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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

New heat flow observations have been made in the Iberia abyssal plain off the Galicia margin along the transect of Ocean Drilling Program Leg 149 drill sites, in order to investigate the nature of this unusually wide and deep continent-ocean transition region. Our results indicate the presence of three separate zones. Average values of 47.5 ± 3 mW m−2 in the westernmost zone III agree with predictions of standard oceanic lithospheric models for its estimated age of 126 Ma. In contrast, the heat flow within zone II is 5–15 mW m−2 higher than predicted, assuming that the mantle heat flow remains constant across the basin. This region of high values is coincident with the location of a major intra-crustal “S”-type reflector east of ODP Site 900, and the anomaly is consistent with the presence of 2–3 km of primarily upper continental crust above the reflector, with concentrations of radiogenic components similar to those from granodiorite samples dredged off Galicia Bank. It is not, however, consistent with the low values of heat production measured on gabbroic samples from its western end at ODP Site 900. In zone I, detailed measurements across the tilted fault block south of ODP Site 901 show consistent variations which closely match predictions due to the effects of basement structure and sediment deposition. There is no evidence for variations due to vertical convective transport along the dipping basement fault block. Once corrected for these variations, measurements in zone 1 yield average values that agree quite well with previous measurements across Galicia Bank, indicating no systematic landward increase in heat flow with decreasing amounts of continental extension.

We conducted a seismic refraction experiment across Flemish Cap and into the deep basin east of Newfoundland, Canada, and developed a velocity model for the crust and mantle from forward and inverse modeling of data from 25 ocean bottom seismometers and dense air gun shots. The continental crust at Flemish Cap is 30 km thick and is divided into three layers with Pwave velocities of 6.0–6.7 km/s. Across the southeast Flemish Cap margin, the continental crust thins over a 90‐km‐wide zone to only 1.2 km. The ocean‐continent boundary is near the base of Flemish Cap and is marked by a fault between thinned continental crust and 3‐km‐thick crust with velocities of 4.7–7.0 km/s interpreted as crust from magma‐starved oceanic accretion. This thin crust continues seaward for 55 km and thins locally to ∼1.5 km. Below a sediment cover (1.9–3.1 km/s), oceanic layer 2 (4.7–4.9 km/s) is ∼1.5 km thick, while layer 3 (6.9 km/s) seems to disappear in the thinnest segment of the oceanic crust. At the seawardmost end of the line the crust thickens to ∼6 km. Mantle with velocities of 7.6–8.0 km/s underlies both the thin continental and thin oceanic crust in an 80‐km‐wide zone. A gradual downward increase to normal mantle velocities is interpreted to reflect decreasing degree of serpentinization with depth. Normal mantle velocities of 8.0 km/s are observed ∼6 km below basement. There are major differences compared to the conjugate Galicia Bank margin, which has a wide zone of extended continental crust, more faulting, and prominent detachment faults. Crust formed by seafloor spreading appears symmetric, however, with 30‐km‐wide zones of oceanic crust accreted on both margins beginning about 4.5 m.y. before formation of magnetic anomaly M0 (∼118 Ma).

The Madagascar Ridge is an elongated aseismic plateau in the southwest Indian Ocean, which separates two ocean basins of mid- to late Cretaceous age. From interpretations of a reversed refraction line on the ridge and a continuous gravity profile across it we conclude that it formed during the Cretaceous as thickened oceanic crust, at a mid-ocean ridge which possibly overlay a mantle hot-spot. The seismic data consist of record sections from five recording sonobuoys. Forty-one shots were fired along a 210 km long NNE—SSW line across the central part of the ridge. Simple horizontal-layer analysis of first arrivals reveals a lower crustal refractor with Vp = 7 km s−1 at a depth of 8–8.5 km and the Moho at an apparent depth of 18 km, but fails to characterize the crustal structure as either oceanic or continental. We obtain a definitively oceanic structure by using synthetic seismogram techniques together with Tau(p) analysis and second arrival correlations. The model which best fits the data consists of 2.2 km of water and 1 km of sediments overlying a velocity gradient from 4 to 7 km s−1. Beneath this, at a depth of 7.5 km, is a 9.5 km thick layer with Vp = 6.9–7.1 km s−1. A 7.5 km s−1 layer, not observed in the first arrival analysis, is found at the base of the crust. The Moho is a transition zone 1 km thick and occurs at ∼ 25 km depth. The gravity data consist of a continuous NE—SW profile across the ridge and on to the flanking basins. Two-dimensional gravity modelling allows a structure consistent with the synthetic seismogram results, and supports the existence of a high-density basal layer in the crust. We use in addition a time-series analysis approach to estimate the depth of compensation assuming an Airy model, and lithospheric thickness assuming an elastic plate model. Convolution of bathymetry along the profile with a range of model filters gives minimum rms gravity residuals for a crustal compensation depth of 23±5 km, and a plate thickness of less than 2–5 km.

Six gravity and bathymetry profiles perpendicular to the Kane fracture zone, each more than 300 km long, were gathered to study the variation in crustal structure in the vicinity of a major fracture zone and the gravitational edge effect at the contact between lithosphere of two different ages. A spectral analysis of the gravity and bathymetric series as a function of wavelength shows that the gravitational edge effect is only significant at the longest wavelengths. For remaining wavelengths the admittance, the ratio of the amplitude of the gravity anomaly to the amplitude of the bathymetry, is best explained by a model of isostasy in which topographic loads are partially supported by the flexural rigidity of an elastic plate, about 6 km in thickness. After subtracting the gravitational attraction of the bathymetry and its compensation, substantial isostatic anomalies remain. We interpret these anomalies as being caused by variations in crustal thickness which have little correlation with surface topography, except at very long wavelengths. The apparent crustal thickness varies by as much as a factor of 2, but there is no evidence indicating systematic thinning of the crust beneath the fracture zone. Our data do suggest that such density variations within the plate are also compensated by the isostatic response of an elastic plate but with very different effect from those at the surface. This indicates that there are two different modes of crustal formation with different gravity and topographic signatures: effusive volcanism which loads the surface of the elastic plate producing both topographic relief and coherent gravity anomalies, and intrusive volcanism or underplating producing gravity anomalies but little topographic relief.

The spectral analysis of a 550 km long ship-board gravity and topography profile, across the central part of King's Trough in the north-eastern Atlantic Ocean, shows that the topography is regionally compensated on lithosphere older than 20 Myr. This feature, with its enigmatic trend oblique to that of the mid-Atlantic ridge, therefore dates from no earlier than the late Oligocene and was formed by intra-plate activity. Such a constraint to its age agrees with the recent analysis of geological samples taken on the northern flank of the trough close to the position of the profile. It is not consistent with previous suggestions that King's Trough was created either by the rotations of the Iberian plate to form the Pyrenees or by a readjustment in the strike of the mid-Atlantic ridge. Furthermore, this analysis shows no evidence for either thin crust or high density mantle under the trough. There is no structural evidence to suggest that it formed as part of a compressional boundary.

In this paper, an analysis of seismic refraction and gravity profiles is presented to define the crustal structure across a ridge-trough deformational feature in the Central Indian Basin. The purpose is to investigate what effects the deformation has had on the crust during the incipient rupture of oceanic lithosphere. The wide-angle seismic reflection/refraction profiles which are modelled in this paper come from two previously reported data sets (three R/V Conrad profiles and two R/V Mendeleev profiles) located south of the ODP Leg 116 drilling sites, where N-S reflection profiles clearly delineate a symmetrically upwarped and faulted basement ridge bounded by two undeformed troughs. Previous analyses of the refraction data gave conflicting accounts of velocity and thickness variations between trough and ridge structures. In this paper, the refraction profiles are displayed in similar manner and both vs and vp structures are determined by forward modelling using 1-D reflectivity synthetics. Results indicate a typically mature oceanic upper crust for all profiles (vp = 4.5–6.3 km s-1; depth=0–1.2 km; σ=0.35–0.27) above a transitional crust (vp = 6.3–6.8 km s-1; depth=1.2–2.6 km; σ=0.25). In the middle and lower crust, there are major changes between trough and ridge profiles: (i) within the basement troughs, there is a sharp transition between layer 3A (vp = 6.7 km s-1; thickness=1.7 km; σ=0.26) and layer 3B (vp = 7.1–7.4 km s-1; thickness=2.0–2.5 km; σ=0.25); (ii) on the ridge crest, vp is increased to 7.2 km s-1 in layer 3A (σ=0.26) but reduced to 6.6 μM 0.2 km s-1 in layer 3B (σ=0.24–0.30), forming a distinct low-velocity zone in both vp and vs. The crustal thickness is reduced progressively at its base from 6.2 μM 0.3 km to 5.4 μM 0.2 km. A 2-D gravity model across the ridge-trough structure, based on the 1-D seismic velocity-depth models, is consistent with an observed anomaly of 40–45 mGal. These results demonstrate that the crust does not thicken beneath the basement ridge crest, as has previously been suggested. The crustal structure is, therefore, more consistent with lithospheric deformation by buckling than by boudinage. The crust beneath the ridge is altered primarily by the development of a low-velocity and low-density zone within the high-velocity lower crust of seismic layer 3B. A possible primary origin for the LVZ is by serpentinization at low temperatures of olivine clasts within the mafic gabbro of layer 3B. Asymmetric folding of the lower crust or lateral flow of the more viscous serpentine might explain the possible crustal thinning under compression. Penetration of water to deep crustal levels within this region is consistent with recent deep multichannel reflection images of reverse faults which penetrate into or through the lower crust. It is also consistent with fluid flow along faults within the sediment which have perturbed the surface geothermal gradients. This process could explain the discrepancy between the high regionally averaged heat flow and normal basement depths when compared to standard lithospheric thermal models.