abstract

Analysis of seismic record sections constructed from recordings of the Early Rise explosions indicates that the stable blocks forming central and eastern Canada and at least the northeast part of the United States have similar upper mantle compressional velocity distributions. For these stable regions, apparent surface P velocities of 6.23, 7.17, 8.13, 8.48, 8.60, 9.27 and 10.34 km/sec were determined with an uncertainty of ± 0.05 km/sec. Rapid increases in velocity were found to exist within the upper mantle at depths of approximately 74, 107, 328, and 431 km. In the depth range from about 94 to 107 km beneath central and eastern North America, there is evidence for a low-velocity channel with an average velocity of 8.00 km/sec. Approximately 900 km southwest of Lake Superior, a rapid transition exists from this thin low-velocity channel to the much larger channel which lies beneath the western United States. Extending west from Lake Superior to a distance of about 850 km, there is an anomalous region having a lower crust of much higher velocity than the velocity of the lower crust beneath most of the continental area of central and eastern North America.

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