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TitreCrystalline lithology across the Kapuskasing uplift determined using in situ Poisson's ratio from seismic tomography
AuteurWhite, D J; Milkereit, B; Salisbury, M H; Percival, J A
SourceJournal of Geophysical Research vol. 97, no. B13, 1992 p. 19993-20006,
Séries alt.Commission géologique du Canada, Contributions aux publications extérieures 19591
Documentpublication en série
Mediapapier; en ligne; numérique
Lat/Long OENS -84.0000 -80.0000 50.0000 48.0000
Sujetslithologie; levés de refraction sismiques; levés sismiques, sol; données sismiques; profils sismiques; vitesse des ondes sismiques; levés de reflexion sismiques; levés sismiques; levés géophysiques; interprétations géophysiques; interpretations sismiques; porosité; roches plutoniques; tonalites; anorthosites; roches mafiques; roches ignées; méthodes analytiques; méthodes statistiques; analyse statistique; gneiss; roches métamorphiques; couverture meuble; quartz; nappe phréatique; Zone de Kapuskasing Structural ; géophysique; géomathématique; pétrologie ignée et métamorphique; Précambrien
Illustrationssketch maps; seismic models
ProgrammeCanadian Continental Drilling Program
Diffusé1992 01 01
Résumé(disponible en anglais seulement)
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 = 6150-350 m/s and Vs = 2700-000 m/s; mafic gneiss, Vp = 6000-250 m/s and Vs = 2775-000 m/s; tonalite, Vp = 5850-000 m/s and Vs = 2950-025 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-.39; mafic gneiss, 0.34-.38; and tonalite, 0.32-.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-.32) and the greater quartz content of tonalite (27-36%) relative to anorthosite and mafic gneiss (0-3%). 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.