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Leg 146A

The Cascadia Margin

During Leg 146, diffuse fluid outflow from the accretionary wedge and the nature of a well-defined bottom-simulating-reflector (BSR - thought to represent the base of sediments containing methane hydrate, a clathrate of water and methane) were investigated at five sites off the northwest coast of North America (Site 888 to Site 892). Upper Pleistocene-to-Holocene slope and slope-basin hemipelagites and fan deposits overlie an accreted section marked by pervasive small-scale fracturing of upper Pliocene to Pleistocene sediments. Geochemical anomalies indicate fluid channeling in a high-porosity zone near the base of the slope-basin sediments. In the lower part of the accreted section, increases in the concentration gradients of calcium and silica, and a decrease in the potassium gradient, indicate diffusion or mixing with a deeper-seated fluid. Temperature measurements revealed a linear increase with depth, implying conductive heat loss rather than significant advection. Vertical seismic profile data defined a rise in velocity just above the BSR with a distinct low velocity zone beneath it, corresponding to the presence of either hydrate or free gas, respectively. A mismatch between the depth of the BSR and the predicted base of the pure water/methane hydrate stability field indicates that a pure water/pure methane composition for hydrate does not apply on this margin. There is little evidence for channeling of flow along faults or permeable beds; fluids are probably dispersed throughout the pervasively-fractured accreted sediments.

Site 891, located at the toe of the wedge where the borehole intersected the frontal thrust fault rising from the dÚcollement, is characterized by two intervals of fracturing (198-278 and 321-375 mbsf), each associated with a fault zone. The interval spanning these fracture zones, and extending to a major fluid-chemistry and porosity discontinuity at 440 mbsf, shows a rapid increase in methane concentrations with depth, high relatively constant chlorine concentrations, no sulfate, and low magnesium. Unstable ethene and maxima in ethane, higher hydrocarbons, and carbon dioxide below 240 mbsf suggest active fluid dispersal and localized advection. No significant geochemical anomaly is present at the depth of the major frontal thrust, indicating that this may be a horizon of little or no active fluid flow. Below 440 mbsf, an inversion to lower porosity and a change in the gas and fluid geochemistry suggest that the active portion of the frontal thrust has stepped down to near the top of this interval which may represent the footwall section beneath the frontal thrust system. Massive gas hydrates were recovered from a hydrologically-active fault in the Pliocene section of the wedge (Site 892). Active flow is indicated by geochemical anomalies in pore waters, fluid pressure measurements, and local higher-temperature excursions from a linear increase with depth. The gas hydrates and elevated levels of H2S may be an indirect consequence of the underlying flow regime. Again, the predicted base of the gas hydrate zone appears to be deeper than the depth of the BSR.

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