Norway Basin Research Articles

Sensitivity analysis of similarity learning models for well-intervals based on logging data

The problem of the interpretability of neural network predictions is crucial for industry, especially in areas where the cost of a mistake is incredibly high, to which oil&gas belongs. In this sphere, similarity models play an important role. One of the pre-required input data for constructing a geological model of hydrocarbon field are interwell correlation results through well logging data analysis. Recent publications address the problem of similarity assessment between distinct wells, which is the manual, time-consuming, and expert-based process. To enhance the interpretability of the developed deep learning models, we propose a visualization tool consisting of two methods. Our first method is an adaptation of a saliency map, which has already shown its visualization and interpretability quality. However, as the possibility of dealing with black-box models exists and is high enough, we propose another method based on the masking of the original well-interval. The case study from the Norway basin and our tool’s visualization quality evaluation demonstrate the effectiveness of the developed tool.

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.

The Jan Mayen microcontinent: an update of its architecture, structural development and role during the transition from the Ægir Ridge to the mid-oceanic Kolbeinsey Ridge

Abstract We present a revised tectonostratigraphy of the Jan Mayen microcontinent (JMMC) and its southern extent, with the focus on its relationship to the Greenland–Iceland–Faroe Ridge area and the Faroe–Iceland Fracture Zone. The microcontinent's Cenozoic evolution consists of six main phases corresponding to regional stratigraphic unconformities. Emplacement of Early Eocene plateau basalts at pre-break-up time (56–55 Ma), preceded the continental break-up (55 Ma) and the formation of seawards-dipping reflectors (SDRs) along the eastern and SE flanks of the JMMC. Simultaneously with SDR formation, orthogonal seafloor spreading initiated along the Ægir Ridge (Norway Basin) during the Early Eocene (C24n2r, 53.36 Ma to C22n, 49.3 Ma). Changes in plate motions at C21n (47.33 Ma) led to oblique seafloor spreading offset by transform faults and uplift along the microcontinent's southern flank. At C13n (33.2 Ma), spreading rates along the Ægir Ridge started to decrease, first south and then in the north. This was probably complemented by intra-continental extension within the JMMC, as indicated by the opening of the Jan Mayen Basin – a series of small pull-apart basins along the microcontinent's NW flank. JMMC was completely isolated when the mid-oceanic Kolbeinsey Ridge became fully established and the Ægir Ridge was abandoned between C7 and C6b (24–21.56 Ma).

Late Permian–earliest Triassic high-resolution organic carbon isotope and palynofacies records from Kap Stosch (East Greenland)

During and after the end Permian mass extinction terrestrial and marine biota underwent major changes and reorganizations. The latest Permian and earliest Triassic is also characterized by major negative carbon isotope shifts reflecting fundamental changes in the carbon cycle. The present study documents a high-resolution bulk organic carbon isotope record and palynofacies analysis spanning the latest Permian–earliest Triassic of East Greenland. An almost 700meter thick composite section from Kap Stosch allowed discriminating 6 chemostratigraphic intervals that provide the basis for the correlation with other coeval records across the world, and for the recognition of basin wide transgressive–regressive events documenting tectonic activity during the opening of the Greenland–Norway Basin. The identification of the main factors that influenced the organic carbon isotope signal during the earliest Triassic (Griesbachian to Dienerian) was possible due to the combination of bulk organic carbon isotope, palynofacies and Rock-Eval data. Two negative carbon isotopic shifts in the Kap Stosch record can be correlated with negative shifts recorded in coeval sections across the globe. A first negative shift precedes the base of the Triassic as defined by the first occurrence of the conodont Hindeodus parvus in the Meishan reference section, and the second one coincides with the suggested Griesbachian–Dienerian boundary. This new organic carbon isotope record from the extended Kap Stosch section from the Boreal Realm documents regional and global carbon cycle signals of the interval between the latest Palaeozoic and the onset of the Mesozoic.

Conjugate volcanic rifted margins, seafloor spreading, and microcontinent: Insights from new high‐resolution aeromagnetic surveys in the Norway Basin

AbstractWe have acquired and processed new aeromagnetic data that cover the entire oceanic Norway Basin located between the Møre volcanic rifted margin and the Jan Mayen microcontinent (JMMC). The new compilation allows us to revisit the structure of the conjugate volcanic (rifted) margins and the spreading evolution of the Norway Basin from the Early Eocene breakup time to the Late Oligocene when the Aegir Ridge became extinct. The volcanic margins (in a strict sense) that formed before the opening of the Norway Basin have been disconnected with the previous Jurassic‐Mid‐Cretaceous episode of crustal thinning. We also show evidence of relationships between the margin architecture, the breakup magmatism distribution along the continent‐oceanic transition, and the subsequent oceanic segmentation. The Norway Basin shows a complex system of asymmetric oceanic segments locally affected by episodic ridge jumps. The new aeromagnetic compilation also confirms that a fan‐shaped spreading evolution of the Norway Basin was clearly active before the cessation of seafloor spreading and extinction of the Aegir Ridge. An important Mid‐Eocene kinematic event at around magnetic chron C21r can be recognized in the Norway Basin. This event coincides with the onset of diking and increasing rifting activity (and possible oceanic accretion?) between the proto‐JMMC and the East Greenland margin. It led to a second phase of breakup and microcontinent formation in the Norwegian‐Greenland Sea ~26 Myrs later in the Oligocene.

Origin and evolution of the Kolbeinsey Ridge and Iceland Plateau, N-Atlantic

Variations in crustal structure along the 700 km long KRISE7 refraction/reflection and gravity profile, straddling 66.5°N across the Iceland Shelf, Iceland Plateau and western Norway Basin confirm that extinct spreading centers coexisted with the now extinct AEgir Ridge prior to the initiation of the Kolbeinsey Ridge at 26 Ma. The western 300 km of the profile, across the Iceland shelf, formed by rifting at the Kolbeinsey Ridge, whereas the eastern 400 km, across the Iceland Plateau and the western Norway Basin, formed by earlier rifting, possibly containing slivers of older oceanic or continental crust rifted off the central E-Greenland margin along with the Jan Mayen Ridge. Crustal thickness increases gradually across the Iceland shelf, from 12 to 13 km near the Kolbeinsey Ridge to 24–28 km near the eastern shelf edge, decreasing abruptly across the shelf edge, to 12–13 km. The Iceland Plateau has crustal thickness ranging from 12 to 15 km decreasing to 5–8 km across the western Norway Basin and 4–5 km at the AEgir Ridge. We suggest that high-velocity lower crustal domes and corresponding gravity highs across the Iceland plateau mark the location of extinct rift axes that coexisted with the AEgir Ridge. Similar lower crustal domes are associated with the currently active rift segments within Iceland and the Kolbeinsey Ridge.

North Atlantic Ocean deep-water processes and depositional environments: A study of the Cenozoic Norway Basin

Despite the enormous areas deep-water basins occupy in modern oceans, our knowledge about them remains poor. At depths of greater than 2000 m, the Cenozoic Norway Basin in the northernmost part of the Atlantic Ocean, is one such basin. Interpretation of 2D multichannel seismic data suggests a three-stage evolution for the Norway Basin. (1) Eocene–Pliocene. This time period is characterised by deposition of ooze-rich sediments in a widening and deepening basin. (2) Early-Middle Pleistocene. A significant shift in sedimentary processes and depositional environments took place in the Early Pleistocene. Mass failures initiated on the Norwegian continental slope, and three Early and Middle Pleistocene slide debrites, with maximum thicknesses of 600 m and sediment volumes of up to 25× 103 km3, were deposited. With ages estimated at c. 2.7–1.7 Ma, 1.7–1.1 Ma and 0.5 Ma, these slide deposits are among the largest identified worldwide, and among the oldest mapped along the entire NE Atlantic continental margin. (3) Late Pleistocene-Present. Since c. 0.5 Ma the Norway Basin has been effected by glacigenic debris flows, the Storegga Slide and hemipelagic-glacimarine sedimentation. These sedimentary processes were active during a time of repeated shelf-edge ice advances along the NE Atlantic continental margin. This study shows that deep-water basins represent dynamic depositional environments reflecting regional tectonic and climatic changes trough time.

Identification of a MIS 6 age ( c. 180 ka) Icelandic tephra within NE Atlantic sediments: a new potential chronostratigraphic marker

Abstract The incidence of volcanic ash (tephra) within marine sediments serves as a useful stratigraphic marker and tool for correlation. In addition, where an independent age estimate exists, tephra layers can provide a means of dating the sediments themselves. Here we present a geochemically characterized, size sorted tephra layer within Marine Isotope Stage (MIS) 6, most likely resulting from primary air-fall from an Icelandic volcanic source. This tephra layer is tentatively correlated to the Kerlingarfjöll volcanic system using major element geochemistry. The ash layer has an interpolated age of 181±6 ka based on the age model for MD04-2822. We briefly review the occurrence of silicic tephra in the North Atlantic region from MIS 7 to MIS 5e inclusive and find potential correlatives to the MD04-2822 MIS 6 ash layer in the Norway Basin and Irminger Sea.

Anatomy of a fluid pipe in the Norway Basin: Initiation, propagation and 3D shape

An exploration 3D seismic data set from the Gjallar Ridge off mid-Norway images a giant fluid seep structure, 3×5km wide, which connects to late Palaeocene magmatic sills at depth. Two of the pipes that have developed as hydrothermal vents reach all the way to the modern seafloor implying that they either were active much longer than the original hydrothermal activity or have been reactivated. We combine detailed seismic analysis of the northern pipe and sandbox modeling to constrain pipe initiation and propagation. Although both the seismic data and the sandbox models suggest that fluids at depth are focused through a vertical conduit, sandbox models show that fluids ascend and reach a critical depth migration where focused migration abruptly transforms into distributed fluid flow through unconsolidated sediments. This indicates that at this level the sediments are intensely deformed during pipe propagation, creating a V-shaped structure, i.e. an inverted cone at depth and a positive relief anomaly, 5 to 10m high, at the seafloor, which is clearly identified on 3D seismic data. Comparison of the geometries observed in sandbox modeling with the seismically observed geometries of the Giant Gjallar Vent suggests that the Giant Gjallar Vent may be a proto-fluid seep at an early stage of its development, preceding the future collapse of the structure forming a seafloor depression. Our results imply that the Gjallar Giant Vent can be used as a window into the geological processes active in the deep parts of the Vøring Basin.

Treitel Ridge: A unique inside corner hogback on the west flank of extinct Aegir spreading ridge, Norway basin

Detailed shipborne mapping (sidescan sonar, swath bathymetry, seismic reflection, 3.5 kHz profiling, gravity and magnetics) of the extinct Aegir spreading axis (ca.55–25 Ma) in the Norway Basin revealed a highly unusual (in a MOR setting), narrow, 130 km long, asymmetrical basement hogback (Treitel Ridge; TR) on the southern west flank of the extinct rift, close to its junction with the Iceland–Faeroe aseismic ridge and the putative paleo-Iceland hotspot center. The ridge seems to continue north another 130 km in the form of a wider, more irregular basement rise, for a combined length of ca. 260 km. The southern, narrow part of TR, approximately following or slightly younger than chron 18n (ca. 39 Ma), attains a basement relief of ca. 1000 m in the south, its summit descending gradually (1.7:100) towards the NE. Although an east-flank conjugate appears generally lacking along both northern and southern parts of TR, asymmetry of the southern portion (steeper slope facing towards the extinct rift valley) suggests volcanic formation and axially dipping normal faulting at or near the active plate boundary. The possibility of continental crust extending close to the southern end of TR may also be important, because some of this crust may have been remelted at depth, contributing magma to feed TR eruptions. Southern TR may record a brief (0.1–1 m.y.) inside-corner tectonic event (probably a flexurally compensated normal-faulted offset), in effect a “mega-abyssal hill”. We suggest TR was probably not an expression of change in Iceland hotspot activity or somehow related to nearby continental crust, but rather represents the volcano-tectonic response of the Aegir accretion axis–at the time it became the eastern edge of the new Jan Mayen micro-plate–to extinction of the Mid-Labrador Sea spreading center. As proposed by Nunns (1983), the annexation of the Greenland plate by the North American plate forced a plate boundary reorganization east of Greenland—with northward propagation of the Mid-Atlantic Ridge rift along the Greenland margin to form the Jan Mayen micro-plate. Simultaneously, two ca. 107° trending transforms developed from the previously 146° trending Faeroes Fracture Zone. Both TR formation and fracture zone trend changes may record Aegir Ridge response to this major plate kinematic reorganization. Because TR was formed during a short interval, spreading along the Mid-Labrador Sea ridge may have ceased abruptly as well.

Neogene magmatism northeast of the Aegir and Kolbeinsey ridges, NE Atlantic: Spreading ridge–mantle plume interaction?

According to mantle plume theory the Earth's interior cools partly by localized large vertical mass transport, causing extensive decompression melting. The Iceland melt anomaly is regarded as a typical example of a mantle plume. However, there are centers of Miocene to recent magmatism in the Norwegian‐Greenland Sea not easily explained by the plume theory. Here we present new data to document diffuse late Miocene magmatic underplating of older oceanic crust located mostly north of the Aegir Ridge, an extinct seafloor spreading axis in the Norway Basin. There is also a region with similar magmatism northeast of the presently spreading Kolbeinsey Ridge north of Iceland. Intraplate magmatism in these locations is not easily explained by local plume models, edge‐driven convection, or by asthenosphere flow–lithosphere thickness interaction. On the basis of correlation between the magmatism and the active or extinct spreading ridges, we propose the mid‐ocean ridge basalt–capture model, in which this magmatism can be understood through plume–spreading ridge interaction: The asthenosphere flow out from Iceland captures deeper, low‐degree partially molten asthenospheric regions from underneath the spreading ridges and carry these across the terminating fracture zones, to subsequently underplate oceanic crust or to intrude and build seamounts. This model is similar to lithospheric cracking models for intraplate magmatism in requiring that low‐degree partial melt can be retained in the asthenosphere over time but differ in that the magma is extracted by internal magma movement processes and not by external tectonic forces.

Rates of continental breakup magmatism and seafloor spreading in the Norway Basin–Iceland plume interaction

In year 2000, an ocean bottom seismometer (OBS) profile was acquired across the Møre margin to the Aegir Ridge, an extinct seafloor spreading axis. The margin is an early Eocene volcanic passive margin, located between the Faeroe‐Iceland Ridge (FIR) and the East Jan Mayen Fracture Zone (EJMFZ). The P wave data were modeled by ray tracing to give a crustal transect showing a 10–11 km thick igneous crust created by breakup magmatism, tapering off to magma‐starved seafloor spreading by C23 time (51.4 Ma). The location of the EJMFZ was reinterpreted from a satellite derived gravity map, and spreading direction in the Norway Basin reevaluated. No other fracture zones were confirmed, and both thin oceanic crust (4–5 km) and lack of fracture zones resemble ultraslow spreading on the Arctic Gakkel Ridge. Magnetic seafloor spreading anomalies were identified from the magnetic track recorded with the OBS profile, and half spreading rates were derived. Early seafloor spreading was slow (15–32 mm yr−1), approaching ultraslow (6–8 mm yr−1) by C20 time (42.7 Ma). A V‐shaped pattern seen in the gravity field located only around the northern part of the Aegir Ridge corresponds to increased crustal thickness in the seismic model, recording northeast transport (3–6 mm yr−1) of more melt‐fertile asthenosphere zones. The magma‐starved character of the Norwegian Basin seen also during slow seafloor spreading may be the result of depletion of the asthenosphere when the Iceland plume constructed the FIR to the south, as the asthenosphere is subsequently transported into the Norway Basin.

The dating and morphometry of the Storegga Slide

The Storegga Slide has affected an area of approximately 95 000 km 2, and a sediment volume in the range of approximately 2400–3200 km 3 has been displaced. Around 250 km 3 of the volume has been deposited as turbidite sediments in the Norway Basin. This volume places the Storegga Slide event as one of the world's largest exposed submarine slides. Based on a comprehensive database, detailed morphological investigations and a dating programme have been performed to reveal the slide process and the timing of the slide. To date the slide event, detailed analyses of 89 cores within the Storegga Slide region have been undertaken. The investigations conclude: (a) The main Storegga Slide event represents one retrogressive event dated to be 7250±250 14C yrs BP or approximately 8100±250 cal. yrs BP. This age also corresponds with the age of a tsunami found along the western coast of Norway. (b) A few minor slide/slump events have been identified along the northern Storegga Slide escarpment, dated to be c. 5000 14C yrs BP and 2500–3000 14C yrs BP or approximately 5700 and 2200–2800 cal. yrs BP. The total volume of these events is interpreted to be less than 1 km 3 or c. 0.1% of the total volume calculated for the main Storegga Slide event. (c) The statistical analyses carried out on the lobes morphometrical parameters show a fairly good correlation ( R 2=0.8–0.9) between the smallest and the medium/large size debris lobes within the Ormen Lange Field area. This means that the rheological properties for this area can be described as fairly uniform for the slide masses and scaling of the morphometrical parameters should be possible with a great confidence.

The Storegga Slide: architecture, geometry and slide development

The detailed mapping of the Storegga Slide morphological elements and the analyses of the slide development are based on high-quality acoustic and sampling data sets acquired through a cooperation between academia and the petroleum industry. The Storegga Slide has affected an area of c. 95 000 km 2 and a sediment volume of minimum 2400 km 3 and maximum 3200 km 3 has been displaced with c. 250 km 3 deposited as turbidite sediments in the Norway Basin. This volume places the Storegga Slide event as one of the world largest exposed submarine slides. The Storegga Slide can be divided into six distinctive morphological provinces. Associated, and superimposed, on these provinces a total of 63 slide lobe phases have been identified and mapped. The morphological investigations have furthermore made it possible to generate a set of numerical values for statistical analysis of slide sediment rheology. This knowledge also makes it feasible to model the Storegga Slide. The analyses of the slide have revealed that the slide has developed through a retrogressive process starting most probably on the lower slope. The most likely location for initializing is in an area close to the Faroe–Shetland Escarpment.

Crustal structure of the southeastern Iceland-Faeroe Ridge (IFR) from wide aperture seismic data

The aseismic Iceland-Faeroe-Ridge (IFR) is of central importance in reconstructing the opening of the North Atlantic Ocean. To investigate the crustal structure of the IFR at its southeastern part we conducted a wide aperture seismic survey across the IFR from the Iceland Basin to the Norway Basin. Seismic energy was generated by two 60-litre sleeve guns and recorded by 42 ocean-bottom seismometers (OBS). Kinematic and dynamic forward modeling by two-point ray tracing was applied to develop a 2D velocity-depth model. The accuracy of the model obtained is better than 5% for both velocity and depth of interfaces. We find a Moho depth of 23 km below the crest of the ridge that decreases to about 15 km at either end of the line. The upper part of the crust exhibits P-wave velocities between 5.7 and 6.3 km/s that increase towards the Norway Basin. The lower crust is more homogeneous with vp ranging from 6.6 to 7.0 km/s. Upper and lower parts of the crust are separated by a first order discontinuity. Amplitudes (PmP) and arrival times (Pn) of wide-angle phases indicate upper-mantle velocities of 7.9 km/s. Furthermore we analyzed PS-converted phases for upper crustal depth levels and find a vp/vs ratio of 1.73±0.04 equal to a Poisson's ratio of 0.25 with no significant lateral variations along the seismic line. On the crest of the ridge we identified a basalt layer embedded within the sedimentary sequence that was also penetrated by deep sea drilling and dated to 43±3.3 million years. Below the basalt layer we observed a low-velocity layer that is identified as Mesozoic sediment that was also observed to either end of the basalt. We conclude that the southeastern part of the IFR is composed of stretched continental crust and thus part of Paleo-Europe rather than oceanic crust formed by the Iceland hot spot. This conclusion is supported by the distribution of magnetic anomalies that allow identifying the passive continental margin approximately 100 km to the NW of our seismic line.

Giant hummocks in deep-water marine sediments: Evidence for large-scale differential compaction and density inversion during early burial

This paper describes large-scale hummock structures from an Oligocene-Miocene succession in the Faeroe-Shetland Trough, off the northern coast of the United Kingdom. These hummocks have a wavelength of 1–2 km, and an amplitude of about 50 m; they are characterized by a polygonal planform geometry. Two successive depositional packages in this probably pelagic-dominated interval have this form of structural expression, but troughs of the younger set are directly superposed on crests of the older set. They occur at a present-day subsea-bottom depth of about 1000 m, and cover an area of ∼6000 km 2 . This extraordinary structural configuration is attributed to a protracted depositional and deformational history in which a density inversion was established, hummock formation was initiated in response to differential loading above an irregular polygonal fault system, and then subsequent collapse produced syndepositional troughs in the overlying units. These structures are similar in geometry to hummocks described from near-surface sediments in the Norway Basin by P. R. Vogt and are the largest type of density inversion deformation structure yet described from a clastic sedimentary succession.

Glacigenic debris flows on the North Sea Trough Mouth Fan during ice stream maxima

Numerous short sediment cores which penetrate through the glacimarine section and into glacigenic debris flow debrites (GDFs) on the North Sea Fan are investigated. The sampled GDFs represent the uppermost of several sequences of these stacked bodies, many hundreds of metres thick. The GDFs occurred over slopes from 0.3 to 0.7° and examples are found from the shelf break at the mouth of the Norwegian Channel to the Norway Basin floor, 300 km distant. The GDF diamicts are structureless and homogeneous in terms of grain size, magnetic susceptibility, NRM, bulk density, water, carbonate, foraminiferal and rock fragment content. This applies both from GDF to GDF across the fan and in a proximal/distal sense. Comparison of the investigated lithologic parameters with Weichselian till from a borehole in the Norwegian Channel (200 km up-ice) shows striking similarity to the GDF debrites. AMS 14C dates at the GDF/glacimarine contact and stratigraphic relationships indicate voluminous deposition before 18 ka, continuing, possibly uninterrupted, until ca. 16 ka, and a final phase halting just before 15 ka when the Norwegian Channel became ice free. This chronology, the comparable lithologic character between till and GDF debrites, and a seismic continuity confirms inferences that the GDFs are directly associated with glacial ice at the shelf break. The glacial efflux has not been subject to subsequent sorting and the homogeneity of the GDF is thought to reflect thorough mixing in a deforming till layer beneath a fast-flowing ice stream. The glacimarine section subsequent to GDFs shows no indications of turbidity flow. Observations are compatible with a slow, non-disintegrating (cohesive) `plug' flow transport mechanism with little sediment/water interaction on the upper surface. Minor distal differentiation might reflect a change in flow process. Deposition on the fan margins from water-borne glacimarine processes simultaneous with GDF deposition is limited.

Mid-Tertiary rifting and magmatism in the Traill Ø region, East Greenland

A number of planar normal faults displace Tertiary basaltic intrusions in the Traill Ø region of East Greenland. This 'post-magmatic' extension occurs over an along-strike length of >350 km and over an across-strike width of >100 km with fault displacements and fault-block rotations indicating that (3 increases in magnitude eastwards from near unity to β ≈ 1.1. The stretching direction was approximately E-W, orthogonal to the strike of major faults in the region, most of which are reactivated Mesozoic structures. Two periods of Tertiary basaltic magmatism are recognized in the area. 40 Ar– 39 Ar step heating suggests that the first event, which gave rise to tholeiitic sills with a composite exposed thickness of c. 1 km, occurred at c. 54 Ma, and the second, which formed smaller volumes of alkaline dykes and two large syenite complexes, occurred at c. 36 Ma. Field observations indicate that 'post-magmatic' extension occurred after 54 Ma and possibly continued after 36 Ma. The tholeiitic intrusions are probably related to a stretching event (β ≤ 1.1) prior to the onset of sea-floor spreading in the Norway Basin, whilst the alkaline basalts and 'post-magmatic' rifting are associated with the separation of the Jan Mayen continental block from the East Greenland margin during the Eocene-Oligocene. Both tholeiitic and alkaline basalts formed by the partial melting of garnet-spinel peridotite with greater degrees of partial melting (8–16%) for the earlier tholeiitic group. Rifting in East Greenland ceased with the development of an oceanic ridge between Jan Mayen and East Greenland in late Oligocene time. Subsequent minor compression, probably the result of ridge-push, is suggested by the development of broad wavelength, low amplitude folds in the Traill Ø region.

Topographic Sensitivity Studies with a Bryan–Cox-Type Ocean Model

Abstract This paper describes a series of four experiments, each run for 10 years at 1° × 1° resolution on a North Atlantic domain, designed to illuminate the sensitivity of a Bryan–Cox-type ocean model to changes in the representation of the ridges that restrict the flow of dense, deep water out of the Greenland–Iceland–Norway (GIN) basin. In reality, much of the outflow takes place through narrow sills, which are subgrid-scale in the model, and small changes in the model topography to reflect these sills have a large impact on the outflow and on the compensating inflow of warm North Atlantic water. The circulation of the GIN basin is dramatically changed depending on the amount of this inflow; with no inflow, the basin cools and freshens, as would be expected, whereas with too much inflow, it becomes warm, salty, and homogeneous to great depths. Moreover, the small changes in topography have wider implications for the simulation. The presence or absence of dense overflows has a great impact on the mixed...

The Aegir Rift: Crustal Structure of an Extinct Spreading Axis

Two seismic refraction and gravity lines were obtained along and normal to the axis of the Aegir Rift, an extinct spreading centre in the Norway Basin. Velocity-depth solutions and crustal structure models are derived from ocean-bottom records using two-dimensional ray tracing and synthetic seismogram modelling techniques. Gravity data are used to generate models consistent with the lateral variations in thickness of the layers in the crustal models. The resulting models require considerable degree of lateral inhomogeneity along and perpendicular to the rift axis. Crust within the extinct spreading centre is found to be thinner and of low P-wave velocity when compared with the crust sampled off-axis. To explain reduced velocities of the lower crust we suggest that, due to the relationship between fracturing and seismic velocity, the decreasing spreading rate leading up to extinction let the mechanically strong layer thicken, so that faulting and fracturing extended to greater depths . Low velocities are also observed in the uppermost mantle underlying the extinct spreading ridge. This zone is attributed to hydrothermal alteration of upper mantle peridotites. Furthermore, after spreading ceased 32-26 my ago, ongoing passive hydrothermal circulation was accompanied by the precipitation of alteration products in open void spaces, thereby decreasing the porosity and increasing the velocity. Consequently the typical low velocities of layer 2 found at active mid-ocean ridges have been replaced by values typical of mature oceanic crust.