Leg 174B Scientific Report
BACKGROUND
Only a handful of Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) holes penetrate more than 500 m into "normal" oceanic crust formed at mid-ocean ridges, and these are all, therefore, important reference holes. Among them, Holes 395A and 504B (Fig. 1) form the most important pair of reference sites for young, upper oceanic crust formed at slow and medium spreading rates, respectively. They are particularly important as reference sites for the hydrogeology of young oceanic crust, which has been studied with extensive downhole measurements and detailed heat-flow surveys at both sites (Fig. 2). Holes 395A and 504B are the best documented of several cases in which ocean bottom water is known to be flowing down open DSDP/ODP holes into permeable levels of upper basement. These examples suggest that young upper oceanic crust under a sediment cover is easily permeable enough to support active circulation of seawater, but we still barely understand the details of such off-axis hydrothermal circulation or its control by the pressure distribution and fine-scale permeability structure. Site 395 is located in 7-Ma crust, in an isolated sediment pond with low heat flow (Hussong et al., 1979; Langseth et al., 1992) that might be considered somewhat typical of the structure and hydrogeological setting for thinly sedimented crust formed at slow spreading rates. Since it was drilled in 1975-1976 (Melson, Rabinowitz, et al., 1979), Hole 395A has been revisited three times for an extensive set of downhole measurements: during DSDP Leg 78B in 1981 (Hyndman, Salisbury, et al., 1984), during ODP Leg 109 in 1986 (Bryan, Juteau, et al., 1988), and during the French wireline reentry campaign DIANAUT in 1989 (Gable et al., 1992). The hole was originally drilled during Leg 45 to a depth of 664 m, or 571 m into basement, but bad hole conditions were encountered in the deepest 50 m (Melson, Rabinowitz, et al., 1979). When the hole was revisited five years later during Leg 78B, the deepest 55 m of the hole were blocked by fill (Hyndman, Salisbury, et al., 1984). However, very similar total hole depths were registered during Leg 109 and the DIANAUT program, indicating that hole conditions apparently stabilized shortly after Leg 45. Total open hole length is ~606 m with 513 m into basement. On each of three prior reentries of Hole 395A, the first order of business was to log the hole with a temperature tool long after it had reequilibrated from any prior disturbance by DSDP/ODP operations and before it was disturbed by new logging. Each of the three temperature logs obtained from the previous cruises showed strongly depressed borehole temperatures, essentially isothermal to a depth of about 300 m into basement (Becker et al., 1984; Kopietz et al., 1990; Gable et al., 1992). Packer and flowmeter experiments conducted during prior reentries indicate that this section of basement is much more permeable than the underlying formation (Hickman et al., 1984; Becker, 1990; Morin et al., 1992). The near-isothermal temperatures in the upper part of the hole indicate a strong downhole flow of ocean bottom water into permeable upper basement, at rates of thousands of liters per hour, virtually unabated over the 21 yr the hole has been open. In that time, it is estimated that a total of over 200,000,000 liters of ocean bottom water has been drawn down the hole into the subseafloor hydrogeologic system at Site 395. In comparison, temperatures measured during the multiple revisits to Hole 504B were initially strongly depressed to a depth of about 100 m into basement, but then rebounded nonmonotonically toward a conductive profile. This indicates that the rate of downhole flow in that hole had decayed since the hole was first drilled and that the downhole flow is directed into a more restricted section of uppermost basement than in Hole 395A (Becker et al., 1983a, 1983b, 1985, 1989; Gable et al., 1989; Guerin et al., 1996). This comparison suggests that Hole 504B penetrates a more passive hydrothermal regime, whereas Hole 395A provides a man-made shunt into a more active circulation system in basement. The various observations at Site 395 generally support a model proposed by Langseth et al. (1984, 1992; Fig. 3) for lateral circulation in the upper basement beneath the sediment pond where the hole is sited, but we have little resolution on any details of such circulation. A number of holes drilled into young oceanic crust have proven to be drawing ocean bottom water down into permeable levels of basement (e.g., Erickson et al., 1975; Hyndman et al., 1976; Anderson and Zoback, 1982; Becker et al., 1983a, 1983b, 1984; Davis, Mottl, et al., 1992). Such downhole flow requires sufficient basement permeability and a differential pressure between the fluids in the borehole and the formation fluids. In general, we surmise that the necessary differential pressures may arise because of some combination of two independent effects: (1) The differential pressure (which should not be termed an "underpressure") between the cold, dense seawater used as drilling fluid in the borehole and the warmer formation fluids; and (2) True, dynamically maintained underpressures caused by active circulation in the basement that would occur even if the borehole were not present. In cases of downhole flow in holes drilled into formations with high geothermal gradients, the driving force is probably dominated by the former effect (e.g., ODP Leg 139 sites in Middle Valley, Davis, Mottl, et al., 1992). For holes such as Hole 504B, both effects may be important. In holes drilled into young crust with low geothermal gradients, such as Hole 395A, the latter effect may be predominant.
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