The CORK experiment in Hole 395A was designed to monitor true formation pressures and temperatures in the upper basement—the zone of large-scale lateral flow—to elucidate the causes and patterns of this flow and to further test the model of Langseth et al. (1984) for off-axis circulation in sediment ponds on the flanks of slow-spreading ridges. Using Nautile, the first 6 months of temperature/pressure data was recovered from Hole 395A on 30 January 1998. No further data have been recovered since then, but a data recovery dive using Alvin is scheduled for the summer of 2001. The first 6 months of data (Fig. F5) clearly show a very slow recovery of temperatures and pressures in the hole. Full recovery will certainly take much longer than 6 mo, given the >21 yr of unabated downhole flow immediately before the installation of the CORK.
The initial 6 months of data allow only lower bounds to be estimated for in situ temperatures and pressures—but these preliminary bounds are themselves quite illuminating. First, although temperatures clearly show signs of recovery throughout the first 6-month recording period, they are not much warmer than the profile characteristic of downhole flow (Fig. F3). Second, in situ pressures appear quite close to hydrostatic under local geothermal conditions and the attenuation of the tidal signal seen in the sealed hole is small. Thus, there appears not to have been a strong formation underpressure "sucking" the pre-CORKing downhole flow into a basement reservoir; instead, it appears that the upper basement beneath North Pond is very well connected in a hydrologic sense to the ocean bottom water via the basement exposures that surround North Pond. These results clearly support the Langseth model; they indicate that much of the uppermost basement beneath the sediment pond is indeed very permeable and kept quite cool by vigorous lateral flow of fluids close to bottom-water temperature. Basement permeability and transmissivity underneath the sediment pond must be high enough that there is virtually no resistance to flow nor pressure losses along the flow path. In this context, the disturbance generated by the >21 yr of downhole flow before CORKing—a total of over 100,000,000 L of seawater flowing down the hole and into the formation—was just a relatively minor perturbation to the natural flow system!
This interpretation is further supported by analysis by Davis et al. (2000) of attenuation and phase lag of the tidal loading signal as recorded by the CORK pressure gauges, even if only 40 days of pressure data were available. Davis et al. (2000) show that the results from Hole 395A are remarkably consistent with those from the younger Juan de Fuca flank CORKs. This pair is located in 0.9 and 1.3 Ma crust under continuous sediment cover within a few kilometers of extensive basement outcrop—a comparable setting to Hole 395A, which is even closer (1 km) to extensive outcrop. Although located in different oceans in crust formed at different spreading rates, these CORKs show tidal loading behavior that remarkably can be fit to a single simple model of lateral transmission from nearby basement outcrop via permeable and transmissive upper basement (Fig. F6). Thus, the CORK results from Hole 395A as well as the Juan de Fuca flank sites strongly support other inferences that there are huge fluxes of low-temperature fluids in very transmissive upper basement in thinly sedimented young oceanic crust, regardless of whether the sediment cover is continuous or patchy and regardless of spreading rate.