A high-quality multichannel seismic reflection profile across the West Iberia margin confirms the existence of a 130-km-wide transition zone between tilted continental fault blocks and unequivocal oceanic crust. The zone is characterized by a 1.0–2.5-km-thick seismically unreflective upper basement layer lying over reflective basement that contains major landward-dipping reflectors that mostly truncate against the base of the upper layer and penetrate to depths of 10–19 km below the basement surface, and minor seaward-dipping reflectors. Moho reflections are absent in this zone. We suggest that the unreflective layer represents tectonically exposed upper mantle that has undergone extensive serpentinization enhanced by vigorous seawater circulation. About 40 km farther north the transition zone narrows to 35 km and the unreflective layer is less continuous and widespread.