Site 1201 was devoted to coring and casing a hole in the seafloor at abyssal depths in the Philippine Sea to install a broadband seismometer for a long-term subseafloor borehole observatory. Because the observatory is an important component of the International Ocean Network (ION) seismometer network, the location of the site was dictated primarily by seismic considerations.
The first of these considerations is largely geographic. One of the greatest problems facing seismologists who use teleseismic waves to study the structure and dynamics of the Earth's interior is the uneven distribution of seismic stations, especially the lack of stations in large expanses of ocean such as the Pacific. A global seismic network with at least one seismic observatory every 2000 km is required in the northwestern Pacific to achieve homogeneous coverage of the Earth's surface. One of the most significant gaps in this coverage lies in the center of the Philippine Sea. A major objective of the ION project was to fill this gap by installing a seafloor seismic observatory in the West Philippine Basin west of the Kyushu-Palau Ridge between existing stations at Ishigaki (ISG) and Baguio (BAG) to the west, Inuyama (INU) and Taejon (TJN) to the north, Minami Torishima (MCSJ) and Chichijima (OGS) to the east, and Pohnpei (PATS) and Jayapura (JAY) to the south (Fig. F1).
A second seismic consideration is to improve the resolution of tomographic imaging of the Mariana subduction zone by placing an observatory in the middle of the Philippine Sea on the overriding plate behind the subduction zone. There are indications that the subducting Pacific plate does not penetrate below the 670-km discontinuity and that it extends horizontally (Fukao et al., 1992; Fukao, 1992), but the resolution of these studies is poor (>1000 km) beneath the Philippine Sea and the northwestern Pacific, especially in the upper mantle. A seismometer at Site 1201 will be crucial to determining whether the Pacific plate is penetrating into the lower mantle in the Mariana Trench but not in the Izu-Ogasawara Trench (van der Hilst et al., 1991; Fukao et al., 1992; van der Hilst and Seno, 1993). In addition, the site will allow imaging of the subducting slab to determine how the stagnant slab eventually sinks into the lower mantle (Ringwood and Irifune, 1988). Better maps of the geometry of the subducting plate and patterns of mantle flow are essential to understanding the dynamics of the mantle and mechanisms of backarc spreading.
Finally, a quiet seafloor observatory at Site 1201 will make it possible to study differences in plate structure under the Philippine Sea and its subbasins (e.g., the Philippine, Shikoku, Japan, and Parece Vela Basins) using surface and body waves. Previous studies with limited resolution in the area suggest that the plate is only ~30 km thick (Kanamori and Abe, 1968; Seekins and Teng, 1977), but this is inconsistent with age vs. heat flow and depth curves (Louden, 1980), and a long-line (500 km) seismic refraction experiment in the West Philippine Basin failed to image the lithosphere/asthenosphere boundary (Goodman and Bibee, 1991).
The site chosen for the seismometer installation is located in the West Philippine Basin ~100 km west of the inactive Kyushu-Palau Ridge and 450 km north of the extinct Central Basin Fault (Fig. F2). Early interpretations of magnetic lineations (Hilde and Lee, 1984) indicate 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 spreading finally ceased at ~35 Ma. Because the earliest magnetic anomalies in the region predate the initiation of subduction at ~45 Ma along the Kyushu-Palau Ridge, Hilde and Lee (1984) considered that the Philippine Sea 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.
Site 1201 is located on flat seafloor at a water depth of 5640 m, where the sediments were predicted to be ~400-450 m thick based on recent seismic reflection surveys (Fig. F3) that showed a two-way traveltime to basement of 0.5 s (Fig. F4). Drilling at other sites in the region during DSDP Legs 31 and 59 (Karig, Ingle, et al., 1975; Kroenke, Scott, et al., 1980) recovered a relatively barren, deepwater section dominated by Holocene to Eocene or Paleocene(?) brown pelagic silty 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 breccia presumably derived from the Kyushu-Palau Ridge. The underlying basement consisted of 80% basalt pillows and 20% diabase. Because Site 1201 lies in a similar setting at the foot of the Kyushu-Palau Ridge, it was considered likely that the section would be similar to that at Sites 290 and 447.
Although the site was located primarily on the basis of seismic constraints, it was recognized that drilling at Site 1201 would also satisfy many other important geological objectives.
Previous drilling in the West Philippine Sea was conducted during DSDP Legs 31 and 59 before the advent of piston coring, and many of the holes were only spot cored. As a consequence, the available core from the region is almost useless for stratigraphic and paleontologic reconstructions. By obtaining a continuous, high-quality record of pelagic sedimentation supplemented by high-quality logs, we hoped to obtain a proxy record of Tertiary climate change for the region. It was anticipated that the upper levels of the section would also contain a record of eolian transport from Eurasia.
Although ash and tuff were present in the sediments recovered in the region during previous legs, it was impossible to reconstruct the ash fall stratigraphy because of core disturbance and the discontinuous nature of the coring. By continuous coring using APC and XCB techniques and correlation with high-resolution FMS and natural gamma spectrometry logs, we hoped to obtain a detailed record of arc volcanism around the Philippine Sea.
Paleomagnetic measurements of sediments and basalt cores are important because oriented samples are difficult to obtain from the oceans. The basalts record the direction of the magnetic field at the time the basalts were emplaced and can be used to infer the paleolatitude of the site (e.g., Cox and Gordon, 1984). Although it was unlikely that enough flow units would be cored at Site 1201 to average secular variation adequately, the results would be useful in determining a Paleogene paleomagnetic pole for the Philippine plate. Sediments are typically a good recorder of the Earth's magnetic field and should contain a continuous record of movement of the Philippine plate through the Cenozoic. By collecting oriented sediment cores, it might be possible to study the rotation of the Philippine plate (Hall et al., 1995) and the initiation of subduction of the Pacific plate.
Although the age of basement in the northern West Philippine Sea has been estimated from magnetic anomalies, paleontologic confirmation has been imprecise because of spot coring, core disturbance, and poor preservation of microfossils. By continuous coring to basement using modern coring techniques, we hoped to obtain an accurate basement age from undisturbed microfossils, magnetostratigraphy, or radiometric dating of ash horizons. This information would be of considerable importance in constraining models of backarc spreading.
Studies of the relationship between mid-ocean-ridge basalt (MORB) chemistry and crustal thickness indicate that the degree of partial melting is strongly controlled by the temperature of the upwelling mantle at the ridge. This means that the temperature of formation and the concentration of major and minor elements in submarine basalts can be estimated from crustal thickness (e.g., Klein and Langmuir, 1987; White and Hochella, 1992). To date, this type of work has been based on the study of young MORBs, and the model has large uncertainties, partly because there are few locations off ridge where rock samples and high-quality seismic data have been collected in the same location. Chemical analysis of the basalts at Site 1201 should provide clues as to why the crust in the Philippine Sea is thinner (3-4 km) than expected and whether it is due to differences in the initial temperature conditions in the lithosphere.