BACKGROUND AND GEOLOGICAL SETTINGThe H2O Program
Within the Ocean Drilling Program (ODP) and 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. 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.
On a cable like Hawaii-2, there are repeaters every 20 nmi to compensate for attenuation on the cable. The repeater boxes are ~0.2 m in outside diameter and 1 m long. The H2O junction box has been located between two of these repeater boxes. Experiments should be carried out within a kilometer or two of the junction box. The borehole should be no closer than ~500 m.
The background, scientific objectives, and other details of the Nuuanu Landslide Site NU-1A are discussed in the addendum at the end of this prospectus.
The Hawaii-2 cable runs south of the Moonless Mountains between the Murray and Molokai Fracture Zones (Fig. 1) (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 varies to within the available resolution (~100 m or less). Prior to our cable survey cruise in August 1997, sediment thickness was not well resolved along the cable 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 observatory 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 at a "fast" half-rate of about 7 cm/yr (Atwater, 1989; Cande and Kent, 1992). 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. The water depth at the junction box is 4979 m. The maximum relief between sites proposed for the borehole observatory is 40 m.
Between 140° and 143°W, the Hawaii-2 cable lies in the pelagic clay province of the North Pacific (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). DSDP Site 39 is north of the cable at latitude 32°48.28'N with an age of 60 Ma. It has a sediment thickness of only 17 m. DSDP Sites 40 and 41 are near the same latitude at 19°50'N with an age of about 67 Ma. DSDP 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. The acoustic basement, the deepest horizon identified on the seismic reflection profiles, corresponded to the chert beds. DSDP 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 meters below seafloor (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 Leg 5.
Site 172 (31°32.23'N, 133°22.36'W) was drilled on DSDP Leg 18 between the Molokai and
Murray Fracture Zones, penetrating basement with an estimated age of 35 Ma that lies east of
140°W and is in the "disturbed" zone (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
Cable Survey Cruise in August 1997
In August 1997, we carried out a survey of the Hawaii-2 cable between 140° and 143°W (Stephen et al., 1997). Our survey strategy consisted of two phases. First, we collected SeaBeam bathymetry, magnetics, and single-channel seismic profiles along the cable track starting at 140°W and heading west. Our site criteria were (1) to have 100 m of sediment thickness for setting the reentry cone; (2) to be in relatively undisturbed "normal" crust in a plate tectonic sense; and (3) 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 by 20 km area around each of three drill sites to map bathymetry, sediment thickness, basement morphology, and magnetics in the vicinity (e.g., Fig. 3).
Figure 4 shows the H2O junction box location with respect to the tracklines 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 single-channel seismic (SCS) lines (Fig. 5). Figure 6 and Figure 7 show the tracklines, 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 specific 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.
Figure 8 and Figure 9 are the latest 3.5-kHz examples from lines north and south, respectively, of the H2O area. This 3.5-kHz data was 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 10 and 11, respectively, for the north line and in Figure 12 and Figure 13, respectively, for the south line. A tenth of a second two-way traveltime corresponds to about 75 m.
We know there are chert layers in this part of the Pacific from early 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 is that the clean event is the seafloor and that the diffuse event is the chert layer. Ten ms of two-way traveltime corresponds to ~8 m thickness of soft sediments. The 3.5- kHz data image nothing coherent below the "chert layer." This was also the experience in the 1960 surveys where "acoustic basement" turned out to be chert.
There is a continuous midsediment reflector at ~0.03 s below seafloor, or ~25 m depth, which does not correspond to the chert layer identified on the 3.5-kHz records. If we interpret the diffraction events at ~0.06 s below seafloor in the SCS data as occurring at the sediment/basement boundary, we get a very uniform sediment thickness of ~50 m. This may get as thick as 75 m in some areas, but in no area did we identify 100-m sediments.
Scientific Objectives | Table of Contents