Variations in Lithospheric Thickness Across the Superior Province, Ontario, Canada: Evidence from Tomography and Shear-Wave Splitting

The Superior Province of the Canadian Shield is the largest contiguous region of Archean crust. It is made up for the most part of east-west trending belts of plutonic, metasedimentary, granite-greenstone, and gneissic rocks; these belts are interrupted by the northeast-trending Kapuskasing Structural Zone which cross-cut the subprovinces of the Superior at circa 2.6 Ga, representing a region of substantial uplift. The degree to which the crustal structure of the Superior Province is reflected in the mantle lithosphere is not well understood, as past seismic studies of the Superior were necessarily somewhat piecemeal in scope. With the advent of the FedNor seismic array, which provides broadly-spaced seismometer coverage over much of the Superior Province in Ontario, it is now possible to examine the lithosphere of a large portion of the Superior using passive seismic techniques. We combine datasets from the FedNor and CNSN arrays with data from previous temporary deployments in the region (the APT89, Abitibi and TW~ST experiments), and examine the mantle beneath the Superior using shear-wave splitting and travel-time tomography. The shear-wave splitting shows a consistent difference between the eastern and western Superior: the western Superior exhibits very large SKS splits (averaging 1.4 seconds) with a consistent ENE fast direction, while the eastern Superior exhibits more-variable (E to NE) fast directions, with smaller split times averaging 0.8 second. Travel-time tomography shows an overall pattern of higher velocities in the western Superior, with the anomaly interpreted by Sol et al. (2002) as a remnant slab possibly representing the eastern edge of the high-velocity region. In the eastern region, overall velocities are lower; we observe the linear low-velocity feature previously interpreted by Rondenay et al. (2000) as the track of the Great Meteor hotspot. A larger low-velocity anomaly in the centre of the model remains enigmatic, though its resolution will improve with the inclusion of additional data from new stations. Interpretation of these results is in progress; one possibility is that the mantle lithosphere beneath the western Superior is well-preserved, including remnant imbricated high-velocity slabs, while the lithosphere in the eastern region has been more extensively disrupted, perhaps due to thermo-mechanical erosion by a mantle plume and processes related to the Kapuskasing uplift. The large shear-wave splits in the west would then represent the combined effect of aligned lithospheric and asthenospheric fabric.