S-wave velocity structure of the Canadian upper mantle

We investigate mantle structure beneath the Canadian landmass using 500 sur face-wave waveforms recorded at stations of the Canadian National Seismic N etwork 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 stu dies 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 i s well resolved to a depth of 400 km over most of the region and displays t wo prominent, large-scale anomalies: a high-velocity structure beneath the Canadian shield and platform associated with the cratonic keel, and pronoun ced low velocities beneath the Cordillera. The contrast between these featu res is strong, up to 10%, and, at more southerly latitudes, is sharp, with the transition occurring over distances of 600 km or less. Laterally cohere nt, high velocities persist to depths of ca. 250 km beneath the North Ameri can 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. Moderat ely low velocities beneath the St, Lawrence valley region and Labrador may be related to intracontinental volcanism and rifting, respectively. (C) 200 1 Elsevier Science B.V. All rights reserved.