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The Queen Charlotte transform fault zone, which lies immediately east of the Queen Charlotte Islands, marks the boundary between the oceanic Pacific and the continental North American plates. Relative plate motions suggest that oblique underthrusting of the Pacific plate beneath North America may be presently occurring along this transform fault. To investigate this plate boundary and the implications of oblique subduction on crustal structure beneath the region, an onshore-offshore seismic refraction survey was conducted in 1983. The survey was designed to sample the crust beneath the Queen Charlotte Islands and across Hecate Strait to the mainland of British Columbia. Six ocean bottom seismographs and 11 land based stations were deployed along a 200 km line extending from 20 km west of the Queen Charlotte Islands to the mainland. Thirteen 540 kg and twenty 60 kg explosive charges were detonated along a 110 km long east-west line in the ocean to the west of the receivers. The multiple shots recorded on multiple receivers, all along the same line, effectively reverses the profile over some of its length. The objective of this study is to provide a model of the deep crustal structure beneath the fault zone, the Queen Charlotte Islands, and Hecate Strait. An exemplary subset of the extensive data set was selected to meet this objective. Beneath the deep ocean the Moho dips at about 2° to the east. At the Queen Charlotte terrace, a 25 km wide zone immediately west of the active Queen Charlotte fault, the dip of the Moho increases to about 5°. The crust is about 12 km thick at the terrace and 18 km thick at the eastern edge of the Queen Charlotte Islands, and in excess of 30 km thick at the mainland. The terrace unit itself is divided into two units - an upper unit with low velocity (4.1 km/s) and high gradient (0.3 km/s/km) and a lower unit with a high velocity (6.5 km/s) and a low gradient (0.05 km/s/km). This model, while not definitive, supports the interpretation of oblique shallow underthrusting of the Pacific plate beneath the Queen Charlotte Islands. The upper terrace unit could represent a sedimentary accretionary wedge and the lower terrace unit - the subducting slab. A model in which compression across the Queen Charlotte transform fault zone is taken up by deformation of the Queen Charlotte Islands in the form of crustal shortening and thickening is not compatible with the thin crust beneath the islands and Hecate Strait. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

In northern British Columbia, the Sylvester Allochthon of the Slide Mountain terrane is the most inboard of Cordilleran suspect terranes, resting as a vast klippe upon miogeoclinal strata of the Cassiar Platform. The Sylvester is oceanic; it comprises gabbro, pillowed and massive basalt, banded chert, carbonate, argillite, ultramafics and minor arenite, which range in age from Late Devonian to Late Triassic. Internal structure in the Sylvester Allochthon is characterized as a stack of innumerable interleaved tectonic slices, bounded by subhorizontal, layer-parallel faults. These lithotectonic units are an order of magnitude smaller than the terrane itself and may consist of only a single or a few repeated rock types. The internal structure of the Sylvester is complex but not chaotic; small numbers of slices occur together in larger second-order packages which are also fault-bounded and lensoidal. However, tectonic juxtaposition of unrelated lithologies and older-over-younger faults are common. The "stratigraphy" of the Sylvester assemblage is thus tectonic. Sliver-bounding faulting within the Sylvester is known to have, at least in part, predated its post-Triassic, pre-mid Cretaceous emplacement. The Sylvester was emplaced onto North America as the roof thrust to a foreland-style duplex within underlying North American strata. vii viii The Sylvester Allochthon is the most inboard of accreted terranes, however it does not represent a simple marginal basin. New microfossil dating demonstrates that most rock types occur through the complete range of Sylvester ages. Coeval but depositionally incompatable lithologies must have accumulated in separate ocean floor paleoenvironments. Lithologies of the allochthon derive almost exclusively from layer 1, only the surface of oceanic crust. Thus, Sylvester slices are telescoped remnants detached from a vast area of ocean crust which ranged in age and width through the upper Paleozoic but which is now otherwise entirely consumed. Similarities of rock type, internal structure, age range, and regional tectonic setting have identified the Sylvester Allochthon as broadly correlative with a discontinuous series of terranes extending the length of the Cordillera. Together, these terranes may represent the remnants of what was once the late Paleozoic proto-Pacific ocean floor.

Strong evidence suggests that the Cascadia subduction zone, off the west coast of Canada and the United States, is strongly seismically-coupled and that a possible megathrust earthquake might occur in that area in the near future. A study of tsunami hazards along the Canadian west coast associated with such a hypothetical earthquake is presented in this report. Numerical simulations of tsunami generation and propagation have been carried out using three models based on shallow water wave theory. Three cases of ground motion representing the ruptures of different crustal segments in the area have been examined. Computed results provide information on tsunami arrival times and a general view of the wave height distribution. The outer coast of Vancouver Island was found to be the most strongly affected area. At the head of Alberni Inlet, wave amplitudes reached up to three times the source magnitude. Inside the Strait of Georgia, the wave heights are significant enough to receive closer attention, especially in low-lying areas. / Science, Faculty of / Physics and Astronomy, Department of / Graduate