Lithospheric assembly and modification of the SE Canadian Shield: Abitibi-Grenville teleseismic experiment

This paper presents the results of a joint Lithoprobe-Incorporated Research Institutions for Seismology (IRIS)/Program for Array Seismic Studies of th e Continental Lithosphere (PASSCAL) teleseismic experiment that investigate s portions of the Grenville and Superior Provinces of the Canadian Shield a long the Quebec-Ontario border. Data from a 600-km-long, N-S array of 28 br oadband seismographs deployed between May and October 1996 have been supple mented with additional recordings from an earlier 1994 deployment and from stations of the Canadian National Seismograph Network and the Southern Onta rio Seismic Network. Relative delay times of P and S waves from 123 and 40 teleseismic events, respectively, have been inverted for velocity perturbat ions in the upper mantle and reveal a low-velocity, NW-SE striking corridor that crosses the southern portion of the line at latitude 46 degrees N and lies between 50 and 300 km depth. Multievent SKS-splitting results yield a n average delay time of 0.57+/-0.22 s and a direction of fast polarization of N93 degrees E+/-18 degrees,.which is consistent with an earlier interpre tation as being due to fossil strain fields related to the last major regio nal tectonic event. Subtle variations in splitting parameters over the low velocity corridor may suggest an associated disruption in mantle fabric. Pr ofiling 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 degrees N and 46 degrees N, which is followed by a n abrupt thinning to 35 km at 46.6 degrees N, some 65 km southeast of the G renville Front. This structure is interpreted as a subduction suture extend ing the full length of the Front and punctuating a major pre-Grenvillian (A rchean-Proterozoic) episode of lithospheric assembly in the southeast Canad ian Shield. The low-velocity mantle corridor, by contrast, is better explai ned as the extension of the Monteregian-White Mountain-New England seamount hotspot track below the craton and is here postulated to represent interac tion of the Great Meteor plume with zones of weakness within the craton dev eloped during earlier rifting episodes.