Figure F1. Reconstruction of the Newfoundland–Iberia rift at Anomaly M0 (Newfoundland plate is fixed in present geographic coordinates), based on the reconstruction pole of Srivastava et al. (2000). Tectonic and other data are compiled from numerous sources (structural data from Ewing Cruise 00-07 in 2000 [Fig. F3] are not included). Ocean crust (center, shaded) is presumed to have formed beginning near Anomaly M3. South-to-north zipperlike opening would account for the observed splayed tectonic trends between the two margins and the northward-narrowing zone of ocean crust. On the Newfoundland side, the U reflection extends throughout the Newfoundland Basin and pinches out seaward near Anomaly M3. Ewing MCS/OBH/S transects across the Newfoundland margin (Fig. F3) are indicated by bold lines labeled with circled numbers. ODP Leg 210 drill sites are along Transect 2. F.Z. = fracture zone, T.Z. = transfer zone, A.P. = abyssal plain.

Figure F2. A. Reconstruction of conjugate seismic sections from the Newfoundland and Iberia margins to the time of Anomaly ~M1. Left: Preliminary interpretation of Ewing line 2MCS (location in Fig. F3). Interpreted postrift sediments are shaded. The deep U reflection is highlighted and labeled. Proposed drill sites landward of Anomaly M1 are shown. Right: Composite seismic section (LG-12) along the conjugate Iberia drilling transect, adapted from ODP Leg 173 Scientific Party (1998). Drill sites are numbered, and recovery at the bottom of holes is indicated. B. Conrad MCS line NB1 (location in Fig. F3) ~150 km south of Ewing line 2MCS, showing another view of Newfoundland Basin basement structure and the overlying U reflection. C. IAM-9 MCS line ~50 km south of LG-12 off Iberia. In A–C, note the marked asymmetries in basement depth and roughness between the Newfoundland and Iberia sides of the rift. D. Preliminary interpretation of the full length of Ewing line 2MCS with magnetic anomaly locations of Srivastava et al. (2000) and proposed drill site locations. TWT = two-way traveltime.

Figure F3. A. Track map showing partial coverage of multichannel seismic (MCS) and single-channel seismic (SCS) reflection profiles, northern Newfoundland Basin (Ewing Cruise 00-07 lines in bold). Bathymetry in meters (1000-m contours). B. Ewing Cruise 00-07 MCS and ocean borehole seismometer (OBH/S) survey on Transect 2 (labeled every 1000 common midpoint [CMP]). Proposed drill sites are indicated by solid circles and labeling. Bathymetry in meters (100-m contour interval).

Figure F4. Schematic models to explain observed deep structural asymmetries between the Newfoundland (left) and Iberia (right) margins. A. Synrift extension of continental crust. In the central part of the rift, lower crust is thinned ductilely (dashes) but brittle upper crust has limited tectonic extension (e.g., Driscoll and Karner, 1998). B. At the time of Anomaly ~M3. The rift evolves asymmetrically, with a thin remnant of continental crust forming an upper Newfoundland plate and serpentinized peridotite and remnants of ductilely thinned lower crust forming a lower Iberia plate. Bending stresses may account for faulting in the cold brittle mantle footwall as it is exhumed. Basement depth differences on the two margins reflect buoyancy of thin continental crust vs. serpentinized mantle. The U reflection may be a synrift unconformity developed near sea level. C. Anomaly ~M3. Mantle is exposed on both sides of the rift at an early stage, but an asymmetric shear then develops. Melt extracted from the lower plate may permeate the Newfoundland upper plate and flood its surface to form the U-to-basement sequence in a submarine setting (dark gray). Basement depth differences reflect buoyancy differences caused by melt intrusion/extrusion on the Newfoundland side. D. Anomaly ~M3: ultra-slow seafloor spreading. Symmetrical spreading as depicted here is unlikely because it would not account for extensive exposure of serpentinized mantle on the Iberia side or the asymmetry in basement structure of the transition zones on the two margins. Rather, ocean crust may have formed in the western part of the rift by seafloor spreading after the initial exposure of mantle, with the ocean crust subsequently being isolated on the Newfoundland side by an eastward jump of the spreading axis. The U–basement sequence might be explained (e.g., by basalt flows capping the ocean crust or as a sedimentary phenomenon peculiar to the Grand Banks side of the rift). The Newfoundland ocean crust would be more buoyant, and thus shallower, than serpentinized mantle on the Iberia side; differences in basement roughness would reflect differing effects and degrees of tectonic extension in ultra-slow-spreading ocean crust (Newfoundland side) and serpentinized mantle (Iberia side). MOHO = Mohorovicic seismic discontinuity.

Figure F5. Section of Ewing line 2MCS showing proposed drill Sites NNB-01A, NNB-01B, NNB-01C, NNB-03A, and NNB-04A. Locations are shown in Figure F3. Crossing profiles are shown in Figures F7 and F8. Arrows at the bottom indicate depths of basement picks. Holes are planned for at least 100-m basement penetration. CMP = common midpoint.

Figure F6. Section of Ewing line 2MCS extending east of the section in Figure F5, showing proposed drill Site NNB-05A. Site NNB-06B is on line 202 ~8 km southwest of the indicated crossing of this line. Location is shown in Figure F3. Crossing profiles are shown in Figure F8. Arrows at bottom of sites locate basement picks. Holes are planned for at least 100-m basement penetration. CMP = common midpoint.

Figure F7. Ewing MCS profiles crossing at proposed drill Sites NNB-01A, NNB-01B, and NNB-01C. Locations are shown in Figure F3. The main profile (2MCS) is shown in Figure F5. Arrows at the bottom indicate depths of basement picks. Holes are planned for at least 100-m basement penetration. CMP = common midpoint.

Figure F8. Ewing MCS crossing lines for proposed drill Sites NNB-03A, NNB-04A, and NNB-05A and profile through proposed Site NNB-06B. Locations are shown in Figure F3. Arrows at the bottom indicate basement picks. Holes are planned for at least 100-m basement penetration. CMP = common midpoint.