Geology, published online on 1 October 2015 as doi:10.1130/G37086.1
The ocean-continent transition in the mid-Norwegian margin: Insight
from seismic data and an onshore Caledonian field analogue
Mansour M. Abdelmalak1*, Torgeir B. Andersen1, Sverre Planke1,2, Jan Inge Faleide1, Fernando Corfu1, Christian
Tegner3, Grace E. Shephard1, Dmitrii Zastrozhnov1, and Reidun Myklebust4
Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Oslo N-0315, Norway
Volcanic Basin Petroleum Research (VBPR), Oslo Science Park, Oslo N-0349, Norway
3
Department of Geoscience, Aarhus University, Aarhus DK-8000, Denmark
4
TGS, Lensmannslia 4, 1386 Asker, Norway
1
2
ABSTRACT
Understanding the structure of the ocean-continent transition (OCT) in passive margins
is greatly enhanced by comparison with onshore analogues. The North Atlantic margins and
the “fossil” system in the Scandinavian Caledonides show variations along strike between
magma-rich and magma-poor margins, but are different in terms of exposure and degree
of maturity. They both display the early stages of the Wilson cycle. Seismic reflection data
from the mid-Norwegian margin combined with results from Ocean Drilling Program Leg
104 drill core 642E allow for improved subbasalt imaging of the OCT. Below the SeawardDipping Reflector (SDR) sequences, vertical and inclined reflections are interpreted as dike
feeder systems. High-amplitude reflections with abrupt termination and saucer-shaped geometries are interpreted as sill intrusions, implying the presence of sediments in the transition zone beneath the volcanic sequences. The transitional crust located below the SDR of
the mid-Norwegian margin has a well-exposed analogue in the Seve Nappe Complex (SNC).
At Sarek (Sweden), hornfelsed sediments are truncated by mafic dike swarms with densities of 70%–80% or more. The magmatic domain extends for at least 800 km along the
Caledonides, and probably reached the size of a large igneous province. It developed at ca.
600 Ma on the margin of the Iapetus Ocean, and was probably linked to the magma-poor
hyperextended segment in the southern Scandinavian Caledonides. These parts of the SNC
represent an onshore analogue to the deeper level of the mid-Norwegian margin, permitting
direct observation and sampling and providing an improved understanding, particularly of
the deeper levels, of present-day magma-rich margins.
INTRODUCTION
Exposed ocean-continent transitions (OCTs)
have contributed significantly to understanding
hyperextended margin development (e.g. Sawyer et al., 2007). Several studies have addressed
the magma-poor margin analogues (e.g.,
Manatschal, 2004), but less is known about the
magma-rich margin analogues. These margins
are characterized by the presence of seawarddipping reflectors (SDRs), an intense network
of mafic sheet intrusions in the continental crust
and adjacent sedimentary basins, and a highvelocity (Vp > 7.0 km/s) lower crustal body
(e.g., Geoffroy, 2005). Most of the present-day
magma-rich margins are submerged offshore
and are therefore difficult to study by direct observation. Furthermore, the thick accumulation
of extrusive and intrusive rocks presents a major
challenge for seismic imaging of deeper levels.
These issues have led to uncertainties in the
interpretations of margin evolutions and their
structure, in particular details of the transitional
crust located beneath the SDRs. In such situations, better seismic resolution combined with
studies of field analogues can improve our understanding of the OCT in magma-rich margins.
*E-mail: m.m.abdelmalak@geo.uio.no;
Abdelmalak​_mansour@yahoo.fr
In this paper we use new and reprocessed
seismic data and Ocean Drilling Program (ODP)
Leg 104 drill core 642E information from the
mid-Norwegian margin to establish better constraints on the nature of the OCT (Fig. 1). These
observations are compared to the field analogue
in the Seve Nappe Complex (SNC) of the Scandinavian Caledonides and with the example of
the East Greenland margin. The field analogues
make it possible to directly study and sample
rocks as well as observe and interpret structural
geometries, which may be similar to those at
depth in present-day passive margins.
REGIONAL SETTINGS
In the mid-Norwegian margin, continental
breakup marks the culmination of an ~350 m.y.
period of predominantly extensional deformation following the Caledonian orogeny (Doré
et al., 1999; Faleide et al., 2008). Through the
late Paleozoic and Mesozoic, lithospheric thinning resulted in large sedimentary sag basins
controlled by regional detachment faults. Final
continental breakup occurred at the PaleoceneEocene transition (ca. 56 Ma), after a 3–6 m.y.
period of intense extrusive and intrusive magmatism (Eldholm and Grue, 1994) in the adjacent
sedimentary basins and preexisting continental
crust (Gernigon et al., 2004; Planke et al., 2005).
SEISMIC INTERPRETATION
New multichannel seismic data allow for better imaging and interpretations of the breakuprelated igneous rocks on the mid-Norwegian
margin. The volcanic succession displays a
variety of seismic facies indicative of the style
of volcanic emplacement, depositional environment, and subsequent mass transport (Planke
et al., 2000; Berndt et al., 2001). Several volcanic seismic facies units have been identified:
(1) Landward Flows, (2) Lava Delta, (3) Inner
Flows, (4) Inner SDRs, (5) Outer High, and
(6) Outer SDR (Figs. 1 and 2). Such volcanic
facies successions are considered to be typical
of magma-rich margins, and record the evolution of the breakup extrusive complex close to
the first magnetic seafloor spreading anomalies.
Undifferentiated lava flows located between the
inner SDRs and the normal oceanic crust are
also mapped (Figs. 1 and 2).
In the Vøring margin, improvements in subbasalt imaging combined with petrological and
geochemical observations from the ODP (Hole
642E) allow the definition of a new seismic facies unit called the Lower Series Flows, characterized by wavy to continuous subparallel
reflections with an internal disrupted and hummocky shape (Fig. 2; our unpublished data).
This facies unit records the transition from
a sediment-dominated nonvolcanic rift to a
magma-rich margin. This facies unit consists
mainly of evolved pepperitic basaltic andesitic and dacitic flows and thick volcaniclastic
deposits. The geochemical analysis combined
with C, Pb, Sr, and Nd isotope compositions of
drill-core samples indicate interaction of midoceanic ridge basalt (MORB)–type melts with
partial melts of highly radiogenic pelagic sediments rich in organic carbon (Meyer et al., 2009;
our unpublished data). Different high-amplitude
reflections with abrupt termination and saucershaped geometries are identified and interpreted
as sill intrusions. Saucer-shaped sills imply the
presence of sediments in the transitional zone
beneath the volcanics. Offshore mid-Norway
(Møre and Vøring margins), the sill intrusions
cover an area of >130,000 km2. An ~30-kmwide OCT zone separates the crystalline basement and the oceanic layers 3A and 3B, located
to the west (Fig. 2). The area between the lower
crustal body and the SDR wedge is slightly
GEOLOGY, November 2015; v. 43; no. 11; p. 1011–1014 | Data Repository item 2015339 | doi:10.1130/G37086.1 | Published online XX Month 2015
©
2015 Geological
Society
America.
permission to copy, contact editing@geosociety.org.
GEOLOGY 43 | ofNumber
11 For
| Volume
| www.gsapubs.org
1011
Geology, published online on 1 October 2015 as doi:10.1130/G37086.1
Continent-Ocean Boundary
18
Magnetic Lineations
Fracture Zone
-5000°Bathymetry
5°E
10°E
15°E
20°E
Lofoten
Basin
5°E
Devonian molasse
Laurentian nappes
Kebnekaise
Outer High
Inner SDR
JN
Bergen
Inner Flows
Escarpment
ODP Hole
100 km 5°E
Oslo
C23 (ca. 52 Ma)
nd
nla
e
e
Gr
°N
dike
stal
Coa arms
w
s
. 3C
Fig
60°N
rd
lba
a
Sv
late
60
ed
nd in
te as
ex e b
r
pe ng
Hy ela
m
Landward Flows
Baltica Precambrian
basement
Vøring
Møre
Faroe
Islands
W
Outer SDR
GNC
M
hy ag
pe ma
se rex -po
gm te or
en nde
t
d
Lava Flows
Trondheim
Baltica platform and
rifted margin
np
Møre
Basin
65°N
10°E
asia
-500
Trø
n
pla dela
tfo g
rm
extent of
sill intrusions
Hyperextended melange
basin
Ba
ffin
Ma
trans gma-ric
h
ition
al cr margin SNC
ust s
egm
ent
Vøring
Basin
Micro continents of Baltic affinity
Pårte
0°
00
-10
Suspect terranes KNC and SNC
Eur
as
nB
stf
jor
de
Ri
ODP
Hole
642E
Sarek
Fig. 3 A and B
Ve
Z
JMF
Fig.2
Early Paleozoic ophiolite/island
arcs / suspect terrane (GNC)
in
bb
an
Ba
sin
Dikes/ or dike
swarms extent
Offshore geology
Onshore geology
Oslo Rift
(Permian-Carboniferous)
Ba
y
0°
KNC
Tromsø
17
30°
-30
00
7
13
Onshore volcanic rocks
Offshore basalt flows
SDR
COB
Figure 1. Onshore and offshore geological map and regional reconstruction ca. 52 Ma (inset;
based on Gaina et al., 2009). Offshore: distribution of the volcanic seismic facies units in the
mid-Norwegian margin. The extents of the dike swarms and sills are indicated. Onshore: simplified geological map of the different nappe complex (NC) defining the Scandinavian Caledonides.
Two distinctive segments are identified for the pre-Caledonian margin of Baltica: the southern
part is interpreted as hyperextended magma-poor segment (Andersen et al., 2012) and the central part is interpreted as transitional crust of magma-rich margin. JMFZ—Jan Mayan Fracture
Zone; GNC—Gula Nappe complex; JN—Jotun Nappe; KNC—Kalak Nappe Complex; SNC—Seve
Nappe Complex; SDR—Seaward-Dipping Reflector; ODP—Ocean Drilling Program; COB—continent-ocean boundary.
OBS 1-96
ODP Hole 642E
W
3
4
5
6
Top Basalt
Lava
Flows
Oceanic
layer 3A
7.8
10 km
Sills
Sediments ?
6
Dike swarm
and faults
Ocean-Continent
Transition zone
Oceanic
8 layer 3B
(S)
Lower Series Flows
SDR
7
9
E
Landward Flows
Neogene sediments
sediments 3.9
Paleogene
1.8
OBS 11-03
Moho
Crystalline
basement
7
LCB
8
Figure 2. Seismic example of the ocean-continent transition in the Vøring margin (profile location in Fig. 1). The Seaward-Dipping Reflector (SDR) wedge is characterized by a divergent
arcuate reflection pattern with increasing dip in the deeper part. The seismic velocity structure is determined using the Ocean Bottom Seismometer (OBS) profiles crossing the line,
OBS 11-03 (Breivik et al., 2014) and OBS 1-96 (Mjelde et al., 2003). The profile is tied to Ocean
Drilling Program (ODP) Hole 642E. The top of the crystalline basement shows velocity, Vp >
6.0 km/s. The Lower Crustal Body (LCB) near the base of the crust shows a high velocity (Vp
> 7.0 km/s). The Moho is associated with a mantle velocity Vp > 8.0 km/s toward the continental crust and a Vp > 7.8 km/s toward the oceanic crust. (See the GSA Data Repository1 for
the uninterpreted seismic profile.)
flexured and is characterized by discontinuous
reflections of variable amplitude (Fig. 2). Additional nearly vertical and inclined reflections are
identified in the reprocessed seismic lines and
interpreted as dikes or dike swarms. The extent
of the dike reflection along the magma-rich margin correlates with the extent of the SDR and
locally with the Landward Flows (Fig. 1). This
area is considered to represent an upper crustal
level of the OCT situated below the SDR.
FIELD ANALOGUES IN THE
SCANDINAVIAN CALEDONIDES
The early Paleozoic Scandinavian Caledonides comprise a stack of nappes formed during the Silurian–Devonian closure of the Iapetus
Ocean and collision of the paleocontinents of
Baltica and Laurentia (e.g., Corfu et al., 2014).
The deeply denudated mountain belt includes
nappes derived from the collided continents as
well as oceanic and suspect terranes. The SNC
is a composite unit of supracrustal, plutonic
rocks and older gneisses with large local variations in metamorphic grade. The ~800-km-long
SNC constituted an OCT zone of a magma-rich
margin segment (e.g., Svenningsen, 2001). In
the Kebnekaise-Sarek-Pårte region (Sweden),
immediately east of the mid-Norwegian margin
(Fig. 1), the SNC comprises some large areas
of mostly contact metamorphosed sedimentary
and intrusive rocks. Structures reflecting extensional processes are preserved in areas large
enough (10 km scale) to provide detailed outcrop-scale and regional information (Svenningsen, 1994; Andréasson et al., 1998). Precise age
determinations point to a voluminous, but relatively short-lived, magmatic event at 610–595
Ma (Svenningsen, 2001; Root and Corfu, 2012;
Baird et al., 2014) inferred to be contemporaneous with the alleged rifting and continental
breakup at the onset of the Caledonian Wilson
cycle (e.g., Cocks and Torsvik, 2005).
An important characteristic of the SNC is
the abundance of composite basaltic dike complexes (e.g., Svenningsen, 2001) truncating
continental basement and cover units (Figs. 3A
and 3B). Extrusive basalts are present in the
structurally higher parts of the nappe (Kullerud
et al., 1990). The host rocks for the dikes are
mainly hornfelsed sediments with a preserved
stratigraphic thickness of as much as ~5 km
(Svenningsen, 1994) and an unknown amount
of older continental gneisses (Paulsson and
Andréasson, 2002). Both basement and cover
rocks are intensely intruded by sheeted dikes,
commonly 60%–80%, but locally up to 100%
are dikes (Fig. 3B). The dikes have transitional
to enriched MORB compositions (e.g., André1
GSA Data Repository item 2015339, the uninterpreted seismic profile, is available online at www​
.geosociety​.org​/pubs/ft2015.htm, or on request from
editing@geosociety.org or Documents Secretary,
GSA, P.O. Box 9140, Boulder, CO 80301, USA.
1012www.gsapubs.org | Volume 43 | Number 11 | GEOLOGY
Geology, published online on 1 October 2015 as doi:10.1130/G37086.1
Different crosscutting
dike generations
1
2
dikes
2
B
2
Different crosscutting
dike generations
dike
200 m
sediments
Toward ocean
1
1
A
sediments
2
dike
50 m
C
200 m
Precambrian basement
Figure 3. A: Different crosscutting dike generations. The sediments show vertical layering
(dashed white lines). The variable angles between dikes and bedding indicate different intrusion stages during tilting of the margin. The lozenge-shaped wall-rock fragments developed
their shape during successive generations of dilational dikes (1 and 2). B: Outcrop of Favoritkammen sedimentary group highly intruded by mafic dike swarm with a dike density to
70%–80%. C: East Greenland coastal dike swarms.
asson et al., 1998; Baird et al., 2014). Syndepositional extensional faults were apparently
contemporaneous with the emplacement of the
earliest mafic dikes (Svenningsen, 1994).
The spectacularly exposed cliff walls in
Sarek and Pårte are as high as 300 m and demonstrate the crosscutting relationships and progressive tilting of older dikes (Figs. 3A and 3B).
It is evident that the multiple and approximately
parallel dilational dikes were emplaced in rapid
succession; they are most commonly several
meters wide, in some cases several tens of meters wide. Composite dikes without intervening
screens of sediments are common (Fig. 3A)
and the accumulated crustal extension caused
by the dikes was locally very large. Early dikes
and their host rocks subsequently underwent
rotation of as much as 60° before renewed dilational dike intrusions (Figs. 3A and 3B). The
stretching during each intrusive stage was apparently accommodated by mode-1 type extension, whereas rotational extensional faulting
may have occurred between separate dike generations. The present orientations and steep dips
of the sedimentary layers are, however, entirely
controlled by the Caledonian deformation (e.g.,
Svenningsen, 1994).
The along-strike continuation of the magmarich SNC into southern Norway is represented
by distal margin rocks consisting primarily of
phyllites and mica schists interlayered with
coarser grained metasediments, highly attenuated slices of Proterozoic basement, and, most
characteristically, a large number of meter- to
kilometer-scale solitary mantle metaperidotite
bodies. These are generally highly serpentinized
and are associated with ophicarbonate breccias
and soapstone, as well as variably hydrated
and carbonated conglomerates and sandstones
formed by erosion and sedimentation of exposed mantle. Collectively these rocks consti-
tute a mélange interpreted to have formed in
magma-poor hyperextended basins filled mostly
by relatively fine grained postrift sediments
(Andersen et al., 2012).
DISCUSSION AND CONCLUSIONS
Despite intense regional deformation and
metamorphism during successive orogenic
events, the Scandinavian Caledonides provide
a remarkably well preserved and rich geological record of the continental rifting, breakup,
and development of a magmatic OCT zone.
The parts of the SNC discussed here were locally little affected by internal deformation and
metamorphism in the Ordovician, Silurian, and
Devonian events and primary relationships are
locally remarkably well preserved (Fig. 3A).
The structure and dike compositions of the
SNC resemble those of the East Greenland dike
swarm, emplaced during the early Cenozoic
opening of the North Atlantic. Crustal uplift and
deep glacial erosion have provided excellent
exposures of the ~350 km of coast-parallel dike
swarm intruding the Precambrian granulite to
amphibolite gneisses and representing the feeder
systems for the Cenozoic basaltic lava and the
SDR sequences (Fig. 3C; Klausen and Larsen,
2002). The internal structure of the coastal dike
swarm, within the coastal flexure in which the
crust bends in a large monocline toward the ocean
in response to crustal thinning, appears to have
been constructed by multiple steps. In general,
more deformed and metamorphosed dikes are
cut by successive generations of less deformed
and steeper dikes (Karson and Brooks, 1999). In
the basement domains, faults accommodate most
of the extension associated with the early rifting
stages, whereas dikes account for most of the
extension during the volcanically dominated rifting associated with continental breakup. Oceanward, the crust records an increasing intensity of
GEOLOGY | Volume 43 | Number 11 | www.gsapubs.org
dikes, locally reaching a dilation of >60%. The
geometry, crosscutting relations, and variations
in deformation and metamorphism of the dikes
suggest that they were intruded before, during,
and after the development of the coastal flexure.
Therefore, the dikes record a protracted history
of progressive intrusion and rotation (Fig. 3C).
In the final stage of continental breakup, normal
faults and magmatic accretion operated together
during SDR growth and coastal flexure development (e.g., Quirk et al., 2014).
In the same way, but even more pronounced,
the SNC has different generations of dikes that
can be distinguished by crossing and rotational
relations. Small-scale brittle extensional faulting was apparently most active during the early
stages of dike emplacement (Svenningsen,
2001). Angular differences of as much as 60° in
the dip of early and later dike generations in the
same outcrops (see Fig. 3) cannot easily be explained by a monoclinal large-scale regional flexure, similar to East Greenland. The wavelength of
such a monocline could not account for an in situ
rotation of 60°. It is more likely that large-scale
normal faults remained active throughout the intrusion history, but the identification of individual
large faults is now obscured by the dense dike
network, and, perhaps most important, they may
remain unrecognized due to lack of detailed mapping in the inaccessible mountain terrain.
On the mid-Norwegian margin, the dike
swarms identified in the seismic profiles represent the feeder systems for the SDR sequences.
The SDR growth was accommodated by extensional faulting during magma intrusion,
although such active fault systems are not systematically observed. The faults are used as
magma conduits, hampering their identification on seismic profiles. The flexed continental
crust beneath the SDR is considerably dilated
by margin-parallel dikes (Fig. 4). Early feeder
dikes were initially emplaced subvertically and
were gradually tilted oceanward, contemporaneously with the growth of the SDR. Dikes that
were emplaced during the formation of this flexure (feeding the SDR) crosscut the older dikes
and lavas and display variable dips (Fig. 4). The
progressive tilting and subsidence of the margin accommodate the growth of the SDR, but
the angular relationships between different dike
generations are never observed to reach the high
angles observed in the SNC analogue (~60°),
suggesting that local rotational faults may be
more common than those observed in the subSDR seismic lines.
Several sill intrusions were identified below
the SDR (Fig. 2), implicitly indicating the presence of a sedimentary basin. Conversely, the
ODP Hole 642E results showing a peraluminous composition of the Lower Series Flows,
combined with radiogenic Pb isotope trends and
coherent Sr and Nd isotope variations, point to
a significant contamination of MORB-like melts
1013
Geology, published online on 1 October 2015 as doi:10.1130/G37086.1
ODP Hole 642E
Subsidence
LF
lows
F
s
e
ri
e
S
er
SDR
Low
ents
im
Sed
Field analogue
Fault
Oceanic
layer 3A
Sill
intrusions
Different crosscutting
dike generations
Transitional
crust
Figure 4. Simplified schematic illustration (not to scale) of different feeder
dike generations emplaced during the seaward-dipping reflector
(SDR) growth. The first dike generation (light gray) is tilted and crosscut
by a newer dike generation (dark gray) showing a vertical to subvertical dip.
Part of the transitional crust below the SDR is composed of highly intruded
sedimentary basin. The black box shows the position of the field analogue. LF—
Landward Flows; ODP—Ocean Drilling Program.
with pelagic sedimentary rocks rich in organic
carbon (our unpublished data). This indicates that
part of the transitional crust below the SDR in the
mid-Norwegian margin is composed of a highly
intruded sedimentary basin, similar to what we
observe in the SNC. In the SNC the dikes also
penetrated an older crystalline basement.
ACKNOWLEDGMENTS
Funding for this work came from the OMNIS Project (Offshore Mid-Norway: Integrated Margin and
Basin Studies, project 210429/E30) and Centre of
Excellence grant 223272 to the Centre for Earth Evolution and Dynamics, both funded by the Norwegian
Research Council. We thank Tony Doré, two anonymous reviewers, and the editor for useful comments
that improved the paper.
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Manuscript received 19 June 2015
Revised manuscript received 11 September 2015
Manuscript accepted 16 September 2015
Printed in USA
1014www.gsapubs.org | Volume 43 | Number 11 | GEOLOGY
Geology, published online on 1 October 2015 as doi:10.1130/G37086.1
Geology
The ocean-continent transition in the mid-Norwegian margin: Insight from
seismic data and an onshore Caledonian field analogue
Mansour M. Abdelmalak, Torgeir B. Andersen, Sverre Planke, Jan Inge Faleide, Fernando Corfu,
Christian Tegner, Grace E. Shephard, Dmitrii Zastrozhnov and Reidun Myklebust
Geology published online 1 October 2015;
doi: 10.1130/G37086.1
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