Proposed Sites | Table of Contents
Structure of the Pacific Ocean Plate and Underlying Mantle
There are many bathymetric highs in the northwestern Pacific (e.g., Shatsky Rise and Hess Rise) whose roots are poorly known. Body wave studies have not been able to determine the thickness of the plate, although large-scale anisotropy and lateral heterogeneity have been detected (e.g., Shimamura et al., 1983). Accumulation of broadband seismic wave data, including information of wave paths through the basin part of the Pacific plate, is needed to obtain a detailed understanding of the structure of the lithosphere and asthenosphere.
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-2 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 seaward side of the northwest Pacific trenches, giving greater accuracy and resolution to earthquake locations and source mechanisms.
MORB Chemistry and Ocean Crust Formation
Drilling at Site WP-2 will core ~100 m of the upper basaltic crust, making it one of a small number of sites at which significant penetration of the ocean crust has been achieved. Geochemical and isotopic analyses of the basalt samples will add to knowledge of mid-ocean ridge basalt (MORB) chemistry and emplacement (e.g., Hart, 1988; Janney and Castillo, 1996, 1997). This site may display the influence of the mantle plume assumed to have formed Shatsky Rise, as is postulated for Site 304 (DSDP Leg 32; Janney and Castillo, 1996, 1997). In addition, planned Formation MicroScanner (FMS) and ultrasonic borehole imager (UBI) logs (see "Logging Plan" section) will image the borehole walls, allowing geologists a detailed view of upper crust igneous structures. Such data, allied with physical properties, geochemical, and other data collected by cores and logs, can be used to better understand the volcanic structure and emplacement of the crust.
Pacific Plate Paleolatitude and Tectonic Drift
Paleomagnetic measurements of basaltic 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-2 to average secular variation adequately, the results are useful in combination with other Deep Sea Drilling Project (DSDP) and ODP basalt core paleomagnetic data, which can be used to calculate a Cretaceous paleomagnetic pole for the Pacific plate (Cox and Gordon, 1984; Sager and Pringle, 1988).
Age of Anomaly M8
Although widely used for dating the ocean lithosphere, anomalies of the M-series are poorly dated because there are few places where the anomalies have been directly dated (e.g., Gradstein et al., 1994). Site WP-2 is located on Anomaly M8 and may allow radiometric dating of this anomaly, if basalts suitable for dating are recovered.
Microbiology of the Deep Ocean Lithosphere
Recent findings from boreholes suggest that microorganisms can be found buried deep (~900 meters below seafloor [mbsf]) within the lithosphere. Several glass samples recovered during Legs 185 and 187 showed textural evidence for microbial alteration, leaving the intriguing question of whether microbiological activity is still active in volcanic basement. Additionally, there is a major effort to determine community composition by DNA extraction, in situ hybridization, characterization of microbes isolated from enrichment cultures, and culturing of microbes from samples collected during Leg 185 that were maintained at in situ pressure. Drilling at Site WP-2 will give the opportunity to examine samples of old Pacific lithosphere for microorganisms and will allow comparison with measurements made in the vicinity of ODP Leg 185 (Plank, Ludden, Escutia, et al., in press).
Age and Geochemistry of Shatsky Rise
Although numerous holes have been drilled on the southern Shatsky Rise edifice (Texas A&M University [TAMU] Massif), none has recovered basalt. In addition, all dredged samples have been badly altered, so no direct dating of this LIP is available (Sager et al., 1999). Understanding LIP formation has been recognized as an important goal for ODP studies (ODP Long Range Plan, 1996). Isotopic studies of dredged samples suggest that Shatsky Rise has both ridge and plume characteristics (Tejada et al., 1995). If basalt is recovered during the engineering tests planned for Leg 191, it will be the first recovery of such material. Geochemical, isotopic, and radiometric dating of these samples would be invaluable for understanding how this and other ocean plateaus were formed.
Proposed Sites | Table of Contents