quartzite (light colored rock).

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74 SreveW. Van Winkle,MarkG.Sre/renpohl& ArildAndresen

Kolsland schist sequence, Thisbasement-cover contact can be traced nearly cont inuously along strike on the west flankofVetten mountain(Fig. 2).

Rocksofthe basement sliver range from a coarse-gra i- ned orthomylonite on northern Sandsoya to a fine-grained muscovite -bearingS-Cgranitemyloniteon southern Sandsoya. Salmo n pink microcline porphyroclasts are characteristic of the low er exposed portion of this unit but become less abundant st ructurally upward toward theupper contact. Locally, the basementsliver contains small «8 m long)lenses of fine-grained actino lite -bea r- ing biot ite gneiss and epidote-biotite quartzofeldspath ic gneissthat areinterpreted to be mineralogicalvariations of the Tysfjo rdbasement protolithscaused by mylonitic processes.

Orthomylonit e is the most com mon rocktypewithin thebasement sliver. Thisrockrang esinthickness from 3 to 25 m and weath ers from pin kishgraytomedium gray in color. Pink microclineporphyroclasts«3cmin lengt h) have exte nded tails of fine-g rained recrystallized micro- c1ine that givethe rock a banded myloni ti cfoliation. In handsam ple, this rockismediu m-tocoarse-grainedwith survivor microcline porphyroclasts const it uti ng 10% to 50%of therock. In thin-section,this rock contains micro- c1ine,plagioclase,bioti t e, quartz,muscovite,epidote,and accessory calcite, c1inozoisit e, allanite, apatite, titanite, and opaques. Fine-g rained muscovite -bearingS-C granite mylon ite consti tut es the uppermost1 m of thebasement sliver. This rockweat hers gray to medium-grayin outcrop and containsmicrocli ne porphyro clast s«2cm indiameter). In thin-secti onthe rockcontains muscovite, microcline, quart z and plagioclase. The backs ofmusco- vite 'fish' defi ne the S-surfaces wit h extended tails of muscovit e and microclinedefining C-surfaces(Fig. 7).

Vetten schist sequence. TheVet ten schist sequence lies structurallyabove thebasement sliveron Sandsoya and below the Slagstad amphibolite (Fig. 2). The dominant rockstypesincludefine-g rainedflaggy quartzit e,garnet- bearing biotite-muscovite-quartz schist, kyanite-garnet- biotite-muscovite schist, and garnet- quart zofeldspathi c schist. Relatively lessabundant are distinctivecarbonate- bearing quartz schist,calcite and dolomite marble,feld- spat hic quartzite,and rare garnet- bioti te amph ib olite.

Well-foliated flaggy quartzite isin direct contact with the st ruct ural upper contact of the basement sliver on Sandsoya (Fig. 8). In outcrop the quartzite ranges in thicknessfrom 1to 15m andlocallyis tan to buff colored wit h alt ernati ng dark-gray bands that are 1to 20 cm thick. These bands are commonly faulted and folded.

The quartzite readily breaks alongmicaceous part ingpla- nesinto slabs2 to 10 cm thick. Diffuse veins, <10 cm thick, oflighter coloredquartz are common.

Above the basal quartzite is approximatel y 50 m of interbedded carbonate-bearing quart z schist,calcite and dolom it e marble,feldspath icquartzite,andraregarnet- biotiteamphibolite. Theserocks crop out on the north andsout h ends of Sandsoya butwere not traceable along

GU-BULL 431 '99'

st rike towardthecenter of theisland.The marblesin this sequence aredisti nctfromot hermarbleson Sandsoya in that they aremoremicaceou s,and contain large muscovite'fi sh' (2-4cm lo ng). With theexception of the marbles, the remain ing unit sare litholog ically ident ical to those describedfor the So rsand schistsequenceand the- reforewill notbe further discussed.

Struct urally above theseunits isa 75m thickgarnet- muscovite-quartz schist. This schist forms the spi ne of Vetten mountain and isthe most exten siveunit st ruct u- rally above the basement sliver. In outcrop, this rock weat herstoa medium-grayto blue-g ray color. Small«2 mm diamete r)garnetsarevisibleonweat heredsurfaces.

Handsamplesare medium-grained and contain phacoid- shapedlensesofmuscovit eup to2 cmlong,

Struct urally overlying the garnet -m uscovite-quartz schistisapproximately 75m ofkyanite-bea ring garnet- biotite-muscovitephyllonit e.Thisrockhas a'flaser' appearance and commo nly weathers to a blue-graycolor. In thin-section kyanite porphyrob last s «3 mm long) are enclosedwit hi nmuscovit elenses.

Slagsta d amphibolite. The Slagstadamphiboliteis athick am phi bolite unit above the 'flaser' schist and belowthe Kolsland marble and schist sequence on Sandsoya and Grytoya. It has a structuralthickness of about 150 m on the northeastend of Sandsoyabut thins southwestward toappr oximately 10 m on Grytoya. Thebanded amphi- boli te locally is interlayered with garnet amphibolite, hornblende-quartzofeldspath ic schist and gneiss, and minor quartziteand marble.Outcrops of the amphibol ite aredist inctlyfoliated,weather dark-gray to black,and are locallycrenulated. In thin-sect iontherock contains hornblende (40%), plagioclase (25%), bioti te (15%), calcite (10%),quartz(5%),epidot e(3%),and accessory chlorite, clinozoisite,titanite,and opaques.

The Kolsland marbleand schist sequence. The Kolsland marble and schist sequence lies stru cturally abovethe Slagstad amp hib oli t e and contains the highest units exposed on Sandsoya(Fig.2). Thissequenceof rocksis great erthan300 mthick and consistsofinte rleavedcalci- teand dolomitemarbles, dolomi t eschist, various garnet- bearing two -m ica schists,and subordinatequartziteand amphibolite. Rocks wit hinthis sequence are litho logical- Iy identical to thosedescribed for the Hallevika marbl e and schistsequence.

Akemyaschistandmarblesequence.An estimated 3500m ofthe overlying litholog icsecti onis under waterbetween Sandsoya and Akeroya. Detail ed mapping on Akereya revealseight major lith ol ogic typesthat arehereinfor- mallycalled the Akeroya schistandmarblesequ ence.The ent ire package of rocksis approximately 1200 m across strike and consistsof alte rnating garnet-micaschist and calcit emarble units, Other lessabundant units observed in outcrop includeamp hibolite,garnet -bear ing amphi- bolit e, 'garbenschiefer',garnet-bearing gneiss, and car-

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NGU-BULL431,1996

bonate-bearing amphibolite gneiss. Thegarnet-bearing amphibolite occursas both sillsand dikes that locally are enveloped and folded in the garnet-mica schist units.

The garnet two-micaschistsrange from 10m to200m in thickness. Good exposuresof theserocks are on the northwest side of the island. These units weather light gray to silver and characteristicallycontain abundant garnet« 2 mm in diameter). Locally, theschist units contain a crenulation cleavagethat overprints the earlier-formed schistosity. Mino rstauroliteandkyanite occurin the two structurally highest schists along the northern and sout- heasternshores of Akereya (Fig.2).

The four calcite marble unitsrange in thicknessfrom 20to 75m.The three structurallylowest marble units are mod erat ely banded,fine- to medium-grained,and weather darkgray to blue-gray in color. Outc ro ps of these low er units commonly have a blocky appearance and generally split along micaceous part ings into layers less than 15cm thick.The st ruct urally low estmarble uniton Akereyacantainsthin discontinuous layers«5cmthick) of pink marble. The structurally high est marble unit in thesequ encediffers from the lowerthree units in that it conta ins lesscalcite and moremicaandquartz, and does not read ily break into slabs. In thin-secti oncalcitegrains

« 2mmacross) make up approximately90%ofthe mode and maybe elon gated with abundant defo rmat ion twins.

Kjema quartzite. Between the islands of Akereya and Kj atta kalve n,approxim at ely300 m of theIith ologicsecti- on isbelow water. Adistinctive banded quartzite unit, herein called the Kj0tta quartzite, cro ps out along the west and nort hwest shores of Kj0tta and Kjettakalven, respectively(Fig.2).The minimumthickness of this unit, asmeasured where exposed abovewater,averages40m and forms a resistant ridge that istraceable along the ent ire length of thetwo islands. Thequartzitehas a dis- tinct flaggy appearance, weathers tan to buff calored, and hasa rusty-brown staining on some exposed surfaces. On Kjot tathisunit contains rare,thin,discontinuous sill-likebodiesof amphibolite and thin layers of garnet- bearing two-mica schist,both of whichare less than 0.5 m thick. In hand sample the rockis medium-grained and well-foliated with the dominant foliation being defined by micaceous partings lessthan 3 mm thick. Garnets up to 5 mm in diameter occurin the micaceous parting pla- nes. In thin-section the rock hasa equigranular texture with up to 90%quartz,garnet,muscovite, and accessory biot ite, chlori te, calcite, plagioclase, epidote, apatite, hornblende,tourmaline,and opaques.

Cravslett schist sequence. The Cravslett schist sequence lies structurallyabove the Kj0tta quartziteon Kj0tta and Kjottakalven (Fig.2).It has a str uct ural thicknessofabout 300 m but thinsto approx imate ly 75 m on sout hw est Kj 0tta. Four diff erent schist units have been recogni zed in this seq uence. Traversing str uct urally upwa rds,these include a kyanite-staurolite-bearing garnet two-m ica schist, staurolite-bearing garnet two-micaschistwith thin

Steve W. Van Winkle, MarkG.Steltenpohl&ArildAndresen 75

quartz ribbons,hornblende-bearinggarnet-biotite schist, and carbonate-bearing garnet two-mica schist. In outcrop the schist units weatherfrom silver-grayto dark gray in calor and commonly have a red-brown staining on exposed surfaces. Minor amounts of kyanite « 1%),and staurolite «2% ) occur in the lowest schist unit on Kj0ttakalven.

Thin slivers of banded quartzite,less than 20 m thick, interlayered within the schistare Iithologically indistingu- ishable from the Kjetta quartzite (Fig.2). Rare discontinuous bands of amphibolite and calcite marble«1 m thick) in this sequence are commonly boudinaged.

Rodmelasen marble and amphibolite sequence. The Rod rnelasen sequence lies structurally above the Cravslettschistsequence (Fig. 2). It has a structural thicknessof about 100 m onthe northeastend of the island butthins to approximate ly 25 m on the southwestshore.

Rock types inclu de int erlayered gray to cream calored calcitemarble and well foliate dtomassive,banded black amphibo lite. In outcrop the marbles weath erto adull- gray to blue-g ray color and com monly contain dimen- sionally alig ned mica scattered throug hout. On the southwestside of the islandmarble outc ropscom mon ly contain small «10cm)vug-Iike pocketsofepidote and c1inozo isite .

Sandvikagarnettwo-micaschist. TheSandv ika schist lies str uctu rally abovetheRod mela sen sequence and has a st ructura l thicknessofappro ximate ly100m (Fig.2). The unit generally weathers silver-gray to dark gray color. Overall the unit is verysimilarin mineralcomposition and appearance to schist units of the Cravslett mica schist sequencedescribed above. In thin-section the onlydis- cernible diffe rences between these rock types are the recognition of myrm ekitic intergrowths of plagioclase and quartz and the comm on occurren ceof microcline feldspar.

Kobbevika gneiss.The Kobbevikagneiss is the structurally highest rockexposed in the study areaand is distinctive because it is the only intrusiverock,other than the basementsliver,found in thecoverallochthons. It has a mini- mal structural thickness of approximately10m along the southeastshore of Kjetta butthe topis not exposed. The gneisscontains abundant felsic and mafic-rich pods,or lenses, « 25 cm in leng th) that parallel the dominant gneissosity and make up approximately30%of the total volu me ofthis unit. Inthin-sect io n the mafic podscon- tain predomi nant lyhorn blende (55%) and biotite (30%) with minor amount sof plagioclase,epido te, and quart z.

The felsic-rich pods contain quartz (40%), plagioclase (30%),micro cline(15%),and accessory epidot e and serici- tic mica. These pods became smaller in size and less abundant to the sout hw est. In outcrop theKobbevi ka gneissis medium-to coarse-g rained and has alternating light and darkcalored bands« 5cm thick)defining the gneissosity. In thin-section the Kobbevika gneisscon-

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76 SieveW.Van Winkle,MarkG. Steltenpoh!&ArildAndresen GU-BULL 431, 1996

tains hornblende (20%), micro cline (20%), plagioclase (oligoclase; 20%), biotite (15%), and quartz (10%).

Micro cline porp hyroclasts have tartan twins and commonly are surrounded by myremekiti c intergrow t hs of plagio claseand quartz.

Lithotectonic correlations

Figure 9illust rates our interpretat ion of how rocksfrom the present study area correlatewit h sim ilar units south of thestudyarea on Hinnoy and with similar rocktypes throughout the Ofoten region. The structurally lowe st exposed units inthe studyarea,the granite andgranitic gneissof the Basement complex,are identical in appea- ranceand composition to the Tysfjord graniteand granite gneissof the Western GneissRegion,which has been described in detail by Gustavson (1966) and Andresen &

Tull(1986). The olderplut on ic rocksare similarto lithologies repor tedin the young er plutonic sequen ceof Hames (1988) on Senja, north of the study area. On both Sandsoya and Senja metagabbroxenoli t hsare enclosed in large (>25 m²), irregularly shaped bodiesof metagr a- nodio rite.

Metav olcanic and metasedimen tary rocks of the

Krakeneset sequence can be roug hly correlated with rocksof simi lar composit io n inthe basementon eastern Hinnoy(Gustavson 1966,Bartley1981a,Hakkinen 1977), and on Senja(Hames 1988),HornfelsesintheKrakeneset sequence are similarin texture and appearance to hornfelses on Hinnoy described byBartley(1981a),Hornf elses in both areas containthin lenses of foliated arnphibol ite.

Bart ley suggested these amphibolite lenses represent sheared sections of the hornfelses. Oth eramphiboli tes occurasdiscont inuou slenses thro ug houtthe Krakeneset sequence,astyl e that hasbeen recognizedin the younger plutonic sequence on Senja by Hames (1988). The remaini ngrock typesof theKrakeneset sequ ence, quartzite,schist,phyllonite,amphibo lite,and marble aresimi- lar in composit ion to the Hesjevann assemblage of Bartley (1981a) and an unnam ed sequence mapped by Hakkinen(1977)inthebaseme nt on easternHinnoy.

TheSorsand schist sequenceoverlying the basement gneisscomprises basal ort hoqu artzite,phyllonit ic flaser schist, biotite-quartzofe ldspat hic schist, quartz-rich schist,feldspathicquartzite,and rneta-arkose. Thesame lith ol ogi es occur within theVettenschistsequence above the basement sliver,alt hough theVett en sequencecon- tains micaceous calcitic and dolomitic marble. The Sorsandand Vette nschistsequencesareassignedtothe

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(11)

NGU-BULL 431,1996

Lower/M iddleAllochthon on the basis of their association withthe underlying Precambrian basement and their lith - ologic make-up, The orthoquartzite at the bases of these sequences is similar to that overlying granitic basement of the Lower Rombak Group in the Harstad-Tjeldsund area(Gustavson 1966,1972). Bartley(1981a,1984) referred to these rocks as part of his Storvann Group on east Hinnoy, Bjorklund (1987) reported comparable units (sub-unit s A, E-I)overlying the basement in the Storvatn- Gausvik area on eastern Hinnoy. The overlying phyllonitic schist, flaser schist, biotite-quartzofeldspathic schist, quartz-rich schist,and feldspathic quartzite of the study area correspond to the mica schistand feldspar-bear ing quartz schist of the Rombak Group (Gust avso n 1966, 1972),to the mica schist of the Lower-Middle Complex (sub-unit s E,F,H)of Bjorklund(1987)and to the phylloni- tes and mica schists of the Fossbakken Nappe (Rom bak Nappe Complex) described by Barker(1986) along the east limb of the Ofoten Synform. The feldspathic quartzite is strikingly similar to the 'hard schist', a strongly banded quartz schist, described by Gustavson(1966) in the Storfjell and Rombak Groups and of the Abisko Nappe in the Swedish foreland(Bax1989).The meta-arkose in this sequence is Iithologically ident ical to the feldspathic schist (sparag mit e schist) of the Storfjell Group (Gustavson 1966),which corresponds to the distinctive sparagmite schists of the Lower and Middle Allochthons in Sweden (Bax1989). The thin lenses of quartzite,and dolomite and calcite marble interleaved with the above units are reported to be typical of the Storfjell Group (Gustavson1966).

The Hallevika and Kolsland marble and schist sequences, which structurally overlie the Sersand and Vetten schist sequences, are characterized by thick, clean and banded dolomite and calcite marble interlayered with thin garnet two-micaschist. These marble sequences are int erpreted to correspond to the Evenes Group of the Upper Allochthon'sOfoten Nappe Complex (Gustavson 1966,Andresen&Steltenpohl 1991,1994)on the basis of lithology and tectonostratigraphic position and because they project in to and can be traced directly into the type units to the south of the study area. Barker(1986) described a similar, carbonate-rich sequence at the same struc- turallevel in the Heqtind Nappe of the Rombak Complex on the east limb of the Ofoten Synform. The base of the Evenes Group typically is marked by the Elvenes or Harstad conglomerates (Gustavson 1966,Steltenpohl et al. 1990) which have a nonconformable contact with underlying mafic complexes that Andresen&Steltenpohl (199 1, 1994) correlated with the Lillevik dike complex (Boyd 1983) and Lyngen Ophilitic Complex (Minsaas&

Sturt 1985), The bases of the Hallevika and Kolsland sequences are marked by amphibolites which we correlate with similar amphibolites below the Harstad conglo-

merateon Hinnoy (G us ta vso n 1974, Bartley1984)and the

above-mentioned mafic complexes at the base of the Ofoten Nappe Complex (Andresen & Steltenpohl 1991, 1994). The fault at the base of the Ofoten Nappe

SteveW VanWinkle, MarkG.Steltenpohl&ArildAndresen 77

Complex(i.e., the 0se thrust of Barker1986and Andersen et. al. 1992) has apparently removed the entire Narvik Nappe Complex and the Heqtind Nappe in the study area.

Correlation of the overlying Altevik sequence is diffi- cult due to the chaotic distributionand relatively thin and discontinuous nature of lithologies.We suggest its correlation with part of the Bogen Group of the Ofoten Nappe Complex (Andresen &Steltenpohl 1991,1994) based on tectonostratigraphic position and the abundance of schist interlayered with relatively thin calcitic marbles and feldspathic quartzite units. The thin sliver of basement structurally overlying the Altevik sequence on Sandseya and Grytoya corresponds to imbricated basement gneiss of the Middle Allochthon of Gustavson (1966),Bjorklund(1987),and Andresen&Rykkelid(199 1).

The calcite marbles, garnet-bearing two-mica schist, and garbenschiefer of the Akeroya marble and schist sequence clearly correspond to units of the Tangen sequence of the Evenes Group reported by Steltenpohl et al. (1990), on the basis of tectonostratigraphic position abovethe thick, clean marble sequences and below the Kjotta quartzite,the latter of which clearly is equivalentto the Be quartzite of the Bogen Group (Gustavson 1966, Steltenpohl et al. 1990). Other similarities with the Tangen sequence are thin layers of amphibolite and garnet-bearingamphibolite,which occur as thinconcordant layers enveloped between the mica schistunits and thin discontinuous quartzites that are commonly boudinaged (Steltenpohl et al.1990).

The remaining, structurally higher units of the st udy area are interpreted to correlate with lithologies of the Bogen Group (Gustavso n 1966,Steltenpohl et al. 1990).

The Kjetta/Be quartzite is an especially important correlation tool and provides further continuity to thisdistincti- ve regional marker unit (Gustavson 1972,Steltenpohl et al. 1990). Garnet two-m ica schist of the overlying Cravslett schist sequence corresponds to the lower mica schist of Steltenpohl et al.(1990). The Rodrnelasen marble and amphibolite sequence corresponds to the Fuglevann marble of Gustavson (1966) and Steltenpohl et al.(1990) on the basis of lithologic similarities and stra- tigraphic position. The overlying Sandvika garnet two- mica schist is interpreted to correspond to the middle mica schist of Steltenpohl et al.(1990)and the Kobbevika gneissis similar to units containing felsic and mafic injec- tions and lenses within the Middle mica schist (St eltenpohl1983,1987).

Structural Geology

Rocks of the study area record at least four episodes of deformation. Detailed structural and petrographic analy-

se s indicate that these events can be broadly grouped

into two early-phase amphibolite-facies events, D₁ and D₂, followed by two post-metamorphic peak events, D₃ and 0_{4 ,}that occurred under retrograde,greenschist-faci-

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78 SreveW Van Winkle,MarkG.Stettenpob!& Arild Andresen

es condit ion s, Early-p hasedefo rmat ion sare interpret ed to beassociat edwit hnapp e emp lacement whereaslate- phase deformationsproduced ductil eshearzonesinthe basement com plex andfoldsinthe alloc hthon ous cover units. 5t ruct ural notati on used indi cates thatthestruct u- res and fabricsresulting from a parti cular deformat io nal eventare numbered the sameasthateven t; for exam ple L" 51'and F,formed during D,andL2, 52'and F2formed during Dband soon.

D

₁

Deformation

Eviden ce for the earliest deformat iona leventrecognized in rocksof the studyarea,D" islimited due to intense overprinting during the later, kyanite-grade, D2event.

Howe ver,mesoscopicand microscop icevidence forD,is found in eachof the Caledonianallochthons of the stu dy area.

Folds. Isoclinal F,fold hinges inquartzite that fold corn- positional layering 50have anaxialplanar metamorphic foliation 5,. F₁folds and their 51fabrics are refolded by tight,shallow northeast-plungingF2folds that contain an axialplanarfoliat io n,52'whichisthe dominantschistosit y in rocksof thestud yarea. Aresultisthegeneralparalle- lismof the

5 a15 ,

^fol^iation ^{and 52}^'referred to belowasthe 5/5 2foli ati on.

Fabrics. Pet rog raph icevide nce for D₁defor mationis rare and limitedto inclusion trailsingarnet andstauro litepoi- kiloblasts. 5_iin the poikilob lasts is commonlyfolded or snow balled and is discordant with the surroundi ng 5_e foliat ion,52'whichisdefined by the peak metamor phi c mineralassemblages.

Fig.10.Outcrop photograph of tight F2fold s in quartzit ealong basement- covercontact.

NGU-BULL431,1996

Fig.11.Outcrop photographofrefoldedF2fold in amphibolite at the base oftheHallevikaschist and marble sequenceexhibitingaRamsay Type 11 interferencepatternbetween F2and F3folds.

D2 Deformation

D2wasthe most pervasive st ruct ural andfabric-fo rm ing event recognized in rocks of the st ud y area. Prograde metamorphism duringthis event developed peak,low er to middleamphibolite-facies(kyaniteand staurolit e grade)mineralassemblages.

Folds. F2folds are common to practicallyall rock types.

They are tightfolds in quartzite(Fig.10)and marb le with thickened hinges and thinned limbs. F2folds and their axial-plana r52fabricare refoldedbyF₃fol dsresultingin Ramsay (196 7) type 11 (Fig. 11) and III interference pat- terns.F2fol ds typically have amplitudesthat range up to severalmeters.

Fabrics.Twomain fabric elements were produ cedduring D2,the dominant52schistosity and various types of L2 lineation s. 52is defined bylowe rto middleamphibo lit e- facies,kyanite-and staurolite-gradeassemblages. The52 foliation parallels the basement-cover contact and the thrust bou ndariesbetween the allochthons. 52istracea- blestru ctura llydownward forapproximate ly50to100m beneath the basement-cover contact into the Tysfjord granite gneiss but gradua llydisappears,a relation recognized regionallyalong the Western GneissRegionbase- ment-covercontact(Tull 1972, Bartle y1981a,Hodg es et al.1982). 5tereographicprojectio nsof52indi categentle dips to the southeast which become sub-horizontal toward the southeast(Fig. 12), indi cati ng the structural positionof these unitsalong thewesternlimbof theregi- onal Ofoten synform;partialp-gird lesdefined bythe52 poles in figure 12 reflect late-phase folding, discussed below. A pronounced L2stretching lineati on liesin 52 andplunges gently to theeast-sout heast(Fig.13),generally correspond ing to L2 Iineations found regiona lly along the western limb of the Ofoten synform (5teltenpohl 1987). The L2lineation is interpreted to record the D2 transport direction along which the

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NGU-BULL431,1996 SreveW.Van Winkle, Mark G. Sre/renpohl & Arild Andresenl 79

N =278

N N = ¹⁴⁷

N = 79 N N

Fig.12.Conrouredlowerhemispheresrereographic projecrions ofpolesroSI/52,Contoursare Sandseya 4% - 14%-17%-22%;Gryteya2%- 4% -15% - 30%;

Akeroya8%-25% -39%-74%;Kj etta11% -31%-47%-97%per1%area.

Caledonian nappes were emplaced(Steltenpohl1987),

Thrusts. All thrustsinthestudy area are interpretedto be 02st ruct uresbased on parallelism of O2fabricsandstr uc- tures acrossthe faults, the lackof detectablemetamo r- phic grade differ encesacross them and becausefabrics and mine ral assemblages observed in the thrusts are compati blewith having for medunder 02deformational condit ions. However,onlytwo thrustboundaries,thatat

thebase and oneat the top of thebasementsliver,were directly observed. The other thrusts are inferred based on Iithologic contrastsacrossthe boundariesand regional lithologic simi larit ies wit h known thrust-bounded nappes.

The contact between the Tysfjord granitegneiss of the Basement com plex and overlying cover rocks of the Sorsandschist on Sandsoyaisint erpret ed as a thrust(Fig.

2). Evidenc eto support thisis thest rong S2foliation and

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80 SreveW.Van Winkle,MarkG.Steltenpohl&Arild Andresen NGU-BULL431,1996

N

=

25 N N

= 43

N N = 5

N N = ²⁶

N

N =9

o '200

o o

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=

10 o

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=

13 N

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Fig.73.Lowerhemisphere srereograp hic projecrion sof syn-and post-metamorp hic stretchinglinea tions.Contoursfor Sandsoya(post-m eta morphic, n= 43) 7% -3%-5% -7%per7%area; (syn -metamorp nic. n

=

²⁶⁾⁷^%^-^3%^-^5%^{- 7%}^per⁷^%area: Circles

=

post-m etam orph ic. Triangles

=

syn-me tamorphic.

associatedL₂mineral elongationlineati o n thatisprogres- sively developed wit hin basement granite as the basement-cover contact is approached and which parallel s fabricsin the overlying cove r units. Amphibolite-fac ies mineral assemb lagesdefining S2in basemen t and cover unitsand the lackof a ret rog ressive deformatio n zone

indi cate thatthisboundaryprobably isa syn-M,thrust. Thrusts areinterpretedto lie betweenthe Sorsandand Vettenschistsequenc esand betweenthe Hallevikaand Kolsland marbleand schist sequences. The amphibo lit e atthe baseof theHallevikasequenceisinthesame recto- nostratigraph ic posit ion beneath theEvenesGroup and

Basement-cover relations and Caledonian tectonostrati- graphy of Sandseya, Gryteya, Akereya, and Kjetta, Western Gneiss Region, North Norway