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The active margin between the continental North American plate and oceanic Pacific plate west of the Queen Charlotte Islands was the site of an extensive onshore–offshore seismic refraction project in 1983. An airgun line shot over two ocean-bottom seismographs (OBS's) and a 32-charge explosion line recorded on the two OBS's and eight land-based seismographs (LBS's) deployed across northern Moresby Island were selected to study the structure of the predominantly transform Queen Charlotte Fault Zone and the associated offshore terrace. Two-dimensional ray tracing and synthetic seismogram modelling produced a pronounced laterally varying velocity structural model showing three major crustal components (oceanic, terrace, and continental) separated by an outer, crustally pervasive fault and active Queen Charlotte Fault, respectively. The 3 km thick block-faulted upper terrace unit, overlain by deformed sediments, is indistinguishable from adjacent oceanic sediments and upper crustal basalts located to the west. The upper part of the 10–17 km thick lower terrace unit has anomalously low velocities relative to the adjacent oceanic and continental crustal units. A high gradient increases terrace velocity rapidly with depth until the contrast becomes negligible at approximately 17 km depth. Changes in depth to Moho beneath the terrace suggest an increase in eastward Moho dip from 2–5 °observed west of the terrace to 19 °below it. Tectonic mechanisms proposed to explain the anomalous terrace structure involve sediment accretion during subduction of oceanic lithosphere, alternating or combined with compressive upthrusting of material along near-vertical fault planes during periods of active transform motion.
The active margin between the continental North American plate and oceanic Pacific plate west of the Queen Charlotte Islands was the site of an extensive onshore–offshore seismic refraction project in 1983. An airgun line shot over two ocean-bottom seismographs (OBS's) and a 32-charge explosion line recorded on the two OBS's and eight land-based seismographs (LBS's) deployed across northern Moresby Island were selected to study the structure of the predominantly transform Queen Charlotte Fault Zone and the associated offshore terrace. Two-dimensional ray tracing and synthetic seismogram modelling produced a pronounced laterally varying velocity structural model showing three major crustal components (oceanic, terrace, and continental) separated by an outer, crustally pervasive fault and active Queen Charlotte Fault, respectively. The 3 km thick block-faulted upper terrace unit, overlain by deformed sediments, is indistinguishable from adjacent oceanic sediments and upper crustal basalts located to the west. The upper part of the 10–17 km thick lower terrace unit has anomalously low velocities relative to the adjacent oceanic and continental crustal units. A high gradient increases terrace velocity rapidly with depth until the contrast becomes negligible at approximately 17 km depth. Changes in depth to Moho beneath the terrace suggest an increase in eastward Moho dip from 2–5 °observed west of the terrace to 19 °below it. Tectonic mechanisms proposed to explain the anomalous terrace structure involve sediment accretion during subduction of oceanic lithosphere, alternating or combined with compressive upthrusting of material along near-vertical fault planes during periods of active transform motion.
A combined multichannel seismic reflection and refraction survey was carried out in July 1988 to study the Tertiary sedimentary basin architecture and formation and to define the crustal structure and associated plate interactions in the Queen Charlotte Islands region. Simultaneously with the collection of the multichannel reflection data, refractions and wide-angle reflections from the airgun array shots were recorded on single-channel seismographs distributed on land around Hecate Strait and Queen Charlotte Sound. For this paper a subset of the resulting data set was chosen to study the crustal structure in Queen Charlotte Sound and the nearby subduction zone.Two-dimensional ray tracing and synthetic seismogram modelling produced a velocity structure model in Queen Charlotte Sound. On a margin-parallel line, Moho depth was modelled at 27 km off southern Moresby Island but only 23 km north of Vancouver Island. Excluding the approximately 5 km of the Tertiary sediments, the crust in the latter area is only about 18 km thick, suggesting substantial crustal thinning in Queen Charlotte Sound. Such thinning of the crust supports an extensional mechanism for the origin of the sedimentary basin. Deep crustal layers with velocities of more than 7 km/s were interpreted in the southern portion of Queen Charlotte Sound and beneath the continental margin. They could represent high-velocity material emplaced in the crust from earlier subduction episodes or mafic intrusion associated with the Tertiary volcanics.Seismic velocities of both sediment and upper crust layers are lower in the southern part of Queen Charlotte Sound than in the region near Moresby Island. Well velocity logs indicate a similar velocity variation. Gravity modelling along the survey line parallel to the margin provides additional constraints on the structure. The data require lower densities in the sediment and upper crust of southern Queen Charlotte Sound. The low-velocity, low-density sediments in the south correspond to high-porosity marine sediments found in wells in that region and contrast with lower porosity nonmarine sediments in wells farther north.
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