Drilled 200 km south of the Costa Rica Rift, DSDP/ODP Hole 504B is the deepest hole ever drilled into young oceanic crust, reaching a maximum depth of 2111 mbsf. As the most recent cruise to date to visit this site, Leg 148 offered a new opportunity to investigate such observations from the previous legs as the slow thermal recovery from the flow of cold seawater into the underpressured upper units of basement and the unexplained decrease of measured vertical conductive heat flow with depth (from 196 to 120 mW/m2).
Because of operational troubles during the measurements, the initial data have been corrected, and the actual temperatures show that the aquifer underlying the sediments is still recovering from the inflow of cold seawater. The decrease with depth of the conductive vertical heat flow was also confirmed, but careful observations of the temperature gradient variations suggest that the interval where the change in heat flow occurs coincides with a listric fault at 800 mbsf, as inferred from downhole measurements. Even though permeability has been shown to be very low over the basement section, the presence of a narrow high-permeability interval at this depth would be compatible with the uncertainty of these measurements. The circulation of warm fluid within this interval could explain the temperature observations, but it would challenge our understanding of the regional thermal regime.
Date of initial receipt: 15 August 1994
Date of acceptance: 24 February 1995
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