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  • 1
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    Focused Fluid Flow Along the Nootka Fault Zone and Continental Slope, Explorer-Juan de Fuca Plate Boundary (2021)
    Details
    Publication Date: 2021-10-11
    Description: Geophysical and geochemical data indicate there is abundant fluid expulsion in the Nootka fault zone (NFZ) between the Juan de Fuca and Explorer plates and the Nootka continental slope. Here we combine observations from 〉20 years of investigations to demonstrate the nature of fluid-flow along the NFZ, which is the seismically most active region off Vancouver Island. Seismicity reaching down to the upper mantle is linked to near-seafloor manifestation of fluid flow through a network of faults. Along the two main fault traces, seismic reflection data imaged bright spots 100–300 m below seafloor that lie above changes in basement topography. The bright spots are conformable to sediment layering, show opposite-to-seafloor reflection polarity, and are associated with frequency reduction and velocity push-down indicating the presence of gas in the sediments. Two seafloor mounds ~15 km seaward of the Nootka slope are underlain by deep, nonconformable high-amplitude reflective zones. Measurements in the water column above one mound revealed a plume of warm water, and bottom-video observations imaged hydrothermal vent system biota. Pore fluids from a core at this mound contain predominately microbial methane (C1) with a high proportion of ethane (C2) yielding C1/C2 ratios 〈500 indicating a possible slight contribution from a deep source. We infer the reflective zones beneath the two mounds are basaltic intrusions that create hydrothermal circulation within the overlying sediments. Across the Nootka continental slope, gas hydrate-related bottom-simulating reflectors are widespread and occur at depths indicating heat flow values of 80–90 mW/m2.
    Keywords: 551 ; fluid flow ; Nootka transform fault ; gas hydrate ; intrusion ; heat flow
    Language: English
    Type: map
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    CITATION GEO-LEO
  • 2
    Electronic Resource
    Electronic Resource
    The Hatton Bank continental margin—I. Shallow structure from two-ship expanding spread seismic profiles (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 96 (1989), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Summary. The Hatton Bank passive continental margin exhibits thick seaward dipping reflector sequences which consist of basalts extruded during rifting between Greenland and Rockall Plateau. Multichannel seismic reflection profiling across the margin reveals three reflector wedges with a maximum thickness near 7 km, extending from beneath the upper continental slope to the deep ocean basin. We present results of the velocity structure within the dipping reflector sequences at eight locations across the margin, interpreted by synthetic seismogram modelling a set of multichannel expanding spread profiles parallel to the margin. At the top of some reflector sequences, we observe a series of 100 m thick high- and low-velocity zones, which are interpreted as basalt flows alternating with sediments or weathered and rubble layers. At the profile locations, the base of the dipping reflectors correlates with P-wave velocities near 6.5 km s−1. However, elsewhere the reflectors appear to extend significantly deeper than the inferred 6.5 km s−1 velocity contour, indicating that the velocity structure may not be controlled solely by lithological boundaries but also by metamorphic effects. Shear-waves were observed on two lines, permitting the calculation of Poisson's ratio. The decrease in Poisson's ratio from 0.28 to near 0.25 in the upper 5 km of crust may also indicate the effect of metamorphism on seismic properties, or alternatively may be explained by crack closure under load.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1989.tb04451.x
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    Paper (German National Licenses)
    Fulltext
  • 3
    Electronic Resource
    Electronic Resource
    The Hatton Bank continental margin—II. Deep structure from two-ship expanding spread seismic profiles (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Geophysical journal international 96 (1989), S. 0 
    Details
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: Summary. The continent-ocean transition adjacent to Hatton Bank was studied using a dense grid of single-ship and two-ship multichannel seismic profiles. The interpretation of the explosive expanding spread profiles (ESPs) which were shot as part of this survey are discussed here in detail. Extensive seaward dipping reflectors are developed in the upper crust across the entire margin. These seaward dipping reflectors continue northwards on the Faeroes and Vøring margins, where they have been shown to be caused by basaltic lavas, as well as on the conjugate margin of East Greenland. The dipping reflectors are an important feature of the rifting history of the margin and show that extensive volcanism was associated with the extension. The ESPs show clear seismic arrivals out to ranges of 100 km. Wide-angle Moho reflections can be seen on all the lines as well as good mid and lower crustal arrivals. The determination of seismic velocity structure was constrained by ray tracing and by amplitude modelling using reflectivity synthetic seismograms. The results from the ESPs show that there is a thick region of lower crustal material beneath the margin with an unusually high crustal velocity of 7.3–7.4 km s−1. This lower crustal material reaches a maximum thickness of 14 km beneath the central part of the margin and is terminated at depth by the Moho. The lower crustal lens of high-velocity material is interpreted as underplated or intruded igneous rocks associated with the large volumes of extrusive basaltic lavas, now seen as dipping reflectors on the margin.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-246X.1989.tb04452.x
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    Decreased stability of methane hydrates in marine sediments owing to phase-boundary roughness (2002)
    [s.l.] : Macmillian Magazines Ltd.
    Nature 420 (2002), S. 656-660 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Below water depths of about 300 metres, pressure and temperature conditions cause methane to form ice-like crystals of methane hydrate. Marine deposits of methane hydrate are estimated to be large, amassing about 10,000 gigatonnes of carbon, and are thought to be important to global change and ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/nature01263
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    Upper Crustal Velocity Structure of the Southwestern Canadian Cordillera from Explosion Recordings on the WCTN Earthquake Seismic Net (2000)
    Springer
    Pure and applied geophysics 157 (2000), S. 1315-1336 
    Details
    ISSN: 1420-9136
    Keywords: Key Words: Seismic refraction, crustal velocity, Canadian Cordillera, tomographic inversion, earthquake seismic net, shear-wave velocities.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract —The digital seismic network used to monitor earthquakes in southwestern British Columbia recorded explosions from Lithoprobe’s Southern Cordillera Refraction Experiment (SCoRE) in 1989 and 1990 and from the USGS Pacific Northwest Refraction Survey in 1991. Both P and S waves were recorded. The geographical distribution of the shots and receiver stations enabled a large three-dimensional area to be sampled. For ray paths considered to be in the top 10 km of the crust, we used a tomographic inversion procedure to estimate the 2-D distribution of upper crustal P- and S-wave velocities within blocks of a refracting velocity grid, located beneath a near surface velocity layer of uniform thickness. The P-wave velocity distribution mapped the subsurface location of several terranes in the southwest Cordillera. Volcanic rocks of Wrangellia terrane, which is the principal component of the Insular Belt, were characterized by upper crustal velocities 〉 6.3 km s −1, whereas plutonic rocks of the Coast Belt had velocities 〈 6.3 km s−1. East of southern Vancouver Island, the velocity distribution indicated that Wrangellia extends in the subsurface beneath the Coast Belt for about 130 km eastward of the surface location of the Insular/Coast Belt boundary. A portion of the southern boundary of Wrangellia was also delineated by the P-velocity map, which suggested that metamorphic rocks of the Northwest Cascades Thrust System may extend beneath sedimentary cover as far west as south-central Georgia Strait. Inverted S-wave velocities were typically 3.6–3.7 km s−1. Corresponding values of Poisson’s ratio were 0.27–0.28 over Vancouver Island and 0.20–0.24 in the Coast Belt. The higher values for Vancouver Island likely reflect the high mafic content of Wrangellia volcanics, compared to the high quartz content of the Coast Belt plutonics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/PL00001121
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    Paper (German National Licenses)
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  • 6
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    Distributed natural gas venting offshore along the Cascadia margin (2018)
    Nature Research
    In:  Nature Communications, 9 (3264).
    Details
    Publication Date: 2021-03-19
    Description: Widespread gas venting along the Cascadia margin is investigated from acoustic water column data and reveals a nonuniform regional distribution of over 1100 mapped acoustic flares. The highest number of flares occurs on the shelf, and the highest flare density is seen around the nutrition-rich outflow of the Juan de Fuca Strait. We determine similar to 430 flow-rates at similar to 340 individual flare locations along the margin with instantaneous in situ values ranging from similar to 6 mL min(-1) to similar to 18 L min(-1). Applying a tidal-modulation model, a depth-dependent methane density, and extrapolating these results across the margin using two normalization techniques yields a combined average in situ flow-rate of similar to 88 x 10(6) kg y(-1). The average methane flux-rate for the Cascadia margin is thus estimated to similar to 0.9 g y(-1) m(-2). Combined uncertainties result in a range of these values between 4.5 and 1800% of the estimated mean values.
    Type: Article , PeerReviewed
    Format: text
    Format: other
    Format: other
    Format: text
    Format: other
    Format: other
    Format: other
    DOI: 10.1038/s41467-018-05736-x
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Gas hydrate on the northern Cascadia margin: regional geophysics and structural framework (2006)
    Texas A & M University
    In:  Proceedings of the Integrated Ocean Drilling Program, 311 .
    Details
    Publication Date: 2020-06-08
    Description: Integrated Ocean Drilling Program Expedition 311 is based on extensive site survey data and historic research at the northern Cascadia margin since 1985. This research includes various regional geophysical surveys using a broad spectrum of seismic techniques, coring and logging by the Ocean Drilling Program Leg 146, heat flow measurements, shallow piston coring, and bottom video observations across a cold-vent field, as well as novel controlled-source electromagnetic and seafloor compliance surveying techniques. The wealth of data available allowed construction of structural cross-sections of the margin, development of models for the formation of gas hydrate in an accretionary prism, and estimation of gas hydrate and free gas concentrations. Expedition 311 established for the first time a transect of drill sites across the northern Cascadia margin to study the evolution of gas hydrate formation over the entire gas hydrate stability field of the accretionary complex. This paper reviews the tectonic framework at the northern Cascadia margin and summarizes the scientific studies that led to the drilling objectives of Expedition 311 Cascadia gas hydrate.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.2204/iodp.proc.311.109.2006
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Data report: A downhole electrical resistivity study of northern Cascadia marine gas hydrate (2008)
    Texas A & M University
    In:  Proceedings of the Integrated Ocean Drilling Program, 311 .
    Details
    Publication Date: 2020-06-08
    Description: Downhole electrical resistivity measurements can be exploited for gas hydrate concentration estimates. However, to do so requires that several assumptions be made, in particular about in situ pore water salinity and porosity. During Integrated Ocean Drilling Program Expedition 311, electrical resistivity was measured in four boreholes along a transect across the northern Cascadia margin, offshore Vancouver Island, Canada. Logging-while-rilling and conventional wireline logging data of resistivity, density, and neutron-porosity are used together with measurements of porosity and pore water salinity of the recovered core to systematically estimate gas hydrate concentrations at these four sites. Using Ar- chie’s law, empirical parameters a and m are determined from gas hydrate–free zones by means of a Pickett plot. The in situ salinity baseline trend for each site was estimated directly from the measured pore water salinity values, as well as indirectly by calculating the trend using Archie’s law and simultaneously solving for gas hydrate saturation and in situ salinity. Results showed that the in situ salinities at Sites U1325, U1326, and U1329 were well determined from a smooth trend through the highest measured salinity values of the recovered core. Only Site U1327 exhibitsstrongly decreasing pore water salinity, reaching 22‰ at the base of the gas hydrate stability field. This regional low salinity requires special analyses to estimate gas hydrate concentration from resistivity and introduces a large uncertainty. It is suggested that the decreased salinity results largely (90%) from a deeper fresh water source with the remaining freshening being the result of dissociation of pervasive gas hydrate (~3% of the pore space). Considering estimates from density porosity to be the most accurate, gas hydrate saturations average ~9% ± 7% at Site U1326, ~10% ± 7% at Site U1325, and 11% ± 7% at Site U1327 over the entire range of gas hydrate occurrence. No significant gas hydrate is inferred at Site U1329, although small am ounts may be present just above the bottom-simulating reflector. In two localized zones at Site U1326 (60–100 mbsf [meters below seafloor]) and Site U1327 (120–150 mbsf), significantly higher gas hydrate concentrations of 〉30% of the pore space were encountered.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.2204/iodp.proc.311.203.2008
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    Fluid flow and origin of a carbonate mound offshore Vancouver Island: Seismic and heat flow constraints (2007)
    Elsevier
    In:  Marine Geology, 239 (1-2). pp. 83-98.
    Details
    Publication Date: 2015-11-18
    Description: A 1.5 km long, 1 km wide and 70–80 m high carbonate mound was identified on the mid-slope region of the subduction accretionary sedimentary prism offshore Vancouver Island ∼ 3.5 km west of Ocean Drilling Program (ODP) Site 889 and Integrated Ocean Drilling Program (IODP) Site U1327. Seabed-video images show the presence of seafloor carbonate as well as chemosynthetic communities. A high-resolution single channel seismic survey with close line spacing, recording coherent reflectivity down to about 400 m beneath the seafloor, provided acoustic images of this mound and of the gas hydrate bottom-simulating reflector (BSR) beneath it. The mound is interpreted to have developed as a structural topographic high in the hanging wall of a large reverse fault formed at the base of the current seaward slope. The fault zone provides pathways for fluids including gas to migrate to the seafloor where diagenetic carbonate forms and cements the near-surface sediments. To examine the thermal effect of possible upward fluid flow beneath the mound, heat flow at the mound and in the neighbouring region was calculated from the depth of the BSR below the seafloor. These data were combined with heat flow calculated over a broader region from previous multi-channel seismic data. Heat flow within the flattest portion of the surrounding 4 km by 8 km region averages ∼ 74 mW/m2. Taking this value to represent the regional or background heat flow, a simple 2D analytical method was used to calculate theoretical heat flow variations due to topography. Across the mound, most of the variability is explained by topographic effects, including a local 6 mW/m2 negative anomaly over the central mound and a large 20 mW/m2 positive anomaly over the mound steep side slope. However, just south of the mound, there is a 6–7 mW/m2 positive anomaly in a 2-km-long band that has predominantly flat seafloor. Most of this anomaly is probably unrelated to topographic effects, but rather likely due to warm upward fluid flow along faults or fracture zones.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1016/j.margeo.2007.01.002
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 10
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    Bayesian inversion of marine controlled source electromagnetic data offshore Vancouver Island, Canada (2016)
    Oxford Univ. Press
    In:  Geophysical Journal International, 204 (1). pp. 21-38.
    Details
    Publication Date: 2019-02-01
    Description: This paper applies nonlinear Bayesian inversion to marine controlled source electromagnetic (CSEM) data collected near two sites of the Integrated Ocean Drilling Program (IODP) Expedition 311 on the northern Cascadia Margin to investigate subseafloor resistivity structure related to gas hydrate deposits and cold vents. The Cascadia margin, off the west coast of Vancouver Island, Canada, has a large accretionary prism where sediments are under pressure due to convergent plate boundary tectonics. Gas hydrate deposits and cold vent structures have previously been investigated by various geophysical methods and seabed drilling. Here, we invert time-domain CSEM data collected at Sites U1328 and U1329 of IODP Expedition 311 using Bayesian methods to derive subsurface resistivity model parameters and uncertainties. The Bayesian information criterion is applied to determine the amount of structure (number of layers in a depth-dependent model) that can be resolved by the data. The parameter space is sampled with the Metropolis–Hastings algorithm in principal-component space, utilizing parallel tempering to ensure wider and efficient sampling and convergence. Nonlinear inversion allows analysis of uncertain acquisition parameters such as time delays between receiver and transmitter clocks as well as input electrical current amplitude. Marginalizing over these instrument parameters in the inversion accounts for their contribution to the geophysical model uncertainties. One-dimensional inversion of time-domain CSEM data collected at measurement sites along a survey line allows interpretation of the subsurface resistivity structure. The data sets can be generally explained by models with 1 to 3 layers. Inversion results at U1329, at the landward edge of the gas hydrate stability zone, indicate a sediment unconformity as well as potential cold vents which were previously unknown. The resistivities generally increase upslope due to sediment erosion along the slope. Inversion results at U1328 on the middle slope suggest several vent systems close to Bullseye vent in agreement with ongoing interdisciplinary observations.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1093/gji/ggv437
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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