HEADLESS SUBMARINE CANYONS AND FLUID-FLOW ON THE TOE OF THE CASCADIA ACCRETIONARY COMPLEX

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 degrees N, 125 degrees 30'W). In September 1993, we carried out a ser ies 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 ant iclines, 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 canyo ns, 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 see p is characterized by chemosynthetic vent clams, tube worms and extens ive authigenic carbonate. Fluids for this seep may utilize high-permea bility flow paths either parallel to bedding within the second thrust ridge or along the underlying thrust fault before leaking into the ove rriding section. Two seaward facing canyons on the third anticlinal ri dge have vent clam communities near the canyon mouths at approximately the intersection between the anticlinal ridge and the adjacent forear c basin. No seeps were found along-strike at the intersection of the s lope basin and anticlinal ridge. We infer that the lack of seepage alo ng 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 s urface.