This paper presents the results of a joint Lithoprobe-Incorporated Research Institutions for Seismology (IRIS)/Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) teleseismic experiment that investigates portions of the Grenville and Superior Provinces of the Canadian Shield along the Qu¿bec-Ontario border. Data from a 600-km-long, N-S array of 28 broadband seismographs deployed between May and October 1996 have been supplemented with additional recordings from an earlier 1994 deployment and from stations of the Canadian National Seismograph Network and the Southern Ontario Seismic Network. Relative delay times of P and S waves from 123 and 40 teleseismic events, respectively, have been inverted for velocity perturbations in the upper mantle and reveal a low-velocity, NW-SE striking corridor that crosses the southern portion of the line at latitude 46 ¿N and lies between 50 and 300 km depth. Multievent SKS-splitting results yield an average delay time of 0.57¿0.22 s and a direction of fast polarization of N93 ¿E¿18¿, which is consistent with an earlier interpretation as being due to fossil strain fields related to the last major regional tectonic event. Subtle variations in splitting parameters over the low-velocity corridor may suggest an associated disruption in mantle fabric. Profiling of radial receiver functions reveals large and abrupt variations in Moho topography, specifically, a gradual thickening in crust from 40 to 45 km between latitudes 45 ¿N and 46 ¿N, which is followed by an abrupt thinning to 35 km at 46.6 ¿N, some 65 km southeast of the Grenville Front. This structure is interpreted as a subduction suture extending the full length of the Front and punctuating a major pre-Grenvillian (Archean-Proterozoic) episode of lithospheric assembly in the southeast Canadian Shield. The low-velocity mantle corridor, by contrast, is better explained as the extension of the Monteregian-White Mountain-New England seamount hotspot track below the craton and is here postulated to represent interaction of the Great Meteor plume with zones of weakness within the craton developed during earlier rifting episodes. ¿ 2000 American Geophysical Union