The nature of the ocean-continent transition along the West Iberia margin varies significantly over a distance of about 200 km, but certain seismic characteristics related to peridotite outcrops along the margin are fairly consistent. In 1997, over 4000 km of two-dimensional seismic reflection data were collected along this margin as part of the Iberia Seismic Experiment (ISE 97). Nine dip lines were recorded at 10-25 km spacing and therefore provide snapshots of the margin at regular intervals. The northernmost line in the survey coincides with the ODP Leg 103 transect, and the southernmost line roughly coincides with the ODP Legs 149/173 transect.

The ISE 97 data image segments R2 and R3 of the peridotite ridge, a margin-parallel series of peridotite outcrops emplaced near the seafloor at the end of rifting (Beslier et al., 1993). ODP core data confirm that basement highs consisting of serpentinized peridotite occur at both the northern and southern extents of the ISE 97 survey. Internal structure is visible within ridge segment R2 in the form of east-dipping reflectors near the top of the ridge. These reflectors may represent bedding planes, which implies that the western flank of the peridotite ridge is a fault that truncates these prerift beds.

On all ISE 97 dip lines with a clearly recognizable peridotite ridge, a basin immediately landward of the ridge displays a constant thickness of sediments on time-migrated sections. The water depth at this location is also constant on these lines. Isostasy implies that the continental crust is the same thickness (possibly zero) everywhere underneath this basin. This is also the deepest basin observed landward of the peridotite outcrops. The depth of the basin may represent the total tectonic subsidence maximum, which suggests a possible control on the location of the peridotite exposures.