We present multichannel seismic reflection and ocean bottom seismometer reflection/refraction data from ISE-9, a margin-parallel, north--south oriented profile ~200 km west of the Portuguese coast. ISE-9 images the boundary between two distinct segments of the Iberia nonvolcanic rifted margin: Galicia Bank (GB) and the Southern Iberia Abyssal Plain (SIAP). The bathymetric contrast between GB (2 km depth) and SIAP (4--5 km depth) spans only 25 km. The crustal thickness transition, however, spans 137 km, from 13--18 km thick beneath GB to <2 km thick beneath SIAP. We define this crustal thickness transition as the segment boundary. Crustal structure along the segment boundary, tilted blocks bounded by normal faults, is surprisingly similar to crustal structure observed along orthogonal, east--west profiles of the Iberia margin. The apparent north--south extension is similar in magnitude to previously calculated east--west extension, implying an overall northeast--southwest extension. However, paleoreconstructions and rift basin orientations constrain lithospheric extension to a nearly east--west direction. We speculate that north--south extension is limited to the crust and is caused by large-scale mass wasting sometime between the Tithonian and Valanginian. This rotational slump spans the 137-km-wide modern segment boundary, emplacing GB continental crust directly onto exhumed, serpentinized mantle of SIAP. Palinspastic reconstruction restores the southern edge of the blocks to coincide within <10 km of a steep Moho transition and a near-vertical fault which extends through the slump and offsets Moho. We interpret this location as the preslump segment boundary that accommodated transform motion between the two rifted margin segments.