Caledonian structural evolution and tectonostrati­graph,Y in the Rombak-Sjangeli Window and itscovenng sequences, northern Scandinavian Cale­donidesGERHARD BAX

Bax, G. 1989: Caledonian structural evolution and tectonostratigraphy in the Rombak-SjangeliWindow and its covering sequences, northern Scandinavian Caledonides. Nor. geol . unders . Bull.415, 87-104.

The tectonostratigraphy and Caledonian structural evolution of an area in and around the north ­eastern corner of the Rombak-Sjangeli Window (RSW) in the northern Scand inavian Caledonidesis described. The investigated area compr ises four main tectonostratigraph ic units , establishedduring southeast-directed, piggy-back thrusting onto the Baltoscandian Platform. The lowermostexposed unit is composed of Svecokarel ian crystalline rocks of the anochthonous RS-Complexwith its Vendian to Cambrian sedimentary cover of the Gearbeljavri Formation. Lithological andstructural comparisons of this unit with the Rautas Complex in the east favour a correlation ofboth units . Slivers of locally derived crystall ine and sedimentary rocks are arrange d in isolatedduplexes of the Hoiqanjavri Comp lex. Detailed mapp ing has revealed the composite nature of theoverlying mylonitized Abisko Nappe Compl ex. The uppermost preserved port ions of the nappesequence are dominated by metasediments of the Seve-Ki:ili Nappe Comple x. Metamorph ic gradeincreases in tectonically higher and consequen tly farther-travelled nappe units. Scandian thrusting(T,) postdated the peak of Caledonian metamorphism.

'Basement' -cover interaction can be studied around several minor windows and klippen at thenortheastern margin of the RSW. Faulting in the RS-Complex induced folding (partly cross-folds) inthe overriding nappes during two (D) and D.l of at least five phases of Caledonian deforma tion .N-S-trend ing high-angle faulting dur ing D. locally caused back-thrust ing structure s and was accom­panied by emplacement of sulphide-bearing quartz veins. Movement along pre-exist ing Caledonianductile shear zones under britt le conditions resulted in the formation of pseudotachy lite.

Gerhard Bax, Inst itut fOr Geolog ie und Peteontoloqie am Fecnbereicb Geowissens chaften derPhilipps-Universitat, Lahnberge , 0-3550 Marburg/Lahn (West-Germany).Present adress: Norges geologiske undersokelse, Postboks 3006-Lade. N-7002 Tronane im (Norway).

IntroductionThe wester ly dipp ing metamorphic nappe pileof the Scandinavian Caledonides is deeplyeroded , thereby exposing several 'basement'culminations (Fig. 1). These are arranged intwo parallel belts (Vogt 1922, fig. 3) along thestrike of the mountain belt. Whereas the eas­ternmost tectonic 'basement' windows, situ­ated less than 100 km west of the presenterosional thrus t front, are commonly rimmedby a sedimentary cover sequence beneath astack of allochthono us units, most of the base­ment culminations along the Norwegian coastlack cover sequences. The latter category,which includes, for example, the Lofoten terra­ne, is often referred to as the western, basalor basement gneisses (Griffin et al. 1978, Bryh­ni & Sturt 1985, Gorbatschev 1985).

A westwa rd increase in the degree of defo r­mation and metamorphism of the cover sed i­ments is related to their successivel y greaterinvolvement in the Caledon ian orogenesis. Inmost of the windows the sediments have beentreated as a continuation of the autochthonoussedimentary sequences occurring in the fore­land in front of the orogen (Holmqu ist 1910,Kulling 1960b & 1972, Nicholson & Rutland1969, Wilson & Nicho lson 1973, Gustavson1978). Some windows are of a compositenature , with thrust repetition of the basemen t­cover relat ionship, e.g. the Nasat jal: Window(Du Rietz 1949, Thelander et al. 1980) and theGrong -Olden Culmination (Asklund 1938, Gee1980). In northern Norway, no lower tectonicunits have been found beneath the Precam-

88 Gernerd Bax

i'!:-..::#~:-r;-=---- B"r9efjell

......12--+-<-- Grcng-Olden

~ Cover

• 'Bcserr. er.t ' wH~CCwS

I.·.··1 Ba l re 5 .etc

J 3 0(,

FIg. 1. Majo r 'basement' w indows in the ScancmavianCaledorudes.

brian crystalline rocks of the Komag fjord (Rei­tan 1963) and Alta-Kvcenangen Windows, butan involvement in major Caledonian deforma­tion resulting in an allochthonous position , atleast for the Komagfjord window rocks, hasbeen suggested by Chapman et al. (1985) andGayer et al. (1987). Previous investigationsof the Berqe fjell (Greiling 1982) and the Rom­bak-Sjangeli Windows (see next section) showthe 'basement' rocks as the lowermost ex­posed unit , directly overlain by the Middle AI­lochthcn (Gee & Zachrisson 1979). In the pre­sent study the tectonostratigraphy of the rocksin and around the Rombak-Sjangeli Windowis described and structural arguments for anallochthonous pos ition of the window rockswill be present ed.

Regional geology in and aroundthe Rombak-Sjangeli WindowThe Rombak-Sjangeli Window (RSW) is adome-shaped 'basement' culmination which is

GU · BULL ~ 5. 989

sligh ly elonga ed along he E-SS rend.A 'binomial' des ignation is used here (insteadof Rombak Window) in ord er to avoid con­fusion wi h he estab lished term Romba Group(Strand 1960, p. 163), which refers to roc sof the over lying allochthon. The RSW coversan area of about 1900 km' north of latitude68° . along both sides of the borde r betweenSweden and orway (Fig. 2).

To the nort hwest. rocks of the RSW disap­pear under the moderately westw ard-dippingf100 .. thrust of the overlying nappe pile of theOfoten Synform (Gustavson 1972) which inclu­des, in its lower part s, roe units corr elatedby Vog (1922) wi h those of the iddle AIIoch­thon (nappe 4 of Binns, 1978) in the Torne­trask area . Accord ing to Binns (1978) the high­est preserved nappe unit (Binns: nappe 7) inthis part of the Scandinavian Caledonidesappea rs along the axial trace of the synform30 km to the west of the RSW. It is represen­ed by he iingen Schis (Vog 1922. 1942 &1950) or he iingen Group (Gustavson 1966).

o remnants of the unconformable sedimen­tary cover have been found along the wes ernmargin of he RS excep or some isola edoccurences along the S jomenfjord (Bir eland1976) and two others between Rombaken andBeisfjorden (Vogt 1950). On the basis of tecto­nic relationships along the western edge ofthe RSW. Hodges et al. (1982) proposed anA-type subduction zone bordering this window.

The westerly dip of the metamorphic nappepile around the RSW is replaced in the sou h(Hodges 1985) and in the north (TulI et al.1985) by a more or less gentle inclination ofthe floor thrust towards the exte rior of thewindow . Traces of the sedimentary cover,sandwiched betwee n the 'basement' rocks andthe cove ring nappes, become quite commoneast of 17°40' E. These units were describedin the south by Kulling (1964, pp . 60-66. figs.30-36 . ct. Kautsk y & Tegengren 1952) andalong the northern margin of the RSW by Tullet al. (1985. fig. 4).

Some minor windows also occur 0 heeast of the RSW. The northernmost, largestone. the Kuokkel Window, occupies an areaof about 80 km' and is separa ed from heRSW by a more than 184m-deep downfoldedsynform (Kulling 1964, p. 90) of the iddleAllochthon (Fig. 2) north of Vassijaure stat ion(Plate 1). This synform can be follow ed (Plate1) to about 20 km sout h of Vassijaure stat ion,and is accompa nied by two miner tecto nic

NGU -BULL.41 5. 1989 Caledonian structural evolution 89

o 10

KotiSeve Noppe Complex

Abisko No pp e ComplexRou to s Com plexDividol Grou pPi: - Bo s em en t

c o m p il ed by Ger hord B ox 1987

Fig. 2. Tectonostrat igrap hic map compilation for the Rombak-Sjangel i Window and .adjacent areas based on Petersson(1897). Vogt (1950). Ocman (1957). Kulling (1964), Gustavson (1966, 1972 and 1974), Blrkeland (1976), Hodges (1985),T ulI etal. (1985). t.lnostrorn et al. (1985), Llndstrorn (1987), Kathol (1987) and unpublished data of M. Johns son. For location see Fig. 1.

windows (each less than 1 km') along its eas­tern margin, Where this Middle Allochthonsynform is split by eros ion along the Hoigan­vaggi valley, the tectonic window (Kulling 1964,fig. 22 and Bax 1986, fig. 1) - her called AlipHoigan Window - is separa ted from the RSWby a downwarped and locally imbricated sedi­mentary cover. The small Vuolip Hoigan Win­dow is located about 2 km to the southeastaround the outlet of Hoiqanjavri. In the upper­most part of the valley Vassevaggi a newlydiscovered window (this study) - the VasseWindow - is exposed east of the synform .Following the trend of the southern marginof the RSW to the southeast , the Singis Win­dow (Kulling 1964, Plate 1) appears sout heastof Kebnekaise. This window was considered

to be parautochthonous by Kulling (1964, p.138), but he did not include it in his RautasComplex (Kulling 1950a) of the Lower Alloch­thon (Gee & Zachr isson 1979), Because of li­thological similarities (see Plate 1), rocks ofthe Rombak-Sjangeli, Kuokke l, Vasse andHoigan Windows are here included in the sametectonostratigraphic unit.

Steeply inclined suprac rusta l sequences (fig.2 in Holmquist 1903), common ly arranged inapprox imately N-S and NNE-SSW-trendingstripes and lenses, are thought to representthe oldest units in the Norwegian part of theRSW (Vogt 1950, Birke land 1976). Metamor­phic grade reached amphibolite facies (P>6kb,T=575-600°C), according to Sawyer (1986).These rocks have been a target for exp lora-

90 Getnerd Bax

tion because they are associated with oredeposits in orw ay (Korneliussen e al. 1986)and S veden (Petersson 1897. Romer, hisvolume). The age relationship between theseuni s and migmatized supracrustal roe s in heSwedish part of the RSW (map of Johansson1955 in Odman 1957, fig. 25) and the Kuokkelwindow (Adame 1975. Plate 1) is still uncer­tain. The sub-vertical. westerly dipping. Koppar­asen greenstone belt, in he Kuo el Window .with its stra abound uraninite. magne ite andsulphide mineraliza ion (Adamek 1975), ap­pears 0 con inue sou h-sou heastwards underthe overlying nappes as suggeste d by Pe ers­son (1897) and to reappear as the Sjangeligreenstones (cl. Romer, this volume). Minoramounts of similar greenstones, present in theHoigan window , seem to confirm this assump­tion, although some lateral and vertical dis­ruption must be taken into account.

The supracrustal rocks are intruded by pluto­nic rocks of different compos itions that formthe main part of the windows. Rb/Sr ages ofthese plutonic rock s are 1715=90 Ma (Heier& Compston 1969) and 1780=85 Ma (Gunner1981).

The part of the Tornetrask section lyingbetween Tornehamn (Bax 1984) in he wesand Stordalen (Dworatzek 1976, t.mostrorn etal. 1985) in the east, represents the shortestdistance (about 19 km) between the Precam­brian rocks of the Baltic craton and he linearbel of basement window s within he Scandina­vian Caledonides (c . Gee e al. 1985). A hewes ern end 0 he Torne ras sec ion. he

iddle AIIoch hon (Abis 0 appe of Kulling1960a: here called the Abis 0 appe Complexbecause of its compos ite ec onic nature),which overlies the Kuokkel Window . dips 0

the southeast beneath the level of Tornetrask(341m.a.s.I.). The base of the Abisko appeComplex reappears 12 km to the southeaston top of the Rautas Complex of the LowerAllochthon (Fig. 2).

The Rautas Complex is composed of variab­ly deformed Precambrian igneous rocks withattac hed sedimentary cover sequences. Itreaches a maximum thickness of about 400 maround Stordalen. but pinches out tectonicallynorth o f Sakas (Lmd s trorn et a t. 1985). At the

present erosional thrust front around Luo­pakte, the Lower Allochthon is representedby tecto nic slices containing calcareous sedi­ments with Lowe r Cambrian trilobites, which

NGU . BULL . 5 989

are also presen in he underlying au oc ho­nous beds (Ahlberg 1979).

The au och honou s sedimen ary sequence(Dividal Group) of Vendian and Cambrian age(Vidal 1979) res s unconformably on Sveco a­relian crystalline roc s 0 he Bal ic Shield 0

the nor h and sou h of Torne ras (Fig. 2). Themost comple e sec ion 0 hese shelf depo­si s. comprising bo h he iddle 0 UpperCambrian Alum Shale and e underlying or­ne ras Forma ion (c . Thelander 1982) is ex­posed a Luopa e (i oberg 1908). To enor nwes he almos un olded sole hrus 0

he overlying nappes runca es success ivelylower por ions of he Dividal Group. AroundStordalen (abou 24 km nor hwes of he hrusfront), only the basal conglomerate is pre­served (Lindstrorn et al. 1985). either thesediments of the Dividal Group nor those ofthe Rautas Complex in this area were ex­posed to PIT conditions exceeding those ofvery low-grade metamorphism (classificationof Winkler 1979).

The variably myloni ized igneous and sedi­mentary rocks of the Abis 0 appe Complexwere me amorphosed up 0 bio i e grade du­ring he Caledonian orogenesis. These roe scan be followed, wi h varying hie ness. romthe thru st fron in he sou eas 0 (at leas)the orwegian border (Bjer lund 1985) in henor hwes .

The overlying. medium-grade (amphiboli etacies) Upper Alloch on (Gee & l ac risson1979) eas 0 Abis o. is do ina ed by ypicalSeve roe s (l achrisson 974). represen edmainly by amphiboli es. olia ed myloni es andgneisses (Linds rem e al. 985). hese Severoc s branch ou to he nor and wes 0

Abisko undernea h he Keli uni s. Consequen ­Iy, no Seve remnan s are nown 0 occur onthe western side of the RS (l achrisson 1973,Hossack 1983). The Keli succession in theTornetrask area is dominated by impure mar­bles and graphi e schists al erna ing wi h micaschists con aining variable amounts of garnet.Minor occurr ences of amphibolite and gar­benschiefer are concen rated in the Upperport ions of the sequence.

NGU - BULL. 415. 1989

Tectonostratigraphy

Rombak-Sjangeli ComplexThe lowermost tectonostratigraphic unit ex­posed in the study area consists of crystallineSvecokarelian rocks of the RSW (Plate 1). Apossible tectonic lower boundary of this unit,here called Rombak-Sjangeli Complex (RSC),is discussed later (structural section).

The dominant NNE-SSW trend of the supra­crustal rocks is clearly demonstrated by anat least 200 m-thick strip of brown, rusty micaschist (Plate 1) and quartzite, which enters thestudy area southwest of Stuor Gearbil. It con­tinues with variable, sometimes sub-vertical,WNW-dip to t.atrecorru, where it is cut of byan westward-dipping, late Caledonian, high­angle normal fault. The supracrustal rockshave a distinct foliation that is defined by theplanar orientation of biotite flakes in a quartz­feldspar matrix. The rusty appearance is dueto disseminated hematite. To the west of thisstripe and tairecorru, an anastomosing net­work of similar mica schists is intruded byapophyses of the younger Vassijaure Granite(Holmquist 1903, Fig. 1), which contains xeno­Iiths of mica schist. The coarse-grained Vassi­jaure Granite shows an augen texture parallelto lithological boundaries. This rock-type, alsoknown as the Rombak Granite, is commonover large areas of both the RSW and theKuokkel Window (Holmquist 1903, Adamek1975, Bax 1984). It is characterized by 1-3cm­long feldspar auqen mantled by biotite, horn­blende and minor amounts of sphene, apatite,epidote, rutile, tourmaline and ore minerals.

The areas to the east of the mica schistand between t.atrecorru and Vuoiddasriiddaare dominated by fine-grained granitoid rockswith a migmatitic appearance, which includelayers of mica schist. These rocks reappearin the Hoigan and Vasse Windows. A lobe ofsupracrustal rocks extends from the Sjangeliarea (Romer, this volume) in the south up toGearbeljavri, were it disappears beneath theoverthrusted rocks. Amphibolite within thissupracrustal sequence is abundantly exposedalong the eastern shore of Gearbellavri, andits northern continuation reappears in the AlipHoigan Window. The Vassijaure Granite domi­nates in the Vuolip Hoigan Window.

Sulphide-bearing veins are concentratedparallel to lithological boundaries along bothsides of the border between Norway and

Caledonian structural evolution 91

Sweden. Most of these calcite-hosted galenaoccurrences were discovered and briefly de­scribed by Lofstrand (1894). Galena-, sphale­rite- and fluorite-bearing calcite veins aroundSvanqeraive were described more recently byJohansson (1983, 1984). Although the age ofthese mineralizations is still unknown, theymust be younger than the Svecokarelian oro­geny, as the veins cut through all accompany­ing structures. No contact relations with thesedimentary cover or Caledonian structureshave been observed and so their relationshipto the Caledonian deformation remains unk­nown. Caledonian sulphide-bearing quartzveins in the RSC are discussed later.

Gearbeljavri FormationThe crystalline rocks of the RSW are overlainunconformably by a clastic succession, hereinformally called the Gearbeljavri Formation,which occurs widely at the base of the overly­ing nappes (Plate 1; et. Brown & Wells 1966,fig. 2). Where this succession is missing, asin Vassevaggi or around parts of the southernedge of Stuor Gearbil, the floor thrust of theMiddle Allochthon is hard to trace.

The impure, quartzitic, basal parts of thesuccession reach a maximum thickness ofabout 10 m in the southern part of the areaand are commonly in depositional contact withthe underlying crystalline rocks. They showevidence for lateral variations in depositionalenvironment. The most common lithology ofthe Gearbeljavrt Formation is an arkosic meta­sandstone with irregularly distributed, roundedquartz pebbles. The depositional contact withthe underlying crystalline rocks is either sharp,with no obvious grading in the metasand­stone, or transitional, where it rests on a pre­Caledonian weathered zone. In places, locallyderived coarse sedimentary breccias are pre­sent. These diamictites (cf. Flint et al. 1960a,1960b) have been described by Brown &Wells (1966) as tillites, but the present authorhas found no convincing evidence (see Har­land et al. 1966, Flint 1975) for a glaciogenicorigin in the field. Channel-deposited, grain­supported, gravel beds occur in the VuolipHoigan Window and around Gearbeliavri, Thebasal metasandstone, which is quite resistantto weathering, is overlain by an alternationof intensively sheard brownish slates andquartzites which is up to a few metres thick.

92 Gerhard Bax

A partly reworked regolith is regarded asthe source of the metasandstone member, andthe alternation of slates and quartzites indica­tes the influence of a more uniform (possiblymarine) environment. A post-Varangerian (Ven­dian) age (Vidal 1979, Fig. 1) can be proposedfor the Gearbehavn Formation based on corre­lation with the lower sandstone member (The­lander 1982) of the autochthonous TornetraskFormation. This implies a maximum age of668 ± 23 Ma (Pringle 1973), as no definite tilliteshave been found in the study area.

In contrast to its counterparts in the autoch­thonous Tornetrask Formation further east orin the Rautas Complex, the Gearbeliavri Forma­tion contains ubiquitous biotite in the form ofmicroscopic flakes arranged along shear pla­nes. In places the biotite is accompanied byepidote, but chlorite is restricted to plans ofyounger movements.

Hoiganjavri ComplexThe Holqanjavri Complex occurs between theGearbeljavri Formation and the floor thrust ofthe Middle Allochthon. It is composed of meta­sediments and locally derived crystalline rocks.The metasediments appear to represent thesedimentary overburden of the GearbefjavriFormation, which has been detached alongdecollements. These rocks were described byKulling (1960a, fig. 5; 1964, fig. 23) as 'possib­ly parautochthonous, quartz-veined, fine-fol­ded phyllite rocks'. They contain variableamounts of detrital quartz and are regardedhere as metagreywackes. It is hard to esti­mate their primary thickness due to internalfolding and imbrication, but it does not seemto exceed 50 m. Where the uppermost partsof the succession are preserved under theirroof thrust, the phyllites become darker up­wards in the section and pass into graphiteschists which are at most 10 m thick.

No lithological counterparts to the phyllitesof the Hoiqaniavrt Complex exist in the autoch­thonous Tornetrask Formation or in the over­thrust Rautas Complex east of Abisko. Thegraphite schists can be correlated with thequite radioactive Alum Shale Formation (Serg­strorn & Gee 1985, p. 259) from the upperpart of the Dividal Group. Radioactivity meas­urements on the metasediments of the Hoigan­[avrl Complex (Author's unpubl. data) supportthis assumption.

NGU - BULL. 415. 1989

Horses of locally derived crystalline rocksare common in two major duplexes of theHoiqanjavri Complex. In the southern part oft.alrecorru a 'herd' of these horses is intercala­ted with strongly sheared graphite schists. Theduplex at the southern shore of Holqanjavri(locus typicus for the complex) consists main­ly of crystalline rocks. In both cases the crys­talline components are concentrated in theforeland-facing parts of the duplexes (Plate1). A single granitoid thrust slice appears southof Likta on top of the Gearbeljavrt Formation.The basal metasandstcne has not been ob­served in depositional contact upon the base­ment horses of the Hoiqaniavr! Complex.

Mineral assemblages in the phyllites of theHoiqanjavri Complex indicate conditions oflow-grade (biotite) metamorphism, similar tothose reported from the Gearbeljavrl Formati­on. Some of the crystalline rocks contain largeamounts of postkinematic stipnomelane.

Abisko Nappe ComplexThe Middle Allochthon is represented in thestudy area by variably mylonitized Precambriancrystalline rocks overlain by quartzo-feldspat­hic so-called 'hardschists' (Pettersen 1887,p. 420; regarded as allochthonous by Tome­bohm in 1901) of the Abisko Nappe Complex(ANC). In the actual study area the affiliationof these rocks to the ANC is based on thecontinuation of this unit along Hoiganvaggitowards the type locality at Abisko (cf. Fig. 2).

In contrast to the general westward thinningtrend of the allochthon in the Swedish Caledo­nides (Gee & Zachrisson 1979), rocks of theAbisko Nappe Complex thicken from north­east to southwest beneath the Upper Alloch­thon in the high mountains west of Abisko.Going southwestwards, successively lowerunits are exposed in the ANC, as the floorthrust of the Middle Allochthon cuts up sectionfrom southwest to northeast. Around Torne­hamn, at the southeastern corner of the Kuok­kel Window, the ANC is, at most, 40m thickand consists mainly of hardschist with some10 m of mylonitized crystalline rocks at thebase (Sax 1984). The metabasites were regar­ded by Kulling (1964) as derivatives of Precam­brian basic igneous rocks and are quite com­mon along the easternmost rim of the RSW(cf. Kulling 1964, Plate 1).

Rocks of the Abisko Nappe Complex are

NGU - BULL 415. 1989

represented in the klippen struc tures of Gatter­oaivi, l.airecorru and Stuor Gearb il. They alsocons titute the up to 300 m and almost verti­cal cliffs around Vassecohkka, Vuoiddasriiddaand Jorba Gearbil. There , two sheets of main­ly gran itoid mylonites occur, separated by alayer of the above-mentioned metabasites thatis up to 100 m thick . The lower gran ito id sheetshows rapid thickness variations and in placesdisappears , due to hanging-wall cut-offs alongthe floor thrust. The upper granitoid sheetthins out to the northeast and is replaced byincreasing amounts of hardschist. Local occur­rences of metabasites cap the upper granitoidsheet. Both granitoid sheets consis t of similarlithologies, and are only distinguishable bytheir field relationships. The rocks of both lay­ers are generally well foliated . However, len­ses of coarse-grained, almost undeformedgranite of the ANC with augen textures arecommon in the hinges of early-generationfolds. The largest occurrence of these relative­ly undeformed rocks caps Satnjarascorru inthe southeastern part of the map area (Plate1). The author regards these rocks as rem­nants of the granitic protolith preserved fromdeformation. However, Brown & Wells (1966)interpreted them as locally derived 'tectonicinclusions of basement gneiss ' surrounded byparautochthonous mica schist. Deformationtook place under conditions of low-grademetamorphism and results in rocks of themylonite series (protomylonite - mylonite - ultra­mylonite) of Sibson (1977),

The less deformed granitoids are L-tecto­nites with grain elongations paralle l to themain NW-SE trend of the main Caledonianthrust movement. In thin-section they are proto­mylonites showing brittle behaviour of the diffe­rent feldspars (mostly microcline , perthite , al­bite and oligoclase) and dynamic recrystallizati­on of quartz (et. White 1977, Simpson 1985).The mylonitization is accompanied by an over­all grains ize reduction (Fig. 3). With increasingdeformation the feldspar are replaced by seri­cite (Andreatta 1954, Williams & Dixon 1982),and quartz clasts show core and mantle struc­tures (White 1976). Deformation of biotite resul­ted in primary kink bands (Etheridge et al.1973). Alternations of protomylonites , myloni­tes and even intensely fol iated ultramylonitescommonly occur on the scale of thin-sections .Epidote is concentrated along C-planes andrecrystallized biotite flakes mark remainingS-planes (et, Berthe et al. 1979).

Caledonian struc tural evolution 93

Fig. 3. Protomylonitic texture in a granito id of the AbiskoNappe Complex normal to S- and C-p lanes. Partly dynami­cally recrystaluzec quartz is left white in the drawing. Feld­spars are stippled and sericite is indicated by hachures .Hematite and ilmenite are shown in black. Sense of shearis sinistral.

Centimetre- to metre-thick intercalations ofhardschists are concentrated in the upperparts of the granitoid layers. Although theor igin of these rocks still remains unclear (et,Holmquist 1903, Quensel 1916, Brouwer 1937,1940a & b), they are regarded here as pro­ducts ') f intense mylonitization of qua rtzo ­feldspathic sedimentary protoliths of unknownage. In thin-section , these well foliated rocksshow ultramylonitic or blastomylonitic textures(Higgins 1971), depending on their primaryquartz contents. Calcareous intercalations inthe hardschists occur along the north-westernedge of Stuor Gearb il and south of Vassijaurerailway station . A thin carbonate layer appearsunderneath the lower granitoid layer andabove the floor thrust in the southern part ofl.airecorru.

Bjbrklund (1985) distinguished up to sixthrust-sheets of Precambrian gneissic granito-

94 Gerhard Bax

ids in the Akka jaure Nappe Comp lex , eachoverla in by a thin sedimentary veneer. Thehardschists in the Middle Allochthon west ofAbisko may represent counterparts of lessdeformed sediments in the Akka jaure NappeComplex. Howeve r, intense shear ing and re­cumbent folding prevent the estab lishment ofa deta iled internal tecto nostratigrap hy in thegran itoid layers of the Abisko Nappe Comp lex(ANG).

The intercalated chlor ite-schist layer alsoconta ins less defo rmed inclusions of the sup­posed protolith. These relics appear in theform of mass ive metabasites that revea l nema­tob last ic textures with hornb lende, biot ite,epidote , tourmaline, minor quartz, oligoclase,calc ite and magnetite in thin-sect ion. Themore common chlor ite schists are characte­rized by lepidob last ic textures, with chlor iteand epidote replacing the pre-existing maficcons tituents of the suspected protolith . Thinintercalations of graphitic schists accompanythe chlor ite schists in varying amounts. Local­ly, marbles are also intercalated in the chlor iteschists east and west of Holqanjavri, east ofthe top of Stuor Gearb il, and along the south­western slope of Vassecohkka.

Lenses of metabasite which overlie the up­per gran ito id layer were previous ly interpretedas remnants of the Seve unit (fig. 1 in Bax1986), but are here cons idered as part of theAbisko Nappe Complex. This reinterpretationis based on the detec tion of similar lithologiesin the lower chlor ite schist layer, and on theinterfi ngering of these metabasites and accom­panying chlor ite schists with hardschists of theANC. Their occurrence in lenses beneath thefloor thrust of the Upper Allochthon can beexp lained as footwall cuto ffs.

The obv ious ly pre-Ca ledon ian, medium­grade assemb lage in the crystalline rocks ofthe ANC is overprinted by Caledon ian low­grade dynamic metamorphism. Minor occur­rences of garnet are restr icted to the nappeboundaries.

Seve-Koli Nappe ComplexLess attention has been paid to the rocks ofthe Upper Allochthon and only a brief summa ­ry is given here. According to Kulling (1960a,Fig. 5), the K61i succession of the Upper Alloch­thon in the present study area begins withan almost cont inuous layer of impure calcitemarble with intercalated, distorted siliceous

GU -BULL.41 5.1989

fragments . This definition is used here, as theseveral metre thick marble seals the footwallcutoffs under the roof thrust of the MiddleAllochthon. Graphitic schists dominate thelowermost 20 m of the Upper Allochthon inthe western part of the study area. The succes­sion continues with mica schists, containingdifferent amounts of quartz , calcite and gar­net. Sheets and slices of amph ibolite and kyani­te-bearing garbenschiefer occur in differentparts of the Upper Allochthon west of Abisko.The age of the K61i rocks in this part of themoun tain belt is unknown but Kulling (1972,p. 263) suggested a Middle Ordovician age,based on lithostratigraphic corre lations withfoss iliferous units in the centra l ScandinavianCaledonides. No higher tecto nic unit, cor rela­tab le with the Narvik Group (et. Fig. 2), seemsto be present in the mountain range west ofAbisko.

Structural developmentThe Caledon ian structura l history of the areainvestigated is illustrated schemat ically in Fig.4, where the creat ion of structures is relatedto 5 phases of deformation (0 , to 0 ,). Structu­ral alements such as thru sts (T), foliations orcleavages (S), folds (F) and lineations (L) areindicated with their respective deformationphase subscript. Despite the temporal sub­division, the differe nt deformat ion phases andrelated structures should be considered aselements of a more or less continuous proce ss.

The chronological classification of the diffe­rent structures is complicated by the fact thatstructures of more than two generations arerare ly present in cases of obvious overpr in­ting. Using only one of the features style, orien­tat ion or symmetry of folds as a cr iterion forestab lishing a structural chronology can leadto erroneous resu lts (et. Park 1969). Structu­ral terms are used here in accord with Boyer& Elliott (1982) and Butler (1982).

From 0 , onward , movements in the RS­Comp lex played an important role for the struc­tural development of the overly ing allocht honby initiating and mod ifying its structures duringthe emplacement (Bax 1984, 1986). Correspon­ding tecton ic featu res can be dated by theinteraction of related structures in the RSC,even when defo rmat ion changes style acrossnappe boundaries (Bax 1987). It is assumedthat pre-ex isting fabrics in the RSW were re­activated in Caledonian times.

NGU · BULL. 415. 1989 Caledon ian structural evolution 95

Seve-K ol i

Ba lt ic Shie l d

. .. . .. . .'"'- - - .... + ..

U:::;';"~~1ji'/,\ Rombok- Sjongel i

?~~~?~~~IJiV1ll~~~ Com pl ex

- ..,.. ..- -+- ............... -+ ...t ime ---+

Fig. 4. S ructura l develop ment in the study area during 5 phases of deformation . looking north. The develop ment of theoldest structures is show n at the left marg in of the figure and the structural evolution can be followed in time from left to right(0 , to 0 ,) in the involved tectonost rat iqrapnic units. Illustrations of overprint ing features are kept to a minimum in orderto preserve clarity in the diagram. The Gearbehavrl Form ation is stipp led and crystalline rocks of the Hoiqan javn Complexare marked by a pattern of dots and hachures. Quartz veins are shown in black .

DJBecause D, with its penetrative foliation (S,)is ubiquitous in both the Upper and the Midd­le Allochthons and in parts of the underly ingunits , most of the older structura l elementsare either extingu ished or their genetic rela­tionships are diff icult to reconstruc t becauseof reorientat ion (Lindstrorn 1961). Traces ofdeformation events older than D" here summa­rized under D" are preserved in the highermetamorphic part s of the Kbli as inclusion tra­ils (S,) in porphyroblasts (mostly garnet) oras root less intrafolial fo lds, present in singleoutcrops, which are scattered throug hout theent ire allochthonou s sequence.

The floor thrust (T,) of the Upper Allochthonwas developed dur ing D1 and underwent defor­mation during all later phases . During thisearly phase the medium-grade Kbli was super­posed upon the low-grade Middle Allochton.Thrus ting postdates the peak of Caledon ianmetamorphism. Hangingwall and footwall cut ­offs along T, represent , in some cases , rema­ins of bedd ing or foliations.

0 ]D, represents the main phase of overthrus­ting, when the Middle Allochthon started tomigra te along T, on top of rocks which wereto become the RSC and its sed imentary co­ver . During this phase the ubiquitous foliationS, was formed during simple shearing of allinvolved tectonic units . A NW-SE lineation (L,on Fig. 4), present with in S" is parallel to thesupposed SE transport direct ion (Lindstrbm1958). L, appears as a preferred orientationof porp hyrob lasts and clasts and is interpreted(ct. Kvale 1953) as an a-lineat ion in the senseof Sander (1948). Pebble elongat ions in theGearbe ljavri Formation parallel to L, test ify tothe influence of the overiding nappe pile du­ring D,.

The Abisko Nappe Complex is characte rizedby the repetition of a success ion consist ingof basa l granito id mylonite, intercalated andover lain by hardschists and capped by var iab­ly mylonitized metabasites. Isoclinal folding canbe exc luded as an exp lanation for the repeti­tion, because the tectonic layering is rhytm ic,

96 Gerhard Bax

Fig. 5. D, structures in the Rombak-Sjangeli Complex(RSC): a) Westward-dipping low-angle ductile shear-zonesin Hoiqanvaqqi, View loo ing SSE. Base of te A C atStuor Gearbil marked by black line. ote the high-anglefault in the centre of the picture. Width of section at thehorizon is 2 km. b) D, shear-zone with extensional crenulati­on cleavage. View looking SSE. Hammer (55 cm) as scale.Sinistral sense of shear.

not symmetric. The mylonitic characte r of thepenetra tive S, foliation makes it almost impos­sible to localize the critical shear hor izons.Even if the super pos ition of the nardschistson the granitoids is of a primary depos itionalnature, this interpretation implies at least im­portant movement beneath every gran itoid sli­ce where both lithologies alternate in verticalsect ion. This alternat ion of basement and co­ver rocks is similar to the internal structureof the Akkajaure Nappe Complex (Bjbrklund1985) and the Middle Allochthon in the north­ce ntral Sca ndinavian Caledo nides (Greiling1985).

Uncertainty concerning the origin of themetabasites further complicates a palinspasticreconstruction. Because they are partly under-

GU · BULL ' 5.1989

lain by hardschists. at least two continuousthrusts are required beneath the metabas itesin the ANC if they are to be regarded as over­thrusted Precambrian basemen (et. Kulling1964). An alterna ive interpreta ioon of hemetabasites as Caledonian magma ic roe swould imply hat he base 0 the upper grani 0 ­

id layer acted as the major internal hrus .resulting in a doubling of he sequence.

In either case . he internal hrus ing andimbrication uccurred prior to the final establish­ment of the penetrative S: foliation. which hasoverprinted any pre-e xisting discordances. Anyinternal thrusts are. together with S:. post­dated by the floor thru st (T,) of the A C.which on a local scale cuts up section to theeast. The strike of hanging-wall cut-e fts alongT, to the northeast and southwest (Plate 1)indicates local-scale. gentle folding aboutNW-SE-trending axes to or during overthrus­ting. Later reactivation of T: (possibly dur ingD,) under an elast ico-frictional regime (Sibson1977) is documented by a thin veneer of pseu­dotachylite at the base of the Abisko appeComplex near Tornehamn (Bax 1984).

DJDuring DJ the RSC started 0 play an act iverole in the structural evolution 0 he s udyarea. Overthrusting along T: continued. andmovements in the RSC began to both ini iateand influence structures in the overr iding nap­pe pile.

The movement toward he SE encroachedon the crysta lline rocks of the RSW and theirsedimentary cover, where imbrication prod u­ced the horses of the Hotqarqavr: Complexuntil they finally became arranged into severallens-shaped duplexes . Accumulation of base­ment-derived horses in the foreland-facing

Fig. 6. a) Leading edge of a RSW hor se underneath undis­turbed Gearbetiavn Formation southwest of Likta. View loo­king . b) Gearbe jjavn Formation overthr usted by crystalli­ne rocks of the RSC south 0 Likta. View looking W. c)Imbrication in the Koll at Skapax te looking . d) S.w crenu­Jation cleavage at v ass tiau re stat ion looking to the . Out ­crop IS about 2 m In diameter. e) Pseudotachylite vein romD. fault bounding the Stuor Gearbil klippe to the E. Forlocation see Fig. 6a. f) D. igh-angle reverse fault bound ingthe t.airecor ru klippe to t e W. View looking W. g) D.high-angle normal fault bounding he t.airecorru lippe tothe E. View looking . Width of colon the horizon about 60 m.

NGU - BULL. 415. 1989 Caledonian structural evolution 97

98 Gerhero Bax

parts of these duplexes suggests a forward ­propa gat ing imbrication, wh ich involved suc­cessively lower s ratigraphical levels duringcoeval updoming of he RSC. Branch- and ip­line configur ations (Pia e 1). toqe her wi h in er­nal or ientations of mylonitic to liations (ct. syn­opt ic diagram south of Hoiqanjavr i on Pia e1) and a-lineations. indicate SE-directed thrus­ing during formation and emplacement of he

Hoiqanjavri Complex.

Segmentation of the RSC con inued undercon ditions causing ductile behaviour in quar ­zo-Ieldspatnic rock s (quasi-plastic reg ime ofSibson 1977). Parts of the Gearbehavrl Formati­on involved in thrust ing in the uppermo st partsof the RSC demonst rate the comp ressionalnature of the deformat ion (Fig. 6b). The or ienta ­tion of NW-SE-elongated pebb les in this forma­tion beneath RSC horses indicate NW-SE­trend ing dislocations, similar to those in theHoiqanjavr i Complex.

Duct ile shear-zones or duct ile deformationzones (DDZ: Mitra 1978) generally follow litho­logical bound aries, or utilize already foliatedrocks . About 100 m below T, on the northernslope of Stuor Gearbil, one flat-lying DDZ (Fig.5b) exhibits SE-dipp ing extensional crenulationcleavages (ecc: Platl 1979, 1984). In the vicini­ty of the leading edges of the resulting horsesmost of the observed DDZs swing into paralle­lism with the roof thrust T, or the depos itionalcontact of he Gearbeliavr! Formation (Fig. 6a).

Geometrical considerations (see next sec­tion) require a master sole thrust (T,) at thebase of the RSC. However . unpublished inves­tigat ions by the prese nt author and publishedgeological maps fro m different parts of theRSW have not revealed any possible outcropsof this assumed sole thrust. The exposedbase of the Rautas Complex may be regardedas the emergent eastward cont inuat ion of T,.

Ourinq D,. parts o f th e napp e comple xes

were deformed round NE-SW-str iking foldaxes . These folds are termed F'D' where bstands for fold axis para llel to the tectonic Sdirection. F'D is always ove rturned to the SEand is usually accompanied by an axial planecleavage (S,).

The window through the Upper Allochthoneast of Gors aj6ke ln exhibits intensely imbrica­ted granito id mylon ites and hardschists of theAbisko Nappe Complex. Previous ly mylon itizedquartzo-te ldspatnic rocks are here truncatedby W-dipp ing shear -zones . Phyllon ites

~JG U · BULL. 5 989

around the tip-lines of quartzo-feldsp athichorses suffered complex folding, wi h axesboth parallel (see below) and norm al 0 heinferred trend of disloca ion. The sou heas ernpar of he s udy area (Pia e 1) is dom ina edby imbrica ions (Fig. 6c) wh ich involve bo h he

iddle and the Upper AIIoch non . I remainsunclear if (and how ) hese imbrica ions arerelated 0 hose in he RSC (c . fig. 2 In Sax1986).

Ano her ype of fold s ruc ure ha is, aleast partly , related 0 0 , is rep rese nted bythe widespread and approximate ly W-SE­striking transverse folds or cross -folds (et.Rast & Pia 1957). These cross- folds are er­med F,a' Lindstr6m (1961, p. 152) foun d thesetransverse folds to be overturned equally 0

the SW and the E and he (1961, fig. 5) inter­preted the axial trends of minor folds as aresu lt of reorientation of earlier folds towardspara llelism with the stretch ing direct ion. Thisexp lanation is widely accep ted (e.g. Sanderson1973, Escher & Watterson 1974) and appearsto be valid for most tight to isoclinal, minorF,a folds observed in the area.

An explanation for he ransverse rend oflarge-scale. open cross-fo lds, wi h wave­leng ths of severa l metres 0 kilome res. byreor ienta ion is more problema ic. In he idd­le Allochthon many of these struc ures areaccompanied by W-SE- rending high-anglefaults in he directly underlying RSC. The resul­ing hor s -and-graben s ruc ures ac ed as side-

wall or la eral ramps (Sax 1984) during finalstages of overthrusting. This mechanism wasclear ly act ive during transta ion in he Li aarea . where par s of he overriding roe smoved obliquely on a s aircase rajec orywhile undergoing transverse folding and im­bricat ion. The enveloping surfaces (FaI ensp ie­gel) of these cross-folds are parallel to heSW-inclined slope of the 'basement surface '.Ve rgenc ies he re te nd to the southwes . A ro un dTorn ehamn (Sax 1984). where this 'basemensurface' is inclined to the north east, envelo­ping sur faces dip and vergencies face likewiseto the northeast.

Lineat ions on the NW-SE faults indicate astrike-slip com ponent of movement . wh ichexceeds the vert ical component. The str ikeof this group of late 0 , faults varies cons iderab­ly. They commonly intersect hor ses of the RSC(south of Vuoiddasriidda). but they are themsel­ves cut off by approximately N-S-trendinghigh-angle D. faults in Lairevaqqi. In places

NGU-BULL.415,1989

they are overlain by undisturbed duplexes ofthe Holqanjavri Complex.

0 4The phase D. is characterized by large-scalefaulting of the RSC (cf. Fig. 5a) accompaniedby folding of the overlying units. One of theresulting (N-S to NNE-SSW-striking) structureshas been mentioned above. Others are, e.g.,the klippen (cf. plate 1.) of Gatteroavi, t.alrecor­ru and Stuor Gearbil. These synformal outliersof the Abisko Nappe Complex owe their preser­vation to downfaulting during D. (cf. Figs 6f &6g). Goldschmidt (1912) used the term Fal­tungsgraben for similar structures, createdwithout ruptural deformation.

Displacement along almost vertical, normaland reverse shear-zones took place initiallyunder a QP-regime, resulting in rocks of themylonite series (Sibson 1977). During the finalstages of continued movement, or perhapscaused by reactivation, dislocation resulted inthe local formation of pseudotachylite (Fig,6e). Sibson (1975) described pseudotachyliteas a product of rapid (seismic) transient sli­ding on extremely brittle faults. Frictional mel­ting at shallow depth (Sibson 1977:<10-15 km)is widely accepted (e.g. Alien 1980, Maddock1983; but cf. Wenk 1978) as the generatingmechanism.

The N-S to NNE-SSW-striking faults are lo­cally accompanied by sulphide-bearing quartzveins. Sheet-like veins are partly involved inlate 0, folding (cf. Johansson 1980). Kappa­configurations (Tischer 1962) in synoptic S­diagrams for the adioining rocks of the Hoigan­[avri Complex on Stuor Gearbil (see Plate 1)testify to the influence of the largest observedvein (about 800 m long and up to 60 m wide)during D•.

High-angle D. faults usually vary in dip bet­ween 90-60° both to the east and to the west.Eastward-inclined reverse faulting producedpop-up back-thrusts in the telescoped RSC(but cf. Andresen & Cashman 1984, and Cash­man this volume). Holmquist described (1903,p. 70) and illustrated (op.cit., figs. 11-13)some of these structures.

D.-related folding (F,) in the graben-boundsynforms distinctly deforms all pre-existingstructures. The originally oblique trending(NW-SE), ubiquitous L2 lineation can be usedto determine pre-D, tectonosnatlqrapnic wayupeven in overturned F, structures. Vergencies

Caledonian structural evolution 99

of F. usually tend to the east, except in areasinfluenced by (pop-up) back-thrusting. Accom­panying axial-plane cleavages (S4) dip to thewest in the western parts of the synforms,but they adopt the orientation of the faultswhich bound the synforms to the east. Closeto these faults, S. can become the dominantfoliation instead of S2' westward-dipping S. iscalled S'w and the S, related to back thrus­ting, S'E (Fig. 6d). Intersection of S,w and S'Eresults in N-S-trending S. lineations, which, inthe phyllitic part of the Holqaniavri Complex,can give rise to pencil cleavage. D.-relatedstructures die out in successively higher le­vels of the allochthonous sequence.

Large-scale, E-W-trending, tight, uprightfolds occur in the Upper Allochthon east ofVuoiddasriidda. Minor, isolated fold structuresare present in different parts of the study area.It remains unclear whether these folds are theresult of a general, post-D. N-S compression,or if they represent D. cross-folds.

o,Generally eastward-dipping, closely spaced(some ten metres) normal faults in the Kolieast of the present study area are related toDj' SE-plunging strlations on the slickensidesindicate the direction of extensional faulting,which was possibly driven by the force ofgravity.

Arguments for an allochthonousposition of the Rombak-SiangeliComplexAs mentioned above, almost all previous tecto­nostratigraphic compilations presume an au­tochthonous position for the rocks of theRSW. Only Asklund (1946, p. 245) and Vogt(1941) discussed an allochthonous position forthe RSW, but this assumption was censuredby Kulling (1950b, p. 482). Gee et al. (1985)regarded the RSW as parautochthonous.

Clarification of the tectonostratigraphic posi­tion of the RSW and related tectonic windows(see Fig. 1) is a fundamental prerequisite topalinspastic reconstructions and nappe corre­lations in the Caledonides. It would also helpto identify migration paths and possible sour­ces for the ore-bearing fluids that led to theformation of hydrothermal deposits in andaround the RSW.

100 Gerhard Bax

The interpretation of the RSW as an non­transported westerly continuation of the BalticShield is commonly (e.g. Kulling 1964) basedon lithostratigraphic correlation of its sedimen­tary cover (Gearbeliavri Formation) with theautochthonous Dividal Group (Hyolithusserieof Kulling 1964) at the eastern margin of theCaledonides. This argument is not convincing,because the undoubtedly allochthonous Rau­tas Complex (Lower Allochthon) contains ex­tensive and far better preserved equivalentsof the Dividal Group. Lithostratigraphic correla­tion between the two latter units is possibledown to the rank of members or even beds(cf. Dworatzek 1976). Up to 80 m-thick alterna­tions of sand and siltstones, characterizingboth the Tornetrask Formation (Thelander1982) of the Dividal Group and the main partsof the sedimentary rocks of the Rautas Com­plex, are reduced in the west to the upper fewmetres of the Gearbeliavri Formation. On theother hand, no counterparts to the metagrey­wackes of the Hoiqanjavri Complex exist inthe Dividal Group below the Alum Shale Forma­tion (Thelander 1982).

Another problem arising with an autochtho­nous interpretation of the RSW concerns thederivation of the Rautas Complex. Kulling(1964, p. 67) supposed a possible root zonefor the Rautas Complex in the RSW aroundthe border between Norway and Sweden. Lit­hostratigraphic comparisons between the Rau­tas sediments with both the Gearbel[avr! For­mation and the Holqanjavn Complex (see abo­ve), however, exclude this possibility. Additio­nally, a higher grade of metamorphism is tobe expected for a Rautas Complex derivedfrom, or transported over the RSW duringdownward-propagating thrusting of the Cale­donian nappe pile. The partly imbricated, upto 200 m-thick succession of Rautas sedi­ments is considered to be generally anchimeta­morphic (Lindstrorn et al. 1985) in contrast tothe low-grade ('biotite grade') Gearbe'[avriFormation and metasediments of the Holqan­[avrt Complex. A root zone for the RautasComplex between hypothetically autochtho­nous windows and the westernmost occurren­ces of autochthonous Dividal Group (eg. betwe­en Tornehamn and Stordalen on Fig. 2) canbe excluded, because the telescoped Rautassediments (cf. figs. 40 & 41 in Kulling 1964),exposed over 25 km along the Tornetrasksection (cf. fig. 2 in Llndstrorn et al. 1985),require a basin larger than the available 19

NGU-BULL415.1989

km-wide zone between Tornehamn and Stor­dalen for their deposition.

The fact that the roof thrusts of the RS­and the Rautas Complexes suffered the samestyle of folding supports a correlation of bothunits. The sole thrust of the nappe pile in theeastern part of the Tornetrask section (probab­ly correlatable with T,) cuts up section to theeast. It remains non-folded below both theRautas Complex and the Abisko Nappe Comp­lex (et. t.indstrorn et al. 1985).

In any case, the Rautas Complex eitherjoins or underlies the RS-Complex (RCS) westof Abisko. Therefore, the RSC is here includedin the Lower Allochthon. SE-directed displace­ment of the Rautas Complex at Stordalen isestimated to exceed the 20 km of its uninter­rupted occurrence on top of the Dividal Groupalong the Tornetrask section (et. fig. 2 in Lind­strorn et al. 1985). Any detected (subsurface)occurrence of Dividal Group (Fig. 4) under­neath the Lower Allochthon west of Stordalenwould increase the minimum estimate of dis­placement for this Lower Allochthon. Internalshortening due to imbrication in the RSC addi­tionally demands westward increasing displa­cement along T,.

Summary and conclusionsAn area of about 200 km' around the northeas­tern corner of the RSW has been mapped ata scale of 1:10,000 in order to clarify the rela­tionships between the structural basement ofthe RSW and its Caledonian nappe cover.Four tectonostratigraphic units, correspondingmainly to Kulling's (1964) classification in theupper parts of the section, were identified.(iv) The medium-grade metamorphosed Koli

of the Seve-Koli Nappe Complex.(iii) The composite Abisko Nappe Complex

overprinted by low-grade dynamometa­morphism.

(ii) The locally derived Holqanlavri Complex,exposed in isolated duplexes.

(i) The imbricated RS-Complex in stratigrap­hie association with the low-grade Vendi­an to Cambrian Gearbe'[avri Formation.

Piggy-back thrusting towards the forelandis regarded as the mechanism for the pilingup of this nappe sequence. Scandian thrusting(T,) at the base of the medium-grade UpperAllochthon (Seve-Koli Complex) postdates the

NGU - BULL.415.19 89

Sev e -KC:ili Noppe Comp lexAb isko NoppeComplexHoigo njOvr i ComplexGear be l jOvr i Forma t ion

~oOu';'t~~~;;~;Rtel i Complex

Caledonian structural evolution 101

Fig. 7. Block diagram showing the geology of the study area (cl. Plate 1) at the surface and in cross-sections. The vertical scaleis 2.5 times that of the horizontal.

peak of Caledon ian metamorphism. Shearingand overthrusting (D,) of the Middle Allochthon(Abisko Nappe Complex) occurred under con­ditions of low-grade metamorphism. Involve­ment (D)) of the RSC (metamorphised at medi­um-grade during the Svecokarelian orogeny),the Gearbe fjavn Formation and the Hoigan­[avri Complex in the Caledon ian orogeny tookplace under similar cond itions . High-angle,N-S-striking fault ing in the RSC and F, foldingin overlying units were accompanied by em­placement of sulphide-bearing quartz veins .Upl ift during D, resu lted finally in rock pro­ducts (pseudotachylites, cataclasites and brec­etas) of the elastico-frictional regime (Sibson1977).

The exposed nappe boundaries (T, and T,)suffered folding due to deformat ion in theunderlying units . (Re)activation of pre-Caledo­nian structures in the RSC during D) resultedin frontal, lateral or oblique ramps formingbeneath coeval southeastward thrust ing alongT,. This footwall (ramp and flat) topography

gave rise locally to F)a cross-folds in the over­riding allochthon. The creation of many morecross-fo lds in the orogen (cf. Lindstr6m 1961)is probably exp licab le by simu ltaneous deve ­lopment of 'basement' cu lminat ions (but et.Steltenpohl & Bartley 1988).

The present structure and pos ition of theRSW 'basement' culm inat ion (and possiblythose of related culminat ions) is exp licable bystra in harden inq, wh ile passing the ductile!brittle trans ition zone during final upthrustingalong T).

AcknowledgementsThis paper summarizes results of my doctoral thesis at thePhilipps-Universitat in Marburg (West Germany) under super­vision of M. unostrom and H. Zankl. To both of them gomy sincere thanks for their encouragement during all pha­ses of my geolog ical education at the Philipps-Universitat.Pans of the research were performed at Stockholms Univer­sitet under the stimulating and gratefully acknowledgedsuperv ision of M. Lindst rorn. I wou ld also like to expressmy deep apprec iation to the numerous members of bothinstitutes for their support to the present study in the form

102 Gerhard Bax

of advice and practical help. The warm hospitality of thestaff of the Abisko Scientific Research Station is great lyacknowledged. D.G. Gee and E. Zachrisson are thankedfor stimulat ing discussions in the field. The English vers ionof the manuscript benefited from linguistic help by A.R.Cruden and K.H. Sulta n. The referees P.H. Casman, S.W.Sawyer and one anonymous referee are thanked for theircomments which greatly improved the manuscr ipt . Partsof the study were financed by a grant from the Philipps­u mversltat and fieldwork was supported by DAAD, KVAand SGU. Teddy M. Blomberg prepaired the photographs.

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