CARBONATE CEMENTS INDICATE CHANNELED FLUID-FLOW ALONG A ZONE OF VERTICAL FAULTS AT THE DEFORMATION FRONT OF THE CASCADIA ACCRETIONARY WEDGE(NORTHWEST UNITED-STATES COAST)

To understand the relation between fluid seeps and structures, sedimen tary rocks were collected with the DSRV Alvin from a vertical fault zo ne that transects the deformation front of the Cascadia accretionary w edge. The rocks contained diagenetic carbonate cement that was precipi tated from fluids expelled during accretion. Carbon, oxygen, and stron tium isotope data are consistent with a fluid source at > 2 km depth. Most carbon isotopes range from -1 parts per thousand to -25 parts per thousand, (PDB [Peedee belemnite] standard) consistent with a thermog enic methane source. Oxygen isotopes show extreme O-18 depletions (-4 parts per thousand to -13 parts per thousand PDB) that are consistent with precipitation from fluids with temperatures as high as 100-degree s-C. Sr-87/Sr-86 values of 0.709 75 to 0.712 79 may be due to strontiu m in fluids derived from clay-rich parts of the stratigraphic section. The ubiquity of carbonate precipitates and the isotope data indicate that the vertical fault zone is an efficient conduit for fluid dewater ing from deep levels of the accretionary wedge.