FLUID EXPULSION SITES ON THE CASCADIA ACCRETIONARY PRISM - MAPPING DIAGNETIC DEPOSITS WITH PROCESSED GLORIA IMAGERY

Point-discharge fluid expulsion on accretionary prisms is commonly ind icated by diagenetic deposition of calcium carbonate cements and gas h ydrates in near-surface (<10 m below seafloor; mbsf) hemipelagic sedim ent. The contrasting clastic and diagenetic lithologies should be appa rent in side scan images. However, sonar also responds to variations i n bottom slope, so unprocessed images mix topographic and lithologic i nformation. We have processed GLORIA imagery from the Oregon continent al margin to remove topographic effects. A synthetic side scan image w as created initially from Sea Beam bathymetric data and then was subtr acted iteratively from the original GLORIA data until topographic feat ures disappeared. The residual image contains high-amplitude backscatt ering that we attribute to diagenetic deposits associated with fluid d ischarge, based on submersible mapping, Ocean Drilling Program drillin g, and collected samples. Diagenetic deposits are concentrated (1) nea r an out-of-sequence thrust fault on the second ridge landward of the base of the continental slope, (2) along zones characterized by deep-s eated strike-slip faults that cut transversely across the margin, and (3) in undeformed Cascadia Basin deposits which overlie incipient thru st faults seaward of the toe of the prism. There is no evidence of dia genetic deposition associated with the frontal thrust that rises from the decollement. If the decollement is an important aquifer, apparentl y the fluids are passed either to the strike-slip faults which interse ct the decollement or to the incipient faults in Cascadia Basin for ex pulsion. Diagenetic deposits seaward of the prism toe probably consist dominantly of gas hydrates.