Drilled in 1975-1976 (Melson, Rabinowitz, et al., 1979), Hole 395A was one of the earliest successful reentry holes in oceanic crust and penetrated 93 m of sediments and over 500 m of predominantly extrusive basalts. It remains one of the deepest penetrations of upper oceanic crust formed at a mid-ocean ridge and is thus one of the most important reference holes for young oceanic crust. The hole is located in 7-Ma crust about 70 km west of the axis of the Mid-Atlantic Ridge, sited in an isolated sediment pond ("North Pond") about 8 km x 15 km in size and completely surrounded by exposed basement with topographic relief up to a kilometer (Fig. F1). Oceanic crust formed at slow spreading rates typically exhibits much greater bathymetric relief than crust formed at fast rates, so this kind of environment might be considered typical for off-axis circulation in the Atlantic and Indian Oceans.
Hole 395A was also one of the earliest and best examples of a phenomenon that is fairly common in holes drilled into young oceanic crust and dramatically illustrates the need for experiments like CORKs to understand off-axis hydrothermal circulation. In the >21 yr it was left open after initial drilling, Hole 395A was reentered four times: during Leg 78B (Hyndman, Salisbury, et al., 1984), Leg 109 (Bryan, Juteau, et al., 1988), the French wireline reentry DIANAUT expedition (Gable et al., 1992), and Leg 174B. Each time, repeat temperature logs, fluid samples, and flowmeter logs clearly demonstrated that ocean bottom water was flowing down the hole at consistent rates of ~1000 L/hr (e.g., Becker et al., 1984; Morin et al., 1992). Downhole flow in an open hole like Hole 395A requires both a pressure differential to drive the flow and sufficient formation transmissivity to accept the flux. The higher the formation transmissivity, the lower the differential pressure required to drive downhole flow. The pressure differential probably results from some combination of two effects: true formation underpressures resulting from natural fluid circulation and a drilling-induced artifact resulting from the density differential between cold drilling fluids and warmer formation fluids. Resolving the former is obviously of greater interest. However, in most cases of downhole flow in crustal holes, the formation temperatures are indeed warmer than the drilling fluid so the drilling-induced artifact must be significant and virtually precludes determining the true formation state if the hole is left open. Hole 395A is important among examples of downhole flow in that it was drilled into an area of low heat flow; thus, the density difference between drilling fluids and formation fluids was small, and a predominantly natural driving force is probably responsible for the prolonged downhole flow.
Early indications of the nature of the subsurface fluid flow system at North Pond were provided by downhole measurements during prior reentries and particularly the detailed heat flow survey conducted by Langseth et al. (1992) long after the drilling of Hole 395A. Prior downhole measurements indicate that the upper 300-400 m of basement in Hole 395A is certainly permeable enough to support a vigorous fluid flow system if there is lateral continuity of such permeability in the crust underlying North Pond. The heat flow survey indicates that heat flow is on average considerably less than the value of ~180 mW/m2 expected for conductive cooling of 7-Ma crust, with a general increase of heat flow from southeast to northwest across North Pond (Fig. F1). Even where the heat flow is high in the northwest, pore-pressure measurements show negative gradients in the sediments, suggesting recharge through sediments everywhere in the sediment pond. These observations corroborate the model put forth earlier by Langseth et al. (1984) for one-pass lateral fluid flow in permeable upper basement beneath North Pond (Fig. F2) and indicate that this flow generally runs from southeast to northwest. In this model, permeability of uppermost basement is quite high and interconnected throughout the sediment pond and the lateral flow is vigorous enough to keep temperatures at the basement contact nearly isothermal, increasing only slightly along the flow path beneath North Pond. In this case, heat flow in North Pond would be predicted to be directly related to distance from the basement exposures to the southeast and inversely related to sediment thickness—just as observed by Langseth et al. (1992).