Abstract

The compressional (Vp) and shear (Vs) wave velocity structure of the Kapuskasing uplift have been determined as a function of depth, propagation direction, and polarization from laboratory velocity measurements to confining pressures of 600 MPa on oriented samples from known structural levels of the complex. Based on the relative field abundances of the lithologies measured, the three principal terranes exposed in the uplift are characterized at depth by the following average values of Vp, Vs, and apparent Poisson's ratio, σa: (i) Michipicoten greenstone bell (greenschist, depth 0–6 km, Vp = 6.6 km/s, Vs = 3.9 km/s, σa = 0.235); (ii) Wawa gneiss terrane (amphibolite, depth 6–17 km, Vp = 6.5 km/s, Vs = 3.8 km/s, σa = 0.24); and (iii) Kapuskasing structural zone (granulite, depth 17–23 km, Vp = 6.9 km/s, Vs = 3.9 km/s, σa = 0.27). Although anisotropic lithologies such as paragneiss or mafic gneiss are present at all levels and tend to increase in abundance with depth, only in the deepest level (the Kapuskasing zone) are they sufficiently abundant and oriented to produce significant regional seismic anisotropy (transversely isotropic with Vp and Vs fast in the horizontal plane) and detectable shear wave splitting (ΔVs = 0.1 km/s).A comparison between the laboratory data and velocity models determined for the same crustal section from Lithoprobe refraction studies shows excellent agreement, confirming that the lithologies exposed in the Kapuskasing uplift can be projected downdip to the upper–lower crust transition, or Conrad discontinuity, at about 25 km. Below this depth, high P-wave velocities (7.0–7.6 km/s) suggest that the lower crust is more mafic or garnet rich. Similarities between the velocity structure of the Kapuskasing uplift and other sites in the Canadian Shield suggest that the observed crustal section is fairly typical of Archean continental crust.

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