Integrated Ocean Drilling Program Expedition 301 was preceded during 2000 and 2002 by three surveys that helped to delineate seafloor and basement relief, sediment thickness, and the nature of ridge-flank hydrothermal conditions and processes on the eastern flank of the Juan de Fuca Ridge. These surveys generated swath map, seismic, and thermal data used to select locations for primary and secondary drilling targets, building from several decades of earlier work. We show compilations and examples of data from several characteristic settings in and around the Expedition 301 work area and use these observations to evaluate sedimentation patterns and thermal conditions in basement. There remain important unanswered questions in this area concerning fluid circulation within the upper oceanic crust, the magnitude of lithospheric heat input, the quantitative significance of advective heat loss from the crust, and relations between basement relief, sedimentation, and sediment alteration. These questions may be resolved through collection of a modest amount of additional data focusing on a few critical locations.

Young and permeable crust along oceanic ridges is often characterized by large-scale fluid migration depending on permeability and heat distribution. The eastern flank of the Juan de Fuca Ridge off the west coast of North America is buried under an unusual thick sediment cover and is therefore suitable for detailed studies on fluid exchange processes. In autumn 1996 a high-resolution seismic survey was carried out in the vicinity of ODP Leg 168 drill sites to collect detailed information about sedimentary structures and acoustic anomalies in relation to basement morphology. The seismic equipment was optimized for high lateral and vertical resolution to identify small-scale features from the surface down to the basement as potential migration paths for fluids. The acquired seismic sections exhibit numerous vertical zones with reduced reflection amplitudes which may represent paths for fluid advection. The lateral amplitude changes are visible using seismic sources with different frequency content and are apparently related to basement topography. Observations on parallel profiles indicate a 2-D geometry. Theoretical studies suggest that local porosity changes may be the reason for the acoustic visibility of the proposed advection zones.