New insights into the lithosphere beneath the Superior Province from Rayleigh wave dispersion and receiver function analysis

We present new models of shear wave velocity structure of the lithosphere and upper mantle beneath northern and eastern Ontario and surrounding regions. The study area is dominated by the Archean Superior Province, with Proterozoic orogenic belts to the south and southeast. Over the course of similar to 3 Ga, the region has been shaped by accretionary and orogenic events, periods of rifting and the influence of a number of mantle hotspots. New data from the broad-band POLARIS/FedNor seismic network, along with permanent stations of the Canadian National Seismograph Network, are used for a seismic study of the crust and upper mantle beneath the region. This article concentrates primarily on a study of the upper mantle, carried out using two-station phase velocity analysis of teleseismic Rayleigh waves. Dispersion curves were measured for 100 two-station paths across the region, of which 30 were analysed to give 1-D path-averaged models of shear wave velocity structure. Dispersion measurements yielded phase velocity data for periods from similar to 25 to similar to 170 s, allowing us to resolve shear wave velocities to a depth of similar to 300 km. The dispersion curves indicate an upper-mantle structure broadly characteristic of continental shield regions, but there are significant variations in the properties of the data across the province. In the central and western Superior Province, inferred lithospheric thicknesses vary between approximately 140 and 200 km. In general, the models are characterized by a well-developed 'lid' of high-velocity mantle underlain by a zone of reduced velocity. The highest lid velocities are modelled along a path whose azimuth corresponds to the fast direction of anisotropy resolved through SKS splitting analysis. The far northeast of Ontario has the thickest (similar to 220-240 km) lithospheric lid. Eastern Ontario has a complex structure that gives rise to a large variation in the structures modelled from the two-station paths. Apparent lithospheric thicknesses range from similar to 100 to similar to 220 km. This large variation is interpreted to arise from highly heterogeneous mantle structure and/or anisotropy across the Abitibi Greenstone Belt, the Southern Province, and the Proterozoic Grenville Province. Receiver function analysis shows variations in Moho depth and bulk Poisson's Ratio in the Superior Province crust. The crustal thickness varies from similar to 34 to similar to 44 km through most of the province, with a zone of anomalously thick crust (48 km) in the Kapuskasing Structural Zone region. Measurements of bulk Poisson's Ratio indicate that the crust is highly felsic in some parts of the province, but includes a significant mafic component in parts of the western Superior and eastern Ontario, regions that have been affected by uplift or rifting. The new results, together with information from other recent studies of shear wave splitting and body-wave tomography, show that the Superior craton is far from a homogeneous entity. Rather it is divisible, on the basis of thickness, anisotropy and velocity structure of the crust and lithosphere, into distinct subdomains that reflect the complex tectonic history of this region. The large range of inferred thicknesses of the crust (34-48 km) and lithosphere (100-240 km) within the Superior craton represents a significantly greater variability than has been previously observed for Archean lithosphere.