Post-Caledonian tectonics along the Billefjorden fault zone, , and implications for the Arctic region

G. M. MANBY School of Earth Sciences, University of Greenwich, Bigland Street, London El 2NG N LYBERIS 1 T */-tTTorir\r^ri I Université Paris VI, DPtde Geotectonique, 4 Place Jussieu, 75252 Paris Cedex 05 J. CHUKUWtCZ, J F. THIEDIG Geologisch-Palaeontologisches Institute und Museum, Universität Münster, Corrensstraße 24

ABSTRACT tensor calculations computed for fault plane In most reconstructions for the North At- populations with slickensides using the lantic region in late Paleozoic time, the Bille- The north-south Billefjorden fault zone of method described by Etchecopar and others fjorden fault zone is interpreted to have been northern in the Svalbard archipel- (1981). The results of these analyses form the a major left-lateral transform (Harland and ago has been widely regarded as one of the basis of a new interpretation of the displace- others, 1974; Harland and Wright, 1979; Zieg- classic areas for transpressional tectonics. The ments along the Billefjorden fault zone and its ler, 1982, 1988). The Billefjorden fault zone fault zone coincides with a late Caledonian evolution and wider regional significance. became active in mid-Devonian time when shear zone and presently constitutes the east- Svalbard, which lies on the extreme north- the Ny Friesland block was transported some ern boundary of the main Devonian basin of west corner of the Eurasian Plate, separated 200 km northward by sinistral strike slip into northern Svalbard. Fold axial traces and from North Greenland along the De Geer its present location (Friend and Moody-Stu- thrust faults that developed in the Devonian to Fracture Zone (DGFZ, Fig. la) with the art, 1972; Harland and others, 1974). As Permo-Carboniferous rocks parallel the strike opening of North Atlantic and Arctic Ocean much as 1,000 km of left-lateral motion took of the fault zone. There is no evidence for the basins. Separation of the two blocks began in place on the Billefjorden fault zone in Late rotation of fold hinges, and nonaxial planar, late Paleocene time (56 Ma), and the forma- Devonian time, and the (Svalbardian) defor- transecting cleavages are lacking. Folds and tion of the West Spitsbergen fold belt to- mation of the post-Caledonian rocks along thrusts with similar geometries aifect the whole gether with the Eurekan structures of North the Billefjorden fault zone and across the De- 60-km-wide Devonian basin fill. These obser- Greenland and the Canadian Arctic islands vonian basin was caused by sinistral vations, together with analyses of fault and fold are attributed to this motion (for example, transpression accompanying this motion patterns, indicate (A) pre-Devonian (Cale- Harland and Horsfield, 1974). The Green- (Harland and Wright, 1979; Harland, 1985). donian) left-lateral ductile shearing; (B) Late land-Svalbard margin was also subjected to Folds with axial traces oblique to the Bille- Silurian-Early Devonian extension; (C) Late an earlier sequence of mid- to late Paleozoic fjorden fault zone are not found, however, Devonian-early Carboniferous inversion ac- (Caledonian and Ellesmerian, respec- and McWhae (1953) proposed that the fault companied by folding of Devonian sedimen- tively) orogenic events that appear to have zone was dominated by contraction followed tary rocks; (D) early Carboniferous extension developed diachronously following the by extension rather than strike-slip displace- followed by platform-wide subsidence into late closure of an earlier ocean (compare Har- ments. Fold axial traces and thrust faults af- Mesozoic time; (E) Late Cretaceous-Pa- land and Gayer, 1972; Christie, 1979). Sev- fecting the Devonian rocks along the north- leocene, east-west contraction, thrusting, and eral major north-south fault zones transect ern part of the fault zone are largely parallel folding; and (E) post-Paleocene east-west ex- Svalbard (Fig. lb), of which the Billefjor- to the main faults (Lamar and others, 1986). tension. The Late Devonian deformation along den fault zone is the most important be- Although Lamar and others (1986) did not the Billefjorden fault zone is compared with cause it has influenced the sedimentary present any detailed structural analysis to the Late Devonian to early Carboniferous and tectonic evolution of central Svalbard support their interpretation, they concluded Ellesmerian orogeny of the Canadian Arctic. since late Caledonian (Late Silurian-Early that the folds and thrusts were formed by re- The Late Devonian megashear model for the Devonian) time (Harland and others, gional east-west contraction rather than by tectonic evolution of the North Atlantic region 1974). The Billefjorden fault zone sepa- sinistral transpression. Paleomagnetic data, is not supported by the evidence from rates the Devonian basin of northern Spits- although somewhat equivocal, suggest that Svalbard. bergen from the high-grade Caledonian the Billefjorden fault zone has not experi- gneisses of the Hecla Hoek succession enced any significant strike slip since Devo- INTRODUCTION (Fig. lb) in Ny Friesland. Recent geophys- nian time (Torsvik and others, 1985). ical investigations have established, how- This paper presents the results of new ever, that Devonian red-bed sequences are GEOLOGIC SETTING mapping and structural analyses of data from also present in the offshore basins south- the Billefjorden fault zone in central Spitsber- east of Svalbard (Skilbrei and Eiken, 1992, The Billefjorden fault zone (BFZ) is best gen. Structural data are combined with stress personal commun.). exposed in central Spitsbergen between

Geological Society of America Bulletin, v. 105, p. 201-216, 13 figs., February 1994.

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Present Day Oceanic crust Continental crust

Figure la. Present-day tectonic setting of Svalbard. DGFZ = De Geer Fracture Zone; KR = Knipovich Ridge.

Austfjord and Billefjord where it parallels the striking, sinistral shear zone, 20 km east of steep limb at a high angle. On the shallow main Caledonian tectonic grain (Fig. 2). In the Billefjord fault zone, separates the limb of the fold, the cleavage is concentrated this area, it also coincides with a wide zone of high-grade Lower Hecla Hoek rocks from along and parallel to the dikes, but the cleav- metamorphic retrogression and intense the weakly to unmetamorphosed Middle age is only weakly developed or is not obvi- shearing in the Hecla Hoek gneiss (base- Hecla Hoek succession. ous a few meters away from the dikes. The ment). We have recognized that the gneiss in The Devonian basin west of the Billefjord- dikes are more competent than the sedimen- the fault zone displays a range of ductile, en fault zone consists of 8-9 km of fluviatile tary rocks, and they appear to have acted as semi-brittle to brittle shear structures, all of red beds (Manby and Lyberis, 1992) depos- "stress risers," accumulating the strain to which are consistent with left-lateral dis- ited during the 410 Ma to 360 Ma interval. produce the cleavage; away from the dikes, placement. The extensive replacement of the Although the Devonian rocks are locally the shortening may have been dissipated Caledonian amphibolite facies by greenschist folded (Friend and Moody-Stuart, 1972, and along the less-competent siltstone and shale facies mineral assemblages within the shear Harland and others, 1974), the entire basin fill layers. These relations suggest that the short- zone suggests that the shearing followed the is affected by west-vergent, kilometric scale, ening responsible for the cleavage formation peak metamorphism possibly in Late Silurian north-south-trending, box-shaped or kink- and, therefore, a part of the folding affecting to Early Devonian time. like folds that plunge gently to the south the basin fill postdated the intrusion of the North of the Billefjord-Austfjord area, the (Manby and Lyberis, 1992). The Devonian dikes (Manby and Lyberis, 1992). Early Car- fault zone is projected to cross Wijdefjord rocks on the southwest shore of Wijdefjord boniferous coal-bearing deltaic and lacustrine near to the western coast of Ny Friesland are cut by mid-Carboniferous (309 ± 5 Ma) sediments (Horbyebreen Formation, Fig. 2) (Fig. lb). Here, the high-grade basement monchiquite dikes (Gayer and others, 1966), crop out in a zone centered on the Billefjord- rocks are strongly sheared and retrograded one of which parallels the bedding in the en fault zone (Harland and others, 1974). over a 2- to 3-km-wide zone close to the steep western limb of a large anticline; others Overlying these rocks and the basement onshore projection of the fault zone (Har- cut the bedding on the long, east-dipping, along and east of the fault zone is a sequence land and others, 1974; Manby, 1990). In shallow limb of the adjacent syncline 300- of fault-controlled, coarse sandstone and addition, the Sorgfjord shear zone 400 m to the west (Manby and Lyberis, 1992). conglomerate (Svenbreen Formation, Fig. 2) (Manby, 1990; Manby and Lyberis, 1992), A well-developed pressure solution cleavage followed by mid- to late Carboniferous a similar 2- to 3-km-wide, north-south- cuts the Devonian rocks and the dike on the evaporites, carbonate rocks, red beds, and

202 Geological Society of America Bulletin, February 1994

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Late Paleozoic- Mesozoic Devonian Late Precambrian Early Paleozoic

Normal fault

Thrust

Syncline

Anticline Overturned Anticline

Figure lb. Geological sketch map of Svalbard with principal lithostratigraphic units and structural elements. Diagonal ruling indicates areas affected by the West Spitsbergen fold belt deformation. HFZ = Hornsund fracture zone; RFZ = RaudQorden fault zone; BBFZ = Briebogen fault zone; BFZ = BilleQorden fault zone; SSZ = SorgQorden shear zone; AFj = AustQorden; WJL = Wedel Jarlsberg Land; LFZ = Lomfjorden fault

Geological Society of America Bulletin, February 1994 203

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Ebbadalen

Svenbreen Billefjorden Horbyebreen Mimerdalen

Wood ¡Bay

contraction fault (thrust) extension fault

synciine

anticline

overturned anticline

monoclinal fold

overturned synciine

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on o o n'

o Mimerdajeh,l ^I|p- B! CO c

Tl 8-

MD

Figure 2. Geology of the Billefjorden fault zone in central Spitsbergen (Svalbard). The principal faults constituting the fault zone are the Balliolbreen fault, the Odellfjellet fault, and the KarnakfieUet fault. A-B, C-D, E-F, and G-H are the locations of the cross sections in Figure 3. Note duplication where cut.

8

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shallow marine quartz sandstone (Ebbadalen traction rather than east-west extension as tensely developed in the fold hinges and best Formation, Fig. 2). suggested by previously published fault plane developed in the finer grained rocks. Late Carboniferous to Permian thick plat- solutions. From the structural data plotted in Figure form carbonate rocks (Gipsdalen Group, 5a, the north-south-trending fold axes in the Fig. 2) overlap and overstep the Billefjorden STRUCTURES ALONG THE Devonian rocks are nowhere clearly fault zone. The overlying Triassic to Jurassic BILLEFJORDEN FAULT ZONE transected by the cleavages. Tension gashes rocks are deep water argillite and black shale, on fold limbs have east-west-oriented tips whereas the Cretaceous sequences exhibit a This study concentrates on the two best- consistent with shortening in that direction. vertical increase in the clastic component exposed segments of the fault zone: the From analysis of the folds, we conclude that (Steel and Worsley, 1985). The east-vergent Alandvatnet-Austfjord and the Petuniabukta the tectonic transport within the Devonian folds and thrusts that characterize the West areas, where the Billefjorden fault zone con- rocks is from east to west, and that they have Spitsbergen fold belt are best developed in sists of a 2- to 4-km-wide array of extensional not been subjected to any significant rotation the near-foreland late Paleozoic to Mesozoic and contractional faults, including the Balli- since their formation. The thrust faults meas- rocks of western Svalbard, where —40 km of olbreen fault, the Odellfjellet fault and the ured in the Devonian rocks are north-south east-west shortening is estimated to be re- Karnakfjellet fault (Fig. 2). to north-northwest-south-southeast oriented corded (Lyberis and Manby, 1993a, 1993b). with a dominant east to west slip direction. Structurally higher nappes dominated by Alandvatnet-Austflord Section The maximum principal stress axis (CTI) cal- metamorphosed (Caledonian) Hecla Hoek culated from the stress tensor method of rocks exhibit postmetamorphic, east-vergent The principal feature of the whole fault Etchecopar and others (1981) is predomi- folds and thrusts (Manby, 1988; Dallmann, zone is the steep (65°), east-dipping reverse nantly horizontal and east-west oriented 1988), which involve some thin slices of Car- fault (Balliolbreen fault, Figs. 2 and 3), which throughout the whole mapped area. The as- boniferous rocks indicating that the estimated has brought a horst-like block of Hecla Hoek sociated minimum principal stress axis (o3) is 40 km of fold-belt shortening is conservative gneiss over Devonian red beds (Fig. 2). East vertical or oblique, indicating an ambient, (Lyberis and Manby, 1993a, 1993b). Well-de- of the Balliolbreen fault (Fig. 2), on the peaks east-west, pure compressional stress pattern. veloped folds, cleavage, and thrust faults in of Odellfjellet and Sentinelfjellet (Figs. 2 and The calculated maximum principal stress axis the late Paleozoic to Mesozoic rocks along 3, cross section A-B), lower Carboniferous is generally parallel to the shortening direc- the Lomfjorden and Billefjorden fault zones coal-bearing strata (Hórbyebreen Formation, tion, which is suggested by the folds and (Fig. 1) in eastern and south-central Svalbard Fig. 2) are affected by broad synclinal flex- cleavages in the Devonian rocks. are ascribed (Andresen and others, 1988) to ures with north-south-striking, axial planar, The basement rocks exposed in the fault the reactivation of the fault zones in post-Ju- widely spaced, vertical pressure solution zone are limited to the east by the Odellfjellet rassic (West Spitsbergen fold belt) time. Seis- cleavages. Although the relationship between Fault (Fig. 3). The gneiss close to this fault mic reflection profiles on the northern off- the Hórbyebreen Formation and the Balliol- line is brecciated with randomly oriented shore area of Svalbard (Eiken and Austegard, breen fault is not clear, the deformation of fragments set in an anastomosing dolomite 1989, personal commun.) indicate that the these rocks provides evidence for some vein-like system, which we interpret to be the Billefjorden fault zone can be traced several shortening across the fault zone in post-early result of extensional faulting. Close to the kilometers north of the mainland. On these Carboniferous time. fault, the breccia is cut by small-scale reverse profiles, the Billefjorden fault zone appears For —2 km west of the basement horst, the faults with east-west-oriented (up-dip) slick- as a single steep, west-dipping extensional Devonian rocks are affected by several folds enside lineations. In one locality, a small, fault displacing undeformed, possibly Meso- and thrusts. Beyond this narrow zone, the centimeter-scale, box-like crenulation struc- zoic or younger, sedimentary rocks. The dis- folds are comparable with the kilometric ture folds the sheared basement rocks and placement of these rocks on the Billefjorden scale structures present in the rest of the De- cuts across the normal fault breccia on a fault zone may be related to the post-Eocene vonian basin to the west. In this 2-km-wide small reverse fault. Similar brecciation and extension of the western margin of Svalbard zone, there are several steep, east-dipping veining have not been observed in the during the accretion of the Knipovich Ridge thrust faults that are laterally continuous over adjacent Carboniferous rocks, but east- (Eiken and Austegard, 1987). The location of most of the mapped area (Figs. 2 and 3, cross northeast-west-southwest-oriented tension a Quaternary volcano on the Breibogen fault section A-B). One of these, the Karnakfjellet gashes are found in these rocks close to the zone (BBFZ, Fig. lb), with related hot spring fault, appears to merge with the Balliolbreen main fault plane and are consistent with a activity together with extensional faulting of fault in the vicinity of Odellfjellet. It is pos- general east-west contraction. It seems rea- glacial debris along the same fault, lead us to sible that the Karnakfjellet fault and the other sonable to suppose that the broad syncline- conclude that other on-land faults, parallel to larger thrust faults are linked at depth as anticline pair with a well-developed north- the Billefjorden fault zone, have continued to splays from the Balliolbreen fault. Folds are south-striking pressure-solution cleavage be active into Holocene time. Fault plane so- in the hanging walls of each of these thrust affecting the Carboniferous to Permian rocks lutions reported by Mitchell and others (1990) faults, but the lack of distinctive marker beds, to the east of the Odellfjellet fault (Fig. 3) also indicate that present-day seismic activity on combined with poor exposure, makes short- developed in response to the same contrac- Svalbard is located, among other sites, on the ening estimates difficult. The folds generally tional deformation. These features indicate southern terminations of the Billefjorden and have wavelength/amplitudes on the scale of a that the Odellfjellet fault may at first have ex- Lomfjorden fault zones. It is noteworthy, few tens of meters and are invariably over- perienced some (pre-Carboniferous) exten- however, that these authors concluded that turned to the west with 65°-70° east-dipping sion followed by later (post-Carboniferous) their data, obtained over the past decade, are axial surfaces (Fig. 4). Axial planar and fan- contraction. The down-to-the-east displace- consistent with an east-west-oriented con- ning pressure solution cleavages are most in- ment of the Carboniferous to Permian rocks

206 Geological Society of America Bulletin, February 1994

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Mittagleffler breen "'VV • \" """^^'iA 1 > Figure 3. Cross sections of the BilleQorden KF fault zone (see Fig. 2 for locations). The key for the rock units is same as in Figure 2. BF = Balliolbreen fault; OF = Odellfjellet fault; KF = Karnakfjellet fault.

Geological Society of America Bulletin, February 1994 207

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dalen Formation) rocks are faulted against the Horbyebreen Formation coal deposits and the Svenbreen sandstone-conglom- erate. The gypsiferous rocks are affected Alandelva by a large anticline-syncline pair with an amplitude of about 150 m (Fig. 7). In two stream sections, 200-300 m east of these folds, the evaporite rocks are again tightly folded and thrust faulted, but the folds overprint a pre-existing sheared, pressure- solution cleavage (Fig. 8). The gypsiferous rocks are imbricated, and each fault- bounded block is internally folded. The presence of these folds close to the Odell- fjellet fault would support the hypothesis that the fault has been involved in some post-Permian contraction. The present ge- ometry of the Odellfjellet fault with Per- mian rocks to the east and early Carbonif- erous rocks to the west, together with the truncation of the folds by normal faults, would imply later extensional reactivation Figure 4. Overturned west-vergent fold in the Devonian rocks of northwest Alandelva (see of the fault. Fig. 2). Scale of fold is indicated by geologist in lower center of the photograph. Well-developed On the south-facing slopes of Pyramiden, pressure solution cleavage can be seen in siltstone in the syncline immediately above geologist. The two shallow, east-dipping unconformities are overturned anticline exhibits a fanning cleavage. exposed within the evaporite rocks that we believe reflect mid- to late Carboniferous synsedimentary fault activity (Fig. 9). The and truncation of the syncline by the Odell- trolled largely by synsedimentary-growth overlying Permian (Nordenskioldbreen For- fjellet fault (Fig. 2) indicate that extensional faulting. The extension of the Balliolbreen mation, Fig. 2) limestone beds are virtually displacements followed the contraction. The fault is suggested by the presence of steep horizontal and spread right across the fault Alandvatnet (extension) fault (compare Har- reverse faults in the lower Carboniferous zone, suggesting that by late Carboniferous, land and others, 1974; Lamar and others, rocks along its southward projection. The the Billefjorden trough had filled and platform 1986), which cuts the steep western limb of contact between the basement and the De- conditions prevailed. Structural data col- the syncline, is restricted to Odellfjellet area vonian sedimentary rocks cannot be ob- lected from the Carboniferous rocks in the and appears to have accommodated a large served in this area, but McWhae (1953) area between Svenbreen and Pyramiden landslip (Fig. 2). suggested that the Hecla Hoek rocks are (Fig. 5b) are consistent with a thrusting ge- thrust upon the younger rocks. The south- ometry and once again indicate that the prin- Petuniabukta Area ern continuation of the Karnakfjellet fault cipal contraction direction was oriented east- truncates the Horbyebreen formation west, perpendicular to the fault zone. The present structure of the Petuniabukta (Figs. 2 and 3, cross section E-F) and may In the valley sections on the east side of area is an asymmetric trough bounded to the merge southward with the Balliolbreen Petuniabukta (Fig. 2), contractional deforma- west by the Billefjorden fault zone and to the fault. tion linked to that in the Billefjorden fault east by upfaulted Hecla Hoek gneiss (Figs. 2 The southward continuation of the Odell- zone can also be recognized. The character of and 3, cross sections C-D, E-F, and G-H). fjellet fault as the eastern boundary to the this deformation can be seen on the north side This trough-like structure occupies the same basement (Hecla Hoek) horst is on the west- of Ebbadalen (Figs. 2 and 3, cross sections site as the early to mid-Carboniferous "Bille- ern flank of Cheopsfjellet (Figs. 2 and 3, cross E-F and G-H, and Fig. 10) where the Eb- fjorden trough" (compare Steel and Worsley, section C-D, and Fig. 6) and at the eastern badalen evaporite and Nordenskioldbreen 1984), which we interpret to have formed by outfall of Svenbreen. Here, the Carbonifer- limestone are folded into a kink-like struc- extensional reactivation of the Odellfjellet ous-Permian rocks east of the Odellfjellet ture. The fold appears to represent ductile de- fault (see also McWhae, 1953). In this area, fault are folded into an asymmetric syncline formation directly ahead of a thrust tip. The the Billefjorden fault zone is 2 km wade (Fig. 3, cross section C-D, and Fig. 6), indi- basement is faulted against the evaporitic se- (Fig. 2), and it is overstepped by the Car- cating contractional deformation close to quences 2 km east of this fold (Fig. 3, cross boniferous rocks southward, so that south of the fault. Traced southward, this fault cuts section E-F). The fault dips steeply to the Svenbreen the basement (Hecla Hoek) rocks through the folds in the Carboniferous and east, and the displacement appears to be re- are only found in deeply incised valleys Permian rocks (Fig. 3, cross sections C-D, verse with —120 m up-dip displacement. Two (Fig. 2). In the Svenbreen area, the coal-bear- E-F, and G-H) and displaces them 350 m other similarly steep reverse faults imbricate ing Horbyebreen Formation lies directly on down to the east, suggesting that some the basement —200 m to the east of this fault. the basement in a narrow north-south-trend- normal faulting followed the folding. These faults and the fault within the fold are ing elongate basin, 50 to 100 m deep, con- Along this fault line, gypsiferous (Ebba- interpreted as splays propagating up from a

208 Geological Society of America Bulletin, February 1994

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Geological Society of America Bulletin, February 1994 209

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along the fault zone described here and that in south and central Svalbard is connected with the generation of the West Spitsbergen fold belt (McWhae, 1953; Harland and others, 1974).

TECTONIC EVOLUTION OF THE Cheopsfjellet BILLEFJORDEN FAULT ZONE

Odellfjeltet Fault The down-faulted Carboniferous to Per- mian strata in the Petuniabukta to Austfjord area form a trough-like structure, which has been controlled by extensional reactivation of the Odellfjellet fault and other faults to the east, which permitted the uplift of the Ny Friesland basement horst (Figs. 2 and 3). The structural and stratigraphic relationships ex- hibited by the Carboniferous to Permian rocks along the Billeijorden fault zone, to- gether with the seismic reflection profiling and fault plane solutions for recent earth- quakes, indicate that an extensional deforma- Figure 6. The Odellfjellet fault 1 km west of CheopsQellet (see Fig. 2). The Permo- tion is superposed on the earlier east-west Carboniferous rocks east of the fault are folded into an asymmetric syncline. contractional structure. Similar and probably related contractional deformation has also been recognized east of the Billeijorden fault detachment at the base of the Carboniferous company these folds and, together with the zone along the Lomfjorden fault zone sequence or a flat within the basement. fold axis and cleavage data, indicate east- (Fig. lb) by Andresen and others (1988). west shortening. The contractional deformation of the South of Pyramiden On the southeastern side of Billeijorden Permo-Carboniferous rocks in the Petunia- and across (Fig. 1), Carboniferous- bukta and southeast Billefjord areas is clearly In the east-west valley sections south of Mesozoic strata are folded and cut by minor connected to the east-vergent folding and Pyramiden (Fig. 2), large upright folds in the thrusts along the southern extension of the thrusting of the Mesozoic rocks south of Is- Devonian rocks are cut by flat-lying Permo- Billeijorden fault zone. The deformation fjord (Andresen and others, 1988). Recent Carboniferous (Nordenskidldbreen Forma- tion) limestone. The contact is normally sharp, but in the southern part of the Ygg- drasilkampen Valley, it is marked by a fault breccia with gouge, and the limestone is thrust-faulted (Fig. 11). Chert layers within the limestone are also affected by spaced pressure-solution cleavages (Fig. 5b), and the limestone has an inhomogeneously distrib- uted, anastomosing layer-parallel shear fab- ric. In the northern branch of the Ygg- drasilkampen Valley, the eastern edge of the Devonian block is cut by down-to-the-east normal faults that also displace the overlying Permian limestone. To the east and north, along the sea cliff, the Ebbadalen evaporite is also strongly folded. Together these relation- ships suggest that the Carboniferous to Per- mian rocks were subjected to east-west con- traction followed by extension. Fold axes in the Carboniferous and Devo- nian rocks in the southern area plunge to the north or to the south (Fig. 5b), and they are associated with north-south-striking cleav- ages dipping steeply to the east or to the west. Figure 7. Folds and thrusts affecting the gypsiferous rocks adjacent (east) to the Odellfjellet East-west-striking tension gashes also ac- fault, north Ferdinandbreen (see Fig. 2). Height of valley side is 300 m.

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shearing of the Caledonian orogen concen- trated along 2- to 3-km-wide, north-south- trending shear zones and retrograded the high-grade Hecla Hoek gneiss (Manby, 1990; Manby and Lyberis, 1992). The western shear zone formed an intrabasinal fault in the Devonian continental sedimentary basin of northern Svalbard. The eastern shear zone juxtaposed the low-grade moderately de- formed Middle-Upper Hecla Hoek rocks against the polymetamorphic and highly de- formed Lower Hecla Hoek rocks. Displace- ment along this, the Sorgfjorden shear zone, occurred when the late Precambrian-early Paleozoic Middle and Upper Hecla Hoek rocks were being folded about north-south-

oriented F, axes. The F2/S2 crenulation folds with north-south axes and steep east-dipping cleavage, which are developed in these rocks close to the shear zone, are overgrown by mica, chlorite, and cordierite in the contact metamorphic aureole of the Chydenius gran- ite. Isotopic dating of biotite from the pluton places its intrusion in the 393-414 Ma interval (Gayer and others, 1966), suggesting that the principal displacement along the Sorgfjorden (and possibly the Billefjorden) shear zone ceased before that time. It is possible, there- fore, that Early Devonian (410-400 Ma Ged- dinian) sedimentation was accompanied by sinistral shearing along one or both of those shear zones. Early Devonian rocks do not crop out in the eastern part of the basin, and Figure 8. Folded gypsum layers in the Ebbadalen Formation in a stream section 200 m east of it is not known whether the fault zone was Odellfjellet fault. The folded fabric is a well-developed shear/pressure-solution cleavage and in active as an extensional or strike-slip struc- ture at the time. The lack of coarse-grained more competent rocks would imply the fold is an F2 structure. View is to the north. sedimentary rocks in the Wood Bay Group of eastern Andree Land would suggest, how- seismic reflection profiling along Isfjord sistent with homogeneously distributed east- ever, that the fault zone was not active in (Orheim and others, 1988) has found that the west shortening. The folds lack evidence of Siegenian-Emsian time (Friend and Moody- Mesozoic sequences in this trough are cut by rotation or transecting cleavage orientations, Stuart, 1972) when these rocks were being several west-dipping imbricate thrusts. All of and the deformation of the Devonian rocks deposited. The presence of Devonian rocks these features taken together suggest that the resulted from contraction perpendicular to in the offshore areas southeast of the Bille- West Spitsbergen fold belt deformation ex- the fault zone. The geometry and mechanism fjorden fault zone demonstrates that the fault tended much farther east than previously rec- of folding and thrusting within the Devonian was not a major basin margin structure when ognized. The Devonian rocks within the 2-km rocks cannot obviously be related to the red beds were accumulating. zone west of the Billefjorden fault zone ex- transpressional deformation arising from sin- Partial inversion of the Devonian Basin in hibit much more intense north-south-trend- istral strike slip along the Billefjorden fault Late Devonian to early Carboniferous time is ing and west-vergent folds and thrusts than zone. The Hecla Hoek gneiss was involved in suggested by the angular unconformity be- observed in the Permo-Carboniferous rocks the fault zone deformation, and Caledonian tween the deformed red beds and the flat-ly- east of the fault zone. The horizontal attitude structures have continued to influence the ge- ing Permo-Carboniferous rocks (Fig. 12B). of the Carboniferous to Permian rocks lying ometry of subsequent structures. The north- The inversion with some folding and erosion over the highly deformed Devonian rocks south, steeply dipping mylonitic rocks in the of the basin fill corresponds to the Svalbard- west of the fault zone indicates clearly that at gneiss along the fault zone between Petunia- ian contractional event when the Balliolbreen least some of the deformation in the Devo- bukta and northwest Ny Friesland (Manby, fault would have experienced some reverse nian rocks took place in Late Devonian time, 1990) represent a large-scale, late Caledonian displacement. Following this inversion, the during the Svalbardian event (Vogt, 1928). sinistral shear zone, which has been reacti- area must have been eroded close to sea level Analysis of the fold and thrust structures in vated as a brittle fracture system. because the early Carboniferous coal basin the Devonian rocks along the fault zone During Late Silurian to Early Devonian became established on the site of the Bille- (Figs. 5a and 5b) indicates that they are con- time (Fig. 12A), ductile to brittle sinistral fjorden fault zone. The Svenbreen sandstone

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result of the deformation, and drill-core from Petuniabukta (unpublished Russian report, Pyramiden) penetrated in excess of 300 m without reaching the base of these rocks. The Devonian rocks west of the fault zone have, as suggested by the clcavage develop- ment around the Carboniferous monchiquite dikes in eastern Andree Land, also been af- fected by the West Spitsbergen fold belt re- lated east-west contraction. The thrust fault- ing and detachment at the base of the Nordenskioldbreen Formation limestone, which overlies the folded Devonian rocks, are of the same age. Since Oligocene time, the Billeljorden fault zone has been dominated by extensional faulting giving rise to the present-day Wijde- fjord and Billefjord troughs (see Fig. 3).

DISCUSSION

The field and geophysical evidence out- Figure 9. Photograph looking north to Pyramiden (the name of the mountain and the mining lined in the preceding sections strongly sug- settlement). The Carboniferous to Permian rocks between the Balliolbreen (BF) and Odellfjellet gests that the Billeljorden fault zone has been (OF) faults are undeformed and exhibit fault-related unconformities in the Ebbadalen to Mink- dominated since Oligocene time by exten- infjellet Formations (see Figs. 2 and 3). sional faulting associated with the east-west opening of the North Atlantic-Arctic Ocean basins. In the Billeljorden area and west of and conglomerate beds, which together with and simultaneous uplift of the Ny Friesland the fault zone, flat-lying Permo-Carbonifer- the overlying evaporites (with their two un- block and the horst-like block of gneiss in the ous limestone overlies strongly folded Devo- conformities) represent the "Billefjorden fault zone would have then produced the nian rocks. This relationship has been inter- trough fill" (Steel and Worsley, 1985), are re- shortening of the intervening rock sequences. preted as an unconformity and used as the stricted to the east of the fault zone, reflecting Thickening of the gypsiferous sequences in basis for recognizing the Svalbardian defor- the reactivation of the Balliolbreen and Odell- the axis of the trough would be expected as a mation event. South of Pyramiden, however, fjellet faults in late Carboniferous time. By Early Permian time, the entire fault zone and the southern part of the Devonian basin had been reduced to sea level, and platform car- bonate rocks (Nordenskioldbreen Forma- tion) were deposited across the entire region. Similar marine conditions prevailed through the Mesozoic Era without any significant tectonism. In Late Cretaceous to Paleocene time (Fig. 12D), uplift and erosion accompanied the West Spitsbergen fold belt deformation (Lyberis and Manby, 1993a, 1993b). East- Ebbadalen west contraction reactivated the Balliolbreen and Odellfjellet faults as thrusts so that the basement blocks acted as buttresses against and between which the adjacent rocks were folded and thrust faulted. The deformation of the Permo-Carboniferous rocks in the Aust- Ijord-Petuniabukta (Billefjorden) trough may have arisen by a combination of two effects: uplift of the eastern margin of the trough (the Ny Friesland block) on steep reverse faults could be expected to trigger gravity sliding toward the west on the extremely weak gyp- Figure 10. View of the kink-like fold and thrust fault affecting Carboniferous to Permian rocks siferous layers in the Ebbadalen Formation, in Ebbadalen.

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ous to late Mesozoic, subsidence affecting the entire Barents sea shelf; Late Creta- ceous-Paleocene east-west folding and thrusting associated with the West Spitsber- gen fold-belt east-west compression; and post-Oligocene normal faulting giving rise to the Austford-Petuniabukta (Billefjorden) Permian trough. Widespread and intense Ellesmerian (mid- Devonian to early Mississippian) deforma- tion affected the thick (as much as 10 km) Devonian sequences of the Canadian Arctic Islands and North Greenland (Thorsteinsson, 1970; Trettin, 1972; Christie, 1979). In the Ca- nadian Arctic and North Greenland, defor- mation of Ellesmerian age is typified by tight to isoclinal folds interpreted as having devel- oped above a décollement (Soper and Hig- gins, 1987). Overprinting these structures are several low-angle thrusts of late Mesozoic to early Cenozoic (Eurekan) age. The Ellesme- rian and Svalbardian events may be consid- ered to have formed as part of a more exten- Figure 11. View of the thrust-faulted unconformity in Yggdrasilkampen (see Fig. 2). Only the sive deformation that can be traced around Permian limestone is found in the hanging wall of the fault; in the footwall, the Devonian rocks the whole North American-Greenland block are tightly folded, and just below the thrust surface, they are brecciated together with some of (Fig. 13). In a wider context, the Early to the limestone. Late Devonian clastic sequences of the Frank- linian Basin across the Canadian Arctic Is- lands/Northern Greenland and the Devonian this unconformity is thrust faulted. The lime- dicular to the Billefjorden fault zone. It is also basins of Svalbard, East Greenland, Nor- stone in the hanging wall of the thrust fault is noteworthy that these structures parallel the way, and Scotland are generally believed to clearly affected by layer parallel shortening Caledonian tectonic fabric in basement rocks represent the same sedimentary responses to and shear, suggesting that significant east- of Ny Friesland and northwest Spitsbergen the extensional collapse of the Caledonian or- west contraction must have taken place in (Fig. 1). In the case of the Billefjorden fault ogen (compare McClay and others, 1986; post-Permian time. The well-developed zone, it is evident that its location was pre- Norton and others, 1987). If this is accepted, cleavage adjacent to the monchiquite dikes determined by a weak zone in the continental then the Billefjorden fault zone should have cutting the Devonian rocks in eastern Andrei crust represented by the late Caledonian accommodated some of the extension in the Land shows that they were also subjected to shear zone. Svalbard region. In addition, deformation some post-Carboniferous east-west shorten- The time of formation and dominant sense of the Devonian basin fill of Northern Sval- ing. We suggest that all of this post-Carbonif- of displacement on the Billefjorden fault zone bard, together with the tectonism along the erous deformation belongs to the Late Cre- as a brittle fracture system is only imprecisely Billefjorden fault zone, may be linked with taceous-Paleocene West Spitsbergen fold known; however, the shear zone on which it the post-Caledonian tectonic evolution of belt deformation event (Lyberis and Manby, is founded has been found by us to have been North Greenland and the Canadian Arctic 1993a, 1993b). dominated by ductile through to brittle sinis- islands. The Ellesmerian shortening, The more intense folding and thrusting re- tral displacements, and significant amounts which affected the Canadian Arctic Basin corded in the Devonian rocks is evidently the of strike slip cannot be excluded for the fault in Devonian time (Christie, 1979), propa- result of the superposition of the Late Creta- zone in Late Silurian to Early Devonian time. gated diachronously across to Svalbard by ceous-Paleocene deformation on that of The inversion and deformation of the Devo- Late Devonian time, implying that the Svalbardian age. Only one phase of folding nian basin fill suggests, however, that by Late Svalbardian event was part of the Elles- can be detected, however, in the Devonian Devonian time the Balliolbreen fault had ac- merian orogeny. rocks, and rotated folds/transecting cleav- commodated some reverse slip. The move- The deformation of the Devonian basin fill ages have not been observed. Indeed all folds ments along the Billefjorden fault zone can be of Northern Svalbard and similar-aged rocks and thrusts parallel the principal faults in the summarized as follows: left-lateral ductile of the Innutian orogen suggests that a con- Billefjorden fault zone, and it appears from shear in late Caledonian-pre-Devonian time; vergent motion between North Greenland the field relations that the Svalbardian event normal faulting in Devonian time as the basin and the Barents Sea block (Fig. 13) may have was also characterized by east-west fill accumulated; Late Devonian reverse been partly driven by sea-floor spreading in shortening. faulting" accompanying the inversion and con- the Sakmarian-Magnitogorsk arc (Zeigler, Throughout its evolution, the maximum tractional deformation of the Devonian basin 1988). In direct contrast to the Late Devonian and minimum strain directions appear, from fill; early Carboniferous to Permian normal left-lateral megashear models, this motion the structures, to have been oriented perpen- faulting arising from the general, Carbonifer- would link to a large-scale dextral motion be-

214 Geological Society of America Bulletin, February 1994

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\\ Late Precambrian - \\\ Early Palaeozoic [Middle - Upper Hecla Hoek) , l © e A Late Silurian - Early Devonian Sorgefjorden Shear Zone © %© Billefjorden - Wijdefjorden Shear Zone

Balliolbreep Fault] Ml

• Late Devonian - Early Carboniferous

West °Late Cretaceous - Palaeocene

Figure 12. Schematic cross sections illustrating the evolution of the Billefiorden fault zone. The late Silurian-early Devonian events for the Sorgfiorden shear zone are drawn from Manby (1990) and Manby and Lyberis (1992).

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Gayer, R. A., Gee, D. G., Harland, W. B., Miller, J. A., Spall, H. R„ Wallis, R. H., and Wisnes, T. S., 1966, Radiometric age determinations on rocks from Spitsbergen: Norsk Po- larinstitutt Skrifter, v. 137, p. 1-39. Harland, W. B., 1985, Caledonide Svalbard, in Gee, D. G., and Sturt, B. A., eds., The CaledonideiOrogen-Scandinavia and related areas: Chichester, England^ Wiley, p. 999^1016. Harland, W. B., and Gayer, R. A., 1972, The Arctic Caledonides and earlier oceans: Geological Magazine, v. 109, p. 289-314. Harland, W. B., and Horsfield, W. T., 1974, West Spitsbergen Or- ogen, in Spencer, A. M., ed., Mesozoic-Cenozoic orogenic belts: Geological Society of London Special Publication No. 4, p. 747-755. Harland, W. B„ and Wright, N.J.R., 1979, An alternative hypoth- esis for the pre-Carboniferous evolution of Svalbard: Norsk Polarinstitutt Skrifter, v. 167, p. 89-117. Harland, W. B., Cutbffl, J. L., Friend, P. F., Gobbet, D. J., Hol- liday, D. W., Maton, P. I., Parker, J. R., and Wallis, R. H., 1974, The Billefjorden Fault Zone, Spitsbergen. The long his- toiy of a major tectonic lineament: Norsk Polarinstitutt Skrifter, v. 167, p. 8SW17. Lamar, D. L., Reed, W. E., and Douglass, D. N„ 1986, Billefjorden Fault Zone, Spitsbergen: Is it part of a major late Devonian Transform?: Geological Society of America Bulletin, v. 97, p. 1083-1088. Lyberis, N., and Manby, G. M., 1993a, The West Spitsbergen Fold Belt: The result of Late Cretaceous-Paleocene Greenland- Svalbard convergence?: Geological Journal, v. 28, 2, p. 125-136. Lyberis, N., and Manby, G. M„ 1993b, The origin of the West Spitsbergen Fold Belt from geological constraints and piate kinematics: Implications for the Arctic: Tectonophysics (in press). Manby, G. M., 1990, The petrology of the Harkerbreen Group, Ny Friesland, Svalbard: Protoliths and tectonic significance: Ge- ological Magazine, v. 127, p. 129-146. Manby, G. M., and Lyberis, N., 1992, Tectonic evolution of the Devonian Basin of Northern Svalbard: Norges Geologisk Tidsskrift, v. 72, no. 1, p. 7-21. McGay, K. R„ Norton, M. G., Coney, P., and Davis, G. H., 1986, Collapse of the Caledonian orogen and the Old Red Sand- stone: Nature, v. 323, p. 147-149. McWhae, J.R.H., 1953, The major fault zone of Central Spitsber- gen: Geological Society of London Quarterly Journal, v. 108, Figure 13. Tectonic map of the Arctic for mid- to Late Devonian time (partly after Zeigler, p. 209-232. Mitchell, B. J., Bungum, H., Chan, W. W„ and Mitchell, P. B., 1988). North America/Greenland-Svalbard/Barents Platform convergence implies dextral rela- 1990, Seismicity and present-day tectonics of the Svalbard region: Geophysical Journal International, v. 102, p. 139-149. tive displacement between Greenland and . Plate motions are related to the opening of Norton, M. G., McClay, K. R., and Way, N. A., 1987, Tectonic the Ural Ocean. evolution of Devonian basins in northern Scotland and south- ern Norway: Norges Geologisk Tidsskrift, v. 67, p. 323-328. Orheim, A., Aronsen, H. A., Jensen, N. L., Skarpnes, O., and Larsen, B. T., 1988, Seismic mapping of Grimmfjellet and Isfjorden, Svalbard; Tectonic implications: Norsk Polarinsti- tween Scandinavia and Greenland in Late tutt Rapportserie No. 46, p. 63-67. Fund Committee (Manby), and the Deutch- Soper, N. J., and Higgins, A. K., 1987, A shallow detachment be- Devonian time. esforschungsgemeinschaft (Thiedig). We neath the North Greenland fold belt: Implications for sedi- mentation and tectonics: Geological Magazine, v. 124, The evidence described here suggests that would like to thank Arild Andresen for help- p. 441-450. Steel, R. J., and Worsley, D., 1985, Svalbard's post-Caledonian throughout its history as a brittle fracture sys- ful discussions and his considerable assist- strata; An atlas of sedimentological patterns and palaeogeo- tem the Billefjorden fault zone has been sub- ance with logistic arrangements. We are also graphical evolution: Petroleum Geology of the North Euro- pean Margin, Norwegian Petroleum Society, p. 109-135. jected to repeated, east-west-oriented exten- grateful to Rick Law, Rob Strachan, Bob Thorensteinsson, R., 1970, Precambrian and Palaeozoic, in Geology of the Arctic archipelago, Geology and economic minerals of sion and contraction. We have been unable to Holdsworth, and Doug Helm for their con- Canada (5th edition): Geological Survey of Canada Economic find any structural evidence to support the structive comments on earlier versions of this Geology Report No. 1, p. 548-590. Torsvik, T. H., Lovlie, R., and Sturt, B., 1985, Palaeomagnetic hypothesis that the fault zone functioned as a paper. argument for a stationary Spitsbergen relative to the British Isles (Western Europe) since Late Devonian and its bearing large-scale, Late Devonian sinistral trans- on North Atlantic reconstructions: Earth and Planetary Sci- form. There is, however, abundant evidence ence Letters, v. 75, p. 278-283. REFERENCES CITED Trettin, H. P., 1972, The Innutian province, in Price, R. A., and for Late Silurian to Early Devonian sinistral Douglas, R.J.V., eds., Variations in tectonic styles in Can- Andresen, A., Haremo, P., and Berg, S. G., 1988, The southern ada: Geological Association of Canada, no. 11, p. 83-179. displacement along the 2- to 3-km shear zone termination of the Lomljorden fault zone: Evidence for Ter- Vogt, Th., 1928, Den Norske fjellkjedes revolusjons-historie: on which the Billefjorden fault zone is tiary compression in East Spitsbergen: Norsk Polarinstitutt Norges Geologisk Tidsskrift, v. 10, p. 97-115. Rapportserie No. 46, p. 75-78. Zeigler, P. A., 1982, Geological atlas of Western and Central Eu- founded. Christie, R. L., 1979, The Franklinian Geosyncline in the Canadian rope: Amsterdam, The , Elsevier, 130 p. Arctic and its relationship to Svalbard: Norsk Polarinstitutt Zeigler, P. A., 1988, Evolution of the Arctic-North Atlantic and the Skrifter, v. 167, p. 263-314. Western Tethys: American Association of Petroleum Geol- Eiken, O., and Austegard, A., 1987, The Tertiaiy orogenic belt of ogists Memoir 43, 198 p. ACKNOWLEDGMENTS West Spitsbergen: Seismic expressions of the offshore sedi- mentary basins: Norges Geologisk Tiddsrift, v. 67, p. 383-394. This contribution has arisen out of field Etchecopar, A., Vasseur, G., and Daigniers, M., 1981, An inverse problem in microtectonics for the determination of stress ten- work funded by the Franco-Norwegian sors from fault striation analysis: Journal of Structural Geol- ogy, v. 3, pt. 1, p. 51-65. Foundation, the Centre Nationale Recher- Friend, P. F., and Moody-Stuart, M., 1972, Sedimentation of the ches Scientifique (Lyberis and Chorowicz), Wood Bay formation (Devonian) of Spitsbergen: Regional MANUSCRIPT RECEIVED BY THE SOCIETY APRIL 6,1992 analysis of a late orogenic basin: Norsk Polarinstitutt Skrifter, REVISED MANUSCRIPT RECEIVED APRIL 28,1993 the University of London Central Research v. 157, 77 p. MANUSCRIPT ACCEPTED MAY 25, 1993

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