Lofoten Margin Research Articles

CAGE20-0 Cruise Report: From Seep to Source – Lofoten Canyons and PL998 license

The cruise to the Atlantic-Arctic gateway region is organized and funded through the Geological Survey of Norway (NGU) and focuses on studies of methane leakage at the seafloor on the Mid Norwegian shelf and continental margin. Cruise 20200108 in June 2020 was directed to two areas to study ongoing seepage of hydrocarbons and their controlling factors. Along the Lofoten margin, to investigate the outcropping bedrock structures in two canyons; on the Mid-Norwegian shelf, to inspect a recently discovered seep site. The cruise may be known as: CAGE20_0

New insights into the Cretaceous-Cenozoic tectono-stratigraphic evolution of the southern Lofoten margin, offshore Norway

Abstract The tectono-stratigraphic evolution of the southern Lofoten margin and its immediate transition towards the northern Voring margin has been studied in detail utilizing 2D multi-channel seismic reflection profiles, available wells, in addition to gravity and magnetic data. New and better refined structural elements have been mapped within the study area, including (informally named) the West Rost High Fault Complex, Rost Syncline, and Sandflesa High. Furthermore, four main rift phases have been recognised and refined. Late Jurassic-earliest Cretaceous rifting controlled the initial structuring of the main structural elements. Mid-Cretaceous rifting was responsible for initiation of faulting in the West Rost High Fault Complex, while a composite Late Cretaceous rift phase took place and led to westward propagation of fault activity. Paleocene rifting generated new faults and reactivated several earlier faults, prior to continental breakup and seafloor spreading initiation at the Paleocene-Eocene transition. The Bivrost Lineament, separating the southern Lofoten and northern Voring margins, has exhibited a distinct morphological expression during Cretaceous-Cenozoic and is recognised as a structural “corridor” which segments highs and basins/sub-basins. Furthermore, two dome-shaped features have been observed on the southern Lofoten margin and have probably experienced several phases of growth from Late Cretaceous to Miocene times. The domes are located in close proximity to the outer Voring margin and Bivrost Lineament that are believed to have facilitated the transfer of imposed compressional deformation on the NE Atlantic margins. Finally, a comparison of the study area to the conjugate Northeast Greenland margin provides valuable insights on the margin evolution in a regional and conjugate setting.

How do pre‐existing normal faults influence rift geometry? A comparison of adjacent basins with contrasting underlying structure on the Lofoten Margin, Norway

AbstractRecent studies of natural, multiphase rifts suggest that the presence of pre‐existing faults may strongly influence fault growth during later rift phases. These findings compare well with predictions from recent scaled analogue experiments that simulate multiphase, non‐coaxial extension. However, in natural rifts we only get to see the final result of multiphase rifting. We therefore do not get the chance to compare the effects of the same rift phase with and without pre‐existing structural heterogeneity, as we may in the controlled environment of a laboratory experiment. Here, we present a case study from the Lofoten Margin that provides a unique opportunity to compare normal fault growth with and without pre‐existing structural heterogeneity. Using seismic reflection and wellbore data, we demonstrate that the Ribban Basin formed during Late Jurassic to Early Cretaceous rifting. We also show that the rift fault network of the Ribban Basin lacks a pre‐existing (Permian‐Triassic) structural grain that underlies the neighbouring North Træna Basin that also formed during the Late Jurassic to Early Cretaceous. Being able to compare adjacent basins with similar histories but contrasting underlying structure allows us to study how pre‐existing normal faults influence rift geometry. We demonstrate that in Lofoten, the absence of pre‐existing normal faults produced collinear fault zones. Conversely, where pre‐existing faults are present, normal fault zones develop strong “zigzag” plan‐view geometries.

A new tectono-magmatic model for the Lofoten/Vesterålen Margin at the outer limit of the Iceland Plume influence

The Early Eocene continental breakup was magma-rich and formed part of the North Atlantic Igneous Province. Extrusive and intrusive magmatism was abundant on the continental side, and a thick oceanic crust was produced up to a few m.y. after breakup. However, the extensive magmatism at the Vøring Plateau off mid-Norway died down rapidly northeastwards towards the Lofoten/Vesterålen Margin. In 2003 an Ocean Bottom Seismometer profile was collected from mainland Norway, across Lofoten, and into the deep ocean. Forward/inverse velocity modeling by raytracing reveals a continental margin transitional between magma-rich and magma-poor rifting. For the first time a distinct lower-crustal body typical for volcanic margins has been identified at this outer margin segment, up to 3.5km thick and ∼50km wide. On the other hand, expected extrusive magmatism could not be clearly identified here. Strong reflections earlier interpreted as the top of extensive lavas may at least partly represent high-velocity sediments derived from the shelf, and/or fault surfaces. Early post-breakup oceanic crust is moderately thickened (∼8km), but is reduced to 6km after 1m.y. The adjacent continental crystalline crust is extended down to a minimum of 4.5km thickness. Early plate spreading rates derived from the Norway Basin and the northern Vøring Plateau were used to calculate synthetic magnetic seafloor anomalies, and compared to our ship magnetic profile. It appears that continental breakup took place at ∼53.1Ma, ∼1m.y. later than on the Vøring Plateau, consistent with late strong crustal extension. The low interaction between extension and magmatism indicates that mantle plume material was not present at the Lofoten Margin during initial rifting, and that the observed excess magmatism was created by late lateral transport from a nearby pool of plume material into the lithospheric rift zone at breakup time.

A review of the NE Atlantic conjugate margins based on seismic refraction data

Abstract The NE Atlantic region evolved through several rift episodes, leading to break-up in the Eocene that was associated with voluminous magmatism along the conjugate margins of East Greenland and NW Europe. Existing seismic refraction data provide good constraints on the overall tectonic development of the margins, despite data gaps at the NE Greenland shear margin and the southern Jan Mayen microcontinent. The maximum thickness of the initial oceanic crust is 40 km at the Greenland–Iceland–Faroe Ridge, but decreases with increasing distance to the Iceland plume. High-velocity lower crust interpreted as magmatic underplating or sill intrusions is observed along most margins but disappears north of the East Greenland Ridge and the Lofoten margin, with the exception of the Vestbakken Volcanic Province at the SW Barents Sea margin. South of the narrow Lofoten margin, the European side is characterized by wide margins. The opposite trend is seen in Greenland, with a wide margin in the NE and narrow margins elsewhere. The thin crust beneath the basins is generally underlain by rocks with velocities of >7 km s −1 interpreted as serpentinized mantle in the Porcupine and southern Rockall basins; while off Norway, alternative interpretations such as eclogite bodies and underplating are also discussed.

Depositional systems in multiphase rifts: seismic case study from the Lofoten margin, Norway

AbstractThe evolution of depositional systems in multiphase rifts is influenced by the selective reactivation of faults between subsequent rift phases. The Middle Jurassic to Palaeocene tectonic history of the Lofoten margin, a segment of the North Atlantic rift system, is characterised by three distinct rift phases separated by long (>20 Myr) inter‐rift periods. The initial rift phase comprised a distinct fault initiation and linkage stage, whereas the later rift phases were characterised by selective reactivation of previously linked through‐going faults which resulted in immediate rift climax. Using 2‐D and 3‐D seismic reflection data in conjunction with shallow core data we present a 100 Myr record of shallow to deep marine depositional environments that includes deltaic clinoform packages, slope aprons and turbidite fans. The rapid re‐establishment of major faults during the later rift phases impacts on drainage systems and sediment supply. Firstly, the immediate localisation of strain and accumulation of displacement on few faults results in pronounced footwall uplift and possible fault block rotation along those faults, which makes it more likely for any antecedent fault‐transverse depositional systems to become reversed. Secondly, any antecedent axially‐sourced depositional systems that are inherited from the foregoing rift phase(s) are likely to be sustained after reactivation because such axial systems have already been directed around fault tips. Hence, the immediate localisation of strain through selective reactivation in the later rift phases restricts fault‐transverse sediment supply more than axial sediment supply, which is likely to be a key aspect of the tectono‐sedimentary evolution of multiphase rifts.

Evolution of a major segmented normal fault during multiphase rifting: The origin of plan-view zigzag geometry

This case study addresses fault reactivation and linkage between distinct extensional episodes with variable stretching direction. Using 2-D and 3-D seismic reflection data we demonstrate how the Vesterdjupet Fault Zone, one of the basin-bounding normal fault zones of the Lofoten margin (north Norway), evolved over c. 150 Myr as part of the North Atlantic rift. This fault zone is composed of NNE-SSW- and NE-SW-striking segments that exhibit a zigzag geometry. The structure formed during Late Jurassic and Early Cretaceous rifting from selective reactivation and linkage of Triassic faults. A rotation of the overall stress field has previously been invoked to have taken place between the Triassic and Jurassic rift episodes along the Lofoten margin. A comparison to recent physical analogue models of non-coaxial extension reveals that this suggested change in least principal stress for the Lofoten margin may best explain the zigzag-style linkage of the Triassic faults, although alternative models cannot be ruled out. This study underlines the prediction from physical models that the location and orientation of early phase normal faults can play a pivotal role in the evolution of subsequent faults systems in multi-rift systems.

Nature of Palaeozoic extension in Lofoten, north Norwegian Continental Shelf: insights from 3‐D seismic analysis of a Cordilleran‐style metamorphic core complex

AbstractAnalyses of 3‐D seismic data reveal that pre‐Triassic basins are present underneath the Mesozoic North Træna Basin (Lofoten Margin, Norway). These are linked to a Cordilleran‐style metamorphic core complex that developed in Palaeozoic times, including rotated fault blocks with hanging wall ‘growth’ wedges, bounded by listric faults that sole into a sub‐horizontal detachment. On the basis of similarity in age, structural style and transport direction, we propose a kinematic link with a Permian mylonitic detachment documented onshore. This study presents the first offshore evidence for Palaeozoic detachment faulting, elucidating the mechanisms behind the long‐lived exhumation history of the Lofoten basement.

Magma productivity and early seafloor spreading rate correlation on the northern Vøring Margin, Norway — Constraints on mantle melting

Continental rifting at the Vøring Margin off mid-Norway was initiated during the earliest Eocene (~ 54 Ma), and large volumes of magmatic rocks were emplaced during and after continental breakup. In 2003, a marine survey collecting ocean bottom seismometer, single-channel reflection, and magnetic data was conducted on the Norwegian Margin to constrain continental breakup and early seafloor spreading processes. The profile described here crosses the northern part of the Vøring Plateau, and the crustal velocity model was constructed through a combination of ray-tracing and forward gravity modeling, the latter corrected for the thermal effects remaining from the seafloor spreading. We found a maximum igneous crustal thickness of 18 km, decreasing to 6.5 km over the first ~ 6 M.y. after continental breakup. Both the volume and the duration of excess magmatism are about twice as large as that of the Møre Margin south of the East Jan Mayen Fracture Zone, which offsets the two margin segments by ~ 170 km. A similar reduction in magmatism occurs to the north over an along-margin distance of ~ 150 km to the Lofoten Margin, but without a margin offset. Both the geochemical data and the mean P-wave velocity indicate that there is active mantle upwelling combined with a moderate temperature increase during the earliest mantle melting at the Vøring Margin. The mean P-wave velocity versus crustal thickness also indicates that there is a transition from convection dominated to temperature dominated magma production ~ 2 M.y. after breakup. The magnetic data were used to derive plate half-spreading rates for the Northern Vøring Margin, which are very similar to that obtained at the Møre Margin. There is a strong correlation between magma productivity and early plate spreading rate, suggesting a common cause. A model for the breakup-related magmatism should be able to explain this correlation, but also the magma production peak at breakup, the along-margin magmatic segmentation, and the active mantle upwelling. Proposed end-member hypotheses comprise elevated upper-mantle temperatures caused by a hot mantle plume, or edge-driven small-scale convection fluxing mantle rocks through the melt zone. Edge-driven convection does not easily explain these observations, but a mantle plume model in which buoyant plume material flows laterally to pond in the rift-topography at the base of the lithosphere close to breakup time is promising: When the continents break apart, the hot and buoyant plume-material can flow up into the rift zone from surrounding areas as the rift transits to drift, and the excess temperature of this material will then cause excess magmatism which dies off as the rift-restricted material is spent. The buoyancy of the plume-material may in addition cause active upwelling which can increase the melting furthermore, and also increase the force on the plate boundaries to enhance plate spreading rate. This conceptual model explains how both excess magmatism and spreading rate will be reduced similarly with time as the plume material is consumed by plate spreading, and thus correlate.

Along-strike variations in rifted margin crustal architecture and lithosphere thinning between northern Vøring and Lofoten margin segments off mid-Norway

An extensive geophysical and geological database is used to integrate observations and structural and stratigraphic modelling along the northern Vøring and Lofoten margins, off mid-Norway. Subsidence and fault analysis demonstrates depth-dependent lithosphere stretching along both margin segments, and the modelling concurs with an emergent or relatively shallow-water emplacement environment for the breakup lavas along the northern Vøring margin. However off Lofoten, subsidence modelling predicts ~ 1500 m of bathymetric relief during breakup in contradiction to the view of subaerial volcanic emplacement environment off mid-Norway. A distinct Early Eocene bathymetric gradient from Vøring to Lofoten margin is suggested exhibiting depths in the order of 500–1000 m on the latter margin, and thus an environment for submarine lava emplacement. The water depth may even have reached ~ 1250 m following breakup due to an initial, rapid margin subsidence of the Lofoten margin along a deep-rooted low-angle detachment fault. The differences between northern Vøring and Lofoten margin segments in pre- and post-breakup evolution are attributed to the Bivrost Lineament, a transfer zone separating the two margin segments, which appears as a major across-margin boundary in terms of margin physiography and paleogeography, differential sediment accommodation space, crustal- and lithosphere-scale structure, and breakup magmatism. Furthermore, gravity inversion results appear to independently predict a significantly greater volcanic addition on the Vøring than on the Lofoten margin.

An improved tectonic model for the Eocene opening of the Norwegian–Greenland Sea: Use of modern magnetic data

Compilation of new and vintage aeromagnetric data from the Norwegian and Greenland Seas of the NE Atlantic provides evidence for a different interpretation of several tectonic elements. The previously interpreted oceanic fracture zones (Gleipne, Surt, Bivrost, Jenegga and Vesterålen) do not exist; these were artefacts of poor navigation and wide line spacing of the vintage dataset. This reinterpretation impacts our understanding of the early spreading history of the North Atlantic, as the opening of the Norwegian–Greenland Sea between the Jan Mayen and Senja-Greenland fracture zones occurred along a stable axis without offsets of the oceanic spreading anomalies or jumps in spreading axis. These results contradict the hypothesis that a spatial relationship exists between transfer zones and fracture zones on the Lofoten margin, and on the NE Greenland margin to which they have been projected. Simplified palaeogeographic reconstruction of the aeromagnetic map to Anomaly 22 reveals that a c. 50 km wide magnetic anomaly cuts across spreading anomalies 24A, 24B and 23 from the Vøring Marginal High on the Norwegian margin to Traill Ø on the East Greenland coast. The anomaly is interpreted to represent an igneous complex referred to as the Traill-Vøring igneous complex (TVIC). The complex crosscuts anomaly 22 on the Greenland margin, suggesting that the igneous activity was active until c. 50 Ma and can be linked up with the NNE-trending initial magmatic lineament (IML) extending between Traill Ø and Kangerlussuaq. The IML has been suggested to relate to a failed attempt of direct linkage between the Reykjanes and Mohns Ridges. The magnetic response of the TVIC along the Vøring margin has previously been interpreted as representing anomaly 24A and 24B. Such an interpretation required the erroneous introduction of an abandoned spreading ridge.

The mid-Norwegian margin: a discussion of crustal lineaments, mafic intrusions, and remnants of the Caledonian root by 3D density modelling and structural interpretation

The high-density lower crustal body (LCB) on the mid-Norwegian margin is almost universally interpreted to represent magmatically underplated material, added to the crust during Early Tertiary opening of the NE Atlantic. The thickness of the LCB is uneven, and its distribution along the margin is sharply limited by margin-perpendicular lineaments. Three-dimensional density modelling constrained by petrophysical and seismic data was performed to investigate the expression of the various lineaments (Bivrost, Jan Mayen, Gleipne, etc.) in the crustal architecture, with and without the LCB. The Bivrost Lineament, separating the Vøring and Lofoten margin segments, is clearly expressed and so is arguably a lineament in the SW Vøring margin. Notably, most of the lineaments can be interpreted as offshore prolongations of major onshore detachments stemming from Late Caledonian orogenic collapse. An alternative interpretation of the LCB is thus that it represents high-grade metamorphic rocks, remnant from the Caledonian root. Determining the nature of the LCB has profound effects for the volume estimate of magmatic rocks in the North Atlantic Igneous Province, and consequently also for the degree of crustal thinning and heat flow.

Timing and magnitude of depth-dependent lithosphere stretching on the southern Lofoten and northern Vøring continental margins offshore mid-Norway: implications for subsidence and hydrocarbon maturation at volcanic rifted margins

Subsidence analysis on the southern Lofoten and northern Vøring segments of the Norwegian rifted margin shows depth-dependent stretching of continental margin lithosphere in which lithosphere stretching and thinning at continental break-up at ~ 54 Ma greatly exceeds that of the upper crust within 100 km landward of the COB. For the southern Lofoten margin lithosphere β stretching factors approaching infinity are required at 54 Ma west of the Utrøst Ridge to restore the top Basalt (inner lava flow) reflectors and top Tare formation (54 Ma) to presumed sub-aerial depositional environments, while for the northern Vøring margin lithosphere β values of 2.5 are required. In contrast, upper crustal extension by faulting shows little stretching with β < 1.1 at break-up or immediately preceding break-up in the Paleocene and Late Cretaceous. The presence of lithosphere depth-dependent stretching and the absence of significant Paleocene and Late Cretaceous upper crustal extension imply that depth-dependent stretching of the southern Lofoten and northern Vøring margins occurred during sea-floor spreading initiation rather than during pre-break-up intra-continental rifting. Depth-dependent stretching, where upper-crustal extension is significantly smaller than whole-crustal or whole-lithosphere extension within 75–150 km of the COB, has been observed worldwide for both volcanic and non-volcanic rifted continental margins. Temperature and hydrocarbon maturation modelling show that the inclusion of depth-dependent stretching has an important effect on temperature and hydrocarbon maturation evolution in depth and time. Failure to include the large β factors for the lower crust and lithospheric mantle (below the less stretched upper crust) leads to a serious under-prediction of temperature and hydrocarbon maturation. While the effect of emplacing thick sill intrusions or magmatic underplating at continental break-up has an important effect on predicted temperature and %VR, their effects can be small in comparison with that of depth-dependent stretching.

Timing of depth‐dependent lithosphere stretching on the S. Lofoten rifted margin offshore mid‐Norway: pre‐breakup or post‐breakup?

AbstractDepth‐dependent stretching, in which whole‐crustal and whole‐lithosphere extension is significantly greater than upper‐crustal extension, has been observed at both non‐volcanic and volcanic rifted continental margins. A key question is whether depth‐dependent stretching occurs during pre‐breakup rifting or during sea‐floor spreading initiation and early sea‐floor spreading. Analysis of post‐breakup thermal subsidence and upper‐crustal faulting show that depth‐dependent lithosphere stretching occurs on the outer part of the Norwegian volcanic rifted margin. For the southern Lofoten margin, large breakup lithosphere β stretching factors approaching infinity are required within 100 km of the continent–ocean boundary to restore Lower Eocene sediments and flood basalt surfaces (∼54 Ma) to interpreted sub‐aerial depositional environments at sea level as indicated by well data. For the same region, the upper crust shows no significant Palaeocene and Late Cretaceous faulting preceding breakup with upper‐crustal β stretching factors <1.05. Further north on the Lofoten margin, reverse modelling of post‐breakup subsidence with a β stretching factor of infinity predicts palaeo‐bathymetries of ∼1500 m to the west of the Utrøst Ridge and fails to restore Lower Eocene sediments and flood basalt tops to sea level at ∼54 Ma. If these horizons were deposited in a sub‐aerial depositional environment, as indicated by well data to the south, an additional subsidence event younger than 54 Ma is required compatible with lower‐crustal thinning during sea‐floor spreading initiation. For the northern Vøring margin, breakup lithosphere β stretching factors of ∼2.5 are required to restore Lower Eocene sediments and basalts to sea level at deposition, while Palaeocene and Late Cretaceous upper‐crustal β stretching factors for the same region are < 1.1. The absence of significant Palaeocene and late Cretaceous extension on the southern Lofoten and northern Vøring margins prior to continental breakup supports the hypothesis that depth‐dependent stretching of rifted margin lithosphere occurs during sea‐floor spreading initiation or early sea‐floor spreading rather than during pre‐breakup rifting.

Vp/ Vs ratio along the Vøring Margin, NE Atlantic, derived from OBS data: implications on lithology and stress field

A total of 13 regional Ocean Bottom Seismograph (OBS) profiles with an accumulated length of 2207 km acquired on the Vøring Margin, NE Atlantic have been travel time modelled with regards to S-waves. The V p/ V s ratios are found to decrease with depth through the Tertiary layers, which is attributed to increased compaction and consolidation of the rocks. The V p/ V s ratio in the intra-Campanian to mid-Campanian layer (1.75–1.8) in the central Vøring Basin is significantly lower than for the layers above and beneath, suggesting higher sand/shale ratio. This layer was confirmed by drilling to represent a layer of sandstone. This mid-Cretaceous ‘anomaly’ is also present in the northern Vøring Basin, as well as on the southern Lofoten Margin further north. The V p/ V s ratio in the extrusive rocks on the Vøring Plateau is estimated to be 1.85, conformable with mafic (basaltic) rocks. Landward of the continent/ocean transition (COT), the V p/ V s ratio in the layer beneath the volcanics is estimated to be 1.67–1.75. These low values suggest that this layer represents sedimentary rocks, and that the sand/shale ratio might be relatively high here. The V p/ V s ratio in the crystalline basement is estimated to be 1.67–1.75 in the basin and on the landward part of the Vøring Plateau, indicating the presence of granitic/granodioritic continental crust. In the lower crust, the V p/ V s ratio in the basin decreases uniformly from southwest to northeast, from 1.85–1.9 to 1.68–1.73, suggesting a gradual change from mafic (gabbroic) to felsic (granodioritic) lower crust. Significant (3–5%) azimuthal S-wave anisotropy is observed for several sedimentary layers, as well as in the lower crust. All these observations can be explained by invoking the presence of liquid-filled microcracks aligned vertically along the direction of the present day maximum compressive stress (NW–SE).

Structure of a Volcanic Continental Margin Derived from Ocean-bottom Seismographic Data: The Northern Vøring Margin, off Norway

—This paper presents a crustal model derived from an Ocean Bottom Seismograph (OBS) study along the northern Voring margin off Norway. The profile was acquired to map the crustal structure in the northernmost part of the Voring Basin, and to link crustal models of the Lofoten and central Voring Basin obtained by previous OBS studies. The Voring margin, as well as the Lofoten margin to the north, was created by continental breakup between Norway and Greenland in late Paleocene-early Eocene. The rifting and continental breakup process were accompanied by intense extrusive and intrusive magmatic activities. The OBS data provide the whole crustal structure along the northern Voring margin, in the area where the deep crustal structure cannot be resolved by conventional multichannel reflection data due to sill intrusions in the sedimentary sequence. The shallow part of the crustal model is characterized by up to 10 km thick sediments, a sequence of flood basalts and sill intrusions. The P-wave velocities in the flood basalts and sill intrusions are estimated to 5.0 km/s and 4.7–5.8 km/s, respectively. The model indicates an abrupt thickening of the upper crystalline crust from approx.3 km in the NE, to about 10 km towards the SE, with velocities of 6.0–6.2 km/s. The lower crustal velocities are not well resolved due to lack of clear refraction arrivals from the lower crust. However, the observed amplitude versus offsets are best explained by a model with a change in lower crustal velocities from 6.8 to 7.2 km/s beneath the Bivrost lineament. The modelling infers the presence of a lower crustal reflector beneath the lineament, which represents the landward continuation of the Bivrost lineament. Reflection arrivals from the Moho reveal a Moho depth of 23 km in the middle of the profile and 18– 20 km in the northeastern part of the profile. A 370 km long crustal section from the central part of the Voring Basin to the Lofoten margin, obtained by the results of this study and previous OBS studies, shows a simple thinned continental crust on the Lofoten margin, and a high velocity lower crust underlying an upper crust of varying thickness in the Voring Basin. The transition between these structures is situated beneath the Bivrost lineament in the lower crust, and beneath the basement high about 40 km south of the lineament in the upper crust.

Crustal structure of Edoras Bank continental margin and mantle thermal anomalies beneath the North Atlantic

We discuss the structure of the continent‐ocean transition of the Edoras Bank area of the NE Atlantic margin and the influence of the Iceland mantle plume during continental breakup. Edoras Bank lies close to the present‐day limit of influence of the Iceland plume. Sequences of seaward dipping reflectors imaged on multichannel seismic profiles are well‐developed over much of the margin. They form aerially extensive, dipping sheets, with individual reflectors imaged on margin‐parallel profiles as laterally continuous and subhorizontal features. We interpret them as basalt flows extruded from linear fissure vents close to sea level at the time of continental breakup. A crustal velocity model for the margin was developed by travel time modeling of wide‐angle refractions and reflections recorded on digital ocean bottom hydrophones and the structure confirmed by gravity modeling. Oceanic crust formed immediately after breakup is 10 km thick and exhibits classic layer 2 and layer 3 seismic velocities. The crustal thickness increases to the SE reaching 17 km beneath the continental fragment of Edoras Bank. The continent‐ocean transition is dominated in the lower crust by a lens up to 8 km thick of high‐velocity material (7.2–7.5 km s−1), which exhibits a very low velocity gradient. It extends over a 60 km wide region beneath the seaward dipping reflectors. The high velocities and densities of the lower crust are probably caused by igneous rock with a high MgO content that underplated the margin or heavily intruded pre‐existing crustal material during continental breakup. Both the addition of large quantities of melt to the margin and the subsidence curves inferred from postrift sediments sampled by Deep Sea Drilling Project (DSDP) boreholes on the margin indicate the presence of abnormally hot mantle beneath the area at the time of continental breakup. Comparison of the structure and subsidence history of the Edoras Bank margin with other areas of the NW European margin allows us to deduce the spatial extent and temporal variation of the thermal anomalies caused by the Iceland mantle plume during and following continental breakup. We find that at the time of breakup a thin layer of anomalously hot asthenospheric mantle ∼150°C hotter than normal lay beneath the 2000 km long rift, extending from the Lofoten margin off Norway in the north to the Edoras Bank margin in the south. Over a period of 5–10 m.y. following the onset of rifting, the mantle thermal anomaly decayed rapidly to only 50°C or so above normal as the thin layer of abnormally hot mantle decompressed beneath the rift and produced a large pulse of magmatism.

The continent/ocean transition of the Lofoten volcanic margin, N. Norway

A detailed model for the continent/ocean transition of the Lofoten passive volcanic margin, N. Norway, has been obtained by use of seismic wide-angle data from Ocean Bottom Seismographs (OBSs), combined with results from other geophysical data. The OBS experiment across the transition zone involved dip- and strike-profiles, as well as a profile crossing the zone obliquely. The modelling reveals that this oblique profile is a key profile to the understanding of the transition zone, since detailed crustal models are difficult to obtain by use of the dipprofiles due to the presence of strong lateral variations in structures and seismic velocities. The Early Cenozoic extension episode on the outer Lofoten Margin ended with extensive submarine volcanism accompanied by intrusions into the stretched continental crust and emplacements of magmatic bodies into the lower crust. The high magmatic activity can be explained by increased asthenosphere temperatures caused by the proximity to the Iceland plume. From the seawarddipping-reflectors (SDRs) to the Vøring Escarpment (VE) a landward decrease in the upper crustal P-wave velocity from about 6.7 to 6.0 km/s is observed from the OBS data. This lateral velocity variation is interpreted to be caused by a gradual landward decrease in the amount of high-velocity “oceanic” intrusions into the stretched continental crust. The same landward decrease in velocity is also observed in the lower crust; from about 7.3 to 6.8 km/s. The modelling suggests that the high-velocity lower crustal body terminates landwards close to the VE. This lower crustal body is interpreted as magmatic underplating of the crust associated with the Iceland plume. The Mono (upper mantle) velocity is inferred to be 8.2 km/s from the shelf to the SDRs, and 7.9 km/s seward of the SDRs. This lateral velocity variation suggests that the upper mantle is of continental origin landward of the SDRs, and of oceanic origin from the SDRs and westwards. This observation indicates that the lower crustal magmatic material is intruded laterally into the ductile lower crust from the oceanic side. It is concluded that the transition from stretched continental crust with non-observable amounts of magmatic intrusions, to oceanic crust takes place over an about 50 km wide zone from the VE to the SDRs on the Lofoten Margin.

VpIVsratio structure of the Lofoten continental margin, northern Norway, and its geological implications

SUMMARY In 1988, an extensive seismic refraction and wide-angle reflection study was performed at the Lofoten passive continental margin, off northern Norway, by using three-component ocean bottom seismographs. The Lofoten margin was formed by continental rifting and sea-floor spreading with extensive volcanism, which is responsible for landward flood basalts and seaward-dipping reflectors (SDR). The P-wave structure beneath the seaward side of the Voting Plateau escarpment at the Lofoten margin is characterized by a thick oceanic lower crust and the existence of an additional high-velocity layer at the bottom of the crust. In this study, the distribution of the P-wave velocity (Vp) to S-wave velocity (Vs) ratio down to the uppermost mantle is obtained by studying P-to-S converted shear-wave data. Vp/Vs ratios of 3.0–5.5 are obtained in the sedimentary layers. These high ratios are attributed to unconsolidated sediments. In the upper crust, a significant difference in Vp/Vs ratios is found between oceanic-side profiles and land-side profiles, i.e. 1.86 oceanward of the SDRs and 1.76 landward of the SDRs. This variation may have been caused by a difference in porosity, due to the difference in extrusive processes: the upper crust of the oceanic side of the SDRs was created by submarine eruptions, while the land side of the SDRs was formed by submarian eruptions. The Vp/Vs ratios in the middle crust and lower crust are estimated as 1.76–1.78 and 1.80, respectively. Comparing Vp and VsV, data from the lower crust to those in previous studies clearly indicates the presence of mafic gneisses. This result represents new seismic evidence for the hotspot hypothesis concerning the formation of passive volcanic continental margins. Clear S-wave upper-mantle refractions can be observed at offsets of up to 100 km, even where P-wave arrivals cannot be identified. In order to model these S-wave arrivals, a VS=4.8 km s-1 layer is necessary 2 km below the Moho.

Crustal structure of the Lofoten continental margin, off northern Norway, from ocean-bottom seismographic studies

SUMMARY In 1988, an extensive geophysical experiment was performed on the Lofoten passive continental margin, which was formed by the continental break-up between Greenland and northern Norway at 58 Ma. The geological units of the outer Lofoten margin are characterized by seaward-dipping reflectors (SDRs) and landward flood basalt, which extends up to 100 km landwards of SDRs. In this study, we obtain the P-wave velocity structure beneath the Lofoten Basin, the SDRs, and the landward flood basalt by use of ocean-bottom seismograph refraction profiling, and we also discuss the formation of the northern Norwegian passive continental margin. In the Lofoten Basin the crust is of oceanic type, consisting of sedimentary layers, oceanic laver 2 (4.9−5.5 km s−1), layer 3A (6.3−6.8 km s−1) and layer 3B (7.0−7.1 km s−1). Beneath the SDRs the crustal layers are identical to those of the Lofoten Basin, but the thickness of the lower crust, which represents the same velocity as layer 3B, increases to 5 km towards the continent side, and a high-velocity lower crustal layer (7.3 km s−1) is formed at the base of the crust. The ocean-continent transition zone is situated between the landward side of the SDRs and the northward continuation of the Voring Plateau Escarpment. In this region the velocity of the lower crust gradually decreases and approaches the lower crustal velocity beneath the Lofoten Islands (6.8 km s−1). The model also indicates that the high-velocity layer disappears in this region. Comparing our model with the crustal structure on the Voring margin, it is clear that the lower crustal body (≥7 km s−1) thickens southwards along the northern Norwegian continental margin. Recent results from petrological and geophysical studies of the generation of the oceanic crust have shown that increasing the temperature of the upwelling asthenospheric material increases the thickness of the oceanic crust. We interpret this as that the oceanic crust in the southern area in the Voring-Lofoten margin was generated by hotter material than that of the northern area.