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Two geophysical expeditions (LOMROG II and III) were carried out in 2009 and 2012 to acquire seismic data in the western Amundsen Basin in the Arctic Ocean, a basin formed by ultraslow seafloor spreading at the Gakkel Ridge. Previous studies show alternating magmatic and amagmatic segments at the ridge but it is unclear if such segmentation persisted throughout the entire opening history of the basin. The seismic refraction data were used to develop P -wave velocity models down to the uppermost mantle using forward modelling of traveltimes. The coincident seismic reflection data were used to constrain the geometry of the sedimentary layers and for characterizing the acoustic basement. 2-D gravity modelling was used to determine the Moho depth in areas when data quality was insufficient to resolve a Moho reflection. The models distinguish three different basement types: oceanic crust with layers 2 and 3, oceanic crust that is lacking a layer 3 and exhumed and serpentinized mantle. The maximum observed crustal thickness is 6 km. Areas with thin crust (<3 km) may be underlain by partially serpentinized mantle. Where exhumed mantle is observed, a serpentinization front separates highly serpentinized mantle at the top from partially serpentinized mantle below. The presence of oceanic crust off-axis of the presently amagmatic sector of the Gakkel Ridge indicates that there is both a spatial and temporal variation of crustal accretion processes at the ridge. [ABSTRACT FROM AUTHOR]

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, [...]

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.

The Brain Imaging Data Structure (BIDS) is a standard for organizing and describing neuroimaging datasets, serving not only to facilitate the process of data sharing and aggregation, but also to simplify the application and development of new methods and software for working with neuroimaging data. Here, we present an extension of BIDS to include positron emission tomography (PET) data, also known as PET-BIDS, and share several open-access datasets curated following PET-BIDS along with tools for conversion, validation and analysis of PET-BIDS datasets. [ABSTRACT FROM AUTHOR]

Histological atlases of the cerebral cortex, such as those made famous by Brodmann and von Economo, are invaluable for understanding human brain microstructure and its relationship with functional organization in the brain. However, these existing atlases are limited to small numbers of manually annotated samples from a single cerebral hemisphere, measured from 2D histological sections. We present the first whole-brain quantitative 3D laminar atlas of the human cerebral cortex. It was derived from a 3D histological atlas of the human brain at 20-micrometer isotropic resolution (BigBrain), using a convolutional neural network to segment, automatically, the cortical layers in both hemispheres. Our approach overcomes many of the historical challenges with measurement of histological thickness in 2D, and the resultant laminar atlas provides an unprecedented level of precision and detail. We utilized this BigBrain cortical atlas to test whether previously reported thickness gradients, as measured by MRI in sensory and motor processing cortices, were present in a histological atlas of cortical thickness and which cortical layers were contributing to these gradients. Cortical thickness increased across sensory processing hierarchies, primarily driven by layers III, V, and VI. In contrast, motor-frontal cortices showed the opposite pattern, with decreases in total and pyramidal layer thickness from motor to frontal association cortices. These findings illustrate how this laminar atlas will provide a link between single-neuron morphology, mesoscale cortical layering, macroscopic cortical thickness, and, ultimately, functional neuroanatomy. Using deep learning to segment the layers of the cerebral cortex, this study presents the first whole brain quantitative atlas of cortical and laminar structure. This laminar atlas provides a novel framework for bridging between the scales of neuroscience. [ABSTRACT FROM AUTHOR]

Orphan Basin, a massive deepwater rifted basin off the northeastern coast of Newfoundland, was one of the targets of the 2009 SIGNAL (Seismic Investigations off Greenland, Newfoundland and Labrador) experiment to collect refraction/wide-angle reflection (RWAR) data from the Bonavista Platform, through the Orphan Basin, to the Orphan Knoll, and beyond into oceanic crust. Both the data from an earlier RWAR acquisition and the new data were jointly analysed in order to improve on the earlier velocity model and extend its coverage landward and seaward. The resulting velocity model is characterized by an 8–9-km-thick sedimentary package immediately outboard of the Bonavista Platform, which thins toward the Orphan Knoll and beyond. The shallowest modelled sedimentary layer, interpreted as Paleocene and younger post-rift sediments, does not show significant thickness variations and velocities do not exceed 3.3 km s–1. The second modelled sedimentary layer with laterally variable velocities ranging from 2.3 to 5.3 km s–1, interpreted as Late Cretaceous post-rift sediments, is thickest over an interpreted failed rift. The deepest modelled sedimentary layer consists of laterally variable velocities that do not exceed 5.9 km s–1 and is interpreted as possibly Jurassic to Early Cretaceous syn-rift sediments. The crust beneath the Bonavista Platform is subdivided into an upper (5.4–5.9 km s–1), middle (5.9–6.4 km s–1) and lower crust (6.4–6.9 km s–1). The middle crust is modelled as disappearing beneath the seaward limit of the Bonavista Platform at an interpreted failed rift, only to re-appear 100 km further seaward beneath the central Orphan Basin and extend to the seaward limit of the Orphan Knoll, beyond which the crust can be modelled by just an upper (5.0–6.7 km s–1) and a lower (6.7–7.0 km s–1) crustal layer. Towards land, for the first 450 km of the model, velocities generally follow the globally averaged velocity trend for rifted continental crust, albeit with slightly elevated velocities suggestive of magmatic contributions. At the failed rift, within the continental domain, hyperextended crust is modelled, overlying a limited zone of serpentinized mantle. Seaward of Orphan Knoll, the interpretation for the velocity structure is less definitive but an 80-km-wide continent–ocean transition zone consisting of either transitional embryonic oceanic crust or thinned continental crust overlying serpentinized mantle is proposed. Upper mantle velocities as low as 7.7 km s–1 are modelled beneath the interpreted failed continental rift as well as beneath the continent–ocean transition zone, while the rest of the crustal model is underlain by typical mantle velocities of 8 km s–1. Analysis of extension and thinning factors based on the velocity model reveal that the failed rift experienced hyperextension and should have achieved full crustal embrittlement, consistent with localized mantle serpentinization. [ABSTRACT FROM AUTHOR]

Alzheimer's disease (AD) patients are known to have heterogeneous clinical presentation and pathologic patterns. We hypothesize that AD dementia can be categorized into subtypes based on multimodal imaging biomarkers such as magnetic resonance imaging (MRI), tau positron emission tomography (PET), and amyloid PET. We collected 3T MRI, 18F-THK5351 PET, and 18F-flutemetamol (FLUTE) PET data from 83 patients with AD dementia [Clinical Dementia Rating (CDR) ≤1] and 60 normal controls (NC), and applied surface-based analyses to measure cortical thickness, THK5351 standardized uptake value ratio (SUVR) and FLUTE SUVR for each participant. For the patient group, we performed an agglomerative hierarchical clustering analysis using the three multimodal imaging features on the vertices (n = 3 × 79,950). The identified AD subtypes were compared to NC using general linear models adjusting for age, sex, and years of education. We mapped the effect size within significant cortical regions reaching a corrected p -vertex <0.05 (random field theory). Our surface-based multimodal framework has revealed three distinct subtypes among AD patients: medial temporal-dominant subtype (MT, n = 44), parietal-dominant subtype (P, n = 19), and diffuse atrophy subtype (D, n = 20). The topography of cortical atrophy and THK5351 retention differentiates between the three subtypes. In the case of FLUTE, three subtypes did not show distinct topographical differences, although cortical composite retention was significantly higher in the P type than in the MT type. These three subtypes also differed in demographic and clinical features. In conclusion, AD patients may be clustered into three subtypes with distinct topographical features of cortical atrophy and tau deposition, although amyloid deposition may not differ across the subtypes in terms of topography. [ABSTRACT FROM AUTHOR]

A new stratigraphic model and estimated sedimentation rates of the western Amundsen Basin, Arctic Ocean, are presented based on multichannel seismic reflection data, seismic refraction data, magnetic data, and integrated with the sedimentary sequence from the central Arctic Ocean, obtained during the Arctic Coring Expedition. This places new constraints on the postbreakup Cenozoic depositional history of the basin, the adjacent Lomonosov Ridge, and improves the understanding of the tectonic, climatic, and oceanographic conditions in the central Arctic region. Four distinct phases of basin development are proposed. During the Paleocene‐mid‐Oligocene, high sedimentation rates are linked to terrestrial input and increased pelagic deposition in a restricted basin. Deposition of sedimentary wedges and mass transport into marginal depocenters reflect a period of tectonic instability linked to compression associated with the Eurekan Orogeny in the Arctic. During the late Oligocene‐early Miocene, widespread passive infill associated with hemipelagic deposition reflects a phase of limited tectonism, most likely in a freshwater estuarine setting. During the middle Miocene, mounded sedimentary buildups along the Lomonosov Ridge suggest the onset of geostrophic bottom currents that likely formed in response to a deepening and widening of the Fram Strait beginning around 18 Ma. In contrast, the Plio‐Pleistocene stage is characterized by erosional features such as scarps and channels adjacent to levee accumulations, indicative of a change to a higher‐energy environment. These deposits are suggested to be partly associated with dense shelf water‐mass plumes driven by supercooling and brine formation over the northern Greenland continental shelf. Key Points: New multichannel seismic reflection data constrain the Cenozoic depositional history of the Amundsen Basin in the Arctic OceanFour key development stages explain the basin evolution based on facies interpretation and estimated sedimentation ratesPlio‐Pleistocene cascading plumes, possibly from brine formation, affected the North Greenland shelf and influenced deep circulation [ABSTRACT FROM AUTHOR]

APPIAN is an automated pipeline for user-friendly and reproducible analysis of positron emission tomography (PET) images with the aim of automating all processing steps up to the statistical analysis of measures derived from the final output images. The three primary processing steps are coregistration of PET images to T1-weighted magnetic resonance (MR) images, partial-volume correction (PVC), and quantification with tracer kinetic modeling. While there are alternate open-source PET pipelines, none offers all of the features necessary for making automated PET analysis as reliably, flexibly and easily extendible as possible. To this end, a novel method for automated quality control (QC) has been designed to facilitate reliable, reproducible research by helping users verify that each processing stage has been performed as expected. Additionally, a web browser-based GUI has been implemented to allow both the 3D visualization of the output images, as well as plots describing the quantitative results of the analyses performed by the pipeline. APPIAN also uses flexible region of interest (ROI) definition—with both volumetric and, optionally, surface-based ROI—to allow users to analyze data from a wide variety of experimental paradigms, e.g., longitudinal lesion studies, large cross-sectional population studies, multi-factorial experimental designs, etc. Finally, APPIAN is designed to be modular so that users can easily test new algorithms for PVC or quantification or add entirely new analyses to the basic pipeline. We validate the accuracy of APPIAN against the Monte-Carlo simulated SORTEO database and show that, after PVC, APPIAN recovers radiotracer concentrations within 93–100% accuracy. [ABSTRACT FROM AUTHOR]

Chronic pain patients present with cortical gray matter alterations, observed with anatomical magnetic resonance (MR) imaging. Reduced regional gray matter volumes are often interpreted to reflect neurodegeneration, but studies investigating the cellular origin of gray matter changes are lacking. We used multimodal imaging to compare 26 postmenopausal women with fibromyalgia with 25 healthy controls (age range: 50-75 years) to test whether regional gray matter volume decreases in chronic pain are associated with compromised neuronal integrity. Regional gray matter decreases were largely explained by T1 relaxation times in gray matter, a surrogate measure of water content, and not to any substantial degree by GABAA receptor concentration, an indirect marker of neuronal integrity measured with [18F] flumazenil PET. In addition, the MR spectroscopy marker of neuronal viability, N-acetylaspartate, did not differ between patients and controls. These findings suggest that decreased gray matter volumes are not explained by compromised neuronal integrity. Alternatively, a decrease in neuronal matter could be compensated for by an up regulation of GABAA receptors. The relation between regional gray matter and T1 relaxation times suggests decreased tissue water content underlying regional gray matter decreases. In contrast, regional gray matter increases were explained by GABAA receptor concentration in addition to T1 relaxation times, indicating perhaps increased neuronal matter or GABAA receptor up regulation and inflammatory edema. By providing information on the histological origins of cerebral gray matter alterations in fibromyalgia, this study advances the understanding of the neurobiology of chronic widespread pain. [ABSTRACT FROM AUTHOR]

Increasingly available high-resolution brain imaging data require specialized processing tools that can leverage their anatomical detail and handle their size. Here, we present userfriendly Python tools for cortical depth resolved analysis in such data. Our implementation is based on the CBS High-Res Brain Processing framework, and aims to make highresolution data processing tools available to the broader community. [ABSTRACT FROM AUTHOR]

This study examines the velocity structure of Makarov Basin and the adjacent Alpha Ridge to determine the tectonic origins of these features and link them to the larger Amerasia Basin. Seismic data from sonobuoys distributed along a 650 km-long line extending from Alpha Ridge and across Makarov Basin to the Lomonosov Ridge were analyzed for this purpose. Forward modelling of traveltimes, supported by coincident multi-channel seismic reflection and shipborne gravity data, were used to determine the P-wave velocity structure along the line. The sedimentary cover averages 0.5 km-thick on Alpha Ridge and 1.9 km-thick in Makarov Basin, but reaches up to 5 km-thick at the base of Lomonosov Ridge. Velocities in the sedimentary section range from 1.6 to 4.3 km s − 1 . As suggested by relatively high velocities, interbedded volcaniclastic or volcanic rock may occur in the deep sedimentary section. The shallow basement of Alpha Ridge (3.3 to 3.6 km s − 1 ) is characterized by semi-continuous high amplitude reflections and is interpreted as volcanic rock possibly intercalated with sedimentary rock. Velocities do not vary significantly in the upper and mid-crustal layers between Alpha Ridge and Makarov Basin. Total crustal thickness decreases from 27 km beneath Alpha Ridge to 5 km-thick in Makarov Basin then thickens to > 20 km over a short distance as part of Lomonosov Ridge. The crustal structure of Alpha Ridge is consistent with previous studies suggesting that the Alpha-Mendeleev ridge complex is part of a large igneous province (LIP) with thick igneous crust. The lack of change in crustal velocities between Alpha Ridge and Makarov Basin suggests that the basin, at least partly, either formed during or was influenced by LIP-related magmatism. The rapid transition of crustal thicknesses from Makarov Basin to Lomonosov Ridge supports the interpretation that this section of the ridge is a transform margin. [ABSTRACT FROM AUTHOR]

The peri-infarct cortex (PIC) is the site of long-term physiologic changes after ischemic stroke. Traditional methods for delineating the peri-infarct gray matter (GM) have used a volumetric Euclidean distance metric to define its extent around the infarct. This metric has limitations in the case of cortical stroke, i.e., those where ischemia leads to infarction in the cortical GM, because the vascularization of the cerebral cortex follows the complex, folded topology of the cortical surface. Instead, we used a geodesic distance metric along the cortical surface to subdivide the PIC into equidistant rings emanating from the infarct border and compared this new approach to a Euclidean distance metric definition. This was done in 11 patients with [F-18]-Flumazenil ([18-F]-FMZ) positron emission tomography (PET) scans at 2 weeks post-stroke and at 6 month follow-up. FMZ is a PET radiotracer with specific binding to the alpha subunits of the type A γ-aminobutyric acid (GABAA) receptor. Additionally, we used partial-volume correction (PVC) of the PET images to compensate for potential cortical thinning and long-term neuronal loss in follow-up images. The difference in non-displaceable binding potential (BPND) between the stroke unaffected and affected hemispheres was 35% larger in the geodesic versus the Euclidean peri-infarct models in initial PET images and 48% larger in follow-up PET images. The inter-hemispheric BPND difference was approximately 17-20% larger after PVC when compared to uncorrected PET images. PET studies of peri-infarct GM in cortical strokes should use a geodesic model and include PVC as a preprocessing step. Hum Brain Mapp 38:326-338, 2017. © 2016 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The article discusses a study that summarises the status of the Continental Shelf Project of the Kingdom of Denmark after the submission for an extended continental shelf in the area to the north of Greenland. Topics mentioned include the 1982 United Nations Convention on the Law of the Sea, the five partial submissions of the Kingdom of Denmark, and documentation of the sediment thickness in the Northern Deep through modern seismic reflection data.

The Davis Strait is located between Canada and Greenland and connects the Labrador Sea and the Baffin Bay basins. Both basins formed in Cretaceous to Eocene time and were connected by a transform fault system in the Davis Strait. Whether the crust in the central Davis Strait is oceanic or continental has been disputed. This information is needed to understand the evolution of this transform margin during the separation of the North American plate and Greenland. We here present a 315-km-long east–west-oriented profile that crosses the Davis Strait and two major transform fault systems—the Ungava Fault Complex and the Hudson Fracture Zone. By forward modelling of data from 12 ocean bottom seismographs, we develop a P-wave velocity model. We compare this model with a density model from ship-borne gravity data. Seismic reflection and magnetic anomaly data support and complement the interpretation. Most of the crust is covered by basalt flows that indicate extensive volcanism in the Davis Strait. While the upper crust is uniform, the middle and lower crust are characterized by higher P-wave velocities and densities at the location of the Ungava Fault Complex. Here, P-wave velocities of the middle crust are 6.6 km s−1 and of the lower crust are 7.1 km s−1 compared to 6.3 and 6.8 km s−1 outside this area; densities are 2850 and 3050 kg m−3 compared to 2800 and 2900 kg m−3. We here interpret a 45-km-long section as stretched and intruded crust or as new igneous crust that correlates with oceanic crust in the southern Davis Strait. A high-velocity lower crust (6.9–7.3 km s−1) indicates a high content of mafic material. This mantle-derived material gradually intruded the lower crust of the adjacent continental crust and can be related to the Iceland mantle plume. With plate kinematic modelling, we can demonstrate the importance of two transform fault systems in the Davis Strait: the Ungava Fault Complex with transpression and the Hudson Fracture Zone with pure strike-slip motion. We show that with recent poles of rotation, most of the relative motion between the North American plate and Greenland took place along the Hudson Fracture Zone. [ABSTRACT FROM AUTHOR]

SUMMARY Baffin Bay represents the northern extension of the extinct rift system in the Labrador Sea. While the extent of oceanic crust and magnetic spreading anomalies are well constrained in the Labrador Sea, no magnetic spreading anomalies have yet been identified in Baffin Bay. Thus, the nature and evolution of the Baffin Bay crust remain uncertain. To clearly characterize the crust in southern Baffin Bay, 42 ocean bottom seismographs were deployed along a 710-km-long seismic refraction line, from Baffin Island to Greenland. Multichannel seismic reflection, gravity and magnetic anomaly data were recorded along the same transect. Using forward modelling and inversion of observed traveltimes from dense airgun shots, a P-wave velocity model was obtained. The detailed morphology of the basement was constrained using the seismic reflection data. A 2-D density model supports and complements the P-wave modelling. Sediments of up to 6 km in thickness with P-wave velocities of 1.8-4.0 km s−1 are imaged in the centre of Baffin Bay. Oceanic crust underlies at least 305 km of the profile. The oceanic crust is 7.5 km thick on average and is modelled as three layers. Oceanic layer 2 ranges in P-wave velocity from 4.8 to 6.4 km s−1 and is divided into basalts and dykes. Oceanic layer 3 displays P-wave velocities of 6.4-7.2 km s−1. The Greenland continental crust is up to 25 km thick along the line and divided into an upper, middle and lower crust with P-wave velocities from 5.3 to 7.0 km s−1. The upper and middle continental crust thin over a 120-km-wide continent-ocean transition zone. We classify this margin as a volcanic continental margin as seaward dipping reflectors are imaged from the seismic reflection data and mafic intrusions in the lower crust can be inferred from the seismic refraction data. The profile did not reach continental crust on the Baffin Island margin, which implies a transition zone of 150 km length at most. The new information on the extent of oceanic crust is used with published poles of rotation to develop a new kinematic model of the evolution of oceanic crust in southern Baffin Bay. [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]