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UBC Theses and Dissertations

Crustal velocity structure in the southern coast belt, British Columbia, from a seismic refraction survey O'Leary, Deirdre


The Coast Belt is one of five morphogeological belts of the Canadian Cordillera. It was created through the complex tectonic processes that accommodated the Mesozoic accretion of the allochthonous Insular superterrane to the Intermontane superterrane, then the leading edge of western North America, and the subsequent overprinting of the suture zone bygranitic intrusions associated with east-dipping subduction. As part of the LITHOPROBE Southern Cordillera transect, seismic refraction data were recorded along a 350 km long strike profile in the Coast Belt. An iterative combination of two-dimensional travel time inversion and amplitude forward modelling was used to interpret crust and upper mantleP-wave velocity structure. The structural model features a thin (0.5 to 3.0 kin) near-surface layer with an average velocity of 4.40 km/s, a significant vertical velocity gradient and lateral variations. This uppermost stratum overlies a crustal velocity structure with three layers of approximately 10.0 km each. The upper and middle crust have average velocities of 6.20 and6.30 km/s, respectively. The lower crust consists of an upper and lower unit with average velocities of 6.50 and 6.65 km/s, respectively. Beneath the lower crust lies Moho, the crust-mantle boundary, which is modelled as an approximately 2 km thick transitional layer with an average depth to its upper boundary of 34.5 km and a maximum depth of 35.5 km in the southeast. The transitional layer exhibits lateral velocity variations (7.40 to 7.80 km/s) and overlies an upper mantle which is poorly defined in terms of velocity, although the data indicate relatively high values (> 8.05 km/s). Interpretations of LITHOPROBE reflection data and other refraction lines which cross,or nearly cross, the Coast Belt profile suggest that this profile is within the region of the collision zone between the Insular and Intermontane superterranes. These interpretations indicate two models for the collision zone: (i) a crustal delamination model in which the Insular superterrane was displaced along east-vergent faults over the terranes below and (ii) a crustal imbrication model in which imbrication of Insular and Intermontane rocks occurs throughout the crust. The latter model involves the presence of thick layers of Insular material beneath the Coast Belt refraction profile. However, the velocity model indicates predominantly Intermontane material, thereby favoring the crustal delamination model and the eastward extent of the Insular Belt being west of the refraction profile. From comparisons of the refraction velocity model with the reflection data, the top of the Moho transition zone corresponds to the top of a prominent band of reflectivity which constitutes the reflection Moho. When combined with the seismic reflection as well as other geophysical studies, three likely sources for wide-angle reflections from the upper, middle, and lower crust, as observed in the refraction data, are structural features (e.g. fault zones), Ethological contrasts (e.g. batholithic granites to accreted volcanics) and the transition zones at the top and bottom of a region of layered porosity in the crust.

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