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UBC Theses and Dissertations

A teleseismic study of the Northern Cordilleran upper mantle beneath the SNORCLE transect Frederiksen, Andrew William


The study area of the SNORCLE Lithoprobe transect comprises the northern Canadian Cordillera and the northwestern Canadian shield. An array of five portable broadband seismographs has been deployed along the trend of the transect, to complement five permanent stations of the Canadian National Seismic Network and seven Alaskan short-period instruments. The objective of the experiment is to examine the physical state of the upper mantle along the transect. P-wave travel-time residuals up to 2 seconds have been measured, and analyzed using a non-linear tomographic technique, thereby recovering velocity structure between 100 and 600 km depth for the western portion of the transect. Two significant P-wave mantle velocity anomalies have been located. The first, a relatively shallow high-velocity feature located at the western edge of the model, has been interpreted as being the edge of the Pacific slab from the southern Alaska subduction zone. The second is a large, tabular low-velocity anomaly centered at 60°N by 136°W, elongate northwest-southwest, dipping southeast, and reaching a depth of 450-500 km. This low-velocity anomaly is judged to reflect a thermal anomaly of the order of 100°C, with a possible compositional component. Multiple interpretations of the low-velocity feature are considered, the two main hypotheses being a mantle plume or a flow feature related to the proximity of the subducting slab and the opening of the northern Cordilleran slab window. The latter hypothesis is favored, due to the absence of other evidence for a plume in this region. In addition, the upper part of the low-velocity anomaly may reflect the influence of strain heating at lithospheric levels, related to the convergence of the Pacific and North American plates and the uplift of the St. Elias Mountains.

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