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Our 2‐D‐layered velocity model along strike northeastern Nova Scotian margin, constrained by wide‐angle seismic Profile OCTOPUS and coincident 9‐km streamer Profile GXT‐5100, displays highly variable basement structures interpreted to define four distinct zones within the continent‐ocean transition: Zone I, with thin (1–2 km) and laterally uniform upper crust (5.2–5.5 km/s) and high‐velocity lower crust (6.5–7.7 km/s); Zone II, with velocities of 5.5–7.5 km/s and a high vertical velocity gradient (∼1.1/s) characteristic of exhumed serpentinized (up to 90%) mantle (ESM) above ∼3.5‐km‐thick slightly (<30%) serpentinized mantle layer of reduced mantle velocities (7.5–8.0 km/s); Zone III, with 1‐ to 2‐km‐thick upper crust and velocity of ∼5.2 km/s above a ∼6‐km‐thick, moderately (<60%) serpentinized mantle layer (velocity ∼6.4–7.9 km/s); and Zone IV, with lower crust (∼0.7–3.0 km thick and with velocity of 6.1–6.6 km/s) between upper crust (velocity ∼5.0–5.4 km/s) and a slightly serpentinized mantle layer (<40%; velocity ∼7.3–8.0 km/s). These continent‐ocean transition structures represent embryonic oceanic or rifted continental crust, and ESM. By combining our results with those from two dip‐oriented crossing profiles, we map a regional track of non‐ESM landward of Profile OCTOPUS with mostly uniform along‐dip width, but a limited extent and variable width section of ESM seaward. Our results indicate that there can be a remarkable amount of short‐wavelength (e.g., 50 km) lateral structural variability within individual rifted margin segments but that mapping it requires a grid of modern wide‐angle profiles. Continent‐ocean transition at the northeastern Nova Scotia Margin displays highly variable short‐wavelength tectonic structures along strikeWidespread serpentinized mantle is found beneath the transitional crust with exhumed serpentinized mantle present only locallyCapturing structural changes relevant to detailed rift tectonic studies requires a dense grid of high‐resolution wide‐angle seismic profiles

To improve constraints on rifting processes resulting in the formation of the southeastern Canadian margin, we interpret the most detailed regional 2‐D velocity model from offshore Nova Scotia constructed using wide‐angle OETR‐2009 data. This 405‐km‐long profile was collected with 78 ocean bottom seismometers. The presented data analysis and interpretation are supported by a reflection image from the coincident long streamer GXT‐2000 profile. We identify a continental zone where the full‐thickness (~30 km), three‐layered continental crust beneath the inner shelf thins sharply seaward by a listric fault that forms a 12‐km‐deep Huron Subbasin beneath a high‐velocity carbonate bank (~5.8 km/s), creating a shadow zone above tilted crustal blocks. Depth‐dependent and variable initial thinning is evidenced in all three modeled crustal layers, which, nevertheless, pinch out together at their seaward ends. More gradual and regional thinning and a local amagmatic thickening are modeled seaward beneath the slope until 70 km from the shelf break, beyond which a deepwater amagmatic continent‐ocean transition shows velocity characteristics not typical of either continental or oceanic crust. The 100‐km‐wide continent‐ocean transition is characterized by a low‐velocity (5.3–5.4 km/s), low gradient, <2‐km thick upper crust, above a high‐velocity (6.3–7.5 km/s), high gradient, <5‐km‐thick lower crust, which can be interpreted as moderately serpentinized mantle. Underneath this layer is a <5‐km‐thick low‐velocity (7.1–8.0 km/s) partially serpentinized mantle layer. A ~5‐km‐thick oceanic crust is modeled seaward. Our results suggest that amagmatic processes dominated the continental breakup in this area. A > 4‐km‐thick serpentinized mantle layer beneath thin crust within continent‐ocean transition indicates limited melt supply during riftingWhile all three continental crustal layers extended depth dependently, they pinched out together against the transitional zone seawardSynrift extension was amagmatic on both conjugate margins but with asymmetry resulting from postrift volcanism on the Moroccan margin

Two seismic refraction lines were acquired along and across the extinct Labrador Sea spreading center during the Seismic Investigations off Greenland, Newfoundland and Labrador 2009 cruise. We derived two Pwave velocity models using both forward modeling (RAYINVR) and traveltime tomography inversion (Tomo2D) with good ray coverage down to the mantle. Slow‐spreading Paleocene oceanic crust has a thickness of 5 km, while the Eocene crust created by ultraslow spreading is as thin as 3.5 km. The upper crustal velocity is affected by fracturation due to a dominant tectonic extension during the waning stage of spreading, with a velocity drop of 0.5 to 1 km/s when compared to Paleocene upper crustal velocities (5.2–6.0 km/s). The overall crustal structure is similar to active ultraslow‐spreading centers like the Mohns Ridge or the South West Indian Ridge with lower crustal velocities of 6.0–7.0 km/s. An oceanic core complex is imaged on a 50 km long segment of the ridge perpendicular line with serpentinized peridotites (7.3–7.9 km/s) found 1.5 km below the basement. The second, ridge‐parallel line also shows extremely thin crust in the extinct axial valley, where 8 km/s mantle velocity is imaged just 1.5 km below the basement. This thin crust is interpreted as crust formed by ultraslow spreading, which was thinned by tectonic extension. Seismic refraction images the thin crust of the Labrador Sea spreading ridgeFaulting becomes dominant as spreading rate decreases toward ridge extinctionExtreme thinning of the crust as well as an oceanic core complex are observed