A 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.