The Juan de Fuca Ridge is a seafloor spreading center that lies a few hundred kilometers off the coast of North America. It supplies crust and lithosphere to the Pacific and Juan de Fuca plates at rates of about 56 mm/yr (Fig. 1; for reviews, see Johnson and Holmes, 1989; Davis and Currie, 1993). The topographic relief of the ridge (Fig. 2) produces a barrier to terrigenous turbidite sediment supplied from Pleistocene glacial sources along the continental margin, primarily at Queen Charlotte Sound, Juan de Fuca Strait, and the Grays Harbor and Columbia River estuaries. This situation has resulted in the accumulation of an onlapping layer of sediment that buries the eastern flank of the Juan de Fuca Ridge. This sedimented region known as Cascadia Basin extends from the base of the continental margin, where accretion of the sediment entering the Cascadia subduction zone begins, to within a few tens of kilometers of the ridge crest, where sediment laps onto crust that is at some locations younger than 1 Ma (Fig. 3). Along the deformation front of the Cascadia accretionary prism the sediment layer is over 3 km thick in places; in general, the fill in the northern part of the basin is sufficient to completely bury the relief of the igneous crust of the Juan de Fuca plate, with the exception of only a few isolated volcanic cones and seamounts.
Beneath the nearly continuous, flat-lying sediment cover, local basement relief is dominated by linear ridges and troughs that were produced by normal faulting and variations in volcanic supply at the time the crust was created. The amplitude of this relief varies across the basin. At a point roughly 100 km east of the ridge axis there is a fairly sharp boundary between areas of relatively smooth and rough basement relief. To the east, basement relief ranges typically from 300 to 700 m, with ridges separated typically by 3 to 7 km. Closer to the ridge crest, the crust is much smoother, with local relief typically only 100-200 m.
The sediments that blanket the eastern flank of the ridge provide a conveniently soft layer in which heat-flow measurements can be made and a relatively low-permeability, porous "filter" from which pore fluids can be extracted. The degree of sediment burial makes fluid venting relatively rare, but highly focused and hence easily studied. The combination of water-column studies, detailed seafloor heat-flow and pore-fluid geochemical measurements sited along closely-spaced seismic reflection profiles, and direct submersible observations has provided strong constraints on the directions and rates of fluid flow through the seafloor and on the thermal structure, patterns of fluid flow, and pore-fluid composition within the upper igneous crust (e.g., Davis et al., 1992; Mottl and Wheat, 1994; Wheat and Mottl, 1994; Thomson et al., 1995). These studies form the framework for the drilling program planned for Leg 168.
Of particular interest are the remarkably simple examples of three general type-examples of crustal fluid-flow regimes found on this ridge flank that have become the focus of several detailed studies. These three type-examples comprise (1) a transition zone between sediment free (permitting open hydrothermal circulation) and sediment-covered (hydrologically sealed) crust, (2) an area where rugged basement topography and large variations in sediment thickness are inferred to exert a dominant influence on the pattern and rate of fluid flow within the upper igneous crust and through the seafloor into the water column, and (3) an area where a uniform and regionally continuous cover of sediments over unusually flat-lying basement should prevent advective heat loss and allow the total lithospheric heat flow to be determined with confidence. Each of these simple and well-characterized examples of common and important subseafloor hydrologic regimes is particularly well suited to the quantitative studies proposed here. Although the crust in which they occur is unusually young, similar situations can be found on virtually all ridge flanks, and the lessons learned will be extremely valuable for understanding the fundamental nature and consequences of crustal fluid flow as it occurs throughout the world's oceans.
To 168 Scientific Objectives and Methodology
168 Table of Contents