| Summary: | The thick and seismically fast Precambrian continental remnants (cratons) provide fundamental clues about the tectonic processes that operated on the early Earth. Eastern Canada is a natural laboratory to study such processes: its geological record spans more than 3 Ga of Earth history, including the assembly of the largest Archean craton in the world, the Superior craton, which is surrounded by global scale Proterozoic and Phanerozoic orogenic belts. To investigate the crustal and lithospheric structure of eastern Canada, earthquake data recorded at a new broadband seismic network were analysed, in conjunction with other permanent and temporary networks. The QM-III (Quebec-Maine Across Three Sutures) network was deployed across the main tectonic boundaries in eastern Canada, extending from Hudson Bay to the Atlantic Ocean. Using Hk stacking and probabilistic inversion of receiver functions, bulk crustal composition (Vp/Vs ratio), crustal thickness and shear wavespeed (Vs) were estimated beneath seismic stations. Post- Archean crust is thicker ( ~40 km), faster (dVs ~ 0.2 km/s), more heterogenous and more ma c (Vp/Vs ~1.76), suggesting increased crustal growth efficiency, possibly stimulated by mafic underplating. Lack of correlation between Moho topography, elevation and gravity anomalies in Proterozoic terranes indicate isostatic imbalance, best explained by strong mantle buoyant support. An anisotropic seismic model of the Precambrian lithosphere was constructed using fundamental mode Rayleigh waves. Phase velocity heterogeneity and azimuthal anisotropy patterns reveal multiple lithospheric layers within the Superior craton, with distinct tectonic origins. The upper lithosphere is seismically fast ( ~2%) and preserves Archean fossil anisotropy ( ~1%), implying that plate-scale deformation occured during the Archean. This layer partially extends beneath the adjacent Proterozoic belt and survived subsequent metasomatism. The lower lithosphere is fast ( ~2%), more homogenous and weakly anisotropic ( < 0.5%), documenting post assembly lithospheric growth in a slow convection regime. Cratonization processes may be episodic and are not exclusively an Archean phenomenon.
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