Basin Research (1997) 9, 303–312 Headless submarine canyons and fluid flow on the toe of the Cascadia accretionary complex Daniel L. Orange,* Brian G. McAdoo,† J. Casey Moore,† Harold Tobin,†1 Elizabeth Screaton,‡2 Hank Chezar,§ Homa Lee,§ Mark Reid§ and Rick Vail§ *Monterey Bay Aquarium Research Institute, PO Box 628, 7700 Sandholt Rd, Moss Landing, CA 95039, USA †Earth Sciences Department, University of California, Santa Cruz, CA 95064, USA ‡Department of Geology, 31 Williams Hall, Bethlehem, PA 18015, USA §USGS, 345 Middlefield Road MS-999, Menlo Park, CA 94025, USA ABSTRACT Headless submarine canyons with steep headwalls and shallowly sloping floors occur on both the second and third landward vergent anticlines on the toe of the Cascadia accretionary complex off central Oregon (45 °N, 125° 30∞W). In September 1993, we carried out a series of nine deep tow camera sled runs and nine ALVIN dives to examine the relationship between fluid venting, structure and canyon formation. We studied four canyons on the second and third landward vergent anticlines, as well as the apparently unfailed intercanyon regions along strike. All evidence of fluid expulsion is associated with the canyons; we found no evidence of fluid flow between canyons. Even though all fluid seeps are related to canyons, we did not find seeps in all canyons, and the location of the seeps within the canyons differed. On the landward facing limb of the second landward vergent anticline a robust cold seep community occurs at the canyon’s inflection point. This seep is characterized by chemosynthetic vent clams, tube worms and extensive authigenic carbonate. Fluids for this seep may utilize high-permeability flow paths either parallel to bedding within the second thrust ridge or along the underlying thrust fault before leaking into the overriding section. Two seaward facing canyons on the third anticlinal ridge have vent clam communities near the canyon mouths at approximately the intersection between the anticlinal ridge and the adjacent forearc basin. No seeps were found along strike at the intersection of the slope basin and anticlinal ridge. We infer that the lack of seepage along strike and the presence of seeps in canyons may be related to fluid flow below the forearc basin/slope unconformity (overpressured by the impinging thrust fault to the west?) directed toward canyons at the surface. (>50%) to porosities of #10% leads to the expulsion INTRODUCTION: CANYONS AND COLD of significant amounts of fluid (Bray & Karig, 1985; SEEPS Moore & Vrolijk, 1992). Excess pore pressure gradients Accretionary complexes are regions of expected high pore create a seepage force that acts in the direction of flow pressure gradients due to the tectonic compression and is proportional to the head gradient. Seepage-induced inherent to sediment accretion by both offscraping and slope failure occurs where the seepage force (proportional underplating; compaction of the original porosity to head gradient), augmented by downslope gravitational forces, exceeds the ability of the sediment to resist downslope movement (Iverson & Major, 1986; Denlinger Present addresses: 1Department of Earth and Environmental & Iverson, 1992). If the slope fails by this internally Science, New Mexico Institute of Mining and Technology, driven mechanism, the resulting notch in the slope affects Socorro, NM 87801 and 2Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA. the pore pressure gradient: the head gradient at the base © 1997 Blackwell Science Ltd 303