The structure and regional setting of the SkeiGroup, Leka, north-central NorwayBRIAN A. STU RT & DONALD M. RAMSAY

Sturt , BA & Ramsay, D.M. 1994: The structure and regional sett ing of the Skei Group, Leka, north-centra l Nor­way . Nor. geol . unders . Bull. 426, 31-46.

The unconformity between the metasedimentary Skei Group and subjacent ophio lite on Leka has importa nt impli­cations for the tectonostratigraphy of the region . As a distinctive marker horizon, it permits correlations within andbetween nappes and provides a datum level on which to establ ish lithostra tigraphi es in the several units of theHelgeland Nappe Comp lex. The unconformi ty records a major change in the pattern of events and partitions thetectonothermal history into two polyphase orogenies, whose individual effec ts can be recogn ised in many nappesalong the whole length of the belt. Deformations predating the unconformity relate to ocean-floor tectonics and asubsequent phase of regional tecto nothermal development.The Skei Group records two fold phases accompanied by low-grade regional metamorphism. The distinctive litho­stratigraphy facilitates de lineation of mac roscopic folds, whose geometry indicates that the whole outcrop lies wit­hin the inverted limb of the major 0 1 Leknes Synclin e. This, in turn, controlled the attitude of mesoscopic 0 2 foldswhich are themselves parasitic to the large-scale Austra Ant iform, one of a regional system of near-symmetricallate folds which affect both the Helgeland Nappe Complex and its substrate.The litho- and tectonostratigraphic relationships estab lished on Leka and extended to the adjace nt mainland reite­rate a theme currently being espoused for the evolution of the whole Scandian belt, namely dissection and disrup­tion by Scandian thrusts of a craton ic substrate, already structured into a Finnmarkian and in part Late Ordoviciannappe pile.

Brian A. Sturt, Norges geologiske unoerseketse, Post Box 3006 - Lade, 7002 Trondheim, Norway.Donald M. Ramsay, Department of Geology, University of Glasgow, Glasgow G12 80 0 , Scotland, UK.

IntroductionThe bedrock geology of the island of Lekacompr ises the Leka Ophiolite Complex(La C) and the unconformably overlying ,probably Lower to Middle Ordovician meta­sediments of the Skei Group (Sturt et al.1985). These rocks were assigned to theUpper Allochthon by Roberts & Gee (1985),though they are most likely to be part of theHelgeland Nappe Complex (HNC) of theUppermost Allochthon (Fumes et al. 1988).This latter interpretation is based on a cor­relation of the ophiolite fragments of theregion (Sturt 1984, Sturt et al. 1984). Manydetails of the LaC are given in Fumes et al.(1988) and the reader is referred to this asan essential source reference .

The metased imentary rocks of the SkeiGroup have been described in some detailby Sturt et al. (1985), who confirmed thegeneral proposition of Birkeland (1958) thatthey were: "a younger sequence overlyingan older complex of basic and ultramaficrocks", the latter now known as the LaC.The break between the two was, however,not noted by Prestvik (1974, 1980) or byPrestvik & Roaldseth (1978). The latter aut-

hors, however, identified the igneous suiteas the LaC. The sedimentary rocks of theSkei Group were reassessed by Prestvik(1980) as the cap-rock to the LaC and hen­ce essent ially eugeocl inal in characte r.Sturt et al. (1985), on the other hand, clearlydemonstrated that the sediments of the SkeiGroup had been deposited unconformablyon a deeply eroded substrate of the LOC.The lithostrat igraphy (Table 1) and palaeo­geographic interpretation of the Skei Groupwere further refined including the recogniti­on of the continental character of the uncon­formity , with a variably preserved regolith inthe upper levels of the substrate . Quartzkeratophyres intruding sheeted maficdykes , immediately below the unconformity,have been dated (U-Pb zircon) at 497±2 Ma(Dunning & Pedersen 1988) and provide amaximum age constra int for the base of theSkei Group.

A number of smaller ophiolite fragments,with a similar tectonostratigraphic setting ,have been identified in the HNC (Sturt1984, Sturt et al. 1984, Bang 1985, Heldal1987, Husmo & Nordgulen 1988, Thorsnes& Loseth 1991). Nordgulen & Schouenborg

32 Brian A. Sturt & Donald M. Ramsay NGU • BULL 426. 1994

SUB- HNCROCKS

MIGMATlTICGNEISS.MARBLE. SC HIST

PORPHYRI( GRANITE AND GRANODIORITE

MONZONITE, MON ZOOIORITEGRMODIORITE, GRANITE

TONAlITE , GR ANOOIORITEGABBRO, DIORITEv V

V / l

L / ,j

0 0'

10 20 KM

THRUS TS

.L.1,__A-l-A. HNC, INTERNAL

A_ • HNC, SOUTHWEST

~ HNC, EAST

RNC

fv)<.

N SKALV£R

'';( 01

~\00

~1(.VEGA l!t)q) ;I:

+ + G + I )\

'<1+ \s\ ' .

+

::\

:1 /.

KO N'j SM~E N

ROOINGSFJALLUNAPPE COMPLEX

4.0 HORTAVi£R

Fig. 1.Simp lified tecton ostrat igraph ic map of northern Vestranden. (Prepared by T Thorsnes.NG U).

(1990) demonstrated that the granitic Heil­hornet Pluton cuts both such ophiolite frag­ments and their deformed/metamorphosed

cover sequences (Fig. 1). They provided aU/Pb zircon age of 444±11 for the Heilhor­net Pluton, thus giving an upper age con-

NGU - BULL 426, 1994 Brian A. Sturt & Donald M. Ramsay 33

PRESTVIK ( 1974 ) STU RT et. al. (198 5 )

1"00' schist s sl ltstoneu. conglomerate

SKEI FORM ATION volcanic sequence . bl ack s late (sands tone. COnglOmerate ) }greywacke & black schis t sandstone (. cong lome rate ) HAVNA FM .marble marbl e } SKEI GR OUPconglomera te conglomerate STEGAFJ ELL FM.

STOR0YA FORMATION ophiolite complex ophiol ite complex LO C

Table 1. Lithostratigraphy of the Skei Group (Sturt et al. 1985) and comparisom with the nomenclature of Prestvik (1974).

straint for both the deposition and the initialdeformation of the Skei Group; assumingthat the correlation of the cover sequencesis viable .

On the wider scale, unconformable coversequences, usually of continental derivationin their lower part , overlying ophiolite com­plexes are widespread in the Scandin avian

Caledon ides, and these occurrences aresummarised in a recent review paper (Sturt& Roberts 1991), The present paper is con­cerned particularly with details of the struc­tural geology of the Skei Group , the geome­trical relationships between the Skei Groupand the LOC and with evidence for pre­unconformity deformation and metamor­phism of the LOC.

THE SKEI GROUP

-«: 8edding LithologicaV-- Slratigraphical

.-/ SChislos ity boundary

..-L Younging - - Faun

IIII

\ '\" "-\ \ ,~... ~ -- -'\,\ \'..., ....::.~"""""" ""

"",,'~~~

"""' -~---

GANGST0LEN

HAVNA FORMATION

STEGAFJELL FORMATION

Intersection uneaton(0 1,02)

Fold axis (0 1, 02)

Black shale with minorsandstone andconglomerate

Sandstone (ooned) with lenses ot coarseconglomerate ltriangels), beddedconglomerate filled circles), and limestone

Braided stream conglomerate(circles) wilh sandstone (ootteo)

r.""9, "Ld

Fig. 2. Geological map of the Skei Group of northeast Leka. The cross-sections A-A', B-B' and C-C' are shown in Fig. 7.

--v-- --v-- 01 ant ilarm (a x ial

----l<-- --;<-- 01 sy n Ia r m

~~ 02 ant ilarm

==§= ==§= 02 syntorrn

34 Brian A. Stun & Donatd M. Ramsay

a 0 .5

-- - -- ...................

\;p.

t km

GU • BULL 426. 1994

Fig . 3 . Structural map of the Skei Group (dotted ornament). Sma ll stereog rams present att itud es of poles to bedding and S l (dots),and of D2 laid axes (c rosses), for four sub-areas.

NGU • BULL 426. 1994 Brian A. Sturt & Donald M. Rar.nsf/.Y 35

Structure of the Skei Group

Fig. 4. Structural map of the V<\tvik-Stegafjell sub-area Stereo­gram presents structura l data : dots - poles to bedding and S1;crosses - poles to S2; open circles - 01 fold axes; triangles ­02 fold axes .

....... \ .~o '". :· ~oo.... .

-,

",-- =250 mo

'.

The structural history of Leka records acomplex sequence of events spread over atleast two orogenies and extending back, inthe case of the ophiolite, to pre-orogenicdeformation associated with the formationof the complex. The unconformity at thebase of the Skei Group denotes a signifi­cant time interval between two orogenicsequences during which uplift and erosionunroofed the metamorphic Solsemey For­mation (Fumes et al. 1988) and the LOC. Inso doing, it distinguished pre- and post­unconformity tectonism. The dominantCaledonian fabric of the island is post-SkeiGroup in age. Polyphasal deformation ingreenschist facies created both large- andsmall-scale folding and associated pheno­mena in the Skei Group. This deformation islocally penetrative , although sizeable areasmay be little affected by structures of anyone tectonic phase.

First phase of deformation

Along its landward margin the Skei Groupdips steeply westwards , beneath the rocksof the LOC, inverted in the limb of a large01 fold (Leknes Syncline) with NNE-SSWaxial trend (Figs 2, 3 & 9). Macroscopic z­folds with amplitudes up to 1 km in theGanqstelen-Hylla, Vatvik and Stegafjell are­as are parasitic structures to the main fold(Figs. 3 & 7). Of these the Ganqstelen-Hyl­la fold is a recumbent , westerly-closingsyncline tightly folded by an upright 02 syn­form (Fig. 7). The form of the 01 syncline isdelineated by the one-sided core of the SkeiGroup, sandwiched between upper andlower developments of ophiolite (Figs. 3 &7), and confirmed by the unconformity andits attendent development of metasol. Inaddition to this, an abundance of sedimen­tary structures young away from the LOCtowards the core of the syncline (Fig. 2).High 01 strains in the isoclinal core arereflected in a strong foliation within themetasol and a strong planar alignment offlattened pebbles in conglomerates of theSkei Group.

36 Brian A. Sturt & Donald M. Ramsay NGU - BULL 426. 1994

At Vatvik and south Stegafjell (Fig. 3) theform of the mesoscopic 0 1 folds is obvious ,delineated by the outcrop pattern of a well­marked stratigraphy and confirmed by sedi­mentary structures and the presence of themetasol beneath the unconformity (Fig. 4).

On Leka, metamorphic reconstitution in theSkei Group is restricted and crystallinity is

N

low. Greenschist facies is indicated by bioti­te and sporadic garnets in some pelitic hori­zons and by chloritoid in the metasol. Thiscontrasts with the situation on the mainlandto the northeast at levels farther removedfrom the base of the cover sequence wherethe Helgeland Nappes and the complexbeneath generally display a metamorphicmaximum in amphibolite facies.

N

• '.a:.. : .. ..

:. e. • e. • .-

..

\...

· . . .a.•• • .a I •

i ...· ...: ... .· .

a

N

b

N

..-,.. '\... .. \... '\

\. \\\_---·r,III •III

/ .. . /

....

, . .• .. ?~.. .. ~~. .... ~

------x

. . \. ..• - .....~. \:x •

x. J( \--... X,(- " • • t. .x~)( •• •• xx ........ ~• • rt:. r- }-........ e. .

e. __ A..e L • .i~ _••...- x ••• .~. __ •

........... -; e. .Jf' x )( " ,/ • a lii: .~ ........ /' )()()( ~ 'K

/ \ . . ,/ ~ \ ,

/ / . . .... -\ -,

x \ : ": ~ "",. \ e.·· .,., . x

)( I )( •

..or: .lit x

a xl · ·

c d

Fig. 5. Synoptic plot of structural data from the Skei Group. (a) D1 fold axes; (b) D2 fold axes; (c) poles to bedding (dots) andSl (crosses); (d) poles to S2.

NGU • BULL 426, 1994

N

x"4

aN

bFig. 6. (a) 0 1 fold axes from 4 sub-areas, VAtvik·Stegafjell ­dots; central sector - crosses ; Havna - open circles; Ganqsto ­len • triangles. Lines 1·4 are the traces of axial spread resul­ting from 02 refolding, (b) Theoretical pattern of reorientatedfold axes resulting from refolding initially Class 1b type butmodified by later homogeneous strain to Class la (K=0.7).The initial axes trended N-S (fold A) and 030-2 100 (fold B) andboth were horizon tal. The axis of refolding was 236/60°, thesame as the modal 0 2 attitude in NE Leka.

Minor structuresThe small-scale structures of the 01 defor­mation phase, e.g. minor folds, mineral andpebble lineation, foliation and boudinage,are not developed uniformly throughout the

Brian A. Sturt & Donald M. Ramsay 37

Skei Group. For example, in the thick con­glomerates, breccias and some sandstonesthere is locally no trace of 0 1 deformation,so that 02 features are the first record ofstrain. 0 1 structures are most prominentlydeveloped in the fine-grained members ofthe Havna Formation and in the weatheredfacies of the LOC. The only conglomeratesto be significantly affected during this phasewere those already mentioned, in the coreof the Ganqstelen-Hylla sypcline.

Minor folds have a flattened concentric style(Class 1c, Ramsay 1967) with axial plungesvarying between SW and SE (Figs. 4 & Sa).While some of this variation is possibly afunction of primary noncylindrism it is alsocompatible with reorientation consequentupon 02 refolding (Fig, 6). Fig. 6a differen­tiates these fold axes from the four sub-are­as featured in Fig. 3. In each, the axes aredistributed in flat great circle spreads whichare compatible with the reorientation to beanticipated from refolding by 02 buckling(Class 1b), subsequently modified to Class1c folds. Fig. 6b is a theoretical reorienta­tion pattern for originally horizontal 01 foldaxes, trending north-south (Fold a) and 030­2100 (Fold B), respectively. The plunge ofthe later 02 fold axis is 255/60°, compara­ble to the mean attitude for the 02 popula­tion throughout the Skei outcrop, while theflattening strain has a k value of 0.7. Thetwo fold axis loci A-A' and B-B' define a fieldwithin which most of the actual 01 fold axesfall (compare Figs. 6a & 6b).

Slaty cleavage and schistosity (S1) arestrongly developed in the metasol and thepelitic horizons of the Skei Group. Thestrong preferred orientation of distortedpebbles within the axial plane schistosity ofthe Hylla fold, contrasts with the more com­mon pattern of randomly orientated, angularand undeformed c1asts in the massive up­sequence developments , such as the len­ses on Havnaholmen, at Vatvik and north­east of Stegafjell (Fig.2). On the mainland,a system of tight folds morphologically com­parable with this phase refolds an older foli­ation, locally mylonitic in some very compe­tent lithologies, and is 02 in the local tecto­nochronology.

38 Brian A. Sturt & Donald M. Ramsay NGU . BULL 426, 1994

Second phase of deformation (02)

Fig. 7. Diagrammatic cross-sections through the SkeiGroup. For locations. see Fig. 2. Ornament as in Fig. 2.

Minor structuresIn profile, the 02 minor folds vary from near­concentric (Class 1B) to strongly modifiedsimilar style (Class 1C). These folds arecongruous with respect to their macroscopichost folds and together with the co-axialmineral and pebble lineation are homoax ialover the whole area of outcrop (Fig. 3).

In conglomeratic horizons at the base of theStegafjell formation on Havnaholmen and atVatvik, 02 strains were the first to imprint anobvious tectonic fabric. Pebbles are strong­ly distorted , chief ly into prolate ellipsoidswith k-values in the range 1 - 4.4, depen­ding on lithology. The long axes of thesepebbles are parallel to the associated foldaxes and intersection lineations. In peliticand carbonate layers within the conglome­rates the S2 foliation is a prominent spacedcleavage.

In the coarse-grained, up-sequence brecci ­as of Havnaholmen and Hylla the effects of02 deformation are confined to thin and dis­continuous, fine-gra ined sand lenses.Angular and irregular c1asts, up to 0.5 m indiameter, preserve th eir orig inal clastic sha­pes, random orientation and pre-unconfor­mity internal fabrics. As was the case withthe 0 1 phase, the 02 strains are not com­pletely penetrat ive and some bodies of rockshow little or no evidence of it.

On a regional scale this phase is expressedin a system of upright, high-amplitude, peri­clinal folds with wavelengths between 3 and10 km and a broadly NE-SW axial trend,deflected locally by later open folds. OnLeka, conspicuous geniculate deflections ofthe strike, between Havne and Stegafjell,delineate macroscopic folds with a step­fold, z-profile (Figs. 3, 4 & 7). These WSW­plunging structures have a constant senseof facing, consistent with the whole SkeiGroup outcrop being situated on the wes­tern limb of a large antiform, the Austra Anti­form, whose axial trace runs through theAustra peninsula on the adjacent mainland,affecting both nappe rocks and the complexbeneath (Figs. 1 & 9). On the mainland,however, this fold is a 03 structure in theregional tectonochronology.

S '

S l

NNW

N

NNW

_--\~~;;;~~~-F.:-~~.,-.:-_SlC'

SSE

A

As mentioned earlier, on cessation of 0 1deformation the Skei Group of eastern Lekalay in the overturned limb of the LeknesSyncl ine, dipping up to 60° westwardsbeneath the ophiolite. This inverted attitudesubsequently contro lled the axial orientationof 02 folds, which plunge towards WSW at60° (Figs. 3, 4 &. 5b, c). The severalmacroscop ic folds which developed , toget­her with their associated parasitic suites,have an homoax ial attitude and congruousrelationships (Fig. 3).

NGU - BULL 426. 1994

Deformation in the LOC

The rocks of the LOC have experienced acomplex sequence of deformation eventsunder conditions varying from high-tempe­rature 'syn-magmatic' to amphibolite/gre­enschist facies probably related to ocean­floor metamorph ism. They have also beenaffected by clearly superimposed deforma­tions related to regional orogenic activityboth prior to and subsequent to the depositi­on of the rocks of the Skei Group.

Fumes et al. (1 988) emphasize the stronglydeveloped tectonite fabric of the hartzburgi­te and suggest that "this was due to high­temperature ductile shearing in the mantlebeneath an active spreading ridge". Thepresent authors have also observed howcross-cutting sheets of dunite and pyroxeni­te in this fabric are also strongly folded withaxial surfaces parallel to the sheet layering.Related to such structures , axis-parallelmineral lineations are part of the linear tee­tonite fabric . The essentially syn-magmaticnature of this deformat ion is clearly seenwhere folded pyroxenite veins are cutabruptly by non-folded pyroxenite veins(see also Fumes et al. 1988), or by gabbroicdykes. In some of the cumulate ultramafichorizons, flattened folds similar to those inthe harzburgite are developed, whereas inthe layered gabbro only certain horizonsshow ductile layer-parallel foliation, thougha strong mineral lineation parallel to thebanding is more prevalent. The isotropicgabbro, and in part the layered gabbro, dis­plays narrow high-temperature and ductileshear-zones with blastomylon itic textures.That some of these shear-zones wereessentially 'syn-magmatic' is seen from evi­dence of small-scale anatexis, producingrootless-topless veins and irregular patchesof leucocratic gabbro pegmatite, associatedwith such zones.

At higher levels in the LOC pseudostrati­graphy, lower-temperature mineralised (sul­phide-bearing) shear-zones , commonly fol­ded, are observed and may well be relatedto ocean-floor metamorphism, particularlyas a number of these can be observed to becut by metabasaltic dykes.

Brian A. Sturt & Donald M. Ramsay 39

This plethora of structura l features reflectsconditions operative during the formativephase of the ophiolite rather than with oro­genic deformation either associated withobduction/in-thrusting of the complex oroccurring later. In the gabbros , dykes andlavas, on the other hand, obvious polypha­sal orogenic strains are reflected in asequence of foliations and diaphthoreticgreenschist -facies metamorphism (see alsoFumes et al. 1988).

The entire outcrop of the Skei Group lies inthe overturned limb of a large-scale N-Strending Z-fold. No evidence of this structu­re can be clearly discerned in the LOC,which probably behaved as a more rigidblock in the core of this fold. Tight, large­scale folding in the layered ultramafic seg­ment has been determined from way-up cri­teria rather than structural observation (Fur­nes et al. 1988) and does not appear to bematched in the structure of the overlyingSkei Group. The shearing and diaphthore­sis which can be attributed to post-SkeiGroup deformat ion in the LOC is not relatedto this folding, and appears to post-date it.

In southern Leka, the layering of the gabbrounit has a mainly ENE-WSW strike and avariable but predominantly steep southeas­terly dip (Fig. 8b). In some situations astrong foliation is developed sub-parallel tothis banding. The earliest post-Skei Groupdeformation (D1) is represented by a suiteof mesoscopic to macroscopic shear zones,a few centimetres to several tens of metresthick, which can locally become a prominentfeature of the fabric. These range from nar­row and sharply defined to broad and morediffuse zones, characterised in the lattercase by phyllonitic cleavage. On the basisof orientation these shear-zones can be dif­ferentiated into two families (Fig. 8a), with acommon intersect ion plunging towardsENE, parallel to their mutual intesection withthe igneous layering (Fig. 8). On the cliffs ofHorrfjell in SE Leka the shear-zones definethe margins of conspicuous large lenticlesin which the original fabric is still preserved.In some instances the sense of displace­ment on the shears can be determined fromthe deflection of the steep igneous layering

40 Brian A. Stun & Donald M. Ramsay NGU - BULL 426. 1994

N N

xx

b

N

a

..

N

e. : • e.:...:

...

..

.. .... ..

c d

Fig. 8. Structural data of post-Skei Group shears in the LOC of southwest Leka. (a) - poles to foliation in shear-zones; (b) • polesto banding in gabbro (crosses) . with tie lines to associated shear-zones (dots). (c) - Lineation of amphiboles and feldspars in the sne­at-zones: (d) - Fold axes (D2) superimposed on shear-zone foliation.

at the margins of the zones. On Horrfjell, forexample , this exhibits a conjugate pattern ofsmall dextral and sinistral offsets. Indeed,there is no evidence of large displacementon anyone of the shears; rather, the bulkstrain in the gabbro is the sum of small off­sets along the many shear-zones .

In their formation these shear-zones wereattended by considerable reconstitution ofthe original fabric and mineralogy. At themargins there is progressive thinning andsharpening in the definition of the deflectedgabbroic layering, emphasized inwards bythe development of a platy schistosity, coa-

NGU • BULL 426, 1994

Leka

SL

Austra

Brian A. Slur! & Donald M. Ramsay 41

Sertjord

Fig. 9. Diagrammatic cross-section illustrating the relationship of D2 folding on Leka to the folding on the mainland.

ted with coarse phacoidal chlorite, in adiaphthoretic assemblage of actinolite , chlo­rite and albite. In this new foliation a promi­nent mineral lineation of actinolite and feld­spar may be present, sub-parallel to theintersection of igneous banding and shear­zone. (Fig. 8 c). Where several of thesezones come close together or intersect, amore general reconstitution has occurredand relics of the protolith are sparse. Thediaphthoretic greenschist-facies metamor­phism reflected in this mineralogy is isogra­dic with the prograde assemblages of theSkei Group.

The steep attitude of layering in the LOCwithin the core of the anticlinal complimentof the D1 Leknes Syncline may relate, inpart, to this folding , but may also be a relicof some pre-Skei Group attitude. While thehighly anisotropic Skei Group deformed bycongruous folding on a wide range of sca­les, the more massive gabbro yielded byformation of multiple shear-zones, whicheffected a bulk shear flow into the core ofthe anticline.

The second phase of deformation, D2, inthe gabbro can only be recognised withinthe strongly anisotropic shear-zones, whereit is expressed as tight angular , upright to

conjugate folds of the schistosity , associa­ted with a rude crenulation cleavage. Underthe influence of the original attitude of theshear-zones these fold axes are somewhatdispersed between N-S and NE-SW (Fig.8d).

Widespread small- and large-scale faultingis a prominent feature of the fabric of theisland, in both the LOC and the Skei Group.This is attended by brittle shearing but nomineral reconstitution is developed. Thelarger faults of this system juxtapose thevarious levels of the LOC in a number ofwell-defined blocks. Although no dating isyet available, it is possible that this faultingequates with the prominent Mesozoic block­faulting which affected much of coastal Nor­way.

Pre-Skei Group orogenesisPrior to the unconformity at the base of theSkei Group the LOC already possessed atectonic fabric, probably dating back to theformation of the complex. This is generallya strong foliation and folding within thecumulate mafic and ultramafic horizons, butit can locally be a mylonitic foliation in high­temperature shear-zones. These structuresbear no geometric or spatial relationship to

42 Brian A. Sturt & Donald M. Ramsay

a regional structural pattern . As ment ionedearlier, Sturt et al. (1985) recorded relictamphibolite-facies mineralogy in thissequence, overprinted by a low-mid green­schist facies, isogradic with the progrademetamorphism of the Skei Group. Theyregarded the higher grade as representingan older pre-Scandian regional metamor­phism.

In the coarse flood breccias of the lowerpart of the Stegafjell formation , at the wes­tern end of Havnaholmen (Fig. 2), angularand irregular clasts are derived from severalmembe rs of the ophiolite pseudostrati­graphy, especially those levels now poorlyrepresented , due to the depth of erosion .Despite two phases of Scandian deformati­on there is no evidence of mechanical reori­entation or shape distortio n of these clasts.The presence of an internal tectonic fabr icin many is a significant pointer to the tecto­nothermal state of the La C prior to erosion.Mindful of the ambiguous status of tectonicfabrics in the plutonic members of the ophi­olite, the search for evidence of older oroge­nic deformation focused on greenschistpebbles. Cobbles with a randomly orienta­ted schistosity, relative to c1ast shape andexternal foliation , or with a folded schistos ity(Fig. 10), provide clear evidence of polypha­sal orogenic strains prior to the formation ofthe conglomerate. A number of psamm ite

NGU - BULL 426 . 1994

pebb les with pre-pebb le fabr ics were alsoobserved. In being exposed to sub-aer ialeros ion the LaC must already have beenelevated to high topog raphic levels and hadexperienced tecto nic strains and greens­chist-facies metamorphism prior to thedepos ition of the cover sequence. Large­scale , pre-unconfo rmity folding of Group 1ophiolites has already been recorded fromelsewhere in the Helgeland Nappe Com­plex, e.g. Hodcy (Bang 1985), in the Lyng­en Nappe in Troms (Minsaas & Sturt 1985),in the Vaqa-Otta area (Sturt et al. 1991) andon Bomlo in Southwest Norway (Brekke1983, Nordas et al. 1985).

Regiona l setting of the LekasequencesPlacing the LaC and its cover in a regionalsett ing is compl icated by the geographicisolat ion of the island . However, the assoc i­ation of ophiolite with an unconformablecover sequence conta ining polymict conglo­merate horizons rich in ophiolite debris istypica l of many nappes of the upper alloch­thons of Norway. This association occurs atmany localities in the Helgeland area (Sturtet al. 1984, Thorsnes & Loseth 1991), withLeka as the largest single occurrence. Theophiol itic bodies are distributed in lineararrays which, on the one hand , delineatenappe boundar ies and, on the other hand,

Fig.l 0.Boulder of folded. foliatedgreenstone with deformed epidoteknots. Havna Formation. SW Hav­naholmen.

NGU - BULL 426, 1994

partition nappes into substrate and coversequences (Fig. 1). The unconformity isalso a valuable datum plane for establishingthe base of the cover sequences, the polari­ty of the sediments and the lithostratigraphyin each nappe.

In reviewing the tectonostratigraphic relati­onships and obduction histories of Scandin­avian ophiolite terranes , Sturt & Roberts(1991) concluded that the obduction of theGroup I ophiolites (Sturt et al. 1984) in thecentral part of the Caledonides was post­Tremadoc and pre-Middle Arenig in age.This postulate was based on a combinationof faunal dating in cover sequences andU/Pb zircon dating of late components ofthe ophiolites. The Heilhornet Pluton (datedat 444±11 Ma), which intrudes the Terrakophiolite fragment and its deformed andmetamorphic cover sequence, provides anupper limit not only for the age of the coversequence but also for its deformation andmetamorphism. Nordgulen et al. (1993)have also provided a series of reasonablyprecise U/Pb zircon dates for other plutonsin the Bindal Batholith in the range 447±7 to430±7 Ma, i.e. during the period Ashgill-

/ Early L1andovery.

The stage of tectonothermal developmentrepresented here, from approximately MidArenig to Mid Caradoc time , apparentlycoincides with deformation of the Arenig­L1anvirn sequence of Smela, west-centralNorway (Gautneb & Roberts 1989). Itwould thus appear to represent a regionalevent of some magnitude. For convenien­ce, we will here refer to this informally as theTerrek Phase. Evidence for a Mid Ordovici­an orogenic event in certain nappe comple­xes or terranes in Central and SW Norway ,broadly equivalent to the Taconian of theAppalachians, has indeed been presentedby Hall & Roberts (1988). Isotopic data hel­ping to identify this event also in the higherparts of the Upper Allochthon in CentralSweden have been reported by Stephens etal. (1993). There is, however, as yet littleevidence for this event from the gneisses oramphibolite-facies supracrustals of theVestranden region" though Dallmeyer et al.

4u (39(1992) record an Art Ar date from a horn-

Brian A. Sturt & Donald M. Ramsay 43

blende concentrate from supracrustal rocksof the southeastern margin of Vestranden at440±2 Ma which they interpret as a coolingage through c 500°C. The peak Caledonianmetamorphism of the Vestranden gneissesis indicated by a Sm/Nd isochron of 432±6Ma from high-pressure basic granulites ofthe Roan Window (Dallmeyer et al. 1992)and a U/Pb lower intercept age of 434±22Ma from discordant zircons in a migmatitefrom Vikna in the northwestern part of Vest­randen (Schouenberg et al. 1991) . Thesedates are interpreted by Dallmeyer et al.(1992) to indicate peak Scandian progrademetamorphism. Uplift cooling ages , in Vest­randen , are indicated at 419±2 - 393±3 Mafor hornblende concentrates and at 395±2 ­390±2 for micas (Dallmeyer et al. 1992) .Unfortunately no 4°Arf39Ar ages are availablefrom the HNC.

The regional development of a majorunconformity, commonly above ophiolitefragments , in the Scandinavian Caledoni­des has been stressed in a number ofpapers (Sturt et al. 1984, Thon 1985, Min­saas & Sturt 1985, Ramsay & Sturt 1986,Sturt et al. 1991, Sturt & Roberts 1991, B0eet al. 1993); and particularly in earlierpapers this has been assumed to representan individual event horizon. However, itbecomes apparent that such unconformitiesassociated with major clastic wedges maybe more spread in time than has previouslybeen assumed. In the Trondheim NappeComplex a major unconformity above ophi­olitic material is pre-Mid Aren ig in age (Sturt& Roberts 1991) whilst in SW Norway themajor clastic wedge is essentially of Ash­gill/L1andovery age (Thon 1985) , and in partoverlies the West Karrnoy Igneous Complexdated in the range 480-470 Ma (Pettersen &Dunning , in press). Thorsnes et al. (inpress) show how the Follafoss Pluton (nearMaim) , giving a U/Pb zircon age of 460±5Ma, is unconformably overla in by a basalconglomerate . This situation is very similarto that in the Gjersvik Nappe where thebasal conglomerates of the Limingen Grouprest unconformably on the Moklevatn gra­nodiorite which has yielded a U/Pb zirconage of 456±2 (Roberts & Tucker 1991) .

44 Brian A. Sturt & Donald M. Ramsay

Conclusions

The geology of Leka plays a significant rolein deciphering the lithostratigraph ic and tec­tonic patterns of the coastal part of thenorth-centra l Scandinavian Caledonides,where neither the Scandian nappe sequen­ce nor its relationships with the cratonicbasement are well understood. The impor­tance of Leka stems from a lower level ofdeformation and metamorphic reconstituti­on than is generally the case. Leka also pro­vided the first record of an ophiolite associa­tion in the region, which prompted thesearch for and subsequent identification ofother fragments . These have assumed asignificant tectonostratigraphic status bydefining the outlines of the constituent nap­pes along the western margin of the Helge­land Nappe Complex. In the source areasof these nappes ophiolite formed the sub­strate for later Ordovician sedimentationand subsequently formed the 'basement'segment in each of the nappes, when pene­trated by the Mid-Late Ordovician (Terrak)thrusts . The present discontinuous natureof the ophiolite levels in the region is due, inpart, to high orogenic strains as well as pri­mary pre-orogenic erosion.

The precise pattern of Scandian deformat i­on and metamorph ism in the Skei Group isnot known. It is probable that 01 and asso­ciated metamorph ism relate to the TerrakPhase and that Scandian deformation doesnot have a penetrat ive imprint. Clear evi­dence for pre-Skei Group deformation andmetamorphism is, however, clearly seen bythe truncation of fabrics by the unconformityand from cobbles in the conglomerates.

The low strains and well-founded lithostrat i­graphy of Leka afford a clearer insight intothe morphology of the large-scale folds andthe geometric patterns of associatedmacroscop ic structures , than can be obtai­ned elsewhere in the region. The wholeoutrop of the Skei Group defines the over­turned limb of a large Z-fold with N-S axialtrend. Although this is the first post-SkeiGroup deformation to affect Leka, it has thegeometrical characteristics of 02 on themainland, in both the nappes and the sub-

GU • BULL 426. 1994

HNC rocks, and no trace of the regional 01phase is present.The second phase of folding on Leka wassuperimposed on the overturned rocks ofthe Skei Group and the steep attitude ofbedding controlled the orientation of foldaxes, with the result that they consistentlyhave steeper plunges than is normallyencountered in the region. The largerasymmetrical step folds of this phase faceeastwards, consistent with their being para­sitic to the large Austra Antiform (03) on themainland, one of a system of upright pericl i­nes of regional development. The strainassociated with each of these post-SkeiGroup phases is rather patchy in distributionand development, and 0 2 may be theoldest tectonic feature in some localities.

It has been widely assumed that deformati­on and metamorphism of the metasedi­ments and ophiolites in Helgeland can beascribed to the Scandian Orogeny, Le. Mid­Late Silurian (Sturt 1984, Stephens & Gee1985). In this, the nappes were viewed ascoeval with the climactic nappe phase of thebelt, whose effects extend all the way to theorogenic front. With the recognition of theTerrak Phase, however, it has becomeapparent that this pattern needs revision.Thus, by the Late Ordovician, the main tee­tonotherma l events of the HNC were overand the nappes were already in contact withthe sub-Helgeland assemblage. This hasconsiderable implications for modelling thesequential evolution of the ScandinavianCaledonides and provides a caveat for cor­relation of events across major tectonicboundaries which may ignore the complexi­ties of palaeogeographies, and of happe­nings at active plate margins which providea diversity of tectonometamorphic proces­ses at some sites whilst continuous sedi­mentation is occurring at others.

AcknowledgementsThe authors would like to than Oystein ordgulen and an un ­nown reviewer for their critical comments on he manuscripwhich have led to many improvements. We wish 0 han Ter­je Thorsnes for allowing us to use Fig. 1. We are also gra efulto Bina Evensen for the final drafting of he illustrations. AVFand NGU are than ed for financial support .

NGU - BULL 426. 1994 Brian A. Stun & Donald M. Ramsay 45

ReferencesBang, N. 1985: The stratigraph y and structural development of

the Rodoy-Haltoy area . outer Vefsn fjord . Unpubl. Cand.SCient. thesis . Un iv. Bergen, 247 pp .

Birke land, T. 1958 : Geological and petrological investigationsin northern Trondelag, western Norway. Nor. Geol. Tidsskr.38.327-420.

Bre kke. H. 1983 : The Caled onian geological patterns of Mosterand southern Bomlo. Evidence for Lower Palaeozoic arcdevelopment. Unpubl. Cand . Real. thesis. Un iv. Bergen,473 pp.

Brekke, H. & Solberg. P.O. 1987: The geology of Alloy, Sunnfjord,western Norway. Nor. geol. unders . Bull. 4tO. 73 -94.

Bruton, D.L. & Bockelie , J .F. 1982 : Geo logy and palaeontolo ­gy of the Ho londa area , we stern Norway - a fragment ofNorth America ? In: Wones, D.R(ed. ): The Caled onid es inthe USA. Virg inia Poly tech. tnst. & State Univ. Dept. Geol.Sci. Mem. 2. 41-47 .

Boe, R , Sturt, BA & Ramsay, D.M. 1993: The con glomeratesof the Sel Group, Otta-Vaqa area , Central Norway: anexample of a terrane-linking succession . Nor. geol. unders.Bull. 425, 1-23.

Da llme yer, RD., Gee, D.G. & Beckholmen, M. 1985 : "'Arf"Armineral age record of early Caledonian tectonothermal act i­vity in the Baltoscandian miogeocline. central Scandinavia .Am. J . SCi. 285, 532-568.

Da llmeyer, RB.• Johansson, L. & Moller, C. 1992 : Chronologyof Caledonian high -pressure granu lite -fac ies metamor­phism. uplift and deformation with in northern parts of theWestern Gneiss Region. Geol. Soc. Am. Bull. t04 . 444­455.

Dunning, G.R . & Pedersen, R B. 1988: U/Pb ages of oph iolitesand arc related plu tons of the Norwegian Caledonides:Impl ications for the development of lapetus. Cont rib. Mine­ral. Petrol. 98. 13-23.

Furnes, H.• Aust rheim . H., Amaliksen , K.G., Nordas , J . 1983:Evidence for an inc ipient early Caledon ian (Cambrian) oro­genic phase in southwestern Norway. Geol. Mag. t20. 607­12.

Fu rne s, H., Ped ersen , R.B . & St illman, C.J . 1988: The LekaOphiolite Complex, central Norwegian Caledonides: fie ldcha racterist ics and geotectonic signi ficance. J. Geol. Soc .London 145, 401 -412.

Furnes, H., Roberts, D., Sturt, BA, Thon , A. & Gale, G.H. 1980:Op hiolite fragments in the Scandinavian Caledonides. In:Pan ayiotou , A. (ed.). Op hiolites Proc. lnt. Ophiolite Symp.Cyprus 1979, 582-600.

Ga utn eb , H. & Roberts, D. 1989 : Geology and petrochemistryof the Smola-Hitra Batho lith, Central Norway. Nor. geol.unders. Bull. 416 ,1-24.

Hall, L.M. & Roberts, D. 1988 : Timing of Ordovician deformat i­on in the Caledonian- Appa lach ian orogen. Geol. Soc. Spe­cial London Publ. 38. 29 1-309 .

Heldal, T. 1987 : Stratigrafi og strukturell utvikling i Saurenom­rsaet, vest for Bronnoysund, sydlige Nordland. Unpubl.Cand. Scient. thesis, Un iv. Bergen. 300 pp .

Hu smo , T. & Nordgulen , 0 .1988: Structural relat ions along thewestern boundary of the He lge land Nappe Complex , north­central Norway (Abstract). VI Annual TSGS Meet ing. Inst.geo logi. Interne skritter 54 , 20-23 .

Minsaas, O. & Sturt, BA 1985: The Ordovician clastic sequ­ence immediately overlying the Lyngen Gabbro Complex,and its env ironmental sign ificance. In: Gee, D.G. & Sturt,BA (eds), The Caledonide Orogen - Scandinavia and rela­ted areas. John Wiley . Chichest er, 379 -394 .

Nissen, A.L. 1986 : Rb-Sr age determination of intrusive rock sin the southeastern part of the Bindal massif, Nord-Tronde­lag , Norway. Nor. geol. unders. Bull. 406, 83-92.

Noroas, J ., Amaliksen, K.B., Brekke, H., Suthren, RJ., Furnes.H., Sturt, BA & Robins, B. 1985: Lithostratigraphy andpetrochemistry of Ca ledonian rocks on Bornlo, sou th-westNorway. In: Gee, D.G. & Sturt , BA (eds.) . The CaledonideOrogen - Scandinavia and rela ted areas. John Wiley, Ch i­cheste r, 679-692.

Nordgu len , 0 . & Sch ou enbo rg, B. 1990: The Cal edon ianHeilho rnet Pluton , north -centra l Norwa y: geological set­ting , radiometric age and imp lications for the SCand inav ianCaledonides. J. Geol. Soc. Lond. 147, 439 -450.

Nordgu len, 0 .. Bickfo rd , M.E., Nissen, A.L. & Wortman. G.L.1993: U/Pb zircon ages from the Bindal Batholith, and thetectonic history of the Helgeland Nappe Complex. Scand in­avian Caledonides. J. Geol. Soc. London, 150.771 -783 .

Pedersen, R B. & Dunning, G. 1991: U/Pb da tering av oybue­ma gmatisme innenfor det sorvest-norske ofiolitt-terreng.Geonytt 18.42-3.

Pedersen, R B. & Dunning , G.R 1993 : Age relat ions betweenoph iolite and island arc complexes. Eettn Planet . Sci.Lett.(in press).

Prestvik, T. 1974: Supracrustal rocks of Leka, No rd-Trondelag .Nor. geo l. unders. 311 , 65-87 .

Prestv ik , T. 1980 : The Caledonian ophiolite complex of Leka,north-cen tral Norway. In: Panayiotou , A. (ed.) , Ophiolites.Proc. tnt. Ophiolite Syrnp. Cyprus 1979,555-566.

Pre stvi k, T . & Roaldseth , E. 1978: Rare earth element abun­dances in Caledonian metavo lcanics from the island ofLeka, Norway. Geoch em. J. 12, 89- 100.

Ram say, D.M ., St urt , BA, Ro berts, D. & Zwaan, B.K . 1985 :Caledonides of no rthern Norwa y. In: Ge e, D.G. & Sturt ,BA (eds) . The Caledonide orogen Scandinavia and rela­ted areas. John Wiley. Chichester , 163-184 .

Ramsay , D.M . & Stu rt , B .A . 1986: The cont rib uti on of theFinnmarkian Orogeny to the frame wo rk of the Scandinavi­an Caledonides . In: Fett es, D.J. & Harris , A.L. (eds) . Syn­thes is of the Caledonian rocks of Britain. Reidel Pub I.Co.,221 -246.

Ramsay, J .G. 1967: Folding and fracturing of rocks . Mc Graw­Hill Co., New York, 568 pp .

Roberts, D. & Gee, D.G. 1985. An introduction to the structureof the Scandinavian Caledonides. In: Gee, D.G. & Sturt ,BA (eds). The Caledon ide Orogen - Scandinavia and rela­ted areas. John Wiley , Chichester, 55-68.

Roberts, D. & Tucker, RD. 1991: U/Pb zircon age of the Moklevatnet granodiorite, Gjersvik Nappe, Central NorwegianCaledonides. Nor. geo l. unders. Bull. 42 1, 33-38.

46 Brian A. Sturt & Donald M. Ramsay NGU . BULL426. 1994

Ryan, PD" Williams, D.M. & Skevington, D. 1980: A revisedinterpretat ion of the Ordovician stratigraphy of Sor-Tronde­lag, and its implications for the evolut ion of the Scandinavi­an Ca ledonides. In: Wones, D.R. (ed.), The Caledonides inthe USA. Virginia Poly. Inst. & State Univ., Dept. Geol. Sci.Mem. 2,99-105.

Schouenborg, B.E ., Johansson, L. & Gorbatchev, R.G . 1991 :U/Pb zircon ages of basement gne isses and discordant fel­sic dykes from Vestrand en, westernmos t Baltic Shield andcentral Norwegian Caledonides . Geo/. Rundschau 80 , 121­134 .

Stephens, M.B. & Gee, D.G . 1985 : A tecton ic model for theevolution of the eugeoclinal terranes in the Scand inavianCaledonides. In: Gee , D.G. & Sturt, BA (eds). The Cale­don ide Orogen - Scandinavia and related areas. JohnWiley , Chichester, 623 -652 .

Stephens, M.B., Kuller ud, K.. & Claesson, S. 1993 : Early Ca ledonian tectonothermal evolu tion in outboard terranes, cen ­tral Scandinavian Caledonides: new constra ints from U-Pbzircon dates . J. Geol. Soc. London, 51-5 6.

Sturt, BA 1984: The accretion of ophiolite terranes in the Scandi­navian Caledonides. Geo/. Mijn b. 63, 201-12.

Stu rt, BA & Thon , A. 1978: An ophi olit e complex of probableearly Caledonian age discovered on Karmo y. Nature 275,538-539.

Sturt, BA & Roberts, 0 .1 991: Tectonostratigraphic relationshipsand obduction histories of Scandinavian oph iolites. In:Peters Tj.. Nicholas, A. & Coleman, R.G. (eds). Ophiolite

genesis and evolut ion of the Oceanic timosptiete. Kluwer ,Amsterdam, 745-769.

Sturt, BA, Roberts , D. & Fumes, H. 1984: A consp ectus ofScandinavian Caledon ian ophiolites . In: Gass. I.G., Lip­pard , S.J . & Skelton, AW . (eds): Ophiolites and ocea nic lit­nospnere. Geo l. Soc. London Special Publ. 13,381 -389 .

Sturt, BA , Andersen, T.B. & Fumes, H. 1985: The Skei Group,Leka: an unconformable clastic sequence over lying theLeka Ophiolite. In: Gee, D.G. & Sturt, BA (eds), The Cale­don ide Orogen - Scandina via and rela ted area s. JohnWiley, Ch ichester, 395-406 .

Sturt , BA, Ramsay, D.M . & Ne umann, R.B . 1991 : The OUaCong lomerate, the Vaga mo Ophiolite - further indicationsof Early Ordovician orogenesis in the Scandinavian Cale­don ides . Nor. Geo l. Tidsskr. 71, 107-115 .

Then, A. 1985: Late Ord ovician and Early Silurian cover sequ­ences to the west Norwegian oph iolites : strat igrap hy andstructural evolution. In: Gee, D.G. & Sturt, BA (eds): TheCaledon ide Orogen - Scandinavia and related areas. JohnWiley , Chichester , 407-416.

Thorsnes, T. & Loseth, H. 1991: Tectonostratigraphy in the Velfjord­Tosen region, southwestern part of the Helgeland NappeComple x, central Norweg ian Caledonides . Nor. geo /.unders . Bull. 421 ,1-18.

Thorsnes, T.. Bickford, M.E. & Wortman, G . 1994 : Age, geo­chemistry and geo logical setting of metatona lite from theFollafoss area , central Scandinavian Caledonides . Nor.Geol. Tidsskr, (in press ).

Manuscript received September 1993; revised m/s received/accepted March 1994.