Fluid expulsion sites on the Cascadia accretionary prism: Mapping diagenetic deposits with processed GLORIA imagery

Abstract

Point‐discharge fluid expulsion on accretionary prisms is commonly indicated by diagenetic deposition of calcium carbonate cements and gas hydrates in near‐surface (<10 m below seafloor; mbsf) hemipelagic sediment. The contrasting clastic and diagenetic lithologies should be apparent in side scan images. However, sonar also responds to variations in bottom slope, so unprocessed images mix topographic and lithologic information. We have processed GLORIA imagery from the Oregon continental margin to remove topographic effects. A synthetic side scan image was created initially from Sea Beam bathymetric data and then was subtracted iteratively from the original GLORIA data until topographic features disappeared. The residual image contains high‐amplitude backscattering that we attribute to diagenetic deposits associated with fluid discharge, based on submersible mapping, Ocean Drilling Program drilling, and collected samples. Diagenetic deposits are concentrated (1) near an out‐of‐sequence thrust fault on the second ridge landward of the base of the continental slope, (2) along zones characterized by deep‐seated strikeslip faults that cut transversely across the margin, and (3) in undeformed Cascadia Basin deposits which overlie incipient thrust faults seaward of the toe of the prism. There is no evidence of diagenetic deposition associated with the frontal thrust that rises from the dècollement. If the dècollement is an important aquifer, apparently the fluids are passed either to the strike‐slip faults which intersect the dècollement or to the incipient faults in Cascadia Basin for expulsion. Diagenetic deposits seaward of the prism toe probably consist dominantly of gas hydrates.