An unequivocal case for high Nusselt number hydrothermal convection in sediment-buried igneous oceanic crust

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doi: 10.1016/S0012-821X(96)00212-9
Author(s): Davis, Earl E.; Wang, Kelin; He, Jiangheng; Chapman, David S.; Villinger, Heiner; Rosenberger, Andreas
Author Affiliation(s): Primary:
Geological Survey of Canada, Pacific Geoscience Centre, Sidney, BC, Canada
University of Utah, Salt Lake City, UT, United States
Universität Bremen, Bremen, Federal Republic of Germany
Volume Title: Earth and Planetary Science Letters
Source: Earth and Planetary Science Letters, 146(1-2), p.137-150. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X CODEN: EPSLA2
Note: In English. Geol. Surv. Can., Contrib. No. 1996217. 32 refs.; illus., incl. sects., sketch map
Summary: New observations of seafloor heat flow, precisely located along seismic reflection profiles crossing a buried ridge on the eastern flank of the Juan de Fuca Ridge, show a nearly exact inverse correlation between heat flow and sediment thickness, such that the basement-sediment contact appears isothermal to within 10 K, despite a factor of three local variation in sediment thickness. We have used these observations with numerical models to infer hydrothermal heat-transport properties of the upper oceanic crust at this 3.5 Ma site. Model results show that, while fluid circulation is stimulated by the effects of basement topography even at sub-critical Rayleigh number conditions, the creation of a nearly isothermal basement surface requires very high heat-transport efficiency. Lower limits for the Nusselt number (Nu≥25), for the Rayleigh number (Ra≥4000), and for the permeability (κ≥10-11 m2), are provided by assuming that high permeability is distributed throughout the uppermost 600 m of relatively low-velocity igneous crust at this site. Relatively high permeability can also be inferred by considering the calculated fluid pressure regime in light of what is known about the relationship between fluid seepage through the sediment section and the underlying basement topography from geochemical data: Local super-hydrostatic pressure and fluid discharge above buried basement ridges can occur only if basement permeability is higher than 10-13 m2. Unfortunately, no constraint on the actual distribution of high permeability below the top of the igneous crust is provided by the thermal regime based on the heat-flow and seismic observations. Equally uniform upper basement temperatures can be produced by fluid flow in a thinner layer (of thickness h) having a correspondingly higher Nusselt number and permeability. Only the products of Nu×h, and κ×h2 are constrained. Bulk permeabilities (averaged over intervals a few hundred meters thick) measured in boreholes that have penetrated the upper oceanic crust are typically less than 10-13 m2. The much higher formation-scale permeability we infer may be a consequence of the relative youth of the crust at this site, although it is more likely that the interconnected fractures and extrusive volcanic unit contacts and voids that contribute most to the bulk formation permeability are relatively infrequent and not representatively sampled by drilling. Abstract Copyright (1997) Elsevier, B.V.
Year of Publication: 1997
Key Words: 07 Marine Geology and Oceanography; 18 Geophysics, Solid-Earth; Bottom features; Convection; Crust; East Pacific; Finite element analysis; Geophysical methods; Geophysical surveys; Heat flow; Heat transfer; Hydrothermal conditions; Igneous rocks; Juan de Fuca Ridge; Marine sediments; Mid-ocean ridges; North Pacific; Northeast Pacific; Numerical models; Nusselt number; Ocean floors; Oceanic crust; Pacific Ocean; Permeability; Rayleigh number; Sediments; Seismic methods; Statistical analysis; Surveys
Coordinates: N474400 N474800 W1274200 W1274900
Record ID: 1997022053
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands