The long-term H2O site satisfies three scientific objectives of crustal drilling: (1) it is located in one of the high-priority regions for the Ocean Seismic Network (OSN); (2) its proximity to the Hawaii-2 cable and H2O junction box make it a unique site for real-time, continuous monitoring of geophysical and geochemical experiments in the crust; and (3) it is on fast-spreading Pacific crust (71 mm/yr, half rate), which represents one end-member for models of crustal generation and evolution as well as crust/mantle interaction.
This site primarily represents the interests of the Joint Oceanographic Institutions (JOI)/IRIS Steering Committee for Scientific Use of Submarine Cables (Chave et al., 1990), OSN group (Purdy, 1995), and the International Ocean Network (ION) group (Montagner and Lancelot, 1995). Drilling at the H2O site will also provide useful background information for the Borehole Observatories, Laboratories, and Experiments (BOREHOLE) group (Carson et al., 1996), the deep biosphere microbiology community, and the oceanic lithospheric processes community (Dick and Mével, 1996).
One reentry hole within 2 km of the H2O junction box in the eastern Pacific Ocean at 27°52.916´N, 141°59.504´W in 4979 m water depth (Figs. F8, F9, F10) (Butler, 1995a) was proposed. The site is roughly halfway between California and Hawaii. A limited geophysical survey of the cable, including SCS, was carried out from the Revelle in August 1997 (Stephen et al., 1997).
The Hawaii-2 submarine cable system is a retired AT&T telephone cable system that originally connected San Luis Obispo, California, and Makaha, on Oahu, Hawaii (Fig. F8). The cable system was originally laid in 1964. IRIS installed a long-term seafloor observatory about halfway along the cable (Fig. F11). The cable was cut and terminated with a seafloor junction box. The location of the junction box defines the H2O seafloor observatory. The junction box has eight underwater make-break connections. About 500 W of power is available from the junction box, and there is ample capacity for two-way real-time communications with seafloor instruments. Data channels from the seafloor can be monitored continuously via the Oahu end of the cable to any laboratory in the world. (The California end of the cable cannot be used because it was cut and removed from the continental shelf.) There is a shallow buried broadband seismometer operating at the site (Duennebier et al., 2000, 2002). Other seafloor observatories, such as a geomagnetic observatory (Chave et al., 1995), a hydrothermal observatory (Davis et al., 1992; Foucher et al., 1995), or a broadband borehole seismic observatory (Orcutt and Stephen, 1993) can be installed at the site as funding becomes available.
Within the Ocean Drilling Program (ODP) and the marine geology and geophysics communities, there has been considerable interest in the past few years in long-term seafloor observatories that include a borehole installation. Prototype long-term borehole and seafloor experiments almost exclusively use battery power and internal recording. The data are only available after a recovery cruise. One exception to this is the Columbia-Point Arena ocean-bottom seismic station (OBSS), which was deployed on an offshore cable by Sutton and others in the 1960s (Sutton et al., 1965; Sutton and Barstow, 1990). For the foreseeable future, the most practical method for acquiring real-time, continuous data from the seafloor will be over cables (Chave et al., 1990). The H2O project provides this opportunity.
The Hawaii-2 cable runs south of the Moonless Mountains between the Murray and Molokai Fracture Zones (Fig. F8) (Mammerickx, 1989). Between 140° and 143°W, water depths along the cable track are typical for the deep ocean (4250-5000 m); the crustal age varies from 45 to 50 Ma (Eocene); and the sediment thickness to within the available resolution is ~100 m or less. Prior to our cable survey cruise in August 1997, sediment thickness in particular was not well resolved along the track (Winterer, 1989).
Tectonically, the cable runs across the "disturbed zone" south of the Murray Fracture Zone between magnetic isochrons 13 and 19 (Atwater 1989; Atwater and Severinghaus, 1989). In the disturbed zone, substantial pieces of the Farallon plate were captured by the Pacific plate in three discrete ridge jumps and several propagating rifts. To avoid this tectonically complicated region and to be well away from the fracture zone to the south of the disturbed zone, the H2O site was situated west of isochron 20 (45 Ma) at ~140°W.
The crust west of 140°W was formed between the Pacific and Farallon plates under "normal" spreading conditions. The average half-rate quoted by Malahoff and Handschumacher (1971) in this region for Anomalies 18-31 is 49 mm/yr. However, a careful analysis of spreading rate based on the magnetic anomalies of Atwater and Severinghaus (1989) and the ages reported by Cande and Kent (1992, 1995) give a half-rate of 71 mm/yr for Anomalies 18-22 (38.5-48.9 Ma), ~49 mm/yr for Anomalies 22-25, and ~35 mm/yr for Anomalies 25-30 (Fig. F12). At the time this crust was formed, the Farallon plate had not split into the Cocos and Nazca plates, and the ridge that formed this crust was the same as the present-day East Pacific Rise.
Between 140° and 143°W, the Hawaii-2 cable lies in the pelagic clay province of the North Pacific Ocean (Leinen, 1989). The sediments here are eolian in origin, consisting primarily of dust blown from Asia. They are unfossiliferous red clays. Deep Sea Drilling Project (DSDP) Leg 5 drilled a transect of holes in the pelagic clay province along longitude 140°W (McManus, Burns, et al., 1970). Site 39 is north of the cable at latitude 32°48.28´N with an age of 60 Ma (Fig. F8). It has a sediment thickness of only 17 m. Sites 40 and 41 are near the same latitude at 19°50´N with an age of ~67 Ma. Site 40 was drilled in an area of ponded sediments at the base of a large abyssal hill. Basement was not reached, and drilling terminated at a chert bed at 156 m. Acoustic basement, the deepest horizon identified on the seismic reflection profiles at Site 40, corresponded to the chert beds. Site 41 was drilled 15 km from Site 40 but was considered to be more representative of the sediments in the general area. Basaltic basement was encountered at 34 mbsf, but there were no cherts. Site 39 is north of the Murray Fracture Zone, and Sites 40 and 41 are south of the Molokai Fracture Zone. The actual "ribbon" of crust on which the cable lies is between the two fracture zones and was not drilled during DSDP Leg 5.
Site 172 was drilled during DSDP Leg 18 between the Molokai and Murray Fracture Zones but east of 140°W in the "disturbed" zone (31°32.23´N, 133°22.36´W), which has an estimated crustal age of 35 Ma (Kulm, von Huene, et al., 1973). Sediment thickness above the basaltic basement was 24 m. The sediment thickness from seismic reflection profiles had been interpreted as 90-105 m. The discrepancy was attributed to "reverberations and thin sediment cover."
In August 1997, a survey of the Hawaii-2 cable between 140° and 143°W was carried out (Stephen et al., 1997). The survey strategy consisted of two phases. First, we collected SeaBeam bathymetry, magnetics, and SCS along the cable track starting at 140°W and heading west. Our site criteria were to have 100 m of sediment thickness for setting the reentry cone, to be in relatively undisturbed "normal" crust in a plate tectonic sense, and to optimize drilling penetration by selecting sites with well-consolidated basement, not rubble or highly altered zones. As a second phase, we carried out a survey in a 20 km x 20 km area around each of three proposed drill sites to map bathymetry, sediment thickness, basement morphology, and magnetics in the vicinity (e.g., Fig. F13).
Figure F14 shows the H2O junction box location with respect to the track lines for the Revelle during the site survey in 1997. The actual site is to the southwest of a well-surveyed block but is bracketed by two parallel SCS lines. Figures F15 and F16 show the track lines, annotated in SCS shot numbers and Julian time, respectively, for the SCS and 3.5-kHz data. Circles at 1, 2, and 3 km radius from the site and the originally proposed drill locations are also indicated. Although cross-tie seismic lines are not available, the parallel seismic lines are sufficiently close together that contiguous structure can be identified across the lines.
Figures F17 and F18 are the 3.5-kHz lines north and south, respectively, of the H2O area. These 3.5-kHz data were acquired on the Revelle in August 1997 at the same time as the SCS data. Unmigrated and migrated SCS profiles from this site are shown in Figures F19 and F20, respectively, for the north line and in Figures F21 and F22, respectively, for the south line. A tenth of a second two-way traveltime corresponds to ~75 m in the water column.
Prior to drilling, we suspected that there were chert layers in this part of the Pacific Ocean from earlier drilling during DSDP Legs 5 and 18. On these 3.5-kHz records, there is a clean single pulse followed 10 ms later by a diffuse event. Our interpretation was that the clean event was the seafloor and that the diffuse event was the "chert layer." Ten milliseconds of two-way traveltime corresponds to ~8 m thickness of soft sediments. These 3.5-kHz data show nothing coherent below the "chert layer," but some inconclusive, subtle arrivals do appear occasionally near the 38-ms depth (Fig. F18). This was also the experience in the 1960 surveys where "acoustic basement" turned out to be chert.
We had interpreted a continuous mid-sediment reflector at ~0.03 s, or ~25 m depth, in the SCS records, which did not correspond to the "chert layer" identified on the 3.5-kHz records. Futhermore, we interpreted the diffraction events at ~0.06 s in the SCS data as occurring at the sediment-basement boundary, and we estimated a very uniform sediment thickness of ~50 m (Figs. F19, F20, F21, F22). As discussed in "3.5-kHz Deep-Source Experiment" and "Core, Physical Properties, Logging, and Seismic Correlation" the horizon at ~8 mbsf is a mid-sediment reflector but is not chert. The subtle arrivals at ~25-30 mbsf are basaltic basement, not a mid-sediment reflector. The diffraction events at ~50-60 mbsf could be originating from the interface between lithologic Units 1 and 2.
Drilling at the H2O site directly addresses the second of three initiatives outlined in the ODP Long Range Plan (JOIDES Planning Committee, 1996): "In situ monitoring of geological processes" (pp. 49-51). It also represents an initial step in accomplishing the oceanic crustal component of the third initiative: "Exploring the deep structure of continental margins and oceanic crust" (pp. 52-54). The drilling is intimately tied to the use of "seafloor observatories" (p. 63) and represents the partnership of ODP with the OSN, ION, and BOREHOLE (p. 74). (Page numbers refer to pages in the Long Range Plan.)
Prior to Leg 200, there was only one deep borehole (>100 m) in the normal Pacific plate, the largest modern tectonic plate. Table T1 summarizes the boreholes drilled on "normal" crust on the Pacific plate that have >10 m of basement penetration and with crustal ages <100 Ma during past DSDP and ODP drillings. Holes in seamounts, plateaus, aseismic ridges, and fracture zones were not included. Holes with crustal ages >100 Ma were also not included because they would have been affected by the mid-Cretaceous superplume event (Pringle et al., 1993). Prior to Leg 200, in 30 yr of deep ocean drilling and in more than 1000 holes worldwide, there were only 13 holes with >10 m penetration into "normal" igneous Pacific plate: only one hole during ODP, only one hole with >100 m penetration, and no holes in crust with ages between 29 and 72 Ma. Furthermore, there were no boreholes off axis in "very fast" spreading crust. At the latitude and age of the H2O area, the spreading rate was 142 mm/yr (full rate). Having a reference station in "normal" (45-50 Ma) ocean crust would constrain geochemical and hydrothermal models of crustal evolution.
Although fast-spreading ridges represent only ~20% of the global ridge system, they produce more than half of the ocean crust on the surface of the planet—almost all of it along the East Pacific Rise. Most ocean crust currently being recycled back into the mantle at subduction zones was produced at a fast-spreading ridge. If we wish to understand the Wilson cycle in its most typical and geodynamically significant form, we need to examine ocean crust produced at fast-spreading ridges. We have also known for >40 yr that fast-spreading crust is both simple and uniform, certainly so in terms of seismic structure (Raitt, 1963; Menard, 1964). Successful deep drilling of such crust at any single location is thus likely to provide fundamental information that can be extrapolated to a significant fraction of the Earth's surface.
Drilling at the H2O area would address both teleseismic, whole-Earth seismic studies, and regional studies. The site is located in a region on the Earth's surface where there is no land within a 1700-km radius. For uniform coverage of seismic stations on the surface of the planet, which is necessary for whole-Earth tomographic studies, seafloor seismic observatories are required. This site is one of three high-priority prototype observatories for the OSN (Butler, 1995a, 1995b; Purdy, 1995). Global seismic tomography provides three-dimensional images of the lateral heterogeneity in the Earth and is essential in addressing fundamental problems in subdisciplines of geodynamics such as mantle convection, mineral physics, long-wavelength gravimetry, geochemistry of ridge systems, geomagnetism, and geodesy. Specific problems include the characteristic spectrum of lateral heterogeneity as a function of depth, the anisotropy of the inner core, the structure of the core/mantle boundary, the role of oceanic plates and plumes in deep mantle circulation, and the source rupture processes of Southern Hemisphere earthquakes that are among the world's largest (Forsyth et al., 1995).
The culturally important earthquakes in California are only observed at regional distances on land stations in North America that restrict the azimuthal information to an arc spanning ~180°. To observe California earthquakes at regional distances to the west requires seafloor stations. Regional observations are used in constraining earthquake source mechanisms. Since the H2O site data will be available in real time, data will be incorporated into focal mechanism determinations within minutes of California earthquake events. Other problems that can be addressed with regional data from Californian and Hawaiian earthquakes are the structure of the 400-km, 525-km, and 670-km discontinuities in the northeastern Pacific Ocean and the variability of elastic and anelastic structure in the Pacific lithosphere from Po and So and pure-path oceanic surface wave studies (Butler, 1995a, 1995b).
At the OSN pilot experiment site in 1998, we deployed seafloor, buried, and borehole broadband seismometers in order to compare the performance of different styles of installation (Fig. F23). Figures F24 and F25 summarize for vertical and horizontal component data, respectively, the improvement that we expect to see in ambient seismic noise on placing a sensor in basement rather than on or in the sediments. Above 0.3 Hz, the seafloor, buried, and borehole spectra at the OSN-1 site show the borehole to be 10 dB quieter on vertical components and 30 dB quieter on horizontal components (Collins et al., 2001). Shear wave resonances (or Scholte modes) are the physical mechanism responsible for the higher noise levels in or on the sediment. The resonance peaks are particularly distinct and strong at the H2O site. (Note the 15-dB peak on vertical and the 35-dB peak on horizontal near 1 Hz on the H2O spectra.) By placing a borehole seismometer in basement at H2O, we expect to eliminate these high ambient noise levels.
There are three important characteristics for a broadband borehole seismic installation:
Our target depth for drilling at the H2O site was 350-400 m into basement. This was a conservative estimate to get into consolidated basalts based on the drilling experience in Hole 504B. In Hole 504B, sonic logs and resistivity measurements indicate poorly consolidated basalt down to 600 m. A hole that penetrated ~400 m into basement would acquire good-quality basalt samples for geochemical studies, provide adequate penetration into oceanic Layer 2 for paleomagnetic analyses, and provide good hole conditions for in situ experiments.