GEOSCAN, résultats de la recherche


TitreSeismic structure of the continental crust based on rock velocity measurements from the Kapuskasing Uplift
AuteurFountain, D M; Salisbury, M H; Percival, J
SourceJournal of Geophysical Research vol 95, no 2, 1990 p. 1167-1186,
Séries alt.Commission géologique du Canada, Contributions aux publications extérieures 54188
Documentpublication en série
Mediapapier; en ligne; numérique
Sujetsdiscontinuités; faciès à granulite; faciès à amphibolite; roches plutoniques; ondes P; mécanique des roches; croûte continentale; gneiss; séismologie; tectonophysique; ondes sismiques; roches métamorphiques; Bouclier Canadien; Province de Superior ; géologie structurale; Précambrien
Illustrationstables; sketch maps
Diffusé2012 09 20
Résumé(disponible en anglais seulement)
The Michipicoten greenstone belt (MGB), Wawa gneiss terrane (WGT), and Kapuskasing structural zone (KSZ) in Ontario, Canada, are regarded as a 25?km?thick partial cross section through the crust of the Superior Province. We measured compressional wave velocities (Vp) at confining pressures up to 600 MPa for representative rock samples from these terranes. Vp for MGB is variable (6.1–7 km/s) and depends on composition and anisotropy. Compressional wave velocities for low anisotropy quartzofeldspathic gneisses and intrusive rocks from the WGT and the KSZ show little variation over a wide range of silica content but mafic gneisses and anorthositic rocks show a wide velocity range over a narrower silica range, a trend corroborated by data for other high?grade rocks. A model of the continental crust based on the laboratory data shows three megalayers with differing seismic characteristics. The shallowest levels (MGB) are seismically heterogeneous with high average velocities (6.6 km/s at 100 MPa) because of the dominance of metabasalts. The balance of the upper crust (WGT) is seismically homogeneous as it is dominated by a variety of isotropic quartzofeldspathic rocks with average velocities of 6.35 km/s at 100 MPa and 6.6 km/s at 600 MPa. The uppermost lower crust (KSZ) is layered, seismically heterogeneous and transversely isotropic. The average velocity at 600 MPa is 6.84 km/s. The boundary between WGT and KSZ is marked by a velocity increase of between 0.2 and 0.3 km/s, an increase similar to that commonly associated with midcrustal discontinuities (i.e., Conrad) in shield areas. Here, this boundary corresponds to the amphibolite?granulite facies transition and to a change from a tonalitic middle crust to a lithologically heterogeneous uppermost lower crust. Refraction results show an anomalous high?velocity zone in the upper crust under the KSZ that, on the basis of laboratory velocity data, can be correlated with KSZ lithologies. These data indicate that KSZ rocks can be traced to the west to depths as great as 20 km. Levels deeper than 20 km, not exposed at the surface in the KSZ, are characterized by velocities greater than 7.0 km/s and may be dominated by mafic and anorthositic lithologies or by more garnet?rich rocks. The velocity model of the Superior Province crust based on geological reconstruction and laboratory velocity data is in general agreement with the velocity structure to depths of about 25 km determined for the crust below the MGB from refraction experiments. The velocity structure of this model is also generally similar to the velocity structure of shield areas.