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New evidence for age variation and scale effects of permeabilities of young oceanic crust from borehole thermal and pressure measurements |
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| Authors: | Keir Becker Earl E Davis |
| Institution: | a Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA b Pacific Geoscience Centre, Geological Survey of Canada, P.O. Box 6000, Sidney, BC, Canada V8L 4B2 |
| Abstract: | In 1996, long-term sealed-hole hydrological observatories with subseafloor temperature and pressure sensors were installed in four cased holes drilled by the Ocean Drilling Program into sedimented young oceanic crust east of the Juan de Fuca Ridge. Data recovered over a year later showed that all four holes displayed temperature profiles indicative of vertical fluid flow immediately prior to their being sealed. Warm water was being produced from basement in two cases, and cool ocean bottom water was being drawn into basement at the others. Linear flow rates of  60–200 m/h were estimated from the perturbation of the temperature profiles relative to undisturbed geothermal gradients at the sites. The pressure differentials driving the flow were also measured at the time of the observatory installations, allowing estimates of permeabilities of the upper crustal sections penetrated by the holes. Estimated permeabilities vary systematically with age, ranging from about 10−10 m2 in the youngest site (0.9 Ma) to 10−12 m2 in the oldest site (3.6 Ma), confirming an apparent reduction of permeability with age determined with packer experiments at three of the same sites. Combined with other estimates of permeabilities in the same holes using methods with different scales of investigation, the new permeability estimates also provide evidence for a significant scale dependence of permeability in the upper oceanic crust. |
| Keywords: | hydrothermal circulation oceanic crust permeability marine hydrogeology Ocean Drilling Program |
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