Late Palaeozoic structural development of the South-western Barents Sea

Marine and Petroleum Geology ELSEVIER

Marine and Petroleum Geology 15 (1998) 73 102

Late Palaeozoic structural development of the South-western Barents Sea S. T. Gudlaugsson *'l, J. I. Faleide, S. E. Johansen 2, A. J. Breivik Department of Geology, UniversiO' of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo, Norway

Received 27 November 1995; revised 14 August 1997; accepted 30 August 1997

Abstract

A regional grid of multichannel seismic reflection profiles records the Late Palaeozoic structure and tectonic development of the south-western Barents Sea. A 300 km wide rift zone, extending at least 600 km in a north-easterly direction, was formed mainly during Middle Carboniferous times. The rift zone was a direct continuation of the north-east Atlantic rift between Greenland and Norway, but a subordinate tectonic link to the Arctic rift was also established. The overall structure of the rift zone is a fan-shaped array of rift basins and intrabasinal highs with orientations ranging from north-easterly in the main rift zone to northerly at the present western continental margin. The structural style is one of interconnected and segmented basins characterized by halfgraben geometries. A less prominent north-westerly fault trend abuts against the main rift zone from the south-east. From the beginning of Late Carboniferous times, the tectonic development was dominated by regional subsidence, and the entire Barents Sea region gradually became part of a huge Permian-Triassic interior sag basin. This development was interrupted by renewed Permian Early Triassic rifting and formation of north trending structures in the western part of the rift zone. The tectonic link between the northeast Atlantic and Arctic rifts, initiated in the Middle Carboniferous, then became the primary locus of deformation. The tectonic relationship of north-east Atlantic-Arctic rifting to the development of Late Palaeozoic basins, which dominate the structure of the eastern Barents Sea, remains poorly understood. The rapid Late Permian Early Triassic subsidence of these earlier fault-controlled basins also affected the western Barents Sea. This suggests possible influence on rifting in the Barents Sea by active-margin processes operating at the eastern Barents Sea margin during subduction of the Uralian Ocean floor. Strong control on the Late Palaeozoic structural development by zones of weakness in the basement is interpreted to be inherited from three major compressional orogens Baikalian, Caledonian and lnnuitian-~converging and partly intersecting at a major tectonic junction in the south-western Barents Sea. Local observations indicate that the Barents Sea Caledonides were affected by a Devonian phase of late-orogenic extensional collapse. © 1998 Elsevier Science Ltd. All rights reserved. Keywords: Barents Sea ; Late Palaeozoic rifting ; Structural styles ; Salt ; Basement structure ; Palaeotectonic reconstruction

1. Introduction

The Barents Sea is the wide epicontinental sea covering the continental shelf o f north-western Eurasia (Fig. 1). B o u n d e d on the west and n o r t h by Cenozoic passive margins, it preserves a relatively complete succession o f sedimentary strata ranging in age from Late Palaeozoic to Quaternary, locally exceeding 15 k m in thickness. The M e s o z o i c - C e n o z o i c structure (Fig. 2) and tectonic history o f the south-western part o f this region is relatively well k n o w n t h r o u g h a n u m b e r o f studies based

* Corresponding author. Present address : National Energy Directorate, Reykjavik, Iceland 2 Present address : Statoil Research and Development Centre, N-7005 Trondheim, Norway. S0264-8172/98/$19.00 © 1998 Elsevier Science Ltd. All rights reserved. Pl1:S0264 8 1 7 2 ( 9 7 ) 0 0 0 4 8 2

mainly on seismic reflection data correlated to offshore boreholes and onshore outcrops (e.g., Gabrielsen et al., 1990; Faleide et al., 1993). A l t h o u g h studied in several early works, the Late Palaeozoic geology has been less well understood, mainly because o f the deterioration in seismic quality with depth and the limited information released f r o m the few boreholes penetrating into Palaeozoic strata. A d v a n c e s in seismic processing and the release o f pertinent borehole information is gradually improving this situation and several papers addressing Late Palaeozoic structure and tectonism have recently appeared (Lippard & Roberts, 1987 ; Jensen & Broks, 1988 ; Gabrielsen & F~erseth, 1989; Stemmerik & Worsley, 1989; G a b rielsen et al., 1990; G6rard & Buhrig, 1990; D e n g o & Rossland, 1992 ; Jensen & Sorensen, 1992 ; Johansen et al., 1993, 1994a, 1994b; N o t t v e d t et al., 1993a; Bugge & Fanavoll, 1995; Bugge et al., 1995).

S.T. Gudlaugsson et al./Marine and Petroleum Geology 15 (1998) 73 102

74

80 °

75'

10"

20"

30 °

40"

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Fig. 1. Location of study area. Structural features of the western Barents Sea identified in Fig. 2.

70

In somewhat simplified terms, the Late Palaeozoic tectonic history of this area is thought to have involved: (1) consolidation of the basement during the Caledonian Orogeny, (2) a Devonian tectonic regime comprising both extensional and compressional events, so far known on Svalbard only, (3) widespread rifting in the Carboniferous and Permian, and (4) gradual development towards non-fault-related regional subsidence in Permian time. M a n y aspects of this development are not well understood, including the structure of the basement, the nature of the Devonian regime, the geometry and structural style of the Carboniferous-Permian rift system as well as the number, timing and relative importance of tectonic phases. The main objective in this study is to develop further insights into the nature of the Carboniferous-Permian rift system by focussing on the geometry and temporal development of the primary rift structures and subsequent regional subsidence and sedimentation. Seismic evidence is also presented for extensional structures which possibly provide a record of the earliest post-Caledonian (Devonian?) tectonic regime. Regional correlations and tectonic models are considered in terms of a palaeotectonic reconstruction to end-Palaeozoic time. The study area (Figs. 1 and 2) corresponds to the area where seismic, gravity and borehole data have been released from the archives of the Norwegian Petroleum Directorate.

2. Geological background Direct information on the nature of the crystalline crust beneath the Barents Sea sedimentary basins is

10 °

20"

30 °

Fig. 2. Mesozoic-Cenozoicstructural features of the western Barents Sea. 1 = Late Jurassic Early Cretaceous extensional basins ; 2 = Late Cretaceous Palaeogene marginal basin; 3 = Palaeogene West Spitsbergen fold-and-thrust belt; 4 = continent-ocean transition zone; 5 = oceanic crust in the Norwegian-Greenland Sea; 6 = diapirs of Upper Palaeozoicsalt ; 7 = boundaries of structural elements. Abbreviated names of structural elements explained in legend to Fig. 4. Study area outlined.

scarce, but the available, mostly indirect, evidence indicates that the basement underlying much of its western part was consolidated during the Caledonian orogeny and that the structural grain within the Caledonian basement may have influenced later structural development (e.g., Ziegler, 1988a; Dor6, 1991). Early studies of the Upper Palaeozoic succession revealed a structure characterized by numerous faultbounded sedimentary basins and highs, the presence of large evaporite accumulations in some of the basins, and general stratigraphical and tectonic similarities with Svalbard have been postulated (Fig. 3) (Ronnevik, 1981; Ronnevik et al., 1982 ; Faleide et al., 1984 ; Gabrielsen, 1984; Gabrielsen et al., 1984; Ronnevik & Jacobsen, 1984; Berglund et al., 1986; Riis et al., 1986). On Svalbard, an archipelago located on the northwestern corner of the Barents Shelf(Figs. 1 and 2), Lower and Middle Devonian sedimentary strata are preserved in a large north-south oriented graben (Friend & Moody-

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Stuart, 1972). The sediments o f the graben fill, which were compressively d e f o r m e d in a Late D e v o n i a n tectonic phase referred to as the Svalbardian movements, are discordantly overlain by C a r b o n i f e r o u s strata (Vogt, 1928; Cutbill & Challinor, 1965). L o w e r - M i d d l e Carboniferous strata were deposited in extensional basins ranging f r o m wide d o w n w a r p s to n a r r o w grabens (Gjel-

berg & Steel, 1981 ; Steel & Worsley, 1984; Johannessen & Steel, 1992 ; N m t v e d t et al., 1993a, 1993b). A tectonic hypothesis involving sinistral transcurrent displacements o f 200-1000 k m along the axis o f the Caled o n i a n O r o g e n in Late D e v o n i a n times (Harland, 1965, 1969, 1972; H a r l a n d et al., 1974) influenced early interpretations o f the structures observed in the western

76

S.T. Gudlau,gsson et al./Marine and Petroleum Geolo#y 15 ( 19982 7 ~ 1 0 2

Barents Sea. By analogy with Svalbard, the basin-forming tectonic phase was thought to be Late Devonian Early Carboniferous in age, initiated during the Svalbardian movements. Building on Harland's hypothesis, it was explained as resulting from a combination of sinistral movements on a major shear system to the west of the Barents Sea and dextral movements on a system of northwest to north-north-west trending strike-slip faults traversing the western Barents Sea (Ziegler, 1978 ; Ronnevik et al., 1982 ; Ronnevik & Jacobsen, 1984). The north-east striking Caledonian structural trends were reactivated as extensional faults bounding graben structures. Thus, Late Devonian Early Carboniferous tectonism was in effect considered as a single coherent phase involving movements on a coupled system of strike-slip and normal faults. The proposed large-scale sinistral movements is difficult to maintain in the face of growing palaeomagnetic and geological evidence to the contrary (e.g., Smith, 1988 ; Dor6, 1991). Palaeomagnetic data no longer support the concept of orogen-parallel megashear during Late Devonian or Early Carboniferous times (Torsvik et al., 1985). Large sinistral strike-slip displacements on the Billefjorden Fault Zone in Svalbard, proposed as an important element of the megashear system (Harland, 1965, 1969, 1972; Harland et al., 1974), have been contested on structural and sedimentological grounds (Lamar et al., 1986; Reed et al., 1987 ; Chorowicz, 1992 ; M a n b y & Lyberis, 1992; M a n b y et al., 1994) and corroborating evidence for the proposed conjugate system of strike-slip faults in the Barents Sea has proved elusive. Furthermore, since there are no conclusive reports of Devonian sedimentary strata in Barents Sea boreholes, direct evidence for the presence of Devonian basins in the western Barents Sea is lacking. Yet, Devonian megashear movements are still retained in some palaeotectonic reconstructions of the Arctic North Atlantic region (Ziegler, 1988a, 1988b, 1989). Jensen & Broks (1988) and Jensen & Sorensen (1992) revoked the concept of linked strike-slip and extension as an important mode of basin development during the Devonian and Early Carboniferous, proposing that the Trollfjord Komagelv Fault Zone was active as a master strike-slip fault during this period. Thus, the nature of the tectonic regime operating in Devonian times during the transition from the late Caledonian compressional regime to subsequent rifting is both controversial and poorly understood. By contrast, much of the recent literature agrees on the dominantly extensional character of the structures observed and on the occurrence of at least two important extensional phases, the first in Late Devonian('?) to Early Middle Carboniferous times and the second in Permian Early Triassic times (Lippard & Roberts, 1987; Gabrielsen et al., 1990; Dengo & Rossland, 1992: Jensen & Sorensen, 1992; Nottvedt et al., 1993a). This tectonic development strongly resembles the tectonic devel-

opment of Svalbard and Bjornoya following the Svalbardian movements (Gjelberg & Steel, 1981; Steel & Worsley, 1984 ; Stemmerik & Worsley, 1989 ; Worsley et al., 1990 ; Dallmann, 1992 ; Johannessen & Steel, 1992 : Nottvedt et al., 1993b). There are also broad stratigraphic similarities between the two regions (Fig. 3) ; stratigraphic equivalents to the Lower Carboniferous Billefjorden Group, the Middle Carboniferous-Lower Permian Gipsdalen Group, and the Upper Permian Tempelfjorden G r o u p have been identified in Barents Sea boreholes (e.g. Nottvedt et al., 1993a : Bugge et Ell., 1995). The frequent references to a Late Devonian onset of basin formation in the Barents Sea are apparently based on the latest Devonian age of the basal part of the Billefjorden G r o u p on Bjornoya and possibly on Svalbard (Worsley & Edwards, 1976; Gjelberg & Steel, 1981), but do not necessarily imply a tectonic correlation with the Svalbardian movements. The Late Devonian(?) Middle Carboniferous rift phase resulted in the formation of several interconnected extensional basins filled with syn-rift deposits and separated by fault-bounded highs (Lippard & Roberts, 1987 : Gabrielsen et al.. 1990 ; Dengo & Rossland, 1992 ; Jensen & Sorensen, 1992; Breivik et al., 1995). Structural trends striking north-east to north dominate in most of the southwestern Barents Sea (Fig. 2) where the Tromso, Bjornoya, Nordkapp, Fingerdjupet, Maud and Ottar basins have been interpreted as rift basins formed at this time (Dengo & Rossland, 1992: Jensen & Sorensen, 1992: Bugge & Fanavoll, 1995 ; Breivik et al., 1995). The Hammerfest Basin may also have been initiated at this time (Dengo & Rossland, 1992: Jensen & Sorensen, 1992). It has further been suggested that coewd faulting on a north-west structural trend affected the offshore areas north of the Trollt]ord Komagelv Fault Zone (Lippard & Roberts, 1987 : Dengo & Rossland, 1992 : Bugge el al., 1995). Much remains, however, to be learned about the deeper geometry and structural style of the rift system. The regional seismic stratigraphic interpretation for levels deeper than the uppermost Carboniferous and calibrated in boreholes is too limited to resolve the tcmporal evolution of the rift system during pre-Permian times in any detail (Fig. 3). The Billefjorden G r o u p is dominated by continental and shallow marine siliciclastics, and is partly coal-bearing (Stemmerik & Larssen, 1992; Bruce & Toomey, 1993; Nottvedt et al., 1993a; Bugge et al., 1995). In view of the assumed importance of Early Carboniferous extension, surprisingly little has been published on the regional seismic mapping of this group, and its tectonic setting must therefore be considered poorly documented. The most detailed study is that of Bugge et al. (1995) which covers the eastern Finnmark Platform (Fig. 2). The Bashkirian Ebbadalen Formation, which lies at the base of the overlying Gipsdalen Group, has been tentatively identified on the Loppa High based on lithostratigraphic correlation as mixed siliciclastic-car-

S.T. Gudlaugsson et al./Marine and Petroleum Geology 15 (1998) 73-102

bonate shelf sediments with a dominance of sandstone (Stemmerik and Larssen, 1992). On the Finnmark Platform, continental sandstones, possibly including overlying marginal marine sandstones with subordinate shale and limestone, are also interpreted as Ebbadalen Formation equivalents and this seems to be supported by biostratigraphic data (Bruce & Toomey, 1993; Cecchi, 1993; Bugge et al., 1995). On Svalbard, the formation fills narrow extensional basins, which developed in response to a phase of intensified fault-controlled subsidence in Middle Carboniferous times, and consists of marginal continental redbeds passing laterally into basinal carbonates and evaporites (Steel & Worsley, 1984; Johannesen & Steel, 1992). A similar tectonic setting has been anticipated in the Barents Sea (Stemmerik & Worsley, 1989 ; Cippitelli, 1990 ; Nmtvedt et al., 1993a ; Bugge et al., 1995) and this is supported by seismic profiles tied to a borehole on the Loppa High (Johansen et al., 1994a). Fault movements ceased in the eastern areas towards the end of the Carboniferous and the structural relief was gradually infilled and blanketed by a platform succession of Late Carboniferous-Permian age (Stemmerik & Worsley, 1989, 1995; Gabrielsen et al., 1990; Dengo & Rossland, 1992 ; Stemmerik and Larssen, 1992 ; Bruce & Toomey, 1993; Cecchi, 1993; Nilsen et al., 1993; Nmtvedt et al., 1993a; Bugge et al., 1995; Cecchi et al., 1995; Stemmerik et al., 1995). The lower part of this succession passes upwards from cyclical dolomites and evaporites to massive limestones and corresponds to the remainder of the Gipsdalen Group. It includes a widespread evaporite layer of latest Carboniferous-earliest Permian age mapped regionally (G6rard & Buhrig, 1990). The change to platform type sedimentation marked the initial development of a regional sag basin which continued to subside in the Late Permian during the deposition of cherty limestones and shales of the Tempelfjorden Group (Stemmerik & Worsley, 1989; Dengo & Rossland, 1992). The western part of the rift system was affected by block-faulting, uplift and erosion in Permian-Early Triassic times (Berglund et al., 1986; Riis et al., 1986; Stemmerik & Worsley, 1989, 1995; Gabrielsen et al., 1990; Johansen et al., 1994a).

3. Data and methods The data used consist of conventional multichannel seismic reflection (MCS) profiles supplemented by borehole data. Some 20,000 km of MCS profiles were selected from a larger database on the basis of quality and penetration. Preference was given to regional profiles oriented perpendicular to the dominating structural trends. An average regional grid spacing of 10-15 km was used, varying from 4 km on the Loppa High to about 40 km in the south-eastern part of the study area.

77

Penetration and resolution generally deteriorates beneath the top Permian level and there are significant problems with multiples. A detailed seismic stratigraphic scheme was worked out for the Loppa High and calibrated in recently released boreholes on the southern part of the high (Johansen et al., 1994a). However, as a result of the Late Palaeozoic structuring, regional stratigraphic correlations between structural elements are difficult and only two intrasedimentary Late Palaeozoic reflectors, top Permian (TP) and near base Permian (NBP), were mapped regionally (Fig. 3). The sequence between these two reflectors is informally referred to as the Permian succession. In addition, the top basement reflector (TB) was mapped locally on many of the highs. All faults which record movement during Late Palaeozoic times were plotted together with several other Late Palaeozoic structural features such as monoclinal flexures and structural axes of basins and highs. Deep reflectivity patterns, both intrasedimentary and within the basement, were also used extensively for correlation and delineation of the major structural elements. In order to determine the distribution of Upper Palaeozoic salt deposits, large salt structures such as salt pillows, domes and diapirs were included. On the basis of this material, a first order structural map was constructed (Fig. 4). A series of interpreted seismic reflection profiles (Figs. 6 15 and 17) serves to illustrate the structural interpretation.

4. Carboniferous-Permian rifting

4.1. Primary rift structures A fundamental feature in the seismic sections is the presence of a faulted structural relief at depth, infilled and buried by a thick sedimentary section which ranges upwards from syn-rift deposits of variable thickness to more widespread and uniform post-rift deposits (Figs. 615). In a general way, this basin architecture indicates a two-phase development of initial fault-controlled subsidence followed by non-fault-related regional subsidence. Three prominent structural trends are present : north, north-east and north-west (Fig. 4). The north trend is confined to the western part of the study area where it dominates a 100 km wide zone east of the continentocean boundary. Associated with this structural trend is a poorly mapped rift basin, referred to as the western rift basin, with intrabasinal highs and an overall northerly orientation. The north-east trend dominates in the central and eastern parts of the study area where the Nordkapp Basin is the most conspicuous rift basin. The Ottar Basin farther north-west is a much more important structure than thought previously, comparable in dimensions and depth with the Nordkapp Basin (Breivik et al., 1995).

78

S.T. Gudlaugsson et al./Marine and Petroleum Geology 15 (1998) 73 102

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AsteriasFault Complex BillefjordenFault Zone BjarmelandPlatform BjerneyaBasin Bjorneyrenna Fault Complex FingerdjupetBasin FinnmarkPlatform HammerfestBasin HarstadBasin HoopFault Complex Hornsund Fault Zone KnoleggaFault LeirdjupetFault Complex LofotenBasin

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Fig. 4. Late Palaeozoic structures. Numbered lines give location of interpreted seismic profiles shown in Figs. 6-15 and 17.

These two basins may be linked through the Hammerfest Basin to the major north-south oriented rift basin beneath the western margin, though the manner of linkage is unclear. The Fingerdjupet and Maud basins are lesser structures but their intermediate position and orientation imparts a distinct fan shape to the array of rift basins mapped, from northerly at the margin area, to northeasterly at the Ottar and Nordkapp basins. A clear north-west structural trend is present in the southeast. Because of sparse data coverage in this area, the fault pattern in Fig. 4 is based on Lippard & Roberts (1987).

A comparison of the structural map (Fig. 4) with other published structural information (Jensen & Sorensen, 1992; Dengo & Rossland, 1992) and anomalies in the gravity field (Fig. 5), lends credence to the first order structural framework obtained. The configuration of the Late Palaeozoic rift system found in this study is compared with that of Dengo & Rossland (1992) in Fig. 5. There is an overall similarity in structural trends as well as in the location of major basins and highs, but also notable differences. Both maps show a large rift basin at the western margin although Dengo & Rossland (1992)

S.T. Gudlaugsson et al./Marine and Petroleum Geology 15 (1998) 73 102

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20 °

25 °

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Fig. 5. Structure of the Carboniferous Permian rift system : (a) this study ; (b) Dengo and Rossland (1992). (c) Correlation between the rift structures mapped in this study and the free-air gravity anomaly field.

locate the eastern boundary faults somewhat farther to the west. Also, a Late Palaeozoic origin is indicated for the Senja Ridge and the Veslemoy High, in agreement with our interpretation. The main discrepancy concerns the Loppa High, where Dengo & Rossland (1992) interpret a narrow gr~Lben extending 200 km in a north-northeast direction from the south-western tip of the Loppa High (Fig. 5). We do not find convincing evidence for the reality of this feature which does not bear any resemblance to the more deeply buried extensional structures observed by us (Fig. 17(b)). The proposed presence of a structural high beneath the Norvarg Dome, directly conflicts with our interpretation and that of Breivik et al. (1995). A fault :gone proposed by Dengo & Rossland (1992) to parallel the north-eastern coastline cuts important north-east striking faults of Lippard & Roberts (1987) and Bugge et al. (1995) and also shown on the map of Sigmond (1992). We also note a similar disagreement with our interpretation of the Nysleppen Fault Complex. Other discrepancies are minor and may result

from differences in data coverage between the two studies and uncertainty in structural correlation. Close to the western continental margin, the gravity field (Fig. 5(c)) exhibits strong positive and negative anomalies related to Late Mesozoic and Cenozoic structures. In the platform areas east of the Ringvassoy Loppa, Bjornoyrenna and Leirdjupet fault zones, where there has been little post-Palaeozoic structuring, the field is more subdued and the gravity signature is largely controlled by Late Palaeozoic structures (Breivik et al., 1995). Areas with a free-air gravity anomaly of - 1 0 mGal or lower show a good correlation with the rift basins (Fig. 5(c)). 4.2. Structures associated with the north trend 4.2.1. Western rift basin The Ringvassoy Loppa, Bjornoyrenna and Leirdjupet fault zones define the eastern flank of a rift basin which extends 300 km from the Harstad Basin to the Stappen

80

S.T.

Gud/augsson et al.,,Marine and Petroleum Geology 15 (1998) 73 102

High. Deep seismic reflection and expanding spread profiles indicate the presence of Upper Palaeozoic strata beneath the Jurassic--Cretaceous basins west of these fault zones (Gudlaugsson et al., 1987; Jackson et al., 1990). They are, however, deeply buried and only the eastern margin of the Palaeozoic rift basin can be studied on conventional seismic reflection data. The continuation of the rift basin towards the north and south is not clear. It may change polarity across the Leirdjupet Fault Complex and continue northward east of Bjornoya as the Fingerdjupet Basin, but the Knolegga Fault indicates a possible northward continuation west of Bjornoya. Southward continuation into the Harstad Basin seems likely, although late Palaeozoic rift structures have not yet been reported. Several observations suggest a Late Palaeozoic age for the western rift basin. The extensional structures in the Fingerdjupet Basin are definitely of Late Palaeozoic origin (Fig. 6). Most of the movements pre-date the Permian, but thickening of the Permian sequence towards faults indicates subordinate movements also during that period. The boundary faults limiting the Loppa High to the west were also active in Permian times (Fig. 7). Similarly, both Carboniferous and Permian fault movements occurred on Bjornoya located on the Stappen High (e.g., Worsley et al., 1990). Moreover, the occurrence of salt diapirs in the western rift basin shows the presence of Late Palaeozoic evaporite sub-basins. By analogy with the age of the wide-

spread evaporite deposits elsewhere in the Barents Sea, this development is expected to have taken place in Late Carboniferous Early Permian tirnes (Stemmerik & Worsley, 1989, 1995; Jensen & Sorensen, 1992: Cecchi, 1993; Nmtvedt et al., 1993a). In fact, salt drilled in the Tromso Basin has been dated to Late Carboniferous earliest Permian (Faleide et al., 1993).

4.2.2. Fingerdjupet Basin Significant Late Palaeozoic extension occurred in the Fingerdjupet Basin (Fig. 6). A basement block in profile 2 indicates extension on the order of 10 15 km on a basin-bounding listric normal l:ault with downthrow towards east. The basin developed as a westward tilted halfgraben in the hangingwall of this fault and is bounded on the south-west by a north trending intrabasinal horst. The position of the horst appears to be controlled at depth by an eastward tilted basement block, marking a shift to the polarity characterizing the eastern flank of the main western rift basin farther south. 4.2.3. Senja Ridge and Veslemoy High Seismic studies and gravity modelling of the Senja Ridge have revealed a high-density core, probably a basement ridge, beneath the sedimentary cover (Riis et al., 1986). Because of the similarity in seismic and gravity signature, this model probably also applies to the Veslemoy High. If correctly interpreted (cf. Faleide et al.,

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3

E

Fig. 6. ln[erpreted seismic profiles 1, 2 and 3 from Stappen High and Fingerdjupet Basin. Location of profiles and nalnes o[" structural elements in Fig. 4.

S.T. Gudlauqsson et al./Marine and Petroh,um Geology 15 (1998) 73 102

81

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Fig. 7. Interpreted seismic profiles4 and 5 from Loppa High. Legend in Fig. 6. Location of profiles and names of structural elements in Fig. 4.

1993), the distribution of salt diapirs in the western rift basin (Figs. 2 and 4) shows that thick layers of salt must have been deposited both east and west of the Senja Ridge. A Carboniferous Permian age for this salt would in turn indicate that the ridge was established as an intrabasinal high in Late Palaeozoic times. It is therefore tempting to postulate that the core of the Senja Ridge and the Veslemoy High consists of tilted basement blocks similar to those observed in the Fingerdjupet Basin. 4.2.4. Loppa High

The eastern flank of the western rift basin is defined by the L o p p a High. Seismic ties to a borehole located on the southern part of the high allow delineation of the basement surface by seismic means. In east-west crosssections (Fig. 7), the basement block has a triangular or trapezoidal shape with a steeply dipping eastern flank and a fault-bounded western flank. The basement is in most areas characterized by bands of reflectors indicating internal layering dipping steeply towards the east. A wedge of Upper Palaeozoic sedimentary layers dipping and thickening eastwards overlies the eastern flank of the basement block. At the crest, both basement and overlying sedimentary strata are truncated at a prominent, flatlying angular unconformity. In addition to the main north-striking fault zones bounding the Loppa High on the west, a small eastward tilted halfgraben oriented along the north trend was mapped on the southern part of the high (Fig. 4). Seismic ties to a nearby borehole (Johansen et al., 1994a) show that it was probably initiated by Middle Carboniferous faulting. The occurrence of this halfgraben, and the gentle undulating flexuring of the Permian sequence (Fig. 7), indicate that the deeply buried south-eastern flank of the high is split into several smaller eastward tilted fault blocks. Upper Permian Lower Triassic infill of the asymmetric troughs between the blocks indicates renewed movements on this fault system associated with tilting and erosion of the Loppa High (Johansen et al., 1994a).

Seismic profiles crossing the western boundary of the L o p p a High show clear evidence of syntectonic sedimentation in response to down-to-the-west normal faulting (Fig. 7, profile 5). Due to difficulties in establishing reliable stratigraphic correlations, the movements can only be constrained to Permian Early Triassic times. Other seismic profiles indicate an even larger throw on the boundary faults farther north. Thus, the data show that significant extensional tectonism affected the western rift basin in Permian-Early Triassic times. In our interpretation, the angular unconformity at the crest of the high was at least in part caused by footwall uplift and erosion in response to tectonic unroofing in this rift phase. 4.3. Structures associated with the north-east trend 4.3.1 Nordkapp Basin.

The N o r d k a p p Basin is the most marked Late Palaeozoic structure east of the L o p p a High (Figs. 8 and 9). Although the rift basin is several kilometers deep, it is in some places difficult to define the basin boundary exactly because of poor penetration of the seismic data and halokinetic overprint. The clearest and most easily mappable boundary criterion, shown in Fig. 4, is a marked increase in the dip of the top Permian reflector as it goes into the basin. The north-east striking basin is at least 300 km long, up to 70 km wide, and consists of three main segments: south-west, central and north-east. A possible fourth segment lies at the eastern boundary of the study area. Separated from the Norsel High by the Nysleppen Fault Complex, which consists of normal faults with large downthrow to the south-east, the south-west basin segment has a markedly asymmetric cross-section (Figs. 8 and 9, profile l 1). On the south-eastern side, the Finnmark Platform basement dips into the basin to an estimated m a x i m u m depth of 12 14 km in a broad zone (Fig. 8, profile 8). In the area 26' 27 30'E, this flank is broken into several tilted fault blocks by normal faults

82

S.T. Gudluugsso#t el al.iMarine attd Petroleum Geolo#y 15 (1998) 73 102 25 km

o

i

t i

i

i

5

W

E

NW

SE

m

5

S

N

Fig. 8. Interpreted seisnficprofiles 6.7 and 8 from Nordkapp Basin and Norscl High. Legend in Fig. 6. Location of proliles and names o1"structural elements in Fig. 4.

with downthrow to the south-east (Fig. 8, profiles 7 and 8). Although the data quality is poor, it seems that similar structures extend farther north-east along the flank. The simplest structural interpretation is therefore that this part of the basin is a large halfgraben developed by northwestward rotation against a basin-bounding listric norreal fault (Nysleppen Fault Complex) and that the deformation of the hangingwall is taken up by a set of synthetic normal faults. The central segment has a more east-west oriented basin axis, displaced approximately 20 km towards the north with respect to that of the south-west segment across an intrabasinal ridge jutting into the basin in a north-westerly direction (Fig. 4). On the south, the segment is bounded by the Thor Iversen Fault Complex which consists of normal faults with downthrow towards north and north-east (Fig. 9, profile 10). This observation shows that the rift changes polarity, but there is no direct evidence for halfgraben geometry at depth. The northeast segment has a north-east oriented basin axis, but because of sparse data coverage its structure is less well known. The N o r d k a p p Basin is clearly of pre-Permian origin (Figs. 8 and 9). An extensional tectonic phase with blockrotation and syntectonic sedimentation that stabilized prior to the Permian is observed at many locations. Locally, unconformities related to the onset of faulting are clearly observed (Fig. 8, profile 7: Fig. 9, profile 11). It has, however, not been possible to correlate these

unconformities between the different basin segments. Most probably they mark the same tectonic phase, and clear evidence of distinct older phases of faulting has not been found. Along the south-eastern flank of the southwest segment, the whole Upper Palaeozoic sedimentary succession down to basement is imaged in the seismic data (Fig. 8, profile 8). The pre-Permian succession shows a continuous increase in thickness towards the basin and reaches a maximum thickness of 7 8 km. A diverging reflection pattern indicates a more or less continuous subsidence from the deposition of the oldest sediments until Permian times. It seems therefore that this basin segment developed during a single major tectonic phase. However. taking into account the lack of stratigraphic control and the limited resolution of the seismic data, it is possible that the basin developed in more than one phase through repeated reactivation of the same fault systems. The tectonic development cannot be studied in as much detail in other parts of the basin, but it appears unlikely that the other segments have experienced a significantly different tectonic history. At pre-Permian level, the basin is rimmed by a wedgeshaped sedimentary unit dipping and thinning into the basin in a narrow zone (Figs. 8 and 9). The unit has an internal reflection pattern converging downdip. Along the southern margin of the central basin segment and the northern margin of the south-west segment, where the unit fills in a faulted relief, a reasonable interpretation is that it consists of sediments deposited into a fault-

83

S.T. Gudlaugsson et al./Marine and Petroleum Geoloqy 15 (1998) 73 102 O

25 km

5] S

t

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,--

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9 N

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~

-->

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N

0

5

NW

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Fig. 9. Interpreted seismic profiles 9, 10 and I 1 from Nordkapp Basin and Norsel High. Legend in Fig. 6. Location of profiles and names of structural elements in Fig. 4.

controlled basin (Fig. 9 ; profiles 10 and 11). An upward transition to a weakly prograding internal reflection pattern is observed locally (Fig. 9, profile 11). Along the opposite low-relief margins, the unit overlies a relatively flat-lying substratum and cannot have been deposited off fault scarps suggesting up- and outbuilding at a palaeoshelf edge, possibly of a marginal reef complex (Fig. 9, profiles 9 and 10). These observations suggest deposition into a considerable bathymetric depression caused by the subsidence of halfgrabens of opposite polarity. The Nordkapp Basin contains a large amount of salt as salt diapirs and marginal salt pillows (Figs. 4, 8 and 9) (Bergendahl, 1989; Jensen & Sorensen, 1992 ; Gabrielsen et al., 1992). The thickening of Lower and Middle Triassic (Smithian-Ladinian) strata into the basin is clearly related to the diapiric rise of salt from the basin (Jensen & Sorensen, 1992). The salt layer deposited in the platform areas surrounding the basin at the Carboniferous Permian transition (Gzelian Asselian/ Sakmarian) may have contributed to the formation of the marginal salt pillows (G6rard & Buhrig, 1990; Jensen & Sorensen, 1992; Stemmerik & Worsley, 1995). The diapirs must, however, mainly have been sourced from a much thicker pre-Permian salt layer confined to the basin itself. The Permian succession is fairly even in thickness across the basin and represents a platform deposit forming a basin-covering lid which subsided later in response to salt withdrawal. Outside the basin, the salt deposits reach up to the base of this unit and most probably also did so in the basin. Estimates of the amount

of salt deposited in the basin have been based both on direct calculation of the volume of the diapirs and indirectly on calculation of the volume of the secondary rimsynclines. The average thickness of the original saltlayer ranges from 1.0 1.25 km to 1.4 1.8 km for the south-western segment, and 2.3 3.1 km for the central segment (Bergendahl, 1989; Jensen & Sorensen, 1992). Maximum thicknesses as great as 2.0-2.5 km in the southwest segment and 4 5 km in the central segment are probable. The salt layer was originally probably located in a stratigraphic position above the marginal wedge and beneath the Permian platform, as the Permian succession is draped flexurally over the basin margin wedge and there is an unconformity between the two. Trying to reconstruct the basin geometry to the CarboniferousPermian boundary with the near base Permian reflector in a horizontal position without the salt layer at this stratigraphic level, results in the base of the marginal wedge dipping away from the basin axis, even in places where lack of faulting rules out any tectonic rotation of the wedge. Repeating the reconstruction for several profiles crossing the basin, assuming that the subsidence of the top Permian reflector was caused solely by salt withdrawal from the stratigraphic level indicated above, yields an estimated average thickness of 1.6 1.8 km ['or the south-west segment and 2.7 2.9 km for the central segment. This is in reasonable agreement with the estimates of the original salt layer thickness cited above, and shows that there is space for the salt layer in the proposed

84

S.T. Gudlaugsson el al.,'Marim, aml Pelroleum Geology 15 (1998) 73 102

position. The strata beneath the wedge also lack the sag form expected if there was significant withdrawal of salt from deeper levels, on the contrary, they seem to be relatively flatlying at most localities in the basin.

coevally with the development of the N o r d k a p p Basin in the hangingwall.

4.3.3. O t t a r Basin 4.3.2. N o r s e l High

The south-west segment of the N o r d k a p p Basin is bounded on the north-west by the Norsel High which at Upper Paleozoic levels is a prominent structural high (Fig. 8, profile 6 ; Fig. 9, profile 11 ; Fig. 10, profile 13). A marked angular unconformity separates a thin cover of relatively flatlying Upper Palaeozoic sedimentary strata from an underlying unit exhibiting internal layering dipping steeply towards north-west (Fig. 8, profile 6). The unconformity is an erosional surface corresponding to the top of the basement, as shown by drilling (Gabrielsen et al., 1990 ; Johansen et al., 1994b). The basement surface is broken by normal faults which terminate within the Carboniferous Permian platform succession (Fig. 9, profile 11; Fig. 10, profile 13). These observations show that the high experienced at least two separate phases of tectonic movements during Late Palaeozoic times: Large-scale normal faulting and block rotation was followed by erosion and sediment burial, and by minor block faulting in the latest Carboniferous Permian. Because of its location along the Nysleppen Fault Complex, it is reasonable to explain the older and more significant tectonic phase as resulting from footwall uplift

25 km 0--

r

A 170 kin long and 50 80 km wide basin striking northeast is situated between the Loppa High and the Mercurius High in the north-west and the Norsel High in the south-east (Fig. 4). The seismic data have limited penetration in this area, but there are several indications that the basin may be deep. Profiles across the basin margins typically show a 2 3 km thick, reflective basin fill unit and throws on the boundary t;aults of 1.5 2.5 km (Fig. 8, profile 6; Fig. 9, profile 11 ; Fig. 10). Two large salt domes are present within the basin, the Samson Dome and the Norvarg Dome (Fig. 4; Fig. 11, profile 16). The domes are apparently created by non-penetrative movements ot" deeply buried Upper Palaeozoic salt (Gabrielsen et al., 1990). The basin coincides with a gravity low of 10 reGal or more in an area of little post-Carboniferous structuring (Fig. 5(c)). Local negative anomalies of more than - 3 0 reGal are found over the salt domes. Gravity modelling of the anomaly associated with the Norvarg Dome has established the presence of a large mass deficiency of much greater areal extent than the dome itself below the Permian level (Breivik et al., 1995). Assuming the mass deficiency to be caused by pure salt with a density of 2.2 g/cm 3, the negative anomaly may bc

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f./)

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SG

~

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14I

SE Fig. 10. Intcrpreted seismic proliles 12. 13 and 14 from Ouaf Basin. Legend ill Fig. 6. Location of profiles and names of structt.u'a[ dements in Fig. 4. NW

85

S.T. Gudlau.qsson et al./Marine and Petroleum Geoloq)' 15 (1998) 73 102

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Fig. 1 I. Interpreted seismic profiles 15 and 16 from Onar and Maud basins and Mercurius High. Legend in Fig. 6. Location of profiles and names of structural elements in Fig. 4.

explained by a 3-km-thick salt body below the near base Permian reflector (Fig. 4). More realistic models invoking combinations of sediments and salt above basement indicate a thickness of4~6 km for the anomalous section. We conclude that the Ottar Basin is a major Late Palaeozoic rift basin, comparable in dimensions and depth to the Nordkapp Basin. In addition to the large salt domes, several lesser structures in the basin are also interpreted as salt structures (Fig. 10, profile 12 ; Fig. 11, profile 16). Stratigraphically, they all occur just below the near base Permian reflector and are therefore derived from a salt layer of the same age as the marginal salt pillows in the Nordkapp Basin. Several observations indicate that Mesozoic faults may be totally decoupled from deeper levels by detachment in this salt layer. A reflection from a listric fault plane that flattens out within the salt layer is seen on profile 12 (Fig. 10). Swaen Graben (Fig. 10) crosses the north-western boundary fault of the basin at a high angle. It is evident from profile 14 (Fig. 10), which runs across the intersection of the two structures, that the graben is confined to post-Carboniferous levels and is separated from the deeper levels by detachment in the salt layer. The configuration of Swaen Graben and several nearby Mesozoic grabens may in fact indicate that they were caused by tensional forces in the post-Carboniferous sedimentary succession induced by the formation of the large salt domes. 4.3.4. Maud Basin and Mercurius High The Maud Basin trends north-east to north north-east west of the Mercurius High. In the south, where it is developed as a south-eastward tilted halfgraben, it is dominated by the Svalis Dome (Fig. 11, profile 15). The dome consists mostly of pre-Permian salt originally deposited as a thick layer west of the Mercurius High

(Baglo, 1989 ; Bugge & Fanavoll, 1995). As in the Nordkapp Basin, some salt was also deposited outside the basin, and remnants of this layer are observed on the Mercurius High (Fig. 10, profile 13; Fig. l l, profile 15). The Svalis Dome is associated with a negative gravity anomaly of more than - 5 0 reGal. A lesser negative anomaly (--30 mGal) indicates that unmobilized salt may be present farther north in the basin. The basin extends south-west into the Loppa High where it is bounded by a major normal fault on the north-western side (Fig. 12; profile 17). This fault was active prior to the Permian and was reactivated in Jurassic Cretaceous times. An eastward dipping reflection pattern within the Mercurius High is similar in appearance to that observed at the Norsel High and is interpreted to represent basement

25 km 0

.

5

.

.

.

i

LH NW

S

'1 .

.

.

.

MB 1Tr SE

N

Fig. 12. Interpreted seismic profiles 17 and 18 from Hammerfest Basin and north-eastern Loppa High. Legend in Fig. 6. Location of profiles and names of structural elements in Fig. 4

86

S.T. Gudlau,qsson el al./Marine and Petroleum Geolo.qy 15 (1998) 73 102

layering (Fig. 11 ; profile 15). The geometric relationship with the Hoop Fault Complex, which marks the western boundary of the high, indicates that the high is cored by a large tilted basement block.

4.3.5. HammetJ~,sl Basin

The Hammerfest Basin constitutes a potential structural connection between the western rift basin and the areas east of the Loppa High that were affected by Late Palaeozoic extension. The quality of the seismic data is poor at pre-Mesozoic levels, but observations at several localities indicate the presence of a much thicker section of pre-Permian strata than on the Finnmark Platform to the south. A small but discontinuous change in the thickness of Permian strata is observed across the Troms Finnmark Fault Complex in the southeastern part of the basin, the largest thickness occurring on the basinward side (Fig. 13). An increase in the thickness ofpre-Permian strata in the basin is observed towards the northern boundary fault (Asterias Fault Complex) and an angular unconformity at the near base Permian reflector on the northern side of the fault is interpreted as resulting from erosional truncation of the footwall in response to fault movements (Fig. 12, profile 18). This part of the Palaeozoic Hammerfest Basin probably has alfinity towards the Ottar Basin, while the status of the western part is still undetermined.

25 km I

4.4. Structures associated with the north-west trend

Normal faults off the north-eastern coast and the Trollfjord Komagelv Fault Zone with its possible continuation into the Hammerfest Basin constitute the main components of the north-west structural trend in the south-eastern part of the study area (Gabrielsen, 1984; Ziegler et al., 1986; Lippard & Roberts, 1987; Max & Ohta, 1988; Gabrielsen & Feerseth, 1989; Bugge et al., 1995). This trend abuts against the north-east trend on the Finnmark Platform. In the south-west continuation of the Nordkapp Basin there is a deep Late Palaeozoic basin bounded on the south by faults belonging to both trends, the Mfisoy Fault Complex and the Trollfjord Komagelv Fault Zone trend, and on the north by an intrabasinal high (Fig. 13, profile 21; Figs. 14 and 15). A pre-Permian sediment thickness of up to 5 km is observed m the basin. An interesting observation on profile 25 is a salt pillow with the top at 0.8 s twt (2 kin) beneath the near base Permian reflector (Fig. 15). The reflector geometry of the basinfill sedimentary strata shows that the pillow stage was reached prior to the Permian. This provides evidence for an earlier stage of salt mobilization than is commonly interpreted for the Nordkapp Basin (Bergendahl, 1989; Jensen & Sorensen, 1992). The source layer is clearly older than, and separated from, the areally extensive sail layer at the Carboniferous Perlnian boundary. Thus, it

I

0

U)

5

S

N

C

5

S

N

S

N

0

5

Fig. 13. Interpreted seismic profiles 19. 20 and 21 froln western Finnmark Platform and Hammerli~st Basin. Legend in Fig. 6. Location of proliles and names of structural elements in Fig. 4.

S.T. Gudlaugsson et al./Marine and Petroleum Geology 15 (1998) 73-102

87

25 km I I

O

i

f

. . . .

5

W

E

0 ~ ~

5

1 ~

l

~

: ~ :

::~ ~

l

.............................

I

~;f~. ;. ~.~~.~I ~.~.lI.I I.I . I ~

III

l

] ] .........

I

I

I

,

................

1 HfB TFFC W

E

Fig. 14. Interpreted seismic profiles 22 and 23 from western Finnmark P]atform. Legend in Fig. 6. Location of profiles and names of structural elements in Fig. 4.

25 km t

I

5

S

N ,

5l U F C S"

/

/

25 as N

Fig. 15. Interpreted seismic profiles 24 and 25 across the basin bounded by Mgtsey Fault Complex and the Trollfjord Komagelv Fault Zone. Legend in Fig. 6. Location of profiles and names of structural elements in Fig. 4.

may constitute an isolated equivalent to the deeper part of the original salt layer in the Nordkapp Basin. PrePermian faults post-dating the salt layer are observed in profile 24 (Fig. 15).

4.6. Timing offiault movements It is not possible at the present to establish with certainty the number and duration of extensional phases or map the syn-rift sequences consistently on a regional scale. Yet, apart from sporadic indications of older extensional tectonism, most of the observations are consistent with two main phases of extension, one in the Middle Carboniferous and the other in Permian-Early Triassic times. The former affected most of the study area, gradu-

ally giving way to non-fault-related subsidence towards the end of the Carboniferous, whereas the latter phase mainly affected the present western margin area. Normal faulting along the western margin of the Loppa High as well as the uplift, tilting and erosional truncation of the high itself (Johansen et al., 1994a) are of sufficient magnitude to indicate a significant Permian Early Triassic rift phase affecting the north-south structural trend. Evidence of fault movements is found as far north as the Fingerdjupet Basin. The erosional surface associated with this rift phase extends from the Loppa High to the Stappen High and Permian tectonic activity is known to have occurred on Bjornoya and on the Sorkapp High in the southern part of Spitsbergen (Fig. 2) (e.g., Steel & Worsley, 1984; Riis et al., 1986). Therefore, the

88

S.T. Gudlaugsson el al./Marine and Petroleum Geology 15 (1998) 73 102

rift phase probably affected a narrow northerly trending zone along the entire present western margin. Based on information from boreholes on the Loppa High and the eastern Finnmark Platform as well as regional seismic reflection profiles and correlations to Svalbard and Bjornoya, it has been suggested that faultbounded subsidence and halfgraben formation during Middle Carboniferous times marks an important event within a latest Devonian Middle Carboniferous rift phase affecting the southwestern Barents Sea (Stemmerik & Worsley, 1989; Nottvedt et al., 1993a). The initial development of the halfgraben on the southern part of the Loppa High took place in the Middle Carboniferous (Fig. 4) (Johansen et al., 1994a). A tectonic unconformity, observed in seismic data from the eastern Finnmark Platform and dated in shallow coreholes as Serpukhovian Moscovian, was interpreted to have resulted from a Bashkirian rift phase afl'ecting the southwestern Barents Sea (Bugge et al., 1995). However, since firm dating of the major rift structures is apparently still lacking, we emphasize the additional support for a significant Middle Carboniferous rift phase that comes from seismic observations of salt in the major rift basins combined with the climatic record of Upper Palaeozoic strata in Svalbard and in the south-western Barents Sea. In Svalbard, arid climatic conditions recognized from a prevailing sabkha environment shows that conditions favourable for evaporite deposition were present in the region during Middle Carboniferous to Early Permian times. Before this, the Early Carboniferous saw a humid climate with the formation of coal beds (Steel & Worsley, 1984). Devonian salt deposits are not thought to be present in the south-western Barents Sea because reformation from boreholes suggests that the area remained continental until Carboniferous times ; the earliest marine deposits are of Visean age on the eastern part of the Finnmark Platform and probably of Bashkirian age on the Loppa High (Stemmerik & Larssen, 1992; Bruce & Toomey, 1993 ; Bugge et al., 1995). In both areas a change to an arid climate is expected to have occurred at the beginning of Middle Carboniferous times. Thus, deposition of salt in the rift basins, which lasted into the Early Permian, probably began no earlier than Bashkirian time. The thickness of the salt layer in the N o r d k a p p Basin, which is inferred to have reached 4 5 km locally, implies that a substantial fault-generated depression was in place or developed during Middle Late Carboniferous times. Not all of this thickness corresponds to a fault-defined relief, however, because salt was also deposited in the basin during the subsequent phase of differential thermal subsidence. Still, a considerable fault-bounded basin must have existed. For example, at the southern margin of the central segment where the total thickness of the original salt layer was approximately 4 kin, a faultdefined topography of 3-3.5 km is estimated (Fig. 4; Fig. 9, profile 10). Since the narrow width and steep slopes of

the marginal wedge rimming the basin are elements of immature depositional topography and the source layer was located just above the marginal wedge, the deposition of salt was initiated shortly after the faulting. Thus, the onset of the rift phase probably post-dates Early Carboniferous and may well correlate with the Middle Carboniferous initiation of the halfgraben on the Loppa High. This interpretation agrees with Stemmerik and Worsley (1989) who proposed that a several km thick evaporite sequence was deposited in the N o r d k a p p Basin during Bashkirian Moscovian times. Moreover, Nottvedt et al. (1993a) reported a thick, syntectonic sequence of Bashkirian sedimentary strata in the Nordkapp Basin. In the basin at the intersection between the Mfisoy Fault Complex and the Troll0ord Komagelv Fault Zone, a Middle Carboniferous throw of approximately 2 km is estimated from the level difference between the base of the salt layer and top of the basement across the Mfisoy Fault Complex, when a correction for later movements is made (Fig. 4 ; Fig. 15, profiles 24 and 25). In the deeper part of the basin, the salt layer is found at the base of a 4 5 km thick pre-Permian sedimentary succession. Similarly, Baglo (1989) estimated that the salt layer in the Maud Basin originally had a thickness of 2 2.5 km close to the H o o p Fault Complex. The thickness of the evaporite sequence in the Ottar Basin, minimum 2 kin, also indicates tectonic subsidence at this time (Breivik et al., 1995). The data thus indicate that a major rift phase affected the entire rift system in Middle Carboniferous time.

4.7. R e g i o n a l sag basin

Middle Carboniferous rifting was followed by regional subsidence and sediment accumulation in a regional sag basin, interrupted only by renewed Permian Early Triassic rifting along the north-south structural trend in the western part of the rift system. Several studies of the uppermost Carboniferous Permian succession cast light on the development of the regional sag basin (Stemmerik & Worsley, 1989, 1995; Gerard & Buhrig, 1990; Stemmerik & Larssen, 1992; Bruce & Toomey, 1993 ; Cecchi, 1993 ; Nilsen et al., 1993 ; Nottvedt et al., 1993a; Bugge et al., 1995; Cecchi et al., 1995; Stemmerik et al., 1995). Following Gzelian Asselian/Sakmarian deposition of basinal evaporites and growth of marginal carbonate buildups, a regional shallow-water carbonate platt~rm was established during Sakmarian Artinskian time. Kungurian-Tatarian development was characterized by gradual demise of the carbonate platform and increased deposition of siliciclastic sediments. Gerard & Buhrig (1990) mapped the thickness and seismic facies of Gzelian-Asselian strata east of the Loppa High focussing on the spatial distribution of eva-

S.T. Gudlau#sson et al./Marine and Petroleum Geology 15 (1998) 73-102

porites and carbonate buildups. Fig. 16 compares their isopach map of this interval and the distribution of Carboniferous-Permian carbonate buildups with the rift structures mapped in this study. A regional basin extends from the Loppa High to beyond 3 2 E (Fig. 16a), and thickness variations within this basin indicate a pattern of differential subsidence that correlates well with the configuration of the deeper rift system. In particular, depocentres are found over the Nordkapp, Ottar and

.

S

89

Maud basins. This correlation is supported by the distribution of the carbonate buildups which are mainly found at the basin margins and on intrabasinal highs (Fig. 16(b)). The spatial correlation between the Carboniferous rift basins and the depocentres in the overlying regional sag basin supports our proposed rift configuration. It also indicates that the formation of the regional sag basin was, at least partially, thermally driven and a result of Middle Carboniferous lithospheric extension. Thermal subsidence may have begun to dominate in Late Carboniferous times while brittle extension continued at diminishing rates. High regional subsidence rates continued until the end of the Middle Triassic, but overlap in time with and may be geodynamically related to the rapid subsidence of large basins in the eastern Barents Sea (Bergan & Knarud, 1993 ; Johansen et al., 1993 ; Vglgnes et al., 1994).

5. Older structures and tectonism

71L

20"

Highs

30"

25"

~

Limit of evaporite basin

Evaporitelayer>200mstwt

| 100km I

•::/..:i i !i

20"

i[

25"

5.1. Svalbard Caledonides

30"

Late Palaeozoic basins I, ,, ,, ,,I Carbonate buildups Clastic shelf edge

I

The structural and tectonic interpretation given so far has concentrated on Middle Carboniferous and PermianEarly Triassic rift phases. Fragmentary evidence of older structures and tectonism is found at the margins of the Carboniferous Permian rift system and on intrabasinal highs. This evidence, combined with other data, regional geological considerations and the spatial configuration of the rift system, suggests the presence of two major basement provinces of Caledonian origin in the western Barents Sea, referred to as the Svalbard and Barents Sea Caledonides. Local observations suggest that the Barents Sea Caledonides, and possibly also the Svalbard Caledonides in the area south-east of Bjornoya, were affected by late-orogenic or early post-orogenic extensional collapse.

I O0 km

I

Fig. 16. (a) Distribution of latest C a r b o n i f e r o u s ~ a r l i e s t Permian evaporites (from G6rard & Buhrig, 1990) in relation to underlying rift structures. (b) Distribution of Permo Carboniferous carbonate buildups (based on G6rard & Buhrig, 1990 ; Bruce & Toomey, 1993 ; Nilsen et al., 1993: Johansen et al., 1993; Johansen et al., 1994a) in relation to rift structures.

On the Svalbard Platform two different basement provinces occur, separated by a transitional zone on the basis of systematic differences in crustal reflectivity observed in deep seismic reflection profiles (Gudlaugsson et al., 1987). This interpretation is supported by aeromagnetic studies which show that the two provinces exhibit different magnetic signatures (Skilbrei et al., 1990; Skilbrei, 1991; Skilbrei et al., 1993b). The western province, between Bjornoya and Spitsbergen, is characterized by an unusual hyperbolic seismic reflection signature interpreted as representing folding and faulting of the entire crust in a compressional tectonic setting (Gudlaugsson & Faleide, 1994). The magnetic signature of the province is dominated by linear north-north-west to north striking magnetic anomalies. Where they intersect a deep seismic profile across the province, magnetic source depths cor-

90

s.T. Gudlauqsson et al./Marine and Petroleum Geoh¥ly 15 (1998~ 73 102

responding to the seismically defined basement surface are indicated (Skilbrei, 1991; Gudlaugsson & Faleide, 1994). The linear magnetic anomaly pattern continues into the Stappen High where shallow depths to basement are predicted by the aeromagnetic data (Skilbrei, 1991). On Bjornoya, located on the Stappen High, Caledonian basement rocks are known from outcrops (e.g., Harland, 1985). East of the island, the top of this basement is identified as an eastward dipping reflector separating subhorizontal intrasedimentary reflections from a more irregular reflection pattern below (Fig. 6; profile 1). We interpret large antiforms defined by the pattern of reflectivity within the basement as having resulted from deformation in a compressional regime. A strong reflection dipping westwards to a depth of 4 s twt (12 kin) beneath the basement surface is probably a thrust fault. Together, these observations show that a wide area of continental crust along the present continental margin was affected by crustal-scale compressional deformation resulting in a north-north-west to north striking basement grain. Both east of Bjornoya and farther north on the Stappen High, the cover of Upper Palaeozoic sediments onlapping the basement surface westwards is largely undisturbed by post-orogenic extension (Fig. 6 ; profile 1) (Gudlaugsson et al., 1987; Gudlaugsson & Faleide, 1994). Based on the seismic profiles, the compressional event can only be constrained to pre-date the top Permian, the oldest stratigraphic horizon identified with reasonable certainty. However, an older age is suggested by an undisturbed sedimentary section of considerable thickness below this horizon. Since the event was compressional and involved the whole crust, it probably dates back to the Caledonian Orogeny, the last major Palaeozoic compressional event recorded on Svalbard and Bjornoya. The alternative is a Late Devonian origin in the Svalbardian movements, but seismic observations combined with regional tectonic considerations render this unlikely. Deep seismic profiles offshore southern Spitsbergen also show a hyperbolic reflection signature indicating a continuation of this type of crust into southern Spitsbergen (Eiken, 1994; Skilbrei et al., 1993a). This is supported by the continuation of the northerly oriented magnetic fabric into Spitsbergen (Skilbrei, 1991, 1992; Skilbrei et al., 1993b). By contrast, seismic profiles across the Devonian Graben of Spitsbergen show that the contractional structures in the graben-fill strata caused by the Svalbardian movements do not give rise to hyperbolic reflections (Nmtvedt, 1994). In our interpretation, this difference in seismic signature is incompatible with a c o m m o n origin of these two types of compressional structures which more likely represent two different compressional events affecting Spitsbergen. We therefore conclude that the basement province underlying the western Svalbard Platform is of Caledonian origin and represents a southward continuation of the

western provinces of the Svalbard Caledonides at least as far south as Bjornoya. Farther south, in the Fingerdjupet Basin, where the seismic profiles show large tilted basement blocks bounded by pre-Permian listric normal faults, significant postorogenic extension took place (Fig. 6; profiles 2 and 3). The dominant north-south trend of these faults indicates basement control by a structural grain belonging to the Svalbard Caledonide trend and this basement province may thus extend even farther south. 5.2. B a r e n t s S e a Cah, d o n i d e s

The dominant north-east striking structural trend in the south-western Barents Sea is aligned with the onshore Caledonian trend and has been interpreted as being of Caledonian origin (Ronnevik et al., 1982 ; Faleide et al., 1984 ; Ronnevik & Jacobsen, 1984 ; Dor6, 1991 ; Dengo 8,: Rossland, 1992). This is supported by our observations. Important fault zones defining the trend (e.g., Troms Finnmark and Nysleppen fault complexes) are basement involved and have been repeatedly reactivated, indicating structural control by zones of weakness within the basement. The basement control probably operated from early post-Caledonian times because in some cases the deposition of the oldest sedimentary sequences above the basement was demonstrably controlled by the same structural elements that were active later in the Palaeozoic (e.g., south-west segment of the N o r d k a p p Basin), and there is little evidence of structures active in early postCaledonian times defining a different structural trend. However, observational evidence for extensional reactivation of known Caledonian structures has been lacking. On the western Finnmark Platform, south of the H a m merfest Basin, the basement exhibits significant reflectivity which has been interpreted in terms of relict Caledonian structures (Johansen et al., 1994b). Reflections within the basement in the area between 23 and 25'E fall into two main groups (Figs. 13, 14 and 17a): (1) A set of reflectors dips steeply southwards to a depth of 2 s twt (6 km) below the top of the basement. The reflectors, which in places appear to represent layering, are truncated by erosion at the basement surface (Fig. 13, profile 19; Fig. 17a). (2) A 0.5-1 s twt (1.5 3 km) thick band of reflectors dipping less steeply to the north floors the seismic fabric defined by the reflectors in the first group. A probable structural interpretation is that layered basement is broken into blocks by listric normal faults with downthrow towards north or north-west. The faults fiatten out at depth where they meet low-angle faults forming a d6collement. Although the delineation of single fault blocks and their correlation from profile to profile is difficult, the general north dip of the ddcollement together with a north-easterly strike of at least one of the normal faults indicates that the fault blocks are oriented parallel

S.T. Gudlaugsson et al./Marine and Petroleum Geolog)' 15 (1998) 73 102

S

NW

O-

SE

1. 2

Ii 3 . 456

Fig. 17. Extensional collapse structures within the basement of (a) Western Finnmark Platform (part of profile 19, Fig. 13), (b) Loppa High (profile 26). Legend in Fig. 6. Location of profiles and names of structural elements in Fig. 4.

to the onshore Caledonian structural trend. More speculatively, the sole faults may originally have been Caledonian thrusts, the abovelying fault blocks having formed by extensional collapse of the Caledonian nappe pile through backsliding and splaying of an array of normal faults up through the pile. The onshore part of the Caledonides experienced little if any extension, because most of the faults splayed to the surface north of the present coastline. If this model is correct, the tectonic development was similar to the phase of late-orogenic extensional collapse interpreted elsewhere in the Scandinavian Caledonides (Norton, 1986; S6ranne & S6guret, 1987; Andersen & Jamtveit, 1990). A problem lies in the timing. Evidence from other parts of the orogen shows that the phase of extensional collapse occurred during latest SilurianEarly Devonian times and that crustal thickness had returned to normal by Late Devonian times (Andersen, 1993; Fossen, 1993). Field evidence for extensional collapse has not been reported from the Scandinavian Caledonides bordering on the Barents Sea where contractional deformation in the Scandian phase is thought to have continued into Early and possibly early Middle Devonian times (Roberts & Sundvoll, 1990). A phase of extensional collapse affecting the Barents Sea Caledonides should be either coeval with or immediately post-date this final phase of thrusting, suggesting a Devonian age. With the present data, the extensional event on the western Finnmark Platform can only be constrained to pre-date the top Permian reflector, but

91

there are indications of an older age. First, seismic stratigraphic correlations suggest the presence of a thin layer of pre-Permian sedimentary strata above the basement (Figs. 13 and 14). Second, the basement is deeply eroded and seismic profiles from the eastern Finnmark Platform show peneplanation prior to the deposition of the oldest post-Caledonian sediments (Fig. 8, profile 8) (Johansen et al., 1994b). Since an Early Carboniferous upper age limit of these strata may be assumed on the basis of information from boreholes (Bruce & Toomey, 1993; Nottvedt et al., 1993a; Bugge et al., 1995), a pre-Carboniferous age of the extensional structures appears likely. The eastward dipping basement fabric observed over much of the Loppa High is locally disrupted by sections exhibiting opposing dip (Johansen et al., 1994a). One such example is shown in Fig. 17(b). A large roll-over structure is observed in the hangingwall o f a listric normal fault dipping eastwards to a depth of 2.5 s twt (7.5 kin) beneath the basement surface over a distance of 15 km where it flattens out. The geometry of this structure indicates extension of 5-10 km. High-angle truncation of the hangingwall at the basement surface shows that the structure is deeply eroded. Other similar examples are found on the Loppa High, but it has not been possible to consistently map these structures. Seismic tie to a well on the southern Loppa High does, however, indicate a preMiddle Carboniferous age for the faulting (Johansen et al., 1994a). These observations show that the Loppa High was affected by an early tectonic phase of large-scale crustal extension. This development apparently pre-dated the development of the Permo-Carboniferous rift system during which the high acted as a more tectonically coherent block. The large extension, listric fault geometry and near-horizontal sole fault are more similar to the structural style observed on the Finnmark Platform (Fig. 17) and this further strengthens the interpretation of a distinct phase of late-orogenic extensional collapse in the Barents Sea Caledonides. 6. R e g i o n a l correlations and tectonic m o d e l s

A palaeotectonic reconstruction to end-Permian time which compensates for seafloor-spreading and the main effects of subsequent lithospheric extension and shortening, depicts the geometric relationships between major geologic provinces and structures in the waning stages of continental rifting at the close of the Palaeozoic (Fig. 18). Cenozoic seafloor-spreading in the Norwegian Greenland Sea and lithospheric extension in the Northeast Atlantic rift (Late Cretaceous-Palaeocene and Late Jurassic-Early Cretaceous events) were restored according to Skogseid et al. (submitted), whereas Late Cretaceous-Palaeogene lithospheric shortening in the Eurekan Orogeny was modified from de Paor et al. (1989).

92

S.T. Gudlaugsson et al./Marine and Petroleum Geolo,qy 15 (1998) 73 102

~.:."~ CRATONS

[11111! LOMONOSOVLAND

OROGENS:

EXTENSIONAL BASINS:

BAIKALIAN

I°~°~i DEVONIAN

7zZ~] BAIKALIAN ?

PERMO-CARBONIFEROUS

"x~"~ CALEDONIAN

marginal

INNUITIAN/SVALBARD CALEDONIDES

mid

URALIAN

~,~

OROGEN BOUNDARIES:

external

LATE CALEDONIAN EXTENSIONAL FAULTS: 1 2 3 4 5 6 7

Billefjorden Fault Zone Central Ellesmere Foldbelt Clements Markham Foldbelt Devonian grabens of Pechora basin East Greenland Fault Zone East Greenland Devonian Basin East Greenland Devonian strike-slip faults

8 9 1(} 11 12 13 14

central LATE PALAEOZOIC .........

internal

llitlllllilltt tit t t

Harder Fjord Fault Zone Hazen Foldbelt Kanin- Timan Ridge Kontozero Graben Mere - Trendelag Fault Zone North Greenland Foldbelt Pearya Foldbelt

15 Spitsbergen Devonian Graben 16 Storstremmen ShearZone 17 Sverdrup Basin 18 Trolle Land Fault Zone 19 Trollfjord - Komagelv Fau~tZone 213 Magnetic discontinuity

Fig. 18. Regional palaeotectonic reconstruction to end-Permian timc. M a p construction described in text.

6.1. Structural trends and basement provinces

The reconstruction shows that the study area in the southwestern Barents Sea lies at the intersection of three major compressional orogens: the Baikalides, Caledonides and Innuitian Orogen-Svalbard Caledonides. The Innuitian Orogen is an amalgamation of Palaeozoic foldbelts which developed at the northern margin of Lau-

rentia during Early and Middle Palaeozoic times. It comprises a complex assemblage of mountain structures extending across the Canadian Arctic islands and which continues through North Greenland (Trettin, 1991a). 6.1.1. Baikalian Foldbelt Riphean Vendian rocks deformed in the latest Precambrian Baikalian Orogeny, and exhibiting north-west-

s.T. Gudlaugsson et al./Marine and Petroleum Geology 15 (1998) 7~102

erly structural grain, border Baltica on the north-east (Siedlecka, 1975; Ulmishek, 1985; Gramberg, 1988; Roberts and Onstott, 1993 ; Torsvik et al., 1993). Farther east, between the Kanin Timan Ridge and the Urals, the basement is probably also Baikalian but is covered by an up to 10-12 km thick sedimentary succession in the Pechora Basin (Siedlecka, 1975; Ulmishek, 1982, 1985). A dominant north-westerly structural trend defined by large Devonian grabens within the basin is probably inherited from the structural grain of the underlying basement (Ulmishek, 1982). This trend can be followed in the gravity and magnetic fields as well as in the structure of the sedimentary cover some distance into the southeastern Barents Sea where it is lost beneath the large Late Palaeozoic basins in the eastern Barents Sea (Gramberg, 1988 ; Dor6, 1991 ; Johansen et al., 1993). Along the border of Baltica, however, the Trollfjord Komagelv Fault Zone and parallel structures offshore represent a continuation of the Kanin-Timan Ridge trend into the study area (Lippard and Roberts, 1987; Dengo and Rossland, 1992 ; Siedlecka and Roberts, 1992). 6.1.2. Caledonian Foldbelt

The major tectonic elements of the Early PalaeozoicDevonian Caledonian Orogen, including shear zones and late-orogenic extensional faults, define a prominent north-easterly structural trend between Laurentia and Baltica. The Late Palaeozoic north-east Atlantic rift developed within the Caledonian domain, and its location and structural expression was undoubtedly strongly influenced by Caledonian structures. The continuation of the Caledonides farther into the Arctic is a matter of debate and two main alternatives have been proposed. Ziegler (1988a) proposed that the Caledonides turn into a northerly direction in the south-western Barents Sea and link up with the Innuitian Orogen through the Svalbard Caledonides. According to Dor6 (1991), the main arm of the Caledonides extends in a north-easterly direction across the Barents Sea (Barents Sea Caledonides) in direct continuation of the structural axis of the Scandinavian-Greenland Caledonides. A separate northerly oriented arm underlies the north-western Barents Sea and western Svalbard (Svalbard Caledonides), providing a connection with the Innuitian Orogen. Our results support the model of Dor6 (1991) (Figs. 4 and 18). The dominant north-easterly trend of the Late Palaeozoic rift structures continues at least 600 km into the south-western Barents Sea and the reconstruction places the 300 km wide rift in direct continuation of the north-east Atlantic rift between Baltica and Laurentia. The simplest explanation of this continuity of trend is to assume that it is caused by zones of weakness in the underlying basement, inherited from the Caledonian Orogeny. We also note that amphibolite-facies basement rocks drilled on the Norsel High are reported to have been deformed in the Caledonian Orogeny (Gabrielsen,

93

1990; Johansen et al., 1994b). The development of the high was intimately related to that of the Nordkapp Basin, which follows the Caledonian trend beyond the study area in a north-easterly direction. 6.1.3. Innuitian Orogen and Svalbard Caledonides

The structural grain of the Innuitian Orogen, Svalbard Caledonides and fault zones such as the Trolle Land and Harder Fjord Fault Zones of north-east Greenland and the Billefjorden Fault Zone of Spitsbergen defines a wedge-shaped crustal province between Laurentia and Barentsia characterized by structural trends striking north-west to north. The reconstruction in Fig. 18 clearly brings out the apparent structural coherence of this province which includes the strip of compressional deformation along the western Barents Sea margin extending southward to Bjornoya. The Innuitian Orogen was affected by intermittent deformation of limited extent from Late Silurian to Middle Devonian time and was terminated by an extensive compressional event of latest Devonian-Early Carboniferous age, the Ellesmerian Orogeny in the most restricted sense (Trettin, 1991 a, 1991 b). In North Greenland, the orogeny is poorly dated and a clear separation between Caledonian, Ellesmerian and Tertiary structures has not yet been made. The Ellesmerian event almost certainly affected the North Greenland fold belt. The problem, whether or not parts of the fold belt also was affected by Late Caledonian movements, cannot be answered because of the absence of Devonian strata in this area (Soper & Higgins, 1991 ; Surlyk, 1991). This interpretation is supported by similarities in tectonic development. Pearya, a composite terrane at the northern edge of the Innuitian Orogen, preserves a record of Middle Ordovician diastrophism similar to that of north-western Svalbard and is thought to have been accreted through sinistral strike-slip to the Hazen Foldbelt during Late Silurian times (Trettin, 1991a, 1991c), i.e. contemporaneously with the Scandian phase of the Caledonian Orogeny. The final Palaeozoic phase of largescale compressional deformation affecting the whole lnnuitian Orogen, including the Central Ellesmere and North Greenland Foldbelts, took place in Late Devonian-Early Carboniferous times and is referred to as the Ellesmerian Orogeny (Soper & Higgins, 1991; Surlyk, 1991; Trettin, 1991a, 1991b, 1991d). This history of deformation is broadly similar to that of western Svalbard, where both Caledonian events (Middle Ordovician and Late Silurian-Early Devonian) and lesser post-Caledonian compressional episodes (Late Devonian Svalbardian movements, Early Carboniferous Adriabukta phase of folding; e.g., Ohta, 1992 ; Dallmann, 1992) are also recognized. In eastern Svalbard, Caledonian deformation is apparently less strong (Ohta, 1994; see, however, Gee et al., 1994a), and the dominant low-grade Upper Proterozoic Lower Ordovician metasediments of

94

S.f. Gudlau:lsson el al./Marhu" and Petroleum Geolo:ly 15 (199~) 73 102

this region are shown to border on a postulated Barentsia Craton (Fig. 18).

6.2. Late Caledonian tectonic regime 6.2.1. Devonian megashear hypothesis

Due to the possibility of sinistral megashear movements between Laurentia and Baltica-Barentsia on the order of 200 1000 km during Late Devonian times (Hatland, 1965, 1969, 1972; Harland et al., 1974) it is not clear at what time the continental assembly of Fig. 18 was in place. This is a major concern in the interpretation of the late Caledonian tectonic setting of the study area and affects our understanding of the relationship between the lnnuitian Orogen, Svalbard Caledonides and the Barents Sea Caledonides. According to the megashear hypothesis, which came to include even larger displacements of 1000 2000 kin, this was a late development (Harland & Wright, 1979). The central East Greenland Caledonides were juxtaposed with the Barents Shelf as late as the Early Devonian and the configuration shown in Fig. 18 developed during Late Devonian times. In the reconstructions of Ziegler (1988a, 1988b, 1989), sinistral megashear movements totalling about 1000 km are more distributed in time, lasting most of the Devonian period and continuing into the Early Carboniferous. Although numerous geological studies show that the Silurian Devonian suturing of Baltica Avalonia with Laurentia involved sinistral transpression and strike-slip movements within the Caledonian domain, documented sinistral displacements are either small (tens of km or less) or pre-date the Late Devonian. This is shown by the regional overviews and references in Smith (1988), Dor~ (1991) and Soper et al. (1992) and other studies of the British--Irish Caledonides (Donovan & Meyerhoff, 1982; Parnell, 1982; Smith & Watson, 1983; Hutton & Dewey, 1986 ; Hutton, 1987 ; Norton et al., 1987 ; Roddom et al., 1989 ; Rogers et al., 1989 ; Thirlwall, 1989 ; Flinn, 1992 ; Lintern et al., 1992; Seranne, 1992b; Vaughan & Johnston, 1992), the Scandinavian Caledonides (Gronlie & Roberts, 1989 ; S6ranne, 1992a ; Robinson & Krill, 1993) and the Greenland Caledonides (Strachan et al., 1993). Interpretations of palaeomagnetic data used to support the concept of orogen-parallel displacements of 1000 2000 km in the Devonian (Morris, 1976; Harland, 1980) and Carboniferous (Kent & Opdyke, 1978, 1979; Van der Voo et al., 1979; Van der Voo, 1980; Van der Voo & Scotese, 1981), were also later shown to be in error (Irving & Strong, 1984a, 1984b, 1985; Kent & Opdyke, 1985) and the most recent palaeomagnetic reconstructions show that large-scale movements had ceased by the Early Middle Devonian (Trench et al., 1989 ; Scotese & McKerrow, 1990; Torsvik et al., 1990a, 1990b, 1992).

In the megashear interpretation, the Svalbardian movements resulted from large-scale sinistral strike-slip on north trending faults in and possibly to the west of Svalbard (Harland, 1972, 1979). In particular, large sinistral displacements are postulated to have occurred on the eastern boundary fault of the Devonian Graben, the Billefjorden Fault Zone. Svalbard and, by implication, parts of the western Barents Sea--were interpreted as an assemblage of fault-bounded terranes of different origin, juxtaposed by strike-slip movements in Late Devonian times (Harland, 1978; Harland & Wright, 1979). This concept has been contested on structural and sedimentological grounds (Lamar et al., 1986; Reed et al., 1987; Chorowicz, 1992; Manby & Lyberis, 1991, 1992; Manby et al., 1994). Palaeomagnetic studies in Svalbard have also failed to detect any significant relative motion between Svalbard and Britain during the Devonian, but detailed comparison with Baltica is difficult because o1" the scarcity of reliable palaeomagnetic poles for that plate (Storetvedt, 1972; Lovlie et al., 1984; Torsvik et al., 1985; Jelenska & Lewandowski, 1986; Torsvik et al., 1990a, 1990b, 1992). In view of this status of research in Svalbard, as well as the observations from the British and Scandinavian Greenland sectors of the orogen, even the minimum displacement of 200 km on the Billefjorden Fault Zone in the Late Devonian claimed by Harland et al. (1988) must still be regarded as unsubstantiated. Yet, the case for the disparate nature of Svalbard's basement provinces has considerable support (Harland, 1978, 1985; Harland & Wright, 1979; Harland et al., 1988; Smith, 1988; Gee, 1991, 1993; Gee et al., 1991, 1992, 1994a; Manby & Lyberis, 1991 ; Manby et al., 1994). Pre-Devonian strikeslip assembly of the archipelago during the Caledonian Orogeny probably offers the best solution to this problem (Gee, 1991; Gee et al., 1991, 1992, 1994a; Manby & Lyberis, 1991 ; Manby et al., 1994). We therefore conclude that large-scale strike-slip movements were important in the Scandian phase of the Caledonian Orogeny, and may have affected the western Barents Sea, but that the configuration of basement provinces shown in Fig. 18 was largely in place by Early Devonian times. This indicates that the Svalbardian movements and possibly also the Adriabukta phase of folding (e.g. Birkenmajer, K., 1975; Dallmann, 1992) can be considered as far-field effects of the Ellesmerian Orogeny. 6.2.2. Con/ugate shear systems

The system of north-north-west to north-west trending dextral strike-slip faults postulated by Ronnevik & Jacobsen (1984) to cross the Barents Sea, has not been confirmed by later studies and our study has not uncovered any evidence for important strike-slip faults. Such structures are, however, notoriously difficult to identify in seismic data. The only candidate conjugate strike-slip

S.T. Gudlaugsson et al./Marine and Petroleum Geology 15 (1998) 73 102

fault system available for study onshore, the Trollfjord Komagelv Fault Zone, is the subject of a rapidly expanding literature (cf., Siedlecka & Roberts, 1992; Anonymous, 1993). Dextral strike-slip in the order of 200 300 km may have occurred on the fault zone, most likely in the Late Silurian Devonian Scandian phase of the Caledonian Orogeny, but any post-Caledonian strikeslip movements linked to basin development in the southwestern Barents Sea were probably minor. However, the apparent alignment of the Innuitian Orogen Svalbard Caledonides with the trend of the Trollfjord-Komagelv Fault Zone and Kanin Timan Ridge may not be coincidental (Fig. 18). If strike-slip movements took place along the Trollfjord-Komagelv trend during the Caledonian Orogeny, it is possible that they linked up with transcurrent fault systems in the Svalbard Caledonides and the Innuitian Orogen across the Barents Sea. The turn of the Caledonides of northern Scandinavia into the Kanin-Timan trend, the offshore continuation of the Trollfjord Komagelv Fault Zone into the Hammerfest Basin (Gabrielsen, 1984; Ziegler, 1986; Lippard & Roberts, 1987; Max & Ohta, 1988; Gabrielsen and Fa~rseth, 1989), the segmentation and relaying of Carboniferous-Permian rift basins in the study area and a magnetically defined tectonic discontinuity in the northwestern Barents Sea (Fig. 18 ; Skilbrei, 1991) may all be expressions of zones of weakness in the basement following a basement trend caused by such movements. 6.2.3. Extensional collapse The dominant tectonic process in the Caledonian domain during Devonian times was extensional collapse of the orogen (McClay et al., 1986; Norton, 1986). This is particularly well documented for the Caledonides of southwestern Norway (Norton, 1987; Norton et al., 1987 ; S6ranne & S6guret, 1987 ; Andersen and Jamtveit, 1990; Andersen et al., 1991, 1994; Fossen, 1992), but has also been recognized elsewhere in the Scandinavian Caledonides (Fossen & Rykkelid, 1992 ; S6ranne, 1992a ; Gee et al., 1994b ; Rykkelid & Andresen, 1994) and in the Greenland Caledonides (McClay et al., 1986 ; Strachan et al., 1991). Our study provides evidence that this process also operated in the Barents Sea Caledonides, and possibly also in the southern part of the Svalbard Caledonides south-east of Bjornoya. Chorowicz (1992) and Manby & Lyberis (1992) have suggested that the Devonian graben of Spitsbergen was also formed in such a process.

6.3. Post-Caledonian rifting and regional sag The location of the Barents Sea rift system (Figs. 4 and 18), its structural configuration and the timing of the main rift phases strongly suggest that its development

95

was related to the evolution of the north-east Atlantic and Arctic rift systems during Late Palaeozoic times (Haszeldine & Russel, 1987 ; Ziegler, 1988a, 1989; Stemmerik & Worsley, 1989, 1995; Dor6, 1991). The rift followed the structural axis of the Scandinavian-Greenland Caledonides into the south-western Barents Sea where northerly oriented structures provide evidence for a structural connection to the Arctic rift (Beauchamp et al., 1989 ; Beauchamp & Morin, 1993). The fan-shaped structural configuration of the Barents Sea rift system probably resulted from the interfering influence of a northerly oriented structural grain inherited from the Svalbard Caledonides-Innuitian Orogen (Fig. 18). Baikalian structural trends, which are truncated by the Barents Sea Caledonides, were also reactivated, but to a lesser degree. Rifting between Norway and Greenland in response to plate divergence and lithospheric stretching was probably initiated at the close of Devonian times and major rift phases took place in the Middle Carboniferous and Permian (Surlyk et al., 1984, 1986 ; Surlyk, 1990 ; Stemmerik et al., 1991, 1993). This is similar to the tectonic development in the Barents Sea region. Fault-bounded basins began to develop on Svalbard and Bjornoya at the Devonian-Carboniferous transition (Gjelberg & Steel, 1981; Steel & Worsley, 1984; N~ttvedt et al., 1993a, 1993b) and on the eastern Finnmark Platform in the Early Carboniferous (Visean; Bugge et al., 1995). This probably applied to some extent to most of the southwestern Barents Sea although major rift structures of this age have yet to be identified. A phase of intensified faultcontrolled subsidence occurred on Svalbard in Middle Carboniferous times (e.g., Johannessen & Steel, 1992; Nottvedt et al., 1993b) and this study argues for a major Middle Carboniferous rift phase in the southwestern Barents Sea. A Permian-Early Triassic rift phase along the present western Barents Sea-Svalbard continental margin is well established (Berglund et al., 1986 ; Riis et al., 1986 ; Stemmerik & Worsley, 1989, 1995; Gabrielsen et al., 1990 ; Johansen et al., 1994a). There is some uncertainty, however, about the tectonic setting of the Lower Carboniferous basins on Svalbard and Bjornoya ; a regime of sinistral transcurrent movements has been proposed (Steel & Worsley, 1984). The history of sedimentation in the Wandel Sea Basin suggests that a structural connection between the north-east Atlantic and Arctic rifts did not start to develop until Middle-Late Carboniferous times (Stemmerik & H~kanson, 1991). If correct, this interpretation and the occurrence of compressional deformation in southern Spitsbergen in the Early Carboniferous Adriabukta phase (Dallmann, 1992) suggests that the tectonic influence of the Ellesmerian Orogeny extended to the Svalbard Caledonide province as late as the Early Carboniferous and delayed the propagation of a rift arm from the Northeast Atlantic rift into the Arctic.

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S.T. Gudlaugsson et al./Marine and Petroleum Geolo,qy 15 (1998) 73 102

Following the collapse of the Innuitian Orogen and the initiation of the Sverdrup Basin in the Middle Carboniferous (Beauchamp et al., 1989), rifting into the Barents Sea ceased and the structural link between the Northeast Atlantic and Arctic rifts along the present western Barents Sea-Svalbard margin became the main locus of deformation. The transfer of extensional deformation during Permian times to the western part of the Barents Sea rift, and evidence of extensional tectonism and inversion in southern Spitsbergen, Bjornoya and the Wandel Sea Basin, all support such an interpretation.

6.4. Eastern Barents Sea Basins and Uralian Foldbelt

The obvious interpretation of the Barents Sea rift as an integral part of the north-east Atlantic and Arctic rift system is complicated by the possibility of a tectonic relationship to the large north-north-east trending Eastern Barents Sea Basins of Late Palaeozoic Triassic age, which dominate the structure of the eastern Barents Sea (Figs. 1 and 18)(Gramberg, 1988; Dor6, 1991 ; Verba et al., 1992 ; Johansen et al., 1993 ; Vfignes et al., 1994). The deeper fault-controlled part of these basins, interpreted as being either Devonian (Johansen et al., 1993) or Carboniferous Permian (Verba et al., 1992) in age, is associated with crustal thinning and high seismic velocities in the crust (Gramberg, 1988 : Verba et al., 1992) indicating an extensional tectonic setting. No structural connection has been established between the basins and the northeast Atlantic Arctic rift system, but does not seem unreasonable considering the probability of tit least partly coeval development and the short distances separating them from the Barents Sea rift. In the south-western Barents Sea, there was a shift to regional subsidence in the central and eastern areas from the beginning of Late Carboniferous times. The close correlation between Carboniferous rift structures and the areal distribution of evaporites and carbonate buildups in the overlying Permian succession (Fig. 16) bears witness to a component of differential thermal subsidence, probably induced by the earlier phase of crustal extension. To first order, however, the pattern of subsidence fits into a much broader regional picture of a huge interior sag basin which at the close of the Palaeozoic came to include the whole Barents Sea (Dengo & Rossland, 1992 : Johansen et al., 1993: Vfignes et al., 1994). The Eastern Barents Sea Basins, which formed the depocentrc of the interior sag basin, underwent rapid non-fault-related Late Permian Early Triassic subsidence, an event to a lesser extent also recognized in the western Barents Sea (Bergan & Knarud, 1993 ; Johansen et al., 1993 ; Vfignes et al., 1994). The areal configuration of the Eastern Barents Sea Basins and their Late Permian Early Triassic subsidence, contemporaneously with inversion and folding on

Novaya Zemlya (Ulmishek, 1982, 1985), suggests that the primary driving forces of the regional subsidence may be related in some way to active-margin and continentalcollision processes culminating in the Uralian Orogeny at the eastern Barents Sea margin (Vfignes et al., 1994). In Devonian times, an extensional tectonic regime prevailed in the Pechora Basin causing large grabens to form there, and on Novaya Zemlya and on the Kola Peninsula where alkaline volcanism is well documented (Ulmishek, 1982, 1985: Ziegler, 1988a; K r a m m et al., 1993). Bordering on the Uralian Ocean, where active subduction was taking place, the region was probably in a back-arc extensional setting (Ziegler 1988a ; Nikishin et al., 1993). If Johansen et al. (1993) are correct in interpreting the extensional structural relief beneath the Eastern Barents Sea Basins as being of Devonian age, back-arc extension affecting most of the eastern Barents Sea may have been responsible for their formation, in order to explain the Late Permian Early Triassic subsidence, this would apparently require continued or renewed extension in the Carboniferous and Early Permian. Carboniferous Permian extension is, however, less readily explained in terms of back-arc extension. The Uralian foldbelt on N o v a y a Zemlya resulted from the continental collision ot" the West Siberian Craton and Kazakhstan tectonic collage with the eastern margin of Baltica beginning in Middle Carboniferous times (Ziegler, 1988a). The collision propagated northward and reached the eastern Barents Shelf in the Late Carboniferous Early Permian. This event shut down subduction systems and back-arc basinal extension which had been active along the eastern margin since Early Palaeozoic times. Plate convergence continued alter the collision, and on Novaya Zemlya the orogeny culminated in the Early Triassic. Vfignes et al. (1994) propose that reorganization of mantle convection patterns associated with these events was the primary driving force for the development of the Eastern Barents Sea Basins. An interesting feature of Fig. 18 is that the basins apparently follow neither Caledonian nor Baikalian trends, suggesting overprinting by the tectonic processes operating at the eastern Barents Sea margin. Most studies show a clear break with the Baikalian structural trend at the southeastern boundary of the basins (Gramberg, 1988: Dor6, 1991 : Verba et al., 1992; Johanscn et al., 1993). If their formation was associated with large crustal extension, as indicated by interpretations of deep seismic refraction profiles (Gramberg, 1988 ; Verba et al., 1992), the crust can be restored along transverse structural trends that are segmenting and offsetting the basins. Doing this brings the original site of rifting into a closer alignment with the Caledonian trend. Considering that the continuation of the Barents Sea Caledonides across the eastern Barents Sea is not well known, control by a Caledonian grain on the structural expression of the basins cannot be excluded.

S.T. Gudlaugsson et al./Marine and Petroleum Geology 15 (1998) 73 102

Summary and conclusions 1. The south-western Barents Sea was the site of a major Caledonian tectonic junction. The main Scandinavian-Greenland arm probably continued across the Barents Sea in a north-easterly direction as the Barents Sea Caledonides, whereas a second arm, the Svalbard Caledonides-Innuitian Foldbelt, covered the western Barents Sea, western Svalbard and North Greenland. By Early Devonian times, the crystalline basement of the Barents Sea was consolidated and large-scale transcurrent movements within the Caledonian Innuitian domain had probably ceased. The south-western Barents Sea was located in a region of converging and intersecting structural trends inherited from the Scandinavian-Greenland Caledonides, the Svalbard Caledonides Innuitian Foldbelt, and the Baikalian Foldbelt. This provided the fundamental structural framework for later tectonic development dominated by crustal extension, subsidence and sediment accumulation primarily in response to northeast A t l a n t i ~ A r c t i c rifting. 2. The oldest extensional structures recognized are large rollovers or tilted fault blocks associated with listric normal faults turning into low-angle sole faults. Deeply eroded, in some cases apparently beneath the level of the original hangingwall basins, these structures are thought to record a phase of late-orogenic or early post-orogenic collapse of the Barents Sea Caledonides. There is some evidence that this process also affected the southern part of the Svalbard Caledonides in the area south-east of Bjornoya. 3. Crustal extension between N o r w a y and Greenland, which lead to the initial development of the north-east Atlantic rift at the close of Devonian times, may also have affected the Barents Sea, but the 300 km wide Barents Sea rift, extending at least 600 km in a northeasterly direction, is interpreted to have been formed mainly during the Middle Carboniferous. The primary cause was a major attempt at rifting into the southwestern Barents Sea in direct continuation of the north-east Atlantic rift, but a tectonic link to the Arctic rift, where the Sverdrup Basin was initiated at this time through collapse of the lnnuitian Orogen, is also suggested. The overall structure of the Barents Sea rift is a fan-shaped array of rift basins and intrabasinal highs with orientations ranging from north-easterly in the main rift zone to northerly at the present western continental margin. The structural style is one of several interconnected and segmented basins characterized by halfgraben geometries. Polarity shifts suggest possible presence of north-west trending relay zones. A less prominent north-westerly fault trend, associated with poorly-mapped basins, abuts against the main array of fault basins from the south-east. This configuration of the Barents Sea rift strongly

97

suggests that the structural development was controlled by zones of weakness in the basement. The dominant north-easterly structural trend is interpreted to be inherited from the structural grain of the Barents Sea Caledonides, the northerly trend from the Svalbard Caledonides-Innuitian Foldbelt, the fan-shaped structural configuration most probably resulting from an interference of the two trends. The Baikalian structural trend was reactivated to a lesser degree. Renewed Permian Early Triassic rifting reactivated mainly north trending structures in western part of the rift system. Rifting through the Barents Sea was then abandoned, and the tectonic link between the northeast Atlantic and Arctic rifts, initiated in the Middle Carboniferous, became the primary locus of deformation. . F r o m Late Carboniferous times, the tectonic development in the south-western Barents Sea was dominated by regional subsidence. The close correlation between Carboniferous rift structures on the one hand, and the areal distribution of evaporites and carbonate buildups in the overlying Permian succession on the other hand, is interpreted to result from a component of differential thermal subsidence induced by the earlier phase of crustal extension. To first order, however, the pattern of subsidence fits into a much broader regional picture of a huge interior sag basin which at the close of the Palaeozoic came to include the entire Barents Sea. The depocentre of this basin lay in the eastern Barents Sea where initially fault-controlled Late Palaeozoic basins experienced rapid non-fault-related subsidence in Late P e r m i a n Early Triassic times. The geodynamic origin of these basins, which dominate the structure of the eastern Barents Sea, was probably related to the closure of the Uralian Ocean. The tectonic relationship of the basins to rifting in the western Barents Sea is not well understood and no structural connections have been established. However, their large size and close proximity to the south-western Barents Sea rift zone, together with the size and form of the area affected by the Late Permian-Early Triassic subsidence, suggest a possible modulating influence on rifting in the western Barents Sea by the active-margin and continental-collision processes operating at the eastern Barents Sea margin. A process that culminated with the Uralian Orogeny.

Acknowledgements This paper is a result of a joint research project between Statoil and the Department of Geology, University of Oslo. Statoil provided funding (contract No. T-171 165) and access to the large seismic data base released by the Norwegian Petroleum Directorate. This support is

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s.T. Gudlau,qsson et al./Marine and Petroh, um Geology 15 (1998) 73 102

gratefully acknowledged. We also thank Saga Petroleum for allowing use of supplementary seismic data. We thank our Statoil contacts Lars N. Jensen, Karl Johan Skaar, A n t h o n y M . S p e n c e r a n d T o r e Sv~.nfi f o r f r u i t f u l c o o p e r a t i o n , a n d O d d E. B a g l o a n d L e i v R . S t e n s e n f o r a s s i s t ance during

the early stages of the project. Finally, we

thank Filippos Tsikalas for valuable computer assistance. Comments from Finn Surlyk and two anonymous reviewers further helped to improve the paper. Published with permission of the Norwegian and Statoil.

Petroleum

Directorate

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