SCIENTIFIC OBJECTIVESMass Transport Processes in Subduction Zones and Forearcs of Nonaccretionary Convergent Margins (MAF-4B)
The drill site on South Chamorro Seamount will help address the variability of fluid transport and composition within the forearc. Field studies indicate that most of the fluid flow in the Mariana forearc is channeled along forearc faults and fault-controlled conduits in mud volcanoes. The pore fluid compositions are expected to vary depending on the nature of the channeling structures (diffuse network of small faults, major faults, and mud volcano conduits). In particular, fluids ascending along mud-volcano conduits, traveling along well-established paths in contact with previously metamorphosed wallrock, should carry the most pristine slab signature. This was certainly the case at Conical Seamount, drilled on Leg 125. The summit Site 780 produced by far the purest deep slab-derived fluids, based on their much lower chlorinity and higher K, Rb, B, H2S, and sulfate, whereas the flank Sites 778 and 779 produced combinations of slab-derived fluid with seawater that had reacted with peridotite and basalt at shallower crustal levels (Mottl, 1992).
Mechanics and Rheology
The mechanics and rheology of serpentine muds in the Mariana forearc seamounts control the processes that formed the seamounts and their morphology. The rheological study of Mariana serpentine muds will place strong constraints on the mechanics that generate and emplace the serpentine muds, maintain the conduits, and construct the seamounts.
Shipboard torsion-vane testing on Leg 125 at Conical Seamount in the Marianas and at Torishima Forearc Seamount in the Bonin forearc showed that the serpentine muds are plastic solids with a rheology that bears many similarities to the idealized Cam clay soil model and is well described by critical-state soil mechanics (Phipps and Ballotti, 1992). These muds are thus orders of magnitude weaker than salt and are, in fact, comparable in strength to common deep-sea pelagic clays. To determine the physical properties of these muds, we must recover a number of whole-round samples from the serpentine seamount and analyze them mechanically at both shipboard and shore-based laboratories. The rate at which the muds rise relative to the fluids will likely influence the water-rock reactions and the character of the slab signal in fluids from these mud volcanoes. Better constraints on the nature of the fluids will permit a more accurate determination of the physical conditions of the décollement where the fluids originate.
Although total fluid budgets are difficult to ascertain in any convergent margin, we suggest that they are more readily determined at nonaccretionary active margins because the hydrologic flow systems operate on longer time scales than do those at accretionary margins. Attempts to determine the total fluid budgets at accretionary active margins have been hindered by the presence of lateral heterogeneity and transient flow processes. Lateral heterogeneity results in different flow rates and compositions along strike of the margin. Transient flow apparently results largely from the valve-like influence of the accretionary complexes themselves.
Sediment properties vary with fluid pressure, and fluid pressure varies as a function of fluid production rate and transient hydrologic properties. Thus, the accretionary system acts as both a seal and a relief valve on the fluid flow system. The absence of such a short time scale, fluid pressure, and formation properties modulator at nonaccretionary systems should allow fluids to escape more steadily. To test this hypothesis, the physical nature of fluid flow at nonaccretionary settings must be determined. Then fluid budgets can be constructed to determine whether the expected long-term flow is consistent with observations or if the flow must occur transiently. The CORK experiment planned for the South Chamorro Seamount site will address this problem.
Spatial Variability of Slab-Related Fluids within the Forearc Environment as a Means of Tracing
Dehydration, Decarbonation, and Water/Rock Reactions in the Subduction and Supra-Subduction Zone Environments (MAF-4B)
The composition of slab-derived fluids and deep-derived rock materials may differ along the strike of the forearc, reflecting regional variations in composition within the slab and supra-subduction zone lithosphere. The pore fluids from several of the forearc mud volcanoes already sampled are chemically distinct (Fryer et al., in press). This difference is probably associated not only with the depth to the slab, but also with the physical conditions under which water-rock reactions occur and the variations in the regional composition of the plate and over-riding forearc wedge.
The geochemistry of the fluids from Conical Seamount is described in detail in several publications (Fryer et al, 1990; Haggerty, 1991; Haggerty and Chaudhuri, 1992; Haggerty and Fisher, 1992; Mottl, 1992; Mottl and Alt, 1992). These investigators have shown the origin of the Conical Seamount fluids to be from dehydration of oceanic crustal basalt and sediment at the top of the subducting lithospheric slab. The compositions of the fluids from Pacman and seamounts further south are reported in Fryer et al. (in press). Pore fluids from these indicate a slab source, as evidenced by their lower chlorinity and higher K and Rb, similar to that observed at Conical Seamount by Mottl (1992).
Metamorphic and Tectonic History of Nonaccretionary Forearc Regions and Physical Properties of the Subduction Zone (MAF-4B)
Pressure and Temperature Indicators from Fluids
The composition of slab-derived and deep-derived metamorphosed rock is useful in defining geochemical processes and estimates of the thermal and pressure regime at depth, and thus, for determining physical properties of the décollement region. It is possible to constrain some of the pressure and temperature conditions under which certain dehydration reactions take place in the subducted slab. Pore fluids from ODP Site 780 at the summit of Conical Seamount are unusual because of geochemical and physical processes at depth. The observed enrichments in alkali elements and B in fluids from Site 780 are unambiguous indicators of a source temperature in excess of 150°C. The fact that these elements are depleted at Sites 778 and 779 on the flanks of Conical Seamount, relative to their concentrations in seawater, indicates that the deep slab signal can readily be overprinted by local peridotite-seawater reactions at lower temperatures. Not all chemical species are affected by this overprinting, however (i.e., sulfur isotopic composition of dissolved sulfate) (Mottl and Alt, 1992). Thus, to avoid potential reactions between sediment and slab-derived fluids, we need to collect fluids from the mud volcano conduits where continued focused flow provides a pathway for slab-derived "basement" fluids to reach the seafloor.
Studies of deep derived minerals and metamorphic rock fragments brought to the surface in mud flows in these seamounts will permit us to constrain the pressure and temperature regimes under which the metamorphism that formed them took place. We know, for instance, that the minimum pressures of formation for incipient blueschist materials from Conical Seamount are 6-7 kbar (Maekawa et al., 1995). We can estimate from the paragenesis of crossite schist recovered in cores from South Chamorro Seamount that pressures in excess of 7 kbar are consistent with their metamorphism (Fryer et al., in press). With stratigraphic control and deeper penetration (than that afforded by gravity and piston coring) of the muds from these sites, we will be able to quantify the assemblages of muds present in the flows and constrain the ranges of pressure and temperature of the source regions of these materials.
Biological Activity Associated with Deep-Derived Subduction Zone Material
The interest in research pertaining to a deep subsurface biosphere has developed as a result of the study of extreme environments and their possible link to the first living organisms that inhabited the Earth. The search for the last common ancestor in the geologic record is moving toward environments at high temperatures like those at spreading centers and hot spots on the ocean floor and on land. Microbes and microbial products are abundant in oceanic hydrothermal environments and are presumed to be representative of a community of thermophilic and hyperthermophilic organisms that originated beneath the seafloor (Fisk et al., 1998). Microbes are also involved in the transformation of minerals in the oceanic volcanic crust and in the cycling of elements in the crust; however, the origin of these microbes is much more controversial.
Drilling at Chamorro Seamount provides a unique opportunity to reexamine the hypothesis that microbes are capable of using alternative energy sources that would support a heterotrophic subsurface ecosystem. In addition, because the pore fluids are more pristine in nonaccretionary convergent margins, it will be easier to assess from the chemistry of both the muds and the fluids whether organic syntheses capable of supporting life are active.
Understanding the origin of the deep biosphere is fundamental to the ODP drilling program and will further address the compelling question of whether life arose in these types of environments rather than on the surface of the early Earth. Although several experimental studies indicate that a thermophilic origin of life is possible, definitive proof will depend on how successful future efforts, such as studying the material on South Chamorro Seamount, are at demonstrating that these conditions or organic components actually exist in the subsurface environment.
Structure of the Philippine Sea Plate (WP-1B)
The observations of seismic surface waves as well as various phases of body waves from earthquakes at the Philippine Sea plate margins will provide sufficient data to map differences in plate structures among 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 (Kanamori and Abe, 1968; Seekins and Teng, 1977; Goodman and Bibee, 1991). Surface wave data suggest that the plate is only ~30 km thick (Kanamori and Abe, 1968; 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).
Uppermantle Structure beneath the Philippine Sea (WP-1B)
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 (WP-1B)
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. 5A). 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 key areas that are now poorly imaged. In addition, this observatory will provide data from the backarc side of the Izu Oagasawara and Mariana Trenches, giving greater accuracy and resolution of earthquake locations and source mechanisms.
Basalt Chemistry and Crustal Thickness (WP-1B)
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 off ridge where the rock samples and high quality seismic data were collected at the same location. Chemical analysis of the basalt samples from Site WP-1B should provide clues as to why the crust is thinner (3 to 4 km) than normal and whether it is due to the differences in the initial temperature conditions of the lithosphere.
Age of Basement (Site WP-1B)
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 (Site WP-1B)
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.
Ashfall Record (Site WP-1B)
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, Rotation, and Tectonic Drift (Site WP-1B)
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. By collecting oriented sediment cores it may be possible to study the rotation of the Philippine plate and the initiation of subduction of the Pacific plate.
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