Interrelationship of fluid venting and structural evolution: <i>Alvin</i> observations from the frontal accretionary prism, Oregon

Abstract

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 500‐m‐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, and backthrusting is not apparent in the submersible observations. 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 assisted by seepage forces and leading to fluid flow out of stratigraphically controlled conduits in the limbs of the overthrust deposits; (3) locking of the frontal thrust due to dewatering or a local decrease in wedge taper associated with development of the large canyon, leading to failure along the backthrust; and (4) redirection of fluid flow by the backthrust. Thus, within ≤0.3 m.y., deformation of the relatively permeable sediments of the Oregon margin results in stratigraphically controlled flow being partially captured by faults.