The western margin of Iberia consists of a fragment of the Variscan orogen (often referred to as the Hesperic massif), above which occur several Mesozoic–Cenozoic basins that were deformed to varying degrees during two Cenozoic compressional episodes (Capdevila and Mougenot, 1988). The evolution of these basins was related to the reactivation of structures within the Variscan basement, in particular of those associated with the late Variscan deformation phases.
The Variscan structures in Iberia, together with those of the Armorican Massif, define the so-called Ibero-Armorican Arc, which was subsequently disrupted by the opening of the Bay of Biscay. In the Iberian Peninsula, the Variscan orogeny started in the middle Devonian and ended in the Carboniferous (Ribeiro et al., 1979). In very broad terms, the Variscan Massif consists mainly of folded, thrusted and metamorphosed rocks of Precambrian and Paleozoic age, which were extensively intruded by large granitoid batholiths during, and after, the Variscan continent-continent collision. The massif has been divided into five main zones, each characterized by distinct paleogeography, structural style, metamorphism, and magmatism (Julivert et al., 1980; Ribeiro et al., 1979) (Fig. 4C). The Central Iberian Zone, the Ossa Morena Zone, and the South Portuguese Zone underlie the west Iberia Margin. The thrust separating the Central Iberian Zone from the Ossa Morena Zone (the Ferreira do Zêzere-Portalegre thrust) is inclined to the southwest, whereas the thrust that marks the northern boundary of the South Portuguese Zone (The Ferreira do Alentejo-Ficalho thrust) dips north or northeast (Ribeiro et al., 1979; Ribeiro and Silva, in press). In the Central Iberian Zone, both synorogenic (350–300 Ma) and post orogenic granitoids (280 Ma) are observed (Ribeiro and Silva, in press). In the Ossa Morena zone, however, the synorogenic magmatism consists of a suite of gabbros, diorites, and granodiorites, followed by intermediate calc-alkaline volcanics of lower Carboniferous age (Ribeiro et al., 1979; Ribeiro and Silva, in press). The South Portuguese Zone consists of low-grade upper Paleozoic metasediments, with an ophiolite marking its northern boundary. First-phase fold axes in all the zones trend predominantly north-northwest to south-southeast, but second-phase structures show a northwest to southeast trend in the Ossa Morena and South Portuguese Zones.
Recently, the Hesperic Massif has been interpreted as consisting of several major tectonostratigraphic terranes (Fig. 4A-B) that were accreted together at different times during the late Paleozoic Variscan orogeny, which resulted from the closure of the Paleotethys Ocean (Ribeiro et al., 1987; 1990; Quesada, 1990; 1991; Ribeiro and Silva, in press). In Portugal, these include: (1) the Iberian terrane, which occupies most of the Massif, shows Gondwanian affinities at least until the late Proterozoic and is considered the Iberian autochthon (it includes the Central Iberian and Ossa-Morena zones, cf. Fig. 4C); (2) the northern ophiolite, which is an exotic terrane or terranes overlying the Iberian Terrane, representing Paleozoic ocean floor and overlying sedimentary sequences that form an ophiolitic thrust sheet obducted eastward onto the Iberian terrane during the Variscan convergence (possible remnants of a Paleozoic ocean lying west and south of the Iberian Terrane) (Ribeiro et al., 1990; Quesada, 1990); (3) the northern continental terrane, another exotic terrane overlying the Iberian Terrane and consisting of several allochthonous units (Precambrian basement rocks overlain by Paleozoic clastic cover sequences) of unknown palinspastic origin, composing thrust sheets that sit on top of the ophiolitic nappes in NW Iberia (Bragança and Morais Massifs in Portugal [Fig. 4A] and Cabo Ortegal and Ordenes in Spain [Fig. 4B]); (4) The South Portuguese terrane, which is a continental block that includes the pyrite belt (quartzites, a volcanic sedimentary complex, and turbidites), the Baixo Alentejo Flysch Group, and the southwest Portugal domain (sedimentary units that represent time equivalents of the Pyrite belt, in condensed facies); and (5) the South Oceanic Terrane, which represents a major suture within the Variscan fold belt and separates the South Portuguese Terrane from the Iberian Terrane. The South Oceanic Terrane includes the southern ophiolite (Beja-Acebuches ophiolite and ophiolite klippen overlying the Iberian Terrane) and the Pulo do Lobo unit. The latter is essentially a metasedimentary unit with minor representation of ophiolite melanges and volcanic rocks with a typical N-MORB chemistry (from the base upwards: serpentinites, flaser gabbros, massive and banded gabbros, sheeted dikes, and pillow basalts) (Quesada et al., 1994; Ribeiro and Silva, in press). The area of basement exposed in and on the seafloor around Berlengas and Farilhões islands, off Peniche (Fig. 4A), has been recently interpreted as a suspect terrane, corresponding to a sliver of the Iberian Terrane transported southwards by a late Variscan movement along a north-south oriented sinistral strike-slip fault, and subsequently thrust eastwards during the Permian (Ribeiro and Silva, in press).
The late Variscan deformation in Iberia was marked by the development of brittle fault systems (Ribeiro et al., 1990). During this phase, the stress field was complex. The Porto-Tomar Fault was reactivated as a dip-slip, high-angle reverse structure, suggesting an essentially east-west maximum compressive direction. At approximately the same time, however, conjugate strike-slip faults were also generated: a sinistral north-northeast–south-southwest to east-north-east–west-southwest set (which is predominant) and its dextral north-northwest-south-southeast to northwest-southeast conjugate set (Ribeiro et al., 1979; Dias, 1986); these indicate a north-south maximum compressive stress (Ribeiro et al., 1990). This complex regime has been interpreted as a corner effect within the second-order Iberian block, which was compressed within a diffuse, large-scale right-lateral shear zone extending between the Urals and the southern Appalachians (Arthaud and Matte, 1975; Ribeiro et al., 1990).
A detailed study of plutonic and metamorphic rock samples from dredge hauls and cores from the west Iberia Margin has led the recognition of the offshore extension of the main zones of the Hesperic Massif observed under Iberia (Capdevila and Mougenot, 1988). The Iberian terrane (which includes the Central Iberian Zone and the Ossa Morena Zone) seems to extend from northernmost Iberia to south of the Nazaré canyon. Low-pressure amphibolite-facies metamorphic rocks of sedimentary origin, a variety of granites, granodiorites, tonalites, and low-to-intermediate pressure granulites have been recovered from Galicia Bank and from the northern Portuguese continental shelf. Of these, only the granulites (1500 m.y. old; Postaire, 1983) apparently have no onshore equivalents in the Iberian terrane and probably form part of the pre-Variscan complex (the granulites from the NW Iberian nappes are of high-pressure type and probably Paleozoic). The tonalites and granodiorites commonly found in the Ossa Morena Zone are well represented on the northwestern Iberian continental slope and in the Galicia Bank area but not on the shelf. Similar basement rocks have also been recovered from the continental shelf to the west of the Nazaré canyon and as far south as the Sines area. In contrast, the abundant peraluminous leucogranites found in Galicia (Central Iberian Zone) are rarely found on the slope but are common on the Galicia and northern Portuguese continental shelf. Based on this evidence, Capdevila and Mougenot (1988) interpreted that rocks from the Central Iberian Zone form the basement of the continental shelf of Galicia and northern Portugal, whereas rocks from the Ossa Morena zone form the basement in the area between the continental slope off Galicia and the continental shelf west of the Nazaré canyon.
Between the Cape of Sines and Cape Saint Vincent, coring yielded flysch sequences, volcano-sedimentary rock sequences, and quartzites typical of the South Portuguese Zone (Capdevila and Mougenot, 1988), suggesting that the offshore basement to the south of Lisbon consists essentially of rocks from the South Portuguese terrane.
Many of the tectonic trends within the Mesozoic-Tertiary basins of western Iberia follow the prominent Variscan features (Fig. 5; cf. Fig. 2C). The late Variscan strike-slip faults, in particular the predominant north-northeast–south-southwest to east-northeast–west-southwest set (see Fig. 2C), have played an important role since the Triassic. They behaved as normal faults during the pre-Alpine extension and as reversed or thrust faults, sometimes with a strike-slip component, during the Alpine compression (Boillot et al., 1979; Alvarado, 1983; Masson et al., 1994). These late Variscan faults appear therefore to constitute the main structural control on the Mesozoic rifting geometry. The major canyons that cut across the continental shelf also exhibit a mainly northeast-southwest trend, roughly parallel to the dominant set of late Variscan strike-slip faults in western Iberia, and appear to lie on the offshore continuation of major late Variscan faults observed on land (e.g., the Nazaré Canyon, on the continuation of the Nazaré Fault, and the Saint Vincent Canyon on the continuation of the Odemira-Ávila Fault, also known as the Messejana Fault; Boillot et al., 1974; Lallemand and Sibuet, 1986).
The main basins and structural highs that can be observed along the west Iberia Margin are (1) the deep Galicia Margin, (2) the Western Banks, (3) the Galicia Interior Basin, (4) the Porto Basin, (5) the Lusitanian Basin, and (6) the Alentejo Basin. The geometry of these features shows a strong relationship to the orientation of Variscan basement structures described above, particularly the late orogenic strike-slip faults. Their location is shown in Figure 5, cross sections across them are presented in Figure 6, and a brief summary of the main features of each of them is given below.
1. Deep Galicia Margin. The fault spacing across the series of half-grabens in this region ranges between 10 and 25 km and the basin-bounding down-to-the-ocean normal faults trend predominantly north-south, with transfer faults oriented north-east-southwest (Fig. 5, Fig. 6A; Montadert et al., 1979; Thommeret et al., 1988; Boillot et al., 1989b).
2. Western Banks. This area is a zone trending north-northwest-south-southeast along which occur a series of elevated areas, including Galicia Bank, and Vasco da Gama, Vigo, and Porto Seamounts (Fig. 5, Fig. 6B; Mougenot et al., 1984; Murillas et al., 1990). These features have been interpreted as horsts that formed initially during Mesozoic rifting and were later reactivated and uplifted during Cenozoic compressive events (Boillot et al., 1979; Mougenot, 1988). Murillas et al. (1990) suggested that the alignment of elevated areas extends to the south under the Portuguese continental shelf as the Berlenga horst that forms the western boundary of the Lusitanian Basin. However, this structure does not form a southward continuation of the north-northwest–south-southeast trend, and in any case is separated from it by two major faults (Aveiro and Nazaré).
3. Galicia Interior Basin. This basin extends over 350 km north-northwest–south-southeast beneath the Galicia Trough (Fig. 5, Fig. 6B-D). It is bounded by down-to-the-basin normal faults with trends varying between north-northeast–south-southwest and northwest-southeast. The associated transfer faults show northeast-southwest to east-northeast–west-southwest orientations (Fig. 5, Fig. 6B, Fig. 6D; Murillas et al., 1990).
4. Porto Basin. This is a relatively narrow basin (~50 km wide) extending beneath the outer continental shelf and slope opposite Porto (Fig. 5, Fig. 6C). It may be the northward continuation of the Lusitanian Basin (or the northernmost sub-basin within the Lusitanian Basin). Its eastern margin is formed by the off-shore continuation of the Porto-Coimbra-Tomar Fault (Murillas et al., 1990). A horst block on its western side separates it from the Galicia Interior Basin.
5. Lusitanian Basin. This basin extends northwards for over 300 km from the Serra da Arrábida, south of Lisbon (Fig. 5, Fig. 6D). The greater part of it is situated onshore, with its eastern boundary coinciding with the Porto-Coimbra-Tomar Fault. South of the Nazaré Fault, its western margin is formed by the Berlenga horst, but, north of this fault, the tectonic character of the western edge is not known. Constituent sub-basins are bordered today either by salt structures or by down-to-the-ocean normal faults (Montenat et al., 1988; Wilson et al., 1989). The former show a dominant north-northeast–south-southwest trend, and the latter show a north-south trend.
6. Alentejo Basin. This is a north-south-trending basin situated offshore to the south of the Serra da Arrábida (Mougenot, 1988); its relationships to the Lusitanian Basin are not known.
In general terms, the continental crust under western Iberia is approximately 30–35 km thick. Recent studies of new data collected during the ILIHA deep seismic sounding experiment (ILIHA DSS Group, 1993), together with a re-interpretation of the preexisting seismic refraction lines (Mendes Victor et al., in press) have resulted in a well constrained model of the crustal structure beneath Iberia. This reveals a fairly heterogeneous upper crust and a reasonably homogeneous lower crust throughout Iberia. The middle crust in the internal zones of the Variscan orogen is thicker than in the external zones. In broad terms, the seismic-velocity/depth structure beneath western Iberia can be summarized as follows: (1) In places, the sedimentary cover is quite thick (5–6 km in the Lusitanian Basin and in the Algarve Basin, respectively) and the observed seismic velocities reach 5.0 to 5.3 km/s; (2) the upper crust is heterogeneous but it is normally characterized by P-wave velocities of 5.9–6.1 km/s; (3) the middle crust generally exhibits P-wave velocities between 6.2 and 6.4 km/s; and (4) the lower crust generally exhibits velocities in the interval 6.6–7.1 km/s (Mendes Victor et al., in press). The P-wave velocities in the top of the upper mantle are generally 8.0–8.1 km/s, but unusually high velocities of 8.3 km/s are observed under the Iberian Massif in northwest Iberia.