The structure and regional setting of the Skei Group, Leka, north-central

BRIAN A. STU RT & DONALD M. RAMSAY

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

The unconformity between the metasedimentary Skei Group and subjacent ophiolite on Leka has importa nt impli­ catio ns for the tectonostratigraphy of the region . As a distinctive marker horizon, it permits correlations within and between nappes and provides a datum level on which to establish lithostra tigraphi es in the several units of the Helgeland Nappe Comp lex. The unconformi ty records a major change in the pattern of events and partitions the tectonothermal history into two polyphase orogenies, whose individual effects can be recogn ised in many nappes along the whole length of the belt. Deformations predating the unconformity relate to ocean-floor tectonics and a subsequent phase of regional tecto nothermal development. The Skei Group records two fold phases acco mpanied by low-grade regional metamorphism. The distinctive litho­ stratigraphy facilitates delineation of mac roscopic folds, whose geometry indicates that the whole outcrop lies wit­ hin the inverted limb of the major 0 1 Leknes Synclin e. This, in turn, controlled the attitude of mesoscopic 0 2 folds which are themselves parasitic to the large-scale Austra Antiform, one of a regional system of near-symmetrical late 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 reite­ rate a theme currently being espoused for the evolution of the whole Scandian belt, namely dissection and disrup­ tion by Scandian thrusts of a cratonic substrate, already structured into a Finnmarkian and in part Late Ordovician nappe pile.

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

Introduction hors, however, identified the igneous suite The bedrock geology of the island of Leka as the LaC. The sedimentary rocks of the comprises the Leka Ophiolite Complex Skei Group were reassessed by Prestvik (La C) and the unconformably overlying , (1980) as the cap-rock to the LaC and hen­ probably Lower to Middle Ordovician meta­ ce essentially eugeoclinal in characte r. sediments of the Skei Group (Sturt et al. Sturt et al. (1985), on the other hand, clearly 1985). These rocks were assigned to the demonstrated that the sediments of the Skei Upper Allochthon by Roberts & Gee (1985), Group had been deposited unconformably though they are most likely to be part of the on a deeply eroded substrate of the LOC. Helgeland Nappe Complex (HNC) of the The lithostratigraphy (Table 1) and palaeo­ Uppermost Allochthon (Fumes et al. 1988). geographic interpretation of the Skei Group This latter interpretation is based on a cor­ were further refined including the recogniti­ relation of the ophiolite fragments of the on of the continental character of the uncon­ region (Sturt 1984, Sturt et al. 1984). Many formity , with a variably preserved regolith in details of the LaC are given in Fumes et al. the upper levels of the substrate . Quartz (1988) and the reader is referred to this as keratophyres intruding sheeted mafic an essential source reference . dykes, immediately below the unconformity, have been dated (U-Pb zircon) at 497±2 Ma The metasedimentary rocks of the Skei (Dunning & Pedersen 1988) and provide a Group have been described in some detail maximum age constra int for the base of the by Sturt et al. (1985), who confirmed the Skei Group. general proposition of Birkeland (1958) that they were: "a younger sequence overlying A number of smaller ophiolite fragments, an older complex of basic and ultramafic with a similar tectonostratigraphic setting , rocks", the latter now known as the LaC. have been identified in the HNC (Sturt The break between the two was, however, 1984, Sturt et al. 1984, Bang 1985, Heldal not noted by Prestvik (1974, 1980) or by 1987, Husmo & Nordgulen 1988, Thorsnes Prestvik & Roaldseth (1978). The latter aut- & Loseth 1991). Nordgulen & Schouenborg 32 Brian A. Sturt & Donald M. Ramsay NGU • BULL 426. 1994

RNC

v) f SKALV£R <. N '';(01 ~ \00 VEGA (. ~1

; I: l! t)q) + I + + G \ ) + s\ '<1' . + \ ::\

:1 /.

4.0 HORTAVi£R

MIGMATlTICGNEISS.MARBLE. SC HIST

PORPHYRI( GRANITE AND GRAN ODIOR ITE THRUS TS 0 0' MONZON ITE, MON ZOOIOR ITE .L.1,__A-l-A. HNC, INTERNAL GRMOD IOR ITE, GRANITE A_ • HNC, SOUTHWES T TON AlITE , GR ANOOIOR ITE v V K O N'j S M~ E N ~ HNC , EAST GABBRO, DIORITE V / l ROOINGSFJALLU SUB- HNCRO CKS L / ,j NAPPE COMPLEX 10 20 KM

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

(1990) demonstrated that the granitic Heil­ cover sequences (Fig. 1). They provided a hornet Pluton cuts both such ophiolite frag­ U/Pb zircon age of 444±11 for the Heilhor­ ments and their deformed/metamorphosed net 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 )

sc hist s 1"u.00co'nglomerate sl ltstone bl ack slate (sands tone. COnglOmerate ) } SKEI FORM ATION volcanic sequence . greyw acke & black schist sandstone (. cong lome rate ) HAVNA FM . marble marbl e } SKEI GR OUP conglomera te conglomerate STEGAFJ ELL FM.

STOR0YA FORMATION ophiolite complex ophiolite 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 initial Caledon ides, and these occurrences are deformation of the Skei Group; assuming summarised in a recent review paper (Sturt that the correlation of the cover sequences & Roberts 1991), The present paper is con­ is viable . cerned particularly with details of the struc­ tural geology of the Skei Group , the geome­ On the wider scale, unconformable cover trical relationships between the Skei Group sequences, usually of continental derivation and the LOC and with evidence for pre­ in their lower part, overlying ophiolite com­ unconformity deformation and metamor­ plexes are widespread in the Scandin avian phism of the LOC.

THE SKEI GROUP Black shale with minor sandstone and conglomerate

Sandstone (ooned) with lenses ot coarse conglomerate ltriangels), bedded HAVNA FORMATION conglomerate filled circles), and limestone Braided stream conglomerate STEGAFJELL FORMATION (circles) wilh sandstone (ootteo) r.""9, " Ld

I I I I \ '\" "- \\ ,~... ~ -- - ~"" "" "" " " '\,\ \'..., ....::. GANGST0LEN "",,' ~~~

Fold axis (0 1, 02) -«: 8edding LithologicaV -- Slratigraphical """' - ~--- Intersection uneaton .-/ SChislosity boundary (0 1,02) ..-L Younging - - Faun

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. 34 Brian A. Stun & Donatd M. Ramsay GU • BULL 426. 1994

-- - -- ...... \;p.

--v-- --v-- 01 antilarm (a x ial ----l<-- --;<-- 01 sy n Ia r m ~ ~ 02 antilarm ==§= ==§= 02 syntorrn

a 0 .5 t km

Fig . 3 . Structural map of the Skei Group (dotted ornament). Sma ll stereog ra ms present att itud es of poles to bedding and S l (dots), and of D2 laid axes (crosses), for four sub-areas. NGU • BULL 426. 1994 Brian A. Sturt & Donald M. Rar.nsf/.Y 35

Structure of the Skei Group

The structural history of Leka records a complex sequence of events spread over at least two orogenies and extending back, in the case of the ophiolite, to pre-orogenic ...... \ .. deformation associated with the formation ~o '" of the complex. The unconformity at the . .:· ~oo. ... -, base of the Skei Group denotes a signifi­ '. cant time interval between two orogenic sequences during which uplift and erosion unroofed the metamorphic Solsemey For­ mation (Fumes et al. 1988) and the LOC. In so doing, it distinguished pre- and post­ unconformity tectonism. The dominant Caledonian fabric of the island is post-Skei Group in age. Polyphasal deformation in greenschist facies created both large- and small-scale folding and associated pheno­ mena in the Skei Group. This deformation is locally penetrative , although sizeable areas may be little affected by structures of any one tectonic phase.

First phase of deformation

Along its landward margin the Skei Group dips steeply westwards , beneath the rocks of the LOC, inverted in the limb of a large 01 fold (Leknes Syncline) with NNE-SSW axial trend (Figs 2, 3 & 9). Macroscopic z­ folds with amplitudes up to 1 km in the Ganqstelen-Hylla, Vatvik and Stegafjell are­ as are parasitic structures to the main fold (Figs. 3 & 7). Of these the Ganqstelen-Hyl­ la fold is a recumbent, westerly-closing syncline tightly folded by an upright 02 syn­ form (Fig. 7). The form of the 01 syncline is delineated by the one-sided core of the Skei Group, sandwiched between upper and lower developments of ophiolite (Figs. 3 & 7), and confirmed by the unconformity and its attendent development of metasol. In addition to this, an abundance of sedimen­ tary structures young away from the LOC towards the core of the syncline (Fig. 2). High 01 strains in the isoclinal core are reflected in a strong foliation within the o",-- =250 m metasol and a strong planar alignment of

flattened pebbles in conglomerates of the Fig. 4. Structural map of the V<\tvik-Stegafjell sub-area Stereo­ Skei Group. gram presents structura l data: dots - poles to bedding and S1; crosses - poles to S2; open circles - 01 fold axes; triangles ­ 02 fold axes. 36 Brian A. Sturt & Donald M. Ramsay NGU - BULL 426. 1994

At Vatvik and south Stegafjell (Fig. 3) the low. Greenschist facies is indicated by bioti­ form of the mesoscopic 0 1 folds is obvious, te and sporadic garnets in some pelitic hori­ delineated by the outcrop pattern of a well­ zons and by chloritoid in the metasol. This marked stratigraphy and confirmed by sedi­ contrasts with the situation on the mainland mentary structures and the presence of the to the northeast at levels farther removed metasol beneath the unconformity (Fig. 4). from the base of the cover sequence where the Helgeland Nappes and the complex On Leka, metamorphic reconstitution in the beneath generally display a metamorphic Skei Group is restricted and crystallinity is maximum in amphibolite facies. N N

.. \. ·. . .a. • '.a •• • .a I • :.. : .. i ...... :·... :. e. • e. • .- ..·. ..

a b N N , . . . .\. .. • .. ?~ • - ..... ~. \:x • ~ ~ . x~ .. .. ~ x. J( \--... X,(- " • • t...... )( •• •• xx ...... ~ -, • • rt:. r- }- ...... e. . . .. '\.. e. __ A..e L • .i~ _ •• ...... \. ...- x ••• .~. __ • .. '\ ...... -; e. .Jf' x )( " , .. / • a lii: .~ ...... \ . /' )()()( ~ 'K . \ / \ . , ~ \ . , / \ / / ...... -\ -, \ x \ : " ~ _---·r .: \"",e.·· . ------, x ,. I , . x I )( I )( • I • ..or: . I a I lit x xl ·· I / . . . / 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) and Sl (crosses); (d) poles to S2. NGU • BULL 426, 1994 Brian A. Sturt & Donald M. Ramsay 37

N Skei Group. For example, in the thick con­ glomerates, breccias and some sandstones there is locally no trace of 0 1 deformation, so that 02 features are the first record of strain. 0 1 structures are most prominently developed in the fine-grained members of the Havna Formation and in the weathered facies of the LOC. The only conglomerates to be significantly affected during this phase were those already mentioned, in the core of the Ganqstelen-Hylla sypcline.

Minor folds have a flattened concentric style (Class 1c, Ramsay 1967) with axial plunges x" varying between SW and SE (Figs. 4 & Sa). 4 While some of this variation is possibly a function of primary noncylindrism it is also compatible with reorientation consequent upon 02 refolding (Fig, 6). Fig. 6a differen­ a tiates these fold axes from the four sub-are­ as featured in Fig. 3. In each, the axes are N distributed in flat great circle spreads which are compatible with the reorientation to be anticipated from refolding by 02 buckling (Class 1b), subsequently modified to Class 1c folds. Fig. 6b is a theoretical reorienta­ tion pattern for originally horizontal 01 fold axes, trending north-south (Fold a) and 030­ 2100 (Fold B), respectively. The plunge of the later 02 fold axis is 255/60°, compara­ ble to the mean attitude for the 02 popula­ tion throughout the Skei outcrop, while the flattening strain has a k value of 0.7. The two fold axis loci A-A' and B-B' define a field within which most of the actual 01 fold axes fall (compare Figs. 6a & 6b).

Slaty cleavage and schistosity (S1) are strongly developed in the metasol and the b pelitic horizons of the Skei Group. The Fig. 6. (a) 0 1 fold axes from 4 sub-areas, VAtvik·Stegafjell ­ strong preferred orientation of distorted dots; central sector - crosses ; Havna - open circles; Ganqsto ­ pebbles within the axial plane schistosity of len • triangles. Lines 1·4 are the traces of axial spread resul­ the Hylla fold, contrasts with the more com­ ting from 02 refolding, (b) Theoretical pattern of reorientated fold axes resulting from refolding initially Class 1b type but mon pattern of randomly orientated, angular modified by later homogeneous strain to Class la (K=0.7). and undeformed c1asts in the massive up­ The initial axes trended N-S (fold A) and 030-2 100 (fold B) and both were horizon tal. The axis of refolding was 236/60°, the sequence developments, such as the len­ same as the modal 0 2 attitude in NE Leka. ses on Havnaholmen, at Vatvik and north­ east of Stegafjell (Fig.2). On the mainland, Minor structures a system of tight folds morphologically com­ The small-scale structures of the 01 defor­ parable with this phase refolds an older foli­ mation phase, e.g. minor folds, mineral and ation, locally mylonitic in some very compe­ pebble lineation, foliation and boudinage, tent lithologies, and is 02 in the local tecto­ are not developed uniformly throughout the nochronology. 38 Brian A. Sturt & Donald M. Ramsay NGU .BULL 426, 1994

Second phase of deformation (0 2) On a regional scale this phase is expressed in a system of upright, high-amplitude, peri­ As mentioned earlier, on cessation of 0 1 clinal folds with wavelengths between 3 and deformation the Skei Group of eastern Leka 10 km and a broadly NE-SW axial trend, lay in the overturned limb of the Leknes deflected locally by later open folds. On Syncline, dipping up to 60° westwards Leka, conspicuous geniculate deflections of beneath the ophiolite. This inverted attitude the strike, between Havne and Stegafjell, subsequently contro lled the axial orientation delineate macroscopic folds with a step­ of 02 folds, which plunge towards WSW at fold, z-profile (Figs. 3, 4 & 7). These WSW­ 60° (Figs. 3, 4 &. 5b, c). The several plunging structures have a constant sense macroscop ic folds which developed , toget­ of facing, consistent with the whole Skei her with their associated parasitic suites, Group outcrop being situated on the wes­ have an homoaxial attitude and congruous tern limb of a large antiform, the Austra Anti­ relationships (Fig. 3). form, whose axial trace runs through the Austra peninsula on the adjacent mainland, affecting both nappe rocks and the complex beneath (Figs. 1 & 9). On the mainland, N however, this fold is a 03 structure in the regional tectonochronology.

Minor structures In profile, the 02 minor folds vary from near­ A concentric (Class 1B) to strongly modified similar style (Class 1C). These folds are congruous with respect to their macroscopic host folds and together with the co-axial mineral and pebble lineation are homoaxial over the whole area of outcrop (Fig. 3).

NNW In conglomeratic horizons at the base of the Stegafjell formation on Havnaholmen and at Vatvik, 02 strains were the first to imprint an obvious tectonic fabric. Pebbles are strong­ Sl ly distorted , chiefly into prolate ellipsoids S ' with k-values in the range 1 - 4.4, depen­ ding on lithology. The long axes of these pebbles are parallel to the associated fold axes and intersection lineations. In pelitic and carbonate layers within the conglome­ SSE NNW rates the S2 foliation is a prominent spaced cleavage. In the coarse-grained, up-sequence brecci­ as of Havnaholmen and Hylla the effects of _--\~~;;;~~~-F.:-~~.,-.:-_S l 02 deformation are confined to thin and dis­ C' continuous, fine-grained sand lenses. Angular and irregular c1 asts, up to 0.5 m in diameter, preserve th eir original clastic sha­ pes, random orientation and pre-unconfor­ mity internal fabrics. As was the case with the 0 1 phase, the 02 strains are not com­ Fig. 7. Diagrammatic cross-sections through the Skei pletely penetrative and some bodies of rock Group. For locations. see Fig. 2. Ornament as in Fig. 2. show little or no evidence of it. NGU - BULL 426. 1994 Brian A. Sturt & Donald M. Ramsay 39

Deformation in the LOC This plethora of structural features reflects conditions operative during the formative The rocks of the LOC have experienced a phase of the ophiolite rather than with oro­ complex sequence of deformation events genic deformation either associated with under conditions varying from high-tempe­ obduction/in-thrusting of the complex or rature 'syn-magmatic' to amphibolite/gre­ occurring later. In the gabbros , dykes and enschist facies probably related to ocean­ lavas, on the other hand, obvious polypha­ floor metamorphism. They have also been sal orogenic strains are reflected in a affected by clearly superimposed deforma­ sequence of foliations and diaphthoretic tions related to regional orogenic activity greenschist -facies metamorphism (see also both prior to and subsequent to the depositi­ Fumes et al. 1988). on of the rocks of the Skei Group. The entire outcrop of the Skei Group lies in Fumes et al. (1 988) emphasize the strongly the overturned limb of a large-scale N-S developed tectonite fabric of the hartzburgi­ trending Z-fold. No evidence of this structu­ te and suggest that "this was due to high­ re can be clearly discerned in the LOC, temperature ductile shearing in the mantle which probably behaved as a more rigid beneath an active spreading ridge". The block in the core of this fold. Tight, large­ present authors have also observed how scale folding in the layered ultramafic seg­ cross-cutting sheets of dunite and pyroxeni­ ment has been determined from way-up cri­ te in this fabric are also strongly folded with teria rather than structural observation (Fur­ axial surfaces parallel to the sheet layering. nes et al. 1988) and does not appear to be Related to such structures, axis-parallel matched in the structure of the overlying mineral lineations are part of the linear tee­ Skei Group. The shearing and diaphthore­ tonite fabric. The essentially syn-magmatic sis which can be attributed to post-Skei nature of this deformation is clearly seen Group deformation in the LOC is not related where folded pyroxenite veins are cut to this folding, and appears to post-date it. abruptly by non-folded pyroxenite veins (see also Fumes et al. 1988), or by gabbroic In southern Leka, the layering of the gabbro dykes. In some of the cumulate ultramafic unit has a mainly ENE-WSW strike and a horizons, flattened folds similar to those in variable but predominantly steep southeas­ the harzburgite are developed, whereas in terly dip (Fig. 8b). In some situations a the layered gabbro only certain horizons strong foliation is developed sub-parallel to show ductile layer-parallel foliation, though this banding. The earliest post-Skei Group a strong mineral lineation parallel to the deformation (D1) is represented by a suite banding is more prevalent. The isotropic of mesoscopic to macroscopic shear zones, gabbro, and in part the layered gabbro, dis­ a few centimetres to several tens of metres plays narrow high-temperature and ductile thick, which can locally become a prominent shear-zones with blastomylonitic textures. feature of the fabric. These range from nar­ That some of these shear-zones were row and sharply defined to broad and more essentially 'syn-magmatic' is seen from evi­ diffuse zones, characterised in the latter dence of small-scale anatexis, producing case by phyllonitic cleavage. On the basis rootless-topless veins and irregular patches of orientation these shear-zones can be dif­ of leucocratic gabbro pegmatite, associated ferentiated into two families (Fig. 8a), with a with such zones. common intersection plunging towards ENE, parallel to their mutual intesection with At higher levels in the LOC pseudostrati­ the igneous layering (Fig. 8). On the cliffs of graphy, lower-temperature mineralised (sul­ Horrfjell in SE Leka the shear-zones define phide-bearing) shear-zones , commonly fol­ the margins of conspicuous large lenticles ded, are observed and may well be related in which the original fabric is still preserved. to ocean-floor metamorphism, particularly In some instances the sense of displace­ as a number of these can be observed to be ment on the shears can be determined from cut by metabasaltic dykes. the deflection of the steep igneous layering 40 Brian A. Stun & Donald M. Ramsay NGU - BULL 426. 1994

N N .. x x ... e. :• e. .. .: :..

a b N N

......

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) • poles to banding in gabbro (crosses). with tie lines to associated shear-zones (dots). (c) - Lineation of amphiboles and feldspars in the sne­ at-zones: (d) - Fold axes (D2) superimposed on shear-zone foliation. at the margins of the zones. On Horrfjell, for In their formation these shear-zones were example, this exhibits a conjugate pattern of attended by considerable reconstitution of small dextral and sinistral offsets. Indeed, the original fabric and mineralogy. At the there is no evidence of large displacement margins there is progressive thinning and on anyone of the shears; rather, the bulk sharpening in the definition of the deflected strain in the gabbro is the sum of small off­ gabbroic layering, emphasized inwards by sets along the many shear-zones . the development of a platy schistosity, coa- NGU • BULL 426, 1994 Brian A. Slur! & Donald M. Ramsay 41

Leka Austra Sertjord

SL

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 a conjugate folds of the schistosity, associa­ diaphthoretic assemblage of actinolite, chlo­ ted with a rude crenulation cleavage. Under rite and albite. In this new foliation a promi­ the influence of the original attitude of the nent mineral lineation of actinolite and feld­ shear-zones these fold axes are somewhat spar may be present, sub-parallel to the dispersed between N-S and NE-SW (Fig. intersection of igneous banding and shear­ 8d). zone. (Fig. 8 c). Where several of these zones come close together or intersect, a Widespread small- and large-scale faulting more general reconstitution has occurred is a prominent feature of the fabric of the and relics of the protolith are sparse. The island, in both the LOC and the Skei Group. diaphthoretic greenschist-facies metamor­ This is attended by brittle shearing but no phism reflected in this mineralogy is isogra­ mineral reconstitution is developed. The dic with the prograde assemblages of the larger faults of this system juxtapose the Skei Group. various levels of the LOC in a number of well-defined blocks. Although no dating is The steep attitude of layering in the LOC yet available, it is possible that this faulting within the core of the anticlinal compliment equates with the prominent Mesozoic block­ of the D1 Leknes Syncline may relate, in faulting which affected much of coastal Nor­ part, to this folding, but may also be a relic way. of some pre-Skei Group attitude. While the highly anisotropic Skei Group deformed by Pre-Skei Group orogenesis congruous folding on a wide range of sca­ les, the more massive gabbro yielded by Prior to the unconformity at the base of the formation of multiple shear-zones, which Skei Group the LOC already possessed a effected a bulk shear flow into the core of tectonic fabric, probably dating back to the the anticline. formation of the complex. This is generally a strong foliation and folding within the The second phase of deformation, D2, in cumulate mafic and ultramafic horizons, but the gabbro can only be recogn ised within it can locally be a mylonitic foliation in high­ the strongly anisotropic shear-zones, where temperature shear-zones. These structures it is expressed as tight angular, upright to bear no geometric or spatial relationship to 42 Brian A. Sturt & Donald M. Ramsay NGU - BULL 426 . 1994 a regional structural pattern . As mentioned pebbles with pre-pebb le fabrics were also earlier, Sturt et al. (1985) recorded relict observed. In being exposed to sub-aerial amphibolite-facies mineralogy in this erosion the LaC must already have been sequence, overprinted by a low-mid green­ elevat ed to high topog raphic levels and had schist facies, isogradic with the prograde experienced tecto nic strains and greens­ metamorphism of the Skei Group. They chist-facies metamorphism prior to the regarded the higher grade as representing depos ition of the cover sequence. Large­ an older pre-Scandian regional metamor­ scale , pre-unconfo rmity folding of Group 1 phism. ophiolites has already been recorded from elsewhere in the Helgeland Nappe Com­ In the coarse flood breccias of the lower plex, e.g. Hodcy (Bang 1985), in the Lyng­ part of the Stegafjell formation , at the wes­ en Nappe in Troms (Minsaas & Sturt 1985), tern end of Havnaholmen (Fig. 2), angular in the Vaqa-Otta area (Sturt et al. 1991) and and irregular clasts are derived from several on Bomlo in Southwest Norway (Brekke membe rs of the ophiolite pseudostrati­ 1983, Nordas et al. 1985). graphy, especially those levels now poorly represented , due to the depth of erosion . Regiona l setting of the Leka Despite two phases of Scandian deformati­ sequences on there is no evidence of mechanical reori­ entation or shape distortio n of these clasts. Placing the LaC and its cover in a regional The presence of an internal tectonic fabric setting is compl icated by the geographic in many is a significant pointer to the tecto­ isolation of the island . However, the assoc i­ nothermal state of the La C prior to erosion. ation of ophiolite with an unconformable Mindful of the ambiguous status of tectonic cover sequence containing polymict conglo­ fabrics in the plutonic members of the ophi­ merate horizons rich in ophiolite debris is olite, the search for evidence of older oroge­ typical of many nappes of the upper alloch­ nic deformation focused on greenschist thons of Norway. This association occurs at pebbles. Cobbles with a randomly orienta­ many localities in the Helgeland area (Sturt ted schistosity, relative to c1ast shape and et al. 1984, Thorsnes & Loseth 1991), with external foliation , or with a folded schistos ity Leka as the largest single occurrence. The (Fig. 10), provide clear evidence of polypha­ ophiol itic bodies are distributed in linear sal orogenic strains prior to the formation of arrays which, on the one hand , delineate the conglomerate. A number of psamm ite nappe boundaries and, on the other hand,

Fig.l0.Boulder of folded. foliated greenstone with deformed epidote knots. Havna Formation. SW Hav­ naholmen. NGU - BULL 426, 1994 Brian A. Sturt & Donald M. Ramsay 43

partition nappes into substrate and cover blende concentrate from supracrustal rocks sequences (Fig. 1). The unconformity is of the southeastern margin of Vestranden at also a valuable datum plane for establishing 440±2 Ma which they interpret as a cooling the base of the cover sequences, the polari­ age through c 500°C. The peak Caledonian ty of the sediments and the lithostratigraphy metamorphism of the Vestranden gneisses in each nappe. is indicated by a Sm/Nd isochron of 432±6 Ma from high-pressure basic granulites of In reviewing the tectonostratigraphic relati­ the Roan Window (Dallmeyer et al. 1992) onships and obduction histories of Scandin­ and a U/Pb lower intercept age of 434±22 avian ophiolite terranes , Sturt & Roberts Ma from discordant zircons in a migmatite (1991) concluded that the obduction of the from in the northwestern part of Vest­ Group I ophiolites (Sturt et al. 1984) in the randen (Schouenberg et al. 1991). These central part of the Caledonides was post­ dates are interpreted by Dallmeyer et al. Tremadoc and pre-Middle Arenig in age. (1992) to indicate peak Scandian prograde This postulate was based on a combination metamorphism. Uplift cooling ages , in Vest­ of faunal dating in cover sequences and randen , are indicated at 419±2 - 393±3 Ma U/Pb zircon dating of late components of for hornblende concentrates and at 395±2 ­ the ophiolites. The Heilhornet Pluton (dated 390±2 for micas (Dallmeyer et al. 1992). at 444±11 Ma), which intrudes the Terrak Unfortunately no 4°Arf39Ar ages are available ophiolite fragment and its deformed and from the HNC. metamorphic cover sequence, provides an upper limit not only for the age of the cover The regional development of a major sequence but also for its deformation and unconformity, commonly above ophiolite metamorphism. Nordgulen et al. (1993) fragments, in the Scandinavian Caledoni­ have also provided a series of reasonably des has been stressed in a number of precise U/Pb zircon dates for other plutons papers (Sturt et al. 1984, Thon 1985, Min­ in the Batholith in the range 447±7 to saas & Sturt 1985, Ramsay & Sturt 1986, 430±7 Ma, i.e. during the period Ashgill- Sturt et al. 1991, Sturt & Roberts 1991, B0e / Early L1andovery. et al. 1993); and particularly in earlier papers this has been assumed to represent The stage of tectonothermal development an individual event horizon. However, it represented here, from approximately Mid becomes apparent that such unconformities Arenig to Mid Caradoc time , apparently associated with major clastic wedges may coincides with deformation of the Arenig­ be more spread in time than has previously L1anvirn sequence of Smela, west-central been assumed. In the Trondheim Nappe Norway (Gautneb & Roberts 1989). It Complex a major unconformity above ophi­ would thus appear to represent a regional olitic material is pre-Mid Arenig in age (Sturt event of some magnitude. For convenien­ & Roberts 1991) whilst in SW Norway the ce, we will here refer to this informally as the major clastic wedge is essentially of Ash­ Terrek Phase. Evidence for a Mid Ordovici­ gill/L1andovery age (Thon 1985) , and in part an orogenic event in certain nappe comple­ overlies the West Karrnoy Igneous Complex xes or terranes in Central and SW Norway , dated in the range 480-470 Ma (Pettersen & broadly equivalent to the Taconian of the Dunning , in press). Thorsnes et al. (in Appalachians, has indeed been presented press) show how the Follafoss Pluton (near by Hall & Roberts (1988). Isotopic data hel­ Maim) , giving a U/Pb zircon age of 460±5 ping to identify this event also in the higher Ma, is unconformably overla in by a basal parts of the Upper Allochthon in Central conglomerate. This situation is very similar Sweden have been reported by Stephens et to that in the Gjersvik Nappe where the al. (1993). There is, however, as yet little basal conglomerates of the Limingen Group evidence for this event from the gneisses or rest unconformably on the Moklevatn gra­ amphibolite-facies supracrustals of the nodiorite which has yielded a U/Pb zircon Vestranden region" though Dallmeyer et al. age of 456±2 (Roberts & Tucker 1991). 4u (39 (1992) record an Art Ar date from a horn- 44 Brian A. Sturt & Donald M. Ramsay GU • BULL 426. 1994

Conclusions HNC rocks, and no trace of the regional 01 phase is present. The geology of Leka plays a significant role The second phase of folding on Leka was in deciphering the lithostratigraph ic and tec­ superimposed on the overturned rocks of tonic patterns of the coastal part of the the Skei Group and the steep attitude of north-central Scandinavian Caledonides, bedding controlled the orientation of fold where neither the Scandian nappe sequen­ axes, with the result that they consistently ce nor its relationships with the cratonic have steeper plunges than is normally basement are well understood. The impor­ encountered in the region. The larger tance of Leka stems from a lower level of asymmetrical step folds of this phase face deformation and metamorphic reconstituti­ eastwards, consistent with their being para­ on than is generally the case. Leka also pro­ sitic to the large Austra Antiform (0 3) on the vided the first record of an ophiolite associa­ mainland, one of a system of upright pericli­ tion in the region, which prompted the nes of regional development. The strain search for and subsequent identification of associated with each of these post-Skei other fragments . These have assumed a Group phases is rather patchy in distribution significant tectonostratigraphic status by and development, and 0 2 may be the defining the outlines of the constituent nap­ oldest tectonic feature in some localities. pes along the western margin of the Helge­ land Nappe Complex. In the source areas It has been widely assumed that deformati­ of these nappes ophiolite formed the sub­ on and metamorphism of the metasedi­ strate for later Ordovician sedimentation ments and ophiolites in Helgeland can be and subsequently formed the 'basement' ascribed to the Scandian Orogeny, Le. Mid­ segment in each of the nappes, when pene­ Late Silurian (Sturt 1984, Stephens & Gee trated by the Mid-Late Ordovician (Terrak) 1985). In this, the nappes were viewed as thrusts. The present discontinuous nature coeval with the climactic nappe phase of the of the ophiolite levels in the region is due, in belt, whose effects extend all the way to the part, to high orogenic strains as well as pri­ orogenic front. With the recognition of the mary pre-orogenic erosion. Terrak Phase, however, it has become apparent that this pattern needs revision. The precise pattern of Scandian deformati­ Thus, by the Late Ordovician, the main tee­ on and metamorph ism in the Skei Group is tonotherma l events of the HNC were over not known. It is probable that 01 and asso­ and the nappes were already in contact with ciated metamorphism relate to the Terrak the sub-Helgeland assemblage. This has Phase and that Scandian deformation does considerable implications for modelling the not have a penetrative imprint. Clear evi­ sequential evolution of the Scandinavian dence for pre-Skei Group deformation and Caledonides and provides a caveat for cor­ metamorphism is, however, clearly seen by relation of events across major tectonic the truncation of fabrics by the unconformity boundaries which may ignore the complexi­ and from cobbles in the conglomerates. ties of palaeogeographies, and of happe­ nings at active plate margins which provide The low strains and well-founded lithostrati­ a diversity of tectonometamorphic proces­ graphy of Leka afford a clearer insight into ses at some sites whilst continuous sedi­ the morphology of the large-scale folds and mentation is occurring at others. the geometric patterns of associated macroscop ic structures, than can be obtai­ ned elsewhere in the region. The whole outrop of the Skei Group defines the over­ Acknowledgements turned limb of a large Z-fold with N-S axial The authors would like to than Oystein ordgulen and an un ­ trend. Although this is the first post-Skei nown reviewer for their critical comments on he manuscrip Group deformation to affect Leka, it has the which have led to many improvements. We wish 0 han Ter­ je Thorsnes for allowing us to use Fig. 1. We are also gra eful geometrical characteristics of 02 on the to Bina Evensen for the final drafting of he illustrations. AVF mainland, in both the nappes and the sub- and NGU are than ed for financial support. NGU - BULL 426. 1994 Brian A. Stun & Donald M. Ramsay 45

Hu smo , T. & Nordgulen , 0 .1988: Structural relat ions along the Refe rences western boundary of the He lgeland Nappe Complex , north­ central Norway (Abstract). VI Annual TSGS Meet ing. Inst. Bang, N. 1985: The stratigraph y and structural de velopment of geo logi. Interne skritter 54, 20-23 . the Rodoy-Haltoy area . outer Vefsnfjord. Unpubl. Cand. SCient. thesis. Un iv. Bergen, 247 pp . Minsaas, O. & Sturt, BA 1985: The O rdovician clastic sequ­ ence immediately overlying the Lyngen Gabbro Complex, Birke land, T. 1958 : Geological and petrological investigations and its env ironmental sign ificance. In: Gee, D.G. & Sturt, in northern Tro ndelag, western Norway. Nor. Geol. Tidsskr. BA (eds), The Caledonide Orogen - Scandinavia and rela­ 38.327-420. ted areas. John Wiley . Chichest er, 379 -394 .

Bre kke. H. 1983 : The Caled onian geological patterns of Moster Nissen, A. L. 1986 : Rb-Sr age determination of intrusive rock s and southern Bomlo. Evidence for Lower Palaeozoic arc in the southeastern part of the Bindal massif, Nord-Tronde­ development. Unpubl. Cand . Real. thesis. Un iv. Bergen, lag , Norway. Nor. geol. unders. Bull. 406, 83-92. 473 pp. Noroas, J., Amaliksen, K.B., Brekke, H., Suthren, RJ., Furnes. Brekke, H. & Solberg. P.O. The geology of Alloy, Sunnfjord, 1987: H., Sturt, BA & Robins, B. 1985: Lithostratigraphy and western Norway. Nor. geol. unders. Bull. 4tO. 73-94. petrochemistry of Caledonian rocks on Bornlo, sou th-west Norway. In: Gee, D.G. & Sturt, BA (eds.) . The Caledonide Bruton, D.L. & Bockelie , J.F. 1982 : Geo logy and palaeontolo ­ Orogen - Scandinavia and rela ted areas. John Wiley, Ch i­ gy of the Ho londa area , we stern Norway - a fragment of cheste r, 679-692. North America ? In: Wones, D.R(ed. ): The Caled onid es in the USA. Virginia Polytech. & State Univ. Dept. Geol. tnst. Nordgu len , 0 . & Sch ou enbo rg, B. 1990: The Cal ed onian Sci. Mem. 2. 41-47 . Heilho rnet Pluton , north -centra l Norwa y: geological set­ ting , radiometric age and imp lications for the SCand inavian Boe, R , Sturt, BA & Ramsay, D.M. 1993: The con glomerates Caledonides. J. Geol. Soc. Lond. 147, 439 -450. of the Sel Group, Otta-Vaqa area , Central Norway: an example of a terrane-linking succession . Nor. geol. unders. Nordgu len, 0 .. Bickford, M.E., Nissen, A.L. & Wortman. G.L. Bull. 425, 1-23. 1993: U/Pb zircon ages from the Bindal Batholith, and the tectonic history of the Helgeland Nappe Complex. Scandin­ Da llme yer, RD., Gee, D.G. & Beckholmen, M. 1985 : "'Arf"Ar avian Caledonides. J. Geol. Soc. London, 150.771-783 . mineral age record of early Caledonian tectonothermal act i­ vity in the Baltoscandian miogeocline. central Scandinavia. Pedersen, R B. & Dunning, G. 1991: U/Pb da tering av oybue­ Am. SCi. J. 285, 532-568. ma gmatisme innenfor det sorvest-norske ofiolitt-terreng. Geonytt 18.42-3. Da llmeyer, RB.• Johansson, L. & Moller, C. 1992 : Chronology of Caledonian high -pressure granu lite -fac ies metamor­ Pedersen, R B. & Dunning , G.R 1993 : Age relat ions between phism. uplift and deformation within northern parts of the oph iolite and island arc complexes. Eettn Planet. Sci.Lett. Western Gneiss Region. Geol. Soc. Am . Bull. t04 . 444­ 455. (in press). Prestvik, T. 1974: Supracrustal rocks of Leka, No rd-Trondelag . Dunning, G.R . & Pedersen, R B. U/Pb ages of oph iolites 1988: Nor. geo l. unders. 311 , 65-87. and arc related plutons of the Norwegian Caledonides: Implications for the development of lapetus. Contrib. Mine­ Prestvik, T. 1980 : The Caledonian ophiolite complex of Leka, ral. Petrol. 98. 13-23. north-cen tral Norway. In: Panayiotou , A. (ed.) , Ophiolites. Proc. tnt. Ophiolite Syrnp. Cyprus 1979,555-566. Furnes, H.• Austrheim . H., Amaliksen , K.G., Nordas , J. 1983: Evidence for an inc ipient early Caledon ian (Cambrian) oro­ Pre stvi k, T . & Roaldseth , E. 1978: Rare earth el ement abun­ genic phase in southwestern No rway. Geol. Mag. t20. 607­ dances in Caledonian metavo lcanics from the island of 12. Leka, Norway. Geoch em. J. 12, 89- 100.

Fu rne s, H., Ped ersen, R.B . & Stillman, C.J. 1988: The Leka Ram say, D.M ., St urt, BA, Ro berts, D. & Zwa an, B.K . 1985 : Ophiolite Complex, central Norwegian Caledonides: fie ld Caledo nides of no rthern Norwa y. In: Ge e, D.G. & Sturt, cha ract eristics and geotectonic signi ficance. J. Geol. Soc. BA (eds) . The Caledonide orogen Scandinavia and rela­ London 145, 401 -412. ted areas. John Wiley. Chichester , 163-184 .

Furnes, H., Roberts, D., Sturt, BA, Thon , A. & Gale, G.H. 1980: Ramsay , D.M . & Sturt , B.A . 1986: T he contrib uti on of the Op hiolite fragments in the Scandinavian Caledonides. In: Finnmarkian O rogeny to the frame wo rk of the Scandinavi­ Pan ayiotou , A. (ed.). Op hiolites Proc. lnt. Ophiolite Symp. an Caledonides . In: Fett es, D.J. & Harris, A.L. (eds) . Syn­ Cyprus 1979, 582-600. thesis of the Caledonian rocks of Britain. Reidel Pub I.Co., 221 -246. Ga utn eb , H. & Roberts, D. 1989 : Geology and petrochemistry of the Smola- Batholith, Centr al Norway. Nor. geol. Ramsay, J.G. 1967: Folding and fracturing of rocks . Mc Graw­ unders. Bull. 416 ,1-24. Hill Co., New York, 568 pp .

Roberts, D. & Gee, D.G. 1985. An introd uction to the structure Hall, L.M. & Roberts, D. 1988 : Timing of O rdovician deformat i­ of the Scandinavian Caledonides. In: Gee, D.G. &Sturt , on in the Caledonian- Appa lach ian orogen. Geol. Soc. Spe­ cial London Publ. 38. 29 1-309. BA (eds). The Caledon ide Orogen - Scandinavia and rela­ ted areas. John Wiley , Chichester, 55-68.

Heldal, T. 1987 : Stratigrafi og strukturell utvikling i Saurenom­ Roberts, D. & Tucker, RD. 1991: U/Pb zircon age of the Mokle rsaet, vest for Bronnoysund, sydlige . Unpubl. vatnet granodiorite, Gjersvik Nappe, Central Norwegian Cand. Scient. thesis, Un iv. Bergen. 300 pp . Caledonides. 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 revised genesis and evolution of the Oceanic timospti ete. Kluwer , interpretation of the Ordovician stratigraphy of Sor-Tronde­ Amsterdam, 745-769. lag, and its implications for the evolut ion of the Scandinavi­ an Caledonides. In: Wones, D.R. (ed.), The Caledonides in Sturt, BA, Roberts, D. & Fumes, H. 198 4: A consp ectus of the USA. Virginia Poly. Inst. & State Univ., Dept. Geol. Sci. Scandinavian Caledon ian ophiolites . In: Gass. I.G., Lip­ Mem. 2, pard , S.J. & Skelton, AW . (eds): Ophiolites and ocea nic lit­ 99-105. nospnere. Geo l. Soc. London Special Publ. 13,381 -389 .

Schouenborg, B.E., Johansson, L. & Gorbatchev, R.G . 1991 : Sturt, BA , Andersen, T.B. & Fumes, H. 1985: The Skei Group, U/Pb zircon ages of basement gne isses and discordant fel­ Leka: an unconformable cla stic sequence overlying the sic dykes from Vestrand en, westernmos t Baltic Shield and Leka Ophiolite. In: Gee, D.G. & Sturt, BA (eds), The Cale­ central Norwegian Caledonides . Geo/. Rundschau 80 , 121­ don ide Orogen - Scandina via and rela ted area s. John 134 . Wiley, Chichester, 395-406 .

Stephens, M.B. & Gee, D.G . 1985 : A tecton ic model for the Sturt , BA, Ramsay, D.M . & Ne umann, R.B. 1991 : The OUa evolution of the eugeoclinal terranes in the Scand inavian Cong lomerate, the Vaga mo Ophiolite - further indications Caledonides. In: Gee , D.G. & Sturt, BA (eds). The Cale­ of Early Ordovician orogenesis in the Scandinavian Cale­ don ide Orogen - Scandinavia and related areas. John don ides . Nor. Geo l. Tidsskr. 71, 107-115. Wiley , Chichester, 623 -652. Then, A. 1985: Late Ord ovician and Early Silurian cover sequ­ Stephens, M.B., Kuller ud, K.. &Claesson, S. 1993 : Early Cale ences to the west Norwegian oph iolites : stratigrap hy and donian tectonothermal evolution in outboard terranes, cen ­ structural evolution. In: Gee, D.G. & Sturt, BA (eds): The tral Scandinavian Caledonides: new constra ints from U-Pb Caledon ide Orogen -Scandinavia and related areas. John zircon dates. J. Geol. Soc. London, 51-5 6. Wiley , Chichester , 407-416.

Sturt, BA 1984: The accretion of ophiolite terranes in the Scandi­ Thorsnes, T. & Loseth, H. 1991: Tectonostratigraphy in the Velfjord­ navian Caledonides. Geo/. Mijn b. 63, 201-12. Tosen region, southwestern part of the Helgeland Nap pe Comple x, central Norwegian Caledo nides . Nor. geo/. Stu rt, BA & Thon , A. 1978: An ophi olit e complex of probable unders . Bull. 421,1-18. early Caledonian age discovered on Karmo y. Nature 275, 538-539. Thorsn es, T.. Bickford, M.E. & Wortman, G . 1994 : Age, geo­ chemistry and geo logical setting of metatona lite from the Sturt, BA & Roberts, 0 .1 991: Tectonostratigraphic relationships Follafoss area , central Scandinavian Caledonides . Nor. and obduction histories of Scandinavian oph iolites. In: Geol. Tidss kr, (in press ). Peters Tj.. Nicholas, A. & Coleman, R.G. (eds). Ophiolite

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