The western continental margin of Iberia extends from Cape Finisterre in the north to Cape Saint Vincent in the south (Fig. 1). The continental margin has a straight narrow shelf and a steep continental slope. South of 40°N, the slope is cut by several large canyons. This simple picture is complicated by several offshore bathymetric features. The largest feature is the Galicia Bank, a 200- to 150-km area within which the seafloor shoals to a minimum water depth of about 600 m. Galicia Bank is characterized by a series of isolated seamounts on its southern edge (Vigo, Vasco da Gama, and Porto) and is separated from northwestern Iberia by a broad submarine valley underlain by the Interior Basin. At 39°N, the Estremadura Spur extends east-west about 150 km from the shelf edge and forms a barrier between the Iberia and Tagus abyssal plains. Last, the east-northeast-trending Gorringe Bank forms the southern boundary of the Tagus Abyssal Plain and marks the surface expression of the seismically active Eurasia/Africa Plate boundary.
Like many rifted or passive margins, the west Iberia margin had a long history of rifting before the separation of Iberia from the Grand Banks of North America. Three main Mesozoic rifting episodes affected the west Iberia margin. These episodes are recorded in the deposits of the Lusitanian Basin, which is probably continuous with the Interior Basin that separates Galicia Bank from northeastern Iberia (Wilson et al., 1989; Murillas et al., 1990). A Triassic to Early Jurassic (Liassic) continental rifting phase created graben and half-graben structures in which evaporites were deposited. The second rifting phase consisted of extension during the Late Jurassic. The last phase of extension occurred during the Early Cretaceous (from Valanginian to early Aptian time), which coincided with the south-to-north breakup of Iberia from the Grand Banks and which has been well documented with offshore geological and geophysical data (Boillot, Winterer, et al., 1988; Whitmarsh, Miles, and Mauffret, 1990; Pinheiro et al., 1992).
The rifting phases were accompanied by only minor volcanism (dikes and flows) within Iberia. Two phases of pre-breakup volcanism were recognized by Ribeiro et al. (1979) and Martins (1991). A tholeiitic phase lasted from 190 to 160 Ma, coeval with Late Jurassic rifting, and a second phase occurred from 135 to 130 Ma in the Lusitanian Basin. This volcanism was relatively minor, and the west Iberia margin has the characteristics of a nonvolcanic margin. For example, tilted fault blocks and half grabens clearly can be observed off Galicia Bank (Mauffret and Montadert, 1987), and no evidence is seen of seaward-dipping reflectors or of substantial subcrustal underplating.
Parts of the west Iberia margin underwent two additional phases of deformation in the Eocene and the Miocene. The Eocene deformation was caused by the Pyrenean orogeny and the abortive subduction of the Bay of Biscay oceanic crust beneath the north Spanish margin; this deformation affected the margin adjacent to the Iberia Abyssal Plain and included the uplifting of Galicia Bank and adjacent seamounts (Boillot et al., 1979). The Miocene deformation accompanied a phase of compression in the Rif-Betic Mountains and led to the gentle folding of sediments in the Iberia and northern Tagus abyssal plains, as seen in reflection profiles (Masson et al., in press; Mauffret et al., 1989).
Several plate-tectonic reconstructions have attempted to show the original positions of North America, Iberia, and Europe (Le Pichon et al., 1977; Masson and Miles, 1984; Klitgord and Schouten, 1986; Srivastava et al., 1988; Srivastava, Roest, et al., 1990; Srivastava and Verhoef, 1992). The along-strike relative positions of the North American and European plates are not well constrained because oceanic crust that formed mostly during the Mesozoic constant-magnetic-polarity interval lies offshore the Grand Banks and Iberia, and no large fracture zones occur at this latitude. However, a useful constraint is provided by the northern termination of the J magnetic anomaly, a probable isochron slightly older than M0, which should have been contiguous with the northern end of the J anomaly in the Newfoundland Basin. The various reconstructions differ by a few tens of kilometers in the north-south direction. The situation is further complicated by intraplate deformation and "jumping" plate boundaries, which imply that Iberia was alternately attached to Africa or to Europe (Srivastava, Schouten, et al., 1990). The reconstruction by Srivastava and Verhoef (1992), which included de-stretching of the continental crust, is now generally regarded as the most closely constrained (Fig. 2).