Solid Earth, 14, 961–983, 2023

https://doi.org/10.5194/se-14-961-2023
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Analogue experiments on releasing and restraining bends and their

application to the study of the Barents Shear Margin
Roy Helge Gabrielsen1 , Panagiotis Athanasios Giannenas2 , Dimitrios Sokoutis1,3 , Ernst Willingshofer3 ,
Muhammad Hassaan1,4 , and Jan Inge Faleide1
1 Department of Geosciences, University of Oslo, Oslo, Norway
2 Univ Rennes, CNRS, Géosciences Rennes, UMR 6118, 35000 Rennes, France
3 Faculty of Geosciences, Utrecht University, Utrecht, the Netherlands
4 Vår Energi AS, Grundingen 3, 0250 Oslo, Norway

Correspondence: Roy Helge Gabrielsen (r.h.gabrielsen@geo.uio.no)

Received: 3 January 2023 – Discussion started: 13 January 2023

Revised: 23 July 2023 – Accepted: 25 July 2023 – Published: 31 August 2023

Abstract. The Barents Shear Margin separates the Sval- 4. The fold pattern was generated during the terminal stage
bard and Barents Sea from the North Atlantic. During the (contraction–inversion became dominant in the basin ar-
break-up of the North Atlantic the plate tectonic configura- eas) and was characterized by fold axes striking parallel
tion was characterized by sequential dextral shear, extension, to the basin margins. These folds, however, strongly af-
and eventually contraction and inversion. This generated a fected the shallow intra-basin layers.
complex zone of deformation that contains several structural
families of overlapping and reactivated structures.
A series of crustal-scale analogue experiments, utilizing The experiments reproduced the geometry and positions of
a scaled and stratified sand–silicon polymer sequence, was the major basins and relations between structural elements
used in the study of the structural evolution of the shear mar- (fault-and-fold systems) as observed along and adjacent to
gin. the Barents Shear Margin. This supports the present struc-
The most significant observations for interpreting the tural model for the shear margin.
structural configuration of the Barents Shear Margin are the
following.
1 Introduction
1. Prominent early-stage positive structural elements (e.g.
folds, push-ups) interacted with younger (e.g. inversion)
structures and contributed to a hybrid final structural The physiography, width, and structural style of the Norwe-
pattern. gian continental margin vary considerably along its strike
(e.g. Faleide et al., 2008, 2015). The margin includes a
southern rifted segment between 60 and 70◦ N and a north-
2. Several structural features that were initiated during the ern sheared–rifted segment between 70 and 82◦ N (Fig. 1a).
early (dextral shear) stage became overprinted and oblit- The latter coincides with the oceanward border of the west-
erated in the subsequent stages. ern Barents Sea and Svalbard margins (e.g. Faleide et al.,
2008) and is referred to here as “the Barents Shear Mar-
3. All master faults, pull-apart basins, and extensional gin”. This segment coincides with the continent–ocean tran-
shear duplexes initiated during the shear stage quickly sition (COT) of the northernmost part of the North Atlantic
became linked in the extension stage, generating a con- Ocean. Its configuration is typical for that of transform mar-
nected basin system along the entire shear margin at the gins where the structural pattern became established in an
stage of maximum extension. early stage of shear, later to develop into an active continent–

Published by Copernicus Publications on behalf of the European Geosciences Union.

962 R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends

ocean passive margin (Mascle and Blarez, 1987; Lorenzo, that culminated with Cenozoic break-up of the North Atlantic
1997; Seiler et al., 2010; Basile, 2015; Nemcok et al., 2016). (e.g. Brekke, 2000; Gabrielsen et al., 1990; Faleide et al.,
Late Cretaceous–Paleocene shear, rifting, break-up, and 1993; Gudlaugsson et al., 1998). Two shear margin segments
incipient spreading in the North Atlantic were associated are separated by a central rift-dominated segment along the
with voluminous magmatic activity, resulting in the devel- Barents Shear Margin (Myhre et al., 1982; Vågnes, 1997;
opment of the North Atlantic Volcanic Province (Saunders Myhre and Eldholm, 1988; Ryseth et al., 2003; Faleide at al.,
et al., 1997; Ganerød et al., 2010; Horni et al., 2017). Ac- 1988, 1993, 2008). Each segment maintained the structural
cording to its tectonic development, the Barents Shear Mar- and magmatic characteristics of the crust during its develop-
gin (Fig. 1b) incorporates, or is bordered by, several distinct ment. Of these the Senja Shear Margin is the southernmost
structural elements, some of which are associated with vol- segment, originally termed the Senja Fracture Zone by Eld-
canism and halokinesis. holm et al. (1987). Here NNW–SSE-striking folds interfere
The multi-stage development combined with a complex with NE–SW-striking structures (Giannenas, 2018). Strain
geometry caused interference between structures (and sedi- partitioning characterizes the shear zone system (e.g. west-
ment systems) in different stages of the margin development. ern Spitsbergen, Leever et al., 2011a, b; the Sørvestsnaget
Such relations are not always obvious, but interpretation can Basin, Kristensen et al., 2017).
be supported by scale models. We combine the interpreta- The Hornsund Fault Zone and western Spitsbergen fold-
tion of reflection seismic data and analogue modelling. Thus, and-thrust belt form the northernmost segment of the Bar-
we investigate structures generated in dextral shear. These ents Shear Margin. It coincides with the southern continua-
were generated during initial dextral shear, the development tion of the de Geer zone and the Senja Shear Margin. The
into seafloor spreading, and subsequent contraction. The later Hornsund Fault Zone belongs to this system and provides a
stages (contraction) were likely influenced by plate reorgani- type setting for transpression and strain partitioning together
zation (Talwani and Eldholm, 1977; Gaina et al., 2009; see with the western Spitsbergen fold-and-thrust belt (Harland,
also Vågnes et al., 1998; Pascal and Gabrielsen, 2001; Pascal 1965, 1969, 1971; Lowell, 1972; Gabrielsen et al., 1990; Ma-
et al., 2005; Gac et al., 2016) or other far-field stresses (Doré her et al., 1997; Leever et al., 2011 a, b). Plate tectonic re-
and Lundin, 1996; Lundin and Doré, 1997; Doré et al., 1999, constructions suggest that the plate boundary accommodated
2016; Lundin et al., 2013). The present experiments were de- ca. 750 km along-strike dextral displacement and 20–40 km
signed to illuminate the structural complexity affiliated with of shortening in the Eocene (Bergh et al., 1997; Gaina et al.,
multi-stage sheared passive margins so that the significance 2009).
of structural elements like fault-and-fold systems observed The Knølegga Fault Zone can be seen as a part of the Horn-
along the Barents Shear Margin could be set into a dynamic sund Fault system extending from the southern tip of Spits-
context. The study area suffered repeated and contrasting bergen (Gabrielsen et al., 1990). It trends NNE–SSW to N–
stages of deformation, including dextral shear, oblique ex- S and defines the western margin of the Stappen High. The
tension, inversion, and volcanic activity. This is a particular vertical displacement approaches 6 km. Although the main
challenge in such tectonic settings that are characterized by movements along the fault may be Tertiary in age, it is likely
repeated overprinting and cannibalization of younger struc- that it was initiated much earlier. The Tertiary displacement
tural elements. Results from the experiments facilitate the may have a lateral (dextral) component (Gabrielsen et al.,
identification and characterization of structural elements at 1990).
the different stages of deformation. Additionally, they allow The Vestbakken Volcanic Province is the main topic of this
identifying the structural elements that were developed at contribution. It represents the central rifted segment of the
stages of deformation preceding the present-day margin con- Barents Shear Margin and links the sheared margin segments
figuration. to the north and south, occupying a right double-stepping
(eastward) releasing bend setting. Prominent volcanoes and
sill intrusions suggest three distinct volcanic events in the
2 Regional background Vestbakken Volcanic Province (Jebsen and Faleide, 1998;
Faleide et al., 2008; Libak et al., 2012). It is constrained to
In the following sections we provide definitions and a short its east by the eastern boundary fault (EBF in Fig. 1b), which
description of the main structural elements constituting the is part of the Knølegga Fault Complex, separating the Vest-
study area. The structural elements are presented in sequence bakken Volcanic Province from the marginal Stappen High to
from north to south (Fig. 1b). the east. To the south and southeast the Vestbakken Volcanic
The greater Barents Shear Margin is a part of the more ex- Province drops gradually towards the Sørvestsnaget Basin
tensive de Geer zone mega shear system which linked the across the southern extension of the eastern boundary fault
Norwegian–Greenland Sea and the Arctic Eurasia system and its associated faults. To the west and north the area is de-
(Eldholm et al., 1987, 2002; Faleide et al., 1988; Breivik lineated by the continent–ocean boundary and/or transition.
et al., 1998, 2003). Together with its conjugate Greenland The Vestbakken Volcanic Province includes both extensional
counterpart it carries evidence of post-Caledonian extension and contractional structures (e.g. Jebsen and Faleide, 1998;

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R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends 963

Figure 1. (a) The Barents Sea is separated from the Norwegian–Greenland Sea by the de Geer transfer margin. The red box shows the
present study area. (b) Structural map of the Barents Shear Margin. Note the segmentation of the continent–ocean transition. Abbreviations
(from north to south) – WSFTB: western Spitsbergen fold-and-thrust belt, HFZ: Hornsund Fault Complex, KFC: Knølegga Fault Zone,
VVP: Vestbakken Volcanic Province, SB: Sørvestsnaget Basin, VH: Veslemøy High, SR: Senja Ridge, SSM: Senja Shear Margin. Blue lines
indicate the position of seismic profiles in Fig. 2, and the red line (X-X’) shows the western border of thinned crust (see also Fig. 3). Chron
numbers are indicated on oceanic crust area.

Faleide et al., 2008; Blaich et al., 2017). Two main episodes Paleocene fast sedimentation (Ryseth et al., 2003). The later
of Cenozoic extensional faulting were identified in the Vest- stages of the basin formation in particular were strongly
bakken Volcanic Province: (i) a late Paleocene–early Eocene influenced by the opening of the North Atlantic (Hanisch,
event, which correlates in time with the continental break-up 1984; Brekke and Riis, 1987). Salt diapirism also contributed
in the Norwegian–Greenland Sea, and (ii) an early Oligocene to the development of this basin (Perez-Garcia et al., 2013).
event that is tentatively correlated with plate reorganization The Senja Ridge (SR in Fig. 1b) runs parallel to the con-
around 34 Ma activating NE–SW-striking faults. Volcanic ac- tinental margin and coincides with the western border of the
tivity coincides with these events. Tromsø Basin. It is characterized by a N–S-trending gravity
The Sørvestsnaget Basin occupies the area east of the COT anomaly which is interpreted as buried mafic–ultramafic in-
between 71 and 73◦ N and is characterized by an exception- trusions associated with the Seiland Igneous Province (Fich-
ally thick Cretaceous–Cenozoic sequence (Gabrielsen et al., ler and Pastore, 2022). The structural development of the
1990). To the west it is delineated by the Senja Shear Mar- Senja Ridge has been associated with shear affiliated with
gin, and to the northeast it is separated from the Bjørnøya the development of the shear margin (Riis et al., 1986), and
Basin by the southern part of the Knølegga Fault Complex though it documented that it was a positive structural element
(Faleide et al., 1988). The position of the Senja Ridge coin- from the mid-Cretaceous to the Pliocene, it may have been
cides with the southeastern border of the Sørvestsnaget Basin activated at an even earlier stage (Gabrielsen et al., 1990).
(Fig. 1b), whereas the Vestbakken Volcanic Province is situ- The Senja Shear Margin was active during the Eocene
ated to its north. An episode of Cretaceous rifting in the Sør- opening of the Norwegian–Greenland Sea dextral shear,
vestsnaget Basin climaxed in the Cenomanian–middle Tur- causing splitting off of slivers of continental crust. These
onian (Breivik et al., 1998), succeeded by Late Cretaceous– slivers became embedded in the oceanic crust during con-

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964 R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends

Figure 2.

tinued seafloor spreading (Faleide et al., 2008). The Senja The structural development of the margin was complicated
Shear Margin coincides with the western margin of a basin by active halokinesis (Knutsen and Larsen, 1997; Gudlaugs-
system superimposed on an area of significant crustal thin- son et al., 1998; Ryseth et al., 2003).
ning. This part of the shear margin was characterized by
composite architecture even during the earliest stages of its
development (Faleide et al., 2008). The basin system ac- 3 Reflection seismic data and structural interpretation
cumulated sedimentary sequences that reached thicknesses
of up to 18–20 km. Subsequent shearing contributed to the The dataset of this study includes 2D seismic reflection data
development of releasing and restraining bends, associated from several surveys and well data from the Vestbakken Vol-
pull-apart basins, neutral strike-slip segments, flower struc- canic Province. Data coverage is less dense in the north-
tures, and fold systems (sensu Crowell, 1974a, b; Biddle and ern part of the study area. Typical spacing of seismic lines
Christie-Blick, 1985a, b; Cunningham and Mann, 2007a, b). is 4 km. Well 7316/5-1 was used to correlate the seismic
The hanging wall west of the Knølegga Fault Complex (see data with formation tops in the study area, while previously
below) of the Barents Shear Margin was particularly affected published correlations provided the calibration and age of
by wrench deformation as seen from several push-ups and each seismic horizon (e.g. Eidvin et al., 1988, 1993; Ry-
fold systems (Grogan et al., 1999; Bergh and Grogan, 2003). seth et al., 2003). Three stratigraphic groups are encoun-

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R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends 965

Figure 2. Seismic examples from the Vestbakken Volcanic Province. (a) Gentle, partly collapsed, NE–SW-striking anticline and/or dome
of uncertain origin in the eastern terrace domain of the southern Vestbakken Volcanic Province. (b, c) Asymmetrical folds (fold family 2;
Giannenas, 2018) situated along the eastern margin of the Vestbakken Volcanic Province. These may represent primary SPE-4 structures
focused in the hanging walls along margins of master fault blocks, representing reactivated SPE-2 structures. (d) Trains of symmetrical
folds with upright fold axes (corresponding to PSE-5 structures) are preserved inside larger fault blocks. See the text for an explanation of
SPE structures. (e) Section through a push-up associated with restraining bend (PSE-4 structure). (f) Flower (PSE-2) structure in the area
dominated by neutral shear.

tered in the well, namely the Nordland Group (between 473– 4 Strike-slip systems and analogue shear experiments
945 m), the Sotbakken Group (between 945–3752 m), and
the Nygrunnen Group (between 3752–4014 m) (Eidvin et
Shear margins and strike-slip systems are structurally com-
al., 1993, 1998; https://www.npd.no, last access: 29 August
plex and highly dynamic so that the ultimate architecture
2023). Several folds of regional significance and with ax-
of such systems contains structural elements that were not
ial traces that can be followed along-strike for 2–3 km or
contemporaneous (e.g. Graymer et al., 2007; Crowell, 1962,
more occur in the Vestbakken Volcanic Province. The folds
1974a, b; Woodcock and Fischer, 1986; Mousloupoulou et
are commonly situated in the hanging walls of extensional
al., 2007, 2008). Analogue models offer the option to study
faults, and the fold traces and the structural grain of the thick-
the dynamics of such relations; they therefore attracted the
skinned master faults are generally parallel. This shows that
attention of early workers in this field (e.g. Cloos, 19281,
the position and orientation of the folds were determined
1955; Riedel, 1929) and have continued to do so until today.
by the pre-existing basement structural fabric. The mapping
Early experimental works mostly utilized one-layer (“Riedel
of the folds is constrained by the spacing of refection seis-
box”) models (e.g. Emmons, 1969; Tchalenko, 1970; Wilcox
mic lines, so each fold trace may include undetected over-
et al., 1973), which were soon to be expanded by the study
lap zones or axial offsets. The folds were identified on the
of multilayer systems (e.g. Faugère et al., 1986; Naylor et
lower Eocene, Oligocene, and lower Miocene levels. All the
al., 1986; Richard et al., 1991; Richard and Cobbold, 1989;
mapped folds are either positioned in the hanging walls of ex-
Schreurs, 1990, 2003; Manduit and Dauteuil, 1996; Dateuil
tensional (sometimes inverted) master faults or are dissected
and Mart, 1998; Schreurs and Colletta, 1998, 2003; Ueta et
by younger faults with minor throws.
al., 2000; Dooley and Schreurs, 2012). The systematics and
dynamics of strike-slip systems have been focused upon in

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966 R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends

a number of summaries like Sylvester (1985, 1988), Bid- tapers off towards the oceanic crust with a relatively con-
dle and Christie-Blick (1985a, b), Cunningham and Mann stant gradient. A sand wedge with a constant dip angle de-
(2007), Dooley and Schreurs (2012), Nemcok et al. (2016), termined by the difference in thickness between the intact
and Peacock et al. (2016). Concepts and nomenclature es- and the stretched crust, and that covered the width of the sil-
tablished in these works are used in the following descrip- icon putty layer, was made to simulate the ocean–continent
tions and analysis. Also, following Christie-Blick and Bid- transition (Fig. 3b). The taper angle was kept constant for all
dle (1985a, b) and Dooley and Schreurs (2012) we apply the models.
term principal deformation zone (PDZ) for the junction be- The precut shape of the plate boundary includes major re-
tween the movable polythene plates underlying the experi- leasing bends positioned so that they correspond to the ge-
ment. The contact between the fixed and movable base de- ometry of the COB and the three main structural segments
fined a nonstationary velocity discontinuity (VD; Ballard et of the Barents Shear Margin as follows. Segment 1 of the
al., 1987; Allemand and Brun, 1991; Tron and Brun, 1991). BarMar experiments (Fig. 4) contained several sub-segments
Several experimental works have particularly focused on with releasing and restraining bends as well as segments of
the geometry and development of pull-apart basins in re- “neutral” (Wilcox et al., 1973; Mann et al. 1983; Biddle and
leasing bend settings (Mann et al., 1983; Faugére et al., Christie-Blick, 1985b) or “pure” (Richard et al., 1991) strike-
1986; Richard et al., 1991; Dooley and McClay, 1997; Basile slip. Segment 2 had a basic crescent shape, thereby defining
and Brun, 1997; Sims et al., 1999; Le Calvez and Vendev- a releasing bend at its southern margin in a position simi-
ille, 2002; Mann, 2007; Mitra and Paul, 2011). The pull- lar to that of the Vestbakken Volcanic Province that merged
apart basin was described by Burchfiel and Stewart (1966) into a neutral shear segment along the strike, whereas a re-
and Crowell (1974a, b) as formed at a releasing bend or straining bend occupied the northern margin of the segment.
at a releasing fault step-over along a strike-slip zone (Bid- Segment 3 was a straight basement segment, defining a zone
dle and Christe-Blick, 1985a, b). This basin type has also of neutral shear, and corresponds to the strike-slip segment
been termed a “rhomb graben” (Freund, 1971) and “strike- west of Svalbard (Fig. 1).
slip basin” (Mann et al., 1983), and it is commonly consid- The experiments included three stages of deformation with
ered to be synonymous with the extensional strike-slip du- constant rates of movement of the mobile sheet at 10 cm h−1
plex (Woodcock and Fischer, 1986; Dooley and Schreurs, in all three stages. The relative angles of plate movements in
2012). In the descriptions of our experiments, we found it the experiments were taken from post-late Paleocene opening
convenient to distinguish between extensional strike-slip du- directions in the northeastern Atlantic (Gaina et al., 2009).
plexes in the context of Woodcock and Fischer (1986) or Dextral shear was applied in the first phase in all experi-
Twiss and Moores (2007, p. 140–141) and pull-apart basins ments by pulling the lower plastic sheet by 5 cm. In the sec-
(rhomb grabens: Crowell, 1974a, b; Aydin and Nur, 1993) ond phase the left side of the experiment was extended by
since they reflect slightly different stages in the development 3 cm orthogonally (BarMar6) or obliquely (315◦ ; BarMar8
in our experiments (see the Discussion section). and 9) to the trend of the shear margin, whereas plate motion
was reversed during the third phase of deformation, leading
to inversion of earlier formed basins that had been devel-
5 Experimental set-up oped in the strike-slip and extensional phases. Sedimentary
basins that develop due to strike-slip (phase 1) or extension
To study the kinematics of complex shear margins, a series (phase 2) were filled with layers of coloured feldspar sand by
of analogue experiments was performed at the tectonic mod- sieving so that a smooth surface was obtained. These layers
elling laboratory (TecLab) of Utrecht University, the Nether- are primarily important for discriminating among deforma-
lands. All experiments were built on two overlapping 1 mm tion phases and thus act as marker horizons. Phase 3 was
thick plastic sheets (each 100 cm long and 50 cm wide) that initiated by inverting the orthogonal (BarMar6) or oblique
were placed on a flat, horizontal table surface. The bound- (BarMar8 and 9) extension of phase 2 to contraction as a
ary between the underlying movable and overlying station- proxy for ridge push that was likely initiated when the mid-
ary plastic sheets had the shape of the mapped continent– oceanic ridge was established in Miocene times in the North
ocean boundary (COB; Fig. 1b). The moveable sheet was Atlantic (Mosar et al., 2002; Gaina et al., 2009). Contrac-
connected to an electronic engine, which pulled the sheet at tion generated by ridge push has been inferred from the mid-
constant velocity during all three deformation stages. Dis- Norwegian continental shelf (Vågnes et al., 1998; Pascal and
placement rates were therefore not scaled. The modelling Gabrielsen, 2001; Faleide et al., 2008; Gac et al., 2016) and
material was then placed on these sheets where the layers on seems to still prevail in the northern areas of Scandinavia
the stationary sheet represent the continental crust including (Pascal et al., 2005), although far-field compression gener-
the continent–ocean transition (COT), whereas those on the ated by other processes has been suggested (e.g. Doré and
mobile sheet represent the oceanic crust. The model layers Lundin, 1996).
were confined by aluminium bars along the long sides and Coloured layers of dry feldspar sand represent the brittle
sand along the short sides (Fig. 3a). The continental crust oceanic and continental crust. This material has proven suit-

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R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends 967

Figure 3. (a) Schematical set-up of the BarMar3 experiment as seen in map view. (b) Section through the same experiment before deforma-
tion, indicating stratification and thickness relations. (c) Standard positions and orientation for sections cut in all experiments in the BarMar
series. Yellow numbers are section numbers. Black numbers indicate the angle between the margins of the experiment (relative to N–S) for
each profile. (d) Outline of the silicone putty layer as applied in all experiments. The inset shows the original structural map of the Barents
Margin used to define the width of the thinned crust. The red line (X-X’) indicates the western limit of the thinned zone.

able for simulating brittle deformation conditions (Willing- forces are negligible when modelling tectonic processes on
shofer et al., 2005; Luth et al., 2010; Auzemery et al., 2021). geologic timescales (see Ramberg, 1981, and Del Ventisette
It is characterized by a grain size of 100–200 µm, a density et al., 2007, for a discussion on this topic). The models were
of 1300 kg m−3 , a cohesion of ∼ 16–45 Pa, and a peak fric- scaled so that 10 mm in the model approximates ca. 10 km in
tion coefficient of 0.67 (Willingshofer et al., 2018). Addition- nature, yielding a length scale ratio of 1.00 × 10−6 . As such,
ally, an 8 mm thick and variable-width silicone putty mixed the model oceanic and continental crusts scale to 18 and
with fillers corresponding to the transition zone (as mapped 26 km in nature, respectively, which, although slightly over-
in reflection seismic data) of “Rhodorsil Gomme GSIR” estimating the oceanic crustal thickness (10–12 km), is in full
(Sokoutis, 1987) was used as a proxy for the thinned and agreement with the estimated thickness of the thinned ocean-
weakened continental crust at the ocean–continent transition ward segment of the continental crust (30–20 km; Breivik et
(Figs. 1b and 3a, b). This Newtonian material (n = 1.09) has al., 1998).
a density of 1330 k gm−3 and a viscosity of 1.42 × 104 Pa·s. The brittle crust made of dry feldspar sand deforms ac-
The experiments were scaled following standard scaling cording to the Mohr–Coulomb fracture criterion (Horsfield,
procedures as described by Hubbert (1937), Ramberg (1967), 1977; Mandl et al., 1977; McClay, 1990; Richard et al., 1991;
or Weijermars and Schmeling (1986), assuming that inertial Klinkmüller et al., 2016), whereas silicone putty promotes

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968 R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends

which BarMar6 and 8 (and some examples from BarMar9)

are illustrated here. They yielded similar results in that all
crucial structural elements (faults and folds) were reproduced
in all experiments as described in the text (Fig. 4). It is em-
phasized that the extensional basins affiliated with the ex-
tension phase (phase 2) were wider for the orthogonal (Bar-
Mar6) compared to the oblique extension experiments (Bar-
Mar8) (Figs. 5 and 6). Furthermore, the fold systems gener-
ated in the experiments that utilized oblique contraction of
315/135◦ (BarMar8–9) produced more extensive systems of
non-cylindrical folds. These folds also had continuous but
more curved fold traces compared to the experiments with
Figure 4. Position of segments and major structural elements as re-
orthogonal extension–contraction (BarMar6). The fold axes
ferred to in the text and subsequent figures (see particularly Figs. 5
and 6). This example is taken from the reference experiment Bar-
generally rotated to become parallel to the (extensional) mas-
Mar6. All experiments (BarMar6–9) followed the same pattern, and ter faults delineating the pull-apart basins generated in defor-
the same nomenclature was used in the description of all experi- mation stage 1 in experiments with an oblique opening and
ments and provides the template for the definition of structural ele- closing angle.
ments in Fig. 7. Examples of the sequential development are displayed in
Figs. 5 and 6 and summarized in Fig. 7. Elongated positive
structural elements with fold-like morphology as seen on the
ductile deformation and folding. The configuration applied surface were detected during the various stages of the present
in the present experiments is accordingly well suited for the experiments. The true nature of those was not easily deter-
study of the COB in the Barents Shear Margin (Breivik et al., mined until the experiments were terminated and transects
1998). could be examined. Such structures included buried push-
When complete, the experiments were covered with a thin ups (sensu Dooley and Schreurs, 2012), anti-formal stacks,
layer of sand to further stabilize the surface topography be- back-thrusts, positive flower structures, fold trains, and sim-
fore the models were saturated with water and cross-sections ple anticlines. For convenience, we use the non-genetic term
that were oriented transverse to the velocity discontinuity “positive structural elements” (PSEs) for such structure types
were cut in a fan-shaped pattern (Fig. 3c). All experiments as seen in the experiments in the following description. In the
have been monitored with a digital camera, providing top- following the deformation in each segment is characterized
view images at regular time intervals of 1 min. for the three deformation phases (Table 1).
All experiments performed were oriented in a N–S co-
ordinate framework to facilitate comparison with the west- 6.1 Deformation phase 1: dextral shear stage
ern Barents Sea area and had a three-stage deformation se-
quence (dextral shear, extension, contraction). All descrip- In segment 1, differences in the geometry of the precut fault
tions and figures relate to this orientation. It was noted that trace between segments 1, 2, and 3 became visible after the
all experiments reproduced comparable basic geometries and first deformation stage. In segments 1 and 3 in particular, an
structural types, demonstrating robustness against variations array of oblique en échelon folds between Riedel shear struc-
in contrasting strength of the “ocean–continent” transition tures (PSE-1 structures) oriented ca. 135◦ (NW–SE) to the
zone, which included a zone of silicone putty with variable regional VD be came visible before rotating towards NNW–
width below an eastward-thickening sand wedge (Fig. 3b). SSE by continued shear (Fig. 8; see also Campbell, 1958;
The experiments were terminated before the full closure Wilcox et al., 1973; Ordonne and Vialon, 1983; Richard et
of the basin system, in accordance with the extension vec- al., 1991; Dooley and Schreurs, 2012). These were simple,
tor > contraction vector as in the North Atlantic (see Vågnes harmonic folds with upright axial planes and fold axial traces
et al., 1998; Pascal and Gabrielsen, 2001; Gaina et al., 2009). extending a few centimetres beyond the surface shear zone
described above. They had amplitudes on the scale of a few
millimetres and wavelengths on the scale of 5 cm. The PSE-1
6 Modelling results structures interfered with or were dismembered by younger
structures (Y shears and PSE-2 structures; see below), caus-
A series of nine experiments (BarMar1–9) with the set-up de- ing northerly rotation of individual intra-fault zone lamel-
scribed above was performed. Experiments BarMar1–5 were lae (remnant PSE-1 structures; Fig. 8). Structures similar to
used to calibrate and optimize geometrical outline, deforma- PSE-1 fold arrays are known from almost all strike-slip ex-
tion rate, and angles of relative plate movements and are periments reported and described in the literature (e.g. Cloos,
not shown here. The optimized geometries and experimen- 1928; Riedel, 1929; see Dooley and Schreurs, 2012, for a
tal conditions were utilized for experiments BarMar6–9, of summary) and are therefore not given further attention here.

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R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends 969

Figure 5. Sequential development of experiment BarMar6 by 0.5, 2.4, 3.5, 4.0, and 5.0 cm of dextral shear (steps a–e), orthogonal extension
(steps f–h), and oblique contraction (steps i–j). The master fault strands are numbered in Fig. 4, and the sequential development for each
structural family is shown in Fig. 7. The reference panel to the upper left shows the positions of the segments.

Figure 6. Sequential development of experiment BarMar8 by 0.5, 2.4, 3.5, 4.0, and 5.0 cm of dextral shear (steps a–e), oblique extension
(steps f–h), and oblique contraction (steps i–h). The master fault strands are numbered in Fig. 3, and the sequential development for each
structural family is shown in Fig. 7. Phases 2 and 3 involved oblique (3150) extension and contraction in this experiment. The reference panel
to the upper left shows the positions of the segments.

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970 R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends

Figure 7. Summary of sequential activity in each master fault in experiment BarMar6 (Fig. 5) (for the position of each fault, see Fig. 4).
The type and amount of displacement are shown in the two upper horizontal rows. The vertical blue bar indicates the stage at which full
along-strike communication became established between marginal basins. The colour code (see inset) indicates the type of displacement at
any stage. The reference panel to the left shows the positions of the segments.

Table 1. Characteristics of positive structural elements (PSE-1–6) as described in the text and shown in figures. Note that the PSE-1 structures
that were developed in the earliest stages of the experiments became cannibalized during the continued deformation. No candidates for these
structures were identified in the reflection seismic sections.

Struct. type Structural configuration Orientation Exp. stage Segment Recognized Figure Figure
in seism. exp. seism.
PSE-1 Open syn-anticline system 135◦ Stage 1 1, 3 ? 5, 6 1a?
PSE-2 Incipient flower or Parallel master fault Stage 1 1, 2, 3 Yes 5, 6, 8 1b
half-flower
PSE-3 Forced folds above rotated Parallel master fault in Stage 2 1,2 Yes 9b
fault blocks releasing bend
PSE-4 Push-up Parallel master fault in Stage 1 2 Yes 9d 1c
restraining bend
PSE-5 Anticlines/snake heads in Parallel master faults Stage 3 1, 2, 3 Yes 9c, d 1d, e
hanging walls
PSE-6 Anticline–syncline trains Parallel master faults Stage 3 1, 2, 3 Yes 12 1f

By 0.25 cm of horizontal displacement in segment 1, development of new faults and fault segments. They thereby
which included releasing and restraining bends separated by acquired the characteristics of Y shears (oriented sub-parallel
a central strand of neutral shear, a slightly curvilinear sur- to the master fault trace), dissecting the PSE-1 structures. By
face trace of a NE–SW-striking, top-NW normal fault in the 2.4 cm of shear, segment 1 had become one unified fault array
southernmost part of segment 1 developed. This co-existed (Figs. 5d and 6d), delineating a system of incipient push-ups
with the PSE-1 structures and became paralleled by a nor- or positive flower structures (PSE-2 structures; Figs. 8 and
mal fault with opposite dip (fault 2, Fig. 4) so that the two 10, Sects. B1 and B3).
faults constrained a crescent- or spindle-shaped incipient ex- The PSE-2 structures had amplitudes of 1–2 cm and wave-
tensional shear duplex (Figs. 5b and 6b; see also Mann et al., lengths of 3–5 cm as measured on the surface with fault sur-
1983). faces that steepened downward, with the deepest parts of the
A system of separate en échelon N–S- to NNE–SSE- structures having cores of sand layers deformed by open to
striking normal and shear fault segments became visible in tight folds. The folds had upright or slightly inclined axial
segment 1 after ca. 1 cm of shear (Fig. 5c, d). These faults planes, dipping up to 55◦ , mainly to the east. The structures
did not have the orientations as expected for R (Riedel) and also affected the shallowest layers down to 1–2 cm in the
R’ (anti-Riedel) shears (that would be oriented with angles of sequence, but the shallowest sequences developed at a later
approximately 15 and 75◦ from the master fault trace) but be- stage of deformation and were characterized by simple gen-
came progressively linked with along-strike growth and the tle to open anticlines. These structures were constrained to

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R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends 971

floor, generating ridges that influenced the basin floor topog-

raphy and hence the sedimentation. By continued rotation of
the fault blocks and simultaneous sieving of sand the crests
of the blocks became sequentially uplifted, generating forced
folds (Hamblin, 1965; Stearns, 1978; Groshong, 1989; Khalil
and McClay, 2017) (Fig. 10a). In the analysis we used the
term PSE-3 structures for these features. Simultaneously, an
expanding sand sequence became trapped in the footwalls of
the master faults, defining typical growth fault geometries.
By a shear displacement of 0.55 cm additional curved
splay faults were initiated from the northern tip of the mas-
ter fault of fault 3f (Fig. 7), delineating the northern mar-
gin of a rhombohedral pull-apart basin (Mann et al., 1983;
Mann, 2007; Christie-Blick and Biddle, 1985) and with a
Figure 8. PSE-1 anticline–syncline pairs in segment 1 of experi- geometry that was indistinguishable from pull-apart basins
ment BarMar6 in an oblique view (see Fig. 4 for the position of or rhomb grabens affiliated with unbridged en échelon fault
segment 1). PSE-1 folds (indicated by relief defined by blue and arrays (Crowell, 1974a, b; Aydin and Nur, 1993). Although
yellow markers) were constrained to the central fault zone (defined sand was filled into the subsiding basins to minimize the
by Y shear and its splay faults) and extended only 3–4 cm beyond graben relief and to prevent gravitational collapse, the sub-
it. PSE-2 structures (incipient push-ups and positive flower struc- basins that were initiated in the shear stage were affected by
tures) were delineated by shear faults (black lines) and completely
internal cross-faults, and the initial basin units remained the
cannibalized PSE-1 structures by continued shear. Yellow and blue
deepest so that the buried internal basin topography main-
reference lines illustrate the rotation of the fold axial trace caused
by dextral shear. The pre-shear distance between the markers (blue tained a high relief with several apparent depocentres sep-
and yellow lines) was already 5 cm. The black arrow indicates the arated by intra-basinal platforms. Systems of linked shear
shear direction. faults and PSE structures became established in the central
part with neutral shear that separate the releasing and re-
straining bends, with development similar to that seen for
a deformation zone directly above the trace of the basement segment 3 (see below). These structures were, however, soon
fault, similar to that commonly seen along shear zones (e.g. destroyed by the interaction between the northern and south-
Tchalenko, 1971; Crowell, 1974a, b; Dooley and Schreurs, ern tips of the extensional and contractional shear duplexes
2012). This zone was 3–4 cm wide, remained stable through- (Fig. 10).
out deformation stage 1, and was restricted to the close vicin- The first structure to develop in the regime of the restrain-
ity of the basement shear fault itself. A horsetail-like fault ar- ing bend (segment 2) was a top-to-the-southwest (antithetic)
ray developed by ca. 3 cm of shear at the transitions between thrust fault at an angle of 135◦ with the regional trend of the
segments 1 and 2 (Figs. 5b–d and 6b–d). basement border as defined by segments 1 and 3 (fault 6).
The structuring in segment 2 was determined by the pre- It became visible by 0.5 cm of displacement. However, the
cut crescent-shaped basement fault (velocity discontinuity), northern part of segment 2 became dominated by a synthetic
which caused the development of a releasing bend along contractional top-to-the-northeast fault that was initiated by
its southern border, and a restraining bend along its north- 0.85 cm of shear (fault 7; Figs. 5 and 6). Thus, faults 6 and
ern border (Fig. 11). The first fault of fault array 3a–e in 7 delineated a growing half-crescent-shaped 5–7 cm wide
the southern part of segment 2 (Fig. 4) was activated af- push-up structure (Aydin and Nur, 1982; Mann et al., 1983)
ter ca. 0.15 cm of bulk horizontal displacement (Fig. 7). It south of the restraining bend (Fig. 9; PSE-4 structures). Con-
was situated directly above the southernmost precut releas- tinued shearing gave these structures the characteristics of an
ing bend, defining the margin of crescent-shaped incipient anti-formal stack.
extensional strike-slip duplexes (in the context of Woodcock Segment 3 defined a straight strand of neutral shear. Its
and Fischer, 1986, Woodcock and Schubert, 1994, and Twiss development in the BarMar experiments strictly followed
and Moores, 2007, p. 140–141). The developing basin got that known from numerous published experiments (e.g.
a spindle-shaped structure and developed into a basin with Tchalenko, 1970; Wilcox et al., 1973; Harding, 1974; Hard-
a lazy-S shape (Cunningham and Mann, 2007; Mann, 2007). ing and Lowell, 1979; Naylor et al., 1986; Sylvester, 1988;
The basin widened towards the east by stepwise footwall col- Richard et al., 1991; Woodcock and Schubert, 1994; Dau-
lapse, generating sequentially rotating crescent-shaped ex- teuil and Mart, 1998; Mann, 2007; Casas et al., 2001; Doo-
tensional fault blocks that became trapped as extensional ley and Schreurs, 2012). A train of Riedel shears, occupy-
horses in the footwall of the releasing bend (Fig. 11). In the ing the full length of the segment, appeared simultaneously
areas of the most pronounced extension the crestal part of on the surface after a shear displacement of 0.5 cm, occu-
the rotational fault blocks became elevated above the basin pying a restricted zone with a width of 2–3 cm. The Riedel

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972 R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends

Figure 9. Cross-sections through PSE-2-related structures. PSE structures are marked with P and PSE numbers as described in the text (see
also Table 1). (a) Folded core of incipient push-up and positive flower structure in segment 1 for experiment BarMar6. The fold structure
is completely enveloped by shear faults that have a twisted along-strike geometry. Note that the eastern margin of the structure developed
into a negative structure at a late stage in the development (filled by a black–pink sand sequence) and that the silicone putty sequence (basal
pink sequence) was entirely isolated in the footwall. (b) Similar structure type in experiment BarMar8. However, the basal silicone putty
layer here bridged the basal high-strain zone so that folding occurred in the footwall as well as in the hanging wall. Folds propagated up-
section into the sand layers (blue). The folds in upper (pink) layers are younger and were associated with the contractional stage (PSE-6
structures). (c) Contraction associated with the “crocodile structure” in the footwall of the main fault in segment 1 for experiment BarMar8.
Note the disharmonic folding with contrasting fold geometries in the hanging wall and footwall as well as at different stratigraphic levels in
the footwall, indicating that a shifting stress situation in time and space occurred in the experiment. (d) Transitional fault strand between two
more strongly sheared fault segments (experiment BarMar9).

Figure 10. (a) Contrasting structural styles along the master fault system in segment 2 in map view and (b) cross-sections of experiment
BarMar9. SL denotes the silicone layer; the stippled line is the boundary between pre- and syn-deformation layers, and the dashed white line
is the boundary with the post-deformation layers.

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R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends 973

Figure 11. Nine stages in the development of the extensional shear duplex system above the releasing bend in experiment BarMar9. The
master faults that developed at an incipient stage (e.g. fault 3 that constrained the eastern margin of the extensional shear duplex, marked with
a ”3” in the figure; see also Fig. 7) remained stable and continued to be active throughout the experiment but became overstepped by new
faults in the footwall. These were reactivated as contraction faults at the later stages (stages h and i in this figure). The developing basement
was stabilized by infilling of grey sand during this part of the experiment. Fault 3 continued to breach the basin infill, also after the basin infill
overstepped the original basin margin. The distance between the markers (dark lines) is 5 cm. The white arrow marks the north direction.
Note that panels (h) and (i) (bottom right) are viewed from different directions than the other panels.

shears dominated the continued structural development of plate tectonic reconstructions of the North Atlantic suggest
segment 3. Riedel shears were absent throughout the experi- an extension angle of 315◦ (Gaina et al., 2009).
ments, as should be expected for a sand-dominated sequence All strike-slip basins widened in the extensional stage, and
(Dooley and Schreurs, 2012). P shears developed via con- as one would expect, the basins generated in orthogonal ex-
tinued shear, creating linked rhombic structures delineated tension became wider than those generated in oblique exten-
by the Riedel and P shears generating positive structural el- sion. In both cases, however, extension promoted enhanced
ements with NW–SE- and NNE–SSE-striking axes (see also relief that had been generated in the shear stage. In the ear-
Morgenstern and Tchalenko, 1967), soon coalescing to form liest extensional stage, the strike-slip basin in segment 2
Y shears. Transverse sections document the fact that these dominated the basin configuration. By continued extension
structures were cored by push-up anticlines, positive half- the linear segments and the minor pull-apart basins in seg-
flower structures, and full-fledged positive flower structures ments 1 and 2 started to open and became interlinked, sub-
in the advanced stages of shear (PSE-4 structures) (Figs. 5 sequently generating a linked basin system that runs parallel
and 6; see also Fig. 10). These were accompanied by the de- to the entire shear margin (Figs. 5f–g, 6f–g). The basins had
velopment of en échelon folds and flower structures as com- become completely interlinked by an extension of 1.25 cm
monly reported from strike-slip faults in nature and in ex- (marked by the vertical dark blue line in Fig. 7). The or-
periments. The width of the zone above the basal fault re- thogonal extension phase also reactivated and linked several
mained almost constant throughout the experiments but was master faults that were established in deformation phase 1
somewhat wider in experiments with thicker basal silicone (Figs. 5a and 6a). This became evident by an extension of
polymer layers, similar to what is commonly described from 0.25–0.50 cm and included the southern fault margin, the
comparable experiments (e.g. Richard et al., 1991). push-up, and the splay faults defining the crestal collapse
graben (faults 6, 11, and 12; Fig. 4). Among the faults that
6.2 Deformation phase 2: extension remained inactive throughout the extension phase was the
antithetic contractional fault delineating the push-ups in seg-
The late Cretaceous–Paleocene dextral shear was followed ment 2 (fault 6; Fig. 4). The Y shear in segment 3 was reac-
by pure extension that accompanied the opening along the tivated as a straight, continuous extensional fault in phase 2.
Barents Shear Margin in the Oligocene. Our experiments fo- Total extension in stage 2 was 5 cm.
cused on the effects of oblique extension, acknowledging that

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974 R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends

6.3 Deformation phase 3: contraction segments that correspond to the Senja Fracture Zone (seg-
ment 1), the Vestbakken Volcanic Province (segment 2), and
In our experiments the extension stage was followed by the Hornsund Fault Zone (segment 3) and three deformation
oblique contraction (parallel to the direction of extension as phases (dextral shear, oblique extension, and contraction).
applied for each experiment). Some of the early-stage con- Several structural families (PSE-1–6) generated in the exper-
traction was accommodated along new faults. More com- iments correspond to structural features observed in reflec-
monly, however, faults that had been generated in the strike- tion seismic sections. In the following discussion we utilize
slip and extensional stages became reactivated and rotated. these two datasets in explaining the sequential development
So was the development of isolated folds, which were of each segment of the shear margin.
commonly associated with inverted fault traces, generating
snake-head or harpoon structures (Cooper et al., 1989; Cow- 7.1 Structures of phase 1 (dextral shear)
ard, 1994; Allmendinger, 1998; Yameda and McClay, 2004;
Pace and Calamitra, 2014; PSE-5 structures). The predom- Segment 1 (corresponding to the Senja Fracture Zone) was
inant structures affiliated with the contractional stage were dominated by neutral dextral shear, although jogs in the (pre-
still new folds with traces oriented orthogonal to the shorten- cut) fault provided minor sub-segments with subordinate re-
ing direction and sub-parallel to the pre-existing master fault leasing and restraining bends. PSE-1 folds seen in the incipi-
systems that defined the margin and basin margins (Fig. 12). ent shear phase were confined to the area just above the basal
Also, some deep fold sets that had been generated during the master fault (VD) and its immediate vicinity (see also exper-
strike-slip phase and seen as domal surface features became iments in series “e” and “f” of Mitra and Paul, 2011). Coun-
reactivated, causing renewed growth of surface structures terparts to the PSE-1 structural population were not identified
(see Fig. 10 and the explanation in figure caption). These in the seismic data, although some isolated, local anticlinal
folds were generally upright cylindrical buckle folds in the features could be dismembered remnants of such. Because
initial contraction with a very large trace-to-amplitude ratio of their constriction to the near vicinity of the master fault
(SPE-6 structures). Some intra-basin folds, however, defined it is reasonable that structures generated at an early stage of
fold arrays that crossed the basins in a diagonal fashion. Par- shear are vulnerable to cannibalization by younger structures
ticularly the folds situated along the basin margins developed with axes striking parallel to the main shear fault (Y shears;
into fault propagation folds above low-angle thrust planes. SPE-2 structures). We therefore conclude that this structure
Such faults aligning the western basin margins could have an population was destroyed during the later stages of shear and
antithetic attitude relative to the direction of contraction. during the subsequent stages of extension and contraction.
During the contractional phase the margin-parallel, linked PSE-1 folds that developed at an incipient stage were imme-
basin system immediately started to narrow, and several fault diately followed by the development of two sets of NNE–
strands became inverted. The basin closure was a continuous SSW-striking normal faults with opposite throws in the re-
process until the end of the experiment by 3 cm of contrac- leasing bend areas (e.g. fault 2, Fig. 4). The two faults defined
tion. The contraction was initiated as a proxy for an ESE- crescent- or spindle-shaped incipient extensional shear du-
directed ridge-push stage. The first effect of this deformation plexes. These structures were stable during the remainder of
stage was heralded by uplift of the margin of the established the experiments, and their master faults became reactivated
shear zone that had developed into a rift during deformation during the extensional and contractional phases (see below).
stage 2. This was followed by the reactivation and inversion The most prominent of these structures corresponds to the
of some master faults (e.g. fault a2; Fig. 4) and thereafter by position of the Sørvestsnaget Basin (Fig. 1b).
the development of a new set of low-angle top-to-the-ESE Segment 2, which was controlled by a precut crescent-
contractional faults. These faults displayed a sequential de- shaped discontinuity in the experiments, corresponds to the
velopment (fault family 1; Fig. 7) and were associated with Vestbakken Volcanic Province and the southern extension of
folding of the strata in the rift structure, probably reflecting the Knølegga Fault Complex of the Barents Shear Margin
foreland-directed in-sequence thrusting (SPE-5 and PSE-6 (Figs. 1b and 4). The Vestbakken Volcanic Province is dom-
fold populations). inated by interfering NNW–SSE- and NE–SW-striking fold-
and-fault systems in its central part, whereas N–S structures
are more common along its eastern margin (Fig. 12a) (Jeb-
7 Discussion sen and Faleide, 1998; Giannenas, 2018). Intra-basinal highs
and other internal configurations seen in the BarMar experi-
The break-up and subsequent opening of the Norwegian– ments mainly reflect stepwise collapse of the intrinsic basin
Greenland Sea constitute a multi-stage event (Fig. 13) that that generated rotational fault blocks, the crests of which sep-
imposed shifting stress configurations overprinting the al- arated local sediment accumulations.
ready geometrically complex Barents Shear Margin. There- Such structures are common in strike-slip basins (e.g.
fore, scaled experiments were designed to illuminate its Dooley and McClay, 1997; Dooley and Schreurs, 2012) and
structural development. The experiments utilized three main are consistent with the intra-basin depocentres seen within

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R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends 975

Figure 12. PSE-5 folds generated during phase 3 inversion for experiment BarMar8. Note that fold axes are mainly parallel to the basin rims
but that they deviate in some cases in the central parts of the basins. The folds are best developed in segment 2, which accumulated extension
in the combined shear and extension stages.

the Vestbakken Volcanic province and in the Sørvestsnaget and do not occur in the central parts of the Vestbakken Vol-
Basin (Knutsen and Larsen, 1997; Jebsen and Faleide, 1998; canic Province. We suggest that the composite geometry of
Fig. 13). The crests of the rotating fault blocks are termed the Knølegga Fault Complex is due to the development of
PSE-3 structures above, and such eroded fault block crests PSE-2 structures within the realm of a pre-existing normal
define the footwalls of major faults in the Vestbakken Vol- fault zone.
canic Province, providing space for sediment accumulation Due to the right-stepping geometry during dextral shear
in the footwalls. The area that was affected by the basin in segment 2, the southern and northern parts were in the
formation in the extensional shear duplex stage seems to releasing and restraining bend positions, respectively (e.g.
have remained the deepest part of the Vestbakken Volcanic Christie-Blick and Biddle, 1985). Hence, the southern part
Province. The part formed by basin widening through se- of segment 2 was subject to oblique extension, subsidence,
quential footwall collapse formed a shallower sub-platform and basin formation, while the northern part was subject to
(sensu Gabrielsen, 1986) (Fig. 11). oblique contraction, shortening, and uplift. The southern seg-
The Knølegga Fault Complex occupies a kilometre-wide ment expanded to the east and northeast by footwall collapse
zone in segment 2. The master fault strand is paralleled by and activation of rotating fault blocks that contributed to a
faults with significant normal throws in its hanging wall side basin floor topography that affected the pattern of sediment
and is part of the larger Knølegga Fault Complex (EBF, east- accumulation (Fig. 9a, b).
ern boundary fault; Giannenas, 2018; Fig. 12a). The EBF The positive structural elements that prevail in segment 3
zone is a top-west normal fault with maximum throw of belong to the PSE-2 structure population. The structures af-
nearly 3000 m. It can be followed along its strike for more filiated with segment 3 in the BarMar experiments are simi-
than 60 km and seems to die out by horse-tailing at its lar to those seen in the reflection seismic sections along parts
tip points. The areas around the master faults of the Knø- of the Spitsbergen and the Senja shear margins (Myhre et
legga Fault Complex locally display isolated elongate posi- al., 1982) as well as elsewhere (Cloos, 1928; Riedel, 1929;
tive structures constrained by steeply dipping faults. These Tchalenko, 1970; Wilcox et al., 1973). In the experiments én
structures sometimes display internal reflection patterns that echelon folds (corresponding to PSE-1 structures) first be-
seem exotic in comparison to the surrounding sequences. came visible, to be succeeded by the development of Riedel
Some of these structures resemble positive flower structures and P shears (R’ shears were subdued as expected for sand-
or push-ups or define narrow anticlines. They are located in dominated sequences; Dooley and Schreurs, 2012). Con-
both the footwall and hanging wall of the boundary faults tinued shear followed by collapse and interaction between
and strike parallel to them, and the axes of these structures Riedel and P shears as well as the subsequent development
are parallel to the master faults. The traces of such structures of Y shears initiated a push-up and flower structure with
can be followed over shorter distances than the master faults N–S axis (PSE-2) structures that were expressed as non-

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976 R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends

Figure 13. Main stages in opening of the North Atlantic. The figure builds on Fig. 5 in Faleide et al. (2008) and has been updated and
redrawn.

cylindrical (double-plunging) anticlines on the surface (e.g. 7.2 Structures of phase 2 (extension)
Tchalenko, 1970; Naylor et al., 1986). Structures similar to
the PSE-2 structures that were initiated in the present experi-
It is expected that (regional) basin subsidence and (local)
ments are common in scaled experiments with mechanically
fault block subsidence became accelerated during phase 2
stratified sequences where viscous basal strata are covered by
(extension), more so in the orthogonal extension experiments
sand (e.g. Richard et al., 1991; Dauteuil and Mart, 1998).
(BarMar6) than in the experiments with oblique extension

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R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends 977

(BarMar8). However, due to stabilization of basins by in- ening during the Eocene (phase 1) was accommodated by
filling of sand, this was not documented in the final pho- regional-scale strain partitioning (Leever et al., 2011a, b).
tographs. The widening occurred mainly by fault-controlled Also, the Vestbakken Volcanic Province is characterized
collapse of the footwalls and dominantly along the master by extensive regional shortening. The onset of this event of
faults that correspond to the Knølegga Fault Complex. How- inversion–contraction is dated to the early Miocene (Jebsen
ever, new transverse faults within the basin that had devel- and Faleide, 1998, Giannenas, 2018), and this deformation
oped during the shear stage (see above) were also reactivated included two main structural fold styles. The first includes
and contributed to the complexity of the basin topography. upright to steeply inclined, closed to open anticlines that
It is unlikely that a stage was reached where all (pull-apart) are typically present in the hanging wall of master faults.
basin units along the margin became fully linked, although These folds typically have wavelengths of the order of 2.5
sedimentary communication along the margin may have oc- to 4.5 km and amplitudes of several hundred metres. Most
curred. commonly they appear with head-on snake-head structures
During the oblique extension stage segment 1 of experi- and are interpreted as buckle folds, although a component of
ments BarMar7–9 the basin subsidence was focused in the shear may occur in the areas of the most intense deformation.
minor pull-apart basins, which soon became linked along the The second style includes gentle to open anticline–syncline
regional N–S-striking basin axis. Remains of several such pairs with upright or steep to inclined axial planes with wave-
basin centres, of which the Sørvestsnaget Basin (Knutsen and lengths on the order of 5 to 7 km and amplitudes of several
Larsen, 1997; Kristiansen et al., 2017) is the largest, are pre- tens of metres to several hundred metres. We associate those
served and found in seismic data (Fig. 1b). During the exper- with the PSE-4-type structures as defined in the BarMar ex-
iments a continuous basin system developed in the hanging periments. These folds are situated in positions where sedi-
wall side of the master fault. It is, however, not likely that mentary sequences have been pushed against buttresses pro-
linking of shear basins occurred prior to the opening stage vided by master faults along the basin margins. The PSE-6
along the Barents Shear Margin. folds developed as fold trains in the interior basins, where
buttressing against larger fault walls was uncommon. Also,
this pattern fits well with the development and geometry seen
7.3 Structures of phase 3 (contraction)
in the BarMar experiments, where folding started in the cen-
tral parts of the closing basins before folding of the marginal
The contraction phase (phase 3) reactivated both normal and parts of the basin. In the closing stage the folding and in-
shear faults in the master fault zone, also causing folding in version of master faults remained focused along the basin
the hanging wall. Simultaneously, rotation of (intra-basinal) margins.
fault blocks and steepening of pre-existing faults occurred. The experiments clearly demonstrated that contraction by
New fold populations (PSE-5 folds) with axial traces parallel buckle folding was the main shortening mechanism of the
to the basin axis and the master faults characterized the inver- margin-parallel basin system generated in phase 2 (orthogo-
sion stage. Remnants of such folds are locally preserved in nal or oblique extension) in all segments. In the Vestbakken
the thickest sedimentary sequences affiliated with the Senja Volcanic Province segments of the Knølegga Fault Complex,
Shear Margin. the EBF and the major intra-basinal faults contain clear evi-
Fold systems with fold axes paralleling the basin margins dence of tectonic inversion, whereas this is less pronounced
as seen in the experiments are also common in the Vest- in others. The hanging wall of the EBF is partly affected
bakken Volcanic Province. Although shortening occurred in- by fish-hook-type inversion anticlines (Ramsey and Huber,
side individual reactivated fault blocks via large-wavelength 1987; Griera et al., 2018) (Fig. 2d, e), isolated hanging wall
bulging of the entire sedimentary sequence, trains of folds anticlines, or pairs or trains of synclines and anticlines (e.g.
with larger amplitude and shorter wavelength also developed Roberts, 1989; Coward et al., 1991; Cartwright, 1989; Mitra,
at this stage (Fig. 12b, c). Thus, the tectonic inversion was 1993; Uliana et al., 1995; Beauchamp et al., 1996; Gabrielsen
focused along the N–S-striking basin margins but also oc- et al. ,1997; Henk and Nemcok, 2008), with the fold style
curred along some pre-existing NE–SW-striking faults and and associated faults probably being influenced by the ori-
in the central parts of the basin. entation and steepness of the pre-inversion fault (Williams
During phase 3 the restraining bend configuration in the et al., 1989; Cooper et al., 1989; Cooper and Warren, 2010).
northern part of segment 2 was characterized by increasing Some structures of this type can still be followed for many
contraction across strike-slip fault strands that splayed out kilometres, having consistent geometry and attitude. These
to the northwest from the central part of segment 2 in an structures are not much modified by reactivation and are in-
early stage of dextral shear. This deformation was terminated variably found in the proximal parts of the footwalls of mas-
by the end of phase 1 by stacking of oblique contraction ter faults, suggesting that these are inversion structures. They
faults (PSE-5 and PSE-6 structures), defining an anti-formal correlate with PSE-5-type structures in the experiments that
stack-like structure. This type of deformation falls outside developed in areas of focused contraction along pre-existing
the mapped area, but to the north this type of oblique short- fault scarps during Oligocene inversion.

https://doi.org/10.5194/se-14-961-2023 Solid Earth, 14, 961–983, 2023

978 R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends

Trains of folds with smaller amplitudes and higher fre- the detailed geometry and width of the pre-existing weak
quency are sometimes found in fault blocks in the central part zone must be mapped and included in the model.
of the Vestbakken Volcanic Province (Fig. 12a). Although
these structures cannot be dated by seismic stratigraphical
methods (on-lap configurations, etc.) we assume that these 8 Summary and conclusions
folds can be correlated with the tight folds generated in the
inversion stage in the experiments (PSE-6 structures) and that Our observations confirmed that the main segments of the
they are contemporaneous with the PSE-5 structures. Barents Shear Margin, albeit undergoing the same regional
Segment 1 in the experiments, which corresponds to the stress regime, display contrasting structural configurations.
Senja Shear Margin, displays a structural pattern that is a hy- The deformation in segment 2 in the BarMar experiments
brid between segments 1 and 2: it contains incipient struc- was determined by releasing and restraining bends in the
tural elements that were developed in full in segments 2 and southern and northern parts, respectively. Thus, the south-
3, with segment 2 being dominated by releasing and restrain- ern part, corresponding to the Vestbakken Volcanic Province,
ing bend configurations and segment 3 dominated by neutral was dominated by the development of a regional-scale ex-
shear. Because of internal configurations, the three segments tensional shear duplex as defined by Woodcock and Fis-
were affected by secondary (oblique) opening and contrac- cher (1983) and Twiss and Moores (2007). Through contin-
tion in various fashions. Understanding these differences was ued shear the basin developed into a full-fledged pull-apart
facilitated by the comparison of seismic and model data. basin or rhomb graben (Crowell, 1974; Aydin and Nur, 1982)
in which rotating fault blocks were trapped. The pull-apart
7.4 Some considerations about multiphase deformation basin became the nucleus for greater basin systems to de-
in shear margins velop in the following phase of extension, also providing the
space for folds to develop in the contractional phase.
The Barents Shear Margin is a challenging target for struc- We conclude that fault-and-fold systems found in the
tural analysis because it represents a geometrically complex realm of the Vestbakken Volcanic Province are in accordance
structural system with a multi-stage history, but also be- with a three-stage development that includes dextral shear
cause high-quality (3D) reflection seismic data are limited followed by oblique extension and contraction (315/135◦ )
and many structures and sedimentary systems generated in along a shear margin with composite geometry. Folds with
the earlier tectono-thermal stages have been overprinted and NE–SW-trending fold axes are dominant in wider areas of the
obliterated by younger events. This makes analogue exper- Vestbakken Volcanic Province and are dominated by folds in
iments very useful in the analysis, since they offer a tem- the hanging walls of (older) normal faults, sometimes char-
plate for what kind of structural elements can be expected. acterized by narrow snake-head- or harpoon-type structures
By constraining the experimental model according to the that are typical for tectonic inversion (Cooper et al., 1989;
outline of the margin geometry and introducing a dynamic Coward, 1994; Allmendinger, 1998; Yameda and McClay,
stress model consistent with the current understanding of the 2004; Pace and Calamitra, 2014).
tectono-sedimentological evolution, we were able to inter- Comparison of seismic mapping and analogue experi-
pret the observations from the reflection seismic data in a ments shows that one of the major challenges in analysing
new light. the structural pattern in shear margins of complex geometry
Continental margins are commonly segmented, containing and multiple reactivation is the low potential for preservation
primary or secondary transform elements, and pure strike- of structures formed in the earliest stages of development.
slip transforms are relatively rare (e.g. Nemcok et al., 2016).
Such margins, however, invariably become affected by ex-
tension following break-up and sometimes contraction due Data availability. The 2D seismic data are released and available
to ridge push or far-field stress, perhaps related to plate re- through DISKOS, the Norwegian National Data Repository for
organization. The complexity of shear margins has ignited petroleum data: https://www.npd.no/en/diskos/seismic/ (DISKOS,
several conceptual discussions. One such discussion con- 2021).
cerns the presence of zones of weakness prior to break-up
(e.g. Sibuet and Mascle, 1978; Taylor et al., 2009; Gibson
et al., 2013; Basile, 2015). In the case of the Barents Shear Author contributions. RHG: contributions to outline, design and
performance of experiments, first writing and revisions of the paper,
Margin the de Geer zone provides such a pre-existing zone
first drafts of figures. PAG: seismic interpretation in the Vestbakken
of weakness, and this premise was acknowledged when the
Volcanic Province, identification and description of fold families.
scaled model was established. The relevance of our model is DS: main responsibility for set-up, performance, and handling of
therefore constrained to cases in which a crustal-scale zone experiments; revisions of the paper. EW: performance and handling
of weakness existed before break-up. Furthermore, in cases of experiments, revisions of the paper, design and revisions of fig-
with pre-existing zones of weakness, our model shows that ure material. MH: background seismic interpretation, discussions
the initial architecture of the margin is indeed important, and and revisions of the paper, design and revisions of figure material.

Solid Earth, 14, 961–983, 2023 https://doi.org/10.5194/se-14-961-2023

R. H. Gabrielsen et al.: Analogue experiments on releasing and restraining bends 979

JIF: regional interpretations and design of experiments, participa- Ballard, J.-F., Brun, J.-P., and Van Ven Driessche, J.: Propagation
tion in performance and interpretations of experiments, revisions of des chevauchements au-dessus des zones de décollement: mod-
the paper, design and revisions of figure material. èles expérimentaux, CR Acad. Sci., 11, 305, 1249–1253, 1987.
Basile, C.: Transform continental margins – Part 1:
Concepts and models. Tectonophysics, 661, 1–10,
Competing interests. At least one of the (co-)authors is a guest https://doi.org/10.1016/j.tecto.2015.08.034, 2015.
member of the editorial board of Solid Earth for the special issue Basile, C. and Brun, J.-P.: Transtensional faulting patterns ranging
“Analogue modelling of basin inversion”. The peer-review process from pull-apart basins to transform continental margins: an ex-
was guided by an independent editor, and the authors also have no perimental investigation, J. Struct. Geol., 21, 23–37, 1997.
other competing interests to declare. Beauchamp, W., Barazangi, M., Demnati, A., and El Alji, M.: Intra-
continental rifting and inversion: Missour Basin and Atlas Moun-
tains, Morocco, AAPG Bull., 80, 1455–1482, 1996.
Disclaimer. Publisher’s note: Copernicus Publications remains Bergh, S. G. and Grogan, P.: Tertiary structure of the Sørkapp-
neutral with regard to jurisdictional claims in published maps and Hornsund Region, South Spitsbergen, and implications for the
institutional affiliations. offshore southern extension of the fold-thrust-belt, Norw. J.
Geol., 83, 43–60, 2003.
Bergh, S. G., Braathen, A. and Andresen, A.: Interaction of
basement-involved and thin-skinned tectonism in the Tertiary
Special issue statement. This article is part of the special issue
fold-and-thrust belt of Central Spitsbergen, Svalbard, APPG
“Analogue modelling of basin inversion”. It is not associated with a
Bull., 81, 637–661, 1997.
conference.
Biddle, K. T. and Christie-Blick, N.: Strike-Slip Deformation, Basin
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Acknowledgements. The work was supported by ARCEx (Research Biddle, K. T. and Christie-Blick, N.: Glossary – Strike-slip defor-
Centre for Arctic Petroleum Exploration), which was funded by the mation, basin formation, and sedimentation, in: Biddle, K.T., and
Research Council of Norway (grant no. 228107) together with 10 Christie-Blick, N. (eds.): Strike-Slip Deformation, Basin Forma-
academic and 6 industry (Equinor, Vår Energi, Aker BP, Lundin tion, and Sedimentation, Soc. Econ. Pa., 37, 375–386, 1985b.
Energy Norway, OMV, and Wintershall Dea) partners. Muhammad Blaich, O. A., Tsikalas, F., and Faleide, J. I.: New in-
Hassaan was funded by the Suprabasins project (Research Council sights into the tectono-stratigraphic evolution of the
of Norway, grant no. 295208). We thank Schlumberger for provid- southern Stappen High and the transition to Bjørnøya
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pretation. Two anonymous reviewers and the editors of this special https://doi.org/10.1016/j.marpetgeo.2017.04.015, 2017.
issue provided comments, suggestions, and advice that enhanced Breivik, A. J., Faleide, J. I., and Gudlaugsson, S. T.: Southwest-
the clarity and scientific quality of the paper. ern Barents Sea margin: late Mesozoic sedimentary basins and
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Breivik, A. J., Mjelde, R., Grogan, P., Shinamura, H., Murai, Y., and
Financial support. This research has been supported by Norges Nishimura, Y.: Crustal structure and transform margin develop-
Forskingsråd grants ARCEx (Research Council of Norway, grant ment south of Svalbard based on ocean bottom seismometer data,
no. 228107) and Suprabasins (Research Council of Norway, grant Tectonophysics, 369, 37–70, 2003.
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