The TWiST Experiment

A teleseismic study of the Western Superior Archean lithosphere in Canada

The Working Group

University of Bristol
Michael Kendall – Dave Francis
Queens University
Colin Thompson – Stephane Sol
The Geological Survey of Canada
Don White – Isa Adudeh


NSERC Canada
The Royal Society

Research components

SKS Splitting
Seismic Tomography
Surface Wave Studies
Polarisation Analyses

The Western Superior province is the largest Archaean crustal block in the world and may have originated as the result of a widespread crustal accretion event (ca. 2.7 Ga) manifested in Archaean cratons worldwide. It is characterized by a pattern of alternating granite-greenstone and metasedimentary belts evident also in other Archaean cratons. An understanding of the acretionary evolution of this craton is the objective on the ongoing Lithoprobe transect in the Western Superior Province. The geophysical components of the transect include seismic reflection and refraction, magnetotellurics and teleseismic experiments. The Teleseismic Western-Superior Transect (TW~ST) was designed to explore the structural and physical properties of the subcrustal lithosphere and their implications for proposed accretionary models. 11 short-period 3-component (3C) seismometers and 14 broadband (BB) 3C instruments were deployed along a 600 km line from west of Thunder Bay to north of Pickle Lake, Ont. Three more BB sites operated at northern communities extending into the Trans-Hudson Proterozoic shear-zone. Anisotropy studies, surface wave analyses and travel-time tomography reveal variations between the southern end of the transect, a region affected by Keweenawan rifting, and the northern part, which lies in the Trans-Hudson. SKS analysis shows large (up to 2secs) amounts of shear-wave splitting with a roughly E-W trend in the fast-shear-wave polarization direction for most stations. This conforms with crustal-deformation trends. The splitting magnitude suggests anisotropy extends well into the upper-mantle, implying coupling with crustal tectonics. Stations in the younger Trans-Hudson Orogen show strikingly different results; both stations show an absence of SKS splitting suggesting a sharp change in the nature of the upper mantle beneath the Superior and Trans-Hudson. Surface waves reveal evidence for a thin very-high-velocity layer 5-20km thick which lies beneath a 37-43km thick crust and above a >200km thick high velocity upper-mantle layer. This thin layer may be evidence of underplating during terrane acretion. The surface wave particle motions also show evidence of an anisotropic mantle. The tomographic models shows anomalies in the southern end of the transect which may be associated with the Keweenawan rift. In contrast to the SKS results, the tomography shows little mantle signature of the transition between the Superior and Trans-Hudson provinces.


Kendall, J-M., S. Sol, C. J. Thomson, D. E. White, I. Asudeh, C. S. Snell, & F. H. Sutherland, Seismic heterogeneity and anisotropy in the Superior Province, Canada: Insights into the evolution of an Archaean craton, The early Earth: physical, chemical and biological development, eds C.M.R. Fowler, C.J. Ebinger and C.J. Hawkesworth, Geol. Soc. London, Spec. Pub. 199 , 27-44, 2002.

Sol, S., C. J. Thomson, J-M. Kendall, D. White, J. C. Vandecar and I. Asudeh, Seismic tomographic images of the cratonic upper mantle beneath the Western Superior Province of the Canadian Shield – A remnant Archean slab?, Phys. Earth Planet. Int., . 53-70, 2002.
Kay, I.,S. Sol, J-M. Kendall, C. J. Thomson, D. White, I. Asudeh, B. Roberts, and D. Francis, 1999, Shear wave splitting observations in the Archean craton of Western Superior, Geophys. Res. Lett., 26 , 2669-2672, 1999