Three, high-quality, migrated, multichannel seismic reflection profiles pass near Site 897 (Fig. 4). These are Lusigal Lines 4 (Fig. 5) and 15 (Fig. 6) and Sonne Line 75-16 (Fig. 3), on which Site 897 is located. The profiles show that Site 897 is located over a roughly north- to south-trending basement ridge that rises to within 560 ms two-way traveltime (510 m) of the seafloor. This ridge is asymmetrical and has a steeper slope on its eastern side. Sedimentary horizons onlap the high on both sides, with most reflectors onlapping higher on the eastern side of the ridge. The troughs adjacent to the ridge contain about 1500 (east) and 2000 (west) m of sediments. Numerous normal faults can be found in the overlying sediments to the east of the top of the ridge. No obvious faults are observed in the sediments to the west of the ridge.
The relationship of Site 897 to the top of the ridge is best seen in Sonne Line 75-16 (Fig. 3). Two local basement highs, to the west of the shallowest point on the ridge, may be offset by west-dipping normal faults. The shape of the highest point on the ridge suggests that it may have been bevelled by erosion, followed by rotation down to the east. We drilled to the west of the main basement high, through an apparently unconformable and transparent packet of sediments about 60 ms (~55 m) thick, between two of the basement highs. We thought that this was our best chance of encountering at least some synrift sediments. We predicted that basement would be encountered at 680 mbsf at this site.
Magnetic data (see Frontispiece; PR. Miles, J. Verhoef, and R. MacNab, pers. comm., 1993) show that Site 897 is located in a magnetic trough. The magnetic anomalies to the west are lineated roughly north to south and have been interpreted as seafloor-spreading anomalies (Whitmarsh et al., 1990). The anomalies to the east are not lineated and were interpreted as the signature of rifted continental crust. Beslier et al. (1993) interpreted seismic reflection profiles to show that the basement under Site 897 is continental upper mantle that had been exposed during rifting. The basement rocks were predicted to be serpentinized peridotite.
While acquiring the JOIDES Resolution seismic profile 149-2, we noted one irregularity in the generally monotonous bathymetry of the Iberia Abyssal Plain. East of Site 897, along Sonne Line 75-16 and the site approach survey track, a 7-m step up to the east was observed in the seafloor (Fig. 7). The seafloor as far as 1 km west of this step is 2 to 3 m lower than typical depths in the area. In Sonne Line 75-16, this feature can be seen as an abrupt step that we had previously dismissed as a timing artifact introduced during acquisition or processing. We have now interpreted it as an indication of active faulting in the sedimentary section. The sense of motion of the fault is equivocal and might be a west-dipping normal fault or an east-dipping reverse fault.