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

Cochrane, Guy R.; MacKay, Mary E.; Moore, G. F.; Moore, J. Casey

doi:10.2973/odp.proc.ir.146-1.003.1994

Cited by 14 publications

(14 citation statements)

References 15 publications

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“…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 …”

Section: Introductionmentioning

confidence: 99%

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

Kastner¹,

Sample²,

Whiticar³

et al. 1995

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

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Pervasive fluid advection is indicated in the geochemical depth profiles at four locations (five sites), two in the Vancouver Island (VI) sector and two in the Central Oregon (CO) sector of the Cascadia Convergent Margin. Diffusive fluid flow prevails at the VI Sites 888 and 889/890, whereas confined fluid flow dominates the CO Sites 891 and 892. Exactly the same deepseated fluid source (>1.5 to <4 km) was found in both areas, regardless of the fluid-flow regime. It is characterized by lower than seawater Cl concentration, higher than seawater Li, Ca, Sr, and silica concentrations, non-radiogenic Sr isotopes and is slightly depleted in I8 O. The upward-advecting fluid that transports thermogenic hydrocarbons mixes with the pore fluids altered by diagenesis. The extent of mixing between these fluids varies spatially.At the CO sector of the Cascadia Margin where confined fluid flow prevails, the frontal thrust at Site 891 is presently not acting as a significant, but only as a leaky, fluid conduit, in contrast to the landward-dipping fault at Site 892.Organic matter-fueled diagenesis dominates over the depth-range drilled. An inverse relationship between sedimentation rate and both total organic carbon content (TOC) and the sulfate reduction rate is characteristic of this convergent margin. An important result of the bacterial degradation of organic matter is a widespread formation of diagenetic carbonates. The upwardmigrating fluid that is enriched in Ca, dissolved inorganic carbonate, and thermogenic hydrocarbons most likely favors the formation of epigenetic carbonates.Although solid gas hydrate was not recovered at the depth of either of the prominent bottom-simulating reflectors (BSRs), their presence is inferred from geochemical (and geophysical) measurements. The geochemistry, particularly the coincidence between pore fluids that have very low Cl concentrations and very high methane concentrations at these depths, indicates that primarily a methane hydrate occupies at least 15% of the pore space above the BSR at the VI Site 889, and at least 10% of the pore space above the BSR at the CO Site 892. The solid gas hydrate recovered at ~2 to 19 mbsf, Site 892, is a mixed CH 4 -H 2 S hydrate of which up to 10% of the gas is H 2 S.

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Section: Introductionmentioning

confidence: 99%

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

Kastner¹,

Sample²,

Whiticar³

et al. 1995

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

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“…Tectonically induced dewatering is found to be highest within the shallower sediments, which appear to accommodate horizontal shortening largely through volume loss. Volume changes appear to increase in the vicinity of the blind thrusts, perhaps, as suggested by Cochrane [1994], in response to enhanced fracture permeability along the faults. Significant dewatering is also still observed in the vicinity of the frontal thrust, however, not as much as that estimated from the first solution.…”

Section: Discussionmentioning

confidence: 84%

Kinematic constraints on porosity change in the toe of the Cascadia accretionary prism: Evidence for cementation and brittle deformation in the footwall of the frontal thrust

Morgan¹

1997

J. Geophys. Res.

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Abstract. Kinematic analyses are performed on a seismic profile across the deformed protothrust zone (PTZ) of the Cascadia accretionary prism, off the central coast of Oregon, in order to quantify the deformation in this zone and to explore mechanisms and controls on deformation and dewatering. The analyses rely on seismically defined stratigraphy and seismic interval velocities to constrain distortional and volumetric changes within the PTZ, permitting the calculation of the complete deformation field. Two solutions are offered: the first is based on porosity estimates derived from seismic interval velocities by Cochrane et al.[1994b] which suggest extremely high volume loss in the immediate footwall of the frontal thrust; the second solution employs a modified volume distribution in which lateral shear deformation is minimized. This latter approach is justified by evidence from seismic profiles and in drill cores for subhorizontal maximum principal stresses in many prisms. The two solutions yield total shortening of about 1.46 km (12%), with a general landward increase in horizontal shortening and vertical extension; this appears to be accompanied by increasing brittle deformation and porosity decrease, perhaps enhanced by dewatering along protothrusts. The greatest difference between the solutions lies in estimates for tectonically induced volume loss in the footwall of the frontal thrust, which is significantly reduced in the second solution. Relying on kinematic and mechanical arguments that support this result, we propose that the high seismic velocities observed in the footwall of the thrust may result from the presence of intergranular cement, perhaps preferentially precipitated adjacent to the frontal thrust fault in sediments with relatively high fracture permeability and enhanced fluid flow. A high degree of lithification in this zone is consistent with the inferred increase in brittle deformation in the footwall of the thrust.

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“…Finally, the porosity decrease and incipient thrusting occurring seaward of the frontal thrust could The correlation of faults defined on the seismic reflection data and the features from cores requires correct depth conversion of the seismic data. The seismic data were depth converted using velocities derived from the ODP drilling, associated logs, and vertical seismic profiles (Cochrane et al, 1994a). We determined the depth of the thrust splays by subtracting the depth of the water-bottom reflector from the reflectors of the fault splays on seismic line OR-41 that trends north-south within 20 m west of Site 891 and intersects line OR-5, 60 m south of the drill site.…”

Section: Tectonic Settingmentioning

confidence: 99%

“…In the vicinity of ODP Site 891, the frontal thrust of the Oregon accretionary prism dips landward, has a vertical throw of about 1.2 km, and controls the geometry of a fault-bend fold that underlies the first ridge of the margin (MacKay et al, 1992;Cochrane et al, 1994a) (Figs. 1 and 2).…”

Section: Frontal Thrust Geometry and Geologymentioning

confidence: 99%

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

Cited by 14 publications

References 15 publications

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

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

Kinematic constraints on porosity change in the toe of the Cascadia accretionary prism: Evidence for cementation and brittle deformation in the footwall of the frontal thrust

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

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