Abstract:

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.