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 t he physical state of the upper mantle along the western half of the SN ORCLE, LITHOPROBE transect. The resulting three-dimensional P-wave vel ocity model resolves structure beneath the southern Yukon and northern most British Columbia between 100 and 600 km depth. Two significant an omalies are identified. The first, a relatively shallow high-velocity feature located at the western edge of the model, is interpreted as be ing the edge of the Pacific slab from the southern Alaska subduction z one. The second is a large, tabular low-velocity anomaly centred at 60 degrees N by 136 degrees W, elongate northwest-southeast, dipping sou thwest, 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 po ssible compositional component. Its northeastern boundary is particula rly sharp and is interpreted to represent the boundary between thin Co rdilleran lithosphere and a colder cratonic mantle root. Our preferred explanation for the low-velocity feature is a thermal anomaly resulti ng from the advective upflow produced by the opening of a slab window beneath the northern Cordillera. A possible alternative is dense downw ard-percolating partial melt generated by sub-orogenic heating, as pas t work [e.g. Stolper et al., J. Geophys. Res. 86 (1981) 6261-6271] sug gests that basic partial melts may be denser than peridotite below 200 km depth. (C) 1998 Elsevier Science B.V. All rights reserved.