UBC Theses and Dissertations
Subduction beneath the Queen Charlotte Islands? : the results of a seismic refraction survey Mackie, David
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.
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