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Two wide-angle seismic reflection/refraction profiles were acquired to determine the velocity characteristics of the Meguma terrane and adjacent Avalonia northeast of the Cobequid-Chedabucto Fault Zone (CCFZ) in eastern Canada. Line 99–1 is located along the Scotian margin whereas line 99–2 crosses the Scotian margin and onshore Nova Scotia and extends into the Gulf of St. Lawrence. Velocity models were derived by forward modelling of travel times. P-wave velocities of 5.5–6.0 km/s and 6.0–6.4 km/s are interpreted to be metasedimentary rocks of the Goldenville and Halifax groups and granitic rocks, respectively. This inference is based on comparison with laboratory velocities of a suite of rock samples, offshore sampling, links to onshore geology with multibeam bathymetry, and gravity modelling. On both lines a low velocity zone (LVZ) >350 km wide and up to 15 km thick is present at mid-crustal levels. The LVZ is not observed beneath onshore Nova Scotia or northeast of Orpheus Graben, the offshore extension of the CCFZ. The crustal thickness of the Meguma terrane varies between 31 and 38 km; in contrast, the Moho is at a depth of 41 km beneath Avalonia. The velocity of the lower crust on line 99–2 is 6.8–7.1 km/s beneath the LVZ and Avalonia, and 6.6 km/s beneath onshore Meguma terrane. Poisson's ratios calculated from P- and S-wave velocities are 0.19–0.23 in the Meguma terrane crust; in contrast, Poisson's ratios of 0.24–0.25 are indicated for Avalonian crust. • Seismic reflection/refraction, gravity, and rock density/velocity data were obtained. • Two velocity/depth models were generated onshore and offshore of Nova Scotia, Canada. • Both models show an extensive low-velocity zone in the offshore Meguma terrane. • Meguma terrane has crustal structure different from that of adjacent Avalonia. • Avalonia extends only 30 km under Meguma terrane in faulted contact. [ABSTRACT FROM AUTHOR]

The Alpha Ridge-Mendeleev Rise (AMR) is the major bathymetric feature in the Amerasia Basin of the Arctic Ocean. Its tectonic history is controversial due to its remote location in ice covered waters making data acquisition difficult, resulting in the lack of diagnostic data. Analysis of the wide-angle reflection/refraction data based on the compressional waves (P-waves) from the AMR indicates that its velocity/depth structure is typical of large igneous provinces (LIPs). LIPs can form on either oceanic or continental crust and can exhibit complex history of development. Here converted shear waves (S-wave) on the Alpha Ridge have been used to calculate Poisson's ratios and many of measured values are within the felsic range in the upper crust. They are comparable to published S-waves from the Mendeleev Rise. They are also consistent with the results of Rayleigh-wave group-velocity analysis that indicate the Alpha Ridge in adjacent northern Canada has an intermediate composition. Based on magnetics, pseudogravity and volumetric considerations the High Arctic Large Igneous Province (HALIP) most closely resembles the Kerguelen Plateau, a LIP with a continental component. The geophysical characteristics of the AMR are compatible with a number of other LIPs that have continental affinities. A variety of offshore information from dredges and cores and onshore geological data support continental fragments incorporated in the AMR. [ABSTRACT FROM AUTHOR]

On the northern passive margin of Ellesmere Island and Greenland, two long wide-angle seismic reflection/refraction (WAR) profiles and a short vertical incident reflection profile were acquired. The WAR seismic source was explosives and the receivers were vertical geophones placed on the sea ice. A 440 km long North-South profile that crossed the shelf, a bathymetric trough and onto the Lomonosov Ridge was completed. In addition, a 110 km long profile along the trough was completed. P-wave velocity models were created by forward and inverse modelling. On the shelf modelling indicates a 12 km deep sedimentary basin consisting of three layers with velocities of 2.1–2.2, 3.1–3.2 and 4.3–5.2 km s−1. Between the 3.1–3.2 km s−1 and 4.3–5.2 km s−1 layers there is a velocity discontinuity that dips seaward, consistent with a regional unconformity. The 4.3–5.2 km s−1 layer is interpreted to be Palaeozoic to Mesozoic age strata, based on local and regional geological constraints. Beneath these layers, velocities of 5.4–5.9 km s−1 are correlated with metasedimentary rocks that outcrop along the coast. These four layers continue from the shelf onto the Lomonosov Ridge. On the Ridge, the bathymetric contours define a plateau 220 km across. The plateau is a basement high, confirmed by short reflection profiles and the velocities of 5.9–6.5 km s−1. Radial magnetic anomalies emanate from the plateau indicating the volcanic nature of this feature. A lower crustal velocity of 6.2–6.7 km s−1, within the range identified on the Lomonosov Ridge near the Pole and typical of rifted continental crust, is interpreted along the entire line. The Moho, based on the WAR data, has significant relief from 17 to 27 km that is confirmed by gravity modelling and consistent with the regional tectonics. In the trough, Moho shallows eastward from a maximum depth of 19–16 km. No indication of oceanic crust was found in the bathymetric trough. [ABSTRACT FROM AUTHOR]

Abstract: The continental margin of Baffin Island was interpreted from a compilation of seismic reflection and potential field data. Based on these data, it was divided into volcanic and non-volcanic segments. In the northernmost area, coast-parallel grabens and continental basement highs are superseded beneath the slope by irregular basement topography. In combination with the velocities from refraction data and the lack of a magnetic slope anomaly, we propose a non-volcanic margin. In the central region, the seismic data show a limited region with a seaward dipping reflector sequence that may be intercalated with a sedimentary section. Both north and south of Cape Dyer extensive volcanic rocks are interpreted at and below seabed consistent with a volcanic margin. In the southern region, the depth to basement drops rapidly offshore. A thick prograding sedimentary wedge produces a high-amplitude long-wavelength free-air gravity anomaly that was previously confused with the continent–ocean boundary. Although some volcanic rocks are mapped in the region, this area is adjacent to the non-volcanic Labrador Sea margin and is interpreted to be of similar structure. Segmentation of the margin by major faults can explain the abrupt transitions from volcanic to non-volcanic margin segments. The continent–ocean boundary for the region is determined based on the potential field and seismic data. Along the Greenland plate in Davis Strait the continent–ocean boundary coincides with the Ungava Fault Zone. The continent–ocean boundary is used in conjunction with published poles of rotation to evaluate plate reconstructions at chrons 33n and 27n. The earlier reconstruction produces an extensive overlap of boundaries in the north. The later reconstruction shows a gap in the southern region. Thus, additional information is required to refine the reconstructions. [Copyright &y& Elsevier]

Canada ratified the United Nations Convention on the Law of the Sea (UNCLOS) in 2003. With that ratification is an obligation to submit data and information to the U.N. pertaining to the limits of the country's extended continental shelf (ECS); the portion of the juridical continental shelf that extends beyond 200 nautical miles. A team of Canadian scientists, managers, and legal experts that included representation from three Federal Departments (Natural Resources Canada, Fisheries and Oceans Canada, and Global Affairs Canada) with additional support from other departments, spent 13 years compiling and acquiring data to provide the scientific evidence to support delineation of Canada's seaward most maritime limit. The submission has the potential to provide Canada with 2.4 million km2 of additional submarine landmass in the Atlantic and the Arctic oceans over which Canada exercises sovereign rights for the purpose of exploring and exploiting its natural resources. Specific information such as the tectonic framework of the continental margin, the geomorphology of the margin and in particular the continental slope, the geologic nature of adjoined ridges, rises, and plateaux, and sediment thickness within adjacent basins are examples of fundamental pieces of geoscientific information needed to substantiate Canada's outermost maritime limits. This paper highlights a number of segments of Canada's continental margins to showcase this scientific evidence and how it is applied in the UNCLOS context. In doing so, the paper demonstrates the geologic complexity of Canada's margins as illustrated in scientific publications that have resulted from these new data collections, while at the same time presenting new scientific evidence and interpretations. This collection of data and information provides a wealth of new knowledge in Canada's offshore regions. The massive data compilation in the Atlantic led to conception of continental margins, in a source-to-sink scenario, as having an equilibrium base level or graded form, comparable to river systems. Departures from this shape relate to the interplay of sedimentary processes and in particular to those processes that do not fit the source-to-sink paradigm. For example, a significant part of the Atlantic margin is shown to be heavily influenced by along-slope geostrophic currents that generated massive contourite drift deposits. These deposits reflect lateral transport of sediment that had a significant impact on the morphology of the margin. The role of mass transport processes in shaping continental margins is also highlighted, and in particular the collapses of entire segments of the margin were observed. The prominent role mass failure processes play in delivering sediment to the adjacent abyssal plain is also critical in the ECS context. These observations challenge the entrenched notion of a continental margin comprising a shelf, slope, and rise and in particular the concept of the "continental rise". Prior to 2006, regions of the Arctic Ocean seaward of the Canadian landmass had fewer than 5000 km of seismic reflection data. The massive efforts of Arctic coastal States to map their margins for ECS purposes have led to a leap in technological advances to acquire data in ice-covered seas and have led to a wealth of new geoscientific knowledge. Perhaps foremost amongst this knowledge is demonstration that Canada Basin is indeed a fully developed ocean basin, albeit significantly infilled with sediment. Based on this knowledge and identification of related structures, new realistic tectonic scenarios for opening of the Amerasia Basin are proposed that include a significant component of transform or strike-slip motions. With seismic velocity and rock sample information, the continental nature of Alpha and Mendeleev ridges has been substantiated. Even bathymetric data were lacking in the Arctic and new editions of seafloor maps now support grids of 500 m spacing; although some regions remain sparse. Once thought to be relatively stagnant, sedimentary processes such as found in many ocean basins were discovered in the Arctic Ocean. Evidence of geostrophic currents, sediment mass failures, and deep-sea turbidity current channels were found to be ubiquitous, even in the deepest parts of the Arctic's basins. [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]

• Gravity-derived basement topography constraint on the Canada Basin spreading system. • Consistent oblique ridge sub-segmentation along northeast-trending non-transforms. • Highly oblique sea-floor spreading in the Canada Basin during the Early Cretaceous. • Termination of seafloor spreading during a geomagnetic reverse polarity chron. • Juxtaposition of Chuckchi Borderland and Sever Spur prior to ocean crust formation. Understanding the evolution of ocean basins is critical for studies in global plate tectonics, mantle dynamics, and sea-level through time, and relies on identifiable tectonic plate boundaries. The evolution of the 2.5 million km2 Amerasia Basin in the Arctic Ocean remains largely unsettled due to widespread overprint by the Cretaceous High-Arctic Large Igneous Province. Traces of an extinct, but deeply buried, spreading centre (herein South Amerasia Ridge, SAR) has been shown to exist in the southern part of the Amerasia Basin, in the Canada Basin. However, structural details of the SAR and, hence, the kinematic evolution of the Canada Basin, are yet to be unraveled. Based on 3D gravity inversion and the vertical gravity gradient of the latest generation of satellite gravity models, we document new structures within the Canada Basin spreading system. Our results are supported by analysis of aeromagnetic and recent marine geophysical data. Evidence is shown of consistent oblique segmentation of the SAR spreading centre in a right stepping en echelon pattern. The spreading segments are offset by northeast-trending non-transforms that are traceable throughout the oceanic crustal domain and parallel to pre-oceanic strike-slip faults in the older part of the Canada Basin. We interpret the SAR to have formed by highly oblique spreading in a northeast-southwest direction. We compare the predicted SAR basement topography with the global ridge systems and produce a detailed magnetic modelling also constrained by the basement topography. The results indicate that the SAR crust formed by a slow-to-intermediate spreading regime and that sea-floor spreading terminated during a reverse polarity chron, most likely in the Early Cretaceous. Our novel plate reconstruction model, adopting a highly oblique spreading in Canada Basin, requires a translational motion of the Alaska/Chukotka tectonic block, replacing the decades-old rotational model of the Cretaceous High Arctic. [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]