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doi:10.2204/iodp.proc.301.102.2005

Setting, planning, and goals

The Endeavour segment of the Juan de Fuca Ridge (JFR) generates lithosphere a few hundred kilometers west of the Olympic Peninsula, Washington State (USA) (Figs. F1, F2). The study area contains structural features common to most ridge flanks: extrusive igneous basement overlain by sediments, abyssal hill topography, high-angle faulting, and basement outcrops. The topographic relief of the young oceanic crust produces barriers to turbidites originating from the continental margin to the east, resulting in the accumulation of thick sediments that bury the eastern flank of the JFR (Davis et al., 1992; Underwood et al., 2005). Igneous basement is exposed to the west, where the crust is young, and the sedimented seafloor to the east is relatively flat, except over basement outcrops. Low-permeability sediment limits advective heat loss from most of the ridge flank, leading to strong thermal, chemical, and alteration gradients in igneous basement. Basement relief is dominated by linear ridges and troughs, oriented subparallel to the spreading center, and was produced mainly by faulting, variations in magmatic supply at the ridge, and off-axis volcanism (Davis and Currie, 1993; Karsten et al., 1998, 1986). Basement relief across this ridge flank tends to be smoother near the active spreading center (±100–200 m), and rougher (±300–700 m) to the east.

Integrated Ocean Drilling Program (IODP) Expedition 301 is the first part of a two-expedition drilling program, with associated nondrilling experiments, intended to assess the nature of fluid pathways in the crust and the dynamic influences of fluid circulation on this hydrothermally active ridge flank. Expedition 301 and related experiments will help to identify the distribution of hydrologic properties in the crust; the extent to which crustal compartments are connected or isolated (laterally and with depth); linkages between ridge-flank circulation, alteration, and geomicrobial processes; and quantitative relations between seismic and hydrologic properties. These and other scientific questions, and the methods used to address them, are discussed elsewhere (Fisher et al. [this volume], and the "Site U1301" and "Expedition 301 summary" chapters). The main goals of this paper are to

• Show locations where swath mapping, seismic, and heat flow data were collected during recent oceanographic expeditions in preparation for Expedition 301 and related experiments;

• Present and describe examples of these data that are characteristic of local and regional hydrothermal environments;

• Analyze selected sets of collocated seismic and thermal data to resolve hydrothermal conditions in shallow basement; and

• Define important questions that remain to be resolved in this area.

Rosenberger et al. (2000) and Davis et al. (1997a) summarized survey results through 1996, and several other papers have discussed subsets of available data, including drilling results from Ocean Drilling Program (ODP) Leg 168 (e.g., Davis et al., 1992, 1999; Underwood et al., 2005; Wheat et al., 2000; Wheat and Mottl, 1994). Readers interested in a detailed presentation of regional geology and hydrogeology are directed to these studies and to other references cited throughout this paper.

There were two oceanographic expeditions in 2000 intended to provide site survey data for Expedition 301. Sonne expedition SO149 (ImageFlux) was led by V. Spiess, L. Zühlsdorff, and H. Villinger (University of Bremen, Germany) and collected swath-map, seismic, and heat flow data. Thomas G. Thompson expedition TN116 (RetroFlux) was led by A. Fisher (University of California at Santa Cruz), E.E. Davis (Pacific Geoscience Center), C.G. Wheat (University of Alaska Fairbanks), and M. Mottl (University of Hawaii) and collected mainly heat flow data and sediment cores, with a small amount of swath mapping. In addition, Maurice Ewing expedition EW0207, led by S. Carbotte (Lamont-Doherty Earth Observatory of Columbia University), R. Detrick (Woods Hole Oceanographic Institution), and G. Kent (Scripps Institution of Oceanography), collected multichannel seismic (MCS) and swath-bathymetry data. This 2002 program included detailed surveys along the axis of the JFR and a suite of ridge-flank profiles (~1500 km total length), of which the northernmost crossed the Expedition 301 work area.

We had originally hoped to complete the ImageFlux survey in advance of the RetroFlux survey, to allow time for processing and interpretation of swath-map and seismic data prior to collection of cores and heat flow data. The two surveys ended up being run at the same time, but we were able to coordinate activities at sea by email, radio, and a hard-copy data swap so as to avoid having both ships working too closely together and to take advantage of new discoveries as the surveys progressed.

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