Norwegian-Greenland Sea Research Articles

Crustal structure and transform margin development south of Svalbard based on ocean bottom seismometer data

The Barents Sea is located in the northwestern corner of the Eurasian continent, where the crustal terrain was assembled in the Caledonian orogeny during Late Ordovician and Silurian times. The western Barents Sea margin developed primarily as a transform margin during the early Tertiary. In the northwestern part south of Svalbard, multichannel reflection seismic lines have poor resolution below the Permian sequence, and the early post-orogenic development is not well known here. In 1998, an ocean bottom seismometer (OBS) survey was collected southwest to southeast of the Svalbard archipelago. One profile was shot across the continental transform margin south of Svalbard, which is presented here. P-wave modeling of the OBS profile indicates a Caledonian suture in the continental basement south of Svalbard, also proposed previously based on a deep seismic reflection line coincident with the OBS profile. The suture zone is associated with a small crustal root and westward dipping mantle reflectivity, and it marks a boundary between two different crystalline basement terrains. The western terrain has low (6.2–6.45 km s −1) P-wave velocities, while the eastern has higher (6.3–6.9 km s −1) velocities. Gravity modeling agrees with this, as an increased density is needed in the eastern block. The S-wave data predict a quartz-rich lithology compatible with felsic gneiss to granite within and west of the suture zone, and an intermediate lithological composition to the east. A geological model assuming westward dipping Caledonian subduction and collision can explain the missing lower crust in the western block by subduction erosion of the lower crust, as well as the observed structuring. Due to the transform margin setting, the tectonic thinning of the continental block during opening of the Norwegian-Greenland Sea is restricted to the outer 35 km of the continental block, and the continent–ocean boundary (COB) can be located to within 5 km in our data. Distinct from the outer high commonly observed on transform margins, the upper part of the continental crust at the margin is dominated by two large, rotated down-faulted blocks with throws of 2–3 km on each fault, apparently formed during the transform margin development. Analysis of the gravity field shows that these faults probably merge to one single fault to the south of our profile, and that the downfaulting dominates the whole margin segment from Spitsbergen to Bjørnøya. South of Bjørnøya, the faulting leaves the continental margin to terminate as a graben 75 km south of the island. Adjacent to the continental margin, there is no clear oceanic layer 2 seismic signature. However, the top basement velocity of 6.55 km s −1 is significantly lower than the high (7 km s −1) velocity reported earlier from expanding spread profiles (ESPs), and we interpret the velocity structure of the oceanic crust to be a result of a development induced by the 7–8-km-thick sedimentary overburden.

Paleomagnetism of the late Quaternary Ontake Volcano, Japan: directions, intensities, and excursions

Study of paleosecular variation was carried out to mostly andesite lavas from the late Quaternary Ontake Volcano which have detailed stratigraphy and accurate radiometric ages. Among 42 sites, some of the results from lava successions were combined due to possible equivalent ages, remaining 35 sites. After excluding two low latitude VGPs, angular dispersion of 15.6° was obtained for a period which is accurately confined between 20 ka and 90 ka. The obtained angular dispersion is not different from the typical value for the last 5 my, and this is not surprising if we consider that the global paleointensity was not necessarily low and rather oscillatory during the period of 40–100 ka which precedes the global low in 20–40 ka. Two low latitude VGPs from 48 ka lava (41.9°N, 196.2°E) and 80 ka lava (15.9°N, 183.3°E) indicate existence of excursions in Japan during the latest Pleistocene. Preliminary paleointensity experiments indicate that the 80 ka excursion is accompanied by a low paleointensity of about 5 μT while the 48 ka one is not. These two excursions are probably related to those reported previously not only from the Ontake Volcano and its proximity but also from other several sites in Japan, although they were not conclusive. The 48 ka excursion probably correlates to the Laschamp excursion. This is consistent with the fact that the VGP position from the 48 ka lava comes to the central Pacific region, similar to those from the Laschamp excursion found in New Zealand. Possible correlation of the 80 ka excursion is the one from Norwegian-Greenland Sea and Arctic Ocean, which are stratigraphically between the Laschamp and the Blake excursions.

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The Late Cretaceous foraminiferal assemblages from five wells drilled in the southwestern Barents Sea were analysed using morphogroup analysis in order to understand the palaeoenvironment and palaeobathymetry of the study area. Morphogroup analysis of assemblages in the Upper Cretaceous Kveite Formation suggests stable, possibly middle to lower bathyal, mesotrophic and relatively well-oxygenated environments in the Tromsø Basin during the Late Cretaceous. The depositional setting of Upper Cretaceous Kviting Formation in the tectonically more stable Hammerfest Basin was probably shallower, but still represents a deep-water environment similar to that of the Kveite Formation. A shallowing regional bathymetry in the southwestern Barents Sea is indicated by the general decreasing trend in the relative abundance of tubular forms and the species diversity of assemblages, which reflects the regional Late Cretaceous–Paleocene uplift prior to the break-up of the Greenland–Norwegian Sea. Changes in the relative abundance of morphogroups and in the main constituent in the cumulative length of tubular taxa from Rhizammina to Rhabdammina near the top of Kveite Formation agree with the possible existence of bottom water currents.

New aerogeophysical study of the Eurasia Basin and Lomonosov Ridge: Implications for basin development

In 1998 and 1999, new aerogeophysical surveys of the Arctic Ocean9s Eurasia Basin produced the first collocated gravity and magnetic measurements over the western half of the basin. These data increase the density and extend the coverage of the U.S. Navy aeromagnetic data from the 1970s. The new data reveal prominent bends in the isochrons that provide solid geometrical constraints for plate reconstructions. Tentative identification of anomaly 25 in the Eurasia Basin links early basin opening to spreading in the Labrador Sea before the locus of spreading in the North Atlantic shifted to the Norwegian-Greenland Sea. With the opening of the Labrador Sea, Greenland began ∼200 km of northward movement relative to North America and eventually collided with Svalbard, Ellesmere Island, and the nascent Eurasia ocean basin. Both gravity and magnetic data sets reconstructed to times prior to chron 13 show a prominent linear anomaly oriented orthogonal to the spreading center and immediately north of the Yermak Plateau and Morris Jesup Rise. This anomaly may mark the locus of shortening and possibly subduction as Greenland collided with the nascent Eurasia Basin and impinged upon the southern Gakkel Ridge. This collision may have contributed to volcanism on the Morris Jesup Rise. By chron 13, Greenland had ended its northward motion and had become fixed to North America, and the plateau north of Greenland had rifted apart to become the Morris Jesup Rise and the Yermak Plateau.

North Atlantic sea-floor spreading rates: implications for the Tertiary development of inversion structures of the Norwegian–Greenland Sea

The Tertiary development of the Norwegian continental margin was dominated by the opening of the Arctic–North Atlantic Ocean. The correct identification of magnetic anomalies and their ages and the analysis of spreading rates during the formation of this ocean are important in understanding the development of the region and specifically the history of its passive margins. Three ocean domains, the Ægir, Reykjanes and Mohns regions, were investigated in an effort to understand the lateral changes in structural development of the passive margin after continental break-up. Spreading rates generally slowed down from 2 cm a −1 after Early Eocene initiation of sea-floor spreading, to values around 0.5 cm a −1 in Oligocene time. An increase in spreading rates to around 1 cm a −1 coincided with the positioning of the Iceland hotspot under the North Atlantic mid-ocean ridge. At the same time, the European plate changed its absolute plate motion from a north-directed drift to a motion more towards the east. The location of inversion structures in the Vøring and Faeroes Basin rather than in the Møre Basin is related to differences in spreading rates. The Mohns and the Reykjanes Ridges produced more ocean floor than the Ægir–Kolbeinsey Ridges. Asymmetric ocean-floor formation in the Ægir Ridge led to differential stress at the base of the lithosphere, which probably explains the absence of inversion features in the Møre Basin (less mantle drag). Furthermore, upper plate margins such as the Vøring Basin and possibly the Faeroe Basin have a lower compressional strength than lower plate margins such as the Møre Basin, and therefore preferentially developed inversion structures. Along the transform boundaries separating the domains, additional stress probably built up along extension of the transform zones into the extended continental crust. This additional stress probably also assisted initiation of the inversion structures in the Vøring Basin and the Faeroes area. The amplification of the inversion structures in the Vøring Basin and the Faeroes Basin was subsequently caused by a variety of processes related to sedimentation and uplift–erosion.

Preliminary analysis of the Knipovich Ridge segmentation: influence of focused magmatism and ridge obliquity on an ultraslow spreading system

Bathymetry, gravity and deep-tow sonar image data are used to define the segmentation of a 400 km long portion of the ultraslow-spreading Knipovich Ridge in the Norwegian–Greenland Sea, Northeast Atlantic Ocean. Discrete volcanic centers marked by large volcanic constructions and accompanying short wavelength mantle Bouguer anomaly (MBA) lows generally resemble those of the Gakkel Ridge and the easternmost Southwest Indian Ridge. These magmatically robust segment centers are regularly spaced about 85–100 km apart along the ridge, and are characterized by accumulated hummocky terrain, high relief, off-axis seamount chains and significant MBA lows. We suggest that these eruptive centers correspond to areas of enhanced magma flux, and that their spacing reflects the geometry of underlying mantle upwelling cells. The large-scale thermal structure of the mantle primarily controls discrete and focused magmatism, and the relatively wide spacing of these segments may reflect cool mantle beneath the ridge. Segment centers along the southern Knipovich Ridge are characterized by lower relief and smaller MBA anomalies than along the northern section of the ridge. This suggests that ridge obliquity is a secondary control on ridge construction on the Knipovich Ridge, as the obliquity changes from 35° to 49° from north to south, respectively, while spreading rate and axial depth remain approximately constant. The increased obliquity may contribute to decreased effective spreading rates, lower upwelling magma velocity and melt formation, and limited horizontal dike propagation near the surface. We also identify small, magmatically weaker segments with low relief, little or no MBA anomaly, and no off-axis expression. We suggest that these segments are either fed by lateral melt migration from adjacent magmatically stronger segments or represent smaller, discrete mantle upwelling centers with short-lived melt supply.

Depositional environment and source rock potential of Miocene strata from the central Fram Strait: introduction of a new computing tool for simulating organic facies variations

Organic-rich sediments were recognized in early Miocene strata from the Norwegian-Greenland Sea during ODP Leg 151. Three organic-geochemical subunits were distinguished in Hole 909C using detailed organic-geochemical and microscopic analyses. TOC (up to 3 wt%), HI (up to 200 mg HC/g TOC), and δ 13C org values (∼24–26‰) indicate the predominance of terrestrial type III organic matter in Subunit 3 (∼18–16.2 Ma). Biomarker and vitrinite reflectance values ( R 0∼0.5%) point to rather fresh immature terrestrial organic matter supplied by river discharge from adjacent vegetated coastal areas. Although the sediments of Subunit 3 have a fair (to good) source rock potential they are insufficiently mature to generate significant amounts of oil or gas. To test if the moderate generation potential and source rock quality of Subunit 3 are applicable to the entire area, the computer software OF-Mod was applied. Several modelling runs testing the most probable depositional scenarios result, even under conservative assumptions, in the formation of good to excellent source rocks towards the basin margins.

Deglaciation and palaeoclimate of the Andfjord‐Vågsfjord area, North Norway

This paper reviews the deglaciation history and palaeoclimate from 22 to 9.5 14Cka BP in the Andfjord‐Vagsfjord area. Eight main glacial events are recorded: The Egga‐I (>22 14Cka BP), the Bjerka, the Egga‐II (>14.6 14Cka BP), the Flesen (14.5 14Cka BP), the D (13.8–13.2 14Cka BP), the Skarpnes (12.2 14Cka BP), the Tromsø–Lyngen (10.7–10.3 14C ka BP) and the Stordal (10.0–9.5 14Cka BP). Onset of the final deglaciation occurred about 14.6 14Cka BP. Most of the western part of the Fennoscandian and Barents Sea Ice Sheets receded from the outer continental shelf 15–14 14Cka BP. The delivery and melting of icebergs at this time to the Norwegian‐Greenland Sea resulted in a low oxygen isotope event recorded in a number of cores in the region. Atlantic water intruded the area 13.2 14Cka BP, and an atmospheric warming commenced 12.9/12.8 14Cka BP. A marked glacial recession occurred before the Skarpnes event. During Allerød time, the glaciers retreated to the fjord heads or even farther inland. The Fennoscandian outlet glaciers readvanced (locally more than 40 km), reached their Younger Dryas outer limit after 10.7 14Cka BP and retreated from this position before about 10.3 14Cka BP.

Late Weichselian depositional processes, fluxes, and sediment volumes on the margins of the Norwegian Sea (62–75°N)

Quaternary continental margin sedimentation in the Norwegian–Greenland Sea has been a response to a combination of tectonic, oceanographic, and glacial activity. Using geophysical survey data (6.5 kHz side-scan sonar and 3.5 kHz penetration echo sounding), in association with bathymetric datasets, the magnitude and frequency of the main forms of margin sedimentation into the Lofoten and Norwegian Basins of the Norwegian Sea during the Late Weichselian and Holocene are estimated. By comparing these geophysically determined estimates of sediment fluxes with numerically modelled ice-sheet sediment delivery, the role of ice-sheets in shaping the processes acting on this margin is assessed. Sediment flux at the shelf edge during the Late Quaternary is dominated by ice-sheet delivery of sediments, focused at the mouths of fast-flowing ice streams formed in bathymetric troughs. Interglacials are, by contrast, characterised by comparatively little sediment accumulation. Submarine fans, at the margins of cross-shelf troughs, are major depocentres on this glacier-influenced margin. However, occasional large-scale failures of the continental slope account for approximately 75% of the basin sediment accumulation during the past 30 000 yr, by eroding substantial quantities of pre-Quaternary deposits. Trough mouth fan and a combination of hemipelagic and glacimarine processes are responsible for the accumulation of the remaining 15% and 7% of basin sediments, respectively.

The Millstone Grit of northern England: a response to tectonic evolution of a northern sourceland

SUMMARY The Carboniferous succession of East Greenland is dominated by sandstones, deposited by a major northward-flowing fluvial system. The succession is divided into two parts by a major unconformity that spans late Visean to early Westphalian A times. This time interval corresponds to the development of the Millstone Grit of the Pennine Basin, which was deposited by a southward-flowing fluvial system. Heavy mineral analysis indicates that there are common elements to the provenance of Late Westphalian A sandstones in East Greenland and the Namurian Millstone Grit. It appears, therefore, that the southward extension of clastic facies into the Pennine Basin during the Namurian occurred when uplift in the Norwegian–Greenland Sea region diverted the northward-flowing fluvial system southwards. Supply of sediment from the northern sourceland diminished when rifting during the Westphalian allowed the northward fluvial system in East Greenland to be re-established.

Carbonate preservation records of the past 3 Myr in the Norwegian–Greenland Sea and the northern North Atlantic: implications for the history of NADW production

Carbonate preservation records from a number of drill sites in the North Atlantic and adjacent Norwegian–Greenland Sea (NGS) are used to reconstruct variations in North Atlantic Deep Water (NADW) production over the past 3 Myr. Before the initiation of major Northern Hemisphere glaciation, good carbonate preservation was recorded in the North Atlantic and the NGS supporting the superconveyor model of accelerated NADW formation in the late Pliocene. After the inception of main Northern Hemisphere glaciation, NADW formation in the NGS was blocked-off during the period 2.8–1.9 Ma. Carbonate was only badly preserved due to (1) low production of carbonate shells in surface waters, (2) sluggish renewal of deep waters induced by a rather stable sea-ice cover, and/or (3) production of carbonate-corrosive dense brines during sea-ice formation (e.g. sea ice dissolution mode). In contrast, contemporaneous good preservation in the adjacent North Atlantic indicates efficient NADW production. During the middle Matuyama (1.9–1.4 Ma), the first intrusions of the Proto-Norwegian Current into a narrow corridor in the southeastern NGS are evidenced by much better carbonate preservation. A decrease both in carbonate content and preservation towards the north and west of this corridor indicates the proximity of the polar front and gives evidence that NADW production was efficiently triggered by the Atlantic water entrainment mode. During the past 1.2 Myr carbonate preservation patterns in the NGS clearly reflect major global events like the Mid-Pleistocene Transition and the mid-Brunhes dissolution event. The onset of the Mid-Pleistocene Transition at 1.2 Ma is characterized by a complete shift to higher carbonate contents in the southeastern NGS and Labrador Sea. Overall, good preservation during both interglacials and glacials is only interrupted by high-frequency, short-term dissolution spikes, which were induced by ice sheet collapse and development of extensive meltwater lids. As a consequence, NADW was produced nearly continuously during glacials in the Nordic Seas. However, due to its lower density it was entrained into intermediate water levels in the North Atlantic and, thus, enforced the intermediate water circulation loop, whereas a decrease in lower-NADW production is observed contemporaneously.

Sediment reworking on high-latitude continental margins and its implications for palaeoceanographic studies: insights from the Norwegian-Greenland Sea

Abstract Geological evidence indicates that sediment reworking is common around the continental margins and abyssal depths of the Norwegian-Greenland Sea, a high-latitude setting with glacier-influenced margins. Detailed analysis of 22 cores up to 5 m long, placed in context by accompanying geophysical data including high resolution sub-bottom profiles, swath bathymetry and backscatter maps, indicates that reworking is variable and ranges from debris flows and turbidity currents, to bottom-current activity, as well as iceberg scouring. Reworking by debris flows appears to be restricted mainly to the main trough-mouth fans and sediment slides. Elsewhere, turbidity-current activity frequently dominates, although iceberg ploughing down to 600 m depth and current winnowing assume increasing significance on continental shelves. Reworking in the Norwegian-Greenland Sea reflects variations in ice-sheet dynamics that, in turn, influence the rate of sediment delivery and location of depocentres. Spatial variations in the style of reworking may also reflect the influence of continental slope gradient and bedrock geology on continental shelves. The widespread nature of sediment reworking has important implications for palaeoceanographic investigations in the region, as reworking can result in erosion and disturbance of the sediment column. It is estimated that less than 7% of material delivered to the Norwegian-Greenland Sea since the Late Weichselian is derived from hemipelagic and pelagic sedimentation. This problem is significant where continuous, high-resolution records of hemipelagic and pelagic sedimentation are required, and attempts are made to correlate with other high-resolution proxy records, such as ice cores, at sub-millennial scales. Bioturbation results in the smoothing of high-resolution records and imposes a maximum resolution for sediment-core time-slices of generally 400 years or more. In the Norwegian-Greenland Sea, areas of high sedimentation such as trough-mouth fans or contourite drifts are commonly associated with extensive reworking. Identification of reworking is particularly important where attempts are made to link records of iceberg-rafted debris to past ice-sheet dynamics, as bottom-current winnowing and mass-flow processes can increase the concentration of coarse-grained iceberg-rafted debris. Such localized accentuation of the iceberg-rafted debris signal may lead to erroneous palaeo-environmental interpretations. It is therefore critical that palaeoceanographic interpretations are firmly underpinned by an explicit sedimentological assessment of reworking.

Early Eocene sea floor spreading and continent-ocean boundary between Jan Mayen and Senja fracture zones in the Norwegian-Greenland Sea

Sea floor spreading anomalies in the Lofoten-Greenland basins reveal an unstable plate boundary characterized by several small-offset transforms for a period of ∼4 m.y. after opening. North of the Jan Mayen Fracture Zone, integrated analysis of magnetic and seismic data also document a distinct, persistent magnetic anomaly associated with the continent-ocean boundary and a locally, robust anomaly along the inner boundary of the break-up lavas. These results provide improved constraints on early opening plate reconstructions, which include a new anomaly 23-to-opening pole of rotation yielding more northerly relative motion vectors than previously recognized; and a solution of the enigmatic, azimuthal difference between the conjugate Eocene parts of the Greenland-Senja Fracture Zone if the Greenland Ridge is considered a continental sliver. The results confirm high, 2.36–2.40 cm yr−1, early opening spreading rates, and are consistent with the start of sea floor spreading during Chron 24r. The potential field data along the landward prolongations of the Bivrost Fracture Zone suggest that its location is determined by a Mesozoic transfer system which has acted as a first-order, across-margin tectono-magmatic boundary between the regional Jan Mayen and Greenland-Senja Fracture Zone systems, greatly influencing the pre-, syn- and post-breakup margin development.

On the architecture of high-latitude continental margins: the influence of ice-sheet and sea-ice processes in the Polar North Atlantic

Abstract The presence of ice during the Late Cenozoic distinguishes the nature and rates of processes on high-latitude margins from those elsewhere. Ice sheets terminating in marine waters deliver icebergs, meltwater and debris to high-latitude seas. Sea ice influences ocean salinity structure and downslope water and sediment transfer, and also transports fine-grained sediments over long distances. These cryospheric processes have led to the development of a distinctive sedimentary architecture on modern high-latitude continental margins. Large submarine fans made up almost entirely of stacked debris flows are present around the Norwegian-Greenland Sea. Large slides are located in a variety of settings relative to rates of sediment delivery from Quaternary ice-sheet margins, but no large slides have been mapped on the East Greenland margin. However, extensive channel systems and sediment-wave fields are present in the Greenland Basin, probably related to intermittent downslope flow of dense water and turbidity currents. The extensive NE Greenland shelf was not innundated by ice-sheet advance during recent full-glacial conditions, allowing sea-ice and deep-water production during both interglacials and full-glacials. Changes in the nature and rate of sedimentation within the Greenland Basin should provide clues on the rate of dense-water production, with implications for thermohaline circulation in the North Atlantic. Other erosional and depositional features on the Norwegian-Greenland Sea margins include canyons and contourite drifts. High-relief tectonic features influence sediment reworking by turbidity currents at abyssal depths. A simple conceptual model for glacier-influenced marine sedimentation summarizes the role of cryospheric processes in high-latitude margin sedimentary environments.

Mid-Cenozoic post-breakup deformation in the ‘passive’ margins bordering the Norwegian–Greenland Sea

Since the initial breakup of the Norwegian–Greenland Sea in Early Eocene time (Chron 24B, 53.7Ma), the conjugate ‘passive’ margins have been subject to compression, extension, magmatism, and broad uplifts and subsidence. Mid- to Late Cenozoic compressional doming basin inversion and reverse faulting occurred predominantly offshore within the rheologically weak deep Cretaceous depocenters. On the Norwegian margin, the domes seem to have formed primarily in Middle Eocene to Early Oligocene, and in Early Miocene, while doming in the Faroes–Rockall region developed in Late Paleocene to Early Eocene, Oligocene, and Mid- to Late Miocene. We suggest that the main driving force was plume-enhanced ridge push. A testable working hypothesis is that discrete phases of compression were associated with periods of high plume flux. In addition, the Oligocene (Chron 13, 35Ma) change in plate motion appears to have generated local shear adjustments along pre-existing basement weaknesses, forming compressional structures in the overlying sedimentary section. This change in plate motion was first manifest by significant extension between the SW Barents Sea and the conjugate NNE Greenland margin, and led to spreading along the Knipovitch Ridge. Mid-Cenozoic extension and magmatism also preceded the development of the Kolbeinsey Ridge. The associated separation of the Jan Mayen microcontinent from East Greenland must have been strongly influenced by the position of the Iceland Plume center, in addition to the change in extension direction. With respect to initial breakup of the Norwegian–Greenland Sea in Early Eocene time, the mid-Cenozoic extension and magmatism is a post-breakup phenomena. Widely spaced areas of Neogene uplift occurred along all bordering margins. These enigmatic uplifted areas acted as nucleation sites for growth of ice caps, and initiated large glacial deposits that built out on the adjacent shelves.

Multicentury-Scale Records of Carbonate (Hydrographic?) Variability on the Northern Iceland Margin over the Last 5000 Years

The waters off northern Iceland are subjected to extreme hydrographic variability on annual to decadal timescales. In years when cold low-salinity water moves coastward and sea ice is prevalent (i.e., the late 1960s), marine productivity of surface waters is low because the water column is well stratified. In the opposite oceanographic mode, warm, salty Atlantic Water dominates the shelf and vertical mixing results in high productivity. We track these two contrasting modes by measuring the carbonate content of marine sediments, a proxy for productivity, in three cores from northern Iceland. The fjord record (B997-328PC) is anchored by eleven 14C AMS dates and has a resolution of one sample every 50 yr. Thirteen oscillations occurred over the last 4800 cal yr with an average length of 370 yr; these are superimposed on a long-term decrease in net carbonate accumulation. The Little Ice Age is marked by the largest decrease in carbonate content and in flux, with smaller minima at 2300 and 3000 cal yr B.P. Marked peaks in carbonate (optima) occurred at 2000 and 3800 cal yr B.P. The carbonate record from B997-328PC can be correlated with records on the inner shelf (B997-330PC) and midshelf (B997-327PC), indicating that significant regional changes in oceanography occurred at the southern margin of the Norwegian–Greenland Sea. The marine carbonate fluctuations closely track temperature reconstructions from the Greenland Summit site for the last 2000 cal yr and show similar, but slightly offset, oscillations between 2000 and ca. 4800 cal yr B.P.

The role of the Spitsbergen shear zone in determining morphology, segmentation and evolution of the Knipovich Ridge

In 1989–1990 the SeaMARC II side-looking sonar and swath bathymetric system imaged more than 80 000 km2 of the seafloor in the Norwegian-Greenland Sea and southern Arctic Ocean. One of our main goals was to investigate the morphotectonic evolution of the ultra-slow spreading Knipovich Ridge from its oblique (115° ) intersection with the Mohns Ridge in the south to its boundary with the Molloy Transform Fault in the north, and to determine whether or not the ancient Spitsbergen Shear Zone continued to play any involvement in the rise axis evolution and segmentation. Structural evidence for ongoing northward rift propagation of the Mohns Ridge into the ancient Spitsbergen Shear Zone (forming the Knipovich Ridge in the process) includes ancient deactivated and migrated transforms, subtle V-shaped-oriented flank faults which have their apex at the present day Molloy Transform, and rift related faults that extend north of the present Molloy Transform Fault. The Knipovich Ridge is segmented into distinct elongate basins; the bathymetric inverse of the very-slow spreading Reykjanes Ridge to the south. Three major fault directions are detected: the N-S oriented rift walls, the highly oblique en-echelon faults, which reside in the rift valley, and the structures, defining the orientation of many of the axial highs, which are oblique to both the rift walls and the faults in the axial rift valley. The segmentation of this slow spreading center is dominated by quasi stationary, focused magma centers creating (axial highs) located between long oblique rift basins. Present day segment discontinuities on the Knipovich Ridge are aligned along highly oblique, probably strike-slip faults, which could have been created in response to rotating shear couples within zones of transtension across the multiple faults of the Spitsbergen Shear Zone. Fault interaction between major strike slip shears may have lead to the formation of en-echelon pull apart basins. The curved stress trajectories create arcuate faults and subsiding elongate basins while focusing most of the volcanism through the boundary faults. As a result, the Knipovich Ridge is characterized by Underlapping magma centers, with long oblique rifts. This style of basin-dominated segmentation probably evolved in a simple shear detachment fault environment which led to the extreme morphotectonic and geophysical asymmetries across the rise axis. The influence of the Spitsbergen Shear Zone on the evolution of the Knipovich Ridge is the primary reason that the segment discontinuities are predominantly volcanic. Fault orientation data suggest that different extension directions along the Knipovich Ridge and Mohns Ridge (280° vs. 330°, respectively) cause the crust on the western side of the intersection of these two ridges to buckle and uplift via compression as is evidenced by the uplifted western wall province and the large 60 mGal free air gravity anomalies in this area. In addition, the structural data suggest that the northwards propagation of the spreading center is ongoing and that a `normal' pure shear spreading regime has not evolved along this ridge.

A recipe for microcontinent formation

Research Article| March 01, 2001 A recipe for microcontinent formation R. Dietmar Müller; R. Dietmar Müller 1University of Sydney, School of Geosciences, Building F05, Sydney, New South Wales 2006, Australia Search for other works by this author on: GSW Google Scholar Carmen Gaina; Carmen Gaina 1University of Sydney, School of Geosciences, Building F05, Sydney, New South Wales 2006, Australia Search for other works by this author on: GSW Google Scholar Walter R. Roest; Walter R. Roest 2Geological Survey of Canada, 615 Booth Street, Ottawa, Ontario K1A 0E9, Canada Search for other works by this author on: GSW Google Scholar David Lundbek Hansen David Lundbek Hansen 3Department of Earth Sciences, University of Aarhus, DK 8200 Aarhus N., Denmark Search for other works by this author on: GSW Google Scholar Geology (2001) 29 (3): 203–206. https://doi.org/10.1130/0091-7613(2001)029<0203:ARFMF>2.0.CO;2 Article history received: 11 Aug 2000 rev-recd: 04 Dec 2000 accepted: 04 Dec 2000 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation R. Dietmar Müller, Carmen Gaina, Walter R. Roest, David Lundbek Hansen; A recipe for microcontinent formation. Geology 2001;; 29 (3): 203–206. doi: https://doi.org/10.1130/0091-7613(2001)029<0203:ARFMF>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyGeology Search Advanced Search Abstract Accreted slivers of continental margins are common in the geologic record, but the processes that lead to their formation are poorly understood. We observe an association of plume-related microcontinent isolation and subsequent long-term asymmetries in oceanic crustal accretion based on four recent examples: the Seychelles in the Indian Ocean, Jan Mayen in the Norwegian-Greenland Sea, and the East Tasman Plateau and the Gilbert Seamount Complex in the Tasman Sea. These microcontinents formed by rerifting of a young continental margin (<25 m.y. old) in the vicinity of a mantle-plume stem, followed by asymmetric seafloor spreading. Two-dimensional numerical stochastic basin modeling suggests that a yield-strength minimum along the landward edge of a rifted margin, thermally enhanced by heating from a mantle plume, may cause a spreading ridge to jump onto this zone of weakness. This action isolates a passive-margin segment. The association of large igneous provinces and microcontinents should be useful for identifying similar events in the geological record. You do not currently have access to this article.

Benthic foraminifera and sediment grain size variability at intermediate water depths in the Northeast Atlantic during the late Pliocene–early Pleistocene

Abstract Sedimentological and faunal records from the transitional period marking the onset of widespread northern hemisphere glaciation have been investigated at Ocean Drilling Program Site 984. The late Pliocene interglacial sediments of the northeast Atlantic are carbonate rich and show evidence of vigorous bottom water circulation at intermediate water depths. Contrasting this, the late Pliocene glacial sediments are characterised by carbonate dissolution and slower bottom current velocities. Weak or “leaky” Norwegian Sea overflows, undersaturated with respect to carbonate, influenced this region during the late Pliocene glacials. The early Pleistocene pattern of intermediate water circulation appears to have changed radically in the northeast Atlantic. At this time, interglacial carbonate values and inferred bottom current velocities are low. This suggests slow-flowing, undersaturated Norwegian Sea water bathing the site. The overflow increased during the early Pleistocene interglacials as the exchange between the Atlantic and Norwegian-Greenland Seas improved. The most significant feature of the early Pleistocene glacials is the increase in inferred bottom current velocity. These changes reflect a switch in deep North Atlantic convection to shallower depths during glacial periods, possibly in a manner similar to the increasing contribution of glacial intermediate water to the North Atlantic during the late Pleistocene glacials. Our results suggest that the late Pleistocene climate variability of the North Atlantic is a pervasive feature of the late Pliocene–early Pleistocene record.

Inferring Maximum Geostrophic Current Velocities in the Norwegian-Greenland Sea from Settling-Velocity Measurements of Sediment Surface Samples: Methods, Application, and Results

ABSTRACT Critical shear velocities and geostrophic current velocities were calculated from measured grain sizes and settling velocities using a Shields-type threshold curve of particle movement and assuming bedload transport for particles that were found to be transported by currents. Because this reconstruction of the current velocities is based on the coarsest transported component that could be recognized in the sediment, the resulting current velocities represent a maximum flow speed that was effective during deposition of sediment. The shape of the settling-velocity distributions identifies the dominant type of deposition that controlled the accumulation of sediment: residual, well-sorted, or low-energy depositional. The resulting pattern of reconstructed maximum geostrophic current velocities in the Norwegian-Greenland Sea reflects the known features of present-day thermohaline circulation, which is influenced by the bathymetry at many places. Reconstructed current velocities are higher in areas where the cross section of meridional flows is restricted by topographic barriers. The distribution of the depositional characters shows that residual and well-sorted sediment types are generally associated with higher current velocities in areas of strong geostrophic currents (e.g., Fram Strait and East Greenland continental slope). The low-energy depositional sediment type is widespread in the basins of the eastern Norwegian-Greenland Sea, where lower reconstructed maximum current velocities occur. The high reconstructed velocities on the Spitsbergen Bank can be attributed to the disturbance from storm waves in shallower water depths.