mid-Norwegian Continental Margin Research Articles

Rapid glacial sedimentation and overpressure in oozes causing large craters on the mid-Norwegian margin: integrated interpretation of the Naust, Kai and Brygge formations

Abstract Along continental margins with rapid sedimentation, overpressure may build up in porous and compressible sediments. Large-scale release of such overpressure has major implications for fluid migration and slope stability. Here, we study if the widespread crater-mound-shaped structures in the subsurface along the mid-Norwegian continental margin are caused by overpressure that accumulated within high-compressibility oozes sealed by low-permeability glacial muds. We interpret 56 000 km 2 of 3D and 150 000 km 2 of 2D-cubed seismic data in the Norwegian Sea, combining horizon picking, well ties and seismic geomorphological analyses of the crater-mound landforms. Along the mid-Norwegian margin, the base of the glacially influenced sediments abruptly deepens to form 28 craters with typical depths of c. 100 m, areal extents of up to 5130 km 2 and volumes of up to 820 km 3 . Mounds are observed in the vicinity of the craters at several stratigraphic levels above the craters. We present a new model for the formation of the craters and mounds where the mounds consist of remobilized oozes evacuated from the craters. In our model, repeated and overpressure-driven sediment failure is interpreted to cause the crater-mound structures, as opposed to erosive megaslides. Seismic geomorphological analyses suggest that ooze remobilization occurred as an abrupt energetic and extrusive process. The results also suggest that rapidly deposited, low-permeability and low-porosity glacial sediments seal overpressure that originated from fluids being expelled from the underlying high-permeability and high-compressibility biosiliceous oozes.

The seismic reflection Moho across the mid-Norwegian continental rifted margin

The Moho – defined as the boundary that separates the Earth’s crust from the underlying upper mantle at depths of about 5 to 7 km beneath the oceans and 30 to 40 km below continents – has been difficult to pin down with observations. Different methods have yield a variety of results, suggesting that this simple definition may be misleading. Here, we present high-resolution and deep penetrating seismic reflection data that image the whole margin basement of the mid-Norwegian rift system, including the lower basement, Moho and upper mantle, from the proximal to the outer margin domains. We propose a determination of the seismic reflection Moho in this modern rifted margin context and describe its structural heterogeneity along the margin. Finally, the seismic facies, reflectivity, geometries and structural details of the seismic reflection Moho are described and discussed in terms of tectonic deformation pattern.

3-D thermal effect of late Cenozoic erosion and deposition within the Lofoten–Vesterålen segment of the Mid-Norwegian continental margin

3-D thermal effect of late Cenozoic erosion and deposition within the Lofoten–Vesterålen segment of the Mid-Norwegian continental margin

3D seismic evidence of buried iceberg ploughmarks from the mid-Norwegian continental margin reveals largely persistent North Atlantic Current through the Quaternary

Over 7500 buried linear and curvilinear depressions interpreted as iceberg ploughmarks were identified within the Quaternary Naust Formation from an extensive three-dimensional seismic dataset that covers ~40,000km2 of the mid-Norwegian continental margin. The morphology and net orientation of ploughmarks were mapped and analysed. These features are up to 28km long, 700m wide and are incised up to 31m deep. On average, ploughmarks are incised 5m deep, with median width of 185m and median lengths ranging from 1.2 to 2.7km for individual palaeo-surfaces. Width to depth ratio ranges from 8:1 to 400:1 and is on average 36:1. The presence of ploughmarks buried deeply within some palaeo-slope surfaces implies the occasional presence of very large icebergs since the middle Quaternary, suggesting that thick ice-sheet margins with fast-flowing ice streams were present in order to calve icebergs of such dimensions into the Norwegian Sea. The wide geographical distribution of ploughmarks suggests unrestricted iceberg drift and an open Norwegian Sea during the periods of iceberg calving since the early Quaternary. Ploughmark trajectory analysis demonstrates that the ocean current circulation, now dominated by the northeasterly flowing Norwegian Atlantic Current (NwAC), has largely persisted throughout the Quaternary. Despite the overall strikingly consistent pattern of iceberg drift, ploughmark mapping also shows evidence for short-lived NwAC reductions possibly related to major phases of iceberg discharge and/or meltwater pulses from the Fennoscandian Ice Sheet during the middle and late Quaternary.

Deep structure of the Mid-Norwegian continental margin (the Vøring and Møre basins) according to 3-D density and magnetic modelling

Deep structure of the Mid-Norwegian continental margin (the Vøring and Møre basins) according to 3-D density and magnetic modelling

Tectonic evolution and extension at the Møre Margin – Offshore mid-Norway

Lithospheric stretching is the key process in forming extensional sedimentary basins at passive rifted margins. This study explores the stretching factors, resulting extension, and structural evolution of the Møre segment on the Mid-Norwegian continental margin. Based on the interpretation of new and reprocessed high-quality seismic, we present updated structural maps of the Møre margin that show very thick post-rift sediments in the central Møre Basin and extensive sill intrusion into the Cretaceous sediments. A major shift in subsidence and deposition occurred during mid-Cretaceous. One transect across the Møre continental margin from the Slørebotn Subbasin to the continent-ocean boundary is reconstructed using the basin modelling software TecMod. We test different initial crustal configurations and rifting events and compare our structural reconstruction results to stretching factors derived both from crustal thinning and the classical backstripping/decompaction approach. Seismic interpretation in combination with structural reconstruction modelling does not support the lower crustal bodies as exhumed and serpentinised mantle. Our extension estimate along this transect is ~188±28km for initial crustal thickness varying between 30 and 40km.

3-D temperature distribution beneath the Mid-Norwegian continental margin (the Vøring and Møre basins)

3-D temperature distribution beneath the Mid-Norwegian continental margin (the Vøring and Møre basins)

Isostasy as a tool to validate interpretations of regional geophysical datasets – application to the mid-Norwegian continental margin

AbstractIsostasy is a well understood concept, yet rarely applied to its full capacity in regional interpretations of crustal structures. In this study, we utilize a recent density model for the entire NE Atlantic, based on refraction seismic data and gravity inversion, to calculate isostatically balanced bathymetry along the mid-Norwegian margin. Since gravity and isostatically balanced elevation are independent observables but both depend on the underlying density model, consistencies and discrepancies point towards model deficits, erroneously interpreted or poorly understood areas.Four areas of large isostatic residuals are identified. Along the outer Vøring Margin, a mass deficit points to more extensive high-density bodies or a shallower Moho than currently mapped. Farther seaward, along the Vøring Marginal High, a mass excess indicates inaccurate mapping of the continent–ocean boundary and surrounding structures. A number of eclogitic bodies along the proximal mid-Norwegian margin have been described in recent publications and their presence is now also confirmed by isostatic calculations. Major elevation and gravity residuals along the transition between the Vøring and Møre margins signify that the structure of this region is poorly understood and modifications to the mapped continent–ocean transition may be required.

Ice-sheet dynamics through the Quaternary on the mid-Norwegian continental margin inferred from 3D seismic data

Reconstructing the evolution of ice sheets is critical to our understanding of the global environmental system, but most detailed palaeo-glaciological reconstructions have hitherto focused on the very recent history of ice sheets. Here, we present a three-dimensional (3D) reconstruction of the changing nature of ice-sheet derived sedimentary architecture through the Quaternary Ice Age of almost 3 Ma. An extensive geophysical record documents a marine-terminating, calving Fennoscandian Ice Sheet (FIS) margin present periodically on the mid-Norwegian shelf since the beginning of the Quaternary. Spatial and temporal variability of the FIS is illustrated by the gradual development of fast-flowing ice streams and associated intensification of focused glacial erosion and sedimentation since that time. Buried subglacial landforms reveal a complex and dynamic ice sheet, with converging palaeo-ice streams and several flow-switching events that may reflect major changes in topography and basal thermal regime. Lack of major subglacial meltwater channels suggests a largely distributed drainage system beneath the marine-terminating part of the FIS. This palaeo-environmental examination of the FIS provides a useful framework for ice-sheet modelling and shows that fragmentary preservation of buried surfaces and variability of ice-sheet dynamics should be taken into account when reconstructing glacial history from spatially limited datasets.

Fluid migration directions inferred from gradient of time surfaces of the sub seabed

The role of sub seabed topographically controlled fluid migration is assessed to improve our understanding of distributions of acoustic chimneys at the Nyegga pockmark field on the mid-Norwegian continental margin. 3D seismic data interpretations resulted in topographic gradients of seismic time surfaces and RMS amplitude maps. Topographical gradient maps and flow tracing allowed identifying migration pathways and trapping locations for free gas within the shallow sub seabed. The occurrence of acoustic chimneys, pockmarks and mounds correlate with identified fluid migration pathways and gas trapping locations. An important factor that controls the trapping locations and the lateral distribution of seeps on the seabed at Nyegga is the variation through time of the depth of the base of the gas hydrate stability zone (BGHSZ). Fluids can derive from gas hydrate systems that are suspected of being a biogenic source and/or Tertiary domes that are considered to show leakage of thermogenic fluids to the shallow geosphere.

Sources of methane inferred from pore-water δ13C of dissolved inorganic carbon in Pockmark G11, offshore Mid-Norway

Pockmark G11 is the most spectacular one among the pockmarks located at the southern border of Vøring Plateau and 1–2 km away from the northern flank of the Storegga Slide, mid-Norwegian continental margin. For the first time, detailed pore-water geochemical studies were conducted to address methane hydrate occurrence, methane seepage and associated geochemical processes, and methane characteristics in the pockmark. Pore-waters collected from five sediments cores inside and one sediment core outside the pockmark, were analyzed for dissolved Cl−, sulfate (SO42−), total hydrogen sulfide (ΣH2S), Ca2+, Mg2+, dissolved inorganic carbon (DIC), δ13CDIC and δ18O. Methane hydrates were recovered in all sediments below 0.75m in a core inside pockmark G11, which is in good accordance with heavy oxygen isotope (1.9 to 2.3‰SMOW) and low Cl−concentrations (84.9 to 16.1mM) in pore-waters. Pore-water profiles indicate that upward-migrating methane fluids are spatially variable in the pockmark, with methane fluxes ranging from below detection in the center and outside, to 0.30–0.54molm−2 a−1 inside. In the cores with active methane fluxes, maximum DIC concentrations (19.4 to 21.5mM) and corresponding minimum δ13CDIC values (−52.3 to −54.6 ‰ PDB) occur within sulfate-methane-transition (SMT) zones from ∼0.40 to 0.50m below seafloor (mbsf), close to the seafloor. Simple mass balance modeling and δ13CDIC measurements within the SMT zones suggest that methane in shallow sediments within pockmark G11 is microbial in origin. Pore-water geochemistry and seabed observations suggest that methane fluxes inside pockmark G11 are episodic, and derived mostly from the recycling of methane hydrate at depth during sediment burial.

The role of sedimentation rate and permeability in the slope stability of the formerly glaciated Norwegian continental margin: the Storegga slide model

Despite the gently dipping slopes (ca 1°), large-scale submarine slope failures have occurred on the mid-Norwegian continental margin (Storegga, Sklinnadjupet, Traenadjupet), suggesting the presence of special conditions predisposing to failure in this formerly glaciated margin. With a volume estimated between 2,400 and 3,200 km3 and an affected area of approximately 95,000 km2, the Storegga slide represents one of the largest and best-studied submarine slides of Holocene age known worldwide. Finite element modeling of slope failure indicates that a large (6.5 < Ms < 7.0) seismic triggering mechanism would not be sufficient to cause failure at more than 110 m below the seabed as observed for the slip planes at Storegga (northern sidewall). This implies that other factors (e.g., liquefaction, strain softening, gas charging, rapid burial) are needed to explain the occurrence of the Storegga slide with a deep surface of failure. In this paper, we discuss the importance of the compaction effect of rapidly accumulated sediments in the slide area. During compaction, sediment grains reorganize themselves, thereby, expelling pore water. Consequently, depending on sedimentation rate and permeability, excess pore pressures might result beneath less permeable sediments. Our modeling and cross-checking illustrate how excess pore pressure generation due to high sedimentation rate could explain the development of layers of weakness, and thus, how such a large slide might have been initiated in deep sediments. Using the highest sedimentation rate estimated in the area (36 and 27 m/kyr between 16.2 and 15 kyr BP), 1D modeling shows excess pore pressure values of around 200 kPa at a depth of 100 m below the seafloor 15 kyr BP and 60 kPa at a depth of 100 m at the time of the slide (8 kyr BP). Excess pore pressure apparently drastically reduced the resistance of the sediment (incomplete consolidation). In addition, 2D modeling shows that permeability anisotropies can significantly affect the lateral extent of excess pore pressure dissipation, affecting, that way, normally consolidated sediments far from the excess pore pressure initiation area.

The Storegga Slide complex: repetitive large scale sliding with similar cause and development

Seven large pre-Holocene slides on the mid-Norwegian continental margin between 62 and 67°N have been investigated. Most of the slides are located in the area of the Holocene Storegga Slide and in the North Sea Fan. The largest of the pre-Holocene slides are comparable in size to the Storegga Slide. With the exception of one slide, the sliding took place after the onset of continental shelf glaciations at 0.5 Ma. Despite limited chronostratigraphic resolution, at least one large slide apparently occurs during every 100 ky, following glacial–interglacial cyclicity. The slides have several common characteristic features, and they detach in fine-grained, seismically stratified hemipelagic deposits, commonly developed as contourite drifts. The instability is most likely created by pore overpressure built up in the hemipelagic deposits by rapid loading from glacial deposits. In some cases fluid flow from underlying oozes, may be a cause of overpressure. Earthquakes are the most likely trigger. The slide morphologies and other characteristic features favour a retrogressive slide development, as also suggested for the Storegga Slide. The area of the Storegga Slide Complex is particularly slide prone because of its structural setting and the preference for fine grained drift deposition in this region.

The complexity of a ramp–flat–ramp fault and its effect on hanging-wall structuring: an example from the Njord oil field, offshore mid-Norway

The Njord oil field of the mid-Norwegian continental margin comprises a hydrocarbon trap structure, which is delineated by segments of master faults with complex geometry. The Vingleia Fault Complex includes the major fault segments that delineate the Njord structure to the southeast and east. Three master fault segments with a change in strike orientation are recognized. Reflection seismic sections reveal that these segments show strong variations in fault plane geometry. The central part is characterized by a pronounced ramp–flat–ramp geometry, which diminishes both southwestward and northeastward, so that the northeastern and southwestern segments have a more listric geometry. Hence, an along-strike profile shows an antiform-like culmination in the master fault surface. To the characteristic deformation of ramp–flat–ramp faults, additional deformation in the Njord Field is obvious and marked by extensional horses, extensive roll-over folding, varying strata rotation, characteristic fault bending and variable influence of synthetic and antithetic faults. The occurrence of the style of deformation and geometrical variations in fault pattern are closely related to the structural position above the detachment fault. The following steps in the development of the Vingleia Fault Complex and the associated deformation of the hanging-wall fault block are proposed. (1) Initiation of the master faults in early Triassic. This stage was characterized by slow, extensional displacement. The positions of the master fault segments were possibly influenced by basement in homogeneities marked by a magnetic/gravity anomaly. (2) It is likely that the ramp–flat–ramp had already become established at this stage and that the master fault geometry and a proposed weak salt layer promoted this. (3) During accelerating extension in mid to late Jurassic, the master fault segments became connected by one-sided, lateral linking of fault tip and fault plane or by mutual curved linking of the fault tips. (4) Simultaneously, a roll-over anticline developed within the hanging wall of the master fault. Its relatively large amplitude is produced by a differential movement on the fault surface in combination with a dip-parallel shift of the vertical displacement maximum, which is positioned above a change in the footwall dip.

Geological controls on the Storegga gas-hydrate system of the mid-Norwegian continental margin

The geologic setting of the formerly glaciated mid-Norwegian continental margin exerts specific controls on the formation of a bottom-simulating reflector (BSR) and the inferred distribution of gas hydrates. On the continental slope the lithology of glacigenic debris flow deposits and pre-glacial basin deposits of the Kai Formation prevent gas-hydrate formation, because of reduced pore size, reduced water content and fine-grained sediment composition. Towards the continental shelf, the shoaling and pinch-out of the gas-hydrate stability zone terminates the area of gas-hydrate growth. These geological controls confine the occurrence of gas hydrates and ensuing formation of a BSR to a small zone along the northern flank of the Storegga submarine slide and the slide area itself. A BSR inside the slide area indicates a dynamically adjusting gas-hydrate system to post-slide pressure–temperature equilibrium conditions. These observations, together with widespread evidence for fluid flow and deep-seated hydrocarbon reservoirs, suggest that the formation of BSR and gas hydrates on the mid-Norwegian continental margin is dominated by an advection of gas from the strata distinctly beneath the gas-hydrate stability zone. Fluids migrate upward within the Naust Formation and are deflected laterally by hydrated sediments and less permeable layers. Gases continually accumulate at the top of the slope, where overpressure eventually results in the formation of blow-out pipes and consequent pockmark development on the seabed.

Large-scale morphological evidence for past ice-stream flow on the mid-Norwegian continental margin

Abstract The Norwegian continental margin has been affected by several cycles of growth and decay of the Fennoscandinavian Ice Sheet. Evidence for this former ice-sheet activity is found in the seismic stratigraphy of the shelf and slope, and in the morphology of seafloor sediments. The regional bathymetry of the mid-Norwegian shelf (63°N to 68°N) comprises a series of cross-shelf troughs, separated by shallower banks. In Trænadjupet and Suladjupet, streamlined, elongate sedimentary bedforms (known as mega-scale lineations) are found, aligned along trough long-axes. Spacing between ridge-tops is 400–500 m, and ridge width and height are about 250 m and less than 10 m, respectively. Streamlined bedforms are not present on the intervening shallow banks, and terminate abruptly at the trough margins, marking the former boundary of fast glacier flow. On northwestern Trænabanken and on either side of Trænadjupet, lateral ridges are inferred to mark the shear margin of ice streams in Sklinnadjupet and Trænadjupet. The Skjoldryggen Moraine records the seaward limit of Late Weichselian ice-sheet growth, and ridges inshore result from deposition in still-stands during ice retreat. The streamlined bedforms in Trænadjupet and Suladjupet are similar in morphology and scale to streamlined Antarctic bedforms, linked to the former presence of fast-flowing ice streams on the continental shelves of Antarctica. Several geomorphological criteria, identified as diagnostic of past ice-stream flow, are observed in our geophysical studies of the mid-Norwegian shelf. Using these criteria, we identify several fast-flowing ice streams on the western margin of the former Fennoscandinavian Ice Sheet. Numerical ice-sheet model predictions of fast-flowing ice coincide closely with the inferred locations of past ice streams based on the distribution of such suites of diagnostic bedforms.

Seismic volcanostratigraphy of the Norwegian Margin: constraints on tectonomagmatic break-up processes

Voluminous volcanism characterized Early Tertiary continental break-up on the mid-Norwegian continental margin. The distribution of the associated extrusive rocks derived from seismic volcanostratigraphy and potential field data interpretation allows us to divide the Møre, Vøring and Lofoten–VesterÅlen margins into five segments. The central Møre Margin and the northern Vøring Margin show combinations of volcanic seismic facies units that are characteristic for typical rifted volcanic margins. The Lofoten–VesterÅlen Margin, the southern Vøring Margin and the area near the Jan Mayen Fracture Zone show volcanic seismic facies units that are related to small-volume, submarine volcanism. The distribution of subaerial and submarine deposits indicates variations of subsidence along the margin. Vertical movements on the mid-Norwegian margin were primarily controlled by the amount of magmatic crustal thickening, because both the amount of dynamic uplift by the Icelandic mantle plume and the amount of subsidence due to crustal stretching were fairly constant along the margin. Thus, subaerial deposits indicate a large amount of magmatic crustal thickening and an associated reduction in isostatic subsidence, whereas submarine deposits indicate little magmatic thickening and earlier subsidence. From the distribution of volcanic seismic facies units we infer two main reasons for the different amounts of crustal thickening: (1) a general northward decrease of magmatism due to increasing distance from the hot spot and (2) subdued volcanism near the Jan Mayen Fracture Zone as a result of lateral lithospheric heat transport and cooling of the magmatic source region. Furthermore, we interpret small lateral variations in the distribution of volcanic seismic facies units, such as two sets of Inner Seaward Dipping Reflectors on the central Vøring Margin, as indications of crustal fragmentation.

Finite-element modelling of stress patterns along the Mid-Norwegian continental margin, 62° to 68°N

The inferred nature of the regional intraplate stress field off the coast of Norway is based primarily on borehole breakout information and earthquake focal mechanisms. The stress indicators reveal a slightly clockwise rotation of the orientation of the principal horizontal stress S H to the northwest with respect to the dominant E-W orientation observed in the northern North Sea. The stress regime is predominantly strike-slip and thrust fault, although two normal faulting events have occurred on the shelf and shelf edge. Within the Mid-Norwegian margin segment (62° to 68°N), the Trøndelag Platform exhibits large variations of observed S H orientations. This may be due to an almost isotropic horizontal stress state ( S H = S h) resulting from the superposition of local and regional stress sources. This paper presents the predicted tectonic stresses along the Mid-Norweigan margin based on a 3-D finite-element analysis. The two principal modelling objectives are to elaborate the effects of: (1) changes in the regional stress magnitude S H; and (2) variations in rock properties (i.e., Young's modulus) on the horizontal stress anisotropy HSA of the Vøring Basin and the Trøndelag Platform. The model incorporates simplified mainland topography, the Trøndelag Platform, the Vøring Basin, the Vøring Plateau, and the Lofoten Basin. Our modelling results indicate that horizontal stress anisotropy (HSA) and the regional S H magnitude within the various structural units is dependent on their strike with respect to the orientation of the present-day regional stress field. Specifically, we find: (1) a near-isotropic horizontal stress magnitude configuration for the southern part of the Vøring Basin where the gradient of topography across the Mid-Norwegian margin is oriented oblique to the regional trend of S H; (2) a maximum horizontal stress anisotropy HSA in the southern part of the Trøndelag Platform, this local HSA maximum bordering the HSA minimum of the Vøring Basin; (3) a narrow band of high HSA at the eastern edge of the Trøndelag Platform imposed by compressive gravitational stresses related to the elevated topography of mainland Norway; (4) a low HSA at the western margin of the Trøndelag Platform, related to spreading stresses associated with the topographic lowering towards the Vøring Basin; and (5) in case of a far-field stress of S H = 1.1 S h, the modelling results predict exchanged principal horizontal stresses S H and S h at the western margin of the Trøndelag Platform.