Newfoundland Ridge Research Articles

Mesoscale variabilities and gulf stream bifurcation in the Newfoundland basin observed by the Geosat altimeter data

Abstract Three years of analysed Geosat altimeter data exhibit mesoscale meanders and eddies in the bifurcation region of the Gulf Stream (GS) into the North Atlantic (NAC) and Azores (AZC) currents over the Newfoundland ridge and Basin. The decorrelation scales of the sea‐surface height variabilities are ∼50 d in time, ∼120 km in the along‐track direction and ∼150 km in the cross‐track direction. Although significant westward propagation is observed in Fourier analysis at speeds of 3 to ∼15 cm s‐1 near the current axes, some individual mesoscale features are chaotic: they are sometimes persistent for several repeat cycles (∼100 d) and then suddenly propagate westward or disappear within one cycle. The GS bifurcation is enhanced by cross‐track smoothing, which minimizes mesoscale features resolved by only one track but retains the larger scale features. The surface geostrophic velocity field with the cross‐track smoothing provides the following description of the bifurcation: the GS starts to split near t...

The deep western boundary undercurrent off the Newfoundland ridge

Overflow water from the Denmark Straits first encounters water of its own density at the tip of the Grand Banks of Newfoundland at the northeastern end of the Sohm Abyssal Plain. Here the θ-S anomally of the overflow water acts as a dye enabling description of its mixing into the North Atlantic Deep Water. This mixing includes streaks at least 10 km long but less than 1 km in width as well as hemogenous structures as large as 100 km. A strong deep western boundary undercurrent meandering along isobaths to the west at 15–20 cm s −1 was observed at three current meter/thermistor string arrays. These arrays were deployed for 13 months and along with CTD and hydrographic sections form the data set.

Effects of lithospheric flow on the formation and evolution of a transform margin

A simple model is developed for the processes occurring at a transform continental margin and its predictions are compared with observation. The model emphasises the importance of ductile flow in the lithosphere near the transform. At initiation within continental lithosphere, the upper part of the crust is assumed to be brittle while the lower crust and mantle behave as a Newtonian fluid. Shearing along the transform induces a flow parallel to the direction of motion. With the approach and passage of the abutting spreading centre, flow beneath the continental side of the transform slows and stops. This results in thinning of the lower crust and lithosphere as material is dragged away by the moving oceanic plate. The thinning produces an initial thermal uplift combined with isostatic subsidence, followed by thermal relaxation and subsidence, at and near the margin. The model is developed for uniform viscosity and for viscosity that decays exponentially with depth. The results are found to be generally consistent with the observed structure at transform margins, in particular at the Southwest Newfoundland Transform Margin. It is suggested that the Newfoundland Ridge and analogous features may be formed partly from continental crust emplaced by this mechanism.

Upper Triassic—Lower Jurassic salt basin southeast of the Grand Banks

A grid of multichannel seismic reflection (MCS) profiles has been used to delineate and map a large, apparently tectonically controlled basin and associated diapiric province beneath the continental slope and rise southeast of the Grand Banks of Newfoundland. The depocenter, for which we propose the name Salar basin, is defined both landward and seaward by pronounced hinges in acoustic basement. It is approximately 400 km long, 35–70 km wide, and covers an area of approximately 20,000 km 2. The southern end of the basin is marked by the intersection of the Southeast Newfoundland Ridge with the South Bank High on the Grand Banks, and the northern end is bounded by the southwestern margin of Flemish Cap. The Salar basin generally parallels the much smaller Carson basin, which lies beneath the Grand Banks immediately to the west, and it merges with this basin north of 45°22′N. Based upon correlation with contiguous deposits sampled by drilling in the Carson basin, we infer piercements filling the Salar basin to be mobilized Late Triassic-Early Jurassic evaporites, probably predominantly halite. A consequent early Mesozoic age for the Salar basin implies a multi-stage rifting history for the Newfoundland Basin, as has been documented for the adjacent Grand Banks. Initial extension, which probably formed the Salar basin's bounding basement hinge zones, occurred in Late Triassic—Early Jurassic time, and was associated with rifting of North America from Africa and extension in western Europe. The presence of early Mesozoic evaporites in a basin southeast of the Grand Banks suggests development of a significant, but shallow and restricted, seaway between the Grand Banks and the Iberian peninsula, probably connected northward with coeval salt provinces of northwestern Europe. A second rift phase related to separation of Iberia and the Grand Banks affected the Grand Banks and Salar basin in Late Jurassic to Early Cretaceous time, after a period of epeirogenic subsidence during the Early and Middle Jurassic. Uplift associated with this extension led to the development of the widespread “U” (Avalon) unconformity across the Grand Banks, the Salar basin and the Newfoundland Basin out to the J-Anomaly and may have induced the first halokinesis in the Salar basin. The second rift phase culminated in the initiation of seafloor spreading in mid-Early Cretaceous (Aptian) time at a location now marked by the J-Anomaly.

Late Quaternary plume, nepheloid and turbidite sedimentation and effect of the Gulf Stream near the tail of the Grand Banks, Newfoundland

Cores collected from the tops of the Fogo Seamounts southwest of the Grand Banks of Newfoundland contain considerable amounts of detrital clay. The seamounts rise at least 1 km above the surrounding sea floor and are an unlikely place for sedimentation by turbidity currents. Sedimentological and micropaleontological evidence suggests that large amounts of detrital clay were injected into the system during glacial and interglacial stages by turbidity currents and surface sediment plumes. The main source of sediment was the Laurentian Channel, although some sediment was injected through the canyons around the Grand Banks. Surface sediment plumes were strongly influenced by the Labrador Current and by the surface circulation of the Gulf Stream. Fine-grained sediments injected into the system by turbidity currents through the Laurentian Channel were probably trapped by the deep circulation of the Gulf Stream and dispersed to the northeast. Effects of the sediment drift induced by the deep Gulf Stream circulation is also reflected in an abrupt eastward bent of the depositional lobes of the Laurentian Fan and in the shape of the Newfoundland Ridge.

Current Variability near the Southeast Newfoundland Ridge

Abstract A cooperative moored array experiment to measure currents and temperatures in the vicinity of the Southeast Newfoundland Ridge was carried out over a 14-manth period starting September 1979 and ending December 1980. Measurements were obtained from an incoherent exploratory army of nine moorings instrumented at 500, 800, 1500 and 4000 m levels. The objectives were to explore the amplitude, spatial variability and time scales of the eddy field in the decay region of the Gulf Stream. An additional experiment and mooring to examine the influx of Norwegian Sea Overflow Water into the western North Atlantic Basin was imbedded in the array. Supplementary CTD and XBT profiles and a single current profile were taken during deployment and recovery cruises to aid in interpretation of the moored measurements and identification of water masses. A description of the array experiment and preliminary results are reported. Mean velocities were weak compared with rms eddy velocities and were not determined with st...

Two Views of the Cold Filament

Abstract Two views of the Cold Filament, first described by Weatherly and Kelley, are presented. The first is a local view near 40°N, 62°W. There its upslope edge is found to be a front which by benthic standards is large (its downslope edge was not sampled). What distinguishes this benthic front from others is that it is a permanent feature in the abyssal ocean. Above the Cold Filament, relatively murky detached bottom layers were observed and tracked to where they separated from the bottom at the benthic front. Apparently these detached layers entrain overlying water (a density jump at their base apparently restricts entrainment of underlying water) primarily during the detachment process with comparably less entrainment thereafter. The second view, a regional one, comes from examining historical hydrographic sections. These indicate that the Cold Filament extends from the Newfoundland Ridge westward then southward to 24°N and possibly to ∼20°N along the base of the continental rise. The Cold Filament i...

The Newfoundland Basin: ocean-continent boundary and Mesozoic seafloor spreading history

The ocean-continent boundary in the Newfoundland Basin is defined as the seaward limit of a continental margin magnetic smooth zone. East of the Grand Banks this boundary is marked by a prominent NNE-trending magnetic anomaly that is correlated with the J-Anomaly (115 m.y.). South of Flemish Cap the smooth zone boundary strikes approximately 060° and is approximately 15 m.y. younger. Magnetic anomaly trends suggest two directions of motion during separation of Iberia and North America. The first phase of motion, commencing at J-Anomaly time with a spreading center strike of 015°, produced a rifted margin along the Grand Banks south of the Newfoundland Seamounts. No spreading occurred north of the seamounts during this phase, implying a counter-clockwise rotation of Iberia and no Grand Banks-Galicia Bank separation. The second phase began at about 102 m.y. with a shift of the pole of rotation to a location near Paris, producing a ridge orientation of approximately 060°. This spreading center extended north and east into the northern Newfoundland Basin and Bay of Biscay, producing a rifted margin south of Flemish Cap and opening of Biscay. This ridge geometry produced a component of extension across the Newfoundland Fracture Zone and the southeastward migration of the resultant “leaky” transform fault between 102 m.y. and the next pole shift produced the volcanic edifice of the Southeast Newfoundland Ridge. Fracture zone trends during this phase also exerted strong control on volcanism within the Newfoundland Seamount province; this activity ceased at about 97 m.y. The date at which the second phase ended is not well defined by presently available data. A RRR triple-junction existed in the northeastern Newfoundland Basin-western Biscay region for a short time prior to anomaly 33/34 (80 m.y.) which marks the inception of a continuous Mid-Atlantic Ridge spreading center between the Newfoundland and Charlie Gibbs Fracture Zones.

Measurements of Sea-State Variations Across Oceanic Fronts Using Laser Profilometry

Abstract As part of the Grand Banks Experiment in May 1979, airborne laser profilometer measurements of the ocean wave field were made across a large cold-water extrusion situated over the Newfoundland Ridge. The feature is actually an extension of the Labrador Current which is bordered on the west side by the Gulf Stream and on the east side by the North Atlantic Current. Star-shaped flight patterns were flown over the fronts on each side of the cold-water feature. A graphic technique was applied to the apparent wavenumber spectra in order to determine the changes in wave energy, wavelength and direction of propagation of the dominant wind-wave and swell components as they move across the fronts. At the western front, the sea state increased abruptly and the results indicate that wave-current interactions were the most important mechanism for wave modification although boundary-layer effects were present and increased wave breaking was observed. At the eastern front, changes in the swell are compared to ...

Geophysical observations bearing upon the origin of the newfoundland ridge

Multichannel reflection seismic profiles extending southward from the Grand Banks show gently dipping reflectors within “basement” features underlying the Newfoundland Ridge. These reflections appear to be from sedimentary strata, indicating that the Newfoundland Ridge is a remnant of a former sedimentary basin, rather than a ridge of oceanic crust as prescribed by plate tectonic models. Probably this feature is underlain, and to some extent surrounded by, continental crust.

On the nature of the crust in the vicinity of the southeast Newfoundland Ridge

This paper presents new seismic reflection, refraction, gravity, and magnetic data bearing on the nature of the crust in the vicinity of the Newfoundland Ridge and the J-anomaly Ridge, immediately south of the Grand Banks. This area experienced a complicated plate tectonic history being the focal point for interactions of the North American, African, and Iberian plates. New data have recently been published for this region and conflicting interpretations have been offered in relation to the oceanic or continental origin of the crust there. The data presented here show that the seismic structure and the most reasonable models for the magnetic anomalies are more consistent with an oceanic origin. The trends and offsets in the magnetic lineations and possible differences in subsidence, north and south of the Newfoundland Ridge, are discussed in relation to possible modes of formation of this feature. It is proposed that similar subsidence histories since mid-Cretaceous time on the Grand Banks and J-anomaly Ridge are related to a similarity in the thermal history of the lithosphere beneath these areas, as the ridge crest migrated eastwards, and do not require the same type of crust to underlie both areas.

An abyssal shear zone

At Geochemical Ocean Sections Study station 28, in the basin east of the Newfoundland Ridge, dramatic evidence for the ‘underthrusting’ of the benthic boundary layer was encountered. Results from the continuous temperature, salinity, and nephel measurements and from discrete water samples are presented in order to document that strong shearing within the near‐bottom water column must occur in this area. These observations amplify previous observations that Denmark Strait overflow water penetrates as a thin wedge far to the south of its place of origin.

Multichannel seismic reflection profiles of the continental crust beneath the Newfoundland Ridge

Multichannel seismic reflection profiles extending south from the Grand Banks indicate that large ‘basement’ features underlying the Newfoundland Ridge are composed of sedimentary strata. The Newfoundland Ridge, therefore, is interpreted as an area of subsided continental crust rather than a ridge of oceanic crust. The J magnetic anomaly intersects this complex, and the physiographic ridge associated with this anomaly may also be continental in origin.

Sedimentary basins of the atlantic margin of North America

Scismic exploration has identified eight distinct basin structures along the North American Atlantic continental margin forming a chain of elongate depocenters parallel to the continental slope and interrupted by transverse basement arches and impinging oceanic fracture zones. From south to north these are: South Florida—Bahamas Basin bounded on the north by Peninsular Arch and Bahama Escarpment fracture zone; Blake Plateau Basin with Cape Fear Arch and the impinging Great Abaco and Blake Spur fracture zones; Baltimore Canyon Trough bounded by the Long Island Platform and impinging Kelvin fracture zone; Georges Bank Basin with the bounding Yarmouth Arch; Scotian Shelf Basin with Scartarie and Canso Ridges and impinging Newfoundland Ridge fracture zone; Grand Banks Troughs and the intervening horst ridges; and the East Newfoundland Basin separated by Cartwright Arch and the impinging Gibbs fracture zone from the Labrador Shelf Basin. All the basins are characterized by great depths to basement filled with from 7 to 14 km of possible Triassic, Jurassic, Cretaceous and Tertiary sediments. Basement faulting controls the basins' boundaries and the faults have affected the overlying sediments. The major boundary faults of the basins undoubtedly formed during the initial rifting of the Atlantic margin in the Jurassic or perhaps Triassic. However, throughout the Mesozoic and Cenozoic these basement faults have moved in response to different orientations of stress and strain rates produced by continued spreading of the Atlantic Ocean. As a result, the basement faults of the Atlantic Margin were apparently influenced by at least three different local stress systems, spatially overlapping but temporally independent. These are the east—west extensional Atlantic Ocean stress system, the northwest—southeast extensional White Mountain stress system, and the north-south extensional Labrador Sea stress system. Some consequences of this basic tectonic setting were differential cross-strike tilts of the basin blocks with each basin moving somewhat independent of its neighbor. The resulting buildup of the basins' sedimentary geometries reflect these tectonic tilts and varying strain rates. Correlations are found between changes in orientation and rates of Atlantic sea-floor spreading with observed major sedimentary events such as progradations, planar bedding episodes, reef platform development, regressive hiatuses, and transgressions. An understanding of this marginal geosyncline could yield a model with predictability.

Sédimentation sur la Ride de Terre-Neuve

The stratification, in part more or less rhythmic, of a fifteen metre long core of predominantly hemi-pelagic sediment from the northern slope of the Southeast Newfoundland Ridge reflects changing distribution patterns of different water masses during the late Quaternary. In particular, the lithological and microfaunal characteristics of the sediments indicate that, in the area of the core, the cold Labrador Current from the north and the continental slope water have had a permanent influence on the sedimentation pattern, whereas the paths of the Gulf Stream water have shifted intermittently. The influence of the Gulf Stream is clearly identifiable during the Holocene and during the last interglacial (faunal zone X). Ice-rafted debris and relatively coarse turbidite-type beds are more prominent in sequences deposited under a glacial régime, notably in those that accumulated near the end of the late Pleistocene and the beginning of the Holocene. The upper limit of faunal zone X (Sangamon-Würm) is placed close to 1000 cm depth in the core on the basis of the disappearance at this level of Globorotalia tumida flexuosa (Koch). The resulting estimate of the subsequent mean sediment accumulation rate is of the order of 10 cm/1000 y.

Seismic studies in the region adjacent to the Grand Banks of Newfoundland

A detailed survey by Texas Instruments Incorporated under contract to the United States Naval Oceanographic Office in the vicinity of the Southeast Newfoundland Ridge suggests that this ridge is structurally controlled and related to the northern extent of the Jurassic Atlantic basin. Possible salt diapirs were found in the Sohm Abyssal Plain and lower continental rise. Evidence of formerly greater bottom current activity in the region is suggested by the presence of buried abyssal anti-dunes.

Mineral Composition and Dispersal Patterns of Deep-Sea Sands in the Western North Atlantic Petrologic Province

The western North Atlantic petrologic province consists of terrigenous sands of glacial and postglacial age on the shelf and in the deep sea, north of a line extending east of Cape Fear, North Carolina. The province includes the northern part of the Hatteras Abyssal Plain, all of the Sohm Abyssal Plain, and an undefined area east of the Mid-ocean Canyon. Within the province a western, light-mineral subprovince extends northward from Cape Fear, and eastward through the Hatteras Abyssal Plain and the central Sohm Abyssal Plain, to the Southeastern Newfoundland Ridge; this area is characterized by numerous glauconitic quartzose sands derived by selective sorting and a brasion of arkosic glacial detritus on the shelf. An eastern light-mineral sub-province contains relatively unmodified arkosic sands transported southward around the Southeastern Newfoundland Ridge to the eastern part of the Sohm Abyssal Plain. The glacially derived, heavy-mineral assemblage of the western North Atlantic province is complex; the nonopaque, nonmicaceous minerals are characterized by amphibole (44 per cent), garnet (19 per cent), and pyroxene (5 per cent). Five heavy-mineral subprovinces are mapped by areal variations in the proportions of kyanite-sillimanite-staurolite, epidote, tourmaline, and pyroxene plus the ratio of garnet to kyanite-staurolite. None of these indices correlates with mean grain size of the sands and silts. Amphibole increases and garnet decreases in the finer sands and silts. In the more quartzose sands the light-mineral assemblages are relatively mature, whereas the heavy-mineral assemblages are immature. This is because modification of the light minerals toward an ultra-stable quartz-chert composition occurs more rapidly than does modification of the heavy minerals toward an analogous zircon-tourmaline-rutile ultrastable assemblage. The light minerals are thus a more sensitive indicator of abrasion history than are the heavy minerals, which better reflect source-rock lithology. Bottom topography largely controls sand-dispersal patterns. Submarine canyons act as sediment traps that funnel the downslope movement of sand to deep-sea fans. The sand is then transported across the abyssal plains.