During Leg 174B in midsummer of 1997, we revisited Hole 395A for a 5-day program of logging and deployment of the thirteenth Ocean Drilling Program (ODP) instrumented borehole seal (known as a "CORK" for Circulation Obviation Retrofit Kit) (Davis et al., 1992) for long-term hydrological investigations. Drilled over 21 yr earlier during Deep Sea Drilling Project (DSDP) Leg 45, Hole 395A is a classic legacy hole in young crust near the Mid-Atlantic Ridge. Prior to Leg 174B, it had been revisited four times for logging and hydrogeological studies: twice by DSDP and ODP (Legs 78B and 109) for logging and downhole experiments, once with Nautile for logging by wireline reentry, and once by Atlantis for detailed heat flow, coring, and pore-pressure surveys. The primary purpose of the Leg 174B logging and CORK installation was to further elucidate the controlling formation properties and driving forces for the vigorous off-axis, low-temperature hydrothermal circulation inferred to be active in the region from previous reentries of the hole and geothermal surveys.
It is now well accepted that off-axis hydrothermal systems account for the majority of hydrothermal heat and chemical fluxes in the oceanic crust and are a primary factor in the chemical and physical evolution of the crust. Off-axis circulation occurs at lower temperatures and slower rates over large areas of the ocean basins, commonly out to crustal ages of tens of millions of years. It only rarely produces identifiable vents and is therefore more difficult to study with measurements at the seafloor. To date, much of our understanding of off-axis circulation is based on deductions from patterns in seafloor heat flow, analyses of pore waters in the sediments overlying oceanic basement, numerical simulations, and drilling results in a few well-studied, well-sedimented off-axis areas. Like Leg 168 a year earlier, Leg 174B was planned specifically to address the physical and chemical processes occurring in off-axis hydrothermal systems. Along with the four CORKs deployed during Leg 168 on the flank of the Juan de Fuca Ridge (Davis, Fisher, Firth, et al., 1997; Davis and Becker, 1998), the CORK deployed in Hole 395A during Leg 174B represents five active CORKs deployed in off-axis systems for the first continuous monitoring of in situ conditions and processes in such systems.
1Becker, K., Bartetzko, A., and Davis, E.E., 2001. Leg 174B synopsis: revisiting Hole 395A for logging and long-term monitoring of off-axis hydrothermal processes in young oceanic crust. In Becker, K., and Malone, M.J. (Eds.), Proc. ODP, Sci. Results, 174B [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/174B_SR/synopsis/synopsis.htm>. [Cited YYYY-MM-DD]
2Rosenstiel School of Marine and Atmospheric Science, University of Miami, Division of Marine Geology and Geophysics, 4600 Rickenbacker Causeway, Miami FL 33149-1098, USA. kbecker@rsmas.miami.edu
3Lehr-und Forschungsgebiet für Angewandte Geophysik, RWTH Aachen, Lochnerstrasse 4-20, 52056 Aachen, Federal Republic of Germany.
4Pacific Geoscience Centre, Geological Survey of Canada, Sidney BC V8L 4B2, Canada.
Initial
receipt: 23 February 2001
Acceptance: 12 March 2001
Publication: 12 June 2001
Ms 174BSR-130