Excellent reviews of the main physiographic and geomorphological features of the Portuguese part of the west Iberia Margin can be found in Vanney and Mougenot (1981), Brum Ferreira (1981), Sibuet et al. (1987) and Mougenot (1988). These features are clearly visible on the bathymetric charts of the northeast Atlantic published by Laughton et al. (1975) and Lallemand et al. (1985). The summary presented here is largely based on the references presented above, to which the reader is referred for further details.
The width of the western Iberian continental shelf varies between 10 and 65 km along its approximate 700 km length (Fig. 3). It can be divided into three distinct segments, separated by the major canyons that cut across the shelf and descend into the abyssal domains (Vanney and Mougenot, 1981; Mougenot, 1988): (1) north of the Nazaré Canyon, (2) between the Nazaré and the Setúbal canyons, and (3) between the Setúbal and the Saint Vincent canyons (Fig. 3). The segment north of the Nazaré Canyon includes the Galicia Margin of northwest Spain and the Portuguese geographical provinces of Minho and Beira Litoral. Between Porto and Cape Finisterra the continental shelf is only about 30 km wide and is oriented roughly north-south. To the south of Porto, the continental shelf widens a little to about 40 km, and south of Aveiro its width increases still further to 50–55 km. In this region, the continental slope is significantly steeper than it is to the north, and its orientation changes to approximately northeast-southwest along the postulated Aveiro fault zone. Beyond the continental slope adjacent to Aveiro, a marginal plateau extends westward, widening to nearly 200 km in the north (Fig. 3). This comprises Galicia Bank and three major seamounts: Porto, Vigo, and Vasco da Gama. These shallow regions are separated from the continental slope by the Galicia Interior Basin. The D. Carlos Valley, in the southeast limit of Galicia Bank, is probably one of the main conduits of sediments from the continental shelf into the Iberia Abyssal Plain (Gardner and Kidd, 1987).The segment between the Nazaré and the Setúbal canyons is characterized by a relatively wide continental shelf (Fig. 3; Mougenot, 1988). In this area, a ridge projects out from the continental slope, forming one of the most important physiographic features of the margin: the Estremadura Spur. This spur separates the Iberia Abyssal Plain in the north from the Tagus Abyssal Plain in the south, and extends nearly as far west as the Tore Seamount, at the northern end of the Madeira-Tore Rise. The origin of the spur is not clear, but it coincides partly with an alignment of small volcanic features, at least as far west as the 2000-m contour (Mougenot, 1988). Besides the Nazaré and the Setúbal canyons, the only other major canyons that dissect this segment of the margin are the Lisboa and Cascais canyons, both situated southwest of Lisboa (Vanney and Mougenot, 1981). Small islands (Farilhões and Berlengas) occur close to the coast (10–15 km) to the northwest of Peniche (see Fig. 4A for location). The segment between the Setúbal and the Saint Vincent canyons is characterized, in general, by a fairly narrow (10–20 km) continental shelf, although between 37°25'N and 38°N it is extended to the west by a platform (Vanney and Mougenot, 1981). The coastline in this segment consists of prominent scarps cut in Paleozoic schists.
Two large abyssal plains occur off western Iberia: the Tagus Abyssal Plain in the south, and the Iberia Abyssal Plain in the north. The Tagus Abyssal Plain is an enclosed, extremely flat-floored basin, bounded to the east by the irregular continental margin of Portugal, onto which sediments have been channelled through the Lisboa and Setúbal canyons (Fig. 3). Piston cores show that the upper few meters of the sedimentary cover consist of thick turbidites, separated by pelagic layers (P. Weaver, pers. comm., 1989). The abyssal plain is surrounded by three major ridges: the Madeira-Tore Rise in the west, the Estremadura Spur in the north, and Gorringe Bank in the south. The Madeira-Tore Rise shoals to less than 1000 m in places, and it was formed very near the Mid-Atlantic Ridge by excess volcanism, contemporaneously with the adjacent oceanic lithosphere (Peirce and Barton, 1991). Gorringe Bank separates the Tagus Abyssal Plain from the Horseshoe Abyssal Plain. This ridge has been interpreted as an uplifted and tilted block of oceanic crust, resulting from the Africa-Europe collision (Purdy, 1975). At DSDP Site 120, on the northern flank of Gorringe Bank (see location in Fig. 3), the oldest sediments drilled were Lower Cretaceous (Barremian), resting on top of an oceanic basement consisting of spilitic basalts, serpentinites and meta-gabbros (Ryan, Hsü, et al., 1973). The Gettysburg Seamount (western peak of Gorringe Bank) appears to be essentially formed of serpentinite, whereas the basement of the Ormonde Seamount (eastern peak of Gorringe Bank) appears to consist of an oceanic section with gabbros of Berriasian age (140-143 Ma), locally covered by alkaline volcanic rocks of Paleocene age (Auzende et al., 1978; Comen, 1982; Féraud et al., 1986).
The Iberia Abyssal Plain is located off central and northwestern Iberia. To the south, the Iberia Abyssal Plain is separated from the Tagus Abyssal Plain by the Estremadura Spur and the Tore Seamount. To the west and northwest, the limits of the abyssal plain are roughly defined by the 4800-m isobath, which approximately marks the transition from the flat topography characteristic of the abyssal plain to the irregular morphology typical of oceanic basement further west. The sedimentary cover beneath the plain consists mainly of thick turbidites separated by pelagic layers. Major turbidite flows were probably channelled to the abyssal plain down the Porto, Aveiro, and Nazaré Canyons, but Galicia Bank and other elevated areas to the east and northeast of the abyssal plain may also have contributed sediment. The margin shows no signs of significant submarine fans or sediment slumping, unlike the conjugate margin (Gardner and Kidd, 1987). This has been interpreted by Gardner and Kidd (1987) as indicating that bottom currents must play an important role in redistributing the sediments transported downslope from the adjacent continental shelf and offshore highs.