The Newfoundland and Iberia margins first experienced significant extension in Late Triassic time when rift basins initially formed within the Grand Banks and on the Iberia margin (Lusitania Basin) (Figs. F2, F3). The Grand Banks basins accumulated siliciclastic "redbed" sediments, and these were succeeded by deposition of evaporite deposits, which extended into the earliest Jurassic in both the Grand Banks and Lusitania basins (Jansa and Wade, 1975; Wilson, 1988; Rasmussen et al., 1998). A second prolonged rift phase in the Late Jurassic through Early Cretaceous extended the crust in several subbasins, but it ultimately focused extension between the Grand Banks and Iberia (Enachescu, 1987; Tankard and Welsink, 1987; Wilson et al., 1989). This culminated in continental breakup and formation of the first oceanic crust no later than Barremian to Aptian time. Excepting the Southeast Newfoundland Ridge at the southernmost edge of the rift, no significant thickness of volcanic rocks or magmatic underplating is known to be present in the rift. Thus, the system is considered to be nonvolcanic.
Plate reconstruction of the Newfoundland–Iberia conjugate margins at the time of Anomaly M0 (Barremian/Aptian boundary; ~121 Ma) (Fig. F3) provides a regional overview of the rift and the conjugate margins. At this time, thick continental crust of Flemish Cap was close to extended continental crust of Galicia Bank at the northern end of the rift. To the south, geophysical studies and magnetic anomaly identifications suggest that ocean crust was present, extending landward from a seafloor-spreading axis to at least Anomaly M3 (early Barremian; see blue-shaded area in Fig. F3). Along axis, the Newfoundland–Iberia rift can be roughly divided into three segments: (1) a northern segment containing Flemish Cap and Galicia Bank, (2) a central segment bounded on the south by the Newfoundland and Tore seamounts, and (3) a southern segment extending south to the Southeast Newfoundland Ridge and the present-day Gorringe Bank off southwestern Iberia. Each of these is reviewed below.
In the northern segment, Flemish Cap has full continental crust thickness of ~30 km (Funck et al., 2003) and it is separated from the shallow Grand Banks by thinned continental crust under the Flemish Pass Basin and Flemish Cap Graben (Enachescu, 1987). Galicia Bank is extended continental crust that has a maximum thickness of ~20 km in its central part and thins to zero thickness at its western margin; it is separated from Iberia by the Galicia Interior Basin, which contains rifted continental crust that is thinned to ~10 km (González et al., 1999; Pérez-Gussinyé et al., 2003). Anomaly M0 appears to occur just seaward of the edges of continental crust in these conjugate segments (Srivastava et al., 2000), but there are no older M-series magnetic anomalies present. At the seaward margin of Galicia Bank (ODP Site 637), the westward transition from continental to ocean crust is marked by a prominent ridge composed of serpentinized peridotite (Boillot, Winterer, Meyer, et al., 1987; Boillot et al., 1995).
On the Iberia margin, the southern limit of this rift segment lies roughly at the southern edge of Galicia Bank. In this location, the shallow crust of Galicia Bank is expressed in a series of rift-parallel ridges that plunge to the south and lose a large portion of their amplitude beneath the southern Iberia Abyssal Plain. It is along this transition that the Leg 149/173 transect was drilled. The seaward edge of known continental crust passes southeastward through this transect near ODP Site 1069, then probably to the south toward Estremadura Spur (Fig. F3). On the conjugate Newfoundland margin, continental crust reaches seaward at least to the Flemish Hinge near Flemish Cap and to a hinge line at the eastern edge of the Salar-Bonnition Basin to the south. Seaward of the Flemish Hinge, no along-strike change has been identified in the basement structure that would correlate with the structural change at the southern margin of the conjugate Galicia Bank.
The central segment on both margins has an abrupt transition from shallow continental shelf to deep basin, although there are rift basins beneath the continental shelves that are completely filled with sediments (Fig. F3). On the Newfoundland margin these are the Jeanne d'Arc, Carson, and Salar-Bonnition basins, and the Lusitania Basin is present on the Iberia margin. These basins are in extended continental crust that reaches seaward an uncertain distance beneath the continental slope and rise, and they contain evaporites of Triassic age (Jansa et al., 1980; Austin et al., 1989). Farther seaward, the basement is deep and is considered to be "transitional" crust out to a point where magnetic Anomalies M3 to M0 are identified (Fig. F4).
The origin of the transitional crust has been a matter of intense debate. Structural trends in the basement are oriented north-northeast to northeast, subparallel to the Anomaly M0 spreading axis. Srivastava et al. (2000) suggested that magnetic anomalies as old as M17 (early Berriasian; ~140 Ma) are present in this segment, but it is unclear whether the low-amplitude anomalies represent polarity reversals or are related to the basement relief. In the transition zone at the northern margin of the segment, Leg 149 and 173 drilling recovered serpentinized peridotite from basement (Fig. F4). To the south (line IAM9; Fig. F4), seismic data reveal a thin (1.0–2.5 km) unreflective basement layer overlying a more reflective layer (Pickup et al., 1996). This upper layer also has been interpreted to be serpentinized upper mantle peridotite, grading downward into less altered and unaltered peridotite. Seismic refraction experiments there seem to agree, defining a low-velocity "crust" that is only 2–4 km thick and that overlies a layer with velocities of ~7.1–7.7 km/s; these layers are thought to be partially serpentinized peridotite (Whitmarsh et al., 1990; Discovery 215 Working Group, 1998; Dean et al., 2000). In the conjugate central Newfoundland Basin, similar "crustal" thicknesses and velocity structure have been detected in the transition zone (Srivastava et al., 2000). The transition zone width in the central segment is ~150 km on both margins.
The southern segment is like the central segment in that it has deep and thin crust in the transition zone and the zone is ~130–150 km wide. On the Newfoundland side, refraction data of Reid (1994) indicate the presence of very thin crust (~2 km) over apparently serpentinized mantle (7.2–7.5 km/s) that extends at least ~50 km east of the seaward hinge of the Salar-Bonnition Basin. Very similar results have been reported for the conjugate Iberia transitional crust beneath Tagus Abyssal Plain (Pinheiro et al., 1992). Srivastava et al. (2000) suggested that the transitional crust in the southern segment is oceanic and that it is Tithonian (Anomaly M20; ~145 Ma) in its oldest part. The landward portions of the southern segment differ from the central and northern segments in that there are no major rift basins in the proximal continental crust, excepting the southern Salar-Bonnition Basin on the Newfoundland side. On the Newfoundland margin, the southeasternmost Grand Banks is intact continental crust that has been in a subaerial or shallow-shelf environment throughout the Mesozoic and Cenozoic (Jansa and Wade, 1975).
Just as the three rift segments differ from one another from north to south, the conjugate sides of each segment also show dissimilarities. In the northern segment, the major distinction is in the amount of crustal extension (i.e., the thick, intact crust of Flemish Cap vs. the extended and structured crust of Galicia Bank). In the central and southern segments, the major differences are in crustal depth and crustal roughness. The Newfoundland transitional basement averages a kilometer or more shallower than the Iberia basement (Fig. F4), even when corrected for sediment loading. In addition, Newfoundland basement is relatively smooth compared to that off Iberia, where >1 km of basement relief is common.
Structural trends in the transitional basement (Fig. F3) tend to show some convergence toward the north. This, together with the northward-narrowing zone of ocean crust in the M0 reconstruction, suggests that the rift may have opened from south to north, which is consistent with a stage pole of opening a short distance north of the rift (Whitmarsh et al., 1990; Srivastava et al., 2000). Considering the segment-to-segment differences in the extent of rifting in the shallower continental crust, the southern part of the rift may have switched from continental rifting to seafloor spreading relatively early and abruptly, while the northern part experienced prolonged continental extension and a delayed change to normal seafloor spreading.
Additional constraints on basement structure and sedimentary stratigraphy of the Newfoundland transition zone were obtained in 2000 during the Study of Continental Rifting and Extension on the Eastern Canadian Shelf (SCREECH) program (Ewing Cruise 00-07). In this program, multichannel seismic (MCS) and ocean bottom hydrophone/seismometer surveys were made in three major transects across the Newfoundland margin (Figs. F3B, F5, F6). Each transect extended from full-thickness continental crust on the landward end seaward to known oceanic crust beyond magnetic Anomaly M0. Transect 2 was located so that it is conjugate to the Leg 149/173 drilling on the Iberia margin (Fig. F3B), and it is along this transect that Leg 210 drilling was conducted (Fig. F7). To provide regional perspective, the principal results for all three transects are summarized below.
Transect 1 across Flemish Cap is in a position conjugate to Leg 103 drilling conducted on the seaward margin of Galicia Bank (Fig. F3B). It shows that continental crust thins rapidly from ~30 km beneath Flemish Cap to ~2 km beneath the lower continental slope at the Flemish Hinge (Funck et al., 2003; Hopper et al., 2004). Farther seaward, probable ocean crust appears first with oceanic Layer 2/3 velocity structure and it is 3–4 km thick. It then changes eastward to Layer 2 velocity structure and is only 1 km thick. This crust reaches to slightly beyond Anomaly M0, where ocean crust of a more normal thickness (5–7 km) is present. Both the thin continental and thin ocean crust overlie a layer of probably serpentinized mantle (VP = 7.6–7.9 km/s) that is 3–5 km thick.
Transect 2, as noted above, is conjugate to Leg 149 and 173 drilling on the Iberia margin and was the focus of drilling during Leg 210 (Figs. F3B, F4, F7). On this transect continental crust thins rapidly seaward beneath the continental slope from 30 km to ~7–8 km over a distance of 60 km, and then over the next 50 km it thins more slowly to ~5 km at the Flemish Hinge (Fig. F3). Beyond this, transition-zone crust out to Anomaly ~M3 is only 3–5 km thick. Like part of transect 1, this crust has velocities characteristic of oceanic Layers 2/3, but there appears to be no significant underlying zone of possibly serpentinized mantle (Nunes, 2002). This contrasts with transect 1 and with the velocity structure on the conjugate Iberia margin, where a thick zone of serpentinized mantle appears to be present at and south of the Leg 149/173 drilling transect. Also unlike the Iberia conjugate, transect 2 seismic reflection data do not image an unreflective upper basement layer that might be highly serpentinized peridotite.
Transect 3 exhibits still another set of basement structures. Although continental crust thins rapidly from 35 to <10 km under the continental slope near the seaward edge of the Salar-Bonnition Basin (Fig. F3B), thin (<5 km) continental crust above serpentinized mantle appears to reach seaward 50 km into the transition zone. The remainder of the transition zone to the east has extremely thin (~2 km thick) "crust" that may be either a serpentinized layer of exhumed mantle or thin ocean crust (Lau et al., 2003).
A common feature of all three transects is that there is thin crust and apparently very limited magmatism in the transition zone. The transects differ, however, in how magmatism was expressed, in distribution and character of tectonic extension, and in development of serpentinized "lower crust." Thus, it appears that the balance between tectonic extension and limited magmatism was heterogeneous both along and across the Newfoundland–Iberia rift.
In the transition zone off Newfoundland there is a very flat, high-amplitude reflection (U) that closely overlies or intersects basement (Figs. F4, F8, F9). The U reflection is observed some 600 km south to north in the basin and up to ~150 km across the transition zone. Where traced landward, it merges with the Lower to mid-Cretaceous Avalon unconformity on the Grand Banks (Tucholke et al., 1989). At its seaward edge it normally pinches out on crust of about Anomaly M3 age (Hauterivian–Barremian) (Figs. F8, F9). These features suggest that the horizon has an Early Cretaceous age.
The lithologic sequence between basement and U is defined here as seismic Sequence A. It is seismically laminated, exhibits strong and laterally coherent reflections, and is as much as ~0.5 s (two-way traveltime) thick along the proximal part of the margin (Figs. F8, F9, F10). At many locations, basement below U is not identified as a distinct reflection but as a downward disappearance of reflections (Fig. F8A, F8B). This suggests that the impedance of Sequence A is high and is probably close to that of the underlying basement. The sequence is often faulted near its seaward margin, but there appears to be less faulting at other locations in the transition zone (Tucholke et al., 1989). The apparent age of U and underlying sediments is similar to that of the Blake-Bahama Formation (Hauterivian–Barremian limestones, capped by Horizon 
) in the western North Atlantic Basin (Jansa et al., 1979; Tucholke and Mountain, 1979). The reflection character is also similar, although the U horizon generates a much stronger reflection than Horizon . These features predict that Sequence A could be equivalent to the Blake-Bahama Formation in the western North Atlantic Basin.
Locally, U appears to truncate the underlying basement (Tucholke et al., 1989). The possible truncations, areal extent, and flatness of the horizon led Tucholke et al. (1989) to suggest that it could be an unconformity eroded at or above sea level on extended continental crust. This interpretation has been evaluated with thermal-mechanical modeling (B. Tucholke and N. Driscoll, unpubl. data). The results indicate that if erosion occurred at sea level, it would have to be on continental crust at least ~18 km thick, given reasonable upper mantle temperatures of 1300°–1400°C. Thus, if the horizon originated in this way, it must be a synrift unconformity and not a "breakup unconformity."
U and Sequence A can also be compared to a similar deep reflection sequence across probable continental crust of southern Galicia Bank on the Iberia margin (see Figs. F11, F12). The sequence there is capped by the "orange" reflection that was drilled at Site 398 (Shipboard Scientific Party, 1979). It is represented by Site 398 lithologic Subunit 4C, of Aptian age, which is composed of bioturbated mudstones, turbiditic mudstones, thin laminated and cross-laminated fine-grained sandstones and siltstones, and debris flows or mud flows. The unit was interpreted as an upward-fining submarine fan sequence with initial fan deposition in Barremian time and waning deposition in the late Aptian. Seismic profiles and the mapped distribution of this sequence (seismic Unit 4 of Réhault and Mauffret [1979]) show that it fills basement depressions (Fig. F13). A deeper lithologic unit just above basement (lithologic Unit 5), consists of nannofossil limestones interbedded with laminated mudstones. Velocity in lithologic Units 4 and 5 at Site 398 probably increases with depth, but a mean assumed velocity of 3.59 km/s results in good fit between seismic reflections and a synthetic seismogram based on borehole physical property data (Bouguigny and Wilm, 1979). Thus, velocities and impedance in the deep part of the section are high and may be close to those of the underlying basement, much as it is interpreted on the Newfoundland margin. However, on a regional scale, the reflectivity of the orange reflection and the upper part of the sequence appear to be lower than the reflectivity of Sequence A on the Newfoundland margin.
Reflection profiles in the Newfoundland Basin show five principal seismic sequences in the section above U. These are differentiated from one another by broad changes in reflection character, which in turn suggest general changes in depositional conditions and/or deformation. The sequences are summarized below, from base to top.
This sequence is conformable to the underlying U reflection and extends upward to the base of a strongly laminated zone near middepth in the sedimentary column (Fig. F8). In the landward part of the transition zone it has a thickness of ~0.5 s two-way traveltime. Reflections in Sequence B have low amplitude compared to reflections in the underlying and overlying sequences, but they are still relatively well defined and are mostly laterally continuous over distances of tens to hundreds of kilometers. Reflections tend to be more coherent and readily traced in the lower part of the sequence than in the upper part, where they are sometimes disrupted or even chaotic. At some locations in the upper part of Sequence B, reflections show seaward downlap and landward onlap that suggest local control of depositional patterns (e.g., in a fan-channel system). The top of the sequence is marked by an apparent unconformity that truncates progressively deeper beds in a landward direction. The predicted age of this sequence is mid-Cretaceous, which would include black shales equivalent to the Hatteras Formation farther south in the central North Atlantic Basin.
Sequence C exhibits a series of very strong, flat, coherent reflections that are easily traced laterally for distances of 100 km or more. The interval represents ~0.3 s of reflection time and occurs midway in the sedimentary section. Reflection character of this sequence suggests that it consists of interbedded high- and low-velocity layers (e.g., sandstone, turbidites, and shale). In its landward portions, the upper part of Sequence C remains strongly layered in its lower section but its upper section expands and contains chaotic reflections; these reflections appear to be caused by debris flows and other downslope mass movements.
The reflection that marks the top of Sequence C, as discussed below, appears to be equivalent to Horizon Au in the western North Atlantic, suggesting that Sequence C is Eocene at its top; it probably extends into the Paleocene or possibly the Upper Cretaceous at its base. The equivalents to the following central North Atlantic formations would be included in this sequence, from base to top: black shales of the Hatteras Formation, reddish and multicolored pelagic shales of the Plantagenet Formation (± Maastrichtian limestones of the Crescent Peaks Member), and siliceous shales and cherts of the Bermuda Rise Formation (Jansa et al., 1979).
This sequence is characterized by reflections with distinctive pinch-and-swell morphology that suggests current-controlled deposition and formation of sediment waves, much like Oligocene–Miocene seismic sequences along the eastern margin of North America to the south (Mountain and Tucholke, 1985). The sequence is thickest close to the margin (~0.4 s reflection time), and inferred sediment waves are best developed there. Seaward, it thins to ~0.1 s. A number of strong reflections are well developed and continuous through the sequence, but weaker intervening reflections are often broken up, particularly in the lower part of the sequence and in its thinner section away from the margin. The semichaotic character of these reflections suggests that debris flows and mass wasting deposits may form part of the sequence.
The base of this sequence is interpreted to be equivalent to Horizon Au along the western margin of the North Atlantic south of the Newfoundland Basin. There, the horizon is a widespread unconformity that was eroded when strong abyssal circulation (Deep Western Boundary Current [DWBC]) developed in the basin (Tucholke and Mountain, 1979). On the Newfoundland margin the reflection shows truncation of the underlying irregular beds of upper Sequence C beneath the inner continental rise, but farther seaward it is mostly conformable to deeper bedding. The source of bottom water for the DWBC is thought to be in the sub-Arctic/Arctic seas, so the Newfoundland Basin has been the "gateway" region through which this current flowed southward and may contain an important record of how abyssal circulation developed in the North Atlantic Ocean. The DWBC is interpreted to have developed in the latest Eocene to early Oligocene (e.g., Miller and Tucholke, 1983; Davies et al., 2001). The predicted age of Sequence D is early Oligocene at the base, extending up into the Miocene at the top.
This sequence is well developed all along the Newfoundland margin. It is consistently thicker close to the margin (~0.8 s two-way traveltime) and thins seaward to as little as ~0.2 s beneath the outermost continental rise and abyssal plain. Sequence E has a very distinctive seismic signature. It is marked by undulating and contorted reflections that usually can be traced for only limited distances. Some reflections have the form of poorly developed sediment waves. In its upper part the sequence contains channels that are filled with chaotic debris, and other portions also show chaotic signature that probably represents rapid deposition of mass-wasting deposits. The sequence is also permeated by small-throw normal faults, almost none of which extend into the underlying or overlying sequences. These features are commonly developed in abyssal fans; they suggest that the sediments were deposited rapidly while trapping pore fluids and that they later failed as the sediments dewatered. Similar seismic sequences appear along the U.S. East Coast margin (Mountain and Tucholke, 1985) and on other margins across the globe in the middle Miocene to Pliocene–Pleistocene. They appear to document a global period of margin progradation (Bartek et al., 1991).
This is the topmost seismic sequence in the Newfoundland Basin and it consists of reflective, flat-lying turbidites that form an abyssal plain seaward of the lower continental rise. The turbidites interfinger with and lap landward onto underlying fan Sequence E. Thus the base of the sequence is time-transgressive, becoming younger landward. Within the turbidites, it is common to observe chaotic beds as thick as ~0.1 s that extend for many tens of kilometers (Fig. 8B, 8C). These appear to be debris flows that originated from the continental rise. The predicted age of the turbidities is late Pliocene to Quaternary.
