Site 901 (Fig. 1) was one of a series of sites drilled during Leg 149 to elucidate the nature of the top of the crust (acoustic basement) within the ocean/continent transition (OCT) beneath the Iberia Abyssal Plain. The regional background of this and the other Leg 149 sites is presented elsewhere (see "Introduction" chapter, this volume; Whitmarsh, Miles, and Mauffret, 1990; Whitmarsh et al., 1993). Site 901 was a new site, not included in the Leg 149 Prospectus, that was chosen at sea when only 36 hr remained for drilling at the end of the leg. Site 901 is located about 45 km east of Site 900 and 40 km southwest of Site 398 on the lower continental rise in multichannel seismic-reflection profile Lusigal Line 12 (Fig. 2; also see "Site Geophysics" section, this chapter). The site is 500 m east of an 80-m-high seafloor fault scarp that appears to represent the surface expression of a reactivated deep normal fault. This fault forms the western side of an asymmetric basement fault block that tilts about 13° to the east. The north-south shape of the block in cross section is unknown. On a pre-stack, depth-migrated seismic section, the fault block appears to consist of a transparent (sediment?) layer 500 to 600 m thick overlying a strong basement reflector (Fig. 3). The sediments overlying acoustic basement thicken to about 2.25 s two-way traveltime (2.8 km) in the basin west of the site and to at least 1.9 s two way traveltime (2.25 km) to the east.
The objective at this site was to drill into the shallow upper surface of the transparent layer on the fault block that we estimated as being about 100 m below the seabed. We had interpreted the high to be a fault block of continental crust. Should the acoustic basement under the site prove to be continental, then the almost 110-km-wide region of crust between the site and the base of the Portuguese continental slope might logically be assigned a continental origin also. Further more, it could be argued that a wide area of thinned continental crust might also exist under the conjugate Newfoundland margin, where conflicting views exist regarding the extent of thinned continental crust (Keen and de Voogd, 1988; Tucholke et al., 1989).