The mechanism of continental extension and breakup, and the role of detachment faults in these processes, are currently the subject of intense debate. One possible detachment fault is the S reflector, imaged as an undulating (in time), locally discontinuous reflection on existing seismic profiles west of Iberia. Here we present new images in depth of the four margin-normal profiles across the west Galicia rifted margin, where the S reflector was originally defined and is best imaged. It is shown that faults bounding wedge-shaped units of late synrift sediments, which hence were active during rifting immediately prior to breakup, appear to detach at shallow levels onto the S reflector. S itself appears as a continuous, locally domal feature and does not generally appear offset. The waveform of S is compatible with a reflection from a single interface such as a sharp tectonic boundary. The depth sections show that S was active at 1--3 km below the seafloor during final rifting; S is interpreted as a brittle detachment fault which controlled the final breakup of the continent west of Galicia. Furthermore, the data provide constraints on the sense of shear of S: analogy with detachment terranes, the present, synrift and structural dips of S, and the identification of a breakaway to S imply that S accommodated top-to-the-west shear. Toward the east of the profiles, S becomes more complex, possibly because of different phases of detachment faulting and the development of both incisement and excisement structures. From the geometry of wedge-shaped sedimentary units deposited during faulting above S it also appears that S was active at an angle of 20¿ or less and hence may be considered a genuine low-angle normal fault. ¿ American Geophysical Union 1996