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 Seaward-Dipping 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.
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 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 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.
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.
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 Paleocene-Eocene 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).
New multichannel seismic data allow for better 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 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 mid-oceanic 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 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 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 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., Andréasson 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 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 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 constitute 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 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 sub-SDR 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 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.
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.