Seismic heterogeneity and anisotropy in the Western Superior Province, Canada: insights into the evolution of an Archaean craton

Abstract The Superior Province, which forms the nucleus of North America, is the largest preserved Archaean crustal block in the world and may have originated as the result of a widespread crustal accretion event (c. 2.7 Ga) manifested in Archaean cratons worldwide. An understanding of the accretionary evolution of this craton is the objective of the continuing Lithoprobe transect in the Western Superior Province, Canada. The geophysical components of the transect include seismic reflection and refraction, magnetotellurics and teleseismic experiments. The Teleseismic Western Superior Transect (TWiST) was designed to explore the structural and physical properties of the subcrustal lithosphere and their implications for proposed accretionary models. A north-south-trending array of 17 broadband three-component seismometers was deployed between May and November 1997. Surface-wave analyses, SKS-splitting studies and travel-time tomography show variations in the velocity structure and anisotropy between the southern end of the transect, a region affected by Keweenawan rifting, and the northern part, which lies in the Proterozoic Trans-Hudson shear zone. Surface waves reveal evidence for a thin high-velocity layer, 5–20 km thick, beneath a 37–43 km thick crust and above c. 250 km of high-velocity continental root. This thin layer is also visible in wide-angle refraction data from the southern end of the line and may be evidence of underplating during terrane accretion. Discrepancies in the Love and Rayleigh waves and surface-wave particle motions show evidence for an anisotropic mantle. SKS analysis shows large amounts (up to 2 s) of shear-wave splitting with a roughly eastwest trend in the fast-shear-wave polarization direction for most stations. This conforms with crustal deformation trends. Stations in the younger Trans-Hudson orogen show much less splitting. Detailed analysis at a permanent station in the Western Superior shows evidence for two layers of anisotropy. A thinner upper layer is aligned with the surface geology, indicating crust-mantle coupling during craton formation, whereas a thicker lower layer is aligned with the direction of absolute plate motion. Tomographic results show a featureless mantle beneath the Sachigo proto-craton and more heterogeneity towards the south end of the line. A steeply dipping slab-like feature in the lithosphere correlates with wide-angle refraction and deep-reflection seismic profiles. A similar high-velocity feature continues well into the transition zone, but its origin remains to be understood. Towards the southern end of the line there is a deep-seated low-velocity anomaly, which may be associated with Keweenawan plume activity. As a whole, the seismic results show many features that support ideas of subduction-related accretion of a thick stable Archaean tectosphere. There are, however, interesting details that are to date unique to the Western Superior Province. These include thicker than normal Archaean crust, a slab-like velocity anomaly in the mantle transition zone, and large SKS splitting in the Archaean Superior Province but little splitting in the surrounding Trans-Hudson Proterozoic shear zone.