Passive, off-axis convection through the southern flank of the Costa Rica Rift

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doi: 10.1029/JB095iB06p09343
Author(s): Fisher, A. T.; Becker, Keir; Narasimhan, T. N.; Langseth, Marcus G.; Mottl, Michael J.
Author Affiliation(s): Primary:
Rosenstiel Sch. Mar. and Atmos. Sci., Div. Mar. Geol. and Geophys., Miami, FL, United States
Other:
Lawrence Berkeley Lab., United States
Lamont-Doherty Geol. Obs., United States
Univ. Hawaii, United States
Volume Title: Special section on Logging and downhole measurements in Deep Sea Drilling Project/Ocean Drilling Program deep crustal holes
Source: Special section on Logging and downhole measurements in Deep Sea Drilling Project/Ocean Drilling Program deep crustal holes. Journal of Geophysical Research, 95(B6), p.9343-9370. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227
Note: In English. 100 refs.; illus. incl. 8 tables, sketch maps
Summary: Pore fluids are passively convecting through young oceanic sediments and crust around Deep Sea Drilling Project (DSDP) site 504 on the southern flank of the Costa Rica Rift, as inferred from a variety of geological, geochemical, and geothermal observations. The presence of a fluid circulation system is supported by new data collected on Ocean Drilling Program (ODP) leg 111 and a predrilling survey cruise over the heavily sedimented, 5.9 Ma site; during the latter, elongated heat flow anomalies were mapped subparallel to structural strike, with individual measurements of twice the regional mean value, and strong lateral and vertical geochemical gradients were detected in pore waters squeezed from sediment cores. Also, there is a strong correlation between heat flow, bathymetry, sediment thickness, and inferred fluid velocities up through the sediments. On an earlier DSDP leg, an 8-bar underpressure was measured in the upper 200 m of basement beneath thick sediment cover. Although the forces which drive passive circulation are not well understood, it has generally been thought that the length scale of heat flow variations provides a good indication of the depth of hydrothermal circulation within the oceanic crust. This assumption was based on analytical and numerical analyses of relatively simple porous media models. Deep crustal convection had been inferred near site 504 based on the geometry of surface heat flow anomalies with a wavelength of 4-7 km but appears to be precluded by low crustal permeability, as measured in DSDP hole 504B. The widely varied geothermal and hydrogeological observations near site 504 are readily explained by a model which combines (1) basement relief, (2) irregular sediment drape, (3) largely conductive heat transfer through the sediments overlying the crust, and (4) thermal and geochemical homogenization of pore fluids at the sediment/basement interface, which results from (5) topographically induced, passive hydrothermal circulation with large aspect ratio, convection cells. This convection involves mainly the permeable, upper 200-300 m of crust; the deeper crust is not involved. This convection is induced through a combination of buoyancy fluxes, due to heating from below, and topographic variations on the seafloor and at the basement-sediment interface. This model was designed to incorporate data from both near the sediment surface and deep within boreholes; it is successful in duplicating numerous field observations. Copyright 1990 by the American Geophysical Union.
Year of Publication: 1990
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
ODP Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; 18 Geophysics, Solid-Earth; Convection; Costa Rica Rift; Crust; DSDP Site 504; Deep Sea Drilling Project; East Pacific; Equatorial Pacific; Fluid dynamics; Heat flow; Hydrothermal conditions; IPOD; Leg 111; Leg 137; Leg 140; Leg 148; Leg 69; Leg 70; Leg 83; Leg 92; Marine sediments; Ocean Drilling Program; Oceanic crust; Pacific Ocean; Permeability; Pore water; Pressure; Rift zones; Sediments; Tectonophysics; Upper crust
Coordinates: N010900 N011600 W0834100 W0834800
Record ID: 1990053614
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute.