INTERRELATIONSHIP OF FLUID VENTING AND STRUCTURAL EVOLUTION - ALVIN OBSERVATIONS FROM THE FRONTAL ACCRETIONARY PRISM, OREGON

Seismic reflection and Sea Beam bathymetric data plus submarine geological measurements define a ramp anticline at the deformation front of the central Oregon subduction zone. At its northern termination the ramp anticline is deeply incised by a large 500m deep submarine canyon and cut by a probable backthrust. To the south along the strike of the fold, a smaller submarine canyon shallowly erodes the anticline. Two Alvin dives along a transect through the southern canyon show active fluid vents demarked by biological communities at the frontal thrust and at the breached crest of the anticline. Along a northern transect, encompassing the large submarine canyon, 10 Alvin dives indicated no venting on the frontal thrust, limited venting in the canyon, but numerous biological communities along a scarp interpreted as the surface trace of the backthrust. These observations suggest a scenario of vent and structural-geomorphic development consisting of 1) frontal thrust faulting and associated venting, facilitated by high fluid pressure; 2) erosion of the oversteepened seaward flank of the ramp anticline; 3) locking of the frontal thrust; and 4) redirection of fluid flow by the backthrust. -from Authors