ODP Leg 146 examines fluid flow in Cascadia margin

For the better part of 2 decades, it has been known that dewatering of sediments accreted to or subducted beneath accretionary wedges is a fundamental aspect of the subduction‐accretion process. Yet, evidence for fluid flow in modern accretionary wedges is largely secondary and based on the presence of geochemical and/or thermal anomalies [e.g., Vrolijk et al ., 1991]; the analysis of seismic velocity as an indicator of porosity, which suggests a progressive loss of pore volume in a landward direction [e.g., Bray and Karig , 1985]; and the occurrence of secondary sediment microstructures characteristic of fluid movement [e.g., Maltman et al ., 1992]. The only quantitative measurements of fluid expulsion at surface vents are based on submersible‐deployed, seepage‐meter data [e.g., Carson et al ., 1990], and these results—coupled with the surface area of the vents—indicate flow rates significantly greater than can be supported by steadystate dewatering [ Le Pichon et al ., 1992]. The fluid budgets and mass fluxes associated with accretion are poorly constrained. Results of previous drilling suggest two distinct modes of fluid flow: channelized flow along fault zones (primarily the décollement [e.g., Mascle and Moore , 1990]) or diffuse flow, which is apparently accommodated by a pervasive fracture permeability [e.g., Taira et al ., 1992].