Generalized structure and composition of the deep crust and upper mantle in Canada

Abstract

Throughout most of Canada the regional Bouguer anomaly is inversely proportional to the depth of the crust-mantle boundary; the constant of proportionality indicates that the density contrast at the crust-mantle boundary is about 0.2 g/cm3. Seismic data suggest that compressional wave velocities of 7.0 to 7.5 km/sec occur near the base of the crust, and interpretations of magnetic and electromagnetic measurements in Canada combined with analyses of heat flow data and upper mantle seismic velocity distributions from other areas suggest that the temperature in the vicinity of the crust-mantle boundary in a stable area is of the order of 1000°K and that the vertical gradient of temperature is about 15° to 20°K/km. The above results, considered together with laboratory measurements of compressional-wave velocities in rocks and petrological investigations of mafic and ultramafic mineral assemblages, are interpreted as follows: in cratonic areas of Canada, rocks such as amphibolite and-or intermediate to basic granulite probably comprise the deep crust; garnet-peridotite is probably an important constituent of the upper mantle; rocks of a given composition should be stable at different levels in the crust and upper mantle, and therefore, assuming that the crust-mantle boundary represents a change in chemical composition, it can be understood why large fluctuations can exist in the depth of the crust-mantle boundary. In Canada, variations of compressional wave velocity in the upper mantle may be due to regional variations of deep heat flow in some areas and, in other areas, to anisotropic propagation of seismic waves. Lateral variations of density and compressional wave velocity in the upper mantle appear to extend to a depth of at least 100 km.