Cross folds and back folds in the - area, north , and their significance for Caledonian tectonics

MARK G. STELTENPOHL Geological Survey of Alabama, P.O. Box O, Tuscaloosa, Alabama 35486 JOHN M. BARTLEY Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112

ABSTRACT mid-Paleozoic (post-collisional) lateral shear, These include south-vergent D3 cross folds commonly with a sinistral sense. Although (folds formed with axes at a high angle to the The outcrop pattern of Caledonian alloch- Paleozoic strike-slip faults have not been structural trend of the orogen) and northwest- thons in is controlled by identified in the Caledonides at this latitude, vergent D4 back folds (folds that have axes par- two late fold sets: cross folds (F3), which have we suggest that the cross folds may be an allel to the structural trend but that verge in the axes at a high angle to the structural trend of alternative expression of large-scale sinistral opposite sense to that of nappe emplacement).

the orogen, and back folds (F4), which have shear. The kinematics of the late-phase folds bear on axes parallel to the structural trend but verg- two aspects of Caledonian tectonics. First, the ing in the opposite direction from that of INTRODUCTION form of Caledonian gneiss domes in Ofoten and nappe emplacement. Interference of cross Tysfjord is defined by interference of the two folds and back folds produces Ramsay type I The Scandinavian Caledonides comprise a late-phase fold sets. We suggest that this inter- (dome and basin) and type II (boomerang or stack of large thrust nappes emplaced to the ference pattern is repeated along the length of canoe) interference patterns on all scales. The southeast upon Precambrian basement rocks the Norwegian Caledonides. This interpretation Ofoten area contains the closure of a region- and autochthonous cover of the Baltic craton. supports a crustal shortening mechanism for the ally extensive, northeast-trending back fold, The orogen can be divided into three longitudi- formation of the Norwegian gneiss domes, the Ofoten synform. Within its core are nal belts (Fig. 1; Ramberg, 1966), a central syn- rather than diapirism of a buoyant granitic layer klippen of the structurally highest nappes formal belt, containing the highest preserved beneath a denser cover (Eskola, 1949; Ramberg, preserved in this portion of the Caledonides. Caledonian nappes, and two flanking belts of 1966). Second, the change in deformational At Tysfjord, 15 km south of Ofoten, some of structural domes that expose Precambrian pattern expressed by the cross folds may be re- the deepest structural levels within the oro- basement rocks underlying the nappes. The cen- lated to "transpression" (combined strike slip gen are exposed in the core of the Tysfjord tral belt is subdivided into three elongate struc- and convergence; Harland, 1971) of the orogen cross-fold culmination. tural basins separated by transverse culminations during late Caledonian time. The Norwegian Caledonides contain two at Grong and Tysfjord. The tectonostratigraphy of the region is not strike-parallel belts of gneiss domes underlain Mapping at 1:20,000 scale and fabric analysis considered in detail here. Rocks composing the by Precambrian basement rocks flanking an in the Ofoten and Tysfjord areas (Fig. 2) distin- nappe stack are summarized in Gustavson elongate structural basin to which the Ofoten guish four Caledonian deformational events (1966, 1972) and Tull and others (1986). synform in part corresponds. In Ofoten- (Bartley, 1984; Steltenpohl, 1985, 1987b). Details of the structural development are in Tysfjord, these domes and basins reflect a These can be divided into two groups, early- Bartley (1984), Hodges (1985), and Steltenpohl large-scale cross-foid-back-fold interference phase and late-phase deformations. Early-phase (1983,1985,1987b). Structural notations in this pattern. Structural characteristics of the cross deformations occurred before (Dj) or during paper reflect the deformational event which folds and back folds indicate that they formed (D2) the amphibolite-facies metamorphic peak produced them, that is, S2, F2, and L2 formed in response to layer-parallel shortening, im- and include thrust emplacement of cover nappes during D2; S3, F3, and L3 formed during D3; plying that gneiss domes in these areas reflect and southeast-vergent recumbent folding. D[ and so on. interference of folds formed in crustal com- structures are restricted to the cover allochthons

pression rather than diapirism. Because this and may have formed before the allochthons F3 CROSS FOLDS dome-and-basin pattern is present along vir- were thrust upon the Baltic continental margin tually the entire length of the Norwegian Cal- during D2; however, some major F2 folds are A long-standing problem concerning the edonides, we suggest that other gneiss domes cored by basement rocks (Bartley, 1984; structural development of the Scandinavian in Norway may have a similar origin. Hodges, 1985; Steltenpohl, 1985, 1987b), indi- Caledonides is widespread folding about axes The orientation of the cross folds in cating that D2 represents deformation during transverse to the trend of the mountain system Ofoten-Tysfjord suggests a component of sin- and after the climactic collision of the Baltic and (Kvale, 1953). Mechanisms for the development istral shear affecting the orogen after the Laurentian cratons (Hodges and others, 1982; of folds parallel to the shear direction have been Early Silurian and before the Late Devonian. Tull and others, 1986). treated by many authors (for example, see Cob- Paleomagnetic and structural data from other Late-phase deformations, the focus of this bold and Quinquis, 1980). In Ofoten-Tysfjord, parts of the Caledonides suggest large-scale paper, occurred after the metamorphic peak. transversely oriented folds are associated with

Geological Society of America Bulletin, v. 100, p. 140-151, 9 figs., January 1988.

140

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emplacement of the nappes. Late-phase cross folds, however, are not similarly explained be- cause they appear to represent shortening di- N rected at a small angle to the trend of the orogen (see below; also Olesen, 1971). Macroscopic F3 cross folds correspond to the ù>„ northwest-trending F2 folds of Gustavson $0 (1972) and are abundant in Ofoten and Tysfjord (Fig. 2). These are particularly well developed in cover rocks on east Hinnay and in west Skaan- land (Figs. 3, 4). D3 effects on basement rocks area of Figure 2 are best illustrated by the Efjord antiform- Kjcpsvik synform fold couple in Indre Tysfjord (Figs. 5, 6). 4PI Cross-fold styles change markedly from one v / Ty s lithology to another. In schists, quartzites, and amphibolites, the cross folds are kinklike or chevron folds that have subhorizontal long limbs and steeply dipping short limbs. In marbles, the folds are tighter and more sinusoidal. In strongly foliated granite, F3 fold styles resemble those in the cover rocks but are more concentric. In all lithologies and at various scales, the folds occur as roughly periodic groups. These characteristics imply that the F3 cross folds formed at least partly by flexure in response to layer-parallel shortening (Johnson, 1978). 1 Gronq On HinnOy and in western Skaanland, two main fabric elements formed during D3: S3, a weak- to well-developed spaced cleavage, and Figure 1. Geologic map of Nor- L3, a well-developed intersection lineation be- way, illustrating the outcrop pat- tween S2 and S3. The S3 cleavage is most tern of Caledonian cover and Pre- strongly developed in schists, in which it may be cambrian basement. the predominant foliation in outcrops. S3 is poorly developed in marbles and amphibolites, most likely owing to the lack of mica and quartz, the main minerals involved in its for- mation. Some granitoid rocks contain S3 as a , 200 km secondary biotite foliation crossing the main schistosity. In thin section, the S3 cleavage is defined by discrete surfaces spaced a few millimetres apart along which earlier grains, including garnet, pla- highest nappes CALEDONIAN gioclase, kyanite, and carbonates, are bent and/or truncated. Retrograde greenschist-facies ( higher nappes COVER assemblages replace amphibolite-facies minerals along S3 surfaces. The typical retrograde as- semblage in mica schists is chlorite+biotite+ white mica+epidote+quartz (Steltenpohl and PRECAMBRIAN BASEMENT Bartley, 1984). Generally, there is a decrease in cross-fold in- tensity passing eastward across Ofoten. This is illustrated by equal-area plots of the predomi- three different Caledonian deformational events, suggests that they formed in response to east- nant schistosity, S2, passing from Hinnoy, on the

Dj, D2, and D3. Early-phase Fj folds are recog- southeast-directed shear (Hakkinen, 1977; west limb of the Ofoten synform, to Haafjell, nized only in complex interference patterns Bartley, 1984; Steltenpohl, 1985, 1987b). Con- which lies on the east limb near the hinge of the

where their hinges are refolded around F2 sequently, in Ofoten-Tysfjord, early-phase synform (Figs. 7, 8). North-northeast-south- hinges. Owing to the strong degree of overprint- transverse folds probably reflect tectonic move- southwest girdles that reflect strong F3 cross

ing, the kinematics of Dj are unknown. F2 folds ments directed at a high angle to the trend of the folding are gradually replaced by northwest-

have sheathlike forms, and kinematic analysis orogen, consistent with east-southeast-directed southeast girdles in the Haafjell area. F3 minor

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steeply northward, consistent with southward vergence of the folds defined by map patterns; Caledonian Nappes (3) S2 poles form fairly complete great-circle girdles, which reflect tight, cylindrical rather Basement and than conical fold forms; and (4) poles to the S2 Autochthonous Cover girdles correspond to point maxima of observed F3 fold hinges (Fig. 9a). Combined with the low

plunges of F2 and F4 axes (Bartley, 1984; Stel- tenpohl, 1985, 1987b), these data indicate that

the S2 schistosity prior to F3 must have been practically planar and horizontal. The homoaxial character of the cross folds indicates an absence of major heterogeneities in the deformational pattern during D3. Kinematic analysis of the cross folds using refolding of earlier fabric elements (for example, Weiss, 1959) is precluded because F3 hinges are nearly parallel to older fold hinges and linea- tions, and as a result, the orientations of earlier lineations are not greatly modified by F3 folds (Bartley, 1984; Steltenpohl, 1985, 1987b). However, because (1) the fold styles suggest a significant component of layer-parallel shorten- ing, (2) hinge surfaces consistently dip to the north-northeast, and (3) both major and minor folds systematically verge to the south-south- west, the cross folds appear to reflect tectonic transport at a small angle to the strike of the orogen during D3.

F4 BACK FOLDS

Back folds have axes that are parallel to the trend of the orogen but verge in the opposite sense from that of nappe transport. D4 folds in Ofoten are true back folds, in contrast to appar- ent back folds reported in the literature that formed with "normal" vergence but were reori- ented by subsequent folding (for example, Milnes, 1974). Late back folds are becoming more widely recognized (Robinson and Hall, 1979; Milnes and Pfiffner, 1980; Hanley and Redwine, 1986; Brown and Journeay, 1987) and may represent a fundamental feature com- mon to orogenic belts that is not yet understood. In Ofoten and Tysfjord, back folds closely resemble cross folds in every regard except orientation. Minor back folds are characterized by the same range of fold styles and fabrics as a function of lithology as that observed in the Figure 2. Simplified map of the basement/cover contact throughout Ofoten-Tysfjord, depict- cross folds. The descriptive similarities between ing major northwest-southeast-trending cross-fold and northeast-southwest-trending back- back folds and cross folds suggest that although fold hinge traces, after Gustavson (1972), Bartley (1984), Steltenpohl (1985,1987b), Hodges the back folds appear to be somewhat younger, (1985), Tull and others (1986), and J. M. Bartley and P. J. Schubert (unpub. data). (Location the two probably formed closely in time.

shown in Fig. 1.) In areas where back folds dominate, F4 fold hinges form strong, gently plunging, northeast- folds and S3 spaced cleavage also are progres- The fabric data illustrate four general charac- oriented point maxima normal to girdles defined sively less prominent from west to east. An ex- teristics of F3 cross folds: (1) cross folds are by poles to S2 (Figs. 7,8,9b). Whereas poles to ception to this general pattern is northeastern nearly homoaxial, bearing west-northwest-east- the late cleavage generally cluster on stereoplots Hinney, where F4 folds and associated fabrics southeast and generally plunging <20°; (2) as- for areas dominated by cross folds (Fig. 7, sub- dominate (Fig. 7, subarea A; Bartley, 1984). sociated cleavage mainly dips moderately to areas B-F), poles to the late cleavage are

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dispersed along northwest-southeast girdles for F4 folds. The spread of the cleavage poles along Ofoten-Tysfjord area, the Ofoten synform and subareas where both fold sets are present (Figs. 7 these girdles suggests that the late cleavage the Mannfjord antiform. The Ofoten synform and 8, subareas A, G, and H). There is generally began to form during cross folding and was re- corresponds to the northernmost central struc- a point maximum on the girdle in the northwest folded by the back folds. tural basin in Figure 1. The consistent gentle quadrant, reflecting the northwest vergence of There are two major F4 back folds in the southeast plunge of cross folds on Hinncty and in

liiiiP Pipili OFOTEN ROCK UNITS SM: B Niingen Group •SÎV'Î-'i• "r Bogen Group

Evenes Group

Narvik Group

Stangnes Amphibolite

Basal Allochthon

Storvann Group

Basement

STRUCTURAL SYMBOLS

Inverted upright Antiform

Synform

"Early phase" fold

"Late phase" fold(F3,F4)

D| thrust (overturned)

*——D2 thrust (overturned)

High angle fault

Schistosity

6 8 10km

Scale

Figure 3. Simplified geologic map of the Ofoten area, after Bartley (1984), Tuli and others (1986), and Steltenpohl (1987b). (Location shown in Fig. 2.)

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western Skaanland reflects their position on the cambrian basement granite and gneiss that have The great circle that contains both the axial trace northwest limb of the Ofoten synform (Figs. 3, not been mapped in detail. and fold axis corresponds to the axial surface 4). The axial trace of the Ofoten synform (Fig. The attitude of the axial plane of the Ofoten and is oriented N41°E, 40° southeast (Fig. 9c). 2) can be extrapolated nearly 100 km northeast synform is depicted in Figure 9c. The bearing of This orientation is corroborated by 18 F4 minor of Skaanland where klippen of the highest Cale- the axial trace (N41°E) was deduced from S2 fold-hinge surfaces from the same area (Fig. 9d). donian nappes mark the core of the fold (Fig. 1; orientations in the core of the synform in south- The results are also consistent with the distribu- Gustavson, 1972). South of the Efjord culmina- ern Ofoten (Steltenpohl, 1983). The axis tion of cleavage orientations described above tion, the axial trace has not been recognized in (N49°E, 10° northeast) was determined from and indicate that the Ofoten synform is an open the field. It would be difficult to identify here, the visual best-fit great circle described by poles back fold (interlimb angle = 90°, Fig. 8, subarea however, because it appears to pass into Pre- to the S2 regional schistosity for the same area. I; see also Fig. 4, C-C').

o—

-1000

Horizontal Scale 0 2 3 4 5km _JL-

Figure 4. Cross sections through the Ofoten area (see Fig. 3 for locations), illustrating cross-fold (A-A') and back-fold (B-B', C-C') geometries. Open diamonds indicate early fold-hinge surfaces; filled diamonds are late hinge surfaces; rock units shown are labeled in Figure 3.

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The Mannfjord antiform in the Tysfjord area Mannfjord antiform is overturned to the west, that characterize the eastern, more external parts appears to be the eastward antiformal counter- the western limb dipping -70° to the east (Fig. of the orogen (Bjorklund, 1986). We have not part of the Ofoten synform, the two together 6, B-B'). This is clearly the original geometry of recognized any evidence for a major structure forming a large-scale back-fold couple. The the fold because the more gently dipping upright that could have reoriented the Mannfjord Mannfjord antiform clearly postdates cross fold- limb continues eastward through several minor antiform. ing becuase it refolds the Kjopsvik synform and upright folds into the relatively flat-lying units The kinematics of the Mannfjord antiform are associated fabrics and minor structures. The

TYSRJORD ROCK UNITS Upper Nappe Complex Storvann Group Metasedlmentary Slivers Bosement o 1 4km Structural symbols as in Figure 3

Figure 5. Simplified geologic map of the Tysfjord area, after Foslie (1941) and J. M. Bartley and P. J. Schubert (unpub. data). (Location shown in Fig. 2.)

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materials indicate that chevron folds form in re- sponse to layer-parallel compression of multi- layers with weak contacts or weak interlayers that permit distributed flexural slip (Johnson and Honea, 1975). When layer-parallel shear is added to the layer-parallel shortening, chevron folds become asymmetrical, verging in the direc- tion of the applied layer-parallel shear. By con- trast, layered media with significant contact strength form kink bands in response to layer- parallel shortening (Honea and Johnson, 1976) because the strong contacts slip only sporadi- cally. Reches and Johnson (1976) showed that 0 2 4 6 8km when a component of layer-parallel shear is » i i i i added in this case, the kinks become asymmetric in a sense opposite that of the imposed layer- parallel shear. Therefore, depending on interpre-

tation of the fold style, F4 fold vergence carries opposite dynamic implications. In particular, if

the F4 back folds are kink folds, their northwest vergence could result from a southeast-vergent shear couple, the same as that assumed for Caledonian nappe transport. In this case, the fold asymmetry observed for the back folds would reflect a change in material properties rather than the dynamic pattern of the orogen. In a practical sense, chevron and kink-fold styles are difficult to distinguish in the field (Ramberg and Johnson, 1976). Two possible criteria exist, however. First, chevron folds should form periodic fold trains with a charac- teristic wavelength, whereas kinks are aperiodic.

Although the development of F4 structures is sporadic across Ofoten-Tysfjord, in domains Figure 6. Cross sections through the Tysfjord area (see Fig. 5 for locations and for labels of where F4 is prominent, the folds occur as rock units). roughly periodic sets. In the Indre Tysfjord area, A-A'. Efjord antiform and Kjepsvik synform. Note F3 transposition of intercalated alloch- however, some smaller, map-scale F4 folds have thonous basement and metasedimentary rocks in the overturned limb. B-B'. Mannfjord mixed vergence that does not follow patterns antiform. Note that basement on the west end of section overlies the cover because it lies in the predicted for folds parasitic to the Mannfjord

overturned limb of the Kjepsvik synform. D4 D.D.Z. (ductile deformation zone) is a zone of antiform (Fig. 5). These folds form isolated fold intense D4 strain in the overturned limb of the antiform. Southward, the hinge of the Kjctpsvik couples with hinge-surface traces that are short synform is sheared out along this zone, which explains Foslie's (1941) observation that there is compared to fold amplitudes. These folds are no mirror-image repetition of cover units across the synform south of Kjepsvik. therefore probably kink folds, but inconsistent overturning implies that layer-parallel shear was minimal and layer-parallel shortening was pre-

dominant in forming these D4 folds. constrained by an intense L4 stretching lineation F4 folds are consistent with the interpretation that plunges east to east-southeast south of that the back folds have not been reoriented by a Second, concentric folds and more sinuous Kjepsvik and northeast north of Kjopsvik. The later event and thus reflect west-northwest- fold forms will not be confused with kink folds. variation of the strain field at the north end of directed transport. F4 folds in marbles and granite gneiss more the fold probably reflects complexities near its closely resemble the former mentioned style termination. If this is taken into account, the FOLD STYLES AND MECHANICS classes. They generally share the sense of ver- Mannfjord antiform resembles the F4 folds on gence of the sharp-hinged F4 folds in schists, Hinnay and in Ofoten, in that D4 transport at On the basis of fold forms alone, the sharp- quartzites, and amphibolites, which implies

Tysfjord appears to have been directed toward hinged folds typical of F4 in quartzites, schists, that the latter are predominantly chevron folds the west-northwest. More intense penetrative de- and amphibolites could be interpreted as either rather than kink folds. These observations formation accompanied F4 at Tysfjord, which is chevron (sharp-hinged periodic folds with a indicate that F4 folds do imply a reversal consistent with the deeper level of exposure im- characteristic wavelength) or kink folds (sharp- in the sense of layer-parallel shear from that plied by regional exposures of basement. hinged aperiodic folds which occur as discrete during nappe emplacement. The back folds are the last Caledonian fold fold couples); to which type they belong has The most important observation regarding F3 phase recognized in Ofoten-Tysfjord. The uni- consequences for assessing the mechanical signif- and F4 fold styles is that they are characteristic form hinge trend and consistent southeast- icance of the back folds (Johnson, 1978). Theo- of folds formed in response to layer-parallel dipping hinge-surface orientation of the minor retical studies and experiments with analog shortening. F3 folds occur in periodic sets on a

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scale o * 4 • • n >•

132=301 S,.4 = 7I • S2= 120 • • S3i4 = 37

0 Fs.4=I74 O F,.„ = 14 L ,4=69 • s • 1-3,4 = 81

S3(4=2I

O FSi4 = I80

• 1-3,4*57

Figure 7. Subarea distribution and equal-area plots of S2 (left-hand plots) and S3, S4, F3, F4, L3, and L4 (right-hand plots) for north Ofoten.

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Figure 8. Subarea distribution and equal-area plots of S2 (left-hand plots) and S3, S4, F3, F4, L3, and L4 (right-hand plots) for south Ofoten. Contours for subarea I are: S2—1%, 2%, 7%, 9%, and 13% per 1% area; F3, F4, L3, L4—1%, 2%, 4%, 5%, 9%, 13%, 18%, and 21% per 1% area.

variety of wavelengths, and there is a general buckling mechanism, in which bending and Cooper and Bradshaw, 1980) or as fold- correspondence between thickness of the folded layer-parallel shortening are localized in the interference structures resulting from basement layers and fold size. Along with the fold forms hinges whereas the limbs undergo rotation and shortening (Ramsay, 1967; Naylor, 1969, 1975; themselves, these are properties of folds initiated layer-parallel shear. Concentration of cleavage Bartley, 1981b; Hodges and others, 1982). Be- in response to a buckling instability; such folds in hinges is unlikely in folds formed in passive cause the amplification rate of an irregularity are unlikely to have formed by other mecha- shear because this generally implies higher due to buoyancy instability depends on wave-

nisms (Johnson, 1978, Chapter 1). Although F4 strains in limbs than in hinges (Ramsay, 1967). length (Biot and Od6, 1965), diapiric gneiss folds locally are aperiodic and resemble kink domes should have characteristic wavelengths bands, these only form when layer-parallel NORWEGIAN GNEISS DOMES just as do structures formed by buckling instabil- shortening dominates over layer-parallel shear AS A CONSEQUENCE OF ity. Ramberg (1966) compared Norwegian (Reches and Johnson, 1976). CRUSTAL SHORTENING gneiss domes to diapirs produced experimentally Layer-parallel shortening is further indicated using analog materials in a centrifuge. Ramberg by the observation that the coeval spaced cleav- Mantled gneiss domes have been interpreted pointed out the presence of two prominent sets age is generally more intense in fold hinges and either as diapirs not directly related to crustal of gneiss domes in the Norwegian Caledonides weaker in limbs. This is also consistent with a shortening (Eskola, 1949; Ramberg, 1966; between the Grong and Tysfjord culminations,

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N N the antiformal crests of major basement-cored folds of both fold sets (for example, the Efjord and Mannfjord antiforms at Tysfjord), indicat- ing that the antiformal crests experienced layer- parallel shortening. By contrast, in Dixon's (1975) analog models, the cover of diapiric crests recorded strong layer-parallel extension; in fact, this was where the maximum finite-strain magnitudes were recorded in the models. Third, the distribution of strain in the basement also appears to conflict with predicted distributions in the buoyant layer of stress (Fletcher, 1972) and strain (Dixon, 1975). The highest stresses and strains in the buoyant layers of model dia- pirs are located at the center of the domes, yet our field observation is that visible evidence for strain, in the form of intense tectonic foliations and lineations, disappears in the cores of the domes. Instead, displacement appears to be con- centrated in high-strain zones along the flanks of N n the domes, consistent with compressive shear zones along the flanks being responsible for up- lift of the domes. Therefore, a diapiric origin for the major late folds appears to conflict with field observations. We interpret the basement highs to be major folds that formed in response to regional hori- zontal shortening. Diapirism may have played a role in the ultimate form of these structures but was not the principal cause. Post-nappe-emplacement layer-parallel short- ening in Ofoten may have been caused by the need for the stack of thrust nappes to thicken as thrusting propagated cratonward (Davis and others, 1983). A thickening mechanism pro- posed by Hodges and others (1982) for this re- gion was downward stepping of the basal Caledonian thrust into the basement during later

Figure 9. Geometric summary of F3 and F4. (a) Cross folds. Great circles are visual best fits phases of Caledonian deformation. Thickening of poles to S2 for indicated subareas; filled circles are visual best fits of combined F3 and L3 of the stack may also have been accomplished modes, (b) Back folds; symbols same as in a. (c and d) Estimate of the axial surface of the by back folding, however (compare with Davis Ofoten synform (see text). Contours are 6%, 12%, 17%, and 23% per 1% area. and others, 1983). This would require that back folding overlapped in time with the youngest and most external thrusts in the orogen. The age of external thrusts is poorly constrained at lat an eastern set that has a spacing of -200 km and is analogous to recent interpretations of base- 68°N., but farther south, movement ceased near a western set that has a spacing of 20-80 km ment massifs in the Scandinavian Caledonides the Silurian-Devonian boundary (Gee, 1975). (Fig. 1), which he interpreted to reflect a charac- and other orogens (Thelander and others, 1980; The rather loose timing constraints on D3 and teristic wavelength resulting from diapirism. Hossack and others, 1981; Chapman and others, D4 (see following section) are consistent with Cooper and Bradshaw (1980) did not consider 1981; Boyer and Elliott, 1982; Hodges and oth- this, but the data do not demand such an the observed distribution to be sufficiently regu- ers, 1982; Steltenpohl, 1987a). We favor this interpretation. lar to support Ramberg's model if the basement interpretation for the gneiss domes in the Regional patterns illustrated in Figure 1 sug- were homogeneous but suggested that irregulari- Ofoten-Tysfjord area for three primary reasons. gest that the basin-and-dome fold interference ties in spacing reflect selective upwelling due to First, the outcrop pattern of the basement-cover pattern we describe in Ofoten-Tysfjord persists compositional heterogeneities. contact (Fig. 2) is demonstrably controlled by along the length of the Norwegian Caledonides On the other hand, Bartley (1981b) argued, the F3 cross folds and F4 back folds, the geomet- in the form of basement windows and structural on the basis of structure, that the Norwegian rical properties of which indicate a substantial basins. We suggest that the mantled gneiss gneiss domes reflect crustal shortening, possibly component of layer-parallel shortening. Second, domes in Norway generally may owe their forming as "hanging wall anticlines" above duc- coeval spaced cleavages occur at a high angle to origin to crustal shortening during post-colli- tile thrust zones in the basement. The suggestion compositional layering in cover rocks forming sional convergence rather than diapirism.

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F3 CROSS FOLDS AS EVIDENCE 2,000 km) in this southern portion of the ing of the basement to produce the D3 antiforms FOR LATE CALEDONIAN Caledonides. is consistent with the general form of the late TRANSPRESSION Several basins of Devonian clastic sedimen- folds and represents an interesting model, which tary rocks in are syndeposi- we unfortunately lack the data to properly If F3 cross folds reflect layer-parallel shorten- tionally bounded by thrusts and high-angle evaluate. Oblique convergence appears to be ing in a direction at a low angle to the Caledo- faults. Nilsen (1973; 1979, personal commun.) necessary in Ofoten-Tysfjord regardless of the nian orogen, then the obliquity of this shortening suggested that the high-angle faults are sinistral basement deformation style, however. The cover direction to the trend of the orogen implies a strike-slip faults of possible major displacement; above an antiformal duplex formed in response component of sinistral shear. Although the de- however, more recent studies of sedimentation to northwest-southeast shortening should be tails are controversial, post-collisional lateral patterns within the Devonian basins suggest that shortened in the transport direction; however, it shear, much of it sinistral, appears to be a wide- horizontal displacements along the high-angle is most likely stretched perpendicular to trans- spread and important aspect of the Caledonian faults are predominantly dextral (Steel and port because draping of the cover across the orogen. On the basis of this evidence (see discus- Gloppen, 1980). The approximate east-west stacked-up basement will increase the length sion below), we propose that F3 may record a orientation of the basins is appropriate for pull- of the basement-cover contact in that direction. transition from simple convergence to sinistral apart basins produced in a dextral shear system. By contrast, observed cleavage and minor folds "transpression" (Harland, 1971) of the orogen We note that both sinistral and dextral dis- in the F3 antiforms indicate that the cover during the late Caledonian. placements may have occurred throughout the was shortened perpendicular to F3 hinges, In the British Caledonides, the Great Glen Caledonian orogen; Mosher (1983) described requiring a major component of shortening in fault has long been recognized as a major such a complex history in the Narragansett basin this direction. Devonian-Carboniferous strike-slip fault. Al- in the northern Appalachians. Against the transpression hypothesis is the ab-

though there is disagreement regarding the mag- The F3 cross folds are constrained to have sence of reported late Caledonian strike-slip nitude and sense of displacement, most workers formed between Early Silurian and Late Devo- faults in northern Norway. On the basis of tim- prefer sinistral movement with a cumulative nian time. The Caledonian metamorphic peak, ing constraints alone, northeast-trending high-

displacement between 100 and 300 km. W. synchronous with D2 deformation, can be in- angle faults that we have recognized on Hinnay Gibbons and R. A. Gayer (unpub. data) sug- ferred to have been after the Early Silurian on and in Ofoten (Fig. 3) could have formed during gested that the Great Glen is but a minor fault in the basis of lithologic correlation of a distinctive, either Paleozoic wrench faulting or Mesozoic a series of eight major transcurrent faults which although highly transposed and metamorphosed, rifting before opening of the Norwegian Sea subdivide the British Caledonides into as many rock sequence in Ofoten ( Group) with (Bartley, 1981c; Steltenpohl, 1985,1987b). The as nine terranes. Although the senses of dis- low-grade Lower Silurian fossiliferous rocks latter hypothesis is favored, however, because placement are unknown, these authors noted (Balsfjord Supergroup, Binns, 1978) 100 km to matching of offset Caledonian fold hinges indi- that the most important movements took place the north (Steltenpohl and Bartley, 1984; Stel- cates that net slip across these faults is predomi-

in the Late Silurian. tenpohl and others, 1985,1986). Because the F3 nantly down dip. Harland and Gayer (1972), Harland and oth- cross folds fold structural elements formed dur- Therefore, we suggest that F3 cross folds indi- ers (1974), and Harland (1978) suggested that as ing this metamorphism (Bartley, 1984; Stelten- cate shortening at a low angle to the trend of the much as 200-1,000 km of sinistral displacement pohl, 1985, 1987b), the cross folds must be orogen and express a component of sinistral occurred along the Billefjorden fault zone on younger than Early Silurian. shear. Interpretation of these folds as indicative Spitsbergen, Svalbard, during the Late Devo- A minimum age is given by Rb/Sr biotite- of regional transpression will remain speculative, nian. Ziegler (1978) suggested that the Bille- whole-rock isochrons and K/Ar biotite ages however, unless more direct evidence for sinis- fjorden fault zone may represent the northern from basement and cover rocks from throughout tral shear is recognized. extension of a major Devonian transform which the region. The ages range from 347 to 393 Ma, connects with the Great Glen fault, Scotland, but most are between 360 and 370 Ma (Bartley, SUMMARY and the Cabot fault, Newfoundland. If so, such a 1980,1981a; Hodges, 1985), which corresponds connection would be located within the Caledo- to the Late Devonian. These ages are interpreted The outcrop pattern of Caledonian alloch- nian orogen to the west of our study area. Lamar to place a minimum age on growth of new bio- thons in the Ofoten-Tysfjord region is primarily and others (1986), however, indicated that strike tite along the S3 and S4 cleavages and therefore controlled by two late-phase fold sets, F3 cross slip probably has not occurred along the Bille- provide a minimum age for the late fold phases. folds and F4 back folds. The cross folds and

fjorden fault zone since deposition of the Old The age and kinematic interpretation of F3 back folds formed in response to layer-parallel Red Sandstone. cross folds thus are compatible with the view compression, and their interference resulted in Paleomagnetic data from Upper Devonian that they formed in response to regional sinistral Ramsay type I and type II interference patterns. continental red beds in the northern Appala- shear of the orogen. An alternative interpreta- This interference pattern appears to be repeated chians indicate a Devonian paleolatitude for tion is that the major cross-fold culminations along the length of the Norwegian Caledonides. New England as much as 20° south of that for reflect underlying basement duplexes with crests The Precambrian basement windows corre- Upper Devonian rocks of the adjacent North that trend at a high angle to the orogen and spond to structural domes, and the central chain American platform (Kent and Opdyke, 1978; intervening synforms reflecting footwall lateral of structural lows, which contain klippen of the Van der Voo and others, 1979). These data are ramps. M. Stevens (1986, personal commun.) highest preserved Caledonian nappes, are the as- interpreted to indicate very large-scale sinistral has suggested that in this case, oblique conver- sociated basins. On the basis of this and our strike-slip displacements (possibly as much as gence during D3 may not be required. Duplex- observations in Ofoten-Tysfjord, which indicate

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1981c, Mesozoic (?) high-angle faults, east Hinnoy, north Norway: Quarterly Journal, v. 109, p. 51-73. that the Precambrian basement highs in these Norsk Geologisk Tidsskrift, v. 61, p. 291-296. Lamar, D. L., Reed, W. E., and Douglass, D. N., 1986, Billefjorden fault zone, areas formed in response to layer-parallel com- 1984, Caledonian structural geology and tectonics of east Hinnoy, north Spitsbergen: Is it part of a major Late Devonian transform?: Geological Norway: Norges Geologiske Undersokelse, v. 396, p. 1-24. Society of America Bulletin, v. 97, p. 1083-1088. pression, we suggest that other Norwegian gneiss Binns, R. E., 1978, Caledonian nappe correlation and orogenic history of Milnes, A. G., 1974, Post-nappe folding in the western Lepontine Alps: Eclogae Scandinavia north of latitude 67N: Geological Society of America Bul- Geologicae Helvetiae, v. 67/2, p. 333-348. domes may owe their origin to crustal letin, v. 89, p. 1475-1490. Milnes, A. G., and Pfiffner, O. A., 1980, Tectonic evolution of the Central Alps shortening. Biot, M. A., and Od6, H., 1965, Theory of gravity instability with variable in the cross section St. Gallen-Como: Eclogae Geologicae Helvetiae, overburden and compaction: Geophysics, v. 30, p. 213-227. v. 73, p. 619-633. Prior to F3 cross folding, early-phase struc- Bjorklund, L., 1986, The Middle and Lower AHochthons in the Akkajaure- Mosher, S., 1983, Kinematic history of the Narragansett basin, Massachusetts Tysfjord area, northern Scandinavian Caledonides, in Gee, D. G., and and Rhode Island: Constraints on late Paleozoic plate reconstructions: tures record east-southeast-directed tectonic Sturt, B. A., eds., The Caledonide orogen—Scandinavia and related Tectonics, v. 2, p. 327-344. transport, which is considered to be the "nor- areas: New York, J. Wiley & Sons, p. 515-528. Naylor, R. S., 1969, Age and origin of the Oliverian Domes, central-western Boyer, S. E., and Elliott, D., 1982, Sub-thrust geology beneath metamorphic New Hampshire: Geological Society of America Bulletin, v. 80, mal" direction for emplacement of the Caledo- terranes, Blue Ridge province, North Carolina: Geological Society of p. 405-428. America Abstracts with Programs, v. 14, p. 450. 1975, Basement rocks in the northern Appalachians and plate-tectonic nian nappes. F3 cross folds formed between the Brown, R. L„ and Journeay, J. M., 1987, Tectonic denudation of the Shuswap hypotheses: Geological Society of America Abstracts with Programs, Early Silurian and the Late Devonian and re- metamorphic terrane of southeastern British Columbia: Geology, v. 15, v. 7, no, l,p. 98-99. p. 142-146. Nilsen, T., 1973, Devonian (Old Red Sandstone) sedimentation and tectonics cord a major shift in the Caledonian displace- Chapman, J. J., Gayer, R. A„ and Williams, G. D., 1981, Structural cross of Norway: American Association of Petroleum Geologists Memoir 19, sections through the central Caledonides and timing of the p. 471-481. ment pattern to a shortening direction at a low Finnmarkian event [abs.j: Terra Cognita, v. 1, p. 39. Olesen, N. O., 1971, The relative chronology of fold phases, metamorphism angle to the trend of the orogen. Later, but Cobbold, P. R., and Quinquis, H., 1980, Development of sheath folds in shear and thrust movements in the Caledonides of , north Norway: regimes: Journal of Structural Geology, v. 2, p. 119-126. Norsk Geologisk Tidsskrift, v. 51, p. 355-377. nearly coeval, F4 back folds reflect another shift Cooper, M. A., and Bradshaw, R., 1980, The significance of basement gneiss Ramberg, H., 1966, The Scandinavian Caledonides as studied by centrifuged domes in the tectonic evolution of the Salta region, Norway, in Phillips, dynamic models: Geological Institute of the University of Uppsala Bul- which resulted in northwest-vergent shearing. W.E.A. and others, eds., Deformation and metamorphism in the Cale- letin, v. 43, p. 1-45. We propose that the F3 cross folds record a shift donide orogen: Geological Society of London Quarterly Journal, v. 137, Ramberg, I. B., and Johnson, A. M., 1976, Part V. Asymmetric folding in p. 231-240. interbedded chert and shale of the Franciscan Complex, San Francisco from simple convergence to transpression along Davis, D., Suppe, J., and Dahlen, F. A., 1983, Mechanics of fold and thrust Bay area, California: Tectonophysics, v. 32, p. 295-320. belts and accretionary wedges: Journal of Geophysical Research, v. 88, Ramsay, J. G., 1967, Folding and fracturing of rocks: New York, McGraw- a plate boundary located somewhere west of the p. 1153-1172. Hill, 568 p. present-day Norwegian coast. Dixon, J. M., 1975, Finite strain and progressive deformation in models of Reches, Z., and Johnson, A. M., 1976, Part VI. Asymmetric folding and diapiric structures: Tectonophysics, v. 28, p. 89-124. monoclinal kinking: Tectonophysics, v. 35, p. 295-334. Eskola, P., 1949, The problem of mantled gneiss domes: Geological Society of Robinson, P., and Hall, L. M„ 1979, Stratigraphic-tectonic boundaries related London Quarterly Journal, v. 104, p. 461-476. to Acadian convergence in southern New England [abs.]: Geological ACKNOWLEDGMENTS Fletcher, R. C., 1972, Application of a mathematical model to the emplace- Association of Canada- Mineralogicat Association of Canada, Joint ment of mantled gneiss domes: American Journal of Science, v, 272, Annual Meeting, Program Abstracts, no. 4, p. 74. p. 197-216. Steel, R., and Gloppen, T. G., 1980, Late Caledonian (Devonian) basin forma- This work was supported by National Science Foslie, S., 1941, Tysfjords Geologi: Norges Geologiske Undersokelse, v. 149, tion, western Norway: Signs of strike-slip tectonics during infilling: In- 298 p. ternational Association of Sedimentologists Special Publication, v. 4, Foundation Grant EAR-8107525 to Bartley and Gee, D. G., 1975, A tectonic model for the central part of the Scandinavian p. 79-103. Caledonides: American Journal of Science, v. 275-A, p. 468-515. Steltenpohl, M. G., 1983, The structure and stratigraphy of the Ofoten synform, by awards from the Reynolds Foundation, Gustavson, M., 1966, The Caledonian mountain chain of the southern Troms north Norway [M.S. thesis]: Tuscaloosa, Alabama, University of Ala- Norges Geologiske Undersôkelse, and the Uni- and Ofoten areas. Part 1. Basement rocks and Caledonian metasedi- bama, 106 p. ments: Norges Geologiske Undersokelse, v. 239,162 p. 1985, The structural and metamorphic history of Skaanland, north versity of Tromsô to Steltenpohl. Thanks go to 1972, The Caledonian mountain chain of the southern Troms and Norway, and its significance for tectonics in Scandinavia [Ph.D. thesis]: Ofoten areas. Part IH. Structures and structural history: Norges Geolo- Chapel Hill, North Carolina, University of North Carolina, 181 p. A. Andresen, R. Boyd, D. Roberts, M. Gustav- giske Undersokelse, v. 283, 56 p. 1987a, Geology of the Pine Mountain imbricate thrust zone, Opelika, son, D. Bruton, W. Griffin, and all our friends at Hakkinen, J. W., 1977, Structural geology and metamorphic history of western Alabama: Geological Society of America Abstracts with Programs, Hinnoy and adjacent parts of eastern Hinnoy [Ph.D. thesis]: Houston, v. 19, p. 131. the University of Tromso, Norges Geologiske Texas, Rice University, 161 p. 1987b, Tectonostratigraphy and tectonic evolution of the Skaanland Hanley, T. B„ and Redwine, J. C., 1986, The Bartletts Ferry and Goat Rock area, north Norway: Norges Geologiske Undersokelse, v. 409 (in press). Undersokelse in Trondheim, and the Mineralo- fault zones north of Columbus, Georgia, in Neathery, T. L., ed., South- Steltenpohl, M. G., and Bartley, J. M., 1984, Kyanite-grade metamorphism in gisk-Geologisk Museum in Oslo for logisti- eastern section of the Geological Society of America centennial field the Evenes and Bogen Groups, Ofoten, north Norway: Norsk Geologisk guide: Boulder, Colorado, Geological Society of America, v. 6, Tidsskrift, v. 63, p. 21-26. cal support, hospitality, and friendship; to p. 291-296. Steltenpohl, M. G., Andresen, A., and Tull, J. F., 1985, Evidence for a litho- Harland, W. B., 1971, Tectonic transpression in Caledonian Spitzbergen: stratigraphic correlation between the Salangen Group (Ofoten) and A. Andresen, K. Hodges, R. Boyd, P. Crowley, Geological Magazine, v. 108, p. 27-41. Balsfjord Group (Troms) and its regional implications [abs.]: Geolog- C. Burchfiel, J. Tull, A. Barker, A. Welbon, 1978, The Caledonides of Svalbard: 1GCP Project 27, Caledonian- nytt, v. 20, p. 47. Appalachian orogen of the North Atlantic region: Canada Geological 1986, Lithologic correlation between the Evenes/Bogen Groups (Of- P. Schubert, and L. Wood for helpful discus- Survey Paper 78-13, p. 3-11. oten) and the Balsfjord Group (Troms): Evidence for the Finnmarkian Harland, W. B., and Gayer, R. A., 1972, The Arctic Caledonides and earlier unconformity, north Norway: Geological Society of America Abstracts sions; to the University of North Carolina, oceans: Geological Magazine, v. 109, p. 289 -314. with Programs, v. 18, n. 6, p. 762. W. Smith, and the Geological Survey of Ala- Harland, W. B., Cutbill, J. L., Friend, P. F., Gobbett, D. J., Holliday, D. W„ Thelander, T., Bakker, E., and Nicholsen, R., 1980, Basement-cover relation- Maton, P. I„ Parker, J. R., and Wallis, R. H., 1974, The Billefjorden ships in the NasalQellet Window, central Swedish Caledonides: Geolo- bama for technical support; to S. Goldberg, fault zone, Spitzbergen: Norsk Polarinstitutt Skrifter 161, 72 p. giska Foreningens i Stockholm Forhandlingar, v. 102, p. 569-580. Hodges, K. V., 1985, Tectonic stratigraphy and structural evolution of the Tull, J. F., Bartley, J. M., Hodges, K. V., Andresen, A., Steltenpohl, M. G., and L. Wood, R. Butler, P. Fullagar, and A. Glazner Efjord-Sitasjaure area, northern Scandinavian Caledonides: Norges White, M. J., 1986, The Caledonides in the Ofoten region (68-69N), for critical review of earlier versions of the Geologiske Undersokelse, v. 399, p. 41-60. north Norway: Key aspects of tectonic evolution, in Gee, D. G., and Hodges, K. V., Bartley, J. M., and Burchfiel, B. C., 1982, Structural evolution of Sturt, B. A., eds., The Caledonide orogen—Scandinavia and related manuscript; and to M. Gustavson, M. Stevens, an A-type subduction zone, -Rombak area, northern Scandina- areas: New York, J. Wiley & Sons, p. 553-568. vian Caledonides: Tectonics, v. 1, p. 441-462. Van der Voo, R., French, A. N., and French, R. B., 1979, A paleomagnetic pole and G. Milnes for comments which greatly Honea, E., and Johnson, A. M., 1976, Part IV. Development of sinusoidal and position from the folded Upper Devonian Catskill red beds, and its improved the manuscript. kink folds in multilayers confined by rigid boundaries: Tectonophysics, tectonic implications: Geology, v. 7, p. 345-348. v. 30, p. 197-239. Weiss, L. E., 1959, Geometry of superposed folds: Geological Society of Amer- Hossack, J. R., Nicketsen, R. P., and Garton, M., 1981, The geological section ica Bulletin, v. 70, p. 91-106. from the foreland up to the Jotun Sheet in the Valdres area, south Ziegler, P. A., 1978, North-western Europe: Tectonics and basin development: REFERENCES CITED Norway: Terra Cognita, v. 1, p. 52. Geologic en Mijnbouw, v. 57, p. 589-626. Johnson, A. M., 1978, Styles of folding: Amsterdam, the Netherlands, Elsevier, Bartley, J. M., 1980, Structural geology, metamorphism, and Rb/Sr geochro- 406 p. nology of east Hinnoy, north Norway [Ph.D. thesis]: Cambridge, Mas- Johnson, A. M., and Honea, E., 1975, Part III. Transition from sinusoidal to sachusetts, Massachusetts Institute of Technology, 263 p. concentric-like to chevron folds: Tectonophysics, v. 27, p. 1-38. 1981a, Field relations, metamorphism, and age of the Middagstind Kent, D. V., and Opdyke, N. D., 1978, Paleomagnetism of the Devonian Quartz Syenite, Hinnoy, north Norway: Norsk Geologisk Tidsskrift, Catskill red beds: Evidence for motion of the coastal New England- v. 61, p. 237-248. Canadian Maritime region relative to cratonic North America: Journal 1981b, Basement thrust ramps and redeformation of the Caledonian of Geophysical Research, v. 83, p. 4441-4450. MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER 22, 1986 nappe slack, north Norway: Geological Society of America Abstracts Kvale, A., 1953, Linear structures and their relation to movement in the Cale- REVISED MANUSCRIPT RECEIVED JUNE 2,1987 with Programs, v. 13, p. 404. donides of Scandinavia and Scotland: Geological Society of London MANUSCRIPT ACCEPTED JUNE 9,1987

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