|Title||Focused fluid flow along the Nootka Fault Zone and continental slope, Explorer-Juan de Fuca plate boundary|
|Author||Riedel, M; Rohr, K M M; Spence, G D; Kelley, D; Delaney, J; Lapham, L; Pohlman,
J W; Hyndman, R D; Willoughby, E C|
|Source||Geochemistry, Geophysics, Geosystems (G3) vol. 21, issue 8, e2020GC009095, 2020 p. 1-26, https://doi.org/10.1029/2020GC009095 Open Access|
|Alt Series||Natural Resources Canada, Contribution Series 20200324|
|Media||paper; on-line; digital|
|File format||pdf; html|
|Province||British Columbia; Western offshore region|
|Area||Pacific Ocean; Vancouver Island; Canada; United States of America|
|Lat/Long WENS||-128.7500 -127.3333 49.5833 48.8333|
|Subjects||marine geology; surficial geology/geomorphology; tectonics; geophysics; Science and Technology; Nature and Environment; tectonic evolution; plate margins; fluid flow; bedrock geology; structural
features; fault zones; submarine features; magmatism; intrusions; basalts; heat flow; hydrothermal systems; submarine hydrothermal vents; continental margins; continental slope; geophysical surveys; seismic surveys, marine; seismic reflection
surveys; photography; seismicity; earthquake risk; seismic risk; earthquake magnitudes; basins; deformation; marine sediments; marine sediment cores; pore fluids; hydrate; methane; ethane; biota; bedrock topography; bathymetry; abyssal plains; Nootka
Fault Zone; Explorer Plate; Juan de Fuca Plate; Maquinna Mound; Haggis Mound; gas hydrates|
|Illustrations||location maps; seismic profiles; geoscientific sketch maps; profiles; plots; models|
|Program||Public Safety Geoscience Assessing Earthquake Geohazards|
|Released||2020 08 07|
|Abstract||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 scientific 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, non-conformable high amplitude reflective zones. Measurements in the water column above one mound revealed a plume of warm, turbid 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.
|Summary||(Plain Language Summary, not published)|
We used geoscientific data from the Nootka fault zone, the most seismically active region off Vancouver Island, to study fault-related fluid flow.
Seismicity down to the upper mantle is linked to near-seafloor fluid flow through faults. Along the two main faults, seismically imaged bright spots lie above abrupt changes in basement topography. The bright spots indicate free gas in the sediments.
Two seafloor mounds ~15 km west of the Nootka slope were studied with a variety of techniques. Seismic data show high reflective zones up to 600 m deep beneath the mounds. Water column data revealed expulsion of warm water above one mound, and
bottom-videos showed hydrothermal vent biota. Pore fluids contain microbial methane with a high proportion of ethane. We infer the reflective zones beneath the mounds are intrusions creating hydrothermal circulation within the overlying sediments.
Across the Nootka slope, gas hydrate related bottom-simulating reflectors indicate upward fluid flow.