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Geological Background

The Newfoundland–Iberia rift first experienced significant extension in the Late Triassic, when basins initially formed within the Grand Banks and accumulated clastics and evaporites (e.g., Jansa and Wade, 1975). A second rift phase in the Late Jurassic through Early Cretaceous focused extension between the Grand Banks and Iberia, culminating in breakup and formation of the first oceanic crust no later than Barremian to Aptian time (Fig. F1). Excepting the Southeast Newfoundland Ridge at the southernmost edge of the rift, no significant thickness of volcanic rocks or magmatic underplating are known to be present, and the system is considered to be a non-volcanic rift.

Plate reconstruction of the Newfoundland–Iberia conjugate margins at the time of Anomaly M0 (early Aptian; ~121 Ma) (Fig. F1) provides a regional overview of the rift and the conjugate margins. At this time, thick continental crust of Flemish Cap was opposite 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 (shaded area in Fig. F1). Off Iberia, the seaward edge of known continental crust is near the western margin of Galicia Bank, and to the south it passes near Ocean Drilling Program (ODP) Site 1069, then east and south toward Estremadure Spur. On the Newfoundland margin, continental crust is thought to reach seaward to at least the Flemish Hinge in the north and to a hinge line at the eastern edge of Salar-Bonnition Basin in the south.

On both margins, seafloor between Anomaly M3 and the most-seaward "known" continental crust is considered "transitional crust," and its origin is a matter of intense debate. Structural trends of basement in this zone are oriented north-northeast to northeast, subparallel to the Anomaly M0 rift axis. In the area of the Iberia Abyssal Plain, ODP coring recovered serpentinized peridotites from basement in the transition zone (Fig. F2). In seismic data to the south, a thin (1.0–2.5 km) acoustically unreflective basement layer is observed overlying a more reflective layer (Pickup et al., 1996), and this has been interpreted to be serpentinized upper mantle peridotite grading downward into unaltered peridotite. Velocities in the Iberia transition zone define a "crust" that is only a few kilometers thick and that has velocity values and gradients characteristic of neither extended continental nor oceanic crust (Whitmarsh et al., 1990; Pinheiro et al., 1992).

Similar "crustal" thicknesses and unusual velocity structure were indicated along the landward margin of the transition zone in the Newfoundland Basin (Todd and Reid, 1989; Reid, 1994; Srivastava et al., 2000). In 2000, detailed multichannel seismic (MCS) and ocean borehole seismometer (OBH/S) surveys were conducted across the entire Newfoundland transition zone on the Ewing cruise 00-07, including drill site surveys in a corridor conjugate to the Leg 149 and 173 drilling (Transect 2 in Fig. F3). Results show that crustal thickness averages only ~3 km. However, the crustal velocity structure appears to differ from that off Iberia. It seems to be more similar to Layer 3 of ocean crust, with Layer 2 either extremely thin or absent.

Despite the similarity of "crustal" thicknesses in the Newfoundland and Iberia transition zones, there are significant asymmetries in deep and basement structure between the two margins (Fig. F2). Newfoundland transitional basement averages a kilometer or more shallower than Iberia basement, and it is comparatively smooth compared to ~1- to 2-km basement relief off Iberia. Off Newfoundland, landward of Anomaly ~M3, there is also a very flat, high-amplitude, basin-wide reflection (U), which closely overlies or intersects the underlying basement; there is no known counterpart to this horizon in the transition zone off Iberia. In the central and southern Newfoundland Basin the U reflection appears locally to truncate the underlying basement, and where traced landward it merges with the mid-Cretaceous Avalon unconformity on the Grand Banks. Because of these characteristics, Tucholke et al. (1989) suggested that the reflection is an unconformity that was eroded at sea level on extended continental crust. Thermal-mechanical modeling, however, shows that such sea level erosion would have to be on continental crust up to ~20 km thick, so if the reflection is a subaerial unconformity it must be a synrift unconformity and not a "breakup unconformity" (B. Tucholke and N. Driscoll, unpubl. data.).

Hypotheses for Origin of Transitional Crust

We have posed three hypotheses to explain crustal structure in the Newfoundland and Iberia transition zones and the cross-rift asymmetries between these zones (Fig. F4):

  1. The Newfoundland transition zone is strongly thinned continental crust. Newfoundland crust is shallower and smoother than Iberia crust, and it could be the upper plate in an asymmetric detachment system (Fig. F4B). The lower Iberia plate east of Anomaly ~M3 would be exhumed lower continental crust and mantle. Strong thinning of the Newfoundland crust without significant brittle extension might be explained by ductile flow of the lower crust (e.g., Driscoll and Karner, 1998) (Fig. F4A). This should be reflected in rapid synrift subsidence, and it would be recorded in the sedimentary section above reflection U if we interpret U as a subaerially eroded unconformity. An alternate explanation of reflection U is that it is the top of basalt flows, emplaced either subaerially (synrift on continental crust) or on the seafloor. If melt was extracted from the rising lower plate and emplaced in the Newfoundland upper plate, the exhumed Iberia mantle would be virtually melt-free, as is suggested by existing Iberia drilling.
  2. The transition zones reflect extreme extension in an amagmatic rift. 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. F4C). This hypothesis differs from the one above in that Newfoundland transitional crust would be (serpentinized) mantle, and the U reflection cannot be a subaerial unconformity because it would be impossible to uplift extending mantle to sea level. The reflection U–basement interval could consist of basalt flows, with melt generated from the rising lower plate in an asymmetric extensional system as noted above.
  3. Transitional crust was formed by ultra-slow seafloor spreading. Slow seafloor spreading (Fig. F4D) is known to expose lower crust and mantle (e.g., in the Labrador Sea), and it could explain the transitional crust in the Newfoundland Basin. However, symmetrical ultra-slow seafloor spreading in the rift (Fig. F4D) seems unlikely because it does not explain the extensive mantle exposures off Iberia, nor does it explain the asymmetries in crustal structure between the conjugate transition zones. It is possible that extension first exposed mantle in the rift, that ultra-slow seafloor spreading then occurred on the Newfoundland side of the rift, and that this ocean crust was then isolated on the Newfoundland margin by an eastward jump of the spreading axis. This hypothesis precludes reflection U from being a subaerial unconformity because it would overlie thin ocean crust and thus it could not have been exposed at sea level. The U reflection could prove to have another, unknown origin, perhaps as a basin-wide sedimentary event that was restricted to the Grand Banks side of the rift.

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