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Abstract: The thermal history and maturation potential of the central Scotian Slope is constrained using a combination of 47 recently acquired seafloor heat flow measurements, two-dimensional (2D) seismic reflection data, [...]

Abstract: Data from a refraction seismic profile parallel to the coast of Labrador (Canada) were used to determine the crustal structure across the boundary of the Nain and Makkovik provinces, [...]

Upwelling or downwelling flow in Earth's mantle is thought to elevate or depress Earth's surface on a continental scale. Direct observation of this 'dynamic topography' on the seafloor, however, has remained elusive because it is obscured by isostatically supported topography caused by near-surface density variations. We calculate the nonisostatic topography of the North Atlantic by correcting seafloor depths for lithospheric cooling and sediment loading, and find that seafloor west of the Mid-Atlantic Ridge is an average of 0.5 km deeper than it is to the east. We are able to reproduce this basic observation in a model of mantle flow driven by tomographically inferred mantle densities. This model shows that the Farallon slab, currently in the lower mantle beneath the east coast of North America, induces downwelling flow that deepens the western North Atlantic relative to the east. Our model also predicts dynamic support of observed topographic highs near Iceland and the Azores, but suggests that the Icelandic high is due to local upper-mantle upwelling, while the Azores high is part of a plate-scale lower-mantle upwelling to the south. An anomalously deep area off the coast of Nova Scotia may be associated with the downwelling component of edge-driven convection at the continental boundary. Thus, several of the seafloor's topographic features can only be understood in terms of dynamic support from flow in Earth's mantle. Keywords: dynamic topography, seafloor depth, mantle flow, North Atlantic, Iceland, Azores, Scotian Basin.

Prestack depth-migrated seismic reflection data collected off Flemish Cap on the Newfoundland margin show a structure of abruptly thinning continental crust that leads into an oceanic accretion system. Within continental crust, there is no clear evidence for detachment surfaces analogous to the S reflection off the conjugate Galicia Bank margin, demonstrating a first-order asymmetry in final rift development. Anomalously thin (3-4 km), magmatically produced oceanic crust abuts very thin continental crust and is highly tectonized. This indicates that initial accretion of the oceanic crust was in a magma-limited setting similar to present-day ultraslow spreading environments. Seaward, oceanic crust thins to Keywords: continental margin, seafloor spreading, extension tectonics, continental breakup.

We present the combined results of deep multichannel reflection and refraction seismic surveys across the Flemish Cap--Goban Spur conjugate margin pair (North Atlantic), which we use to infer rifting style and breakup. Profiles on both margins cross magnetic anomaly 34 and extend into oceanic crust, making it possible to observe the complete history from continental rifting through to the formation of initial oceanic crust. The deep multichannel seismic (MCS) reflection data have previously been used to support a model of symmetric pure shear extension followed by asymmetric breakup and a sharp continent-ocean boundary. Using both types of seismic data, our results indicate instead that asymmetric structures are formed during all stages of rifting, breakup, and complex transition to oceanic spreading. The differing nature of the two ocean-continent transition zones is particularly striking. For Flemish Cap, our reprocessed image of the MCS profile clearly shows tilted fault blocks beneath back-tilted sediment packages, consistent with a wide region of highly thinned continental crust inferred from wide-angle seismic data. In contrast, normal incidence and wide-angle seismic data for the Goban Spur transition zone indicate the presence of exhumed serpentinized mantle. [ABSTRACT FROM AUTHOR]

The crustal structure of the NE Flemish Cap margin was determined along a 460-km-long refraction/wide-angle reflection seismic transect (FLAME Line) to define the thickness, structure and composition of the crust and uppermost mantle along the line. A P-wave velocity model was developed from forward and inverse modelling of dense airgun shots recorded by 19 ocean bottom seismometers. A coincident multichannel seismic profile was used to guide the modelling as reflections could be identified down to Moho. The model displays a sediment cover of up to 3.6-km-thick, subdivided into three layers with velocities of 1.8-1.9 km s, 2.8-3.1 km s and 4.7-4.8 km s. For the western part of the FLAME Line over Flemish Cap, the P-wave velocity model displays an up to 32-km-thick, three-layer continental crust. The continental crust has velocities of 5.8-6.1 km s, 6.3-6.45 km s and 6.65-6.85 km s and thicknesses of about 5 km, 7 km and 20 km in the upper, middle and lower layers, respectively. The thick continental crust thins to a two-layer, 6-km-thick crust (upper layer is 5.55-6.0 km s and the layer below is 6.65-6.8 km s) over a distance of 45 km. S-wave velocities are determined in the upper layer of the thick continental crust over Flemish Cap and the transition zone by assigning Poisson's ratios in the P-wave velocity model. Comparison of calculated to observed arrival times gives a Poisson's ratio of 0.27 in the upper layer and 0.28 in the layer below, which suggests that the composition of the crust is primarily continental in both the thick crust and the thin crust of the transition zone. The thin continental crust is stretched over a width of 80 km and is underlain by a layer with velocities of 7.5-7.9 km s. We interpret this layer as partially serpentinized mantle, which is consistent with observations from the Newfoundland margin to the south. The serpentinized mantle terminates 30 km seaward of the thick continental crust. At the seaward-most end of the thin continental crust, a prominent ridge feature is observed. The seismic refraction and multichannel seismic data results indicate a mixed character between serpentinized mantle with volcanic extrusions or continental crust. The reflection seismic data show a high relief basement from the ridge feature and seaward. The FLAME Line crosses magnetic anomaly 34 and extends another ∼50 km seaward well into oceanic crust. The ridge is flanked seaward by a two-layer oceanic crust. The upper layer (Layer 2) has velocities of 4.8-5.0 km s for the landward-most 35 km of oceanic crust and 4.8-6.2 km s for the seaward 60 km. The average thickness of Layer 2 is ∼2 km. The lower layer (Layer 3) has velocities of 6.7-7.2 km s and a thickness of ∼3.5 km. The velocity model is consistent with a sharp onset of seafloor spreading seaward of the ridge feature. [ABSTRACT FROM AUTHOR]

The crustal structure in the southern Davis Strait and the adjacent ocean–continent transition zone in NE Labrador Sea was determined along a 185-km-long refraction/wide-angle reflection seismic transect to study the impact of the Iceland mantle plume to this region. A P-wave velocity model was developed from forward and inverse modelling of dense airgun shots recorded by ocean bottom seismographs. A coincident industry multichannel reflection seismic profile was used to guide the modelling as reflectivity could be identified down to Moho. The model displays a marked lateral change of velocity structure. The sedimentary cover (velocities 1.8–3.9 km s−1) is up to 4 km thick in the north and thins to 1 km in the south. The segment of the line within southern Davis Strait is interpreted to be of continental character with a two-layered 13-km-thick crust with P-wave velocities of 5.6–5.8 and 6.4–6.7 km s−1 in the upper and lower crust, respectively. The crust is underlain by a 2- to 4-km-thick high-velocity layer (7.5 km s−1). This layer we interpret as underplated material related to the Iceland plume. The southern segment of the line in Labrador Sea displays a 2-km-thick layer with a velocity of 4.5 km s−1. This layer can be correlated to a well about 100 km to the west of the line, where Palaeocene basalts and interbedded sediments were drilled. Underneath is a 12-km-thick crust with a 2-km-thick upper layer (5.8–6.6 km s−1) and a 10-km-thick lower layer (6.8–7.2 km s−1). This crust is interpreted to be of oceanic character. S-wave modelling yields a Poisson's ratio of 0.28 for the lower crust, compatible with a gabbroic composition. The igneous crust is 5 km thicker than normal oceanic crust. We suggest that the increased magma production was created by buoyancy-driving flow. We propose a model in which initial seafloor spreading occurred between Labrador and West Greenland, when the Iceland plume arrived in the area at ∼62 Ma and caused enhanced magma production. Shortly afterwards (chron 27–26), plume material was channelled southward underplating part of Davis Strait and forming basaltic flows interbedded with sediment. [ABSTRACT FROM AUTHOR]

A P-wave velocity model along a 565-km-long profile across the Grand Banks–Newfoundland Basin rifted margin is presented. Continental crust ∼36 km thick beneath the Grand Banks is divided into upper (5.8–6.25 km s−1), middle (6.3–6.53 km s−1) and lower crust (6.77–6.9 km s−1), consistent with velocity structure of Avalon zone Appalachian crust. Syn-rift sediment sequences 6–7 km thick occur in two primary layers within the Jeanne d'Arc and the Carson basins (∼3 km s−1 in upper layer; ∼5 km s−1 in lower layer). Abrupt crustal thinning (Moho dip ∼35°) beneath the Carson basin and more gradual thinning seaward forms a 170-km-wide zone of rifted continental crust. Within this zone, lower and middle continental crust thin preferentially seawards until they are completely removed, while very thin (<3 km) upper crust continues ∼60 km farther seawards. Adjacent to the continental crust, high-velocity gradients (0.5–1.5 s−1) define an 80-km-wide zone of transitional basement that can be interpreted as exhumed, serpentinized mantle or anomalously thin oceanic crust, based on its velocity model alone. We prefer the exhumed-mantle interpretation after considering the non-reflective character of the basement and the low amplitude of associated magnetic anomalies, which are atypical of oceanic crust. Beneath both the transitional basement and thin (<6 km) continental crust, a 200-km-wide zone with reduced mantle velocities (7.6–7.9 km s−1) is observed, which is interpreted as partially (<10 per cent) serpentinized mantle. Seawards of the transitional basement, 2- to 6-km-thick crust with layer 2 (4.5–6.3 km s−1) and layer 3 (6.3–7.2 km s−1) velocities is interpreted as oceanic crust. Comparison of our crustal model with profile IAM-9 across the Iberia Abyssal Plain on the conjugate Iberia margin suggests asymmetrical continental breakup in which a wider zone of extended continental crust has been left on the Newfoundland side. [ABSTRACT FROM AUTHOR]

The central Nova Scotia margin off Eastern Canada is located at a transition from a volcanic margin in the south to a non-volcanic margin in the north. In order to study this transition, a wide-angle refraction seismic line with dense airgun shots was acquired across the central Nova Scotia margin. The 500-km-long transect is coincident with previous deep reflection profiles across the Lahave Platform and extending into the Sohm Abyssal Plain. A P-wave velocity model was developed from forward and inverse modelling of the wide-angle data from 21 ocean bottom seismometers and coincident normal-incidence reflection profiles. The velocity model shows that the continental crust is divided into three layers with velocities of 5.5–6.9 km s−1. The maximum thickness is 36 km. A minor amount (∼5 km) of thinning occurs beneath the outer shelf, while the major thinning to a thickness of 8 km occurs over the slope region. The seaward limit of the continental crust consists of 5-km-thick highly faulted basement. There is no evidence for magmatic underplating beneath the continental crust. On the contrary, a 4-km-thick layer of partially serpentinized mantle (7.6–7.95 km s−1) begins beneath the highly faulted continental crust, and extends ∼200 km seawards, forming the lower part of the ocean-continent transition zone. The upper part of the transition zone consists of the highly faulted continental crust and 4- to 5-km-thick initial oceanic crust. The continent–ocean boundary is moved ∼50 km farther seawards compared to an earlier interpretation based only on reflection seismic data. The oceanic crust in the transition zone consists of layer 2 and a high-velocity lower crustal layer. Layer 2 is 1–3 km thick with velocities of 5.6–6.0 km s−1. The high-velocity lower crustal layer is 1–2 km thick with velocities of 7.25–7.4 km s−1, suggesting a composite layer of serpentinized peridotite and gabbroic layer 3. Oceanic crust with normal thickness of 5–7 km and more typical layer 3 with velocities of 6.95–7.3 km s−1 is observed at the seaward end of the profile. [ABSTRACT FROM AUTHOR]

Seismic reflection and refraction data were acquired along the southeast margin of Flemish Cap at a position conjugate to drilling and geophysical surveys across the Galicia Bank margin. The data document first-order asymmetry during final break-up between Newfoundland and Iberia. An abrupt necking profile of continental crust observed off Flemish Cap contrasts strongly with gradual tapering on the conjugate margin. There is no evidence beneath Flemish Cap for a final phase of continental extension that resulted in thin continental crust underlain by a strong ‘S’-like reflection, which indicates that this mode of extension occurred only on the Galicia Bank margin. Compelling evidence for a broad zone of exhumed mantle or for peridotite ridges is also lacking along the Flemish Cap margin. Instead, anomalously thin, 3–4-km-thick oceanic crust is observed. This crust is highly tectonized and broken up by high-angle normal faulting. The thin crust and rift structures that resemble the abandoned spreading centre in the Labrador sea suggest that initial seafloor spreading was affected by processes observed in present-day ultra-slow spreading environments. Landwards, Flemish Cap is underlain by a highly reflective lower crust. The reflectivity most likely originates from older Palaeozoic orogenic structures that are unrelated to extension and break-up tectonics. [ABSTRACT FROM AUTHOR]

The Eastern Canadian Shield Onshore–Offshore Transect (ECSOOT) of the Lithoprobe program included 1200 km of normal-incidence seismic profiles and seven wide-angle seismic profiles across Archean and Proterozoic rocks of Labrador, northern Quebec, and the surrounding marine areas. Archean crust is 33–44 km thick. P-wave velocity increases downwards from 6.0 to 6.9 km/s. There is moderate crustal reflectivity, but the reflection Moho is unclear. Archean crust that stabilized in the Proterozoic is similar except for greater reflectivity and a well-defined Moho. Proterozoic crust has similar or greater thickness, variable lower crustal velocities, and strong crustal reflectivity. Geodynamic processes of Paleoproterozoic growth of the Canadian Shield are similar to those observed in modern collisional orogens. The suturing of the Archean Core Zone and Superior provinces involved whole-crustal shearing (top to west) in the Core Zone, linked to thin-skinned deformation in the New Quebec Orogen. The Torngat Orogen sutures the Nain Province to the Core Zone and reveals a crustal root, in which Moho descends to 55 km. It formed by transpression and survived because of the lack of postorogenic heating. Accretion of the Makkovik Province to the Nain Province involves delamination at the Moho and distributed strain in the juvenile arcs. Delamination within the lower crust characterizes the accretion of Labradorian crust in the southeastern Grenville Province. Thinning of the crust northwards across the Grenville Front is accentuated by Mesozoic extension that reactivates Proterozoic shear zones. The intrusion of the Mesoproterozoic Nain Plutonic Suite is attributed to a mantle plume ponding at the base of the crust.Le transect du programme Lithoprobe des zones continentales et extracôtières de la partie est du Bouclier canadien (ECSOOT) comprend 1200 km de profils sismiques à incidence normale et sept profils sismiques à grand-angle à travers les roches archéennes et protérozoïques du Labrador, du nord du Québec et des régions marines avoisinantes. La croûte archéenne a une épaisseur de 33 à 44 km. La vitesse de l'onde P augmente vers le bas de 6,0 km s[sup -1] à 6,9 km s[sup -1] . La réflectivité crustale est modérée, mais la réflexion du Moho n'est pas claire. La croûte archéenne qui s'est stabilisée au Protérozoïque est semblable, à l'exception d'une plus grande réflectivité et une discontinuité Moho bien définie. La croûte protérozoïque présente une épaisseur similaire ou plus grande, des vitesses variables pour la croûte inférieure et une forte réflectivité de la croûte. Les processus géodynamiques de croissance du Bouclier canadien au Paléoprotérozoïque sont semblables à ceux observés dans des orogènes de collision modernes. La suture de la zone noyau archéenne et de la Province du Supérieur implique du cisaillement de la croûte entière (du haut vers l'ouest) dans la zone noyau, relié à de la déformation en couches minces dans l'orogène du Nouveau-Québec. L'orogène Torngat suture la Province de Nain à la zone noyau et révèle une racine crustale dans laquelle le Moho descend à 55 km. Cet orogène s'est formé par transpression et a survécu en raison du manque d'élévation de température post-orogénique. L'accrétion de la Province de Makkovik à la Province de Nain implique une délamination au Moho et une distribution des contraintes dans les arcs juvéniles. La délamination à l'intérieur de la croûte inférieure caractérise l'accrétion de la croûte labradorienne dans le sud-est de la Province de Grenville. Un amincissement de la croûte vers le nord à travers le front du Grenville est accentué par une extension, au Mésoprotérozoïque, qui a réactivé les zones de cisaillement protérozoïques. L'intrusion de la Suite plutonique de Nain (Mésoprotérozoïque) est attribuée à un panache du manteau stagnant à la base de la croûte.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

The crustal structure of the eastern Grenville and Makkovik provinces was determined using two onshore–offshore refraction seismic lines of the Lithoprobe Eastern Canadian Shield Onshore–Offshore Transect (ECSOOT). A gravity high in the Hawke River terrane correlates with increased P-wave velocities in the upper 30 km of the crust (6.2–6.7 km/s in the upper and middle crust and 6.9–7.1 km/s below) which we interpret as structure inherited from the Labradorian orogen. Velocities in the adjacent Groswater Bay terrane are 6.0–6.55 km/s in the upper and middle crust and 6.6–6.95 km/s in the lower crust. The entire Grenville crust is underlain by a 15–20 km thick high-velocity lower crustal (HVLC) wedge consisting of an upper layer (7.1–7.4 km/s) and a lower layer (7.6–7.8 km/s). The HVLC wedge is interpreted as an underplated layer formed during Iapetan rifting. This interpretation is based on the correlation with the 615 Ma Long Range dykes onshore and the eastward termination of the wedge at the Cartwright Arch. Similar HVLC layers are found offshore western Newfoundland, suggesting that the underplating may be a continuous feature along the passive Grenvillian margin. The Cartwright Arch is characterized by velocities of 6.4 km/s and 4 km thick sediment sequences (4.3–5.7 km/s) in the surrounding basin, interpreted as an extensional basin with basaltic magmatism within the arch. The Grenville front is clearly marked by a decrease of velocities in the Makkovik Province (5.8–6.4 km/s in the upper and middle crust, 6.65–6.85 km/s in the lower crust) and a gradual thickening of the crust (not including the HVLC layer) from 30 km in the Grenville Province to 35 km in the Makkovik Province.La structure crustale de l'est des provinces de Grenville et de Makkovik a été déterminée en utilisant les données de deux lignes de réfraction sismique du transect Lithoprobe des zones continentales et extracôtières de la partie l'est du Bouclier canadien (ECSOOT, Eastern Canadian Shield Onshore-Offshore Transect). Un maximum gravimétrique dans le terrane de la rivière Hawke concorde avec des augmentations des vitesses de l'onde P dans les 30 km supérieurs de la croûte (6,2–6,7 km/s dans la croûte supérieure et médiane et 6,9–7,1 km/s pour la croûte inférieure) ce que nous interprétons comme une structure héritée de l'orogène labradorien. Les vitesses dans le terrane adjacent de Groswater Bay sont de 6,0–6,55 km/s dans la croûte supérieure et médiane et de 6,6–6,95 km/s dans la croûte inférieure. La croûte du Grenville au complet recouvre un coin de haute vitesse de la croûte inférieure (HVLC) de 15 à 20 km d'épaisseur, lequel comprend une couche supérieure (7,1–7,4 km/s) et une couche inférieure (7,6–7,8 km/s). Ce coin HVLC est interprété comme une couche infraplaque formée au cours de la dérive de l'Iapetus. Cette interprétation est basée sur une corrélation avec les dykes Long Range (615 Ma) sur la côte et l'extrémité est du coin à l'arche Cartwright. Des couches HVLC semblables se retrouvent au large de l'ouest de Terre-Neuve, suggérant que le sous-placage peut être une caractéristique continue le long de la marge passive du Grenville. L'arche Cartwright est caractérisée par des vitesses de 6,4 km/s et les séquences sédimentaires du bassin environnant (4,3–5,7 km/s et 4 km d'épaisseur) sont interprétées comme un bassin d'extension avec du magmatisme basaltique à l'intérieur de l'arche. Le front du Grenville est nettement défini par une diminution des vitesses dans la province de Makkovik (5,8–6,4 km/s pour la croûte supérieure et médiane et 6,65–6,85 km/s pour la croûte inférieure) ainsi qu'un épaississement graduel de la croûte (n'incluant pas la couche HVLC) de 30 km dans le Grenville à 35 km dans la province de Makkovik.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]