Drilling objectives for Leg 210 were twofold. The primary objective was to sample the deep sedimentary structure and basement in order to investigate early rift development. A related objective was to study the shallower stratigraphy and to elucidate the postrift sedimentation processes and paleoceanographic history of this gateway between the North Atlantic and the sub-Arctic seas. The background for each of these objectives is summarized below.
Drilling results from the Iberia margin and geophysical data from both sides of the Newfoundland–Iberia rift show clearly that the rift was characterized by very limited volcanism, that there are marked asymmetries between margin conjugates, and that there is significant structural variability along strike between rift segments. These features are particularly manifested in the transition zones between known continental and known oceanic crust on the opposing margins. We posed three hypotheses to explain the crustal structure and basal stratigraphy in the Newfoundland and Iberia transition zones and the cross-rift asymmetries between these zones (e.g., Tucholke et al., 1999) (Fig. F14). Leg 210 provided the first direct test of these hypotheses by drilling in the transition zone along transect 2 on the Newfoundland margin (Figs. F3, F4, F8B).
Newfoundland transitional crust is shallower and has less roughness than Iberia crust, and it could be the upper plate in an asymmetric detachment system (Fig. F14B). According to this hypothesis, the lower Iberia plate east of Anomaly ~M3 would be exhumed lower continental crust and mantle (e.g., Whitmarsh et al., 2001). Strong thinning of the Newfoundland crust without significant brittle extension might be possible if the lower crust was thinned by ductile flow (e.g., Driscoll and Karner, 1998) (Fig. F14A). This should be reflected in rapid synrift subsidence of the Newfoundland basement. If U corresponds to a subaerially eroded unconformity, the rapid subsidence would be recorded in the sedimentary section above the reflection.
There are two other possible explanations for U. One is that it corresponds to the top of basalt flows emplaced either subaerially (i.e., it is synrift on continental crust) or on the seafloor (Enachescu, 1988). If melt was extracted from the rising lower plate and emplaced in the Newfoundland upper plate (e.g., Fig. F14C), the exhumed Iberia mantle could be virtually melt-free, as has been suggested by existing Iberia drilling (Whitmarsh and Sawyer, 1996). Although it seems unlikely that smooth basalt flows could be as widespread as is indicated by the distribution of U, there are documented instances where such flows are known to be extensive (e.g., Larson and Schlanger, 1981; Driscoll and Diebold, 1999).
Another explanation is that U corresponds to the top of high-velocity sedimentary deposits that were shed from the Grand Banks, probably in Early Cretaceous time. As already noted, this sequence could be similar to the Aptian fan deposits recorded on the conjugate Iberia margin (see also "Comparison with Iberia Margin Stratigraphy"), although the stronger seismic signature on the Newfoundland margin suggests much higher–velocity beds that possibly are very coarse grained or carbonate rich.
According to this hypothesis, continental extension proceeded under nearly amagmatic conditions to a state where only mantle was exposed and at some point an asymmetric shear developed within the exposed mantle (Fig. F14C). This hypothesis differs from the one above in that Newfoundland transitional crust would be exhumed, probably serpentinized mantle. U could not correspond to a subaerial unconformity because it would be impossible to uplift extending mantle to sea level. The U–basement interval could correlate with basalt flows, with melt generated from the rising lower plate in an asymmetric extensional system, or it could be high-velocity sedimentary beds, as noted above.
Slow seafloor spreading (Fig. F14D) is known to expose lower crust and mantle (e.g., in the Labrador Sea [Chian and Louden, 1995; Osler and Louden, 1995]), and it could explain the transitional crust in the Newfoundland Basin. However, symmetrical ultra-slow seafloor spreading in the rift seems unlikely because it does not explain the extensive mantle exposures off Iberia, nor does it explain the asymmetries in crustal structure and depth between the conjugate transition zones. It is possible that extension first exposed mantle in the rift, that ultra-slow seafloor spreading then initiated on the Newfoundland side of the rift, and that this ocean crust was subsequently isolated on the Newfoundland margin by an eastward jump of the spreading axis (Fig. F14D). This hypothesis precludes U from corresponding to a subaerial unconformity because it would overlie thin ocean crust. As in the above hypotheses, U might correlate with either basalt flows or the top of high-velocity sedimentary beds.
Rifting between Labrador and Greenland, and between Greenland and Eurasia (Rockall Trough), began in the Early Cretaceous, leading to Late Cretaceous seafloor spreading in the Labrador Sea and Paleocene spreading east of Greenland (e.g., Eldholm et al., 1990; Srivastava and Roest, 1999). The Newfoundland–Iberia rift was a gateway between the main North Atlantic and these developing ocean basins, so it is in a key position to investigate sedimentary history and paleoceanographic links through the northward-expanding ocean basins.
Two features of the predicted sedimentary record above U were of particular interest during Leg 210. The main basin of the adjacent North Atlantic was accumulating black shales of the Hatteras Formation in Barremian–Cenomanian time, followed by deposition of the Plantagenet Formation under oxygenated seafloor conditions in the Late Cretaceous (Jansa et al., 1979). The Newfoundland Basin Cretaceous sedimentary record provides an opportunity to examine whether this record of reduced and then increased ventilation of the deep basin extended northward into the developing ocean basins, as well as information on the timing of that record. It also allows investigation of paleobiogeography in a zone where Tethyan and boreal flora and fauna were expected to have mixed.
The second feature of interest was the upper Eocene–lower Oligocene sedimentary record, which could contain important information on the first development of strong abyssal circulation in the North Atlantic. As already noted, the source of the bottom water for this developing circulation has been interpreted to be the sub-Arctic seas and the timing has been estimated as latest Eocene to early Oligocene (Miller and Tucholke, 1983; Davies et al., 2001). However, these predictions are based largely on the occurrences of hiatuses in boreholes farther south in the North Atlantic; the lack of sedimentary records in the critical intervals there makes it difficult to verify the predictions. New data from the gateway region could help to constrain the source and timing of the circulation event.