The Seismic structure of Tofino Basin and underlying accreted terranes

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dc.contributor.authorLangton, Stewart Gordonen_US
dc.date.accessioned2024-08-14T20:59:43Z
dc.date.available2024-08-14T20:59:43Z
dc.date.copyright1995en_US
dc.date.issued1995
dc.degree.departmentDepartment of Physics and Astronomy
dc.degree.levelMaster of Science M.Sc.en
dc.description.abstractTofino Basin, located west of Vancouver Island, is a sedimentary basin overlying the Cascadia subduction zone. Convergence of the Juan de Fuca plate with the North American plate has resulted in an accretionary prism formed from sediments scraped from the subducting Juan de Fuca plate. The sediments have been trapped against a backstop of two allochthonous terranes, the Eocene Crescent Terrane and the Mesozoic Pacific Rim Terrane. Multichannel seismic reflection data, traversing Tofino Basin and extending into Cascadia Basin beyond the continental shelf, were acquired in 1985 and 1989 by the Geological Survey of Canada. Six of these seismic profiles, along with previously collected geological and magnetic field data, were used to study the 3-D structure and development of Tofino Basin in relation to the accretionary prism and accreted terranes. According to critical wedge theory, vertical growth of an accretionary prism occurs if the dip of the subducting oceanic plate is less than a critical angle of 11°. The seismic data indicate that the dip of the oceanic crust increases landward from 3° near the shelf edge, to 15° at the eastern limit of Tofino Basin, near Vancouver Island. The critical angle of 11° is reached at the western limit of Tofino Basin near the shelf-slope transition. West of this point, growth and uplift of the accretionary wedge continues, with associated entrapment of overlying sediments and increase in lateral size of Tofino Basin. The lower boundary of the Crescent Terrane was clearly imaged on many seismic sections. The Crescent Terrane was observed to terminate against the subducting oceanic plate, indicating that little or no sediments are being eroded from the accretionary prism. The thickness of Crescent Terrane to the west of Vancouver Island is at least 6-9 km. This supports a model whereby a sliver of oceanic crust was detached from the top of the subducting oceanic plate. However, the very thick Crescent Terrane on the Olympic Peninsula supports a second model in which the complete lithosphere was faulted and underthrusting of the oceanic plate stepped seaward. In the southeast portion of Tofino Basin, a linear trench parallel to Vancouver Island contains up to 3500 m of sediments, the thickest accumulation of sediments in the basin. This feature is interpreted as a fossil trench, a relic of past subduction of the Crescent Terrane beneath the Pacific Rim Terrane. The northern limit of the fossil trench occurs near the Zeus Structure, where the thickness of Tofino Basin sediments overlying the Crescent Terrane is less than 500 m. This suggests that after the time of emplacement, the Crescent Terrane was an exposed island, with water depths increasing to the south into the fossil trench. Growth of the accretionary prism and uplift of the Zeus Structure have resulted in variable motion and erosion of the Crescent Terrane which constrained the thickness of Tofino Basin sediments. Deformation of Tofino Basin sediments is minimal over the Crescent Terrane which acts as a stable buttress , restricting deformation. Deformation increases toward the outer shelf, over the accretionary prism. Over the fossil trench, possible motion along the Tofino Fault that separates the Crescent and Pacific Rim terranes may have influenced the greater amount of deformation of Tofino Basin sediments observed in the seismic profiles.
dc.format.extent98 pages
dc.identifier.urihttps://hdl.handle.net/1828/18536
dc.rightsAvailable to the World Wide Weben_US
dc.titleThe Seismic structure of Tofino Basin and underlying accreted terranesen_US
dc.typeThesisen_US

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