Through a wide range of studies beginning with the classic RV Pioneer magnetics survey (Raff and Mason, 1961), the Juan de Fuca Ridge and flank has become one of the most thoroughly studied areas in the oceans, effectively a type-area for seafloor spreading and related processes. Over the past decade, a great deal of effort has been devoted to the study of ridge-flank hydrothermal circulation on the eastern flank of the ridge, because many simple examples of typical hydrologic regimes as defined by basement structure and sediment cover occur in this relatively small and accessible region. Geophysical and geochemical studies have employed submersibles for fluid sampling from seafloor springs (e.g., Mottl et al., 1998), piston and gravity coring for interstitial fluid sampling (Wheat and Mottl, 1994), heat-flow probes for thermal studies (Davis et al., 1992), multichannel seismic reflection and ocean-bottom seismometer seismic refraction for crustal lithology and physical properties determination (Hasselgren and Clowes, 1995; Rohr, 1994; Rohr et al., 1994), and Ocean Drilling Program (ODP) drilling for deep sediment, rock, and fluid sampling and in situ measurements and monitoring. A brief review of recent work on the eastern flank, including preliminary results of drilling, has been provided by the Shipboard Scientific Party (1997).
Permeability is the primary factor influencing crustal fluid flow, and one of the most fundamental permeability contrasts in the oceanic crust is that between the upper, highly fractured and permeable igneous crust and the hydrologically resistive sediments that bury the igneous rocks progressively with time. Definition of this fundamental hydrologic boundary in oceanic environments relies on high-quality seismic reflection data. Hence, all of the coring, heat-flow, drilling, and numerical modeling studies on the eastern Juan de Fuca Ridge flank have been carried out in the context provided by a number of high-resolution seismic reflection surveys. The importance of this context has been demonstrated clearly in many instances (Davis, Wang, et al., 1997; Fisher and Becker, 1995; Wheat and Mottl, 1994), particularly in the case of studies carried out during and following ODP Leg 168 (Davis and Becker, 1998; Shipboard Scientific Party, 1997).
Although much has been learned in this area, many well-defined problems concerning ridge-flank fluid flow remain to be addressed through additional seafloor and drilling studies. To facilitate planning and provide the structural and hydrologic context for currently ongoing and future studies, we provide in this report single-channel seismic reflection data collected during cruises in 1995 and 1996. These data add considerably to the definition of regional and local sediment thickness and basement depth variations in the region permitted by older single- and multichannel data (e.g., Davis et al., 1992; Rohr et al., 1992; Spence et al., 1990). Complementary heat-flow data collected through 1995 have been published and discussed briefly in Davis, Fisher et al. (1997).