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TitleThe northern Cascadia Subduction Zone At Vancouver Island: Seismic Structure and Tectonic History
AuthorHyndman, R D; Yorath, C J; Clowes, R M; Davis, E E
SourceCanadian Journal of Earth Sciences vol. 27, no. 3, 1990 p. 313-329, Open Access logo Open Access
Alt SeriesLithoprobe Publication 57
Alt SeriesGeological Survey of Canada, Contribution Series 13689
PublisherCanadian Science Publishing
Mediapaper; on-line; digital
File formatpdf
ProvinceBritish Columbia; Western offshore region
NTS92B; 92C; 92D; 92E/SW; 92F/SW; 92E/SE; 92F/SE
AreaVancouver Island; Juan De Fuca Strait
Lat/Long WENS-127.0000 -123.0000 49.5000 48.0000
Subjectsstructural geology; tectonics; geophysics; tectonic history; subduction; subduction zones; oceanic crust; seismic interpretations; seismic reflection surveys; seismic profiles; seismic surveys, marine; seismic refraction surveys; seismic surveys; magnetic surveys, marine; magnetic interpretations; magnetic surveys; gravity surveys; geophysical surveys; seismicity; magnetotelluric data; accretion; heat flow; geothermal surveys; continental shelf; bathymetry; magnetic anomalies; gravity anomalies; modelling; faults; faults, thrust; folds; structural features; Cascadia Subduction Zone; Pacific Rim Terrane; Crescent Terrane; Tofino Basin; Crescent Formation; LITHOPROBE; Quaternary
Illustrationssketch maps; seismic profiles; cross-sections
AbstractThe structure and Tertiary tectonic history of the northern Cascadia subduction zone have been delineated by a series of new multichannel seismic lines acquired across the continental shelf to the deep sea, combined with adjacent land multichannel seismic data and results from a wide range of other geophysical and geological studies. The top of the downgoing oceanic crust is imaged for a remarkable distance downdip from the deep ocean basin to a depth of 40 km beneath Vancouver Island. The reflection depths are in good agreement with seismic refraction models and Benioff-Wadati seismicity. Two broad reflective bands imaged as dipping gently landward at depths of about 15 and 30 km on the land lines merge to a single reflector band offshore. They may represent underplated oceanic material or, alternatively, they may not be structural but may be zones of contrasting physical properties, perhaps representing trapped fluid. Two narrow terranes, the Mesozoic marine sedimentary Pacific Rim Terrane and the Eocene marine volcanic Crescent Terrane, have been thrust beneath, and accreted to, the margin in the Eocene, about 42Ma, near the start of the present phase of subduction. They provide a landward-dipping backstop to the large sediment wedge accreted since that time. The deformation front is characterized by mainly landward-dipping thrust faults that cut close to basement. This result and the mass balance of the incoming sediment compared with that present in the accreted wedge suggest that there is little subduction of sediment into the mantle. The Tofino Basin sediments, up to 4 km in thickness, have been deposited on the continental shelf over the accreted terranes and the developing accretionary wedge.

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