4. Site 11791

Shipboard Scientific Party2


The main objective at Site 1179 was to drill a borehole ~100 m into the basaltic top of the ocean crust, install a guidebase and casing in the hole, and emplace a seismometer with a recording package that would allow later data retrieval by submersible vehicles. Site 1179 was chosen as one of three locations in the northwest Pacific Ocean where broadband seismometers are to be installed in Ocean Drilling Program (ODP) boreholes (Fig. F1). The other two locations are the inner wall of the Japan Trench, where two seismometers were installed during Leg 186 at Sites 1150 and 1151 (Sacks, Suyehiro, Acton, et al., 2000), and another to be installed at proposed Site WP-1 on the Philippine Sea plate during ODP Leg 195. Together, these seismometers augment the local network of land seismometers in Japan, eastern Russia, China, the Philippines, and elsewhere. The Leg 191 and other ocean borehole seismometers fulfill two needs identified by proponents of the International Ocean Network (ION) (Purdy and Dziewonski, 1988). They provide better coverage of earthquakes occurring at the subduction zones of the northwest Pacific, and they fill a gap in the global coverage of earthquake ray paths needed for more accurate tomographic studies of the Earth's mantle. For local seismicity and tectonic studies, oceanic seismometers lessen the asymmetry of station coverage, making it possible to determine more accurate earthquake locations and focal mechanisms. In its northwest Pacific location (Fig. F2), Site 1179 fills a gap in the global array of seismic stations for tomographic studies. Such studies would be more accurate and have better resolution if the global array of seismometers was regularly spaced rather than having stations clustered on the 29% of the Earth's surface covered by land. By filling a large gap, the Site 1179 seismometer will record seismic rays passing through parts of the mantle that would not be imaged otherwise.

Another compelling reason for emplacing borehole seismometers in the oceans, an expensive undertaking, is that submerged boreholes have been shown to be extraordinarily quiet environments (e.g., Stephen et al., 1999). For this reason, a seismometer situated at the bottom of an ocean borehole should have a better signal-to-noise ratio than similar land stations, allowing smaller earthquakes to be recorded and more accurate studies of earthquake waveforms. Both are the basis for detailed studies of the Earth's interior structure.

Naturally, a borehole into oceanic basement in the northwest Pacific also allows the addressing of many scientific problems more commonly attacked during ODP drilling: sediment and basalt geochemistry, regional history of sedimentation, heat flow, fossil distribution and time scales, paleoceanography, paleolatitude and plate tectonics, physical properties of the basaltic crust and sediment column, and occurrence of microbes in the crust. Although the 100-m penetration into the igneous crust at Site 1179 is not great, it is nonetheless one of a small number of boreholes to sample more than a few meters into the upper igneous crust, making the data collected in this section of particular value. Studies of the igneous section undertaken by Leg 191 scientists include igneous geochemistry and isotopic signatures to learn about magma sources and emplacement, radiometric geochronology for better dating of the crust and magnetic lineations, paleomagnetism for plate tectonic drift studies, seismic velocity and other physical properties measurements to better characterize the geophysical properties of the crust, and structure and fracture geometry to illuminate volcanic processes and hydrology. Sediment column studies include sedimentation history of the northwest Pacific, paleontology of siliceous and calcareous microfossils and palynomorphs to broaden our knowledge of fossil time scales and paleoceanography, the occurrence and timing of ash layers to understand the eruptive histories of western Pacific island arcs, magnetic stratigraphy to better define the polarity reversal time scale, magnetic reversal transitions to understand the geodynamo, and the occurrence of microbes to define the depth and extent of the deep biosphere. In addition to these studies with their regional and global implications, scientific description of the boreholes and their properties was necessary to characterize borehole wall properties both for finding the best depth to install the seismometer and for making local corrections to propagation paths.

Geologic and Tectonic Setting

Site 1179 (proposed Site WP-2A) is located on abyssal seafloor northwest of Shatsky Rise, ~1650 km east of Japan (Fig. F2). This part of the Pacific plate was formed during the Early Cretaceous, as shown by northeast-trending M-series magnetic lineations that become younger toward the northwest (Larson and Chase, 1972; Sager et al., 1988; Nakanishi et al., 1989). As shown by recent anomaly mapping, the site is situated on magnetic Anomaly M8 (Nakanishi et al., 1999), corresponding to an age of ~129 Ma and the Hauterivian stage of the Early Cretaceous (Gradstein et al., 1994, 1995). We plotted the magnetic anomaly data around Site 1179 and confirmed its location near the center of Anomaly M8 (Fig. F3). Paleomagnetic data indicate that the Pacific plate has drifted northward ~30 since the Cretaceous (Sager and Pringle, 1988; Larson et al., 1992), which suggests that the crust at Site 1179 likely formed ~10 north of the equator. If plate drift continues at its present speed and direction, after 7-8 m.y. the lithosphere, including Site 1179, will be subducted beneath the southern Kuril Trench.

Northwest Pacific Stratigraphy

Cores collected in the northwest Pacific basin by the Deep Sea Drilling Project (DSDP) (Legs 6, 20, 32, and 86) and ODP (Legs 185 and 191) over the last 30 yr show similar stratigraphy with three primary layers (Fisher, Heezen, et al., 1971; Heezen, MacGregor, et al., 1973; Larson, Moberly, et al., 1975; Heath, Burckle, et al., 1985; Plank, Ludden, Escutia, et al., 2000). At the top is a Miocene to Pleistocene blanket of siliceous clay and oozes that contains numerous ash layers. In these sediments, diatoms and radiolarians are common to abundant but few calcareous microfossils are found. This Neogene layer can be >200 m thick. Comparison with holes located southeast of Shatsky Rise (Fig. F4) indicates that this layer is largely absent or attenuated in that region. This observation implies that the thick Neogene layer results from productivity in divergent waters northeast of the western boundary current. In its lower reaches, the Neogene clayey layer may contain zeolite. The gray to olive siliceous clays and oozes typically pass downward to barren brown or reddish brown clays. Although the age of these clays is usually undetermined, at some sites it belongs to the mid- to Late Cretaceous (e.g., Sites 51, 194, and 195) but it may contain a highly condensed Tertiary section as well (e.g., Site 576). Beneath the barren clays is an often poorly recovered layer consisting of calcareous oozes, chalk, or marl deposited soon after the formation of the crust while it was at a depth above the calcite compensation depth (CCD). This layer suffered poor recovery because it is associated with chert and porcellanite layers that are ubiquitous in the northwest Pacific. During rotary drilling using water as a flushing agent, the chert causes the formation to be ground up and the softer parts to wash away, generally leaving only rounded chert fragments and slight traces of the softer matrix. In many holes, the top of the chert layer seems to correspond to the top of the calcareous section (Fig. F4) but this relationship is difficult to discern in some holes owing to poor recovery. In some other holes, however, the chert appears higher in the section with the barren brown clays.

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