Two-station Rayleigh wave tomography of Canada

Abstract

Seismic tomography is a common method used to study the lithosphere and can reveal evidence of past and present tectonic and geodynamic processes. In largely aseismic areas such as central and eastern Canada, single-station measurements of surface-wave dispersion will provide limited and uneven resolution; two-station measurements allow the use of a larger selection of earthquakes (i.e., extending to teleseismic events) while still preserving a regional-scale measurement. This work examines seismic velocity and anisotropy variations of the Canadian lithosphere using two-station-based surface-wave tomography, to obtain new insights into the formation history of the continent. A cross-correlation method was used to obtain new two-station dispersion curves (a total of 117) which were added to a dataset of previously published measurements. These were chosen to fill in gaps in ray coverage and to maximize azimuthal distribution, particularly across the Prairies and the Cordillera. Tomographic inversion was used to calculate anisotropic phase velocity maps for periods of 15-400 s, following outlier exclusion and resolution testing. The phase velocity maps show that the low velocity Cordilleran lithosphere contains two southern zones (west and east) with different anisotropy, which are interpreted to be due to the influence of the subducting Juan de Fuca plate in the west. A sharp Cordillera-craton boundary with the Canadian Shield is also noted, which shifts eastward near the US border. A very low velocity anomaly is observed beneath southeastern Saskatchewan which is likely linked to a mantle suture in this area. The highest velocities are seen beneath the craton, with particularly high velocities in Nunavut (northern Canada) and the western Superior Province. Strong E-W anisotropy is observed beneath the western Superior resulting from frozen lithospheric fabric. A clear distinction between the Precambrian Shield and the lower velocity Appalachian domains is also noted. This Appalachian low velocity zone is thought to be associated with the present-day Northern Appalachian Anomaly, rather than the Great Meteor Hotspot track.