Lithosphere Structure of Canada: Constraints From Ambient Seismic Noise Tomography

English Abstract:

The continental lithosphere of Canada contains a record of the events that have shaped its formation and evolution over the last 4 Gyr. In this study, we provide an updated assessment of the crust and uppermost mantle structure of Canada using shear wave velocities (Vs) from a recent continental-scale ambient noise tomography model (Kao et al., J. Geophys. Res., v. 118, p. 5865-5887, 2013). The model is derived from Rayleigh waves recorded from 2003 to 2009 at 788 seismic stations throughout Canada and adjacent regions. The more uniform data coverage and higher frequency content of ambient noise provides better resolution of crustal structure than images based on earthquake waveforms. The ambient noise tomography model shows both regional and local spatial variations in Vs, with generally high crustal and mantle velocities in the Canadian Shield and lower velocities in surrounding areas, especially in the Cordillera. In many areas, the Moho is characterized by a velocity increase over ~2 km depth, but in some places (e.g., the southern Western Canada Sedimentary Basin, WCSB), a 6-8 km thick velocity gradation is observed, suggesting that this may be a zone of mixing between crustal and mantle rocks or an area where the lowermost crust is partially eclogitized. Moho depth varies between 30 and 55 km, with the thinnest crust below the Cordillera and thickest crust below the southern WCSB. Within the Canadian Shield and Cordillera, the Moho depth exhibits significant relief over 100-500 km spatial scales, which is not reflected in the surface topography. Such Moho relief may be compensated by lateral variations in crust/mantle density structure. We are currently inverting the Vs model for lithosphere thermal and density structure to examine the implications for lithosphere composition, strength and surface topography.