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

Abstract

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.