Scientific ObjectivesStructure of the Philippine Sea Plate
Uppermantle Structure Beneath the Philippine Sea
Previous studies of spreading scenarios for the Philippine Sea have focused on kinematic processes. There is no consensus as to how marginal seas open, whether or not a single mechanism explains all backarc basins, or how the basins disappear. The mapping of the mantle flow and the subducting plate geometry is essential for understanding the dynamics of the mantle.
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). Site WP-1B will be a crucial network component in determining whether the Pacific plate is penetrating into the lower mantle in the Marianas Trench but not in the Izu Ogasawara (Bonin) Trench and, if so, to understanding why (van der Hilst et al., 1991; Fukao et al., 1992; van der Hilst and Seno, 1993). In addition, Site WP-1B will allow imaging of the subducting slab to determine how the stagnant slab eventually sinks into the lower mantle (Ringwood and Irifune, 1988). Also, the mantle heterogeneity that causes the basalts sampled from the western Pacific marginal basins to have Indian Ocean ridge type isotopic characteristics (Hickey-Vargas et al., 1995) may be inferred from the detailed image of the mantle flow.
Important Component of ION
A global seismographic network was envisioned by the Federation of Digital Seismographic Networks to achieve a homogeneous coverage of the Earth's surface with at least one station per 2000 km in the northwestern Pacific area (Fig.1). Thus, the Site WP-1B seismic observatory will provide invaluable data, obtainable in no other fashion, for global seismology. Data from this observatory will help revolutionize studies of global Earth structure and upper mantle dynamics by providing higher resolution of mantle and lithosphere structures in areas now poorly imaged. In addition, this observatory will provide data from the backarc side of the Izu-Oagasawara, Mariana Trenches, giving greater accuracy and resolution of earthquake locations and source mechanisms.
Basalt Chemistry and Crustal Thickness
Recent studies on the relationship between midocean 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 oceanic 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, this type of work has concentrated on young MORBs. The chemical model on which these predictions are based still has large uncertainties, partly because there are few cases where the rock samples and high-quality seismic data were collected at the same location. Chemical analysis of the basalt samples from WP-1B should provide clues as to why the crust is thinner (3 to 4 km) than normal and whether it is because of the differences in the initial temperature conditions of the lithosphere.
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 hope to obtain an accurate basement age from undisturbed microfossils, magnetostratigraphy, or radiometric dating of ash horizons. This information will be of considerable importance in constraining models of backarc spreading.
Tertiary Climate Record
Previous drilling in the west Philippine Sea was conducted on 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 hope to obtain a proxy record of Tertiary climate change for the region. It is anticipated that the upper levels of the section may also contain a record of aeolian transport from Eurasia.
Although ash and tuff were present in the sediments recovered in the region on previous legs, it was impossible to reconstruct the ashfall stratigraphy because of core disturbance and the discontinuous nature of the coring. By continuous coring using advanced hydraulic piston coring (APC) and extended core barrel (XCB) techniques and correlation with high-resolution Formation MicroScanner (FMS), natural-gamma spectrometry tool (NGT), and ultrasonic borehole imager (UBI) logs, we hope to obtain a detailed record of arc volcanism around the Philippine Sea.
Philippine Plate Paleolatitude 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 is unlikely that enough flow units will be cored at Site WP-1B to average secular variation adequately, the results will 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.
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