GEOLOGY | Volume 43 | Number 11 | www.gsapubs.org 1011

The ocean-continent transition in the mid-Norwegian margin: Insight from seismic data and an onshore Caledonian field analogueMansour M. Abdelmalak1*, Torgeir B. Andersen1, Sverre Planke1,2, Jan Inge Faleide1, Fernando Corfu1, Christian Tegner3, Grace E. Shephard1, Dmitrii Zastrozhnov1, and Reidun Myklebust4

1Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Oslo N-0315, Norway2Volcanic Basin Petroleum Research (VBPR), Oslo Science Park, Oslo N-0349, Norway3Department of Geoscience, Aarhus University, Aarhus DK-8000, Denmark4TGS, Lensmannslia 4, 1386 Asker, Norway

ABSTRACTUnderstanding 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 Seaward-Dipping Reflector (SDR) sequences, vertical and inclined reflections are interpreted as dike feeder systems. High-amplitude reflections with abrupt termination and saucer-shaped ge-ometries are interpreted as sill intrusions, implying the presence of sediments in the transi-tion 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 densi-ties 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.

INTRODUCTIONExposed ocean-continent transitions (OCTs)

have contributed significantly to understanding hyperextended margin development (e.g. Saw-yer 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 seaward-dipping reflectors (SDRs), an intense network of mafic sheet intrusions in the continental crust and adjacent sedimentary basins, and a high-velocity (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 ob-servation. 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 situa-tions, better seismic resolution combined with studies of field analogues can improve our un-derstanding of the OCT in magma-rich margins.

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 con-straints on the nature of the OCT (Fig. 1). These observations are compared to the field analogue in the Seve Nappe Complex (SNC) of the Scan-dinavian 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 SETTINGSIn the mid-Norwegian margin, continental

breakup marks the culmination of an ~350 m.y. period of predominantly extensional deforma-tion following the Caledonian orogeny (Doré et al., 1999; Faleide et al., 2008). Through the late Paleozoic and Mesozoic, lithospheric thin-ning resulted in large sedimentary sag basins controlled by regional detachment faults. Final continental breakup occurred at the Paleocene-Eocene transition (ca. 56 Ma), after a 3–6 m.y. period of intense extrusive and intrusive magma-tism (Eldholm and Grue, 1994) in the adjacent sedimentary basins and preexisting continental crust (Gernigon et al., 2004; Planke et al., 2005).

SEISMIC INTERPRETATIONNew multichannel seismic data allow for bet-

ter imaging and interpretations of the breakup-related igneous rocks on the mid-Norwegian margin. The volcanic succession displays a variety of seismic facies indicative of the style of volcanic emplacement, depositional environ-ment, and subsequent mass transport (Planke et al., 2000; Berndt et al., 2001). Several vol-canic 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 evolu-tion 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 sub-basalt imaging combined with petrological and geochemical observations from the ODP (Hole 642E) allow the definition of a new seismic fa-cies unit called the Lower Series Flows, char-acterized by wavy to continuous subparallel reflections with an internal disrupted and hum-mocky 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 andes-itic 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 mid-oceanic ridge basalt (MORB)–type melts with partial melts of highly radiogenic pelagic sedi-ments rich in organic carbon (Meyer et al., 2009; our unpublished data). Different high-amplitude reflections with abrupt termination and saucer-shaped 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-km-wide OCT zone separates the crystalline base-ment 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

*E-mail : m.m.abdelmalak@geo.uio.no; Abdelmalak _mansour@yahoo.fr

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flexured and is characterized by discontinuous reflections of variable amplitude (Fig. 2). Addi-tional 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 mar-gin 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 Cale-donides comprise a stack of nappes formed dur-ing 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 varia-tions 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 ex-tensional processes are preserved in areas large enough (10 km scale) to provide detailed out-crop-scale and regional information (Svenning-sen, 1994; Andréasson et al., 1998). Precise age determinations point to a voluminous, but rela-tively short-lived, magmatic event at 610–595 Ma (Svenningsen, 2001; Root and Corfu, 2012; Baird et al., 2014) inferred to be contempora-neous 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 com-plexes (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., André-

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1GSA Data Repository item 2015339, the unin-terpreted 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.

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: simpli-fied 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 cen-tral 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—con-tinent-ocean boundary.

Lower Series FlowsNeogene sediments

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Figure 2. Seismic example of the ocean-continent transition in the Vøring margin (profile lo-cation 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 struc-ture 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 conti-nental crust and a Vp > 7.8 km/s toward the oceanic crust. (See the GSA Data Repository1 for the uninterpreted seismic profile.)

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asson et al., 1998; Baird et al., 2014). Synde-positional 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 dem-onstrate the crosscutting relationships and pro-gressive 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 me-ters 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 di-lational dike intrusions (Figs. 3A and 3B). The stretching during each intrusive stage was ap-parently accommodated by mode-1 type exten-sion, whereas rotational extensional faulting may have occurred between separate dike gen-erations. 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 magma-rich SNC into southern Norway is represented by distal margin rocks consisting primarily of phyllites and mica schists interlayered with coarser grained metasediments, highly attenu-ated 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 ex-posed 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 CONCLUSIONSDespite intense regional deformation and

metamorphism during successive orogenic events, the Scandinavian Caledonides provide a remarkably well preserved and rich geologi-cal record of the continental rifting, breakup, and development of a magmatic OCT zone. The parts of the SNC discussed here were lo-cally 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 rift-ing associated with continental breakup. Ocean-ward, the crust records an increasing intensity of

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 develop-ment (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 fault-ing 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 ex-plained by a monoclinal large-scale regional flex-ure, 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 in-trusion 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 map-ping in the inaccessible mountain terrain.

On the mid-Norwegian margin, the dike swarms identified in the seismic profiles repre-sent the feeder systems for the SDR sequences. The SDR growth was accommodated by ex-tensional faulting during magma intrusion, although such active fault systems are not sys-tematically observed. The faults are used as magma conduits, hampering their identifica-tion 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, contempora-neously with the growth of the SDR. Dikes that were emplaced during the formation of this flex-ure (feeding the SDR) crosscut the older dikes and lavas and display variable dips (Fig. 4). The progressive tilting and subsidence of the mar-gin 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 sub-SDR seismic lines.

Several sill intrusions were identified below the SDR (Fig. 2), implicitly indicating the pres-ence of a sedimentary basin. Conversely, the ODP Hole 642E results showing a peralumi-nous 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

A

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Figure 3. A: Different crosscutting dike generations. The sediments show vertical layering (dashed white lines). The variable angles between dikes and bedding indicate different intru-sion 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 Fa-voritkammen sedimentary group highly intruded by mafic dike swarm with a dike density to 70%–80%. C: East Greenland coastal dike swarms.

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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.

ACKNOWLEDGMENTSFunding for this work came from the OMNIS Proj-ect (Offshore Mid-Norway: Integrated Margin and Basin Studies, project 210429/E30) and Centre of Excellence grant 223272 to the Centre for Earth Evo-lution and Dynamics, both funded by the Norwegian Research Council. We thank Tony Doré, two anony-mous 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

LF

Fault

SDRSubsidence

SillintrusionsSediments

Different crosscuttingdike generations

ODP Hole 642E

Transitional crust

Oceanic layer 3A

Field analogue

Lower Series Flows

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.

as doi:10.1130/G37086.1Geology, published online on 1 October 2015

Geology

doi: 10.1130/G37086.1 published online 1 October 2015;Geology

 Christian Tegner, Grace E. Shephard, Dmitrii Zastrozhnov and Reidun MyklebustMansour M. Abdelmalak, Torgeir B. Andersen, Sverre Planke, Jan Inge Faleide, Fernando Corfu, seismic data and an onshore Caledonian field analogueThe ocean-continent transition in the mid-Norwegian margin: Insight from  

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as doi:10.1130/G37086.1Geology, published online on 1 October 2015