Permeability, underpressures, and convection in the oceanic crust near the Costa Rica Rift, eastern equatorial Pacific

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doi: 10.1029/JB087iB04p02860
Author(s): Anderson, Roger N.; Zoback, Mark D.
Author Affiliation(s): Primary:
Lamont-Doherty Geol. Obs., Palisades, NY, United States
Volume Title: Journal of Geophysical Research
Source: Journal of Geophysical Research, 87(B4), p.2860-2868. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227
Note: In English. 34 refs.; illus. incl. sketch map
Summary: In situ permeability and pore pressures were measured 200 m deep beneath the top of the oceanic crust at DSDP site 504B. These measurements have relevance for the transition from convective to conductive heat flow on the south flank of the Costa Rica Rift. Conventional "slug" and constant rate injection tests were made below a hydraulic packer set at various depths in the hole. The packer was first set in a massive flow unit 37 m below the sediment-basement interface. The bulk permeability of the 172.5 m of pillow basalts and basaltic flows below the packer was found to be about 40 millidarcys (4 × 10-10 cm2). Measurements over 3- and 15-m intervals at the bottom of the hole in an altered pillow zone indicated a bulk permeability of 2-4 millidarcys. These values are thought to be accurate to ±30%. Formation pore pressures were found to be approximately 8 bars (∼2%) below hydrostatic. Interpretation of the data with respect to simple numerical convection models suggests that the transition from convective to conductive heat flow is controlled by the cessation of convective heat transport through the sedimentary layer rather than the cessation of convection in the sediment. Furthermore, the agreement between observed and modeled underpressures implies that hole 504B penetrated an active ocean crustal convection system. The thick sedimentary layer, layers of basal chert, and massive flow basalts above the layer 2A pillow flows apparently form an impermeable lid, effectively isolating the convection system from the ocean.
Year of Publication: 1982
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
ODP Ocean Drilling Program
Key Words: 18 Geophysics, Solid-Earth; Boreholes; Convection; Costa Rica Rift; Crust; DSDP Site 504; Deep Sea Drilling Project; East Pacific; East Pacific Rise; Equatorial Pacific; Geophysical surveys; Heat flow; Heat flux; Hydrothermal conditions; IPOD; Leg 111; Leg 137; Leg 140; Leg 148; Leg 69; Leg 70; Leg 83; Leg 92; Ocean Drilling Program; Oceanic crust; Pacific Ocean; Permeability; Physical properties; Pore pressure; Pressure; Rift zones; Surveys; Tectonophysics; Well-logging
Coordinates: N010000 N020000 W0833000 W0840000
Record ID: 1982048072
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