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Journal of the Geological Society, London, Vol. 160, 2003, pp. 197–208. Printed in Great Britain.
197
Extension, crustal structure and magmatism at the outer Vøring Basin, Norwegian
margin
L. GERNIGON 1, J. C. RINGENBACH 2, S . PLANKE 3, B. LE GALL 1 & H. JONQUET-KOLSTØ 4
1Institut Universitaire Europeen de la Mer, Place Nicolas Copernic, 29280 Plouzane, France
(e-mail: [email protected])2TotalFinaElf Exploration Libya, Dhat El Imad building, tower 3, P.O Box 91171, Tripoli, Libya
3Volcanic Basin Petroleum Research (VBPR), Forskningsparken, Gaustadalleen 21, N-0349 Oslo, Norway4TotalFinaElf Exploration Norge AS, P.O. Box 168, 4001 Stavanger, Norway
Abstract: Regional analysis of new 2D and 3D multichannel seismic data has improved interpretation of the
crustal configuration and structural style along the Norwegian margin. Five domains with different structural
styles and evolutions are defined along the outer Vøring Basin: (1) the Nyk High–Naglfar Dome; (2) the
north Gjallar Ridge; (3) the Gleipne saddle; (4) the south Gjallar Ridge; (5) the Ran ridge. Early Campanian–
Early Paleocene and Early to mid-Cretaceous extensional events are evidenced. Timing of deformation and
structural styles observed along each segment reflect a lateral variation of the rifted system, probably affected
by magma-tectonic processes. Correlation with the deep structures of the outer Vøring Basin shows that the
shallow structure in that basin is directly controlled by a deep-seated, strong, high-amplitude reflection (the T
reflection), marking the top of a high-velocity body (Vp .7 km s�1). The relation between the lower-crustal
architecture and the subsurface basin structures has implications for the margin evolution and for the nature of
the high-velocity body.
Keywords: Norwegian margin, rifting, magmatism, underplating.
The Norwegian margin is part of the NE Atlantic rift system
(Fig. 1). It is characterized by a long period of episodic rifting
initiated after the Caledonian orogeny (Dore et al. 1999; Brekke
2000; Brekke et al. 2001). The overall tectonic framework of the
margin between 628 and 698N consists of a central NE-trending
deep Cretaceous basin (the Vøring and Møre basins), flanked by
palaeo-highs, platforms and the elevated mainland (Blystad et al.
1995).
In a regional context, the Trøndelag Platform and the Halten
Terrace, to the east (Fig. 3), are the best areas to illustrate the
Jurassic evolution, which was strongly influenced by Triassic salt
tectonics resulting in the development of both low-angle detach-
ment and steeply dipping fault structures (Koch & Heum 1995;
Pascoe et al. 1999). A major unconformity at the base of the
Cretaceous marks the Late Jurassic–Early Cretaceous rifting
episode, which is commonly considered as the major rifting
event in the Halten Terrace (Blystad et al. 1995). Although not
proven by well control, the Triassic–Jurassic sediments may be
present below the thick Cretaceous series in the central Vøring
Basin and in the outer Vøring Basin (Blystad et al. 1995). The
Cretaceous evolution of the Norwegian margin is also a subject
of debate. Dore et al. (1999) proposed separate Late Jurassic and
Cretaceous rift episodes in the Vøring Basin, whereas Blystad et
al. (1995) and Brekke (2000) argued for a continuous rift episode
from Mid-Jurassic to Early Cretaceous time. According to Pascoe
et al. (1999), extension in the northern part of the Halten Terrace
continued until Late Aptian to Early Cenomanian time.
The previous structural works described the outer Vøring
Basin mainly in the northern part (Skogseid & Eldholm 1989;
Lundin & Dore 1997; Mogensen et al. 2000) or discussed the
Gjallar Ridge in detail (Ren et al. 1998) but few studies were
performed in the southern part. Several regional reviews and
modelling studies of the Norwegian margin have been published
(Eldholm et al. 1989; Skogseid 1994; Blystad et al. 1995; Koch
& Heum 1995; Bjørnseth et al. 1997; Robert et al. 1997; Walker
et al. 1997; Hjelstuen et al. 1999; Brekke 2000; Tsikalas et al.
2001), but few structural studies have focused on the early rifting
evolution and the structure of the outer Vøring Basin, investi-
gated in this paper (Figs 1 and 3).
The outer Vøring Basin consists of a NE-trending system of
ridges generally assigned to a Maastrichtian–Paleocene episode
(Lundin & Dore 1997; Ren et al. 1998) or to an Early
Cenomanian–Paleocene rifting event (Bjørnseth et al. 1997)
(Fig. 1). It is overlain to the west by Tertiary flood basalts
(seaward-dipping reflectors) extruded before and during the
continental break-up in a short period of time (63–54 Ma)
(Saunders et al. 1997). Before this volcanic event, a regional
uplift close to the Cretaceous–Tertiary boundary is observed
along the North Atlantic rift system and is interpreted to reflect
interaction of the Iceland mantle plume with the base of the
lithosphere (Dam et al. 1998; Skogseid et al. 2000).
The outer Vøring Basin is also an interesting area for
investigating the relationships and interactions between crustal
magmatism and basin deformation in space and time. Large
amounts of seismic refraction and reflection data have greatly
improved knowledge of the crustal structure along the outer
Vøring Basin and the Vøring Marginal High (Mutter et al. 1984;
Planke et al. 1991; Skogseid et al. 1992; Eldholm & Grue 1994).
Commonly observed below the margin, a high P-wave velocity
body (Vp .7 km s�1) (lower-crustal body) is interpreted as
homogeneous, underplated mafic crust associated with perva-
sively intruded crust (White & McKenzie 1989; Mjelde et al.
1997; Eldholm et al. 2000). Previous studies have pointed out
the isostatic impact of the lower-crustal body on the subsidence
of the margin (Skogseid 1994; Robert et al. 1997; Walker et al.
1997). At the mid-crustal scale, Lundin & Dore (1997) inter-
preted the geometry of the Late Cretaceous–Paleocene normal
faulting along the Gjallar Ridge as a mid-Cretaceous to Paleo-
cene metamorphic core complex caused by magmatism at the
base of the crust.
This paper is a regional overview of the entire outer Vøring
Basin with broader implications for the deformation of rifted
systems and/or the development of volcanic margins. The aims
of this study are: (1) to constrain and refine the structural and
stratigraphic history of the outer Vøring Basin; (2) to describe
the along-strike structural variation of the sub-basins and their
genetic relationships with the deep crustal geometry; (3) to
attempt to document the variability of the structural styles and
their links with the Tertiary magmatism or/and the structural
inheritance. Because the official nomenclature proposed by
Blystad et al. (1995) is not sufficiently detailed to describe the
structural complexity of the outer Vøring Basin, additional
informal names are proposed for specific structures (Table 1).
Data, methods and interpretation confidence
This work is mainly based on the interpretation of a regional
database of more than 100 000 km of 2D seismic reflection, at
2 km 3 1 km spacing, tied to wells on the Trøndelag Platform
and to some released information from recent wells in the Møre
and Vøring basins. Three-dimensional seismic reflection data are
also used, particularly a 3D seismic survey covering the northern
part of the Gjallar Ridge (Fig. 1). Interpretation was carried out
within a single GeoframeTM project covering the region between
628N and 688N, and was undertaken in the framework of the
preparation of the 16th Norwegian concession round (2001).
Compilation of proprietary ship track (Norwegian Petroleum
Directorate (NPD) database and TGS-NOPEC surveys V2R96,
MWT96, HT97, KFW98 and RSW98) and public gravity data
(Sandwell & Smith 1997) are also used and correlated with the
regional structure.
The Early and pre-Cretaceous markers are calibrated with the
numerous Halten Terrace exploration wells but the pre-Turonian
basin history of the outer Vøring Basin is mostly based on
seismic interpretation. In addition to Deep Sea Drilling Program
(DSDP) and Ocean Drilling Program (ODP) data (Eldholm et al.
1989; Hjelstuen et al. 1999), released data from recently drilled
wells (Fig. 1) on the Nyk High (6707/10-1) (e.g. Kittilsen et al.
1999), Vema Dome (6706/11-1), north Gjallar Ridge (6704/12-1)
and Helland Hansen Dome (6505/10-10) supply further calibra-
tions. Wells 6707/10-1 and 6706/11-1 are drilled on a structural
high position and do not penetrate the lower section of the synrift
sequence below the Coniacian–Turonian series. The Gjallar well
(6704/12-1) reached the Lower Campanian series. Around the
south Gjallar and Ran ridges, the interpretations rely mostly on
regional correlations of seismic facies.
Fig. 1. Structural map of the outer Vøring Margin and location of regional cross-sections in Figure 8. BCU, base Cretaceous unconformity; BL, Bivrost
Lineament; FFC, Fles Fault Complex; FG, Fenris Graben; GFZ, Gleipne Fracture Zone; GS, Gleipne saddle; HG, Hel Graben; HT, Halten Terrace; JMFZ,
Jan Mayen Fracture Zone; ND, Naglfar Dome; NGR, north Gjallar Ridge; NH, Nyk High; NS, Nagrind Syncline; RaB, Ran basin; RR, Ran ridge; Rym
FZ, Rym Fault Zone; SGR, south Gjallar Ridge; TP, Trøndelag Platform; VD, Vema Dome; VMH, Vøring Marginal High; VS, Vigrid Syncline. Volcanic
facies map of the Vøring Marginal High has been modified after Berndt et al. (2001).
L. GERNIGON ET AL .198
Table 1. Characteristics of the outer Vøring Basin
Structural element Main geological features Deep reflections Ages of the main tectonic events
Nyk High� NE-trending structural high Not observed? High-velocity sills Latest Cretaceous–PaleoceneHel Graben–NaglfarDome�
Cretaceous–Paleocene depocentreinverted during Tertiary. High positionduring the Maastrichtian
Poorly expressed. High-velocity sills Collapse during Paleocene
Inversion Early Eocene–PresentNorth Gjallar Ridgey Major tilted blocks affected by low-angle
faulting, partly controlled by a shaledecollement layer
The north Gjallar Ridge is influenced bythe dome-shaped T reflection (top of thehigh-velocity body (Vp .7.1 km s�1)
Early Campanian?– Paleocene
Fenris Graben� Graben onlapped by the inner flows.Located in the western part of the northGjallar Ridge and south Gjallar Ridgesegments
T reflection observed and expectedbelow the inner lava flows
Early Campanian–Paleocene
Gleipne saddley Synform, saddle structure, influenced bymagmatic sill injection. Minor faulting
Progressive plunging of the T reflection Early Campanian?– Early Paleocene
South Gjallar Ridgey Tilted panels controlled by step faultsconnected with Palaeozoic–Jurassicpanels
The T reflection observed in highposition
Early Campanian–Early Paleocene
Early–mid-CretaceousRan basiny Cretaceous depocentre influenced by
voluminous sill intrusion. Subaerialvolcanism is reduced compared withthe other sub-basins
Transition zone between the T reflectionand the top basement
Late Jurassic–mid-Cretaceous?
Early Campanian–Early PaleoceneRan ridgey Uplifted Triassic–Jurassic depocentre
sealed by the base Cretaceousunconformity. Tilted panels controlledby a decollement layer
Top faulted basement correlated withthe T reflection
Late Jurassic–mid-Cretaceous?
Early Campanian–Early Paleocene
�Nomenclature after Blystad et al. 1995.yInformal names used in this paper.
Fig. 2. Bouguer residual gravity anomaly
map, 50 km high-pass filtered (courtesy of
TGS NOPEC/VBPR). Abbreviations as in
Figure 1. Numbers represent the location of
the five domains.
VØRING BASIN, NORWEGIAN HIGH 199
Structure and evolution of the outer Vøring Basin:evidence for along-strike variation
Gravity data
The 50 km high-pass filtered Bouguer residual gravity anomaly
map of the outer Vøring Basin shows various trends and
structures (Fig. 2). Negative anomalies are found to be consistent
with the sub-basins, whereas positive anomalies reflect the
structural highs and lows mapped in this study (Figs 1 and 4).
The complex pattern of gravity anomalies can be used together
with the structural map to define five structural domains,
comprising from north to south: (1) the Nyk High–Hel Graben
domain, with a NE–SW positive anomaly for the Nyk High and
a north–south to NE–SW negative trends for the Hel Graben;
(2) the north Gjallar Ridge domain, bounded to the north by the
Rym Fault Zone and showing a main positive rounded anomaly;
(3) the Gleipne saddle domain, characterized by a gravity low;
(4) the south Gjallar Ridge domain, marked by a broad NE–SW-
trending positive anomaly, adjacent to a negative anomaly
situated below the inner lava flows; (5) the Ran ridge domain,
with NW–SE and east–west orientations located in the prolonga-
tion of the oceanic Jan Mayen Fracture Zone that also coincides
with major NW–SE positive trends in the oceanic domain
(Fig. 3).
The five domains coincide with high or low structures
characterized by a gradual along-strike variation without any
clear boundaries. Nevertheless, approximate limits can be pro-
posed to better define the five zones or domains (Fig. 4),
illustrated below by a summary of their tectonostratigraphic
evolution (Fig. 5) and detailed geoseismic profiles (Fig. 8).
Nyk High–Hel Graben
The morphology of this segment is dominated by the Nyk High,
which separates the Nagrind Syncline from the Hel Graben (Fig.
6a). The Nyk High is bounded to the west by a major normal
fault complex whereas the Hel Graben has subsequently been
inverted to form the Naglfar Dome (Fig. 1). The Nyk High is,
together with the Gjallar Ridge and the Vema Dome, one of the
few structures recently drilled in the Vøring Basin. The Nyk
High is a large SE-tilted faulted block affected by an array of
Early Campanian–Early Paleocene, NW-dipping normal faults.
The normal faults are interpreted to detach at depth in Lower
Cretaceous(?) shales and they appear to be located above faulted
blocks of similar polarity at base Cretaceous level.
Well 6707/10-1 was drilled in a faulted block SE of the Nyk
High and bottomed in Santonian strata. The constant thickness of
the Turonian–Santonian series and their apparent continuity
towards the Hel Graben suggest minor south-dipping normal
faulting before the Early Campanian in the Hel Graben (Fig. 8a).
In the northern prolongation of the Nagrind Syncline, the Nyk
High probably remained unfaulted until Late Maastrichtian time
during the deposition of the thick Nise formation sand in
Santonian–Early Campanian time. As interpreted here, the uni-
form thickness of the Upper Cretaceous seismic package sug-
gests also that most of the tilting and uplift of the Nyk High
occurred during the Late Paleocene–Eocene (Fig. 5).
At 10–20 km west of the Nyk High, the Cretaceous basin was
mainly eroded during Maastrichian to Early– (mid?)-Paleocene.
During this period, the uplifted and eroded area represented the
Fig. 3. Regional cross-section of the Vøring margin from the Vøring
Marginal High to the Trøndelag Platform. IF, inner flows; LF, landward
flows; SDR, seaward-dipping reflectors. Location is shown in Figure 1.
L. GERNIGON ET AL .200
highest position of the segment. Partly concomitant with the
uplift of the Nyk High, the structure of the Hel Graben and the
presence of Paleocene sediments above the base Tertiary uncon-
formity argue for a sudden collapse of the basin during the
mid?–Late Paleocene–Eocene, in agreement with Lundin &
Dore (2002). The mid–Late Paleocene deformation is coeval
with emplacement of high-velocity sills in the Hel Graben, which
is thus interpreted as a probable igneous centre during this period
(Berndt et al. 2000). The break-up magmatism is expected to be
involved in the sudden uplift of the Nyk High structure. By
analogy with structures deduced from sand–silicone models
(Bonini et al. 2001), a lateral flow and a boudinage of the melted
lower crust, triggered by underplated magma, may explain both
the Nyk High uplift and the Hel Graben collapse (Fig. 6). A
slightly different magma-tectonic model has been also proposed
by Lundin et al. (2002), who interpreted the Hel Graben as a
major cauldron that collapsed during Paleocene time.
North Gjallar Ridge
The north Gjallar Ridge domain represents the ridge itself and
part of the Fenris Graben (Figs 1 and 8b). The ridge is well
expressed at the base Tertiary level (Fig. 6) and it is mostly
dissected by arrays of NW-dipping faults (Figs 8b and 9). Strongly
truncated by the base Tertiary unconformity, above the circular-
shaped gravity high (Fig. 4), each block is characterized by well-
layered reflections that are interpreted as synrift deposits (Fig. 9).
Swieciki et al. (1998) proposed a Triassic–Jurassic age for these
deposits, but recent drilling and in-house dating show it to be of
Campanian–Maastrichtian age so that the Base Tertiary unconfor-
mity is assumed to represent a hiatus between Lower Maastrich-
tian and Upper Paleocene (or/and Lower Eocene) series.
Contrary to the models established by Walker et al. (1997) and
Ren et al. (1998), continuous detachment crosscutting the entire
upper crust is not observed and an intricate system of detachment
faults, controlling block rotation during the Atlantic rifting, is
mainly limited to the upper part of the basin (Fig. 9). In the
eastern limb of the ridge, strongly rotated small blocks are
observed above a chaotic transparent seismic facies, believed to
represent (middle?) Cretaceous mobile shales (Fig. 9). The
shales, interpreted to have acted as a decollement for the
overlying rollover feature, seem to die out laterally to the north,
where faults are steeper. The rollover structure predicts a
westward thickening of the synrift sequences and is at least
partly concomitant with onlap and pinch-out of Turonian–
Paleocene formations within the Vigrid Syncline on the eastern
flank (Fig. 8b). In the western part of the ridge, a sigmoidal fault
pattern at the base Tertiary unconformity level (Fig. 9) also
suggests dextral strike-slip deformation along NE-trending faults.
The Paleocene interval largely onlaps the north Gjallar Ridge
and thus was only slightly faulted in Early Paleocene to Early
Eocene time. Faulting along the ridge probably took place a few
million years before the break-up in the latest Paleocene; this
suggests a progressive focus of crustal deformation to the west
(e.g. Lundin & Dore 1997). The westward shift may be explained
by a weakening of the crust induced by the starting magmatism,
as has been proposed for the west Greenland margin, where the
pre-break-up deformation was progressively focused toward the
igneous centres (Geoffroy et al. 2001).
Fig. 4. Strike-parallel section along the Gjallar ridges and Naglfar Dome
illustrating the lateral evolution and corrugation of the T reflection (TR)
along the outer Vøring Basin and its relation to sill intrusions. Legend as
for Figure 3. Location is shown in Figure 1.
VØRING BASIN, NORWEGIAN HIGH 201
Gleipne saddle
The Gleipne saddle forms a transition zone between the north
and south Gjallar ridges (Figs 1 and 8c). At Cretaceous level, it
is characterized by an embayment NW of the Vigrid Syncline
(Fig. 1). As observed in Figure 4, the transition zone coincides
with both a marked gravity low and an offset of the Vøring
Escarpment (Fig. 1).
Faulting in the Gleipne saddle is not significant in comparison
with the north Gjallar Ridge and no major significant block
rotation is observed (Fig. 8c). However, seismic interpretation
within the saddle is rendered difficult by the presence of
numerous magmatic intrusions in the Cretaceous section. Never-
theless, the Cretaceous series appear to thicken from the north
Gjallar Ridge into the Gleipne saddle (Fig. 4).
During Early Paleocene–Mid-Eocene time, a sedimentary
wedge infilled westwards from the Vigrid Syncline into a large
part of the subsiding Gleipne saddle (Figs 4 and 8c). Moreover,
inner volcanic flows, emplaced during the break-up, flowed
around the north Gjallar Ridge and preferentially infilled the
southern part of the Fenris Graben (Fig. 1), hence suggesting its
low structural position with regard to the north Gjallar Ridge
(e.g. Lundin & Dore 1997).
South Gjallar Ridge
The south Gjallar Ridge lies between the Gleipne saddle and the
Ran basin (Figs 1 and 8d) and is bounded to the east by the
Fenris Graben. The structural history is not constrained by
proximate well control and is based only on regional seismic
facies correlation.
The south Gjallar Ridge is a prominent elongated high (75 km
3 50 km) at base Tertiary unconformity level. The south Gjallar
Ridge is expressed by steep NW-dipping normal faults bounding
elongated rotated blocks (Fig. 8d). The faulting activity is
assumed to be of Early Campanian–Early Paleocene age as
similarly stated for the north Gjallar Ridge. The structural style
suggests domino deformation rather than the listric faulting
evident in the north Gjallar Ridge. The layered seismic facies of
the synrift sediments are less well developed than in the north
Gjallar Ridge, whereas the faults show less displacement (Fig.
8). The Vigrid Syncline to the east is marked by Early Cretac-
eous faulting, sealed near the base Cenomanian level (Fig. 8d).
The gravity low, observed in the western part of the segment,
is inferred to correspond to the progressive down-faulting of the
Cretaceous basin in the Fenris Graben. However, this area is
poorly imaged because of the emplacement of the inner lava
flows (Fig. 8d).
Ran ridge
The Ran ridge is the southwesternmost domain of the outer
Vøring Basin and appears to be different from the other ridges
(Figs 1 and 8e). The domain is composed of the Ran ridge and is
Fig. 5. Tectonic calendar of the Vøring margin. Time scale after Berggren et al. (1995).
L. GERNIGON ET AL .202
bounded by the Ran basin to the west and lies to the SE of the
Jan Mayen Fracture Zone (Fig. 1). A significant part of the Ran
ridge lies beneath the inner flows to the west and numerous
intrusions affect the Cretaceous and older series (Fig. 8e).
The axis of the Ran ridge is shifted eastwards relative to the
Gjallar ridges (Fig. 1). On its eastern flank, the Ran ridge is
defined by a clear unconformity, not yet drilled but interpreted to
be the base Cretaceous unconformity. The feature is a broad high
limited by major NNE-trending listric faults facing the WNW
and cut in the south by the Jan Mayen Fracture Zone (Fig. 1). To
the east, low-angle normal faults are detached between the base
Cretaceous unconformity and a strong seismic marker, which is
structured into tilted blocks and is interpreted as the top base-
ment (Fig. 8e).
Furthermore, Early to mid-Cretaceous (Aptian–Cenomanian?)
normal faulting is observed, concomitant with onlaps, wedging
and erosion features on the eastern flank of the Ran ridge (Fig.
8e). To the west, thinning of the series lying between the base
Cretaceous unconformity and the top basement interval suggests
that the Ran ridge was a depocentre during pre-Cretaceous time.
Its current morphology may result from uplift and inversion of
the pre-Cretaceous depocentre during Early to mid-Cretaceous
time.
During the Late Cretaceous, some faults in the Ran ridge were
reactivated (Fig. 8e). Normal faults associated with the late phase
of rifting are mainly located to the west and do not overprint the
faults belonging to the Ran ridge system. The fault array of the
south Gjallar Ridge cannot be confidently traced into the Ran
basin because of a loose seismic grid and (magmatic) sill
intrusions.
To the NW of the Ran ridge, the Ran basin is a narrow NE-
oriented Cretaceous depocentre severely intruded by sills (Figs 1
and 8e). Several NW-trending transfer zones deduced from
unpublished gravity data affect the Ran basin. This basin is
connected to the north with the small half-graben previously
described in the western limb of the south Gjallar Ridge
(Fig. 8e).
Jurassic to mid-Cretaceous rift events in the outerVøring Basin
The rifting in the outer Vøring Basin has been assigned
previously to a Late Cretaceous–Paleocene episode (Lundin &
Dore 1997; Walker et al. 1997), likely to be Maastrichtian–
Paleocene in age (Ren et al. 1998). In this study, an Early
Campanian age is inferred for the initiation of the late rifting
event. It coincides with a sudden decrease of the sand influx in
the Nyk High and the Vema Dome area, as well with faulting
and uplift in the north Gjallar Ridge, and it is finally linked to a
sudden and major kinematic change in the North Atlantic at
80 Ma, as suggested by Torsvik et al. (2001).
Earlier deformation (mid-Cretaceous and Late Jurassic–ear-
liest Cretaceous?) is also involved in the development of the
outer Vøring Basin. Mid-Cretaceous faulting is observed along
the Ran ridge (Fig. 8e), whereas the axis of the Paleocene rifted
zone is shifted slightly further to the west in the Vøring margin.
Fig. 6. A sand–silicone model of lateral
extension and doming formation (after
Bonini et al. 2001). Lateral flow of a low-
viscosity lower crust (magma chamber),
initially emplaced below the main rift may
have induced uplift of the upper crust and is
accommodated at depth by listric border
faults. This process may explain the
geometry of the Nyk High, which shows
similar structures (Fig. 6a). LC, lower crust;
M, mantle; UC, upper crust.
Fig. 7. Base Tertiary two-way travel time map mixed with a coherence
attribute computed from the 3D seismic survey between the north Gjallar
Ridge and the Gleipne saddle. The north Gjallar Ridge coincides with the
crest of the T reflection mapped at 7.5 s (dashed line). Arrows indicate
fold axes related to Early Eocene inversions. Location of 3D survey is
shown in Figure 1. Late tectonic activity within the Gleipne saddle is
mainly related to minor Early Eocene north–south anticlinal structures in
the eastern part of the survey.
VØRING BASIN, NORWEGIAN HIGH 203
Fig. 8. Seismic line drawing through various segments of the outer Vøring Basin, defined in Table 1. TR, T reflection. Location is shown in Figure 1.
L. GERNIGON ET AL .204
The deep structure of the Ran ridge exhibits low-angle faults
with pre-Cretaceous displacement that could be related to a
Jurassic–earliest Cretaceous rifting controlled by Triassic salt as
documented on the Halten Terrace. Mid-Cretaceous deformation
(mid-Aptian to Cenomanian?) is recorded in the Ran ridge area.
It is considered as a significant rifting phase in the southern
segment and may be correlated with a mid-Cretaceous phase
evidenced in the northern Halten Terrace by Pascoe et al. (1999).
A progressive thinning of the interval between the top base-
ment and the base Cretaceous unconformity to the east in the
Ran ridge suggests that it was a depocentre during pre-Cretac-
eous time (Fig. 8e). Even if the nature of the pre-Cretaceous
series is not well determined, the Ran ridge domains interpreted
as a depocentre during Permian–Late Jurassic(?) was later
uplifted during mid-Cretaceous time synchronously with the
progressive downflexure of the south Vøring Basin. This agrees
with the palaeogeographical interpretations of Brekke et al.
(2001) and Martinius et al. (2001) suggesting an emerged Early–
Mid-Jurassic hinterland located to the east in the current central
Vøring Basin.
Relation between deep and shallow structures
The crustal architecture along the outer Vøring Basin suggests a
direct relationship between the shallow deformation of the basin
and the high-amplitude reflection observed at mid-crustal depth
(Lundin & Dore 1997; Ren et al. 1998). The strike section along
the outer Vøring Basin (Fig. 4) illustrates its mid-crustal
structure, which is mainly characterized by the strong, high-
amplitude T reflection (Gernigon et al. 2001). The T reflection is
clearly mapped from the north Gjallar Ridge to the Ran ridge
whereas it is not observed in the Nyk High on the conventional
seismic data available in this study (Fig. 8).
Mapping of the T reflection reveals three broad high zones,
lying respectively below the north Gjallar Ridge, the south
Gjallar Ridge and the Ran ridge segments (Fig. 10). These mid-
crustal highs strictly correlate with positive gravity anomalies
(Fig. 10). The T reflection exhibits a dome shape with a flat top
at 7.5 s two-way travel time (TWT) beneath the northwestern
part of the north Gjallar Ridge and the Fenris Graben and deeps
below the Vigrid Syncline (Fig. 8). To the south, the T reflection
deepens below the Gleipne saddle, but shallows below the south
Gjallar Ridge (Figs 4, 8 and 10). In this area, the reflection forms
mostly a NW-trending elongated high with two peaks at 7.6 s
TWT, separated by a trough interpreted as a transfer zone that
also affects the shape of the T reflection below the Vigrid
Syncline (Fig. 10). Below the Ran ridge, the underlying T
reflection can be mapped continuously and coincides with the
top basement previously described at 7–8 s TWT (Fig. 10).
However, it shows a morphology characterized by faulted blocks
that differ from the smooth shape observed below the south and
north Gjallar ridges. The faulted blocks occur as NE-oriented
highs, 20–25 km wide, and limited to the south by the Jan
Mayen Fracture Zone and to the north by a NW-trending transfer
zone. The most prominent high corresponds to both the main
Bouguer positive anomaly (Fig. 10) and the highest position of
the Ran ridge at base Cretaceous level.
The geometry of the T reflection is believed to have controlled
the structural evolution of a large part of the outer Vøring Basin.
Cretaceous–Paleocene depocentres preferentially developed
Fig. 9. Depth-converted 3D seismic profile across the north Gjallar Ridge. Rollover structures are controlled at depth by probable (mid?)-Cretaceous
mobile shales (MS). The base Tertiary unconformity truncates a large part of the fault activity. A deep tilted block system is interpreted below the
decollement layer. Between 8 and 12 km, a high-Vp lower-crustal dome is observed, bounded on its top by the T reflection. Seismic depth conversion was
applied using semi-regional seismic interval velocities interpolated with check-shot velocities measured in well 6707/12-1.
VØRING BASIN, NORWEGIAN HIGH 205
where the T reflection is deep, whereas most of the erosional
features at the base Tertiary level occur on the north and south
Gjallar ridge, where the T reflection is shallower (Figs 4 and 8).
Despite uncertainties concerning the Mesozoic picking and
calibration, it is believed that the entire Cretaceous series may be
thinner above the north and south Gjallar ridges compared with
the Gleipne saddle. The domal shape of the T reflection appears
genetically associated with the faulting in the outer Vøring Basin
(Fig. 8) and its structural influence may have started as early as
Campanian–Maastrichtian time. Furthermore, a correlation be-
tween the intensity of intrusions and the geometry of the T
reflection is also inferred from the preferred distribution of sills
in the synform structures overlying the T reflection (Fig. 4).
Implications for the nature of the lower-crustal body ina volcanic margin context
At 7–8 s TWT (10–15 km) below the north and south Gjallar
ridges, the T reflection coincides with the lower-crustal reflection
described by Skogseid & Eldholm (1987), whereas it is shallower
than the Moho estimated by Mjelde et al. (1997) at 20 km depth
below the north Gjallar Ridge. The T reflection coincides with
the top of the lower-crustal body (Vp .7.1 km s�1) determined
by ocean bottom seismometers (OBS) (Mjelde et al. 1997).
Below the Ran ridge, recent OBS data recording shows also that
Vp velocities measured below the T reflection are relatively high,
approximating 6.5–8 km s�1 (T. Raum, personal communica-
tion).
Lundin & Dore (1997) suggested that the dome-shaped T
reflection may represent the top of a lower-crustal diapir,
triggered by mafic underplating, and inducing a fall of viscosity
and a rising of the lower continental crust. This magma-tectonic
process is similar to the model suggested above for the formation
of the Nyk High (Fig. 6). Such a model fits the timing of both
the main magmatic event (commonly dated between 63 and
54 Ma) and the Nyk High uplift (Fig. 5).
In the north Gjallar Ridge, fault patterns above the dome
suggest activity before Paleocene time (Fig. 5). Assuming that
the dome is a magmatic-induced feature (Lundin & Dore 1997)
implies that instantaneous emplacement of thick underplated
bodies had started already during Campanian–Early Maastrich-
tian time in the north Gjallar Ridge domain. However, we are not
aware of any evidence supporting such a young and huge
magmatic activity during this period (e.g. Saunders et al. 1997).
Because the dome strongly influences the structure of the pre-
break-up rifted system in the outer Vøring Basin, the T reflection
is interpreted as probably being a structure emplaced before the
main magmatic event, considered to be Late Paleocene to Early
Eocene in age in the North Atlantic domain (Saunders et al.
1997). From the overall characteristics mentioned above, we
suggest that the crustal dome bounded by the T reflection may be
partly characterized by high-pressure granulite or eclogitic
material that also displays high Vp (7.2–8.5 km s�1) (Fountain et
al. 1994) as documented in the Caledonides of Western Norway
(Austrheim 1990) or below the Mesozoic rifted basins in the
North Sea (Christiansson et al. 2000). This interpretation has
Fig. 10. Time structure map (TWT) of the
T reflection from the north Gjallar Ridge to
the Ran ridge domain. The free air gravity
anomalies fit the shape of the mid-crustal T
reflection.
L. GERNIGON ET AL .206
important implications for thermal history, the mantle tempera-
ture and the magmatic production along the Vøring volcanic
margin because it challenges and minimizes the size of the
Tertiary underplated lower crust, which is generally correlated
with the whole lower-crustal body.
Conclusion
Description of the various segments along the outer Vøring Basin
shows that the Norwegian margin architecture varies along-strike.
Five NE-trending domains, marked by contrasted structural styles
and evolutions, correspond to ridges and sub-basins, named, from
north to south: (1) the Nyk High–Hel Graben; (2) the north
Gjallar Ridge; (3) the Gleipne saddle; (4) the south Gjallar
Ridge; (5) the Ran ridge.
The Ran ridge differs from the other domains as it is a pre-
Cretaceous structure that was subsequently uplifted and affected
by west-verging listric faults during Jurassic?– Early Cretaceous
time. This domain evolved independently with regard to the
Gjallar ridges and Nyk–Hel Graben systems, which were mostly
affected by the Early Campanian–Paleocene rifting leading to
the break-up in Early Eocene time. A strong high-amplitude
reflection at mid-crustal depth is mapped between the north
Gjallar Ridge and the Ran ridge. The so-called T reflection
coincides both with the positive gravity anomalies and the top of
the high-Vp lower-crustal body, previously interpreted as mafic
underplating below the north Gjallar Ridge. A strong correlation
between the shape of the T reflection and the upper-crustal
deformation sealed by Upper Paleocene series suggests that the
lower-crustal body influenced the sedimentary basin before the
break-up and probably started as early as the onset of the latest
rifting in Early Campanian time.
The T reflection is interpreted as the top of a lower-crustal
crystalline body, inferred to be partly Caledonian(?) in age and
displaying an eclogitic nature that may also account for its high
Vp values.
This work was initiated in the framework of the 16th Norwegian
Licensing Round and is part of a PhD thesis at the University of Brest
funded by the TotalFinaElf Exploration Norge Research Centre. This
paper is a contribution to the ‘Sub-Basalt Imaging Project’ involving
TotalFinaElf. The regional interpretation was initiated in 1999 in collab-
oration with the geologists and geophysicists of TotalFinaElf Exploration
Norge in Stavanger and this work has benefited from discussions with
them and their support. We are grateful to C. Ebinger, O.P. Hansen, E.
Lundin, S. Matthews, an anonymous reviewer and the editor A. Maltman
for their constructive criticisms and helpful comments. We thank TGS-
NOPEC, the Norwegian Petroleum Directorate and Norsk Hydro for
permission to publish seismic sections and gravity data.
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Scientific editing by Alex Maltman
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