Observations of natural-state fluid pressures and temperatures in young oceanic crust and inferences regarding hydrothermal circulation

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doi: 10.1016/S0012-821X(02)00982-2
Author(s): Davis, E. E.; Becker, K.
Author Affiliation(s): Primary:
Geological Survey of Canada, Pacific Geoscience Centre, Sidney, BC, Canada
Other:
Rosenstiel School of Marine and Atmospheric Science, United States
Volume Title: Earth and Planetary Science Letters
Source: Earth and Planetary Science Letters, 204(1-2), p.231-248. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X CODEN: EPSLA2
Note: In English. 49 refs.; illus., incl. sketch map
Summary: Four boreholes, drilled a few tens of meters into igneous basement on the eastern flank of the Juan de Fuca Ridge during ODP Leg 168, were sealed and instrumented for long-term monitoring to observe the hydrologic state of young sediment-sealed oceanic crust. The thermal regime is dominated by the effects of rapid fluid circulation in uppermost igneous basement driven by very small non-hydrostatic pressure gradients. Upper basement temperatures are uniform laterally between pairs of holes over distances of hundreds of meters to kilometers. In the case of two holes drilled into a sediment-buried basement ridge and adjacent valley, basement temperatures differ by less than 2 K despite the 2.2 km lateral separation of the sites and the 2.5:1 contrast in sediment cover thickness. Under conductive conditions, upper basement temperatures would differ by roughly 50 K. By comparison with modeling results, the observed degree of isothermality suggests a fluid flux of at least 10-6 m s-1 (30 m yr-1), and an effective permeability in the range of 10-10-10-9 m2 in the uppermost igneous crust. The pressure difference available to drive this rapid flux between the ridge and valley, estimated by comparing the observed pressures via the isothermal upper basement hydrostat that is inferred to connect the two sites, is small (≈2 kPa) and also suggests high permeability. Relative to the hydrostats defined by the local conductive sediment geotherms, substantial super-hydrostatic pressure (+18 kPa) is present within the buried basement ridge, and sub-hydrostatic pressure is present in the adjacent valley (-26 kPa). Such pressure differentials are the direct consequence of the advection-dominated thermal regime and small pressure losses in high-permeability basement, and are available to drive fluid seepage through sediment sections vertically up above and horizontally away from buried ridges, and down above valleys. No constraints are provided by any of the observations on the depth in the crust to which thermally or chemically significant flow might extend, although just as in the overlying sediments, the pattern of deep flow may be affected by the near-isothermal and near-hydrostatic conditions present in the permeable uppermost crustal section. Abstract Copyright (2002) Elsevier, B.V.
Year of Publication: 2002
Research Program: ODP Ocean Drilling Program
Key Words: 18 Geophysics, Solid-Earth; 20 Geophysics, Applied; Basement; Boreholes; CORK; Circulation Obviation Retrofit Kit; Crust; East Pacific; Fluid dynamics; Fluid pressure; Geothermal gradient; Heat flow; Heat flux; Hydrostatic pressure; Hydrothermal conditions; Igneous rocks; Juan de Fuca Ridge; Leg 168; Marine sediments; Mid-ocean ridges; North Pacific; Northeast Pacific; ODP Site 1024; ODP Site 1025; ODP Site 1026; ODP Site 1027; Observations; Ocean Drilling Program; Ocean floors; Oceanic crust; P-T conditions; Pacific Ocean; Permeability; Sediments; Seepage; Temperature; Temperature logging; Thermal regime; Upper crust; Well-logging
Coordinates: N475427 N475427 W1284502 W1284502
N475310 N475310 W1283858 W1283858
N474556 N474556 W1274526 W1274526
N474524 N474524 W1274351 W1274351
Record ID: 2003010990
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands