The crust across each margin of the Labrador Sea, a conjugate rift margin pair, consists of three well-defined zones: thinned continental crust, 70–80-km-wide transitional crust, and oceanic crust. The transition zone is characterized by a low-velocity upper crust (4–5 km/s) underlain by a 6.4–7.7 km/s layer. We propose that the lower layer is serpentinized upper mantle; it is less likely to be gabbroic igneous lower crust. The low-velocity upper crust may be either an oceanic basaltic layer or very thin (<2 km) continental crust. Results from this first combined seismic reflection and refraction study of a conjugate margin pair support very slow initial continental rifting (∼1 mm/yr on average) resulting in an asymmetric crustal-thinning profile, with breakup occurring against the West Greenland shelf. This asymmetry may be an important feature of rifting at nonvolcanic margins and may be related to strain hardening of the cooling and thinning lithosphere or to small-scale convection in the lower lithosphere and asthenosphere at the sides of the rift. The thin crust in the transition zone implies that the amount of melt generated in the upwelling mantle is negligible, consistent with slow spreading or rifting. The transition from this slow formational period to normal oceanic accretion processes is marked by a sharp decrease in basement depth, signaling an increase to normal oceanic crustal thickness.