We present a wide-angle seismic refraction study of an 80¿40 km region of the southern Iberia Abyssal Plain, south of Galicia Bank. An intersecting grid of two E-W and four N-S wide-angle reflection/refraction profiles is used to define variations of the basement velocity structure within this unusually wide ocean-continent transition (OCT). These structures can be systematically linked to variations in acoustic basement morphology and to results from Ocean Drilling Program (ODP) boreholes. Lateral changes in the velocity structure of the basement occur abruptly over distances of ~20 km where complex variations may be found. Thinned upper continental crust, 2--5 km thick with velocities of 5.0--6.6 km/s, is limited to a series of N-S fault blocks immediately south of Galicia Bank. This crust is underlain by a high-velocity layer (7.3--7.9 km/s) of weakly serpentinized (i.e., 0--25%) periodotite, which exists throughout the eastern part of the survey area. Basement within the OCT appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally. In the southeast sector of our survey where basement topography deepens and becomes subdued, continental fault blocks are absent; instead, basement contains an upper layer of more pervasively serpentinized (i.e., 25--45%) peridotite that is ~2 km thick. This layer is characterized by low velocity at the top of basement (4.2 km/s) that increases rapidly with depth, and it probably corresponds to a seismically unreflective layer, previously identified in reflection profiles to the south of our survey. In the western section of our survey, beneath a series of elevated basement ridges, velocities are reduced within both the upper basement layer (3.5--6.0 km/s) and lower layer (6.4--7.5 km/s). These changes suggest that both upper and lower layers have become more highly serpentinized (with values of 60--100% in the upper layer and 25--45% in the lower layer) probably during the last stages of rifting and immediately before formation of oceanic crust. A normal or slow spreading oceanic crustal structure is not found within the survey region. Thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at ODP Site 897 and east of the J magnetic anomaly. ¿ 1999 American Geophysical Union