One of the most controversial issues in geology today is the way the lithosphere extends. The Galicia margin is a particularly good place to address this problem as it is fairly sediment starved, has clear upper crustal structure well imaged by a variety of seismic reflection profiles and has well control to calibrate seismic data. Observed subsidence is compared with extension of the uppermost crust of the entire margin as computed from the geometry of tilted fault blocks imaged on a series of parallel seismic lines. The pure shear (or stretching) model clearly accounts for the extension of the upper continental margin but not for the lower margin where the S reflector zone, a prominent zone of intra-crustal reflectors, is present. I suggest that the S reflector zone is a datachment zone accommodating decreasing displacement towards the continent and that this detachment zone, active since the beginning of rifting, outcrops on the Canadian side and corresponds to the steep southeastern Flemish Cap continental slope. Pure shear is the mechanism which affects the whole lithosphere, but in the upper part of the crust, the rifting mechanism is locally simple shear where the detachment zone is active. ¿ American Geophysical Union 1992