INTRODUCTION

The Barents Sea consists of a large epicontinental sea bounded by young passive continental margins in the north and west and covers an area of approximately 1.4 million km2 [3]. It is bounded by Svalbard archipelago in the north and the Norwegian and Russian coasts in the south. The Norwegian–Greenland Sea lies to the west and Novaya Zemlya forms the eastern boundary of the Barents Sea. The Barents Sea contains one of the deepest basins in the world which developed due to different regional tectonic events from Paleozoic to Cenozoic within the North Atlantic–Artic region [4].

Its western part constitutes the northern Norwegian continental shelf and is located between the mainland Norway and Svalbard, informally termed as southwestern Barents Sea [9]. Most of the fault complexes in the southwestern Barents Sea have an overall NE‒SW to ENE‒WSW structural trend in its eastern and central parts, whereas the western part of the area consists of NNW‒SSE to N‒S fault complexes [7, 9]. These fault complexes formed from late Proterozoic to Cenozoic in response to several rifting and collision events and indication of inversion structures were reported by earlier investigators [[3, [4, [9, [15‒[17].

The development of inversion structures has been suggested to be related to strike-slip tectonics or head-on inversion [1, 8, 9, 14]. The aim of present study is to identify and decipher eventual Cretaceous inversion structures in the Bjørnøyrenna Fault Complex by means of structural restoration. To these aims, 2D MOVETM [20], a structural modeling and analysis software by Midland Valley Exploration Ltd. (Glasgow, Scotland), is used and three key seismic lines crossing the central and northern segments of the Bjørnøyrenna Fault Complex are restored.

GEOMENTRY AND STRUCTURAL EVOLUTION OF THE BJØRNØYRENNA FAULT COMPLEX

The NE‒SW striking Bjørnøyrenna Fault Complex is located in the western Barents Sea between 72° N, 19° E and 73°15′ N, 22° E and is considered to be the northern continuation of the Ringvassøy–Loppa Fault Complex [5, 7]. The fault complex marks the boundary between the platform-like Loppa High to the southeast and deep Cretaceous basins to the northwest [10, 12] (Fig. 1).

Fig. 1.
figure 1

Regional setting and major structural elements of the study area (after [21], modified). Indicated: AFC—Asterias Fault Complex, BB—Bjørnøya Basin; BFC—Bjørnøyrenna Fault Complex; COB—Continent‒Ocean Boundary; FSB—Fingerdjupet Sub-Basin; HB—Harstad Basin; HfB—Hammerfest Basin; HFC—Hoop Fault Complex; HrFC—Hornsund Fault Complex; KFC—Knølegga Fault Complex; LFC—Leirdjupet Fault Complex; LH—Loppa High; MB—Maud Basin; MFC—Måsøy Fault Complex; MH—Mercurius High; NB—Nordkapp Basin; NFC—Nysleppen Fault Complex, NH—Norsel High; OB—Ottar Basin; PSB—Polhem Sub-Platform; R-L FC—Ringvassøy–Loppa Fault Complex; SB—Sørvestsnaget Basin; SFZ—Senja Fracture Zone; SR—Senja Ridge; TB—Tromsø Basin; T-F FC—Troms-Finnmark Fault Complex; TIFC—Thor Iversen Fault Complex; VH—Veslemøy High; VVP—Vestbakken Volcanic Province.

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The Bjørnøyrenna Fault Complex is an extensional structural feature which lies over a crustal zone of weakness and displays complex geometries due to multiple phases of deformation, including reactivation and inversion phases [5, 7, 9]. Based on fault geometry and structural trend, subdivided the fault complex into four major segments [9]. The southern segment (NNE‒SSW and NE‒SW), central segment (NE‒SW) and northern segment (NNE‒SSW) are separated from the northwestern shoulder of the Loppa High, which consists of narrow horsts and grabens [9]. According to Gabrielsen et al. [8], these grabens have flower-like structures (Fig. 2, segment 4 (Loppa High)).

Fig. 2.
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Subdivision of the Bjørnøyrenna Fault Complex based on structural trend and geometry (after [9], modified).

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Regional extension and minor strike-slip adjustments along old lineaments in the western Barents Sea started again in Middle-Late Jurassic [4]. At regional scale, the Jurassic extensional structures of the Barents Sea belonged to the larger Arctic-North Atlantic rift system [2]. The Middle-Late Jurassic extensional phase created the Hammerfest and Bjørnøya basins following pre-existing structures and caused block faulting [4]. This was also coeval with renewed subsidence in the Tromsø and Bjørnøya basins [4]. The Bjørnøyrenna Fault Complex is of extensional origin and presents listric fault geometry [4, 7, 9]. The main phase of subsidence along the fault complex started in Callovian [4, 7].

The more complex structuration occurred in the western Barents Sea at the end of Jurassic due to the development of regional scale extensional fault blocks and the influence of shear movements in parts of the North Atlantic [11, 14].

In Early Cretaceous, subsidence went on in the western Barents Sea and, in particular, rapid subsidence occurred along the Bjørnøyrenna and Ringvassøy–Loppa Fault complexes [9].

Faleide et al. [4] described three tectonic rifting phases affecting the major basins of the western Barents Sea (e.g. Hammerfest, Tromsø, and Bjørnøya basins) during Early Cretaceous. The first two phases (Berriasian/Valanginian and Hauterivian/Barremian) strongly affected the Tromsø and Bjørnøya basins. The last phase included thermal subsidence in the Tromsø Basin which is evidenced by gradually increased thickness of the Barremian sediments (i.e. Kolje Formation) westwards in the Hammerfest Basin and into the Ringvassøy‒Loppa Fault Complex.

In the western Hammerfest Basin, uplift and thinning of the Aptian sequences (i.e. Kolmule Formation) towards the Ringvassøy–Loppa Fault Complex evidence further the occurrence of an Aptian tectonic event in the area [4]. In general, the development of Early Cretaceous structures in the western Barents Sea were coeval with the opening of the Amerasian Basin and the North Atlantic rifting and mainly characterized by extensional faults with large downthrow to the west with minor wrench component.

Faleide et al. [4] described sinistral transtensional strike-slip along the Bjørnøyrenna Fault Complex which caused formation of the Senja Ridge and the Veslemøy High as positive structural elements.

Riis et al. [14] also suggested sinistral shear which caused compressional faulting and folding in the Senja Ridge during Early Cretaceous.

Gabrielsen et al. [9] suggested that the local thinning of the Hauterivian–Aptian sequence along the Bjørnøyrenna Fault Complex is associated with mild inversion of local depocentres and reverse faults associated with minor hanging wall folds.

Gabrielsen and Færseth [6] also suggested dextral transpressional strike-slip movement during Early Cretaceous.

According to Gabrielsen et al. [8] the fault pattern in the central segment of the Bjørnøyrenna Fault Complex is similar to positive (semi)-flower structures. Indication of local Early Cretaceous inversion is also observed along the Ringvassøy Fault Complex and its junction with the Asterias Fault Complex [7].

During Late Cretaceous, opening of the Labrador Sea started and regional subsidence occurred along the North Atlantic rift basins. These deep and broad basins ended at the Dee Geer Zone where they were prolonged by pull-apart basins as a response to dextral oblique slip [4].

Most of the basins in the western Barents Sea (i.e., Tromsø and Sørvestsnaget basins) continued to subside during the Late Cretaceous. Although extension was dominant at regional scale during the Late Cretaceous, compressional deformation evidenced by reverse faults and folds was also observed along the Bjørnøyrenna Fault Complex and the Ringvassøy Fault Complex [7, 9]. Seismic stratigraphy analysis suggested that compression started to act after the deposition of the Intra-Cenomanian reflector and the Late Cretaceous strata, which was also involved in folding and reverse faulting was eroded and uncomformably sealed by the Cenozoic strata [9].

The compressional phase continued into Cenozoic and early Cenozoic sequences also experienced head-on inversion with NW‒SE compression direction [9]. The Late Cretaceous inversion phase is characterized by e.g. upright open folds, close to tight inclined to recumbent folds and compressional footwall shortcuts.

In contrast, Rønnevik and Jacobsen [16] and Riis et al. [14] suggested sinistral shear movement due to the opening of the North Atlantic Ocean, causing reactivation of the Bjørnøyrenna Fault Complex in the Late Cretaceous and Cenozoic. The Ringvassøy Fault Complex was also reactivated during the Late Cretaceous [1, 7].

DATA AND METHODS

We used 2D MOVETM [20] (structural modeling and analysis software by Midland Valley Exploration, Ltd (Glasgow, Scotland) for structural restoration of Cretaceous inversion structures in the Bjørnøyrenna Fault Complex, western Barents Sea. Three key seismic profiles with different orientations have been selected for this purpose (Fig. 3). These seismic lines belong to two different surveys (NBR08, NBR10) and are of variable quality and variable depth resolution. The data were provided by the Department of Geosciences, University of Oslo, Norway.

figure 3

Fig. 3. Base map of the study area showing locations and orientations of the restored seismic profiles and locations of used well.

Indicated: inverted structures(red rectangles); structural boundary (red line); faults (black lines); a well location (pink); basins (green), structural highs (yellow-green); deep basin (light blue).

The main motivation for the modeling is to restore the sections backwards, to locate null point positions and to isolate inversion events at Lower and Late Cretaceous levels. To these aims the 2D kinematic module has been used and different restoration techniques (i.e. 2D unfolding or flexural slip, 2D move on fault or simple shear) have been adopted. In general, two objectives can be obtained from restoration or backward modeling of a particular structure. The structural restoration can validate the interpreted geometry in cross section and can provide information about the processes of progressive deformation in the region.

In order to identify inversion events and to locate null points the models need to be decompacted and restored. The workflow used in the current restored models includes digitizing of seismic cross sections. Different horizons including sea bottom and top basement were digitized and polygons representing the different sediment packages were created. The details of all digitized horizons, age and thickness (according to Well 7219/9-1) are given in Table 1.

Table 1.   Digitized horizons representing color and age
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The 2D depth-conversion tool was used to convert the seismic sections into depth. The 2D decompaction tool was used to decompact the rock units and footwall blocks were unfolded using the flexure slip method. The hanging-wall blocks were moved upwards along the fault to restore the latest pre-compressional sedimentary layer, i.e. the Hekkingen Formation, to its original position.

Structural Restoration Techniques

Structural restoration allows to validate an interpreted section through back stripping of depositional and tectonic events by applying certain geometric rules. The robustness of the restoration is checked by means of identifying space problems during unfolding restoration. The 2D unfolding tool, involving flexural slip unfolding, to restore footwall block and the move-on-fault technique for hanging wall block were selected in the present study.

The flexural slip unfolding algorithm maintains bed thickness between the template horizon and other passive objects. The algorithm is also built to maintain the line length of the template horizon in the direction of unfolding and maintain the area of the fold. The move-on-fault module is used to restore deformation and allows the user to model pre- and syn-tectonic successions and different displacements. Simple shear maintains the relationship between fault geometry and hanging-wall deformational features.

It diffuses deformation throughout the hanging wall instead of partitioning it into discrete slip between beds (i.e. flexural slip).

RESULTS

Based on the interpretation of seismic cross-sections, calibrated by exploration wells, the structural restoration permits to investigate the Early and Late Cretaceous inversion events in the western Barents Sea. Total three key seismic profiles located in the central and northern segments of the Bjørnøyrenna Fault Complex have been used for this purpose (Fig. 3).

Restoration of the Key Profile-1

The WNW‒ESE seismic cross-section crossing the central segment of the Bjørnøyrenna Fault Complex, was imported in MOVETM (Figs. 2, 4). Ten key horizons and two faults were digitized using horizon and fault options (Table 1, Fig. 5). Stratigraphy and rock properties were assigned for each sedimentary package according to data available from Well 7219/9-1 (Table 2).

Fig. 4.
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Interpreted seismic cross-section of the central segment of the Bjørnøyrenna Fault Complex (Data courtesy of TGS (Oslo, Norway) [22] and Spectrum (Oslo, Norway) [23]). For location see Fig. 3.

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Fig. 5. Digitized key profile-1: seismic cross-section showing sediment packages and major faults.

Indicated: fault (red line) with half-arrows (sense of movement). For location see Fig. 3.

Table 2. Rock properties of sedimentary packages adopted from Well 7219/9-1
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The polygons of the different sediments packages, ranging in age from Late Triassic to Late Cretaceous, were created using the “create auto polygon tool” [20]. The seismic image then converted into depth using 2D depth conversion tool. The tool used for conversion of a seismic section from time to depth or vice versa.

The equation implemented is:

$$Z = {{V}_{0}}\left( {{{e}^{{kt}}} - {\text{ }}1} \right)/k.$$
(1)

Where:

Z ‒ depth in meters,

V0 ‒ initial velocity (m/s),

k ‒ rate of change in velocity with increasing depth,

t ‒ one-way travel time (s).

If “k” is equal to zero then the formula becomes:

$$Z = {{V}_{0}}t.$$
(2)

All the sediment packages (polygons) were then decompacted using Sclater and Christie’s laws [17]. In present study, most of the used decompacted parameters (e.g. surface porosity and depth coefficient) are given by the program (Table 2). The top-most horizon (Earth surface) was deleted after decompaction.

The footwall block is then unfolded using the flexural slip method (Fig. 6). The base Cretaceous reflector (top Hekkingen Formation) is used as a ‘template bed’ and unfolded to an arbitrary depth of ~1550 m. The rest of all horizons are kept as ‘passive features’ which follow the template bed and preserve interbed volumes. The resulted restored section shows the unfolded footwall block with anticlockwise rotation and the hanging wall block on the left side remains unaltered (Fig. 6). The restored section resulted in ~100‒150 m gap between the two fault blocks.

Fig. 6.
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Unfolded footwall block at the base Cretaceous level (top of the Hekkingen Formation).

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The hanging-wall block is moved along the fault to connect the base Cretaceous horizon (Fig. 7). A simple shear method is adopted and a shear angle of 32° with respect to vertical [19] is applied. The hanging wall block was moved ~300 m upwards and the resultant horizontal elongation or restored shortening is ~200‒250 m. All the sediment packages up to the base Cretaceous level are well connected with each other (Fig. 8).

Fig. 7.
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Hanging-wall block is moved along the fault up to the base Cretaceous level.

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Fig. 8.
figure 8

Position of “null point” indicates positive inversion affecting Lower and Upper Cretaceous sediment packages.

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The thickness variations of Lower Cretaceous sediment package (Knurr Formation) from the hanging wall block and to the footwall fault block indicate positive inversion. The extensional fault reversed the sense of motion during compression which caused Lower Cretaceous sediments to turn inside out and to become a positive feature (Fig. 8).

As a result, the fault retains net extension at a deeper level and net contraction associated with an anticline in the upper portion.

The restored section shows the null point at the base of the Cretaceous (Fig. 8, Hekkingen Formation). The position of the null point evidences the progressive compressional inversion of Lower Cretaceous syn-rift sequence (Knurr Formation). Below the null point, the geometry of the restored fault shows normal faulting while above the null point the geometry points to reverse. The eroded part of Lower Cretaceous (Knurr Formation) is interpolated which suggests reverse faulting.

The restored section shows also positive inversion as a fold in the hanging wall at the base of the Upper Cretaceous (Kolmule Formation). The eroded Kolmule Formation is interpolated from the footwall towards the hanging wall (Fig. 8). The interpolation suggests reverse faulting of the base of the Upper Cretaceous. The restored section is slightly modified and space issues (~100‒200 m) between the hanging wall and the footwall are solved (Fig. 8).

Restoration of Key Profile-2

The WNW–ESE seismic cross section located north of key profile 1 and passing through the central segment of the Bjørnøyrenna Fault Complex is used for structural restoration (Figs. 3, 9). Eight horizons including top Fruholmen Formation (base Jurassic), top Tubåen Formation (Early Jurassic), top Nordmela Formation (Mid-Jurassic), top Fuglen Formation (Upper Mid-Jurassic), top Hekkingen Formation (base Cretaceous), top Knurr Fomation (Lower Cretaceous), top Kolmule Fomation (base Upper Cretaceous) and seafloor are digitized (Fig. 10).

Fig. 9.
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Interpreted seismic cross-section of the central segment of the Bjørnøyrenna Fault Complex (Data courtesy of TGS (Oslo, Norway) [22] and Spectrum (Oslo, Norway) [23]). For location see Fig. 3.

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Fig. 10.
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Digitized key profile-2 showing major faults and formations. For location see Fig. 3.

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After creating polygons of all sediment packages the seismic image is converted into depth using the 2D depth conversion tool. The footwall is then unfolded using the flexural slip method (Fig. 11). A similar procedure to the one used in the restoration of key profile-1 is applied and the base Cretaceous reflector is used as a ‘template bed.’ The footwall is unfolded to the 1600 m depth level. The other horizons are kept as ‘passive features.’ The restored section shows an unfolded footwall with anticlockwise rotation and the hanging wall remains unaltered (Fig. 11). The restored section resulted in ~150‒200 m gap between the two fault blocks. The base of the Upper Cretaceous (Kolmule Formation) shows clockwise tilting on the footwall as a response to the underlying folded Knurr Formation (Fig. 11).

Fig. 11.
figure 11

2D unfolding (flexure slip method) using the base Cretaceous (top of Hekkingen Formation) as template bed.

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The hanging wall is moved ~700 m along the fault in order to the base Cretaceous horizon in the footwall block (Fig. 12). A simple shear method is adopted and a shear angle of 32° from the vertical is applied as proposed by Yamada and McClay [19]. The resultant horizontal elongation or restored shortening is ~500 m. The eroded part of Lower Cretaceous (Knurr Formation) and the Upper Cretaceous (Kolmule Formation) are interpolated. The eroded sediment packages on the footwall (Knurr Formation and Kolmule Formatio) are extended to the hanging wall (Fig. 12).

Fig. 12.
figure 12

Hanging-wall block is moved along the fault up to the base Cretaceous (top of the Hekkingen Formation).

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In the upper part of the Lower Cretaceous sequence, the extensional fault inverted its sense of motion during compression and, as a result, the restored section suggests a positive inversion structure (anticline, reverse fault) at the level of Lower Cretaceous (Fig. 12). The restored section also indicates a compressional structure (reverse fault) at the base of the Upper Cretaceous (Kolmule Formation).

Restoration of Key Profile-3

The WSW‒ENE seismic cross section crossing the northern segment of the Bjørnøyrenna Fault Complex was used for structural restoration (Figs. 3, 13).

Fig. 13.
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Interpreted seismic cross-section of the northern segment of the Bjørnøyrenna Fault Complex (Data courtesy of TGS (Oslo, Norway) [22] and Spectrum (Oslo, Norway) [23]). For location see Fig. 3.

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Seven horizons including top Tubåen Formation (Early Jurassic), top Nordmela Formation (Mid Jurassic), top Fuglen Formation (Upper Mid Jurassic), top Hekkingen Formation (base Cretaceous), top Knurr Fomation (Lower Cretaceous), top Kolmule Fomation (base Upper Cretaceous) and seafloor are digitized (Fig. 14).

Fig. 14.
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Digitized key profile-3 showing major faults and formations. For location see Fig. 3.

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“Auto polygon tool” is used and polygons are created representing different sediment packages ranging in age from Early Jurassic to Late Cretaceous. A similar procedure to the one used for the depth conversion and decompaction of profile-1 and profile-2 is applied. In key profile 3, the footwall block is eroded upto the base of the Upper Cretaceous (Kolmule Formation) due to the uplift of the Loppa High, located in the east of the Bjørnøyrenna Fault Complex. After depth conversion and decompaction, 2D unfolded technique is applied on hanging wall block.

The base Cretaceous horizon (Hekkingen Formation) is used as a ‘template bed’ and the section is unfolded at the depth of ~1430 m. The resulted restored section shows compressional structure (anticline) associated with Early and Late Cretaceous inversion events. Both the Lower Cretaceous (Knurr Formation) and the base of the Upper Cretaceous (Kolmule Formation) moved downward showing compression. A minor gap (~50‒75 m) occurred between the hanging wall block and the Loppa High due to unfolding (Fig. 15). The base of the restored section is slightly modified (20‒30 m) to avoid further gaps.

Fig. 15.
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2D unfolding (flexure slip method) using the base Cretaceous horizon (top of Hekkingen Formation) as template bed.

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The restored section shows downward movement of hanging wall depicting inversion structure (anticline) at Lower Cretaceous (Knurr Formation) and at the base upper Cretaceous (Kolmule Formation).

DISCUSSION

The Bjørnøyrenna Fault Complex is defined as extensional by origin with listric fault geometries [4, 7, 9]. The main phase of subsidence along the fault complex started in late Middle Jurassic i.e. Callovian [4, 7]. The subsidence along the Bjørnøyrenna Fault Complex is locally interrupted in (Hauterivian–Aptian) which is evidenced by local thinning of Lower Cretaceous (Hauterivian–Aptian) sediments [9]. The inversional event is marked as a strike-slip origin (dextral wrenching) on the basis of interpreted basin ward tilt of strata which define half-flower-like structures [1, 6, 8, 9, 14].

Indrevær et al. [14] also interpreted inversion structures of early Barremian to mid-Albian age (ca. 131‒105 Ms) along the margins of the Loppa High and concluded that these inversion structures developed due to uplift of the Loppa High along its inclined boundary fault (e.g. Bjørnøyrenna Fault Complex). The uplift of the Loppa High may be due to the contemporaneous extreme lithospheric thinning going on in the Tromsø and Bjørnøya Basin and somehow the heat created from this thinning caused the Loppa High to be uplifted more than its surroundings and hence cause local tectonic inversion in the area as it forced itself upwards like a wedge [13].

The restored seismic sections along the Bjørnøyrenna Fault Complex also showed inversion in Early Cretaceous. The unfolding of footwall block helped to mark the position of null point in key pr-ofile-1 and key profile-2 and restored eroded parts of Lower Cretaceous (Knurr Formation).

The position of “null point” is showing the progressive compressional inversion of extensional syn-rift of Lower Cretaceous sequence (Knurr Formation). Below “null point,” the geometry of the restored fault showing normal faulting while above the null point the geometry is reverse.

The Bjørnøyrenna Fault Complex also affected by Late Cretaceous tectonics and was reactivated in compression in the Cenomanian, and again in the Late Cretaceous [9, 17]. The Late Cretaceous sediments were also involved in folding and reverse faults and were eroded and diconformably overlain by the early Cenozoic strata. The Late Cretaceous inversional phase is resulted from head-on, southeasterly-directed contraction and characterised by minor fold trains associated with assumed thrust faults at the base of the Upper Cretaceous (Kolmule Formation) in the central segment of the Bjørnøyrenna Fault Complex [9].

Restoration of key profiles also suggested folds and reverse faults associated with Late Cretaceous inversion event. The results also suggested horizontal elongation or restored shortening ~200‒250 m in the central segment of the Bjørnøyrenna Fault Complex (key profile-1) and ~500 m shortening in the northern part of the central segment (key profile-2).

CONCLUSIONS

Inversion in the Bjørnøyrenna Fault Complex is evidenced by thickness vatiations of Lower Cretaceous (Hauterivian‒Aptian) sediments and by folds associated with assumed reverse faults in Late Cretaceous. The restored seismic sections also confirmed two inversion events i.e. Early and Late Cretaceous. The thickness variations of the Lower Cretaceous sediment package (Knurr Formation) in the hanging-wall block and footwall fault block (key profile-1 and key profile-2) indicate positive inversion. The extensional fault reversed the sense of motion during compression which caused Lower Cretaceous sediments to turn inside out and became positive feature. As a result fault retains net extension at a deeper level and got net contraction associated with anticline in the upper portion.