Lithospheric structure and composition of the Archean western Superior Province from seismic refraction/wide-angle reflection and gravity modeling



[1] In 1996, LITHOPROBE acquired two ∼600 km long seismic refraction/wide-angle reflection profiles oriented parallel and orthogonal to the regional geologic strike of the Archean western Superior Province of the Canadian Shield. We present results from ray-based travel time inversion of these data, combined with modeling of the long-wavelength gravity field and independent estimates of Vp/Vs (γ), all of which constrain the structure and composition of the lithosphere to depths of 120 km. Large variations in lower crustal Vp (6.7–7.5 km s−1), lower crustal γ (1.72–1.86), upper mantle Vp (8.0–8.8 km s−1), and Moho depth (32–45 km) are determined. Velocities along orthogonal profiles indicate 8% azimuthal Vp anisotropy (fast propagation normal to strike) in part of the lower crust and >6% azimuthal Vp anisotropy (fast propagation parallel to strike) in a 15–20 km thick layer (layer H) in the upper mantle which dips northward at ∼10° from a minimum depth of 48–50 km. Wide-angle reflections identify a deep boundary at ∼110–120 km depth along both lines. Lower crustal chemical compositions estimated from Vp, γ, and density are generally intermediate (57–66% SiO2) requiring the presence of 25–60% granitic rocks, which is significantly higher than previous estimates (30% intermediate or felsic granulite) for average Archean crust. The anisotropic lower crustal layer characterized by anomalously high Vp, high γlc, and low (−7%) relative density values is interpreted as an amphibolite (basic-intermediate composition) having a subhorizontal lineation to account for 8% azimuthal Vp anisotropy. The high Vp and intermediate anisotropy of upper mantle layer H requires a harzburgite peridotitic composition with the a axis of olivine aligned E-W. We suggest that both of these layers are relic oceanic lithosphere accreted at the base of the crust during the final stages of lithospheric assembly in the western Superior Province at ∼2.7 Ga.