Seismic tomographic images of the cratonic upper mantle beneath the Western Superior Province of the Canadian Shield—a remnant Archean slab?

Knowledge of the velocity structure of the upper mantle beneath the Western Superior Province (WSP) is key to under-standing better the accretionary processes active during the Archean. To that end, teleseismic P- and S-wave travel times recorded as part of the Teleseismic Western Superior Transect (TWST) were inverted for their respective seismic velocities. This experiment involved 17 portable broadband stations arrayed in northern Ontario, Canada, so as to cross-cut the strike of many subprovinces as well as the boundary with the Proterozoic Trans-Hudson Orogen to the north. The 5-month deployment yielded 1423 P-wave and 651 S-wave high-quality residuals for inversion. The resulting tomographic images reveal three apparently-robust velocity anomalies represented by: (i) a dipping tabular high-velocity anomaly; (ii) a relatively shallow low-velocity anomaly directly above the positive anomaly; and (iii) a deep low-velocity body. The first anomaly may be interpreted in the Western Superior context as a 30–50 km thick eclogite/dunite layer representing remnant subducted oceanic lithosphere. The presence of such a body within the cratonic root would suggest its origin at around 2.7 Ga and the apparent SE–NW strike is noticeably oblique to the main EW trend of the subprovince boundaries. The low-velocity anomalies may be related to processes that occurred at the edges of the descending slab or they may be expressions of later upwelling material. The presence of a thick cratonic root (≈300 km) may also be revealed by the tomographic images. Overall, these travel time results are considered compatible with late Archean structures at depth resembling those of modern subduction tectonics