Crystalline lithology across the Kapuskasing Uplift determined using in situ Poisson's ratio from seismic tomography

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Seismic refraction tomography has been used to image lateral velocity variations (both Vp and Vs) within the shallow crystalline basement using travel times from a high‐resolution reflection profile across the Kapuskasing uplift structure in northern Ontario. Comparison of these in situ velocities and the outcrop geology indicates a correlation of velocity with the major lithologic units: anorthosite, Vp = 6150–6350 m/s and Vs = 2700–3000 m/s; mafic gneiss, Vp = 6000–6250 m/s and Vs = 2775–3000 m/s; tonalite, Vp = 5850–6000 m/s and Vs = 2950–3025 m/s. Vp measurements on core samples from the main lithologic units confirm that tonalite can be be clearly distinguished from mafic gneiss and anorthosite based on Vp. The observed Vp values are attributed to mineralogy, based on modal analyses of core samples from the same region. Poisson's ratio calculated from in situ Vp and Vs helps to discriminate between velocity variations caused by porosity effects from those due to lithology. Values of Poisson's ratio obtained for the three lithologic units, within the upper 100 m of the basement, are anorthosite, 0.35–0.39; mafic gneiss, 0.34–0.38; and tonalite, 0.32–0.34. These relatively high values of Poisson's ratio indicate that the shallow basement is porous and water‐saturated. Poisson's ratio is a particularly effective discriminator in this instance because of its extremely low value in quartz (0.08) relative to other silicate minerals (0.21–0.32) and the greater quartz content of tonalite (27–36%) relative to anorthosite and mafic gneiss (0–23%). Reflection coefficients calculated for the contrasting lithologies range up to 0.065 for tonalite‐mafic gneiss contacts and up to 0.044 for tonalite‐anorthosite contacts, confirming that an interlayered sequence of tonalitic, anorthositic and mafic rocks mapped at the surface may be the cause of the shallow reflections observed in the coincident seismic reflection data.