Site 1201 is located in the West Philippine Basin in 5711 m of water ~100 km west of the inactive Palau-Kyushu Ridge and 450 km north of the extinct Central Basin Fault (Fig. F15). Early interpretations of magnetic lineations (Hilde and Lee, 1984) indicated that the site lies on 49-Ma crust near Chron 21 and formed by northeast-southwest spreading on the Central Basin Fault. The spreading direction then changed to north-south at ~45 Ma and finally ceased at ~35 Ma as volcanism ceased on the Palau-Kyushu Ridge. Because the earliest magnetic anomalies in the region predate the initiation of subduction at ~45 Ma along the Palau-Kyushu Ridge, Hilde and Lee (1984) considered that the Philippine Sea initially formed by entrapment of an older Pacific spreading ridge. More recent bathymetric and magnetic surveys (Okino et al., 1999) show that the site lies at the transition from well-defined anomalies south of the Oki-Daito Ridge to more complicated anomalies to the north, which implies that the crust to the north may have formed at a different spreading center. Analysis of paleolatitude and declination data from the Philippine plate and its margins suggests that the plate has drifted about 15° to the north and rotated clockwise by up to 90° since the middle Eocene (Hall et al., 1995).

The sediment section at Site 1201 was predicted to be 400 to 450 m thick based on recent seismic reflection surveys showing a two-way traveltime to basement of 0.45 s (Fig. F16). Drilling at other sites in the region during Deep Sea Drilling Project (DSDP) Legs 31 and 59 (Karig, Ingle, et al., 1975; Kroenke, Scott, et al., 1981) recovered a relatively barren deep-water section dominated by Holocene to Eocene-Paleocene(?) brown pelagic clays overlying basement near the Oki-Daito Ridge (DSDP Sites 294 and 295). At DSDP Sites 290 and 447 to the south, the section consists of a barren interval of Pliocene clays underlain by Oligocene nannofossil-bearing silty clays mixed with ash. This was underlain by a thick section of polymict and volcanic breccias presumably derived from the Palau-Kyushu Ridge to the east. The underlying basement consists of 80% basalt pillows and 20% diabase. Because Site 1201 lies in a similar setting at the foot of the Palau-Kyushu Ridge, it was considered likely that the section would be similar to that at Sites 290 and 447.