Interrelationship of fluid venting and structural evolution: Alvin 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 northem 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 southem 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 structuralgeomorphic 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. The initial series of Alvin dives in 1984 first discovered chemosynthetic biological communities and associated authigenic carbonate deposits at a convergent margin [Suess et al., 1985; Kulm et al., 1986]. Other components of the Alvin program have examined the chemistry of fluids from the vents [Suess and Whiticar, 1989]; the role of expulsed fluid in global geochemical cycles and diagenesis [Hah and Sues& 1989], the association of carbonate deposits with fluid venting processes [Kulm and Suess, this issue], and the relationship of venting to large scale geologic structure [Lewis and Cochrane, this issue].

Organic Geochemistry of Gases, Fluids, and Hydrates at the Cascadia Accretionary Margin

“…In contrast to VIM, the COM exhibits extensive thrusting at the deformation front of the accretionary wedge (Snavely, 1987;Moore et al, 1990;MacKay et al, 1992). A result of this is a focused pore fluid expulsion along fracture zones at the COM.…”

Diagenetic Carbonates from Cascadia Margin: Textures, Chemical Compositions, and Oxygen and Carbon Stable Isotope Signatures

“…This type of plate kinematics causes evolution of a complex interplay of sedimentation, structural evolution, diagenesis and fluid flow. Convergent motion in the Cascadia forearc results in sediment compaction, over-pressuring and expulsion of pore-fluids, and development of fault zones to structurally accommodate shortening and perhaps dewatering (e.g., Moore et al, 1990). Sediment offscraping and accretion takes place predominantly by landward-vergent thrusting along most of the Cascadia Margin.…”

Geochemical Evidence for Fluid Flow and Diagenesis at the Cascadia Convergent Margin

“…That fluids affect virtually all aspects of the geologic evolution of convergent margins has been well established (e.g., Shipley et al, 1979;Bray and Karig, 1985;Kulm et al, 1986;Carson et al, 1990;Moore et al, 1990;Kastner et al, 1991). Prior to Leg 146, most attention was given to surficial manifestations of confined fluid venting in the CO sector of the margin, which supports benthic communities and is associated with mud volcanoes, diapirs, and widespread carbonate deposition (e.g., Kulm et al, 1986;Ritger et al, 1987;Moore et al, 1990;Carson et al, 1990Carson et al, , 1994.…”

“…Prior to Leg 146, most attention was given to surficial manifestations of confined fluid venting in the CO sector of the margin, which supports benthic communities and is associated with mud volcanoes, diapirs, and widespread carbonate deposition (e.g., Kulm et al, 1986;Ritger et al, 1987;Moore et al, 1990;Carson et al, 1990Carson et al, , 1994. Information about fluid expulsion by diffusive-flow through porosity reduction by tectonic compaction, deformation, and thermal mineral dehydration and transformation processes was provided through seismic studies of arcward thinning of the sediment sections, by mapping the distribution of carbonate crusts, and by studies of the physical and thermal states of the sediments (e.g., Hyndman and Davis, 1992;Moore et al, 1990;Cochrane et al, 1994;Tobin et al, 1994;Carson et al, 1994). On the basis of these studies the main objectives for drilling four locations (five sites) at the Cascadia Convergent Margin, Ocean Drilling Program (ODP) Leg 146, were (1) to document the fluid-flow …”