Deep seismic sounding of the earth’s crust was conducted along a line of seven recording sites running from Flin Flon to Mafeking, Manitoba. Two interfaces, the Conrad and the Mohorovičić discontinuities, were distinguished. The following head‐wave types were observed: P*, SPP, SPS, S*, [Formula: see text], PSPSP, SSPSS, SPPPS, SSPPS, and PSPSS. Even though only a single shotpoint was used, it was found possible to determine velocities, layer thicknesses, and structure, using the interpretational possibilities of converted head waves. Discontinuities were determined as follows: Conrad—rises from 15.5±1‐km depth near Flin Flon to something just under 10‐km depth near Mafeking; Mohorovičić—rises from 34±1‐km depth near Flin Flon to 31±1‐km depth near Mafeking. Velocities are as follows upper layer—[Formula: see text], [Formula: see text]; intermediate layer—[Formula: see text], [Formula: see text]; lower layer—[Formula: see text]. Ratio of vertical to horizontal radial displacement was experimented with as a method of identifying converted head waves, and was found to be definitive for head waves arriving from the Mohorovičić as an S wave. Traveltime was found to be effective in wave identification. Comparison of the observed gravity curve with the curve computed for the seismic model indicates that depths and densities related to the model do not totally explain the observed gravity. There is reason to believe that masses of anomalous density within the granite layer are responsible for the disagreement.
This paper presents an overview of six geophysical projects (seismic reflection and refraction, gravity and magnetic anomaly interpretation, specific gravity and magnetic property measurements) carried out in an area in Manitoba and northwestern Ontario bounded by 93 and 96°W longitude, and 49 and 51°N latitude.The purpose of the surveys was to define crustal structure in the Kenora–Wabigoon greenstone belt, the Winnipeg River batholithic belt, the Ear Falls – Manigotagan gneiss belt, and the Uchi greenstone belt. The following conclusions emerge.In all of the belts, a major discontinuity divides the crust into the commonly found upper and lower crustal sections. At the top of the lower crust, a seismically distinct layer (the mid-crustal layer) occurs. Seismic velocities in this layer suggest either intermediate to basic igneous rocks or metamorphic rocks of the amphibolite facies.Crustal geophysical characteristics vary sufficiently among the four belts to justify the classification of all four as distinct subprovinces of the Superior Province.Cet article présente une vue générale sur six projets de géophysique (réflexion et réfraction sismique, interprétation d'anomalies de gravité et magnétiques, déterminations de densité et de propriétés magnétiques) réalisés dans une région du Manitoba et du nord-ouest de l'Ontario encadrée par les longitudes 93 et 96°O et les latitudes 49 et 51°N.
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