Yuan, Tianson2024-08-152024-08-1519901990https://hdl.handle.net/1828/20252The Queen Charlotte Islands region is located on the Canadian western margin near the triple junction between the Juan de Fuca ridge system, the Cascadia subduction zone, and the Queen Charlotte transform fault. The evolution and interactions of the continental and oceanic plates have played an important role in the structural development of the region. 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 region. Simultaneously with the collection of the multi­-channel reflection data, refractions and wide-angle reflec­tions from airgun array shots were recorded on single channel seismographs distributed on land around Hecate Strait and Queen Charlotte Sound. For this thesis a subset of the resulting data set was chosen to study the crustal structure in Queen Charlotte Sound and the adjacent subduction zone. Two-dimensional raytracing and synthetic seismogram modelling produced a well constrained velocity structure model across Queen Charlotte Sound. Moho depth is modelled at 27 km off southern Moresby Island but only 23 km north of Vancouver Island. Excluding the approximately 3 km of the Tertiary sediments, the crust in the latter area is less than 20 km thick, indicating substantial crustal thinning in Queen Charlotte Sound. Such thinning of the crust suggests an extensional mechanism for the origin of the sedimentary basin. On a margin-parallel line, in the southern portion of Queen Charlotte Sound a mid-crustal event with apparent velocity of more than 7.2 km/s was modelled as a high velocity sliver at a depth of about 17 km. On an unreversed refraction line normal to the continental margin, an upper crust layer with velocity more than 7 km/s also was interpreted at depths above about 13 km. The interpretation of these high velocity layers is uncertain, but they could represent high velocity material imbedded in the crust from earlier subduction episodes or mafic underplating associated with the Masset volcanics. Refraction 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 and multichannel reflection velocity analyses indicate a similar velocity contrast. Gravity models along the reflec­tion line require lower sediment and upper crust densities, consistent with the crustal thinning implied by refraction data. The low velocity/low density sediments correspond to high porosity marine sediments found in wells in the southern part of the region, and contrast with lower porosity non-marine sediments in wells further north. The contrast in upper crust velocity and density from north to south can be explained if the Mesozoic or Tertiary volcanics that appear to floor the basin are underlain by thick and dense volcanic sequences in the north, and by a predominantly sedimentary sequence in the south.121 pagesAvailable to the World Wide WebThesis