[1] The Nova Scotia continental margin off eastern Canada marks a transition from a volcanic to a nonvolcanic style of rifting. The northern (nonvolcanic) segment of the margin was studied by a 490-km-long refraction seismic line with dense air gun shots, coincident with previous deep reflection profiles. A P wave velocity model was developed from forward and inverse modeling of the wide-angle data from 19 ocean bottom seismometers and coincident normal incidence reflection profiles. The continental crust has a maximum thickness of 36 km and is divided into three layers with velocities of 5.7-6.9 km/s. Crustal thinning down to 3 km occurs in a 180-km-wide zone and the sediment cover in this area is up to 15 km thick. Farther seaward, a 150-km-wide transition zone is observed with a 5-km-thick lower layer (7.2-7.6 km/s) interpreted as partially serpentinized mantle. At the landward end, this layer is overlain by highly altered continental crust (5.4 km/s) extending up to the seaward limit of the Jurassic salt province. Farther seaward, the upper layer is interpreted as exhumed and highly serpentinized mantle (5.1 km/s) separated from the lower layer by subhorizontal reflectivity, which probably represents a serpentinization front. Oceanic crustal thickness is 4 km with layer 2 velocities of 4.6-5.0 km/s. Layer 3 velocities of 6.4-6.55 km/s are lower than typical lower oceanic crust velocities but consistent with a low magma supply and increased tectonism as observed on the reflection profile. This reduced magma production might be related to the proximity of the Newfoundland transform margin.

S U M M A R YThe 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 oceancontinent 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.

The Nova Scotia and Morocco margins formed within a transition between volcanic margins to the south and non-volcanic margins to the north. We present deep seismic profiles to document the nature of this transition. Profiles on the Nova Scotia margin show two major transitions. The first transition represents a sharp reduction in syn-rift volcanism coincident with major changes in the East Coast Magnetic Anomaly and with the southern limit of the Slope Diapiric Province. The second transition represents a further restriction in syn-and post-rift volcanism leading to exposure of serpentinized mantle or the creation of highly tectonized oceanic crust. This transition is marked by highly extended and faulted continental crustal blocks. Revised plate reconstructions show similar transitions along the Moroccan margin. The southern transition occurs at a major change in the West Africa Coast Magnetic Anomaly and the southern limit of the Morocco Salt Basin. The second transition occurs at a major basement high (Tafelney Plateau), which is considered a high relief accommodation zone and contains highly extended faulted crustal blocks similar to those in a conjugate position off central Nova Scotia. This transition marks a major change in rifting asymmetry and separates the margins into two fundamentally distinct segments.

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