We present a joint analysis of seismic reflection and refraction profiles across an extinct spreading center in the Labrador Sea. The refraction data are used to constrain the crustal and subcrustal velocity structure, and the deep multichannel reflection data are used to determine the nature of the tectonic fabric within the crust. The resulting crustal model shows that major reductions in crustal thickness and velocity across the rift are associated with a broad zone of rotated fault blocks. Enhanced faulting persists over a width of 100 km, creating a deep rift valley and adjacent elevated ridges. The location of crustal-scale faults beneath the sides of the rift valley bound a region of reduced mantle velocity, consistent with its formation by 5%–10% serpentinization of mantle peridotite. The variation in crustal thickness across the rift indicates a 30%–40% reduction in the crustal thickness for full spreading rates <20 km/m.y. However, our results suggest that most of the observed crustal thinning is produced by postaccretionary tectonic extension of the crust, during an extended interval when the rate of melt supply did not keep pace with the rate of plate separation. Such a discontinuous process may not be properly represented by theoretical models of crustal formation that assume steady-state viscous flow.