S-wave velocity structure of the Canadian upper mantle

We investigate mantle structure beneath the Canadian landmass using 500 surface-wave waveforms recorded at stations of the Canadian National Seismic Network and Global Seismic Network. The data are processed using the method of partitioned waveform inversion and afford a comprehensive sampling of S-velocity structure beneath the North American craton and adjacent orogenic belts. Our study thereby extends results from previous regional, S-wave studies north of the coterminous US, and provides a characterization of mantle structure beneath Canada which is complementary to the body-wave model of grand [J. Geophys. Res. 99 (1994) 11591]. The resulting 3D velocity model is well resolved to a depth of 400 km over most of the region and displays two prominent, large-scale anomalies: a high-velocity structure beneath the Canadian shield and platform associated with the cratonic keel, and pronounced low velocities beneath the Cordillera. The contrast between these features is strong, up to 10%, and, at more southerly latitudes, is sharp, with the transition occurring over distances of 600 km or less. Laterally coherent, high velocities persist to depths of ca. 250 km beneath the North American craton which we interpret to represent the base of the continental keel. Smaller-scale variations are observed within the cratonic mantle but, in general, show little correlation with surface geological provinces. Moderately low velocities beneath the St, Lawrence valley region and Labrador may be related to intracontinental volcanism and rifting, respectively. (C) 2001 Elsevier Science B.V. All rights reserved.