The West Iberia continental margin contains an enigmatic north-south ridge of serpentinized peridotite located within the ocean-continent transition. We used prestack depth migration and total tectonic subsidence analysis to constrain the basement type adjacent to the ridge in order to learn more about its emplacement. Prestack depth migration was performed on segments of the nine east-west lines from the Iberia Seismic Experiment (ISE 97), and total tectonic subsidence analyses were performed on the depth sections. The seismic data and subsidence analyses are consistent with exhumed upper mantle occurring immediately landward of the ridge, suggesting that the ridge is located within a broad zone of exhumed mantle that has been serpentinized. The subsidence analyses also provide minimum bounds on the thickness of the serpentinized layer, which extends from the zone of exhumed mantle landward under thinned continental crust. Exhumed upper mantle has previously been identified in the wider ocean-continent transition to the south in the southern Iberia Abyssal Plain, and we conclude that similar but narrower exposures occur to the north in the Galicia Bank area. While upper mantle appears to have been tectonically exhumed along all the ISE 97 lines, it does not always form a distinct basement ridge. There is no well-defined ridge on three seismic profiles in the middle of the ISE 97 survey. Where it is well-developed, the peridotite ridge parallels a deeply-penetrating, west-dipping normal fault, and the three middle lines appear to represent a transfer zone where this normal fault is not well-developed.