S U M M A R YA 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.

S U M M A R YNew multichannel seismic reflection data were collected over a 565 km transect covering the non-volcanic rifted margin of the central eastern Grand Banks and the Newfoundland Basin in the northwestern Atlantic. Three major crustal zones are interpreted from west to east over the seaward 350 km of the profile: (1) continental crust; (2) transitional basement and (3) oceanic crust. Continental crust thins over a wide zone (∼160 km) by forming a large rift basin (Carson Basin) and seaward fault block, together with a series of smaller fault blocks eastwards beneath the Salar and Newfoundland basins. Analysis of selected previous reflection profiles (Lithoprobe 85-4, 85-2 and Conrad NB-1) indicates that prominent landward-dipping reflections observed under the continental slope are a regional phenomenon. They define the landward edge of a deep serpentinized mantle layer, which underlies both extended continental crust and transitional basement. The 80-km-wide transitional basement is defined landwards by a basement high that may consist of serpentinized peridotite and seawards by a pair of basement highs of unknown crustal origin. Flat and unreflective transitional basement most likely is exhumed, serpentinized mantle, although our results do not exclude the possibility of anomalously thinned oceanic crust. A Moho reflection below interpreted oceanic crust is first observed landwards of magnetic anomaly M4, 230 km from the shelf break. Extrapolation of ages from chron M0 to the edge of interpreted oceanic crust suggests that the onset of seafloor spreading was ∼138 Ma (Valanginian) in the south (southern Newfoundland Basin) to ∼125 Ma (Barremian-Aptian boundary) in the north (Flemish Cap), comparable to those proposed for the conjugate margins.

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.Subsequent to these previous studies, a dense wide-angle profile OETR-2009

We used first arrivals and Moho reflections from the 500km-long Orphan Basin Wide-Angle Velocity Experiment (OB-WAVE) profile with 3-5-km instrument spacing to construct a traveltime tomography section and to delineate the Moho discontinuity across the Orphan Basin. The Orphan Basin is a failed rift located offshore Newfoundland, Canada, showing thinned continental crust over an unusually wide region. We observed (1) a zone of extreme crustal thinning (<7-km-thick crust) with no evidence for mantle serpentinization, (2) basement morphology exhibiting tilted blocks linked to the crustal thinning, and (3) a thicker central crustal segment that is probably related to prerift structural inheritance. Comparison with the adjacent Jeanne d'Arc Basin to the southeast suggested the presence of a decoupling zone between the two basins accommodating the difference in extension rates. There was a good correlation between the tomographic velocities and the reflection structure derived from a coincident seismic reflection profile except in an area in which the reflection seismic data suggested the presence of a deep sedimentary basin. The velocity model computed in this work indicated that this area consists of prerift basement rather than Jurassic or older sediments. Tomographic models computed by varying the density of the recording instrument array gave insight into the relationship among the target size, the instrument spacing needed to resolve it, and the velocity model uncertainty. These results may help guide the design of future wide-angle reflection and refraction surveys across rifted structures.

SUMMARY New seismic reflection data from the Grand Banks of Newfoundland and the Newfoundland Basin add to the growing knowledge of the composition, structure and history of this non‐volcanic margin. Geophysical imaging is now approaching the extent of that done previously on the conjugate margin along Iberia, providing a valuable database for the development of rifting models. Two parallel profiles over the shelf platform image deep crustal fabric representing Precambrian or possibly Appalachian deformation as well as Mesozoic extension. Progressively more intense extension of continental crust is imaged oceanwards without the highly reflective detachments frequently seen on profiles off Galicia. A landward‐dipping event ‘L’ is imaged sporadically and appears to be analogous to a similar event on the Iberian IAM9 profile. The transition zone is probably exposed serpentinized mantle as interpreted off the Iberian margin although there appears to be a difference in the character of ridge development and reflectivity. The distinctive ‘U’ reflection identified previously at the base of the Newfoundland Basin deep water sedimentary section and recently identified as one or more thin basalt sills is imaged on newly presented profiles that connect previously published profiles SCR3 and SCR2 showing that ‘U’ is highly regular and continuous except where interrupted by basement highs. ‘U’ is also seen to have a major impact on the ability to image underlying basement. A full transect beginning over completely unextended continental crust through to oceanic crust has provided a data set from which estimates of extension and the pre‐rifting location of the present continental edge can be made. Two estimates were obtained; 85 km based on faulting and 120 km based on crustal thickness.