The Newfoundland and Iberian Margins are similar. Each margin contains two sets of basins: one landward of the continental shelf break and the other seaward of the shelf break. Thus, four basin sets form the conjugate margins. They are, from west to east, the basins of the Grand Banks of Newfoundland, Newfoundland Basin, Iberia Abyssal Plain, and Lusitanian Basin.
The Grand Banks is a 450-km-wide continental shelf off Newfoundland (Tankard and Welsink, 1987) with an average water depth of less than 200 m (Welsink et al., 1989). The Grand Banks is underlain by continental crust between 32 and 38 km thick (Tankard and Welsink, 1987, 1989; Reid and Keen, 1990).
The Newfoundland Basin extends about 200 km to the southeast from the Grand Banks continental slope (Tucholke et al., 1989). Its bathymetry ranges from 2000 to 5000 m, but averages about 4000 m. This basin contains between 2 and 7 km of sediments (Tucholke et al., 1982, 1989). Many workers (Tucholke et al., 1989; Austin et al., 1989; Sullivan, 1983; Masson and Miles, 1984; Meador and Austin, 1988) have interpreted most of the crust under the Newfoundland Basin to be highly extended continental crust with a thickness of 4 to 8 km (Tucholke et al., 1989), with the ocean/continent boundary at the "J anomaly" (Pitman and Talwani, 1972; Tucholke and Ludwig, 1982; Malod and Mauffret, 1990; Srivastava et al., 1990; Austin et al., 1989). This interpretation is not universally accepted (Parson et al., 1985); several authors (Keen and de Voogd, 1988; Malod and Mauffret, 1990) place the ocean/continent boundary closer to the shelf break, implying that the crust underlying the Newfoundland Basin is oceanic. In our efforts here, we attempted to model this crust as extended continental crust.
The Iberia Abyssal Plain lies seaward of the narrow Iberian continental shelf and is about 350 km wide. Water depth ranges from 3000 to more than 5000 m, crustal thickness appears to be 5 to 8 km, and sediment thickness is 1 to 3.5 km (Whitmarsh et al., 1990). As in the Newfoundland Basin, the location of the ocean/continent boundary is uncertain, but may coincide with a ridge of serpentinized peridotite that parallels the Portuguese coast (Boillot et al., 1987). Whitmarsh et al. (1990, 1993) proposed that much of the Iberia Abyssal Plain is underlain by thinned continental crust, based on its seismic velocity structure and a magnetic model.
The Lusitanian Basin is completely underlain by continental crust with a thickness of about 30 km (Banda, 1988; Cordoba et al., 1988; Whitmarsh et al., 1990). The basin parallels the Portuguese coast, and is more than 250 km long and 50 to 100 km wide. The part of the basin landward of the Iberia Abyssal Plain lies onshore; it was uplifted primarily because of compression related to Pyrenean and Alpine mountain building (Wilson et al., 1989). (Please note that while some [e.g., Murillas et al., 1990] assert that the Galicia Interior Basin to the north is an offshore continuation of the Lusitanian Basin, the analyses performed in the research documented here concern only the portion of the Iberian Margin in the direct vicinity of the Iberia Abyssal Plain, from about 39°30'N to 41°N latitude.) Sediment thickness varies from 0 to 4 km in the Lusitanian Basin (Wilson et al., 1989).
The evidence indicates that rifting progressed from south to north (Srivastava et al., 1988; Murillas et al., 1990). Extensional structures were localized along Avalonian and Hercynian crustal weaknesses (Keen et al., 1987; Tankard and Welsink, 1989). Rifting began in the Carnian (235 Ma) in the Lusitanian Basin (Wilson et al., 1989; Leinfelder and Wilson, 1989) and in the Grand Banks basins (Tankard and Welsink, 1987); this phase of extension ended at about 210 Ma. (Absolute ages are based on Harland et al., 1990.)
A period of thermal subsidence followed on both margins. At about 160 Ma, extension was renewed on the Grand Banks (Tankard and Welsink, 1988; Tankard et al., 1989; Grant et al., 1988; Keen et al., 1990) and in the Lusitanian Basin (Wilson, 1975; Montenat et al., 1988; Leinfelder and Wilson, 1989; Murillas et al., 1990; Hiscott et al., 1990; Mauffret and Montadert, 1987) and was most intense through about 150 Ma.
Extension became more concentrated in the zone of the future ocean/continent boundary in the Early Cretaceous. Extension had begun in the Newfoundland Basin (Tucholke et al., 1989; Austin et al., 1989) and in the Iberia Abyssal Plain (Whitmarsh et al., 1990; Montenat et al.,1988) by 140 Ma. During the final 20 m.y. or so of rifting, there was little additional extension in the shelf basins of both margins. By the end of the Barremian (124 Ma), creation of oceanic crust had begun, and rapid subsidence in the seaward basins occurred; a breakup unconformity (the Avalon unconformity in the basins of the Grand Banks) marked this event (Murillas et al., 1990; Groupe Galice, 1979). Serpentinized peridotite was emplaced on the Iberian Margin at the ocean/continent boundary (Boillot et al., 1987), and although no evidence has been found of it, perhaps on the Newfoundland Margin as well. In general, then, the earlier phase of rifting was limited to the shelf basins, whereas the later phase was marked by a shift in extension from the shelf basins to the deep-water basins, culminating in seafloor spreading.
Masson and Miles (1986) provided a possible explanation for this pattern of rifting. During the late Triassic phase of rifting, Iberia moved with Africa, so that extension took place simultaneously between the eastern United States and Africa and between Newfoundland and Iberia. After 195 Ma, a transform boundary developed between Iberia and Africa and along the southern margin of the Grand Banks, allowing Africa and North America to continue rifting and finally to separate (at about 175 Ma), while little or no extension occurred between Newfoundland and Iberia (Masson and Miles, 1986; Tankard and Welsink, 1988). Srivastava and Verhoef (1992) suggested that, during this period, the southern Grand Banks high was dragged away from the rest of the Grand Banks by Africa. After 160 Ma, extension resumed again between these margins and progressed to the point of continental breakup and seafloor spreading. Srivastava et al. (1990) indicated that Iberia was moving as a separate plate when spreading began. (Ziegler, 1989, provides a well-illustrated map-view history of Mesozoic North Atlantic rifting.)
Little rift-related magmatism is observed on either side of the Atlantic. The central Newfoundland Basin is dotted by the mid-Cretaceous Newfoundland Seamounts, but these are unrelated to the rifting of these margins. Also, aside from some dike intrusion in southeastern Newfoundland during the Early Jurassic and some minor volcanic rock found in wells, rift-related igneous activity is scarce on the Grand Banks (Keen et al., 1990; Tankard and Welsink, 1988). Mesozoic magmatism in the Lusitanian Basin is insignificant (Montenat et al., 1988).
The conjugate margins display mild asymmetries. Figure 1 shows a cross section of the margins, perpendicular to the direction of rifting. The Iberia Abyssal Plain appears to be slightly wider than the Newfoundland Basin (350 vs. 200 km); this probably indicates that the site of seafloor spreading was somewhat off-center in a broad area of highly extended crust. In addition, the subsidence amounts for the Iberia Abyssal Plain and the Newfoundland Basin (labelled "TTS" in Fig. 1) are somewhat different (Tett, 1993). These asymmetries are limited to the highly thinned areas adjacent to the ocean/continent boundary, however, and are minor if the margins are considered as a whole. Figure 1 also may give the impression that basement underlying the Iberia Abyssal Plain is rougher; this difference merely reflects the levels of detail in the data shown in this figure. The observed subsidence profile is compared to the modeled subsidence in Figure 15.
Lastly, the continental shelf on the Newfoundland margin (not shown in Fig. 1) is 1 order of magnitude wider than that on the Iberian side; at first glance, this variance seems to be a significant asymmetry. Much of the Grand Banks, however, is the unextended Bonavista platform (Srivastava and Verhoef, 1992), and the Iberian shelf has become emergent largely because of Cenozoic compression. Thus, although the basins are wider on the Grand Banks than the Lusitanian Basin, the asymmetry of the shelf widths is not as extreme as the bathymetries would suggest.
These observations notwithstanding, the Newfoundland and Iberian Margins are grossly symmetric. During both phases of extension, rifting was distributed symmetrically through time on both margins, first on the shelf basins, and later in the more seaward basins. Common lithologies are found on both sides of the Atlantic throughout the rifting history. In addition, the two phases of extension were centered in the same location, so that the resulting conjugate margins are roughly mirror images of one another.