Carbonate cements indicate channeled fluid flow along a zone of vertical faults at the deformation front of the Cascadia accretionary wedge (northwest U.S. coast)

To understand the relation between fluid seeps and structures, sedimentary rocks were collected with the DSRV Alvin from a vertical fault zone that transects the deformation front of the Cascadia accretionary wedge. The rocks contained diagenetic carbonate cement that was precipitated from fluids expelled during accretion. Carbon, oxygen, and strontium isotope data are consistent with a fluid source at >2 km depth. Most carbon isotopes range from -1‰ to -25‰ (PDB standard) consistent with a thermogenic methane source. Oxygen isotopes show extreme ¹⁸O depletions that are consistent with precipitation from fluids with temperatures as high as 100°C. ⁸⁷Sr/⁸⁶Sr values of 0.709 75 to 0.712 79 may be due to strontium 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 dewatering from deep levels of the accretionary wedge. -Authors