Geological map (modified from Larsen, 1990) showing distribution of seaward-dipping reflector sequences and continental flood basalts of the North Atlantic Volcanic Province. ODP and DSDP drill sites along the volcanic rifted margins of the North Atlantic are shown. Subaerially erupted basalts show flood basalt structure landward of the inferred continent/ocean boundary and have a SDRS structure seaward of the boundary. The part of Northwestern Iceland that shows SDRS-like structure is included. Note that the spreading history of the Iceland Plateau north of the GIR is different from that south of the GIR. In the young crust north of the GIR, the typical SDRS structure is not continuously present and extends to a depth of only about 2 km. However, below the GIR itself, the SDRS may attain a thickness of 10 km (Larsen and Jakobsdóttir, 1988).
Figure 2. Seismic track map and regional bathymetry. Previously drilled holes and Leg 163 drilled sites and planned sites are shown. Ice-free, subaerial bedrock outcrop is stippled.
Figure 3. Seismic cross section through Site 988 (top). Interpretations shown in line drawing (middle) and migrated section (bottom). The Eocene age of the postrift sediments is inferred through correlation with the EG63 transect (Larsen, Saunders, Clift et al., 1994). Seismic velocities used in the depth conversion and migration are in km/s.
Figure 4. Seismic cross section through Site 989 (top). Interpretations shown in line drawing (middle) and migrated interpretation (bottom). Steeply dipping to subvertical prerift sediments were encountered within the rotated fault block located below the lava sequence at Site 917. ODP Sites 914-917 are described in Larsen, Saunders, Clift, et al. (1994). Seismic velocities used in the depth conversion and migration are given in km/s.
Figure 5. N-MORB normalized (Sun and McDonough, 1989) minor-element and trace-element spidergrams of cored basalts from the SDRS along the EG66 transects (Site 988) and EG63 (Sites 915, 917, 918, 989, and 990). The Site 917 data include only samples with 6%-9% MgO; samples with high Nb/Zr were also excluded. The fields for basalts from Sites 917 and 918 are based on data in Fitton et al. (in press). The Site 990 lavas are virtually identical in composition to lavas recovered from Site 918, 72 km farther offshore and in the center of the seaward-dipping reflector sequence (Larsen, Saunders, Clift, et al., 1994; Fitton et al., in press). This observation implies that seafloor-spreading-type magamtism was established soon after breakup of the continental margin (represented by the Site 917 Upper Series). The Site 988 lavas are considerably enriched in the incompatible elements as compared to those from the EG63 transect. The Site 988 lavas are about 300 km closer to the Greenland-Iceland Ridge than lavas from the EG63 transect and are similar to Tertiary basalts from Iceland and Scoresby Sund, East Greenland (Larsen et al., 1989; Fitton et al., in press). The offset from the Iceland plume and its palaeoposition (Iceland-Greenland Ridge), at which we observe this marked enrichment are similar to that mapped by DSDP Sites 406-408 (Fig. 2) and along the Reykjanes Ridge (Schilling, 1986). This observation suggests that the generation of enriched, Iceland-type tholeiites has been limited to about the same (~200-300 km) offset from the center of the Iceland hot-spot since the inception of rifting and ocean floor formation.
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