Application of a critical wedge taper model to the Tertiary transpressional
fold-thrust belt on Spitsbergen, Svalbard
Alvar Braathen*
Geological Survey of Norway, PO Box 3006, N-7001 Trondheim, Norway
Steffen G. Bergh
University of Tromsø, Institute of Geology, N-9037 Tromsø, Norway
Harmon D. Maher, Jr. University of Nebraska at Omaha, Nebraska 68182-0199
ABSTRACT
The Tertiary opening of the North Atlantic
Ocean involved major and long-lived overall
dextral transpression between the Svalbard
and Greenland plates. In Spitsbergen, this tectonic event is manifest as a 100–200-km-wide
contractional fold-thrust belt in the form of an
east-pinching prism. This belt can be subdivided into (1) a western, basement-involved
hinterland province that reveals more complex deformation, including thrust, transcurrent, and normal faulting, and (2) an eastern
thin-skinned fold-thrust belt with structures
oriented subparallel (north-northwest–southsoutheast) to the transform plate boundary.
The time-space distribution and interaction
of different structural styles of Tertiary
deformation evident on Spitsbergen support a
model with linked, long-term and short-term
(episodic) dynamic growth of a composite contractional and transcurrent fold-thrust wedge.
The growth of a narrow, high-taper (criticalsupercritical) contractional wedge occurred
during northward-directed crustal shortening
(stage 1) in an oblique, dextral transcurrent
setting. Crustal thickening in the form of
thrust uplift and basin inversion and strikeslip duplexing during the main contractional
event (stages 2 and 3) created an unstable, supercritical wedge of basement and cover rocks
in the hinterland. At the same time, a broader
and more homogeneous frontal part of the
wedge developed eastward by in-sequence imbrication in order to reduce the taper angle.
Local erosion and lateral wedge extrusion
(stages 3 and 4) modified the oversteepened
hinterland wedge to a critical taper angle.
Continued tectonic activity in the hinterland
caused renewed internal imbrication of the
frontal wedge, where deformation was accom*E-mail: alvar.braathan@ngu.no.
A
Figure 1. (A) Definitions of key variables of a Coulomb wedge influenced by a push from behind.
Wedge taper (θc) is the sum of surface slope (α) and the angle of basal decollement (β). (B) Simplified evolutionary model of a wedge-shaped fold-thrust belt illustrating various deformation
processes of wedge response to supercritical taper shape: (1) extensional collapse and minor forelandward propagation (minor wedge growth); (2) extension, out-of-sequence thrusting and moderate foreland propagation (moderate wedge growth); or (3) out-of-sequence thrusting and major
foreland thrust propagation (large wedge growth). Shaded areas represent the new shape and/or
response of the wedge.
modated by tear faulting and out-of-sequence
thrusting (stage 4). Adjustment toward a stable taper geometry included local extension
(stage 5) and erosion and sedimentation.
In a transpressional fold-thrust belt, as in
Spitsbergen, out-of-plane (orogen oblique to
parallel) transport in the hinterland may
cause local and lateral supercritical and subcritical wedge tapers. Hinterland geometries
could trigger adjustments in a frontal thrust
wedge in a decoupled situation, and/or orogen
oblique or parallel motions in a coupled situation. Changing kinematics may thus be expected along strike in such an orogen.
INTRODUCTION
Fold-thrust stacks involving crystalline basement and overlying sedimentary rocks in contractional orogens typically form wedge-shaped
prisms. The prism is characterized by a frontal
imbrication zone and a basal decollement dip-
GSA Bulletin; August 1999; v. 111; no. 8; p. 000–000; 10 figures.
2
ping toward the hinterland, and an upper topographic surface dipping toward the foreland (e.g.,
Elliott, 1979; Chapple, 1978; Davis et al., 1983).
Mechanical thrust models (Fig. 1A), based on the
assumption that the material involved in the
wedge behaves by Mohr Coulomb failure
(Suppe, 1981; Davis et al., 1983; Dahlen et al.,
1984; Dahlen, 1990), postulate that the wedge
will evolve toward a critical taper (θc) geometry,
defined by the slope of the topographic surface
(α) and the dip of the basal decollement (β). A
critical wedge taper occurs when the gravitational and compressive forces in every segment
of the wedge balance the basal friction force
(Platt, 1986). Mass transfer by erosion, sedimentation, or faulting, isostatic responses, and
changes in boundary conditions can cause a
wedge to become unstable. A wedge with an unstable geometry may evolve toward a critical taper angle by frontal imbrication or hinterland extension (latter if taper is >20°) to reduce the taper
angle, or by internal wedge thickening to increase
B
Figure 1. (Continued).
Geological Society of America Bulletin, August 1999
3
BRAATHEN ET AL.
the angle (Fig. 1B) (Platt, 1986; Woodward,
1987). These processes may operate in an interlocked manner, thus causing sequential overlap
of structural styles and movement directions in
the prism.
The wedge model has proven successful in interpreting large-scale kinematics of active thrust
belts and accretionary prisms (Chapple, 1978;
Davis et al., 1983; Stockmal, 1983; Platt, 1986).
However, application to ancient fold-thrust belts
requires caution, because the geological structures observed may diverge considerably from
those predicted by the model (Woodward, 1987;
Meigs, 1993; DeCelles and Mitra, 1995). We use
a growing wedge model to explain complex
time-space distributions and interactions of foldthrust structures in a transform plate setting dominated by transpression, i.e., in the Tertiary foldthrust belt of Spitsbergen (Harland, 1969). Some
qualitative key variables used to evaluate wedge
behavior through time are: (1) the geometry and
spatial distribution of structures and structural
domains, (2) kinematic development, (3) tectonic
transport directions, (4) interaction of forelandhinterland deformation, and (5) erosion and sediment accumulation.
ASPECTS OF WEDGE SHAPE AND
BEHAVIOR
Stable wedge behavior during deformation requires mechanical equilibrium and counterbalance of the forces that reduce or increase θ (see
Platt, 1986). Key parameters are wedge growth
rate (Fig. 1B), rock strength (competence),
strength of the basal units and detachment surface, and erosion and flexural subsidence of the
upper surface (Davis et al., 1983; Dahlen et al.,
1984; Woodward, 1987; DeCelles and Mitra,
1995). The strength of the basal layer is of particular importance for the ability of the wedge to adjust to stable taper. A weak basal layer will reduce the value of θc required to move the thrust
wedge, and a strong layer will increase θc.
Wedges can be critical (θ = θc), subcritical (θ
< θc), or supercritical (θ > θc). A critical wedge
maintains a balance of forces and θc with either
static wedge movement or by continuous accretion of material along the wedge base. Examples
of processes leading to a subcritical wedge are
erosion and flexural subsidence. A supercritical
wedge forms by internal wedge thickening. Temporal changes in the growth rate or basal detachment strength (due to pore-pressure changes) can
also cause changes in θ and θc.
The change in state of a tapered wedge through
time (subcritical, critical, supercritical) may explain some of the structural complexities that appear in fold-thrust belts, such as temporal and spatial changes in structural geometry, kinematic
4
development, and transport directions. The basic
premise of wedge theory is that deformation will
proceed within the fold-thrust belt until a critical
taper is achieved. Wedge growth by foreland
propagation requires a supercritical wedge shape
(DeCelles and Mitra, 1995). Generalized models
of stable wedge behavior (Fig. 1B) advocate
growth of the wedge by thrusting and/or imbrication at the front of the wedge concurrent with underplating (Platt, 1986). Crustal uplift and thickening processes may be localized by contractional
inversion of former basins, renewed movement
along preexisting fabric, and/or duplexing of the
basal decollement during foreland thrust propagation. A supercritical wedge may change its taper
by three deformation mechanisms that may operate synchronously: (1) frontal foreland imbrication, which lengthens the wedge, (2) superposed
thrust imbrication (out-of-sequence), and (3) extension or collapse near the top of the wedge,
where the extension is triggered by gravitational
instabilities (Fig. 1B). Regional dynamics of the
fold-thrust belt can drive long-term deformation
episodes, and local differential tectonic responses
produce short-term deformation phases (DeCelles
and Mitra, 1995).
This work aims at tentatively applying an evolving wedge taper model to explain complex geometric and kinematic interactions in the Tertiary
transpressional fold-thrust belt of west-central
Spitsbergen, Svalbard (Fig. 2). This deformation
province is expressed as a belt as much as 200 km
wide with alternating thin-skinned and thickskinned deformation zones, a major strike-slip
fault zone, and localized extensional basins. A
well-established structural chronology, relative
timing constraints, kinematics of thrusting, and
shortening amount (see Braathen and Bergh,
1995; Bergh et al., 1997; Maher et al., 1997) are
utilized in developing the model. We specifically
explore how the interaction in time and space of
the different styles of the hinterland and foreland
deformation may be linked as parts of an evolving composite wedge. We also address the effects
of a transpressional regime in the hinterland on
wedge behavior, and associated parameters affecting supercritical wedge growth. We argue that
fold-thrust belt contractional uplift, superposed
imbrication, and extension in Spitsbergen can be
associated with hinterland strike-slip tectonics.
Such considerations may represent a fruitful general expansion of the wedge model.
SVALBARDS TERTIARY
DEFORMATION PROVINCE
Svalbard (Fig. 2) plays a key role in understanding the tectonic evolution of the North Atlantic region, because of its well exposed, long,
complex, and diverse geologic record. The is-
Geological Society of America Bulletin, August 1999
land is underlain by pre-Devonian metamorphic
basement (Hecla Hoek) and an overlying, semicontinuous sequence of Devonian through
Paleogene sedimentary strata (Fig. 3; Steel and
Worsley, 1984). During the early Tertiary opening of the North Atlantic Ocean, when the midoceanic spreading ridge migrated northward
into the area (Talwani and Eldholm, 1977;
Faleide et al., 1984; Muller and Spielhagen,
1990), Svalbard and Greenland separated along
a major, intracratonic, dextral transform (Hornsund) fault zone (Harland, 1969; Lowell, 1972;
Faleide et al., 1984). On Spitsbergen, this event
produced (1) a 100–200-km-wide contractional
fold-thrust belt, involving both basement and
cover strata on land (e.g., Dallmann et al., 1993;
Bergh et al., 1997); (2) a transcurrent hinterland
zone to the west and offshore, including extensional and transtensional grabens (e.g., Forlandsundet graben; Fig. 2); and (3) a central foreland
basin (Central Tertiary basin) (Steel et al., 1985;
Eiken and Austergaard, 1987; Gabrielsen et al.,
1992). The location of the fold-thrust belt adjacent to a transform plate setting led previous
workers to propose it as a type example of a
transpressional orogen (e.g., Harland, 1969;
Lowell, 1972). Subsequent models, however,
suggest that Svalbard’s Tertiary deformation
zone developed due to kinematic decoupling of
the contractional and transcurrent elements
close to the intercratonic paleotransform fault
(Maher and Craddock, 1988; Faleide et al.,
1988; Nøttvedt et al., 1988; Maher et al., 1995).
Regional structural work supports a five-stage
kinematic evolution of the fold-thrust belt and adjacent transcurrent fault zone, including (1) early
north-south orogen-oblique contraction (Late
Cretaceous or Paleocene?), (2) major west-southwest–east-northeast orogen-normal contraction
and uplift (Eocene?) with in-sequence folding
and thrusting, followed by, or concurrent with
(3) local orogen-parallel transcurrent faulting,
(4) local orogen-oblique transtension and transpression (out-of-sequence thrusting) both in the
west and east, and (5) locales of orogen-normal
and oblique extension in an overall transtensive
setting in the west (Oligocene?) (e.g., Braathen
and Bergh, 1995; Bergh et al., 1997; Maher et al.,
1997). The complex time-space distribution and
interaction of Tertiary structures, in particular the
location of Tertiary thrusts and transcurrent fault
zones, appear to have been controlled by preexisting basement fabric and structural grains, and
by inversion and/or uplift of Paleozoic grabens
along margin-bounding normal faults (Steel and
Worsley, 1984; Bergh and Andresen, 1990;
Haremo et al., 1992; Welbon and Maher, 1992;
Braathen et al., 1995).
The following consists of three parts: (1) a
summary of the pertinent structural data set, and
Geological Society of America Bulletin, August 1999
5
Figure 2. (A) Regional location map of Svalbard with respect to the Barents Sea. Spitsbergen is the main island of Svalbard. Some of the major onshore and offshore structural lineaments of Tertiary age are shown. Note location of the Hornsund fault zone (HFZ) acting as a
transform fault during dextral separation of Svalbard and Greenland in Eocene time. (B) Geologic map of the central part of Spitsbergen (see location in A). Map areas of Figure 4 (A and
C), and the cross-section line of C, are outlined by boxes and a heavy line. (C) Schematic cross
section along an east-west transect of central Spitsbergen illustrating the main Tertiary struc-
tural domains referred to in the text. Abbreviations are as follows: BFZ—Billefjorden fault
zone; CTB—Central Tertiary basin; LFZ—Lomfjorden fault zone; FG—Forlandsundet
graben; RB—Renardodden basin; LT—Lappdalen thrust front; NB—Nordfjorden block;
NL—Nordenskiøld Land; OL—Oskar II Land; SEDL—Svartfjella-Eidembukta-Daudmannsodden lineament; SJ—St. Jonsfjorden; SK—Sørkapp; B-SFZ—Bjørnøya-Sørkapp
fault zone; SFZ—Senja fracture zone; TFP—Troms-Finnmark platform.
BRAATHEN ET AL.
review of Svalbard’s Tertiary structure from west
to east (hinterland to foreland); (2) a discussion
of evidence for the structural chronology, tectonic transport directions, interacting forelandhinterland deformation, and erosion and sediment accumulation; and (3) development of a
modified, general wedge model for this transpressional orogen.
Structural Framework
The Tertiary deformation province in central
Spitsbergen (Fig. 2) can be subdivided into the
following zones of contrasting tectonic styles,
from west to east: (1) a hinterland zone of predominantly basement (Hecla Hoek) strata, with
local Tertiary basins (Forlandsundet graben) and
a major, orogen-parallel transcurrent fault zone
with cover-strata involvement, the Svartfjella-Eidembukta-Daudmannsodden lineament (Maher
et al., 1997); (2) a major thick-skinned (basement-involved) fold-thrust complex or antiformal
stack system; (3) a central thin-skinned zone of
fold-thrust structures above a basal decollement
in Permian gypsum; and (4) an eastern foreland
province, with structures indicating interplay
between thin-skinned tectonics and reactivation
of underlying basement-rooted reverse faults,
the Billefjorden and Lomfjorden fault zones
(Haremo and Andresen, 1992).
The fold-thrust belt portion of the transect
(Fig. 4, A and B) displays an overall geometry of
a low-angle, eastward-tapering wedge, as illustrated by the composite cross section of Oskar II
Land (Fig. 4B). The map and cross section
demonstrate a transition from steep, basement-involved transcurrent and reverse faults in the hinterland, to low-angle detachment faults that climb
from Carboniferous strata in the hinterland to
Permian and Triassic-Jurassic age strata in the
front portions to the east. A general east-northeast
shortening direction is estimated in the foldthrust belt (Fig. 4; see Dallmann et al., 1993).
However, local tectonic transport directions varying from 0° to 80° are recognized (Braathen et al.,
1997). Shortening estimates range from 20–40
km in the hinterland and central parts of the foldthrust belt (Bergh and Andresen, 1990; Welbon
and Maher, 1992; Wennberg et al., 1994) to
20–25 km in the front part, yielding a total of
~45% (Bergh et al., 1997).
Western Hinterland Zone. The western hinterland zone represents the deepest eroded part of
the transect, composed of various basement
rocks, semiductilely deformed slivers of Carboniferous-Permian strata, and several Tertiary
grabens, including the Forlandsundet graben
(Fig. 2; Steel et al., 1985; Gabrielsen et al., 1992;
Kleinspehn and Teyssier, 1993; Eiken and
Austegård, 1987) and Øyrlandet graben at
6
Figure 3. Stratigraphic scheme for western part of Nordenskiøld Land (after Braathen and
Bergh, 1995). Symbols of the stratigraphic column are used in other figures. The structural role of
different stratigraphical units with respect to rock strength and style of deformation are indicated
to the right. Note the presence of at least four detachment-decollement levels (arrows). Thicknesses
are from Ohta et al. (1992), with additions from Steel and Worsely (1984) and Nøttvedt et al. (1992).
Geological Society of America Bulletin, August 1999
A
B
Figure 4. (A) Structural map of southeastern Oskar II Land showing main folds and thrusts and subdivision of tectonic domains. IY—IsfjordenYmerbukta fault; MT—Mediumfjellet out-of-sequence thrust; PT—Protektorfjellet out-of-sequence thrust. (B) Composite cross section illustrating the wedge nature of the fold-thrust belt. Map and cross sections are after Bergh et al. (1997). (C) Detailed geologic map of a part of the transcurrent fault system (Svartfjella-Eidembukta-Daudmannsodden lineament) near Svartfjella, after Guddingsmo (1996) and Sørhaug (1996).
Location is shown in Figure 2B.
Geological Society of America Bulletin, August 1999
7
BRAATHEN ET AL.
Sørkapp, in south Spitsbergen (Dallmann et al.,
1993). The Forlandsundet graben trends northnorthwest and contains as much as 5 km of
Paleogene sediment fill. The Paleogene basin
strata are influenced by both syndepositional and
postdepositional extensional deformation as well
as folding and thrusting (Gabrielsen et al., 1992).
The western margin of the basin is characterized
by several steep, down-to-the-east normal faults,
separating blocks that contain a basal unconformity between basement and Eocene cover strata.
The eastern fault margin is vertical, as are adjacent Tertiary strata, and may be linked to the
Svartfjella-Eidembukta-Daudmannsodden lineament fault zone along strike to the southeast. The
lineament is a major north-northwest–striking,
transcurrent lineament with a strike length greater
than 60 km (Fig. 2), which consists of steeply
dipping, intensely deformed slivers of Carboniferous through Lower Permian strata within Caledonian basement rocks (Fig. 4C; Maher et al.,
1997). Basement and cover strata along the lineament were influenced by east-northeast–directed
thrusting (as in the fold-thrust belt), and later
sinistral and dextral strike-slip fault movements.
Maher et al. (1997) considered the lineament and
the fold-thrust belt as associated parts of a decoupled transpressional orogen. A favorable model
for the Forlandsundet graben and adjacent Svartfjella-Eidembukta-Daudmannsodden lineament
is that they formed synchronously, by decoupling
of a transtensional basin, bounded on one side by
a normal fault and on the other side by an orogenparallel transcurrent fault. A similar polyphase
decoupling history is invoked for the Øyrlandet
graben and adjacent structures at Sørkapp
(Bergh, 1996).
Basement-Involved Fold-Thrust Complex.
The basement-involved fold-thrust complex
marks a transition to where east-northeast contractional structures dominate the transect (Figs. 2
and 4, A and B). In western Oskar II Land and
Nordenskiøld Land the overall structure is that of
chevron-style anticlines and synclines and monoclines with wavelengths up to 5 km (Maher et al.,
1989; Braathen et al., 1995; Bergh et al., 1997). At
St. Jonsfjorden (Fig. 2) the complex is made up of
east-northeast–rotated thrusts forming an antiformal stack (Welbon and Maher, 1992). The rotated
thrusts are interpreted as early formed, leadingedge ramps that were progressively modified during foreland propagation and uplift of the complex. Cross-strike variations in facies and thickness
of the Carboniferous strata within the thrust sheets
indicate that the fold-thrust complex is located at
the eastern margin of an inverted Lower Carboniferous basin (St. Jonsfjorden, Maher and Welbon,
1992; Bellsund, Braathen et al., 1995). North of Isfjorden the fold-thrust complex is replaced by a
major syncline at the transition to the central zone,
8
Figure 4. (Continued).
which is bound by oppositely verging thrusts in a
pop-up structure (Bergh et al., 1997). These forethrust and back-thrust pairs are connected to a
steep, west-dipping ramp in the subsurface, as inferred from offshore seismic data, and thus mark a
transition from basement-involved to thin-skinned
tectonics (Fig. 4B).
Central Zone. The central zone of the transect defines a thin-skinned fold-thrust belt (e.g.,
Bergh et al., 1997) within exposed Permian and
Mesozoic strata of Oskar II Land (Fig. 4, A and
B). Open to tight, upright folds with subhorizontal enveloping surfaces characterize the deformation. Exposed thrusts typically tip out in the cores
of anticlines. More open to gentle folds occur in
the Paleogene-Eocene strata of the Central Tertiary basin, along strike to the south of Isfjorden
(Fig. 2). The regional map (Fig. 4) and seismic
data from Isfjorden (Eiken, 1994; Bergh et al.,
1997) suggest that these surface folds are developed above a regional decollement in Permian
evaporites of central Oskar II Land, and a roof
detachment in Triassic-Jurassic strata south of
Geological Society of America Bulletin, August 1999
Isfjorden. The evaporites are extensively developed in central Oskar II land and toward the
foreland, but are thinner to absent in the western
hinterland (Lauritzen, 1981).
A major thrust-ramp front marks the transition from the central, thin-skinned tectonic
province to nearly flat lying strata of the foreland
province (Figs. 2 and 4, A and B). At Lappdalen
and Mediumfjellet the thrust front is a hinterland-dipping duplex with at least six horses that
dip gently to steeply west-southwest and merge
against a sole thrust in the Permian evaporites
(Bergh and Andresen, 1990; Wennberg et al.,
1994). Two out-of-sequence thrusts decapitate the
duplex horses, and flatten into Triassic-Jurassic
shales farther east (Fig. 4B).
Eastern Foreland Province. The eastern
foreland province is characterized by dominantly
subhorizontal Paleozoic and Mesozoic strata.
Considerable intrastratal deformation of Triassic,
Jurassic, and Cretaceous shale horizons, however, suggests that Tertiary contractional strain
was transmitted eastward as decollements at
TRANSPRESSIONAL FOLD-THRUST BELT ON SPITSBERGEN, SVALBARD
higher levels. At Billefjorden and Lomfjorden
(Fig. 2), macroscale folding of the decollements
and near-surface sedimentary strata are directly
related to movements on subsurface, basementinvolved reverse faults (Andresen et al., 1994).
Components of sinistral and dextral transcurrent
movements along the faults are inferred from
obliquity and drag of adjacent fold-thrust structures (McCann and Dallmann, 1995). In a few
places, uplift flexures are decapitated by lowangle, out-of-sequence thrusts (Haremo and
Andresen, 1992; Andresen et al., 1994).
Kinematics and Structural Timing
A tentative summary of the overall structural
patterns and kinematic chronology of Tertiary
folding and thrusting along the central Spitsbergen transect is shown in Figure 5. Although conflicting views exist on plate location and absolute
timing of tectonic events (Muller and Spielhagen,
1990; Maher et al., 1995; Lyberis and Manby,
1996), recent reconstructions advocate a fivestage structural chronology, initiating in Late
Cretaceous or early Paleocene time (stage 1;
Kleinspehn et al., 1989; Braathen and Bergh,
1995). The late Paleocene–Eocene main phase of
fold-thrust development (stages 2, 3, and 4) included local sinistral and dextral orogen-parallel
strike-slip movements in the hinterland during
transpression and transtension (stages 3 and 4).
These phases were succeeded by late Eocene to
Oligocene extensional collapse and basin formation (stage 5).
The structural nature and kinematics of these
stages vary between the different structural domains (see Fig. 5). In the western hinterland
zone Maher et al. (1997) established a structural
chronology of the Svartfjella-EidembuktaDaudmannsodden lineament (Fig. 4C) initiating
with a dominant east-northeast–directed contractional folding and overthrusting event (Fig. 5;
stage 2), followed by sinistral strike-slip overprint along the steep limb of the overthrust complex (stage 3). A west-side-down component on
the Svartfjella-Eidembukta-Daudmannsodden
lineament at this time is consistent with coeval
formation of the adjacent Forlandsundet graben
as a local transtensive feature. Dextral strike-slip
motion (stage 4) and local extension ended the
deformation along the lineament (stage 5)
(Guddingsmo, 1996; Sørhaug, 1996).
In the basement-involved fold-thrust complex a
preuplift event (stage 1) is demonstrated by northto north-northeast–verging folds and detachment
thrusts (Fig. 5) that are consistently rotated by
stage 2 fold-thrust structures. The main contractional event included orogen-normal shortening in
the fold-thrust complex, and was probably coeval
with the first shortening in the lineament, as sug-
gested by a similarity in structural style and thrust
transport direction (east-northeast).
The main east-northeast–verging fold-thrust
complex can be attributed to a temporal deformational sequence of forward-breaking thrust propagation and duplex formation (Fig. 5; stages 2
and 3). Further deformation led to the formation
of conjugate transcurrent faults (stage 4) at high
angles (east to north-northeast) to the fold-thrust
complex. Most of these faults show dextral
movements, e.g., the north-northeast–striking Isfjorden-Ymerbukta fault (Fig. 4A). Associated
reverse faults (Protektorfjellet and Mediumfjellet
thrusts, Fig. 4A; Bergh et al., 1997) decapitate
obliquely the underlying fold-thrust stack (stage
3), defining stage 4 out-of-sequence thrusts. The
kinematics of stage 4 structures in the fold-thrust
belt are consistent with orogen oblique motions
in a coupled dextral transpression setting, and are
thus most likely coeval with dextral movement
on the Svartfjella-Eidembukta-Daudmannsodden
lineament (Fig. 5).
In the central zone and foreland of eastern
Spitsbergen, the stacked imbricate thrusts and underlying gypsum decollement of the thrust front
and higher detachments in the east all root in the
thick-skinned (stages 2 and 3) reverse faults in
the west. The decollements formed in an eastward-propagating sequence, climbing to successively higher stratigraphic levels (e.g., Bergh et
al., 1997).
Decapitating out-of-sequence thrusts (stage 4)
have been identified both in the western-central
zones and in the foreland. In the foreland, largescale monoclines of strata along the Billefjorden
and Lomfjorden fault zones postdated the insequence (stages 2–3, Fig. 5) decollement
thrusts, and were probably caused by late-stage
reactivation of deep-seated reverse faults (stages
3–4). Similarly, the uplifted strata and monoclines are cut by low-angle thrusts correlatable
with the out-of-sequence thrusts of the central
and western zones, thus constraining the deformational chronology.
The formation of Tertiary extensional basins in
the western hinterland can be ascribed to (1) local
transtension, (2) gravitational collapse, or (C) regional changes in the boundary conditions of the
uplifted hinterland portion of the thrust belt. The
exact timing and linkage with the other structural
events, however, are uncertain. Some of the orogen-parallel normal faults in the hinterland
clearly postdate the main uplift events, and thus
can be considered as late orogenic or relaxation
structures (stage 5). However, other extensional
faults near the steep eastern limb of the foldthrust complex reveal downdrop offset patterns
that are both parallel (east-northeast) and orthogonal (southeast and northwest) to the strike of the
main fold-thrust complex, thus suggesting a tem-
poral overlap with the stage 4 strike-slip and outof-sequence faults (see following; Fig. 5).
Tectonic Transport Directions
Local strain and tectonic transport directions
have been estimated for each structural stage on
the basis of fold-thrust interrelationships and
fault-slip striae populations (Fig. 6). The data
help to establish a time-space correlation of the
structural stages, reflecting changes in wedge
mechanics. Stage 1 fault data, when rotated with
bedding to a subhorizontal attitude, yield consistent north-south to north-northeast–south-southwest–directed (0°–035°) shortening oblique to
the average strike of the Tertiary deformation
zone, consistent with a dextral transpressional
setting (Lepvrier 1992; Braathen and Bergh,
1995; Braathen et al., 1997). For the stage 2
thrusts a more east-northeast (~080o) directed uplift and overthrust transport direction is evident in
most of the transect. Stage 3 shortening structures
also show a predominant east-northeast (080°)
transport direction, but upon more steeply dipping thrusts in the western parts, corresponding
with the main uplift and fold-thrusting event, and
as low-angle thrusts in the central zone.
Along the Svartfjella-Eidembukta-Daudmannsodden lineament, sinistral strike-slip faults
with south-southeast–plunging displacements
(stage 3) were activated along older rotated
thrusts and steep fold limbs of a ramp antiform
(Guddingsmo, 1996; Sørhaug, 1996). In the foreland province, stage 3 uplift was provided by
east-west–directed movement on deep-seated reverse faults with a weak, oblique-sinistral component, thus confirming a possible temporal
overlap in strain with the lineament.
Stage 4 of the fold-thrust belt consists of conjugate transcurrent faults where the acute bisector
trends northeast (045°), parallel to the strike of associated down-to-the-southeast and northwest
normal faults (Ingebrigtsen, 1994; Haabet, 1995).
A similar northeast shortening axis is obtained for
the out-of-sequence thrusts of the central zone,
demonstrating a temporal change in tectonic
transport direction compared to the underlying insequence thrust stack, which is east-northeast directed (Fig. 6). Such a strain pattern of orogen
oblique transport in the central zone would be
consistent with a more coupled dextral transpressive setting and most directly linked to dextral
transcurrent motion in the hinterland, along the
Svartfjella-Eidembukta-Daudmannsodden lineament (Fig. 6; see following).
Extensional faults of stage 5 postdate uplift in
the hinterland portion of the transect, and may
have been related to formation of upper parts of the
Tertiary basins (see following). These faults consistently reveal down-to-the-west and west-south-
Geological Society of America Bulletin, August 1999
9
10
Geological Society of America Bulletin, August 1999
Figure 5. Summary diagram suggesting spatial and temporal linkage of the various structural patterns and kinematic chronology in the central Spitsbergen transect. Numbers in cir-
cles refer to structures related to the various structural stages. See discussion in text. FG—
Forlandsundet graben; SEDL—Svartfjella-Eidembukta-Daudmannsodden lineament.
Geological Society of America Bulletin, August 1999
11
Figure 6. Summary diagram of fault data. The equal-area, lower hemisphere stereonets present mesofaults with slip lines as slip-linear plots (pole and line with arrow) and M-plane plots
(contoured). In the case of the basement-involved fold-thrust complex the stage 4 data are separated into strike-slip and normal fault subpopulations. The stereo plot of data from the fore-
land province (Lomfjorden) shows fault planes (great circle, N = 53) and fold axes (dots, N =
29). Number of faults (N), orientation of composite movement planes, and maximum density
of M-planes are presented. The method was described in Goldstein and Marshak (1988) and
Braathen and Bergh (1995). SEDL—Svartfjella-Eidembukta-Daudmannsodden lineament.
BRAATHEN ET AL.
west displacement directions, which differ from
the slip senses of stage 4 normal faults (Fig. 6), indicating rapidly changing extensional directions in
the hinterland.
Interaction of Foreland-Hinterland
Deformation
A thick-skinned (basement involved) transpressional welt with a spatially and temporally evolving pattern of decoupling (stage 1–3 increasing
decoupling with time and localization of strike-slip
components on developing weak surfaces, e.g.,
Svartfjella-Eidembukta-Daudmannsodden lineament) was likely linked to east-northeast thrust
translation along a basal gypsum decollement in
the foreland central zone. In the foreland province,
bilaterally symmetric, north-south–trending basement uplifts at Billefjorden and Lomfjorden fault
zones postdated the thin-skinned thrust translation.
Later deformation in the west occurred along
basement-involved out-of-sequence faults that
have a shortening axis similar to conjugate transcurrent faults, indicating a temporal linkage.
The latter structures locally acted as oblique
ramps and tear faults (e.g., Isfjorden-Ymerbukta
fault, Fig. 4A) between thrust-related structures
in the central zone. In the foreland, the out-of-sequence thrusts propagated as higher level
decollements that crosscut the earlier formed
basement-involved monoclines. Local extensional and transtensional structures developed at
various locales (i.e., in the Svartfjella-Eidembukta-Daudmannsodden lineament and foldthrust complex) coeval with the contractional
episodes in the fold-thrust belt. Thus, the data
suggest intimately related episodes of (1) orogen-normal thrusting in the fold-thrust belt (insequence followed by out-of-sequence thrusting;
stages 2, 3, and 4) and (2) orogen-parallel, transcurrent movements in the hinterland (transpressional-transtensional; stages 3 and 4 of the Svartfjella-Eidembukta-Daudmannsodden lineament).
Tertiary Erosion and Sedimentary
Accumulation
The sedimentary strata of the Central Tertiary
basin and Forlandsundet graben range in age
from late Paleocene to Eocene and possibly
Oligocene time (Manum and Throndsen, 1986;
Steel and Worsley, 1984; Gabrielsen et al., 1992),
spanning the time interval of the inferred Tertiary
deformation (Fig. 7). An underlying Late Cretaceous low-angle (<1°) regional unconformity
progressively truncates Aptian-Albian strata
northwestward. This unconformity, dating crustal
uplift, reflects north-up and south-down regional
tilting. Possible explanations (Maher et al., 1995)
are crustal thinning and thermal doming prior to
12
dextral transtensional rifting between Svalbard
and Greenland, or, less likely, initial transpression along the northern edge of the Barents Shelf
(i.e., perhaps early, stage 1 deformation).
Subsequent Paleocene strata in the Central
Tertiary basin, mostly marine, record rapid basin
subsidence and sediment derivation from a
source area to the north, northeast, and east
(Muller and Spielhagen, 1990). Sedimentological signals indicative of the main contractional
uplift (stage 3) in western Spitsbergen in late
Paleocene to Eocene time include a profound
shift to a western source area (Steel et al., 1985)
and the appearance of metamorphic clasts in conglomerates and sandstones, indicative of basement exhumation. Most of the Paleocene strata,
at least in the west and northwest (Forlandsundet
graben and at Brøggerhalvøya; Fig. 1), were
tilted, folded, and imbricated during the structural stages 1 to 3 (Bergh et al., 1997). In the Central Tertiary basin, however, internal angular unconformities in the Tertiary sequence have not
been identified. Regional flexural subsidence is a
likely cause for this basin, but the sedimentary fill
was not folded before Eocene time (Fig. 7). Steel
et al. (1997) noted a rapid, west-east change in
depositional environment within early Eocene
strata of the Central Tertiary basin, indicated by
synchronous coastal plain deposits, deltaic sandstones, slope mudstones, and deep-water turbidites. Repeated depositional events were associated with an eastward shift of the main
depocenter. These facies characters and their distribution are consistent with erosion from an uplifted, foreland-migrating (east-northeast) foldthrust belt and deposition of the sediments in a
foreland basin (Helland-Hansen, 1990; Steel et al.,
1997). Eventually, the aforementioned roof thrust
developed beneath the basin, thin-skinned thrusting emerged on its eastern side, and it developed
a piggy-back status.
Consistent with their hinterland position, the
thick (>5 km) Paleogene sediments of the Forlandsundet graben (Fig. 7) record a more complex polyphase structural history than do strata of
the Central Tertiary basin. This includes obliqueextensional and margin-parallel and oblique foldthrust structures (Lepvrier, 1990; Gabrielsen et al.,
1992; Kleinspehn and Teyssier, 1992). Alongstrike variations, as expected in a transcurrent setting, explain the complexity. Seismic images indicate a half-graben fill, thicker to the east, and
bounded by stepped normal faults to the west
(Nøttvedt et al., 1994). The exact timing of sediment deposition and tectonic activity is uncertain.
However, on the west side Eocene strata locally
unconformably overlie Hecla Hoek basement,
suggestive of erosion of more than 3–5 km of
Mesozoic-Paleocene cover prior to deposition of
Eocene strata. Paleocene-Eocene rocks along the
Geological Society of America Bulletin, August 1999
eastern basin margin are subvertical, and are possibly influenced by sinistral strike-slip faulting
(i.e., Svartfjella-Eidembukta-Daudmannsodden
lineament; our stage 3). The presence of mid-late
Eocene sediments in the graben, affected by
contractional and late dextral transtension
(Gabrielsen et al., 1992; Lepvrier, 1992), give evidence for continued transcurrent faulting (stage
4). Flexural subsidence of the Forlandsundet
graben (Manum and Throndsen, 1986), and
formation by faulting of the Oligocene(?) Renardodden basin, which shows weakly lithified sediments overlying basement (Fig. 2B; Dallmann,
1989), represents the final Tertiary deformation
(stage 5 or younger).
APPLICATION OF A CRITICAL WEDGE
TAPER MODEL
In applying a wedge model to the Tertiary deformation province of central Spitsbergen, qualitative information about potential wedge behavior can be obtained by evaluating parameters
such as the present structural framework, the
kinematic history, change of lithology, relative
strengths of basal layers and decollements, rampflat geometry of thrusts, and erosion levels and
sediment deposition.
Present Wedge Geometry and Mechanics
In terms of its present geometry, Spitsbergen’s
Tertiary fold-thrust belt displays a composite
wedge-shaped prism (Fig. 4B). An inferred basal
detachment (β) dips ~7° west, whereas the present
topographic surface (α) is nearly horizontal (1°–2°
east dip, local high relief) and cuts into successively older rocks to the west. Thus, the present
shape is that of a wedge that has been deeply
eroded. Its initial form has been strongly modified.
The varying styles of deformation and the nature
of rocks involved along the west-east transect
(Figs. 2 and 4) suggest the presence of two juxtaposed wedges, a hinterland wedge (western hinterland and basement-involved fold-thrust complex) with high taper, and a foreland wedge
(central zone and foreland) characterized by a low
taper. A high taper in the hinterland is supported by
the involvement of mechanically strong metamorphic basement rocks, closely spaced thrusts, contractional antiformal stacks and large folds, and
significant foreland-dipping enveloping surfaces.
A much lower taper is evident in the frontal
wedge, which consists entirely of layered sedimentary rocks, with an extensive, weak evaporite
unit and several shale-silt units that are ideal
decollements. Tilted Tertiary strata in the western
portion of the Central Tertiary basin mark the transition between the hinterland and foreland wedges.
Uplift and thickening of a mechanically strong
Geological Society of America Bulletin, August 1999
13
Figure 7. Diagram showing tentative time-space history and nature of fault movements
(arrows with hachures related to structural stages, indicating uplift-thrusting, subsidencenormal faulting, and transcurrent displacements) and accumulation of synorogenic sediments in the Forlandsundet graben (shaded areas) and in the central Spitsbergen foreland
basin. Outlined sedimentary wedges indicate the spatial distribution of more coarse clastic
(sandstone) deposition (Steel et al., 1997). The links between the sedimentary wedges and
deformation stages are indicated. Note that the deformation propagated, from west to east,
underneath the Central Tertiary basin (CTB). Schematic stratigraphic columns of the sequences in the Central Tertiary basin and Forlandsundet graben are after Manum and
Throndsen (1986), Steel et al. (1985), Rye-Larsen (1982), and Gabrielsen et al. (1992). T, toward; A, away.
BRAATHEN ET AL.
Figure 8. Schematic diagram illustrating the possible evolution and changes in state and shape of Spitsbergen's tapered wedge through time.
Growth of the wedge reflects either a long-term, progressive deformation event and/or a short term, episodic behavior. Circled numbers refer to
stages of Figure 7. See text for further explanations.
hinterland wedge underlain by steep basement
ramps, and the decreased strength of both an extensive detachment flat and the overlying rock
units in the foreland direction, may have resulted
in the wedge becoming supercritical and propagating forward (as in other orogens; DeCelles
and Mitra, 1995). This would explain the composite wedge geometry (Figs. 2 and 4). This hypothesis is based on the fact that the basal detachment emerges from a steep, basementinvolved ramp at the western boundary of the
central zone (Fig. 4B) and turns into a flat, first in
the Permian evaporites, then at successively
higher (Mesozoic) levels toward the frontal portion of the fold-thrust belt (Figs. 3 and 4).
The ramp-flat geometry of the basal detachment may be ascribed to two related factors:
(1) the westward pinch-out of the Permian evaporite horizon (Lauritzen, 1981) at the transition
between the western and central zones, and an
eastward decrease in thickness of Carboniferous
sandstones of the St. Jonsfjorden Trough toward
the eastern basin margin, and (2) the presence of
Carboniferous basin-bounding normal faults,
dipping steeply west and emerging from the
steep foliation within the basement (Maher and
Welbon, 1992; Braathen et al., 1995). The lack of
continuous low-strength detachments would
maintain overall steeper dips of thrusts emerging
from steep basement foliation flats into the cover
strata, and would be associated with greater uplift
in the hinterland (Bergh et al., 1997). East of the
major ramp, the presence of evaporites reduced
the basal friction, thus forcing thrusts to proceed
as low-angle decollements at various levels and
time intervals (see Gutscher et al., 1996). The
structural style of the central zone, with upright
folding of the Permian and Triassic units, and a
lack of emergent thrusts (Fig. 4B), supports the
presence of an extremely weak, basal evaporite
decollement. Estimates of fold-thrust belt tapers
bound by a sole evaporite are in the range of
1°–2° (Davis et al., 1983), which may apply to
the central fold-thrust belt in Spitsbergen.
14
Further evidence of decreasing strength of the
basal rocks and detachment toward the frontal
wedge is a decrease in shortening and increase in
the initial spacing of thrusts and thrust ramps (see
Dahlen, 1990). In the hinterland wedge, thrust
ramps are closely spaced, producing antiformal
stack geometries with significant crustal shortening (to 46%; Braathen et al., 1995) that formed
beneath appreciable overburden of strong rocks
(Costello and Hatcher, 1985). However, the less
shortened (20%–30%) frontal wedge (Central
zone) is characterized by gently westward dipping imbricate thrusts with broad spacing of
thrusts, allowing less deformed frontal thrust
sheets to develop (Fig. 4B). This is possibly due
to thickness reduction and weakening of the
decollement unit during deformation, rather than
from increasing strength of the overriding rocks.
A local increase of shortening and closer imbricate stacking of Permian units are demonstrated within the frontal wedge, near the transition between the central zone and foreland (at
Lappdalen; Fig. 4B). There, the detachment steps
to a higher stratal level (Mesozoic). Because the
strata above the evaporite decollement of the central zone do not display any marked change in
thickness or composition, a mechanical explanation for the thrust step is strengthening of the
rocks beneath or ahead of the decollement. The
latter is consistent with the presence of competent Devonian sandstones and conglomerates belonging to a Carboniferous high, the Nordfjorden
block (Fig. 2), probably serving as a structural
buttress against strain transfer into the foreland
(see Bergh et al., 1997). In the foreland province,
fold-thrust belt deformation is accommodated in
the form of layer-parallel decollements in Mesozoic shales. Such a deformation style is consistent with a thick succession detached along a
weak basal thrust, in which the value of θc has
been reduced to a minimum.
In summary, the present geometry of the
fold-thrust belt suggests that uplift and duplexing (stage 3) of the hinterland wedge produced
Geological Society of America Bulletin, August 1999
a supercritical taper. This resulted in renewed
frontal imbrication along one or more highlevel detachments and lagging hinterland erosion that served to return the wedge to a critical
or subcritical state. The overall timing of these
events is consistent with paleogeographic reconstructions wherein the hinterland portion
was uplifted and exhumed in late Paleocene–
Eocene time, while marine, low-relief conditions prevailed in the Central Tertiary basin
(e.g., Helland-Hansen, 1990).
Wedge Shape and Behavior Through Time
In the long term (~40[?] m.y.), Spitsbergen’s
fold-thrust belt propagated more than 100 km
eastward, a process that could not have taken
place unless the taper had been maintained at
critical to supercritical values episodically over
the same time span (Fig. 8; DeCelles and Mitra,
1995). We propose that there were changes in the
state of the wedge due to both long-term and
short-term processes: (1) long-term behavior, encompassing a time period of nearly 40 m.y., and
the entire structural history (stages 1–5), whereas
(2) short-term behavior applies to local changes
in wedge shape, which in turn forced the rest of
the wedge to respond. Experimental studies (e.g.,
Lallemand et al., 1994; Gutscher et al., 1996)
show that accretionary wedges with a high basal
friction evolve through rapid and significant
changes in the shape and surface slope (α), which
may well apply to the hinterland wedge of Spitsbergen. This would include rapid and episodic
uplift and subsidence, deformation, and erosion
(i.e., stages 2–4), over a time period of <20 m.y.
(Fig. 7). A low basal friction favors a low-angle
and more stable wedge, which could be the case
for the central fold-thrust belt (Fig. 4B). In the
following we discuss possible tectonic processes
operating in Spitsbergen’s evolving wedge and
probable responses to changes in kinematics and
paleostress conditions through time.
Three main structural processes may be respon-
TRANSPRESSIONAL FOLD-THRUST BELT ON SPITSBERGEN, SVALBARD
sible for the long-term change of a wedge taper
geometry: (1) thickening of the hinterland wedge
during crustal shortening and uplift from basin inversion, antiformal stacking, and strike-slip duplexing (i.e., our stages 1, 2, and 3), (2) lengthening of the frontal wedge by renewed, forward
imbrication (stage 2–3) and superposed out-ofsequence thrusting (stage 4), and (3) thinning of
the hinterland wedge by lateral extrusion (transcurrent faulting) and extension or transtension
(stages 4 and 5). Variation in taper geometry along
strike is expected in a transpressive hinterland. Local supercritical taper geometries could then result
in short-term and local orogen-oblique or orogen
parallel (out of plane) adjustments (see MartinezTorres et al., 1994; Gray and Stamatakos, 1997;
Maher et al., 1997; Enlow and Koons, 1998). This
could explain the observed transtensional faulting
in the hinterland.
An initial, critical wedge shape may have
evolved from movement oblique (0°–035°; Fig. 5,
stage 1) to the main trend of the regional deformation zone, in a narrow belt near the paleotransform boundary (Braathen and Bergh, 1995).
Stage 1 shortening may have caused localized
thickening, and even formation of a supercritical
taper acting as a precursor contributing to later
east-northeast–directed shortening and forelandward propagation of thrusts (i.e., stage 2). It is feasible that an additional effect of stage 1 was to create an initial zone of through-going weakness that
would later concentrate orogen-parallel movement and permit decoupling (Jones and Tanner,
1995) in stages 2 and 3. A similar process occurred during stages 2 and 3 along the SvartfjellaEidembukta-Daudmannsodden lineament, with
initial orogen-perpendicular contraction and subsequent migration and localization of an orogenparallel movement component along a developed
weak surface.
Major contraction (stages 2 and 3) of the hinterland fold-thrust complex likely produced an
overall, unstable supercritical wedge in the hinterland. Stage 2, 3, and 4 structures in the western
fold-thrust complex, and subsequently in the central and foreland zones, suggest that the oversteepened hinterland wedge adjusted by lengthening and imbrication of the frontal wedge (see
Fig. 1B). Such mechanical readjustments may
have occurred in two modes. (1) One mode is an
episodic process of eastward thrust propagation
(Fig. 8), where hinterland thickening occurred
first and triggered frontward in-sequence thrusting (stage 2), then renewed out-of-sequence
thrusting (stages 3 and 4), which in turn caused
frontal lengthening, and last, extension and collapse of the wedge. (2) The other mode is a continuous, progressive deformation. We favor a
combination of the two modes, in which a shortterm episodic behavior was superposed on an
overall long-term evolution toward a critical taper
(Fig. 8).
Changes in the strain-stress pattern are well
documented by the kinematic data of the foldthrust complex (Fig. 6). Notable is the change
from dominant east-northeast–directed (075°–
080°) thrusting and imbrication (plane strain of
stages 2 and 3; regional intermediate stress σ2
subhorizontal) into northeast- directed (045°)
conjugate strike-slip faulting (σ2 subvertical)
and out-of-sequence thrusting (non-plane strain
of stage 4). One plausible explanation is buildup (stages 2 and 3) of the hinterland wedge until a supercritical taper was reached. Increased
gravitational instability may have caused lateral
extrusion (northwest-southeast directed; stage
4) associated with a change in the paleostress
field. This may be related to evolving detachment strength, and the degree of decoupling of
the orogen (see next section).
Local topographic highs would be subjected to
increased erosion (Fig. 7). Erosion is considered
an important factor in controlling long-term
wedge behavior (England and Molnar, 1990;
Beaumont et al., 1992). The erosional durability
of the hinterland wedge surface partly controls α
and is a function of the lithologies present at the
surface of the wedge (DeCelles and Mitra, 1995).
In western Spitsbergen, durable Paleozoic and
basement rocks gradually became more widely
exposed, thereby enhancing the tendency of the
wedge to become supercritical. Eocene and
Oligocene strata overlying basement rocks in the
hinterland (Forlandsundet Graben and Renardodden basin, respectively) suggest that this had
largely been accomplished by that time.
A change from a north to easterly source area
for Paleocene sedimentation to the onset of clastic sediment accumulation from the west in the
Central Tertiary basin documents uplift and regional unroofing in the western hinterland
wedge (stage 3) in early Eocene time. The Central Tertiary basin may have formed as a low-relief foreland basin to the fold-thrust belt, with
subsidence driven by tectonic and sedimentary
loading and downwarping in front of the belt. At
a given stage (3–4) this foreland basin became a
piggy-back basin (Helland-Hansen 1990) incorporated in the low-taper frontal wedge. In contrast, the thick accumulation of Paleocene(?)Eocene strata in the Forlandsundet graben (Fig 7)
indicates an orogen internal basin with local and
rapid, renewed uplift and erosion (stages 2, 3,
and 4), and deposition in multiple fans
(Gabrielsen et al., 1992). Episodic or continued
changes into a supercritical status of the hinterland wedge is needed in order to produce this
prolonged history of the Forlandsundet graben
infilling coeval with foreland wedge and basin
development (Fig. 7).
Transpression as a Cause of Wedge
Modification
Spitsbergen’s Tertiary fold-thrust belt formed
adjacent to a dextral transpressional plate margin
(e.g., Talwani and Eldholm, 1977; Steel et al.,
1985), where structures were partitioned through
time and space (e.g., Maher et al., 1997). We discuss here supplementary causes of wedge growth
and modification in view of the transpressional
regime. A regional decoupling model is well constrained in Spitsbergen, expressed by linked contraction and transcurrent tectonism followed by
transtension or extension. The result is a two-fold
structural division. (1) A broad zone of distributed
orogen-normal (contractional) fold-thrust belt
structures constitutes the central and foreland portions of the transect. (2) A narrow and complex
zone with orogen-parallel (strike-slip) and orogenoblique structural patterns is present in the hinterland (Svartfjella-Eidembukta-Daudmannsodden
lineament) closest to the paleotransform (Fig. 2).
Similar decoupled tectonic patterns are suggested
for the San Andreas fault system (e.g., Mount and
Suppe, 1987; Namson and Davis, 1988). This
scheme of a composite wedge with a steeper hinterland taper characterized by transpressive complexities and a decoupled and simpler, lower-taper
frontal wedge dominated by orogen-perpendicular
contractional structures is consistent with the theoretical analysis of Enlow and Koons (1998) of critical wedges in three dimensions. They found that
“steep wedges.…. are unlikely to show extensive
parallel displacement partitioning, when they are
laterally sheared,” whereas “displacement partitioning is predicted for weak base wedges.” For
Spitsbergen’s composite wedge this translates into
increased decoupling toward a weaker foreland.
In terms of general wedge behavior, a decoupled orogen could produce a two-part composite wedge; one typified by local and rapid
(episodic short term) wedge growth and readjustments to a critical taper of the transpressional
zone (hinterland wedge), and another wedge in
the contractional foreland (foreland wedge),
where more continuous and homogeneous
wedge growth prevailed (i.e., long-term adjustment to a stable, critical taper). Several arguments indicate local and rapid (episodic, shortterm) wedge growth and adjustment in the
hinterland wedge of Spitsbergen (Fig. 9). First,
the early (stage 1) transpressional strains adjacent to the paleotransform boundary, in combination with initial north-south–trending basin
geometry of Paleozoic strata of the Carboniferous St. Jonsfjorden Trough (Welbon and Maher,
1992; Maher et al., 1997), may have created lateral differences and segmentation of the hinterland wedge. Furthermore, the preexisting basinbounding normal faults of the St. Jonsfjorden
Geological Society of America Bulletin, August 1999
15
BRAATHEN ET AL.
Figure 9. Block diagrams illustrating the
structural development and importance for
the wedge shape (stages 1 through 5) of the
western, hinterland part, including SvartfjellaEidembukta - Daudmannsodden lineament
(SEDL), of the central Spitsbergen transect.
Shaded areas show basement, thin lines show
the bedding in the St. Jonsfjorden Trough, and
dot pattern shows the Tertiary Forlandsudet
graben (FG). See text for details.
Trough (Fig. 9) certainly made this portion of the
transpressional regime a favorable site for
episodic reactivation. Second, during the main
deformation of the hinterland (stages 2 and 3),
Paleozoic normal faults of the St. Jonsfjorden
Trough were progressively reactivated as steep
thrusts, then as strike-slip faults to the west
(Fig. 9). Branching of the strike-slip faults could
have produced local uplift and subsidence domains (steep wedges) along the active SvartfjellaEidembukta-Daumannsodden lineament fault
strand, which became extinct during subsequent
linkage of irregular strike-slip faults (Davison
1995) at adjacent locales. An important factor
for supercritical wedge growth at this stage may
be the anomalous sinistral structures (Fig. 5;
Svartfjella-Eidembukta-Daudmannsodden lineament, stage 3) within a dominant dextral
strike-slip fault strand of the Hornsund fault
zone. Maher et al. (1997) suggested that the
sinistral strike-slip faults developed by a lowangle wedge extrusion or lateral extrusion
process, as discussed by Powell (1993) and Martinez-Torres et al. (1994). For example, if sinistral faults joined dextral faults that stepped left,
as commonly observed in the Svartfjella-Eidembukta-Daudmannsodden lineament, then an
oblique, local supercritical wedge may have developed, the tip line of which propagated into the
adjoining crustal block, causing additional
wedge growth until delamination evolved (i.e.,
double wedge kinematics according to MartinezTorres et al., 1994). In this context, lateral wedge
extrusion within the reactivated basin-bounding
transpressional zone of Spitsbergen’s hinterland
may have influenced the overall foreland wedge
taper, as well as the formation of Tertiary basins.
A laterally segmented, transpressional
wedge that temporally and locally adjusted to
critical taper is also suggested for the Forlandsundet graben (Fig. 9; Maher et al., 1997),
which possibly evolved under a transtensional
basin mode of formation. This is supported by
an elongate (low width to length ratio) basin
geometry and complex kinematics of internal
structures (Gabrielsen et al., 1992; Kleinspehn
and Teyssier, 1992; Lepvrier, 1992).
Readjustment of the hinterland wedge by
16
Geological Society of America Bulletin, August 1999
Figure 10. Summary of structural evolution and inferred wedge behavior for Spitsbergen's Tertiary fold-thrust belt in terms of changing regional dynamics and tectonic setting, as discussed in the text. The tectonic settings are based on diverse plate reconstructions (e.g., Ziegler, 1988;
Muller and Spielhagen 1990; Skilbrei and Srivastava, 1993). Open arrows show the s1 axis, whereas black arrows show the relative motion between the Svalbard and Greenland plates. Shaded areas reflect sediment accumulations. Abbreviations are as in Figure 2. T-toward; A-away;
SJT-St. Jonsfjorden Trough; RB-Renardodden basin; SEDL-Svartfjella-Eidembukta-Daudmannsodden lineament; CTB-Central Tertiary
basin; FG-Forlandsundet graben; HF-
Geological Society of America Bulletin, August 1999
17
BRAATHEN ET AL.
transverse faulting (stage 4 of the fold-thrust
complex) marks a change in the shortening direction (maximum horizontal stress σ1 045°), which
is oblique to the lineament and parallel with the
direction of out-of-sequence thrusting in the foreland (Fig. 6).
Two competing hypotheses may be entertained for the stage 3 to 4 transition. The change
could be in response to lateral differences in the
wedge geometry, or it could reflect a change
from a decoupled to a coupled status so that the
oblique kinematics more directly reflect dextral
transpressive plate motion. In the first case a supercritical more hinterland portion that did not
persist along strike drove oblique thrust motion in
the foreland at the same time it produced conjugate strike-slip and normal faults toward the hinterland. The hinterland strain field would be consistent with oblique escape and gravitational
collapse. Changes in cover thickness (decreasing) and the change in structural trend along
strike to the northwest (Brøggerhalvøya) support
this possibility.
In the second case, changes in detachment
and/or wedge strength may have changed the degree of decoupling (Enlow and Koons, 1998). A
possibility in the case of the central zone is that
salt mobility during stage 2–3 deformation and/or
solution grounded the thrust sheet and increased
the detachment strength, causing the wedge to become more coupled and producing hinterland
conjugate strike-slip faults and foreland out-ofsequence thrusting. At numerous locales the gypsum is largely missing, even though the surface
clearly acted as part of an extensive flat.
CONCLUSIONS AND WEDGE MODEL
Based on the time and space distribution and
interaction of different structural styles in Spitsbergen’s fold-thrust belt, we tentatively suggest a
model advocating linked, long-term and shortterm (episodic) dynamic growth of a composite
(partitioned-decoupled) contractional and transcurrent fold-thrust wedge taper (Fig. 10). Growth
of a narrow, initial low-taper (critical-supercritical) contractional wedge occurred during northward-directed (σ1 0°–035°) crustal shortening
(stage 1) in a coupled dextral transpressive setting.
This early stage wedge created a zone of weakness that allowed later decoupling concentration
of contractional deformation along the margins
of the Carboniferous St. Jonsfjorden Trough
(subparallel with the Svartfjella-EidembuktaDaudmannsodden lineament).
Combined crustal thickening in the form of
thrust uplift and inversion of the Carboniferous
St. Jonsfjorden Trough, and strike-slip duplexing
(stages 2 and 3) during the main event (stages 2
and 3; σ1 075°–080°), created an unstable, su-
18
percritical wedge of basement and cover rocks in
the hinterland. At the same time a broader, more
homogeneous frontal part of the wedge developed eastward by in-sequence decollement
thrusting and frontal imbrication in order to
lengthen the overall wedge topography. A weak
detachment not only promoted a lower frontal taper, but also facilitated wedge decoupling at this
time. The hinterland wedge adjusted to a critical
taper by maintaining erosion, transcurrent faulting, lateral wedge extrusion, and extensional
faulting (stages 3 and 4). Either a change in the
regional strain field (in plate motions) to transtension, or an increase in strength of the basal detachment may have resulted in these wedge adjustments.
Renewed internal shortening of the frontal
wedge was accommodated by out-of-sequence
thrusting and possibly, reactivation of higher
level decollements (stage 4; σ1 045°). The final
adjustment to a stable taper geometry included
local extension and sediment deposition (stage
5). Such an evolutionary model of Spitsbergen’s
decoupled fold-thrust wedge is largely in accordance with the proposed moderate wedge-growth
model (Fig. 1B) and with Platt’s (1986) twodimensional model for wedge dynamics. Modification of these models to account for lateral taper
changes due to the complex transpressive character of the hinterland may be fruitful in analyzing
other fold-thrust belts.
ACKNOWLEDGEMENTS
We thank Jonny Guddingsmo, Tora Haabet,
Arild Ingebrigtsen, and Petter Sørhaug for fruitful discussions and for their contribution of fault
data. Logistical support by Norsk Polarinstitutt,
and financial contributions by the University of
Tromsø, Geological Survey of Norway, University of Nebraska at Omaha, Statoil-VISTA, Saga
Petroleum ASA, Norwegian Research Council
and Petroleum Research Fund are acknowledged.
Constructive reviews by Dan Davis, Peter DeCelles, and an anonymous reader improved the
manuscript.
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REVISED MANUSCRIPT RECEIVED SEPTEMBER 30, 1998
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