INTRODUCTION

Mariana Subduction System
Geologic processes at convergent plate margins control geochemical cycling, seismicity, and deep biosphere activity within subduction zones. The study of input into a convergent plate margin by sampling the down-going plate provides the geochemical reference necessary to learn what geochemical factors influence the production of supra-subduction zone crust and mantle in these environments. The study of the output in terms of magma and volatiles in volcanic arcs and backarc basin settings constrains processes at work deep in the subduction zone, but these studies are incomplete without an understanding of the throughput, the nature of geochemical cycling that takes place between the time the subducting plate enters the trench and the time it reaches the zone of magma genesis beneath the arc. Tectonically induced circulation of fluids at convergent margins is a critical element in the understanding of chemical transport and cycling within convergent plate margins and ultimately in understanding global mass balance (e.g., COSOD II 1987; Langseth et al., 1988; Kulm and Suess, 1990; Langseth and Moore, 1990; Kastner et al., 1992; Martin et al., 1991). In the shallow to intermediate supra-subduction zone region, dehydration reactions release fluids from pore water and from bound volatiles in oceanic sediments and basalts of the down going plate (Fryer and Fryer, 1987; Peacock, 1987, 1990; Mottl, 1992; Liu et al., 1996). Fluid production and transport affect the thermal regime of the convergent margin, metamorphism in the supra-subduction zone region, diagenesis in forearc sediments, biological activity in the region, and ultimately the composition of arc and backarc magmas. Furthermore, these fluids, their metamorphic effects, and the temperature and pressure conditions in the contact region between the plates (the décollement) affect the physical properties of the subduction zone where most major earthquakes occur.

The discovery of the earth's deep biosphere is recognized as one of the most outstanding breakthroughs in biological sciences. The extent of this biosphere is currently unknown, but we are becoming increasingly aware that life has persisted in environments ranging from active hydrothermal systems on midocean ridges to deep ocean sediments, but so far no detailed investigations have been made of the potential for interaction of the deep biosphere with processes active in convergent plate margins.

Determining unequivocally the composition of slab-derived fluids and their influences over the physical properties of the subduction zone, biological activity, or geochemical cycling in convergent margins requires direct sampling of the décollement region. To date, studies of décollement materials, mass fluxes, and geochemical interchanges have been based almost exclusively on data from drill cores taken in accretionary convergent margins (e.g., Kastner et al., 1992; Carson and Westbrook 1995; Maltman et al., 1997). Large wedges of accreted sediment bury the underlying crystalline basement, making it inaccessible by drilling, and the wedges interact with slab-derived fluids altering the original slab signal. The dehydration reactions and metamorphic interchanges in intermediate and deeper parts of the décollements have not been studied in these margins. By contrast, nonaccretionary convergent margins permit direct access to the crystalline basement and produce a more pristine slab-fluid signature for two reasons: (1) the fluids do not suffer interaction with a thick accretionary sediment wedge and (2) they pass through fault zones that have already experienced water-rock interactions, thus minimizing interaction with subsequently escaping fluids. Regardless of the type of margin studied, the deeper décollement region is directly inaccessible with current or even foreseeable ocean drilling technologies. We need a locality where some natural process brings materials from great depths directly to the surface. The Mariana convergent margin provides precisely the sort of environment needed, as the South Chamorro Seamount (Fig. 4), located on the southern Mariana forearc, is the only known site of active blueschist mud volcanism in the world.

The Mariana subduction system is nonaccretionary and the forearc is pervasively faulted (Fig. 2). It contains numerous large (30 km diameter and 2 km high) mud volcanoes (Fryer and Fryer, 1987; Fryer, 1992; 1996) (Fig. 2, Fig. 3). The mud volcanoes are composed principally of unconsolidated flows of serpentine muds with clasts of serpentinized mantle peridotite. Some have also brought up blueschist materials (Maekawa et al., 1995; Fryer, in press). Faulting of the forearc to great depth produces fault gouge that generates a thick gravitationally unstable slurry of mud and rock when mixed with slab-derived fluids. The slurry rises in conduits along the fault plane to the seafloor (Fig. 2) (Lockwood, 1972; Bloomer and Hawkins, 1980; Fryer et al., 1990; Phipps and Ballotti, 1992; Fryer, 1996). One of these faults supports the first discovered megafaunal community associated with a serpentine/blueschist mud volcano (Fryer, in press; Fryer et al., in press). These mud volcanoes are our most direct route to the décollement and episodically open a window through protrusion events, which provides a view of processes and conditions at depths of up to 35 km beneath the forearc (Fig. 2).

ION Seismometer in the Philippine Sea
Tomographic studies using earthquake waves propagating through the Earth's interior have revolutionized our understanding of mantle structure and dynamics. Perhaps the greatest problem facing seismologists who wish to improve such tomographic models is the uneven distribution of seismic stations, especially the lack of stations in large expanses of ocean such as the Pacific. The International Ocean Network (ION) project, an international consortium of seismologists, has identified "gaps" in the global seismic net and is attempting to install digital seismometers in those locations. One of the highest ION priorities is to install a station beneath the deep seafloor of the Philippine Sea (Fig. 5A, Fig. 5B).

Site WP-1B, situated in the west Philippine Basin west of the Kyushu-Palau Ridge (Fig. 6), is slated to become a long-term borehole seismic observatory, which will be neighbored by stations at Ishigaki, (ISG) and Tagaytay (TAG) to the west, by many Japanese stations to the north, by Minami-Torishima (MCSJ) Island station to the east, and by the stations at Ponpei (PATS) and Port Moresby (PMG) to the south (Fig. 5A). A seismic station at the center of the Philippine Sea plate is an essential addition to the surrounding stations and, together with existing land stations, will aid in understanding the global dynamics operating in the western Pacific (Fig. 6). Like other existing oceanic borehole observatories (Sites 1150 and 1151) (Suyehiro, Sacks, Acton et al., 2000), there is a nearby coaxial transoceanic telephone cable (TCP-2) to use for data recovery and power. However, the Site WP-1B installation is designed as a stand-alone system with its own batteries and recorder. Thus, once instruments are installed in the hole, they will be serviced for data analyses, distribution, and archiving. We plan to connect data, control, and power lines to the TPC-2 cable owned by the University of Tokyo after confirmation of data retrieval. This will be done under the auspices of an ongoing national program within Japan (Ocean Hemisphere Network Project). Initially, power will be supplied to the observatory by a battery pack, and data will be retrieved by a remotely operated vehicle (ROV) (Fig. 7). The data will eventually become accessible worldwide through the Internet. Although data recovery will be costly and the data will not be available in real time until the system is connected to the TCP-2 cable, the scientific importance of the site to the ION concept is such that this is worthwhile.

Proposed Site WP-1B is also important because it will provide samples representative of the Eocene/Paleocene crust of the northern west Philippine Basin. Results from this site will augment those obtained on Deep Sea Drilling Project (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 is also anticipated that drilling at Site WP-1B will 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.