NEW INSIGHTS INTO THE LITHOSPHERE BENEATH THE SUPERIOR PROVINCE FROM RAYLEIGH WAVE DISPERSION AND RECEIVER FUNCTION ANALYSIS

Abstract

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 ∼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 ∼25 to ∼170 s, allowing us to resolve shear wave velocities to a depth of ∼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 (∼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 ∼100 to ∼220 km.