Seismic structure of the upper mantle beneath the northern Canadian Cordillera from teleseismic travel-time inversion

Teleseismic tomography is used to image upper-mantle structure beneath the northern Canadian Cordillera, with the objective of determining the physical state of the upper mantle along the western half of the SNORCLE, LITHOPROBE transect. The resulting three-dimensional P-wave velocity model resolves structure beneath the southern Yukon and northernmost British Columbia between 100 and 600 km depth. Two significant anomalies are identified. The first, a relatively shallow high-velocity feature located at the western edge of the model, is interpreted as being the edge of the Pacific slab from the southern Alaska subduction zone. The second is a large, tabular low-velocity anomaly centred at 60 degrees N by 136 degrees W, elongate northwest-southeast, dipping southwest, and reaching a depth of 450-500 km. This low-velocity anomaly is judged to reflect a thermal anomaly of 100-200 degrees C, with a possible compositional component. Its northeastern boundary is particularly sharp and is interpreted to represent the boundary between thin Cordilleran lithosphere and a colder cratonic mantle root. Our preferred explanation for the low-velocity feature is a thermal anomaly resulting from the advective upflow produced by the opening of a slab window beneath the northern Cordillera. A possible alternative is dense downward-percolating partial melt generated by sub-orogenic heating, as past work [e.g. Stolper et al., J. Geophys. Res. 86 (1981) 6261-6271] suggests that basic partial melts may be denser than peridotite below 200 km depth. (C) 1998 Elsevier Science B.V. All rights reserved.