NORWEGIAN JOURNALOF Tectonic structures of the conglomerates 245

Tectonic structural features of the Fauske conglomerates in the løvgavlen quarry, , Norwegian Cale­ donides, and regional implications

David Roberts, Tom Heidal & Victor Melezhik

Roberts, D., Heidal, T. & Melezhik, V. A.: Te ctonic structural fe atures of the Fauske conglomerates in the Løvgavlen quarry, Nordland, Norwegian Caledonides, and regional implications. Norsk Geologisk Tidsskrift,Vo l. 81, pp. 245-256. 2001. ISSN 029-196X.

A quarry excavated in a multilayered, carbonate conglomerate fo rmation of apparent Cambrian age in the Fauske of the Uppermost , Nordland, provides evidence fo r three principal phases of Caledonian deformation. The 60 m-thick Fauske conglomerates are char­ acterised by a rapid, vertical fa cies change from carbonate debris and carbonate breccia to rhythmite conglomero-breccia and graded calcareous greywacke.The carbonate clasts were deposited from southeast-directed turbidity currents on the palaeoslope of a basin that deepened to the southeast. Both the basin physiography and the palaeocurrent directions are opposite with respect to fe atures to be expected along the fo rmer Baltoscandian margin of . The earliest tectonic structures are NW-directed, ramp-and-flatthrust fa ults and fa ult-bend fo lds, that progressed into a higher-strain regime involving clast flattening and elongation, and the development of an amphibolite-facies schistosity, S1. The early staircase thrusts and fo lds there­ fo re record a tectonic reversal of the basinal regime, as well as documenting an opposite structuralvergence with respect to thrust-sheet assembly during Scandian orogenesis. The early structures are overprinted by SE-verging fo lds and small-scale thrusts, and a penetrative, NW-dipping, S2 cleavage. Later minor structures are mainly extensional. The combined palaeodepositional and early structural picture is highly reminiscent of the geological development that has been recorded along the Laurentian margin of the Iapetus Ocean in Cambro-Ordovician time, involving an extensive carbonate shelf and adjacent slope-and-rise. We postulate that the Fauske conglomerates originated along that fo rmer passive margin. The staircase thrusting and early fo lds, and earliest meta­

morphic fabric, s1 , are Jikely to be Taconian (Mid to Late Ordovician). The Fauske Nappe itself, on the other hand, is a part of the SE-translated Caledonian allochthon, generated during the Scandian (Late Silurian-Early Devonian) as a result of Baltica- continent-conti­ nent collision. The second-generation, SE-verging fo lds and minor thrusts, and associated NW-dipping cleavage (52), are almost certainly of Scandian age. Later, extensional structures at Fauske are probably a reflection of the WSW-directed, Early to Mid Devonian shear regime recorded widely in this part of the Norwegian Caledonides.

David Roberts, Tom Heidal & Victor A. Melezhik, Norges geologiske undersøkelse, 7491 Trondheim, .

lntroduction rocks and carbonate conglomerates in this part of the Caledonide allochthon. Some of the highest nappe complexes and thrust sheets The first unit to be described in some detail, within in the tectonostratigraphy of the Scandinavian Caledo­ the NGU project, was the Fauske conglomerate (Mele­ nides are extensively exposed in the county of Nord­ zhik et al. 2000; see also Heidal et al. 1999 for a conden­ land, in north- (Nicholson & Rutland sed version, in Norwegian). This is essentially a mono­ 1969, Stephens et al. 1985). Rock types within these mict carbonate conglomerate, where both the clasts and are extremely varied but a prominent compo­ the matrix are composed of carbonate material. The lith­ nent, in the Uppermost Allochthon and some of the ology is well known in Norway and abroad as an attrac­ Kali nappes of the Upper Allochthon, is that of meta­ tive, colourful, dimension stone, and has been exploited morphosed carbonate rocks and polymict to monomict, at the Løvgavlen quarry near Fauske (Figure l) since carbonate-silicate conglomerates. Until recently, little 1870. Several lithological varieties are present, and research work had been done on such carbonate forma­ quarried, each with its own commercial name. The tions and their carbonate-clast conglomeratic deriva­ diverse lithofacies have been described, bed by bed, in tives. At the Geological Survey of Norway (NGU), an Melezhik et al. (2000). That publication dealt specifi­ ongoing project is dealing specifically with the detailed cally with the sedimentology, depositional environment, mapping, depositional environments, metamorphism isotopic geochemistry and age, deriving a depositional and age of selected carbonate formations in Nordland model that can be employed in estimation of reserves and neighbouring county, aimed at determining and as a prospecting philosophy for deposits of similar the commercial potential of these and other carbonate quality. 246 D. Roberts et al. NORWEGIAN jOURNAL OF GEOLOGY

In the present contribution we report on the main fea­ tory are then discussed in the context of regional geolo­ tures of the polyphase, Caledonian, tectonic deformation gical and inter-plate palaeogeographical development. At of the Fauske conglomerate, including some strain data the outset, we note that the name Fauske conglomerate on clasts of differing viscosity, based on its exposure in (FC) is an informal designation; and its lithofacial, bed­ the Løvgavlen quarry and immediate vicinity. The to-bed variation is such that we will, from time to time, combined basin depositional scenario and structural his- use the plural form - Fauske conglomerates (FCs).

Geological setting

In earlier literature dealing with the geology of this dis­ trict, the principal, thick, carbonate unit was called the Fauske (Vogt 1927, Strand 1972) or the Fauske (Rutland & Nicholson 1965), encompassing both dolomite and an overlying calcite marble. Subsequent detailed mapping and map compilation resulted in the two carbonate formations being united under the term Group (Gustavson 1996). In a more regional context, Nicholson (1974) had earlier introduced the term Fauske Nappe for all the medium­ grade, carbonate-rich successions lying structurally above the high-grade Gasak Nappe and below rocks of the Rodingsfjiillet Nappe Complex (Figure 1). Within the Fauske Nappe, Gustavson (1996) established a lithostra­ tigraphy consisting of 4 separate groups; in ascending order, the Pålsfjell, Rognan, Kjerketind and Øynes Groups. The first three groups each contain marble for­ mations, with the Rognan Group comprising just two unnamed formations - the dolomite marble and calcite marble, mentioned above. The Fauske conglomerate of the present study constitutes a c. 60 m-thick lenticular unit, or member, within the upper, calcite marble forma­ tion of the Rognan Group. A feature of this calcite mar­ ble is the local presence of a particular, pink to maroon, colour-banded, carbonate lithofacies known as the 'Leiv­ set-type' marble. To the northwest of the Løvgavlen quarry, the carbo­ nate rocks of the Rognan Gro up are succeeded by some of the formations of the Kjerketind Group, but to the

Quaternary deposits southwest, and on the Øynes peninsula west of Fauske, D Moralne l::diii Caldte marble/FC the Kjerketind rocks are successively cut out and the polymict conglomerates of the Øynes Group lie uncon­ Caledonian units D Dolomlte marble NAPPE Mlca sdtlst and gneiss formably upon the Rognan carbonate lithologies. In establishing a modem tectonostratigraphy for the ll:�:�_�j schlst and gneiø GASAKNAPPE , Gee et al. (1985), Roberts & R

here, and in Melezhik et al. (2000), are based on natura} outcrops and quarry faces exposed during the years 1997-1999.

Lithologies and lithofacies In the quarry, only the lower contact of the FC is expo­ sed, against the subjacent dolomite marble. Twenty-five beds with thicknesses of 5 cm to 3 m have been recorded and described in the FC, which is characterised by rapid, vertical and lateral, facies changes (Melezhik et al. 2000). The unit comprises carbonate debris, carbonate breccia, rhythmite breccia and rhythmic, graded, calcareous

100metres greywacke. Large blocks, boulders, cobbles, pebbles and smaller clasts are composed mainly of white dolomite marble and colour varieties of calcite marble; white, Clast and matrixcomposition of theFauske conglomerates pink, beige and dark grey ('blue'). A few small clasts of Pink andwhite calcitc anddolomite marblcin quartz�hloritc�citc matrix Prcdominantly "'bbue"'calcite marblc in cak:ite matrix quartzite and vein quartz are present in one particular - Predominantlywhite dolomite marblcin calcitematrix - Pink andwbiæ dolomite and calcite marble in calcite matrix bed. Clasts derived from a local source (white dolomite

Local "basement" and 'blue' calcite marble) are characterised by angular, - Dolomitc marble subangular and platy shapes in cross section, whereas the "( Steike& dip of bedding clasts incorporated from unidentifiable sources (pink, ...--- Thrustfuuhs beige and white calcite marbles) are moderately roun­ ded. Platy fragments commonly exhibit a form of 'imbri­ cation', though with the clasts dipping in the opposite Fig. 2. Geological map of the Løvgavlen quarry, showing the trases of the main thrust fa ults. The Jour main prospecting channels are sense to that expected from the dominant palaeocurrent indicated (Ch.l-4). Thisshows the quarry topography as it was in flow direction; thus suggesting a tectonic rather than 1999. Contour interval- l metre. sedimentary origin for the apparent imbrication. The matrix to the FCs shows a similar range in lithology, con- the isotope geochemical study, by Melezhik et al. (2000), of marble clasts and calcite matrix in the intraformatio­ nal Fauske conglomerate and its immediate dolomite N marble substrate. This showed that the least altered B7SrfB6Sr isotopic value plotted on the Sr-isotope, world­ wide reference curve is consistent with any one of three groups of apparent depositional ages ranging from 520 to 470 Ma, whereas the least altered 013Ccarb value inter­ sects only at 520 Ma; i.e., indicating a probable late Early Cambrian (Landing et al. 1998) age of deposition for these carbonates. o + +tj The Løvgavlen quarry +<>r + t (;eneralfeatures In the immediate vtctmty of the Løvgavlen quarry, exposure is poor (Gustavson et al. 1995). In the quarry itself, which covers an area of c. 500 m x 200 m, quarry­ ing and production strategy has involved the incision of four, 4-6 m-deep, prospecting channels (Figure 2) with near-vertical, smooth walls, which provide excellent sec­ tions through the and lithofacies of Fig. 3. Structural data (and poles to dykes) fromthe Løvgavlen the conglomerate unit. There are also lang, near-verti­ quarry. Lower hemisphere, equal-area projection. Large dots- poles cal, quarry walls cut almost at right-angles to these to bedding; circles- poles to early thrust- surfaces; crosses­ channels. The descriptions and photographs presented poles to S2 cleavage; squares- poles to maficdykes; small dots (in SW)- clast elongation lineation. 248 D. Roberts et al. NORWEGIAN JOURNAL OF GEOLOGY

Structural sequence Inspection of the artificial channel walls in the quarry, and especially those oriented approximately normal to the general NE-SW strike of bedding, provides a fairly clear indication of the sequence of tectonic events follo­ wing the deposition of the FCs. In addition, in parts of the quarry there are quartz veins and a few mafic dykes that cut across pre-existing structures but were themsel­ ves deformed to a minor degree during later compo­ nents of movement. A prominent feature is that the conglomerate clasts show evidence of variable reaction to tectonic deformation, dependent on lithological com­ position. Whitedolomite marble clasts, in particular, are commonly aligned at a moderate to low angle to bed­ ding, dipping northwest; an alignment which appears to Fig. 4. The nature of clast stretching, relating to D 1 deformation, as seen on a quarry wall betweenchannels l and 2, Løvgavlen; looking derive from a mechanical rotation of these comparati­ c. NNW. Thesurface is within 15° of the true clast elongation trend, vely rigid clast types in a yielding matrix. Calcite marble clasts, on the other hand, have responded to this same and is thus dose to the XZp/ane of the D 1 strain ellipsoid. The white clasts are of dolomite marble; the remainder are diverse colour varie­ component of externally imposed deformation by small­ ties of calcite marble. For scale, the large, grey marble clast, bottom scale folding, rather than rotation. Although some clasts, leftand centre, is 6-7 cm thick. particularly those of white dolomite and 'blue' calcite marble, were originally platy, it is apparent that all clasts, whatever their lithology, were strongly deformed and sisting of variable amounts of quartz, muscovite, calcite, elongated during a tectonic event that predated the rota­ sericite, fuchsite and chlorite. A normal grading of clasts tional and/or small-scale folding event noted above. In can be recognised in many beds, and the calcareous grey­ addition to this ubiquitous clast deformation, the quarry wacke layers show both cross-bedding and channelling faces provide text-book examples of ramp-and-flat indicating a normal way-up and with palaeocurrents thrusting with a movement sense which is opposed to directed approximately towards the southeast. The depo­ that generally found elsewhere in the allochthon of the sitional rnodelinvolves a tectonically unstable, carbonate Scandinavian Caledonides. shelf margin traversed by a submarine channel, with the marine basin deepening to the present-day southeast. Full details of the lithofacies and the depositional model can be found in Melezhik et al. (2000).

Structural geology

General features In this part of the region, the many formations within the Fauske Nappe generally show intermediate to steep dips to the northwest, by virtue of their location on the we­ stern flank of an antiformal tectonic window of basement granitic gneisses; the Rishaugfjell window of Nicholson & Rutland (1969) (Figure 1). Local upright folds with stee­ ply NW-dipping axial surfaces interrupt this pattern, and at Løvgavlen the FCs are situated in the SE-dipping short limb of one such fold. In the area of the quarry, the lower o 1'.5 lo parts of the FC unit dose to the subjacent dolomite mar­ logZ!Y ble dip at 20-30° to the SSE, whereas dips increase to 35- 450, and are directed more to the SE, in the highest expo­ Fig. 5. Deformation p lot showing fini te strains measured from sed beds. While the metamorphic grade of carbonate deformed clasts of calcite marble and dolostone, Løvgavlen quarry. The tie lines link measured dolostone (circles) and calcite marble rocks is notoriously difficult to judge, mica schists in ( squares) clasts fromtwo separate localities within the quarry ( n = adjacent groups and formations in this same nappe con­ 20 fo r each clast gro up). The dots represent fivedolostone clasts tain garnet and, in places, staurolite, indicating that PT extracted froma quarried block in Channell. The cross marks the conditions attained those of amphibolite facies. mean value of these clasts. NORWEGIAN JOURNAL OF GEOLOGY Tectonic structures of the Fauske conglomerotes 249

Early deformation episode In terms of the three principal axes of the deformation Structures that can be ascribed to the earliest recogni­ ellipsoid, where X > Y > Z, o ne quarry face is dose to the XZ sable tectonic deformation (D1) in these metasedimen­ plane while the other approximates to YZ. The data tary rocks are of two disparate, though not entirely unre­ are presented in a logarithmic deformation plot (Figure 5), and show dear differences in the finite strains of the lated types: meso-scale, ramp-and-flat type thrusts (l) dolomite and calcite marble dasts. Whereas the mean with associated, thrust-related, fault-bend folds; and (2) deformation ellipsoids of the dolomite marble dasts in strongly deformed (flattened and stretched) calcite and both channels are similar at c. 1.95: 1: 0.30, the ellipsoids dolomite marble dasts. Taking the dast deformation for the calcite marble dasts range from 1.65: 1: 0.22 to first, it is evident that the small-scale falding and dast 2.19: 1: 0.19. The average dimensional shortening across rotation noted above had affectedan already fairly highly Sl (parallel to ranges from 65 to 75% and stretching strained conglomerate. All the dasts, even the originally Z) in the direction of greatest finite elongation varies from platy dasts of white dolomite and 'blue' calcite marble, 125% to almost 200%. Extension has also occurred along show features of extreme flattening though with partly the intermediate axis, Y, by some 40%, indicating that prolate shapes, ascribed to an early phase of deforma­ the overall deformational mode involved a significant tion. The pebbles and cobbles are dearly tectonically component of flattening. stretched, and in the quarry this prominent, dast elonga­ An attempt to extract dasts from blocks, for indivi­ tion lineation plunges at a few degrees to the southwest dual measurement, proved too difficult because of the (Figures 3 and 4). On quarry walls normal to this linea­ extent of recrystallisation. However, just five dolomite tion, the flattened dasts dip at c. 4° to 10° steeper than marble dasts recovered from quarried blocks in channel bedding, and also to the southeast, which accords with l were measured; and their mean ellipsoid value (3.24: 1: the principal metamorphic, schistose fabric (S1) in the 0.38) is dose to that of a plane strain deformation mode groundmass and in calcarenite and silty greywacke inter­ (Figure 5). It is important here to note that had the beds. This relationship, of S1 steeper than bedding (S0), quarry wall been perfectly parallel to the dast elongation also accords with the fact that the beds are younging trend, then the resulting increased X:Z ratios would have normally. brought the other mean values doser to the median, In an attempt to quantify the approximate finite plane strain line, in the field of S/L rather than S> L tec­ strain state of the rock, measurements were made of tonites. Nevertheless, the deformed dasts of calcite mar­ dasts of white dolomite marble and pink to white calcite ble are dearly far more oblate than those of the dolomite marble at separate sites in two of the quarried channels marbles. This is a relationship which conforms with that (channels l and 2). Clasts were measured in the same of reported strain studies, in particular of tie-lines bet­ horizon in two dimensions on two near-vertical, sawn, ween dasts of different viscosities at one and the same quarry walls orientated more or less at right angles to conglomerate locality, and of the predicted displacement each other. These faces are within 15° of the dast-stret­ vectors (Freeman & Lisle 1987); i.e., for a given strain, ching trend, and the normal to that trend, respectively. the K-value for the dast shape should increase in accor-

Fig. 6. (A) Thrust fault cutting up through the multilayered Fauske conglomerate; Channel3, Løvgavlen quarry, August 1997, looking in a direction betweenNE and ENE. The displacement along the thrust surfaceis 7. 6 metres. A fault-bend fold has developed in the hangingwall along the flat,forward of the ramp, the latter cut by a thin maficdyke. Note that a small normal fault has developed just beneath the toe of the fold. The dark layers within the conglomerates are calcareous greywackes. (B) Small thrust fault (bottom right to top left) showing twosepa­ rate rampsand ane short flat (centre of pho to), offsettingalong the fault by c. 2.3 m, in multilayered car bonate conglomerateand darker calca­ reous greywacke; Channe/3, southeasternmost thrust fault (cf. Figure 7), looking ENE. Scale -- the maficdyke to the leftis 8 cm thick. 250 D. Roberts et al. NORWEGIAN JOURNAL OF GEOLOGY

footwalls of ramps. The folds (F1) have c. NE-SW axial trends and face NW, and the fault ramps dip at c. 35-50° towards SE. One of the hangingwall anticlines has deve­ loped furtherin to a tight overturned structure where the beds in the inverted limb are strongly attenuated and dragged into the fault surface (Figure 7). In this particu­ lar case, it is clear that the clast flattening (XY plane) and matrix schistosity are broadly axial planar to the fold. The overall impression, therefore, is that the deforma­ tion mode has evolved gradually, with increasing strain and ductility, from the inaugural thrust detachments along flats and ramps with coeval birth of folds, into a higher grade domain involving initial clast deformation and the generation of a pervasive sl schistosity. Fig. 7. Small thrust fault in conglomerate beds and intercalated Because of the limited exposure, the true extent of calcareous greywackes; looking NE. The thrust displacement here is this staircase thrusting is impossible to judge. Within 1.9 m. Theprominent alignment of clasts fromtap right to bottom channel 3 (53 min length in August 1999), four separate leftcorresponds to the S2 fabric. thrust faults were recorded (Figure 8), two of which had thrust displacements toward NW of 4.3 m and 7.6 m. Although an array of stacked thrusts is almost certainly dance with the viscosity contrast. Here, it should be involved, there are no thrust surfaces yet exposed which noted that there are several unknown factors involved in would qualify as definite floor or roof thrusts - which any quantitative appraisal of finite strain in conglomera­ would otherwise help to define the definite presence of a tes, namely, initial clast shape, possible volume change duplex. At the stage of quarrying in 1999, two of the during deformation, viscosity contrast between clast and thrusts at the southeast end of channel 3 merged at the matrix, clast/matrix volume percentages, and the extent surface, to the 'east' of the channel, at a branch point, in of early compaction. In the case of the FCs, one of the the manner of a rejoining splay. Future quarrying acti­ most important and fundamental conditions in asses­ vity may help to shed further light on the precise geome­ sing the state of strain, namely that there should be no try involved in this imbricate thrust system. ductility contrast between clasts and matrix, appears to Summing up, the early deformation in the FC invol­ be met, at least for the calcite marble clasts in the calcite ved NW-directed, ramp-and-flat thrusting, fault-bend marble matrix. Thus, the data presented, although folding, and the progressive flatteningand NE-SW stret­ sparse, and the evident differencesin strain between clast ching of clasts during the generation of the pervasive lithological types, do provide a useful indication of the metamorphic fabric. Although the staircase faulting was deformation mode and finite strain state of the FCs. evidently the earliest manifestation of the deformation, An important feature, and one most clearly seen in response to an imposed, NW-directed compressive along channel 3, is the presence of thrust faults (Figure stress, this is believed to have given way progressively, 6). These take the form of ramp-and-flat fault structures, with increasing strain, to a more ductile, higher PT i.e., faults that climb up through the multilayers from a regime involving clast flattening and stretching, such bedding-parallel 'flat' via an inclined 'ramp' to a new that we prefer to view this 'early' deformation as one gra­ 'flat', and thus definea staircase trajectory. In three cases, dually evolving process rather than separate, unrelated prominent anticlinal, fault-bend folds have developed in events. The increasing strain may largely relate to the the hangingwall of such thrust faults (Figure 6a); and in stacking, and thus loading and compactional effect, of two cases there are transected synclines preserved in the higher thrust slices during this particular orogenic event.

NW SE

-----

0 metres 1 O 20 30 40 50 60 70

Fig. 8. Sketch of the northeast wall of Channel3, August 1999, drawnto scale, showing the Jour principal thrust faults. The fault-bend fold visible at 20-25 metres fromthe leftis that shown in Figure 6a. NORWEGIAN jOURNAL OF GEOLOGY Tectonic structures of the Fauske conglomerates 251

ted into subparallelism with S2, the more ductile calcite marble clasts are commonly buckled, with the axial sur­ faces of these small folds parallel to S2 (Figure 10). Larger mesoscopic (F2) folds relating to this S2-forming event, and verging SE, are uncommon in the quarry area, but are more prominent elsewhere in the region (Nicholson & Rutland 1969) where they are also seen to deform nappe boundaries. Careful inspection of the actual sur­ faces of the earlier, NW-facing thrusts reveals local examples of very small-scale falding (Figure 11) , with S2 cutting cleanly across both hangingwall and footwall. In other parts of the quarry, two cases of SE-directed reverse-faulting were recorded; one along the short limb of a small F2 fold (Figure 12) and the other parallel to S2• Recognition of structures postdating FiS2 is facilita­ ted by the local presence of useful markers such as mafic dykes and quartz veins. The dykes, up to 15 cm in thick­ ness, intruded after the early phase of thrust-related deformation (Figure 6). They dip to the NW and gene­ rally 10-15° steeper than s2, but they also carry the s2 fabric. Since the dykes are aligned dose to the XY plane of the theoretical F2 strain ellipsoid, they have deformed largely by shear along S2; accordingly, no folded dykes Fig. 9. Conglomero-breccia beds with interlayered calcarenite show­ would be expected in this particular strain regime and ing the (re-)orientation of white dolostone clasts parallelto and none has been recorded. The quartz veins are commonly definingthe S2cleavage. Photograph taken of part of the northeast only a few millimetres thick and they also dip NW, sub­ faceof Channell. Bar scale = 15 cm. parallel to S2• In a few cases, dykes are cut by small, bed­ ding-parallel, extensional faults, with offsets of just a few centimetres. Such late extensional structures would have Later deformation structures been difficult to detect without the help of the mafic The most conspicuous structure that postdates the dykes, although there are also a few cases of extensional thrusts and related folds, and ductile clast deformation displacements of conglomerate clasts, again parallel to described above is represented by a rotation and realign­ bedding surfaces. Quartz veins and veinlets are particu­ ment of white dolomite marble clasts within a spaced larly common in the vicinity of channel 3 and in many cleavage, S2, dipping towards the northwest (Figure 9). In cases show extensional displacements along the vein-fil­ same beds rich in dolomite material, this may appear to led fracture, i.e., downstepping by 2-3 cm to the NW. A be a truly penetrative fabric. However, whereas the dolo­ few veins, however, show reverse (to p SE) offsets. By and mite marble dasts have largely been mechanically rota- large, though, these later components of movement are

Fig. l O. Fauske conglomerate in Channel4, Løvgavlen quarry, showing the small-scale bu ekle falding of the calcite marble clasts. Thewhiter and generally smaller, more competent dolostone clasts, on the other hand, tend to have been reorientated within the prominent S2fabric. (A) Looking NE (B) looking SW. 252 D. Roberts et al. NORWEGIAN jOURNAL OF GEOLOGY extensional. Outside of the carbonate formations in this extensive carbonate bank accumulations in Cambrian to same district, we have recorded late, extensional shear Early Ordovician time along the Laurentian margin of bands in NW-dipping schist units and banded quartzitic the Iapetus Ocean from the Appalachians to central East lithologies, with a general top-WSW sense of shear. Far­ (Rodgers 1968, Swett, 1969, Swett & Smit ther south, on the western flanks of the Nasafjallet base­ 1972, Schwab 1974). There, in some areas, bank-edge ment window, late-Scandian, WSW- to NW-directed, breccias and proximal basin-slope, carbonate conglome­ extensional structures have been reported (Albrecht rates have been reported (Rodgers & Neale 1963, James 2000, Essex & Gromet 2000, Osmundsen et al. 2000, and & Stevens 1986, Schwab et al. 1988), and many of them, in prep.); and >100 km farther northeast, top-W exten­ for example in western Newfoundland, Quebec and New sional displacements characterise the western margin of England, are involved in NW- to W-transported thrust the Rombak window (Rykkelid & Andresen 1994). sheets (Williams 1975, 1995, Stanley & Ratcliffe 1985) which form an integral part of the Taconian (or Taconic) orogen. In , the general tectonostratigraphy for Discussion the Caledonides (Roberts & Gee 1985) comprises the Lower and Middle in the east, which consist The sequence of structural events recorded in the muti­ of rocks indigenous to the Baltoscandian margin. The layered FCs commenced with a distinctive early episode Upper Allochthon has, at its base, the palaeo-conti­ of NW-directed, ramp-and-flat thrusting and coeval nent/ocean transition zone (Seve Nappes) and is succee­ fault-bend folding that, in its later prograde stages, invol­ ded by a series of accreted suspect comprising ved transition into a more ductile, higher strain domain ophiolites and diverse magmatosedimentary associati­ where carbonate clasts were both flattened and elonga­ ons (Koli Nappes) of varied faunal affiliation (Lauren­ ted. It is essential, at this point, that this deformation tian or, in places, Baltican) and unknown, intra-Iapetus cycle be viewed in conjunction with the depositional origins (Roberts 1988, Stephens & Gee 1989). At the top model for the carbonates of the Rognan Group. As of the SE-transported nappe pile, the Uppermost Alloch­ noted, this involved a tectonically unstable carbonate thon is deservedly the most suspect and exotic of all, and bank or shelf margin, submarine slide deposits and has been interpreted as having probably derived from debris flows on the palaeoslope of a basin that deepened the Laurentian side of the Iapetus Ocean (Roberts et al. to the present-day southeast (Melezhik et al. 2000). This 1985, Stephens & Gee 1985, 1989, Stephens et al. 1985, direction is opposite to that expected, and indeed recor­ Andresen & Steltenpohl 1994, Grenne et al. 1999). The ded, from basins fringing Baltica. The NW staircase depositional model and early structural development for thrusting thus represents a form of tectonic reversal of the FCs thus strengthen the notion of a Laurentian affi­ the basinal regime, and a tectonic scenario that is charac­ nity for these rocks, and for the Rognan Group as a teristic of the external zones of passive margins of many whole. Since the Fauske Nappe is reported to contain a orogens, not least the Caledonian-Appalachian belt. continuous lithostratigraphy embracing 4 groups of The orogen-scale implications of these depositional diverse lithologies (Gustavson 1996), then it is reason­ and early structural developments in the FCs are thus, in able to assume that all these rocks were deposited along our view, quite significant. We note here, briefly, that the either the carbonate-dominated shelf or the adjacent, situation described above is most reminiscent of the deeper water, slope-rise environment of the passive Lau­ rentian margin. The highest unit, however, the polymict conglomeratic Øynes Group, marks a changing prove­ nance with a sudden input of igneous material. All these features would also help to support the inference that the rocks now forming the overlying Rodingfjallet Nappe Complex, as well as the Nappe Complex far­ ther south (Gustavson 1975, Nordgulen et al. 1993), have also derived from outboard of this same palaeoconti­ nent, though most likely from a palinspastically more distal, magmatic arc-dominated location similar to that described and elegantly illustrated by Stanley & Ratcliffe (1985). Although the lithological successions and earliest structures in the Fauske Nappe thus appear to be of Lau­ rentian continental-margin origin, the nappe itself is one of several nappes and nappe-complexes that together Fig. 11. Small-scale falding of the thrust surface shown in Fig. 7; constitute the SE-translated Caledonian allochthon. looking NE. The axial surfacesof these small buckles are parallel to Accretion of these nappes, onto the Baltoscandian mar­ the prominent S2fabric in the rock (roughly tap right to bottom left). gin or upon each other, or even, in part, as out-of- NORWEGIANJOURNAl OF GEOlOGY Tectonic slructures of the Fauske conglomerates 253

structures and fault-bend folding are most likely to be Ta conian (Mid to Late Ordovician), as in the Appalachi­ ans of New England, USA, and eastern Canada (Willi­ ams & Stevens 1974, St.Julien & Hubert 1975, Stanley & Ratcliffe 1985, Williams 1995, Robinson et al. 1998). In New England, the age of the main Taconian thrusting and metamorphism is Caradoc-Ashgill (Bradley 1989, Hames et al. 1991), in Quebec it is Llanvirn-Caradoc (Whitehead et al. 1996, Prave et al. 2000), whereas in Newfoundland the assembly and transport of the Taco­ nic allochthons began slightly earlier, in Late Arenig­ Llanvirn time (Stevens 1970, Williams 1995), though with final emplacements extending into the Late Ordovi­ cian. Nearer to Scandinavia, in the Laurentian-margin Fig. 12. Small, SE-verging fold and thrust, looking southwest. successions of Scotland and Ireland, the equivalent Grampian orogeny (or Grampian p hase of the Caledo­ nian orogeny; McKerrow et al. 2000) is now considered sequence thrust sheets, is generally considered to have to be of Late Arenig-Llanvirn age (Friedrich et al. 1999, occurred during two main orogenic episodes; the Late Soper et al. 1999, Oliver et al. 2000). Cambrian to earliest Ordovician Finnmarkian, and the In the case of the Fauske conglomerates, if, as we have Late Silurian-Early Devonian Scandian, tectonothermal suggested, the initial NW thrusting, clast flattening and events. In the case of the Fauske Nappe, forming part of elongation, and s1 fabric developed during one protrac­ the highest allochthon, southeastward translation during ted 'event', then this, too, would almost certainly relate to just the Scandian event seems reasonable, at a time when the Ordovician, Taconian orogenic phase. An alternative Baltica was colliding with, and subducting beneath Lau­ is that the metamorphism peaked separately, in Early rentia. During this stage of orogenic development, the Silurian time, as has been suggested in western New­ Laurentia-margin packages, with their early, Laurentian foundland - corresponding to the Salinian event of (Taconic) structures, were in some way detached and Dunning et al. (1990) and Cawood et al. (1994). Farther transferred on to what are now the upper levels of the north in Norway, however, in the Uppermost Allochthon Scandinavian Caledonide metamorphic allochthon. In in the county of Troms, there is growing evidence from some cases, these rocks and structures are surprisingly some of the nappes pointing to a significanttectonother­ well preserved, as at Fauske, perhaps in (Scandian) low­ mal event in Mid to Late Ordovician time (Dallmeyer & strain zones such as one can observe, e.g., in the short Andresen 1992, Coker et al. 1995, 2000, Selbekk et al. limbs of many macroscopic folds, or in other, favourable, 2000, F.Corfu, pers. comm. 2001), also involving obduc­ strain-shadow situations. In other parts of the Upper­ tion of the Lyngen Ophiolite (Oliver & Krogh 1995). In most Allochthon, the Scandian strains are assumed to our opinion, this metamorphism and deformation have been much higher and pervasive enough to have would equate with the Taconian; and these processes, eradicated almost all signs of the Laurentian origins of including obduction of the Lyngen ophiolitic rocks, are these rocks. Several workers in the central Nordland inferred to have occurred along the Iapetan margin of region have described polyphase Caledonian deforma­ Laurentia. Similar geological and isotopic dating con­ tion involving up to S-6 fold phases, with ductile recum­ straints favouring an intra-Ordovician orogenic event bent fold structures dominating in the earlier stages have been reported from the Helgeland Nappe Complex (Nicholson & Rutland 1969, Farrow 1974, Cooper 1978, in central Norway (Nordgulen & Schouenborg 1990, Tragheim 1982). Marbles, in particular, are isoclinally Nordgulen et al. 1993). folded and refolded in some areas of intense deforma­ The age of the SE-verging F2 folding, associated pene­ tion. Late-stage fold nappes have also been reported trative S2 fabric, and small-scale SE-directed thrusting is (Nicholson 1973). To our knowledge, the only indication also unknown at the present time, but we suggest that this of possible early thrust faults akin to those at the Løvgav­ is likely to relate to, and partly postdate, the Scandian len quarry are the imbricate, pre-F1 slides described by emplacement of the Fauske Nappe onto the developing, Tragheim (1982) from the Sørfolda area c. 50 km north subjacent Upper Allochthon, coeval with and immedia­ of Fauske. These may have been generated as early stair­ tely following Baltica-Laurentia collision. Later structures case thrust faults, i.e., Taconian, and subsequently been in this area, on the other hand, are of extensional charac­ reactivated, ironed out and transformed into ductile ter, and are considered to be part of the widespread slides during the pervasive Scandian orogenesis. Devonian, WSW- to W -directed, extensional regime that While the true age, or ages, of the structures and superceded the main Scandian nappe translation p hase in metamorphic fabrics in the Rognan Group carbonates this part of the north-central Scandinavian Caledonides and FCs are, as yet, unknown, from the above compari­ (Rykkelid & Andresen 1994, Albrecht 2000, Braathen et sons we postulate that the early NW-directed, thrust al. 2000, Osmundsen et al. 2000, and in prep.). 254 D. Robertset al. NORWEGIAN JOURNAL OF GEOLOGY

The mafic dykes that cut across the early, inferred The Fauske Nappe itself - lying above the Gasak Taconian structures in the Fauske conglomerate are the Nappe and below the RodingsfjalletNappe Complex -- is subject of an ongoing study. They can be considered as a part of the SE-translated, Caledonian allochthon, ari­ feeders to volcanic or subvolcanic units in higher level sing in this case from Baltica-Laurentia collision that lithostratigraphical successions, which may not exist produced the Scandian orogeny, in Late Silurian-Early now in this particular region. These sequences most Devonian time. The SE-verging F2 folds and penetrative, likely accumulated in a successor basin, or basins, follo­ NW-dipping S2 fabric in the FCs are inferred to be of wing the Taconian orogenic phase. A possible analogy Scandian age since, within the region, equivalent but lar­ farther north is that of the Balsfjord Group in Troms ger 'F2' folds with associated cleavages deform the nappe (Bjørlykke & Olaussen 1981, Steltenpohl et al. 1990, boundaries and early foliations. The later, extensional Coker et al. 2000), which lies with marked unconformity structures can be viewed as part of the Early to Mid above the deformed Lyngen Ophiolite (Minsaas & Sturt Devonian, WSW- to W-directed, extensional regime that 1985). This -bearing lithostratigraphic unit, of Late is now so widely documented in this part of the Norwe­ Ordovician-Early Silurian age, contains both metabasalts gian Caledonides. Taken as a whole, the tectonic defor­ and carbonate rocks, locally with zinc and lead minerali­ mation sequence in the Fauske Nappe is thus both sations, a combination which is reminiscent of several of polyphase and polyorogenic. the late- to post-Taconic successor basins in the Cana­ dian Appalachians. Acknowledgements. -- We owe an enormous debt of gratitude to Ankerske NS, not only fo r granting access to property but also for carving such superb prospecting channels through the breccias and conglomerates, which helped to reveal this fascinating story of Conclusions sedimentary fa cies development and polyorogenic structural history. Sadly, however, some of the hetter examples of thrusts and fa ult-bend Polyphase tectonic structures affecting a multilayered, fo lds are gradually being removed. We are grateful to Robin Nichol­ carbonate conglomerate formation in the Fauske Nappe son fo r his most helpful and constructive comments on an early ver­ of the Uppermost Allochthon in Nordland are described, sion of the manuscript; and to Peter Robinson fo r our frequent dis­ based largely on exposures revealed during quarrying cussions on northern Appalachian geology. Reviews by Winfried Dall­ operations at Løvgavlen, near Fauske. Deposition of mann, Tony Prave and Peter Robinson provided many useful sugges­ these Cambrian-age, Fauske conglomerates occurred at tions which were helpful in improving the final manuscript. the tectonically unstable margin of a carbonate bank and adjacent palaeoslope of an ocean basin deepening to the southeast. The very earliest structures recognised are References NW-directed, ramp-and-flat, thrust faults and fault­ Albrecht, L.G. 2000: Early structural and metamorphic evolution of bend folds. As a result of a progressively more ductile, the Scandinavian Caledonides: a study of the eclogite-bearing Seve higher strain regime, these folds became tight and Nappe Complex at the Circle, . Dodoral thesis, Uni­ recumbent and the carbonate clasts were both flattened versity of Lund, Sweden. and elongated within the developing, axial-planar schis­ Andresen, A. & Steltenpohl, M.G. 1994: Evidence for ophiolite obduc­ tosity, the pebble stretching lineation parallelling the tion, accretion and polyorogenic evolution of the north NE-SW fold axes. The early staircase thrusting and fold Scandinavian Caledonides. Tedonophysics 231, 59-70. development is thus a form of tectonic reversal of the Bjørlykke, A. & Olaussen, S. 1981: Silurian sediments, volcanics and basinal regime. mineral deposits in the Sagelvvatn area, Troms, North Norway. Norges geologiske undersøkelse 365, 1-38. The combined depositional and early structural pic­ Braathen, A., Nordgulen, Ø., Osmundsen, P.T., Andersen, T.B., Solli, A. ture is reminiscent of that widely documented from the & Roberts, D. 2000: Devonian, orogen-parallel, opposed extension Laurentian passive margin of the Iapetus Ocean in Cam­ in the Central Norwegian Caledonides. Geology 28, 615-618. brian to Early Ordovician time, for example in the north­ Bradley, D.C. 1989: Ta conic kinematics as revealed by fo redeep strati­ eastern US and Canadian Appalachians, northern Scot­ graphy, Appalachian orogen. Tectonics 8, l 03 7 -l 049. land and East Greenland. We interpret the described Cawood, P.A., Dunning, G.R., Lux, D. & van Gool, J.A.M. 1994: Timing of peak metamorphism and deformation along the Appa­ conglomerates, and most of the lithostratigraphy now lachian margin of Laurentia in Newfoundland: Silurian, not Ordo­ composing the Fauske Nappe, as having originated on vician. Geology 22, 399-402. the Laurentian side of Iapetus. Although isotopic dating Coker, J.E., Steltenpohl, M.G., Andresen, A. & Kunk, M.J. 1995: An is lacking, we suggest that the early, NW-directed thru­ 40Arf39 Ar thermochronology of the -Troms region: implica­ sting and related falding is most likely to be of Taco­ tions fo r terrane amalgamation and extensional collapse of the nian (Mid to Late Ordovician) age, and occurred along northern Scandinavian Caledonides. Tedonics 14, 435-447. the Laurentian continental margin as a result of arc­ Coker-Dewey, J,, Steltenpohl, M.G. & Andresen, A. 2000: Geology of continent collision (cf. Stanley & Ratcliffe 1985). The western Ullsfjord, North Norway, with emphasis on the develop­ ment of an inverted metamorphic gradient at the top of the early S1 schistosity is also likely to be Taconian; but a Lyngen Nappe Complex. 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