Implications for focused fluid transport at the northern Cascadia accretionary prism from a correlation between BSR occurrence and near-sea-floor reflectivity anomalies imaged in a multi-frequency seismic data set

A high-resolution seismic survey was carried out at the accretionary prism on the continental slope off Vancouver Island, Canada. Two GI-Gun data sets with different source frequency ranges of 50-150 and 100-500 Hz were combi ned with 4 kHz narrow-beam echosounding data (Parasound), The data allow sp atial correlation between a gas hydrate bottom simulating reflector (BSR) a nd distinct areas of high near-seafloor reflectivity. An integrated interpr etation of the multi-frequency data set provides insight into the regional distribution of tectonically induced fluid migration and gas hydrate format ion in the vicinity of ODP Leg 146 Sites 889 and 890. The BSR at the base o f the gas hydrate stability field is observed within accreted and deformed sediments, but appears to be absent within bedded slope basin deposits. It is suggested that these basin deposits inhibit vertical fluid flow and prev ent the formation of a BSR, whereas the hydraulic conductivity of the accre ted sediments is sufficiently high to allow for pervasive gas migration. An elevation of the BSR beneath the flanks of a topographic high is interpret ed as an indicator fur local upflow of warm fluids along permeable pathways within outcropping accreted sediments. Parasound data reveal discontinuous zones of high reflectivity at or directly beneath the sea floor, which may indicate local cementation of surface sediments. III combination with GI-G un data, the occurrence of these reflective areas can be related to the loc ation of slope sedimentary basins acting as hydraulic seals. It is proposed that the seals sometimes fail along faults extending beneath the BSR, lead ing to focused upflow of methane-bearing fluid and the formation of carbona te pavements at the sea floor.