The H2O observatory is situated on oceanic lithosphere between the Murray and Molokai Fracture Zones within Isochron 20R (46.264 Ma) at ~140°W (Stephen, Kasahara, Acton, et al., 2003), avoiding the tectonically complicated region to the east and the fracture zone to the south. The observatory lies in the pelagic clay province of the North Pacific (Leinen, 1989). The sediments are reddish colored clays, mainly eolian in origin, consisting primarily of dust blown from Asia.
In more than 30 yr of deep ocean drilling prior to Ocean Drilling Program (ODP) Leg 200 and at more than 1200 sites worldwide, there had been only 13 holes with >10 m penetration into "normal" igneous Pacific plate. Among theses holes, only one was deeper than 100 m into basaltic basement, Deep Sea Drilling Project [DSDP] Leg 65 Hole 483B, and none of the holes were in crust with ages between 29 and 72 Ma.
At the latitude and age of the H2O area, the spreading rate was 142 mm/yr (full rate). Prior to drilling we thought that a reference station in "normal ocean," very fast spreading Pacific crust at 45–50 Ma would constrain geochemical and hydrothermal models of crustal evolution. Surprisingly, chemical analysis of the basalts obtained at Site 1224 showed far from the expected normal mid-oceanic-ridge basalt (N-MORB) characteristics, and it is difficult to say how the shallow part of the crust at the H2O site was formed.
Recent detailed bathymetric surveys have revealed the presence of huge landslides on the ocean floor, caused by volcanic eruptions in the Hawaiian Ridge (Moore et al., 1994), Reunion Island (Lenat et al., 1989), and the Canary Islands (Watts and Mason, 1995; Krasetel et al., 2001; Masson et al., 2002). It has been suggested that the gravitational instability of the volcanic edifices resulted in collapse. In the Canary Islands, the debris from the landslides extends to 30 km, and landslides on both island chains indicate generation of huge turbidity currents (Moore, 1964; Moore and Moore, 1988; Moore et al., 1989). In the case of Hawaii, the collapse of Koolau Volcano on Oahu caused the Nuuanu Landslide, which extends more than 200 km from the island (Fig. F2). The debris avalanche of the Nuuanu Landslide contains enormous blocks such as the Tuscaloosa Seamount, which is ~30 km long, 17 km wide, and at least 2 km thick. The Nuuanu Landslide is spread over a 23,000-km2 area (Normark et al., 1993; Naka et al., 2000), with distal portions extending up onto the Hawaiian Arch. To reach the upper portion of the arch, the target site for drilling, the landslide would have had to traverse the deep moat on the northeast side of Oahu and travel more than 100 km uphill. Gravity and piston cores in the landslide are difficult to obtain because the deposit is overlain by a carapace of younger debris such as turbidites and associated deposits. Because of such difficulties, the thickness and depositional history of the Nuuanu Landslide are poorly known. The thickness of the distal portion of the landslide was estimated to be 1–100 m based on seismic profiles (Rees et al., 1993) (Fig. F3). The upper 100 m of sediment at Site 1223 was thought to contain a record of the Nuuanu Landslide, a catastrophic event or series of events that removed ~40% (3000–4000 km3) of Koolau Volcano on the island of Oahu (Herrero-Bervera et al., 2002; Normark et al., 1993). On the other hand, piston Core P3 in Figure F2 shows <1 m deposits from the Nuuanu slide (Naka et al., 2000). This ambiguity in the thickness of the Nuuanu sedimentary deposits creates a large uncertainty in the estimation of landslide volume. Although the Nuuanu Landslide apparently occurred near the end or after the formation of the Koolau Volcano, which has surface flows that are 1.8–2.7 Ma based on K-Ar dating (Doell and Dalrymple, 1973), the exact age of the landslide was not well constrained prior to drilling.