Delineating the nature of crustal variations across passive continental margins is fundamental to our understanding of how the uppermost lithosphere deforms under extension. This information is best obtained across margins where the extensional fabric within the crust has not been significantly modified by large–scale synrift volcanism. Previous studies of such margins have shown a range of extensional styles, which have been interpreted by various combinations of pure and simple shearing of continental crust and lithosphere. This paper reanalyses several key transects in the North Atlantic, including the conjugate margin pairs of Newfoundland–Iberia, Flemish Cap–Goban Spur and Labrador–W. Greenland. Analysis of both deep multichannel seismic reflection and wide–angle reflection/refraction profiles allows us to produce new joint constructions of velocity and reflectivity depth sections. These sections indicate major asymmetries in the width and faulting style of continental rifting between margin conjugates. They also show a general occurrence of a complex transition zone between extended continental and oceanic crust that is dominated by exposed and serpentinized upper mantle. Because serpentinite has a much lower strength than is typical of continental or oceanic crustal rocks, its existence within the transition zone may have a profound influence on how these margins develop. The possible effects that such time and spatially variable rheologies have on the development of these margins needs to be considered in future geodynamical models.