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Scientific Objectives

The principle objective at Site 1201 was to install a long-term borehole seismic observatory in the middle of the Philippine plate to improve global seismic coverage, to study the structure of the upper mantle under the Philippine Sea, and to study plate interactions in the western Pacific. It was also expected that drilling at Site 1201 would provide samples representative of the Eocene/Paleocene crust of the northern West Philippine Basin. Results from this site would thus augment those obtained during DSDP Legs 31 and 59, which were the first legs to sample and estimate the age of basement in the region and to confirm that the seafloor formed by backarc spreading. Results from this site will also add to our knowledge of backarc crustal structure and geochemistry, microplate tectonics, magnetic lineations, and sedimentation. Because core quality and dating techniques have vastly improved since these early legs, it was also anticipated that drilling at Site 1201 would provide better age control on backarc spreading as well as detailed records of Northern Hemisphere climate change, aeolian transport, and arc volcanism in the region during the Tertiary.

Establishment of a Borehole Seismic Observatory

As outlined above, one of the main reasons for installing a borehole seismic observatory in the middle of the Philippine plate was to achieve homogeneous seismic coverage of the Earth's surface with at least one station per 2000 km in the northwestern Pacific area (Fig. F14). Aside from plugging an important gap in the global seismic array, the Site 1201 observatory will produce high-quality seismic data. Tests with other borehole seismometers show that the noise level for oceanic borehole instruments is much lower than for most land stations (e.g., Stephen et al., 1999). High-quality seismic data from this site will be used for several purposes.

Earthquake Mechanisms

First, an observatory at Site 1201 will provide data from the backarc side of the Izu-Ogasawara and Mariana Trenches, giving greater accuracy and resolution of earthquake locations and source mechanisms. The observatory will also be valuable for resolving events in the Ryukyu and Philippine Trenches because its location is analogous to that of station WP-2 off the Japan Trench.

Structure of the Philippine Plate

Observations of seismic surface waves as well as various phases of body waves from earthquakes along the margins of the Philippine plate will provide sufficient data to map differences in plate structure among the different basins comprising the plate (e.g., the West Philippine, Shikoku, Japan, and Parece Vela Basins). Only a few previous studies with limited resolution exist on the lithospheric structure of these areas (Seekins and Teng, 1977; Goodman and Bibee, 1991). Surface wave data suggest that the plate is only ~30 km thick (Seekins and Teng, 1977). Such a value is inconsistent with predicted values from age vs. heat flow and age vs. depth curves (Louden, 1980). A long-line (500 km) seismic refraction experiment in the West Philippine Basin could not image the lithosphere/asthenosphere boundary (Goodman and Bibee, 1991).

Mantle Structure and Dynamics

Finally, Site 1201 will provide higher seismic resolution of mantle and lithosphere structures in key areas that are now poorly imaged. 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, where significant discontinuities and lateral heterogeneities exist (Fukao, 1992). Data from Site 1201 will be crucial in determining whether the Pacific plate is penetrating into the lower mantle in the Mariana Trench but not in the Izu-Ogasawara (Bonin) Trench (van der Hilst et al., 1991; Fukao et al., 1992; van der Hilst and Seno, 1993) and in determining how the stagnant slab eventually sinks into the lower mantle (Ringwood and Irifune, 1988). Detailed images of mantle flow patterns may also help explain how backarc basins open and close and explain the mantle heterogeneity that causes the basalts sampled from western Pacific marginal basins to have Indian Ocean Ridge isotopic characteristics (Hickey-Vargas et al., 1995).

In addition to the seismic objectives at Site 1201, we recognized that coring at the site might accomplish a number of important geologic objectives.

Age of Basement

Although the age of the 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.

Basalt Chemistry and Crustal Thickness

Recent studies on 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. The volume of the melt (represented by the crustal thickness) and its chemical composition are sensitive to the temperature. This means that a knowledge of crustal thickness in an ocean basin makes it possible to estimate the temperature at which the crust was formed and the concentration of major and minor chemical elements in the resulting basalts (e.g., Klein and Langmuir, 1987; White and Hochella, 1992). To date, these studies have concentrated on young MORBs. The chemical model on which these predictions are based still has large uncertainties, partly because there are few cases off ridge where rock samples and high-quality seismic data have been collected at the same location. Chemical analysis of the basalt samples from Site 1201 should provide clues as to why the crust in the Philippine Basin is 3 to 4 km thinner than normal.

Tertiary Climate Record

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 might also contain a record of aeolian transport from Eurasia.

Ash Fall Record

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 Formation MicroScanner (FMS), natural gamma spectrometry tool (NGT), and ultrasonic borehole imager (UBI) logs, we hoped to obtain a detailed record of arc volcanism around the Philippine Sea.

Philippine Plate Paleolatitude, Rotation, and Tectonic Drift

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, it was thought that 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 the movement of the Philippine plate through the Cenozoic. By collecting oriented sediment cores, we hoped to study the rotation of the Philippine plate and the initiation of subduction of the Pacific plate.

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