Site Survey of the Northern Oregon Continental Margin and Astoria Fan

Kulm, L. D.; Prince, Roger A.; Snavely, Parke Detweiler

doi:10.2973/dsdp.proc.18.app1a.1973

Cited by 12 publications

(6 citation statements)

References 5 publications

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“…3 and 7). The proto-décollement appears to have propagated out along a stratigraphic layer that is deeper in the section than the unconformity between Units 1 and 2 of Kulm et al (1973).…”

Section: Focused Flow In the Accretionary Prismmentioning

confidence: 93%

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Consolidation and Deformation of Sediments at the Toe of the Central Oregon Accretionary Prism from Multichannel Seismic Data

Cochrane¹,

MacKay²,

Moore³

et al. 1994

Proceedings of the Ocean Drilling Program

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Interpretation of data from a 1989 multichannel seismic (MCS) survey of the central Oregon Margin indicates that pore fluids escaping from the accretionary prism follow paths that are largely fracture controlled and fault guided. MCS evidence of this control includes reversed-polarity, fault-plane reflections, P-wave-velocity reversals, and upward deflection of gas hydrate reflections where they cross fault traces on some seismic sections. Site 891 was located on the frontal thrust of the accretionary prism. At this site the thrust dips 8° landward and produces a high-amplitude, reversed-polarity reflection that suggests the thrust may be a conduit for fluids from the décollement. Site 892 is situated on an out-of-sequence thrust within the prism that is known to be hydrogeologically active. The thrust dips 14° landward. Upward deflection of the gas hydrate reflection is present where it crosses the fault trace.In Cascadia Basin, west of the Leg 146 Oregon drill sites, velocity measurements increase within all strata in a landward direction, suggesting that diffusive flow is the dominant dewatering process. In the area of the basin nearest the prism, where incipient deformation was observed, proto-thrusts may augment diffusive flow in the section above a proto-décollement. A velocity inversion in the sedimentary section occurs below the proto-décollement. The depth of the proto-décollement may be at the top of an overpressured sedimentary section below a layer with low permeability.

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“…3 and 7). The proto-décollement appears to have propagated out along a stratigraphic layer that is deeper in the section than the unconformity between Units 1 and 2 of Kulm et al (1973).…”

Section: Focused Flow In the Accretionary Prismmentioning

confidence: 93%

“…The stratigraphy of the eastern Cascadia Basin was determined by Kulm et al (1973) on Deep Sea Drilling Project (DSDP) Leg 18. DSDP Site 174 penetrated 879 m of sediment.…”

Section: Diffusive Dewatering In Cascadia Basinmentioning

confidence: 99%

Consolidation and Deformation of Sediments at the Toe of the Central Oregon Accretionary Prism from Multichannel Seismic Data

Cochrane¹,

MacKay²,

Moore³

et al. 1994

Proceedings of the Ocean Drilling Program

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“…Here, the age of the oceanic crust is about 9 Ma, converges at a rate of approximately 4 cm/yr along 070°, and is covered by about 3.5 km of sediment (Kuhn et al, 1984;Demets et al, 1990). This area of central Oregon contains distinct domains of landward-and seaward-verging structures (Kulm et al, 1973;Snavely and Miller, 1986;MacKay et al, 1992). Our study is from a province of seaward-vergent structures.…”

Section: Tectonic Settingmentioning

confidence: 90%

Frontal Thrust, Oregon Accretionary Prism: Geometry, Physical Properties, and Fluid Pressure

Moore¹,

Moran²,

MacKay³

et al. 1995

Proceedings of the Ocean Drilling Program, 146 Part 1 Scientific Results

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The frontal thrust of the Oregon accretionary prism scrapes off about 2 km of incoming sedimentary section and underlies a fault-bend fold of the first ridge landward of the abyssal plain. Scarps with associated chemosynthetic biological communities at the surface and pore-water anomalies at depth indicate that the frontal thrust is tectonically and hydrologically active. At Site 891, near seismic line OR-5, the thrust splits into at least three splays from 375 to 500 mbsf. The upper splay is defined in the cores by strongly developed scaly fabric and is mostly positive polarity on seismic reflection lines. The middle splay shows low to moderate deformation in the cores. Drilling did not penetrate the lower splay. The middle and lower splays are mostly negative polarity on the seismic reflection data. At Ocean Drilling Program (ODP) Site 891 the upper splay is marked by increased velocity and density on borehole logs, which explains its positive polarity reflection. In core samples the middle splay is associated with a velocity and density decrease about 10 m thick that explains its negative polarity reflection.Four techniques were used to estimate fluid pressure from the anomalously porous interval associated with the middle splay of the frontal thrust: (1) comparison of porosity and effective overburden to an undeformed reference section in Cascadia Basin; (2) comparison of the porosity-depth profile over this interval with the normally consolidated porosity-depth function determined from one-dimensional consolidation tests on recovered samples; (3) comparison of the undrained shear strength/ overburden pressure ratio (S u /P 0 ') with the same ratio for normally consolidated clays; and (4) direct measurement of sediment stress history from consolidation tests. These four methods result in a range of fluid pressure ratios (λ) from 0.88 to 0.99 that indicates nearly lithostatic fluid pressures associated with the middle splay of the frontal thrust.Overall the frontal thrust at Site 891 evolves from a fluid-rich, high-fluid-pressure state, as exemplified by the middle and lower splays, to the dense, collapsed mature state of the upper splay. The frontal thrust is thus undergoing "in sequence" or seaward migration into the footwall, associated with dewatering and densification. Rapid burial of the highly porous sediment associated with the middle splay preserved near-surface effective stresses by high fluid pressures, which were perhaps temporarily sealed by the low transverse permeability of the enveloping fault strands.

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“…Here, the oceanic crust is about 9 Ma in age, converges at a rate of approximately 2.5 cm/yr along 040 ø, and is covered by nearly 4 km of sediment [Kulm et al, 1984]. This area of central Oregon contains distinct domains of landward and seaward verging structures [Kulm et al, 1973;Snavely and Miller, 1986]. Our dives were concentrated in a province of seaward vergent structures; a transition to landward vergent thrust faults occurs approximately 20 km to the north.…”

Section: Introductionmentioning

confidence: 93%

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

Moore¹,

Orange²,

Kulm³

1990

J. Geophys. Res.

117

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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].

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Site Survey of the Northern Oregon Continental Margin and Astoria Fan

Cited by 12 publications

References 5 publications

Consolidation and Deformation of Sediments at the Toe of the Central Oregon Accretionary Prism from Multichannel Seismic Data

Consolidation and Deformation of Sediments at the Toe of the Central Oregon Accretionary Prism from Multichannel Seismic Data

Frontal Thrust, Oregon Accretionary Prism: Geometry, Physical Properties, and Fluid Pressure

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

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